Atomic Fluorescence in Environmental Analysis
Yong Cai
in
Encyclopedia of Analytical Chemistry
R.A. Meyers (Ed.)
pp. 2270–2292
 John Wiley & Sons Ltd, Chichester, 2000
1
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
Atomic Fluorescence in
Environmental Analysis
1 THEORETICAL ASPECTS
1.1 Types of Atomic Fluorescence
Yong Cai
Florida International University, Miami, USA
1
Theoretical Aspects
1.1 Types of Atomic Fluorescence
1.2 Intensity of Fluorescence Radiance
1.3 Quenching
1.4 Interferences
1
1
1
2
3
2 Instrumentation
2.1 Excitation Sources
2.2 Atomizers
2.3 Wavelength Selection and Detection
Systems
3 Environmental Applications
3.1 Mercury
3.2 Hydride-forming Elements
3.3 Non-hydride-generation Methods
3
3
5
6
7
7
11
15
4 Conclusions
Acknowledgments
Abbreviations and Acronyms
Related Articles
References
16
17
17
17
18
Atomic fluorescence is a spectroscopic process which is
based upon the absorption of radiation of a certain
wavelength by an atomic vapor and subsequent radiational deactivation of the excited atoms toward the
detection device. Both the absorption and the subsequent atomic emission processes occur at wavelengths
which are characteristic of the atomic species present.
Atomic fluorescence spectroscopy (AFS) is a very sensitive and selective method for the determination of a
number of environmentally and biomedically important
elements such as mercury, arsenic, selenium, bismuth,
antimony, tellurium, lead, and cadmium. This technique
has become one of the most important analytical tools
for trace element analysis in environmental samples, such
as mercury, owing to its advantages over other methods
in terms of linearity and detection levels. Several books
and a number of book chapters and review articles have
been published dealing with the theory and instrumentation of AFS. This article will provide up-to-date AFS
information regarding its application in environmental
analysis.
Encyclopedia of Analytical Chemistry
R.A. Meyers (Ed.) Copyright  John Wiley & Sons Ltd
The main types of atomic fluorescence process are illustrated in Figure 1(a – c), which shows diagrammatically
the transitions involved. Resonance fluorescence occurs
when atoms absorb and re-emit radiation of the same
wavelength (Figure 1a). Many of the measurements made
by analytical chemists employing AFS involve this type
of fluorescence. The most analytically useful fluorescence
line is the resonance fluorescence corresponding to the
transition between an electronic excited state and the
ground state of the atom..1/ The wavelengths of the
absorption (lA ) and fluorescence (lF ) can also be different. If the photon energy of fluorescence is less than
the photon energy of absorption, i.e. lF > lA , the process is called Stokes-type fluorescence. If the reverse is
true, i.e. lF < lA , this process is called anti-Stokes type
fluorescence..1 – 3/ Direct line fluorescence is nonresonance
fluorescence, which is observed when a common upper
level is involved for both excitation and fluorescence.
Figure 1(b) shows an example of direct line fluorescence
in which an atom is excited from the ground state to
a higher excited electronic state and then undergoes a
direct radiational transition to a metastable level above
the ground state. Stepwise line fluorescence (Figure 1c)
occurs when the upper energy levels of the exciting
and of the fluorescence line are different. The excited
atoms may undergo deactivation, usually by collision,
to a lower excited state rather than return directly to
the ground state. There are several more fluorescence
lines which are not of much analytical use. Details
about those fluorescence lines can be found in the
literature..1 – 6/
Generally, an atomic fluorescence spectrum consists of
only a few lines because not all of these fluorescencegenerating processes are active simultaneously..7/ Therefore, the atomic fluorescence spectrum is much simpler
than the one obtained using atomic emission spectroscopy
(AES).
1.2 Intensity of Fluorescence Radiance
It is beyond the scope of this article to provide details
about the derivations of the intensity expressions. Only
the results of the derivations will be given in order to
understand the various dependences of the fluorescence
signal. For a more comprehensive discussion and a
complete derivation of the expressions, articles by
Winefordner et al..4,6,8/ can be highly recommended.
The intensity of the fluorescence radiation produced
in a transition depends on a number of factors, the
most important of which are.5,7/ (a) the intensity of the
excitation source, (b) the concentration of the atom in
2
ENVIRONMENT: WATER AND WASTE
radiance.4,5,7/ can be given by Equation (1):
Z 1
l
Y21 En12
BF D
kn dn
4p
0
1
A
0
(a)
F
Resonance fluorescence
2
F
1
A
0
where BF is the absolute radiance; l is the pathlength
in the direction of the detector; Y21 is the fluorescence
power efficiency, Wfluoresced /Wabsorbed ; En12 is the spectral
irradiation of source at absorption line of frequency n12 ;
and kn is the absorption coefficient.
The integration term gives the integrated absorption
coefficient over the absorption line, which is a function
of the concentration in states (1) and (2), the statistical
weights of state (1) and state (2), and the Einstein B12
absorption coefficient.
The above equation gives some analytically important
information. The fluorescence radiance BF is linearly
dependent on the source irradiance and the fluorescence
quantum efficiency of the transition as long as En12 is very
much below the saturation value. Therefore, in order
to obtain a large fluorescence signal it is necessary to
use a highly irradiant light source. It can be further
demonstrated that if the atomic concentration is low, BF
is linearly related to the total concentration of atoms in
all states..3/
Direct line fluorescence
(b)
1.3 Quenching
2
1
A
0
(c)
.1/
F
The intensity of the atomic fluorescence can be diminished
by the collision between excited atoms and other
molecules in the atomization sources. This process is
called quenching. This problem is accounted for by the
inclusion of the term Y21 in the fluorescence radiance
equation given above. Since the quenching process is
very important in atomic fluorescence, a few examples
are given below. For a more comprehensive discussion
on quenching processes, the reader is referred to several
good reviews..2,9/
Stepwise line fluorescence
1.
Figure 1 Basic types of atomic fluorescence lines.
the atomizer, (c) the quantum efficiency of the process
(i.e. the ratio of the energy emitted in fluorescence to the
energy absorbed, per unit of time), and (d) the extent of
self-absorption in the atomizer.
In the derivation of intensity expression, it has been
assumed that the atom under consideration has only
two energy levels, ground state (1) and first excited
state (2)..4/ Other important assumptions are that the
atoms are uniformly distributed in the atomizer and
the concentration of the analyte atom is low, and the
temperature of the atomizer and the radiation density
of the source are spatially uniform. The fluorescence
The excess electronic energy of the excited atoms is
converted into translational energy of the colliding
species without involving the internal energy of the
latter. For instance, this quenching can be processed
by collision with free atoms (Equation 2):
AŁ C B
!ACB
.2/
or with free electrons (Equation 3):
AŁ C e
2.
!ACe
.3/
The excess electronic energy of the excited atoms
is released on collision, resulting in changes of the
electronic or vibrational/rotational energy of the
colliding species, for instance quenching by collision
3
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
2.1 Excitation Sources
with other atoms (Equation 4):
AŁ C B
! A C BŁ
.4/
or with molecules (Equation 5):
AŁ C BC
! A C BCŁ
.5/
Depending on the experimental conditions used, some
of these quenching processes can significantly affect the
fluorescence radiance.
1.4 Interferences
As with atomic absorption spectroscopy (AAS) and AES
methods, interferences of two major types are encountered in atomic fluorescence..10/ Spectral interferences
arise when lines in the source overlap lines of the matrix
elements in the atomizer. Chemical interferences result
from various chemical processes during atomization that
reduce the population of free atoms.
Spectral interferences are seldom caused by line
sources, but may be a problem with nondispersive
instruments or in dispersive apparatus with continuum
sources. Since generally there are several fluorescence
lines available, it is sometimes necessary to select
different lines to avoid the spectral interference. Chemical
interferences caused by matrix components are one of
the major problems in atomic fluorescence. This type
of interference can be reduced by introducing different
chemicals into the matrix, such as releasing agents and
protective agents..10/ Coupling to vapor generation can
also significantly reduce the problem.
2 INSTRUMENTATION
The basic layout of an AFS instrument is similar to
those for AAS and AES except that the light source and
the detector are located at a right-angle. A schematic
diagram of an AFS instrument is shown in Figure 2.
Although the configuration of an AFS instrument can
be modified for different purposes, the basic apparatus
consists of a radiation source, an atomizer, a wavelengthselection system, a signal detector, and an electronic
readout system.
Atomizer
hν Wavelength
selector
hν
Detector
I
hν
Light source
Figure 2 Schematic diagram of an AFS instrument.
Readout
A number of excitation sources have been used in AFS
and can be categorized into two types, spectral line
sources and continuum sources. Since the intensity of
the fluorescence radiance is proportional to the intensity
of the exciting radiation, sources with high radiance
are required in order to achieve a good sensitivity
and linear range. Indeed, a major part of the effort
devoted to improvement of AFS instrumentation in
recent years has been concerned with the excitation
source. Winefordner discussed the criteria of importance
in choosing excitation source for AFS and provided some
general considerations..3,4/ Briefly, in addition to a high
radiance over the absorption line (in order to obtain a
good detection limit (DL) and linear range), the source
should have good short- and long-term stability. In view
of the need for convenience and ease of use, the source
used should also require a minimum of maintenance and
adjustment to obtain optimum performance. Only the
most commonly used and recently developed sources are
discussed here.
2.1.1 Hollow-cathode Lamps
Although the conventional, and commercially available,
hollow-cathode lamp (HCL) is the most widely used line
source used in AAS,.2,5/ the early version of the HCL
did not show impressive potential as a useful radiation
source for AFS because it does not possess sufficient
intensity. Different efforts have been made to improve
the design of HCLs..1,2,5,11,12/ For instance, the lamps can
be operated using short-duration pulses of current with a
sufficient off-period between pulses to maintain the mean
lamp current at a low level. The pulsed lamps provide
an increase in peak intensity and a reduction of the total
signal observation time..1/ Compared with regular HCLs,
good results have been obtained with pulsed HCLs for a
large number of elements..1,5,13/
The boosted discharge hollow-cathode lamp
(BDHCL), or boosted output HCL as it is sometimes
called, produces intense spectra with narrow line widths
and can be manufactured to provide an excellent source
for many elements..1,5,14/ Since its introduction as an AFS
excitation source by Sullivan and Walsh,.15/ it has received
a great deal of attention and a number of modifications
to this type of source have been made..1,11,16/ Recently,
BDHCLs have become commercially available and this
contributed significantly to the availability of commercial
AFS instruments..12/
The operation principle of the BDHCL is illustrated
in Figure 3..14/ The lamp consists of an anode mounted
behind a cylindrical cathode. A primary discharge is
struck between the cathode and anode to sputter
atoms of the element of interest, as in a normal
4
ENVIRONMENT: WATER AND WASTE
Cylindrical
cathode
Most or all
atoms excited
Normal hollowcathode discharge
Light emitted from
front of cathode area
with negligible self-absorption
Anode
Electron emitter
available for many years..1,5/ The lamp produces intense
radiation by the passage of current through an atmosphere
of xenon. The spectrum is continuous over the range
250 – 600 nm, with the peak intensity about 500 nm..10/
The advantage of this type of continuum source is that
it can be readily employed for multielement analysis
along with the use of a monochromator or interference
filters. However, its intensity falls off severely below about
210 nm, thus making it unsatisfactory for the analysis of
some environmentally important elements such as arsenic
and selenium.
Auxiliary boost
discharge
2.1.4 Laser Sources
Figure 3 Schematic diagram of the boosted discharge HCL
lamp. (Reproduced from PSA 10.055 Millennium Excalibur, User Manual, Issue 1.0, March 1998, by permission of
P. S. Analytical.)
HCL. However, a secondary discharge (boost) is then
struck between an efficient electron emitter and the
anode, passing through the primary atom cloud. The
use of a specifically developed high-efficiency electron
emitter permits operation of the secondary discharge so
that concentration broadening and line broadening are
minimized.
2.1.2 Electrodeless Discharge Lamps
Electrodeless discharge lamps (EDL) are useful sources
of atomic line spectra and provide radiant intensities that
are usually one or two orders of magnitude greater than
the normal HCLs..10/ This type of lamp has been widely
employed for AFS. A typical lamp is constructed from a
sealed quartz tube containing a few torr of an inert gas
such as argon and a small quantity of the metal of interest
(or its salt). The lamp does not contain an electrode but
instead is energized by an intense field of radio frequency
or microwave radiation. Radio and microwave frequency
electromagnetic fields are very efficient for generating and
accelerating electrons and thereby maintaining a gaseous
glow discharge without internal electrode contact with
the plasma.
The characteristics of EDLs have been studied and
discussed in some detail..5,17/ Useful recommendations
have been given by Van Loon.5/ to predict lamp behavior
and to select operating conditions for the EDLs for
AFS. The main drawbacks of the EDLs are that their
performance does not appear to be as reliable as that
of the HCLs (signal instability with time) and they are
commercially available only for some elements.
2.1.3 Xenon Arc Lamps
High-pressure xenon arc lamps, producing a stable
and intense continuum source, have been commercially
Lasers have become important sources in analytical
instrumentation because of their high intensity and
narrow bandwidths and the coherent nature of their
outputs. As a consequence of these unique properties,
laser-induced fluorescence (LIF) offers a very sensitive
and selective spectroscopic method, which has a low
susceptibility to spectral interferences. Compared with
conventional light sources, laser excitation also allows
nonresonance transition lines to be used for many
elements, which significantly reduces any laser radiation
scattered off the vaporized sample.
LIF has been used for the determination of many
elements in a variety of samples,.18 – 23/ including environmental matrices..21,24,25/ The most sensitive DLs that have
been reported have been as low as a few attograms..21,26/
The basic requirements for the laser include wavelength
tunability across the ultraviolet (UV) and visible spectrum to allow for the determination of as many elements
as possible. The most commonly used LIF is the pulsed
excimer- or yttrium aluminum garnet (YAG)-pumped
tunable dye laser because it provides sufficient peak
energy and wavelength tunability. However, this type of
source has some drawbacks, such as the need to replace
the dry solution, which is often toxic and flammable,
before a wavelength change of more than 20 – 30 nm,
and dye degradation with time..21/ These disadvantages
have significantly limited the application of this type of
laser source to routine sequential multielement analysis. Research has been conducted to find alternatives to
the pulsed dye laser. Recently, Zhou et al..21/ reported a
pulsed (10 Hz) optical parametric oscillator (OPO) laser
system based on b-barium borate crystals and equipped
with a frequency-doubling option for use in LIF. This
all-solid-state laser has a narrow spectral line width and
wide spectral tuning range (220 – 2200 nm). It has been
used for cobalt, copper, lead, manganese, and thallium
analysis in Buffalo River sediment samples.
The production of far-UV radiation is required for excitation of some elements, such as arsenic at 193.70 nm and
selenium at 196.03 nm. With the availability of b-barium
5
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
borate crystals, tunable UV radiation is now generated
at wavelengths down to 205 nm by frequency-doubling.
Sum frequency generation or stimulated Raman shifting
techniques can further extend the UV range down to
187 nm with reasonable efficiency, making the determination of metalloids feasible..22/ Recently, Swart et al..22/
reported an LIF technique for selenium, arsenic, and
antimony analysis. The far-UV radiation required was
accomplished by stimulated Raman shifting of the output
of a frequency-doubled dye laser operating near 230 nm.
2.1.5 Mercury Arc Lamps
The mercury arc lamp is a type of vapor-discharge lamp
that forms simple, robust sources of line spectra..10/ The
low-pressure mercury-vapor lamp equipped with a fusedsilica window has been the most common source for
filter fluorometers. This source produces intense lines at
254, 366, 405, 546, 577, 691, and 773 nm. Individual lines
can be isolated with suitable absorption or interference
filters. The mercury lamp has been used in commercially
available instruments with a 254-nm filter for mercury
analysis..27/
2.2 Atomizers
Many atomizers used for AFS are similar to those
used for AAS and AES. The design and properties of
these atomizers have been discussed comprehensively
in terms of their applications for AFS..1 – 3,7/ The basic
requirements of an atomizer for AFS are an efficient
and rapid production of free atoms with minimum
background noise, long residence time of the analyte
atoms in the optical path, low quenching properties, low
cost of operation, and ease of handling. The quenching
process is especially important for AFS and has been
taken into consideration in the design of the atomizer.
The order of quenching efficiency for some gases is
Ar < H2 < H2 O < N2 < CO < O2 < CO2 ..28/ A number
of atomizers have been developed and used for AFS, and
some improvements have been made in recent years.
2.2.1 Flame Atomizers
Perhaps the most commonly used flame atomizer.5,7/
in early studies was a premixed laminar flame with
inert gas sheaths. Without using inert gas sheaths, the
premixed laminar flame, in particular the acetylene-based
flame, exhibits high background emission due to radicals
present in the secondary reaction zone (OH, C2 , CH,
CN) resulting in poor DLs. By preventing entrainment
of the surrounding atmosphere into the flame gases,
the secondary reaction zone may be shifted away from
the primary reaction zone, thus extending the region of
interconal zone, which is the most used part of the flame
for spectroscopy. Separation of the secondary reaction
zone can be effected using either nitrogen or argon.
Argon is favored over nitrogen because of its greatly
reduced quenching cross-section.
Flame emission and the associated flame flicker noise
considerably increase the limits of detection in AFS.
Flames with low background emission are therefore necessary. Diffusion flames, sometimes called cool flames,
were developed and used for this purpose. The term
diffusion flame is generally applied to flames in which
the oxidant necessary for combustion is fully supplied
by diffusion and/or entrainment from the surrounding
atmosphere..1/ Argon is generally used as carrier gas and
hydrogen is used as fuel gas. The temperature of these
argon – hydrogen – air flames ranges from 280 to 850 ° C,
depending on the flame region selected. Despite the relatively low temperature achieved by these flames, they are
still widely used as efficient atomizers for the analysis of
some environmentally important elements. Indeed, a very
simple atomizer based on an argon – hydrogen diffusion
flame has recently been employed in a commercially
available AFS system for hydride-forming elemental
analysis..12/ The argon – hydrogen diffusion flame used
emits very low background radiation over the wavelength
region of interest, and the hydride compounds are easily
decomposed in such a low-temperature flame. In addition,
the quenching effect is relatively low in the argonsupported flame and in hydrogen.
Different designs of diffusion flame atomizers have
been proposed and studied..12,29 – 32/ A simple glass or
silica tube (i.d. 4 – 8 mm) has been used to support the
argon – hydrogen flame..31/ Corns et al. recently investigated four different types of diffusion flame atomizers
for arsenic analysis using hydride-generation/atomic fluorescence spectroscopy (HG/AFS) technique..12/ From
this comprehensive study, they concluded that the
AFS signals were observed only in the presence of
argon – hydrogen flames. The attempt to achieve atomization using an electrically heated atomizer alone, without
an argon – hydrogen diffusion flame, failed. These results
provided further evidence to support the atomization
mechanism proposed by several workers..1,33 – 36/ The
atomization at such low temperature (about 800 ° C) is
not due to thermal decomposition, but to free radicals in
the flame.
2.2.2 Electrothermal Atomizers
Although the flames discussed above are very convenient and have been widely used as atomization cells
in AFS, they do have disadvantages for some analytical work. The high dilution of the sample by flame
gases limits the atom concentration attainable. The large
sample requirement with continuous nebulization makes
6
it unsuitable for small-volume sample analysis, and the
flame cells are not convenient for the direct atomization
of solid samples. Precise control over the chemical environment in the flame is impossible. These drawbacks of
the flame atomizer can be overcome by using electrothermal atomization (ETA) techniques. These electrothermal
atomizers are designed in different shapes and are generally made of graphite. Graphite atomizers are operated
at high temperature and the atoms are contained in an
inert gas atmosphere. This provides a high atomization
efficiency and reduced quenching, consequently offering
high sensitivity for small volumes of sample. It has been
suggested that the most successful atomic atomizer for
LIF spectrometry is the graphite furnace..19/
LIF coupled with ETA has been one of the main
research fields for many scientists in the past two
decades..19,20,22 – 24,37 – 40/ The development of electrothermal atomizers used for AFS benefits greatly from the
analytical knowledge accumulated over the years with
the well-established atomic absorption technique. Generally, certain modifications of the conventional graphite
furnaces are required if they are to be usable in LIF.
The optical detection of fluorescence needs to be made at
90° with respect to the excitation axis. The following are
the typical forms of the graphite furnace studied for AFS:
graphite cup,.37/ Massmann cup,.41/ graphite rod,.42,43/ and
graphite tubes..38,39/ There seems to be a general consensus that graphite-tube atomization is preferable to other
graphite atomization techniques,.39/ and it has been used
in most of the later studies..19,20,22/ With graphite-tube
atomization, the atoms are contained in a hot environment while being excited by the laser, and therefore much
better analytical performance can be achieved.
Besides examining a variety of designs and shapes
for the furnace, other studies, such as those atomizing
at a pressure-controlled.20/ and pyrolytically coated.44/
graphite atomizer, have also been conducted to improve
the performance of these furnaces. The use of lowpressure vaporization to remove matrix interferences
associated with the analysis of solid samples has been
coupled with laser-induced fluorescence/electrothermal
atomization (LIF/ETA)..45/ A lower DL was found by
using the low-pressure vaporization technique. Using a
coating on graphite furnaces was initially conducted in
ETA in the AAS technique to inhibit reaction between
the analyte and the graphite..46/ Among the different
coating methods used for the graphite atomizer in AFS,
a pyrolytic coating has been the most popular..19,20,38/
2.2.3 Cold-vapor Technique for Mercury
Mercury is the only metallic element that exhibits an
appreciable atomic vapor pressure at ambient temperature. The mercury has a vapor pressure of 0.0016 mbar
ENVIRONMENT: WATER AND WASTE
(1 mbar D 102 Pa) at 20 ° C, corresponding to a concentration of approximately 14 mg m 3 in air..46/ This unique
property gives rise to the possibility of measuring mercury without the additional thermal energy supplied by a
flame or electrothermal heating. The cold-vapor atomic
absorption spectrometry (CVAAS) technique for mercury analysis has been studied extensively. This method is
readily applicable to AFS [cold-vapor atomic fluorescence
spectrometry (CVAFS)]. Briefly, the mercury vapor, produced by reduction of the metal from its compounds
with suitable reductants [sodium borohydride or tin(II)
chloride], is swept out of the reaction vessel using argon
carrier gas and introduced into the optical beam where
the mercury atoms can be excited with a suitable source.
2.2.4 Miscellaneous Atomization Techniques
The inductively coupled plasma (ICP) has been proposed
as an atomizer for AFS..47,48/ Plasmas have better
vaporization and atomization efficiency than flames
and give less interference problems. Within an argon
atmosphere, ICP is characterized by a high fluorescence
yield because of the small quenching cross-section.
Background emission is dependent on the viewing region
but is generally higher than in flame..28/ Although the
ICP shows some advantages as an atomizer, the high cost
compared with flame and electrothermal atomizers has
discouraged its widespread application to routine AFS.
The atomization of an analyte can be also carried out
on the principle of an HCL in which the sample is vaporized by a glow discharge..46/ Recently, a microsecond
pulsed glow discharge was studied as an analytical spectroscopic source for solid-sample analysis..49/ The results
showed enhanced efficiency for analytical response of the
sputtered sample atoms.
2.3 Wavelength Selection and Detection Systems
The simplicity of atomic fluorescence spectra makes the
isolation and detection of the selected atomic fluorescence
lines easier than in other atomic spectrometric techniques
such as AAS and AES. Both a monochromator and an
interference filter have been used as wavelength selectors
with different advantages and limitations. The requirements for wavelength selection depend on the light source
employed. With conventional line sources (HCL, EDL)
or laser excitation, low-dispersion monochromators or
nondispersive systems can be used. The nondispersive
instruments are simple and cheap, and readily adaptable to multielemental analysis. They also have a high
energy throughput and thus high sensitivity..10/ However,
to realize these advantages, it is necessary that the output
of the source is free of contaminating lines from other
elements, and no significant background radiation should
occur. In most of the instruments, a filter located between
7
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
the source and detector has been used to remove background radiation. With a continuum excitation source, a
dispersive system is required to discriminate against the
scattering of source radiation and the fluorescence radiation from other species over the entire ultraviolet/visible
(UV/VIS) region..28/
Fluorescence radiation is usually detected with a
photomultiplier tube (PMT) as in many other spectrometers..7,10/ The PMTs are highly sensitive to UV and
visible radiation and have extremely fast response
times.
Since the wavelength selection and detection systems
used in AFS are similar to those of AAS and AES,
readers are referred to other comprehensive discussions
in the literature..10,46/ Over the years, a number of
AFS instruments have been developed, and have either
been used exclusively in research laboratories or have
only recently become commercially available. To the
author’s knowledge, AFS instruments for the analysis
of mercury and some other elements are currently
being manufactured by three companies: P. S. Analytical
Ltd, Sevenoaks, Kent, UK, Brooks Rand Ltd, Seattle,
WA, USA, and Tekran Inc., Ontario, Canada. Different
features of these instruments will be discussed in
the following sections along with their applications in
environmental analysis.
3 ENVIRONMENTAL APPLICATIONS
3.1 Mercury
Mercury pollution has become a global problem because
of its wide occurrence in the environment. The toxicological effects of mercury compounds on environmental
systems have long been recognized. Therefore, mercury
monitoring is a special concern in the field of heavy metal
analysis. AFS-based techniques have been widely used
in environmental analysis. The resonance fluorescence
line at 253.7 nm (3 P1 – 1 S0 transition) has been generally
used in most atomic fluorescence studies on mercury.
AFS instruments are now commercially available for
both total mercury and organomercury species analysis.
3.1.1 Total Mercury
3.1.1.1 Instrumentation
Traditionally, CVAAS has
been the most commonly used method for determining mercury at trace levels. This technique has
been well established and several commercial systems
are available..46/ CVAFS was proposed almost three
decades ago as a method for mercury analysis..50 – 52/ The
advantages of AFS over AAS, in terms of sensitivity, linear range, and spectral interferences, have been demonstrated both theoretically.53/ and experimentally..50,54/
However, it was only during the beginning of the
1990s that AFS became commercially available..55,56/
AFS detection, especially coupled with the vapor generation technique, is becoming popular and replacing
AAS for mercury analysis in many research and service
laboratories owing to its unique sensitivity, specificity,
and simplicity.
Similarly to CVAAS, a general CVAFS system consists
mainly of two parts, the mercury vapor generator
and the AFS detector. A schematic diagram of a
continuous-flow vapor generator is shown in Figure 4..27/
The reductant, blank (1% HCl), and sample solutions
are delivered by variable-speed multichannel peristaltic
pumps. An electronically controlled switching valve
alternates between blank and sample solutions, and two
of the liquid streams (reductant and sample or reductant
and blank) are butt-mixed in the sample valve, where
the reaction starts to occur. The steams and all gaseous
products are continuously and rapidly pumped into a glass
gas – liquid separator, from which the gaseous products
are carried, by argon gas, through a dryer system, finally
reaching the AFS detector.
The design of the AFS detector for mercury analysis
is relatively simple owing to the absence of a thermally
energized atomizer..55,57/ Generally, a UV mercury vapor
lamp is used as excitation source and the fluorescence light
is detected by a PMT, which is positioned perpendicular
to the excitation source. Figure 5 is a schematic diagram
illustrating the optical configuration of an AFS system..27/
A glass chimney is used for introduction of mercury
vapor into the optical path. The chimney is shielded with
a high flow rate of argon. A 253.7-nm interference filter
is employed between the introduction chimney and the
PMT to keep stray light away from the latter.
Pump 1
Blank
Sample
Recycle
Reductant
Sample valve
Dryer
gas Dryer
out
gas in
Pump 2
Waste
Argon
carrier
gas
Gas−liquid
separator
Figure 4 Schematic diagram of the continuous-flow vapor/
hydride generator. (Reproduced from PSA 10.055 Millennium
Excalibur, User Manual, Issue 1.0, March 1998, by permission
of P. S. Analytical.)
8
ENVIRONMENT: WATER AND WASTE
borohydride (Equations 6 and 7):
Collimator
Photomultiplier
tube
Hg2C C Sn2C
Hg
Filter
Lens
2C
! Hg0 C Sn4C
C 2NaBH4 C 6H2 O
Reference cell
Cold vapor
Introduction chimney
Figure 5 Schematic diagram illustrating the optical configuration of an AFS system for mercury analysis. (Reproduced from
PSA 10.025 Millennium Merlin, User Manual, Issue 1.0, October
1997, by permission of P. S. Analytical.)
3.1.1.2 Liberation and Reduction of Mercury
Since
this system is used to determine total inorganic mercury
(Hg2C ), the first requirement for performing analysis is to
liberate all mercury compounds from the sample matrices
and convert all organic forms of mercury to Hg2C by
various digestion/oxidation procedures.
For natural water sample analysis, the previously employed hot oxidizing method using permanganate – peroxodisulfate has been found unsuitable for
low-level mercury determination because of the high
blanks found in these reagents..58/ Bromine monochloride
has been found to be an excellent oxidant and preservative for total mercury in water samples,.59/ working faster
and more efficiently on many organomercurials..58,59/
Hydroxylamine hydrochloride is added to destroy the
excess bromine before analysis with CVAFS. This method
has often been used for converting organic mercury to
Hg2C ..59 – 64/
Both open-vessel and closed-vessel digestion/oxidation
techniques have been described for solid sample (soil, sediment, and tissue) analysis..59,65 – 67/ Open-vessel digestion
results in considerable loss of organic mercury compounds by volatilization at elevated temperature or by
incomplete oxidation of the sample matrix at ambient
temperature. The open-vessel method is also complex
and requires large amounts of glassware. These drawbacks can be overcome by using a closed decomposition
system at elevated temperature. Microwave-assisted.65,66/
and autoclave-assisted.67/ closed-vessel digestion methods have been reported to achieve complete oxidation
while minimizing losses of mercury. By using these closedvessel methods, the preparation of the sample is effective
and allows for a large number of samples to be prepared
simultaneously and efficiently.
Once all forms of mercury have been converted to
Hg2C , the latter is reduced to elemental mercury (Hg0 )
by using either acidified stannous chloride or sodium
.7/
The mercury vapor produced is carried by argon gas
to the AFS instrument for detection. A typical curve
obtained using this type of CVAFS system is shown in
Figure 6. Peak area is generally used for quantification.
This system has been successfully applied to a wide
range of environmental samples, such as water,.56,60,63,66/
soil,.60,66,67/ and biological samples..60,67/
3.1.1.3 Amalgamation
The concentrations of mercury in natural waters are often found to be at sub-parts
per trillion (nanograms per liter) levels. The determination of such low concentrations frequently requires a
preconcentration step prior to AFS detection. Many of
the enrichment techniques are based on the fact that mercury can be deposited easily on metals such as gold and
copper..46/ The amalgam technique was used simply to
improve the DLs of the cold-vapor technique. Normally,
as shown in Figure 6, the mercury vapor is liberated
slowly from solution and thus generates a broad signal. If
instead the mercury vapor is collected by amalgamation,
2000
Output
Mercury
lamp
! Hg C 7H2 C 2H3 BO3
C 2NaC
Shield gas
.6/
0
19
38
57
76
95
Time (s)
Figure 6 A typical response curve for mercury analysis using
CVAFS.
9
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
Flow meter
Fluorescence
region
Wire-wound
gold column
Septum
Power supply
*
Lamp
housing
Quartz
windows
Septum
Gold trap
column
Transformer
Oxygen
removal
trap
Fluorescence
gas cell
PMT housing
Photometer
Needle
valve
Integrator
Helium
carrier gas
Figure 7 Schematic diagram of the Hg detection system. The boxed area on the left is the nondispersive resonance atomic
fluorescence system. The boxed area on the right shows the two-stage gold amalgamation gas train used to introduce elemental Hg
vapor (trapped on a gold column during the purging step) into the gas cell of the detection system. The carrier gas is high-purity
helium, from which oxygen and Hg have been removed with trap columns. The flow rate is controlled by a needle valve positioned
before the oxygen trap column and is monitored at the exit of the fluorescence cell with a flow meter. All connections are made
with Teflon or silicone-rubber tubing. The gas flow direction is denoted by arrows. (Reprinted with permission from G.A. Gill,
K.W. Bruland, Environ. Sci. Technol., 24, 1393 (1990). Copyright 1990 American Chemical Society.)
on a gold trap, for example, and then subsequently
released by heating the trap rapidly to 500 – 700 ° C, a
higher signal-to-noise ratio (S/N) is obtained. A two-stage
gold trap technique coupled with AAS for determining
total mercury in environmental samples was described..68/
Gold-trap preconcentration of Hg0 followed by AFS has
also been reported..57/ Figure 7 shows a schematic diagram of the two-stage CVAFS system..69/ Because of
the excellent sensitivity provided by this technique, goldtrap CVAFS is currently widely used in environmental
analysis..61,62,64,69 – 71/ The United States Environmental
Protection Agency (USEPA) has proposed, in the Federal
Register, a method (Method 1631) for the determination
of mercury in waters using oxidation, purge and trap, and
CVAFS..71/ However, trace levels of mercury have been
found in most reagents used. The application of this technique is often limited by the fact that mercury present as
impurity in these reagents is also preconcentrated.
3.1.1.4 Miscellaneous Atomic Fluorescence Spectrometric
Techniques for Total Mercury Determination
LIF
with ETA has been reported for total mercury
determination with a 0.09-pg absolute DL. However,
because of the small size of the sample to be used
(only 10 µL), the concentration DL in water was only
9 ng L 1 ..72/ This is not sufficient for total mercury determination in most natural waters, therefore limiting the
application of this method.
Several flow-injection (FI) atomic fluorescence methods incorporating an on-line bromide – bromate oxidation
step to determine mercury in water samples have been
described..73,74/ The CVAFS-based technique uses a
heated reaction coil in the FI manifold to increase the
conversion of organic mercury into Hg2C . However, the
DL (6 – 25 ng L 1 ) achieved by these techniques cannot,
in many cases, meet the requirements for the analysis of
environmental samples because only a small volume of
sample can be used.
3.1.2 Mercury Speciation
Speciation is the identification and quantitation of the
individual physicochemical forms of an element in a sample that together constitute its total concentration. It
is well known that the various chemical forms (organic
and inorganic) of mercury in the natural environment
10
ENVIRONMENT: WATER AND WASTE
3.1.2.1 Chromatographic Methods
Gas chromatography/electron capture detection (GC/ECD) was traditionally used for the determination of MeHgCl or
MeHgBr..75/ However, a major problem with GC/ECD
is that the halogen-bearing compounds co-extracted
with organic mercury interfere with the determination
because of the nonspecificity of the ECD. Use of the
very sensitive and selective AFS as a GC detector has
successfully addressed this problem..60,76 – 79/ Gas chromatography/atomic fluorescence spectroscopy (GC/AFS)
systems have been used for determination of methylmercury and ethylmercury in various environmental samples.
A schematic diagram of the GC/AFS system is shown in
Figure 8. A commercially available gas chromatograph,
equipped with a megabore fused-silica column (DB-1 or
BD-5), was used to separate organomercury chlorides or
bromides. A pyrolyzer heated electrothermally at 800 ° C
was added between the column outlet and the AFS detector, converting organomercury compounds to Hg0 . The
pyrolyzer was made of deactivated fused-silica tubing.
The mercury atoms formed in the pyrolysis unit were
transferred via a 0.5-mm i.d. Teflon tube into the AFS
detector, which has a similar design to that shown in
Figure 5. Argon gas was used as make-up and shield
gases, and both helium and argon could be used as the
GC carrier gas. A typical chromatogram obtained using
this system for the determination of methylmercury and
ethylmercury compounds is given in Figure 9. Column
5
8
6
7
4
3
11
9
10
2
12
1
Figure 8 Block diagram of a GC/AFS system for the determination of organomercury compounds. (1) argon; (2) mass
flow controller; (3) sheath gas; (4) make-up gas; (5) carrier gas;
(6) injection port; (7) split value; (8) autosampler; (9) injector
controller; (10) column; (11) pyrolyzer; (12) AFS detector.
0.8
EtHgCl
0.6
MeHgCl
0.4
Volts
behave differently, thereby affecting its biogeochemistry
and toxicity to organisms. The development of analytical techniques able to distinguish different inorganic
and organomercury species has been a main research
area for scientists dealing with mercury in the environment. Basically, chromatography [gas chromatography (GC) and high-performance liquid chromatography
(HPLC)]-based AFS, and purge and trap-based AFS are
the two most widely used techniques for measuring inorganic and different forms of organomercury compounds,
especially methylmercury.
0.2
0.0
−0.2
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14
Minutes
Figure 9 Typical chromatogram obtained using GC/AFS for
the determination of methylmercury chloride (MeHgCl) and
ethylmercury chloride (EtHgCl).
maintenance is always an important step when a GC
method is used for organomercury halide determination.
With regard to the GC/AFS system, the treatment and
maintenance of the GC injection port, the column, and
sample preparation, have been discussed..77,79/
In order to improve GC performance, organomercury
halides can also be converted to fully alkylated derivatives
by different derivatization reactions before GC/AFS
analysis. Two common derivatizing methods are ethylation using sodium tetraethylborate (NaBEt4 ) and
alkylation with Gringard reagents (e.g. butylmagnesium
chloride, BuMgCl)..78,80,81/ The application of NaBEt4
is limited because this procedure does not distinguish
between ethylmercury and inorganic mercury (Hg2C ).
The method of Grignard reactions has to be carried
out in completely dried organic solvents and therefore
involves tedious workup steps, which inevitably introduce analytical errors. Other derivatization reagents,
such as sodium tetraphenylborate (NaBPh4 ).82/ and
sodium tetrapropylborate (NaBPr4 ),.83/ have also been
studied and used for organomercury speciation by
GC/AFS..84/
Sample preparation is one of the critical steps for MeHg
determination in environmental samples. For water samples, liquid – liquid extraction (LLE) with an organic
solvent has been a popular preconcentration method.
However, the large volumes of both water sample and
organic solvent required make it impractical for real sample analysis, especially when a large sample throughput
is needed. The two currently used methods are based
on steam distillation.85/ and solid-phase extraction with
sulfhydryl cotton fiber..77/ Isolation of organomercury
compounds from soil and biological samples requires
11
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
special attention because mercury species cannot be
alternated during the isolation procedure. Recently, evaluation of some isolation methods for organomercury
determination in soil and fish samples by GC/AFS has
been reported..79/
3.1.2.2 Purge and Trap-based Gas Chromatographic/
Cold-vapor Atomic Fluorescence Spectrometric Methods
Purge and trap methods for the determination of
methylmercury requires conversion of ionic MeHgC to
nonionic species. The generally used method is aqueous ethylation with NaBEt4 . The use of NaBEt4 as
an aqueous derivatization agent was first reported by
Rapsomanikis et al. for lead determination using a
purge and trap AAS system..86/ Bloom reported an
analytical method for determination of MeHg in aqueous samples by aqueous-phase ethylation with NaBEt4 ,
followed by cryogenic gas chromatography/cold-vapor
atomic fluorescence spectrometry (GC/CVAFS)..87/ Several studies and modifications have been conducted
in terms of the trap materials and methods. In typical room-temperature trapping experiments, the volatile
derivatives are trapped on a Carbotrap or TenaxTA trap, thermally desorbed, separated by isothermal
packed-column GC, decomposed at 700 – 900 ° C by a
pyrolyzer, and then detected by CVAFS..88/ In the cryogenic trapping method, the derivatives produced are
purged by helium gas, trapped directly on a packed GC
column immersed in liquid nitrogen ( 196 ° C), and then
separated by electrothermally heating the column..89/ The
ethylation approach with room temperature trapping is
at present the most popular method for speciation of
mercury, but ethylation followed by cryogenic trapping is
also effective.
Purge and trap CVAFS is an extremely sensitive
method for mercury determination. A DL (3s of the
mean blank) of 0.016 ng L 1 for MeHg determination
in water samples has been reported..61/ However, this
method cannot distinguish between inorganic mercury
and ethylmercury because they both give diethylmercury
as products. In addition, severe interferences are frequently encountered in the ethylation step and MeHg
artifact has sometimes been observed when environmental samples with a high inorganic mercury content were
analyzed..90/
Hydride-generation with NaBH4 for MeHg analysis
was reported in 1992 by Filippelli et al..91/ and Craig
et al..92/ MeHg was converted to volatile MeHgH, and
Hg2C was converted to Hg0 (Equations 8 and 9):
MeHgC C NaBH4
Hg2C C NaBH4
! MeHgH
.8/
! Hg0
.9/
Several research groups have demonstrated the presence of MeHgH in the gas phase by using different techniques..91,92/ Hydride-generation followed by
CVAFS was reported for mercury determination in various environmental samples..93/
3.1.2.3 Miscellaneous Atomic Fluorescence Spectrometric Techniques for Mercury Speciation
AFS can
also be used as an HPLC detector. Hintelmann and
Wilken described a method for the determination of
organomercury compounds using HPLC with on-line
AFS detection..94/ The organomercury compounds separated by HPLC were converted to Hg0 in a continuousflow system by an oxidizing, and a subsequent reducing,
solution. The Hg0 generated was separated in a gas – liquid
separator and swept into the cell of an AFS detector by a
stream of argon. This system was successfully used to separate eight organomercury compounds. The absolute limit
of detection (LOD) for MeHg was reported to be about
0.02 ng. This corresponds to a concentration LOD of
0.8 µg L 1 for a 25-µL injected volume. Although HPLC
has an advantage over GC in that formation of volatile
derivatives is not necessary, the LOD achieved do not
appear to meet the requirement for most environmental
samples, especially for natural waters.
Recently, Cai et al. reported an analytical method
involving ethylation using NaBEt4 followed by solidphase microextraction (SPME) and GC/AFS detection
for the rapid determination of methylmercury in
fish..81/ The procedure involves aqueous ethylation with
NaBEt4 and adsorption of Et2 Hg and MeHgEt from
aqueous solution or headspace on a fiber coated with
poly(dimethylsiloxane). After retraction of the fiber into
a syringe needle, the analytes are desorbed in an injection
port of a gas chromatograph, separated, and detected by
AFS using the system shown in Figure 8. At present this
procedure cannot be used for the determination of MeHg
in natural waters owing to the limited sensitivity.
3.2 Hydride-forming Elements
Many environmentally important elements, such as
arsenic, selenium, bismuth, antimony, tellurium, germanium, tin, lead, and mercury, can form volatile and
covalent hydrides with ‘‘nascent’’ hydrogen. Hydridegeneration is a preferred technique when hydride-forming
elements are to be analyzed by atomic spectrometry. The
advantage of volatilization as a gaseous hydride clearly
lies in the separation and enrichment of the analyte
element and thus in a reduction or even complete elimination of interferences..46/ Since many features of these
hydride-generation techniques are similar between different elements in terms of the reactions and instrument
designs, the analytical aspects of all these elements using
12
hydride-generation followed by AFS are discussed in this
section. Of course, these elements can also be determined
using other AFS techniques, which will be summarized
in the next section along with the determination of other
non-hydride-forming elements by AFS.
Since Holak in 1969 first applied hydride-generation for
the determination of arsenic using AAS,.95/ a number of
papers have been published describing modifications and
optimization of the techniques. Currently, the number
of metals and metalloids that can be determined by
the hydride-generation technique has increased to about
nine (arsenic, selenium, bismuth, antimony, tellurium,
germanium, tin, lead, and mercury). Although the major
research was carried out using AAS for detection,.46/ most
of the elements have also been studied using hydridegeneration followed by AFS detection. Tsujii and Kuga
in 1974.96/ were the first to describe hydride-generation
coupled to nondispersive AFS. They reported a DL of
2 ng for arsenic analysis. Thompson in 1975.97/ was the first
to apply a dispersive AFS system for the determination of
arsenic, selenium, antimony, and tellurium after hydride
generation. DLs using this system ranged from 0.06 to
0.1 µg L 1 .
Although various metal – acid reactions (e.g. Zn– HCl)
have been used as a means of producing hydride, NaBH4
is currently used exclusively. By using NaBH4 as the
reductant, the technique is easy to automate, since only
solutions are involved, so that a high sample throughput
can be achieved. In contrast to the manual injection
of NaBH4 or sample solution into the reaction, all
the solutions can be delivered by a continuous-flow
hydride-generation system..12/ Such a system is currently
commercially available..14/ The design of this system is
similar to that of the cold-vapor generation system used
in CVAFS for mercury analysis as shown in Figure 4.
Once the hydride has been formed and driven out of the
solution, it can be directly delivered by an inert gas such as
argon to the atomizer where it is excited by a fluorescence
light source and measured by a detector such as solar
blind PMT. The hydride produced can be first collected
in a trap cooled in liquid nitrogen before it is warmed
up to vaporize it for measurement. The configuration
of this design is similar to that used in the purge and
trap system for mercury determination discussed above.
For routine analysis, the former (direct) method offers
the advantage of a higher sample throughput and easy
operation. However, the latter approach provides the
highest yield and best sensitivity..46/ Both procedures
have been widely used in HG/AFS.
When determination is needed for different ionic
species of elements present in a sample, various types
of HPLC are employed to separate these forms of compounds, which are then subjected to hydride generation,
followed by direct or purge and trap AFS detection.
ENVIRONMENT: WATER AND WASTE
Sometimes an on-line decomposition step is added
between HPLC and hydride generation to convert nonhydride-forming species into hydride-forming species. A
chemical reagent, such as potassium peroxodisulfate,
assisted by UV photochemical oxidation or microwave
digestion has been used for this purpose..98 – 101/
Although occasionally an electrothermal atomizer has
been used, an argon – hydrogen diffusion flame atomizer is the usual choice because of very low background radiation, easy decomposition of hydrides in
such a low-temperature flame, and the low quenching
effect in the argon-supported flame. The atomization
mechanisms of hydrides in different atomizers have
been studied extensively. Evidence has been presented
showing that the atomization of hydrides in a cool
argon – hydrogen diffusion flame is not the result of
a simple thermal dissociation process..12,35,46/ Dedina
and Rubeška concluded that atomization is brought
about by free radicals produced in the primary reaction zone of the oxygen/hydrogen flame according to
Equations (10 – 12):.35/
H C O2
! OH C O
.10/
O C H2
! OH C H
.11/
! H2 O C H
.12/
OH C H2
It can be shown that the concentration of H radicals
is several orders of magnitude higher than that of OH
radicals. Taking selenium as an example, the atomization
is via a two-step mechanism with the predominating H
radicals (Equations 13 and 14):
SeH2 C H
SeH C H
! SeH C H2
.13/
! Se C H2
.14/
It has already been shown that there is a large number of
H radicals present in an argon – hydrogen diffusion flame,
so the same reaction mechanisms can be expected..46/
As with any other technique, HG/AFS suffers from
interferences. Welz.46/ gave an extensive discussion about
various interferences encountered in hydride-generation
methods. However, the sensitivity of the AFS system
allows dilution of the sample so that the interferences
are reduced significantly. Spectral interference is not a
problem for HG/AFS because the analyte element passes
into the atomizer as gaseous hydride, while concomitants
normally remain in the reaction vessel. In most currently
used nondispersive AFS detectors, an interference filter
is located between the atomizer and the PMT, to reduce
the flame background and emissions from the excitation
source that do not produce intense fluorescence lines..12/
The flame background can also be minimized by choosing
suitable flow rates of gases. Kinetic interferences are
13
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
an interference filter. A solar blind PMT was used as the
detector. A DL (3s) of 0.1 µg L 1 was reported for arsenic.
To date, about a dozen arsenic compounds have been
found in environmental samples..101/ Extensive studies
on the toxicity and biogeochemistry of arsenic have
clearly depicted the importance of the chemical speciation of this element. To determine these different forms
of arsenic in environmental samples, a powerful separation technique, such as HPLC, has to be used. A number
of analytical systems based on high-performance liquid
chromatography/hydride-generation/atomic fluorescence
spectrometry (HPLC/HG/AFS) have been reported.
Recently, Gomez-Ariza et al..98/ described an anionexchange HPLC/HG/AFS system for the speciation
of arsenite, arsenate, dimethylarsinic acid (DMAA),
and monomethylarsonic acid (MMAA). The products
of hydride-generation of arsenite, arsenate, DMAA,
and MMAA are arsine (AsH3 ), arsine, Me2 AsH, and
MeAsH2 , respectively. DLs of 0.17, 0.38, 0.45 and
0.30 µg L 1 were reported for arsenite, arsenate, DMAA,
and MMAA, respectively. A schematic diagram of
this system is shown in Figure 10. Arsenobetaine, a
non-hydride-forming species of arsenic, was also determined by introducing an on-line photooxidation step
after the chromatographic separation. Le et al..100/
described an on-line microwave derivatization coupled
with HPLC/HG/AFS for arsenic speciation. They studied the separation of 11 arsenic compounds by using
ion-pair chromatography at 30, 50, and 70 ° C column temperature and found that the use of elevated
column temperature improved the separation efficiency
and dramatically reduced the chromatographic time for
some arsenic species. This speciation technique was successfully applied to a study of metabolites of arseno
sugars present in commercial seaweed products. A typical chromatogram obtained for speciation of seven arsenic
compounds is shown in Figure 11.
Hydride-generation followed by purge and trap AFS
can also be used for speciation of hydride-forming arsenic
caused by varying rates of development or liberation
of the hydride from solution. These interferences only
occur in the direct hydride-generation system, and not in
the purge and trap system. A typical kinetic interference
is the retardation of hydride liberation because of the
sample matrix effect. Chemical interferences are perhaps
the most frequently encountered problem. It appears
that transition metals interfere significantly with hydridegeneration reactions.
3.2.1 Arsenic
Arsenic was the first element to be determined using the
HG/AFS technique..96/ A number of papers have been
published on arsenic determination in environmental
samples. Azad et al..102/ described a procedure for the
determination of arsenic in soil digests in which arsine was
generated with NaBH4 , passed to an argon– hydrogen airentrained flame and an atomic fluorescence signal, excited
by a modulated EDL, and measured using nondispersive
AFS. The authors reported a DL of 3 µg L 1 calculated
as the mass of arsenic required to produce a S/N of 2
for the atomic fluorescence signal. Most papers published
in the past utilized an EDL as the excitation source
because this type of lamp provides intense and narrow
lines. However, the EDL is often unpredictable in use
and requires careful temperature control to achieve
good stability. A BDHCL seems to be a reliable highintensity excitation source and this type of lamp has been
used in commercial AFS instruments. Corns et al..12/
described a fully automated continuous-flow hydrogen
generation AFS system for the determination of hydrideforming elements, principally arsenic and selenium. A
minature argon – hydrogen diffusion flame was used as
the atomizer and a BDHCL was used as the excitation
sources. The hydrogen for the flame was chemically
generated as a byproduct of the NaBH4 reduction.
Fluorescence wavelengths of interest were selected using
H2
Air
Argon
Column
HPLC
pump
Injector
Perma pure dryer tube
AFS
detector
Integrator
HCl NaBH4 Argon
Gas−liquid separator
Figure 10 Operating scheme of the HPLC/HG/AFS system for arsenic speciation. (Adapted from J.L. Gomez-Ariza,
D. Sanchez-Rodas, R. Beltran, W. Corns, P. Stockwell, Applied Organomet. Chem., 12, 1 (1998). Copyright 1998 John Wiley
& Sons Limited. Reproduced with permission.)
14
ENVIRONMENT: WATER AND WASTE
species such as arsenite, arsenate, DMAA, and MMAA.
A pH-selective hydride-generation process should be
used..103/ This process comprises two steps. First, arsenite
is reduced to AsH3 by NaBH4 at pH 6. The product is
swept out of the solution and collected with a cooled
trap under liquid nitrogen. The trap is then allowed to
warm to room temperature and the arsine is released into
the AFS system for detection. Second, arsenate, DMAA,
and MMAA in the sample solution are reduced to the
corresponding arsines at pH 1, then followed by purge
and trap AFS detection.
elements in water, SRM 1643d from the National Institute
of Standards and Technology (NIST) (Gaithersburg,
MD, USA), was used to test the performance of this
system..84/ The concentration of selenium was found to be
11.40 š 0.07 µg L 1 (n D 3), which was in close agreement
with the certified value of 11.43 š 0.17 µg L 1 .
Selenium can also exist as different forms, both inorganic and organic, in environmental samples. Volatile
organic selenium compounds can be directly determined
using purge and trap followed by AFS without derivatization (see the next section). The determination of inorganic
forms, Se(IV) and Se(VI), at low concentration levels is
generally carried out using the hydride-generation technique. Since only Se(IV) is able to generate the hydride,
if speciation data are required two procedures are usually
used. In the first procedure, the sample has first to be analyzed without pretreatment, which provides the Se(IV)
concentration. Then the sample is treated by heating
it with 6 mol L 1 hydrochloric acid, which reduces any
Se(VI) to Se(IV). The analysis is performed again, to give
a total concentration of inorganic selenium present in the
sample. The concentration of Se(VI) is then calculated
by difference. The obvious disadvantage of this method
is its indirect determination of Se(VI), which involves the
errors induced by determination of Se(IV) and total inorganic selenium..106/ The speciation of Se(IV) and Se(VI)
can also be carried out using HPLC separation followed
by detection with different detectors..106,107/ Pitts et al..99/
recently described an on-line method for determination
of inorganic selenium species in aqueous samples
using HPLC/HG/AFS. Separation of these species was
achieved using HPLC, after which the analyte was acidified, and passed through an on-line microwave system.
The latter stage left the Se(IV) unaltered, but transformed
Se(VI) to Se(IV). The selenium hydride was then determined using an AFS detector. DLs of 0.2 and 0.3 µg L 1
were reported for Se(IV) and Se(VI), respectively.
3.2.2 Selenium
3.2.3 Other Hydride-forming Elements
The AFS determination of selenium was first reported
by Dagnall et al..104/ using a dispersive spectrometer
equipped with an air – propane flame giving a DL of
0.25 µg mL 1 of selenium on aspiration of aqueous solution using a pneumatic nebulizer. A dramatic improvement in DL (10 µg L 1 ) was obtained by using a similar
experimental arrangement, but using an argon– hydrogen
diffusion flame and the hydride-generation technique..105/
This HG/AFS system was used for the determination of
selenium in soil samples. A current commercially available HG/AFS instrument.12/ utilizes a BDHCL as the
excitation source and also an argon– hydrogen diffusion
flame as the atomizer. A DL (3s) of 0.05 µg L 1 was
reported. A standard reference material (SRM) for trace
In addition to arsenic and selenium, other elements such
as antimony, bismuth, tellurium, germanium, tin, and lead
have also been determined using HG/AFS. It should be
noted that the arrangements of the instruments used
for the analysis of these elements are identical with
those for arsenic and selenium except for the utilization
of a corresponding excitation source and wavelengthselection system. Both dispersive and nondispersive AFS
have been used..56,97/ Kobayashi et al..108/ published
the first paper on the determination of bismuth using
an HG/AFS system. They used an argon – hydrogen
diffusion flame as the atomizer and a microwave-excited
EDL as the excitation source, and achieved a DL
of 5 µg L 1 or 0.1 ng of bismuth. Nakahara et al..109/
7
5
Fluorescence (arb. units)
3
1
2
0
2
4
4
6
6
8
10
12
14
Retention time (min)
Figure 11 Chromatogram of seven arsenicals obtained on a
reversed-phase C18 column (300 ð 3.9 mm i.d., 10-µm particles) with 10 mM hexanesulfonate, 1 mM tetraethylammonium
hydroxide, and 0.5% methanol as eluent (pH 4.0). Microwave
digestion combined with hydride generation and AFS was used
for detection. Column temperature, 70 ° C. Peaks: 1, arsenate (10 ng); 2, arsenite (40 ng); 3, arsenobetaine (15 ng); 4,
MMAA (10 ng); 5, DMAA (40 ng); 6, arsenocholine (15 ng); 7,
tetramethylarsonium (40 ng). (Reprinted with permission from
X.C. Le, M.H. Ma, N.A. Wong, Anal. Chem., 68, 4501 (1996).
Copyright 1996 American Chemical Society.)
15
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
described an analytical method based on HG/AFS for
the determination of tellurium using an argon– hydrogen
diffusion flame and an EDL excitation source. The
comparison of the Zn and NaBH4 reduction methods
was discussed. The best attainable DLs for tellurium
were 0.1 µg L 1 (2 ng) and 1.5 µg L 1 (30 ng).
Nakahara et al. have also conducted a series of
studies on the determination of other hydride-forming
elements.110,111/ using hydride generation coupled with
nondispersive AFS. D’Ulivo and Papoff.112/ described a
method using nondispersive AFS combined with hydride
generation for the determination of lead. They used a
radio-frequency-excited EDL as the excitation source
and a small argon – hydrogen flame as the atomizer. The
DL was 0.06 µg L 1 and the calibration curve was linear up
to 300 µg L 1 , superior to HG/AAS or inductively coupled plasma atomic emission spectrometry (ICP/AES).
Using a similar HG/AFS system, D’Ulivo et al..32/ determined dialkyllead and trialkyllead. The DLs obtained
for trimethyl, triethyl, dimethyl, and diethyllead (3s)
were 3 – 5 ng L 1 . Although HG/AFS is an appropriate analytical technique for the determination of these
hydride-forming elements, its application in environmental analysis seems to be very limited. This is due, in part, to
the fact that until recently no commercial HG/AFS instruments were available. Most of the research was carried out
using laboratory-made systems. However, a continuousflow HG/AFS system, recently made commercially
available, provides a capability for arsenic, selenium,
antimony, tellurium, and bismuth determination with a
DL in the sub-parts per billion region..12,14,56/
3.3 Non-hydride-generation Methods
3.3.1 Laser-induced Fluorescence
Determinations of hydride-forming elements in environmental samples are usually carried out using HG/AFS
because of its high sensitivity and simplicity. Another
very sensitive type of AFS, which has been used for the
determination of many elements (both hydride-forming
and non-hydride-forming), is LIF. LIF provides the lowest
absolute DLs for a number of elements..23/
Although different types of atomizers, such as flame,
plasma, and electrothermal, can be coupled with LIF, it
appears that the most sensitive practical measurements
by LIF are performed in those ETA instruments, such as
graphite-tube furnaces, that are currently used in AAS.
With an ETA as atomizer, absolute DLs below femtogram level can be obtained experimentally for some
elements. Omenetto.23/ collected pertinent data for 22
elements investigated with LIF/ETA and the DLs were
compared with those given by inductively coupled plasma
mass spectrometry (ICP/MS). For most elements, the data
obtained using LIF/ETA are superior to those obtained
using ICP/MS. Recently, a DL of 10 fg mL 1 (100 ag absolute) was reported for lead determination in whole-blood
samples using LIF/ETA..19/ A schematic diagram of the
experimental design for LIF/ETA is shown in Figure 12.
Doubling
crystal
Power
supply
Pierced
mirror
Lens
Lens
Filter
Dye
laser
Graphite
furnace
Copper vapor
laser
Lenses
Photodiode
for laser
absorption
measurements
Photodiode
trigger
CuSO4
filter
To detection electronics
Monochromator
To detection
electronics
PMT
HV
Current to
voltage AMP
To detection electronics
Figure 12 Experimental design for LIF/ETA. (Reprinted with permission from E.P. Wagner II, B.W. Smith, J.D. Winefordner,
Anal. Chem., 68, 3199 (1996). Copyright 1996 American Chemical Society.)
16
LIF/ETA has been used for the determination
of a number of elements in various environmental
samples. Bolshov et al..24/ reported the results for the
determination of cadmium in Antarctic and Greenland
snow and ice by LIF/ETA. The DL was found to be
0.5 fg or 0.01 pg g 1 using a 50-µL sample volume. Silver
was determined in seawater by LIF using novel diffusive
graphite-tube ETA..113/ A 10-µL volume of sample was
applied to a small graphite boat attached to one of the
graphite electrodes and inserted into the graphite tube,
which was then sealed by the electrodes and heated. The
vaporized silver passed through the graphite wall and
was excited by a laser beam a few millimeters above the
surface of the tube. A DL of 90 fg for 1 : 1 diluted seawater was obtained. Other examples of recent applications
of LIF/ETA include the determination of aluminum and
lead.114/ in atmospheric aerosol samples, antimony.40/
in biological and environmental samples, bismuth,.115/
cobalt,.116/ and lead.25/ in seawater, thallium.117/ and
lead.118/ in natural water, and mercury.119/ in soil.
LIF coupled with flame atomization has also been
used for environmental sample analysis. Zhou et al..21/
described methods for the determination of cobalt,
copper, lead, manganese, and thallium in Buffalo River
sediment using flame LIF. A standard, long-path flame
atomic absorption burner was used and DLs for cobalt
(2 ng mL 1 ), copper (2 ng mL 1 ), lead (0.4 ng mL 1 ), and
thallium (0.9 ng mL 1 ) were reported.
One of the major problems facing the future use of
LIF instruments is that lasers are difficult to use. They
are costly and require highly skilled operators. This certainly limits the development of commercially available
instruments.
3.3.2 Atomic Fluorescence Spectrometry as a
Chromatographic Detection Method
AFS can be conveniently used as a detection method for
chromatography, such as GC and HPLC. Details can be
found in an excellent paper by D’Ulivo,.7/ who summarized the research regarding the utilization of AFS in chromatographic detection. Brief information is given below.
Van Loon et al..120/ first described a method for
the simultaneous speciation of several elements using
an atomic fluorescence spectrometer as an elementspecific detector for chromatography. A three-channel
nondispersive atomic fluorescence spectrometer with an
HCL as the excitation source and a nitrogen-sheathed
air – acetylene flame as the atomizer was used. It was
interfaced to an HPLC system by direct connection of the
column outflow to the nebulizer capillary of the burner.
The column flow rate was compatible with the nebulizer
flow rate. The technique was successfully applied to the
simultaneous detection of several metals as their EDTA
ENVIRONMENT: WATER AND WASTE
(copper, zinc, nickel), glycine (copper, zinc, nickel), and
trien complexes. More research has been carried out to
study metal speciation using different models of coupling
of HPLC with AFS..121/
Coupling AFS to GC greatly simplifies the task of the
atomizer since the effluents supplied to the AFS system are already in the vapor phase..7/ Different types of
atomizers have been designed and applied to the speciation of metals..122/ D’Ulivo and Papoff.123/ described a
technique using GC coupled with multichannel nondispersive AFS for the simultaneous determination of alkyllead, alkyltin, and alkylselenide compounds. Recently,
Pécheyran et al..124/ described a GC/AFS method for
the determination of ultratrace volatile selenium species
in aqueous solutions. The method, optimized for field
measurements of volatile selenium species in aquatic
environments, involved a shipboad purge of the water
samples collected under clean conditions, a cryogenic
trapping stage followed by separation of the analytes by
low-temperature chromatography. Under routine operating conditions, absolute DLs of 4 and 4.5 pg of Se for
Me2 Se and Me2 Se2 were obtained, respectively.
It is expected that recent developments will strengthen
the role of commercial AFS in the determination of
trace elements in environmental samples. Indeed, AFS
has already been shown to be the most sensitive and
important detection technique for mercury determination, especially for samples with low concentration levels.
4 CONCLUSIONS
Clearly, the most important and widely used AFS systems in environmental analysis have so far been CVAFS
for mercury and HG/AFS for the determination of the
hydride-forming elements. In practical terms, for environmental and other areas of analysis, the broader adoption
of AFS will depend on the introduction of standardized
commercial instrumentation and the ability to compete
with other techniques that are commonly used, such as
AAS, AES, and ICP/MS. Currently, several companies
manufacture AFS detectors used either for mercury or
hydride-forming elements, or both. These AFS detectors
are of simple design and easy operation. Most importantly, these cost-effective instruments provide excellent
DLs for some environmentally important elements, such
as mercury. Important progress is, therefore, expected in
using these instruments for environmental analysis.
Although LIF has practical limitations at present, such
as high cost and difficult operation, the exceptional low
DLs achieved using LIF for some elements are very
attractive. This is especially true when only small sample
amounts are available.
17
ATOMIC FLUORESCENCE IN ENVIRONMENTAL ANALYSIS
ACKNOWLEDGMENTS
It is a pleasure to acknowledge the encouragement
given to me during preparation of this article by
Dr Rudolf Jaffé. Special thanks are due to Anita Holloway for her assistance in the preparation of the
manuscript. I wish to express my thanks to the many
authors and publishers who have given permission for the
reproduction of figures from the original literature. This
is SERP contribution number 106.
ABBREVIATIONS AND ACRONYMS
AAS
AES
AFS
BDHCL
CVAAS
CVAFS
DL
DMAA
EDL
ETA
FI
GC
GC/AFS
GC/CVAFS
GC/ECD
HCL
HG/AFS
HPLC
HPLC/HG/AFS
ICP
ICP/AES
ICP/MS
LIF
LIF/ETA
LLE
Atomic Absorption Spectroscopy
Atomic Emission Spectroscopy
Atomic Fluorescence Spectroscopy
Boosted Discharge Hollow-cathode
Lamp
Cold-vapor Atomic Absorption
Spectrometry
Cold-vapor Atomic Fluorescence
Spectrometry
Detection Limit
Dimethylarsinic Acid
Electrodeless Discharge Lamps
Electrothermal Atomization
Flow-injection
Gas Chromatography
Gas Chromatography/Atomic
Fluorescence Spectroscopy
Gas Chromatography/Cold-vapor
Atomic Fluorescence Spectrometry
Gas Chromatography/Electron
Capture Detection
Hollow-cathode Lamp
Hydride-generation/Atomic
Fluorescence Spectroscopy
High-performance Liquid
Chromatography
High-performance Liquid
Chromatography/Hydridegeneration/Atomic
Fluorescence Spectrometry
Inductively Coupled Plasma
Inductively Coupled Plasma
Atomic Emission Spectrometry
Inductively Coupled Plasma
Mass Spectrometry
Laser-induced Fluorescence
Laser-induced Fluorescence/
Electrothermal Atomization
Liquid– Liquid Extraction
LOD
MMAA
NIST
OPO
PMT
S/N
SPME
SRM
USEPA
UV
UV/VIS
YAG
Limit of Detection
Monomethylarsonic Acid
National Institute of
Standards and Technology
Optical Parametric
Oscillator
Photomultiplier Tube
Signal-to-noise Ratio
Solid – phase Microextraction
Standard Reference Material
United States Environmental
Protection Agency
Ultraviolet
Ultraviolet/Visible
Yttrium Aluminum
Garnet
RELATED ARTICLES
Clinical Chemistry (Volume 2)
Atomic Spectrometry in Clinical Chemistry
Environment: Water and Waste (Volume 3)
Biological Samples in Environmental Analysis: Preparation and Cleanup ž Capillary Electrophoresis Coupled
to Inductively Coupled Plasma-Mass Spectrometry for
Elemental Speciation Analysis ž Flame and Graphite
Furnace Atomic Absorption Spectrometry in Environmental Analysis ž Gas Chromatography with Atomic
Emission Detection in Environmental Analysis ž Heavy
Metals Analysis in Seawater and Brines ž Hydride Generation Sample Introduction for Spectroscopic Analysis
in Environmental Samples ž Inductively Coupled Plasma
Mass Spectrometry in Environmental Analysis
Environment: Water and Waste cont’d (Volume 4)
Mercury Analysis in Environmental Samples by Cold
Vapor Techniques ž Organometallic Compound Analysis in Environmental Samples ž Sample Preparation for
Elemental Analysis of Biological Samples in the Environment ž Sample Preparation for Environmental Analysis
in Solids (Soils, Sediments, and Sludges) ž Sample Preparation Techniques for Elemental Analysis in Aqueous
Matrices ž Solid-phase Microextraction in Environmental
Analysis
Food (Volume 5)
Atomic Spectroscopy in Food Analysis ž Fluorescence
Spectroscopy in Food Analysis
Forensic Science (Volume 5)
Fluorescence in Forensic Science
18
ENVIRONMENT: WATER AND WASTE
Atomic Spectroscopy (Volume 11)
Atomic Spectroscopy: Introduction ž Laser Spectrometric Techniques in Analytical Atomic Spectrometry
17.
18.
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