Trends in Analytical Chemistry, Vol. xxx, No. x, 2010
Trends
Automation of radiochemical
analysis by applying flow
techniques to environmental
samples
Yamila Fajardo, Jessica Avivar, Laura Ferrer, Enrique Gómez,
Montserrat Casas, Vı́ctor Cerdà
We review the state of the art of flow analysis applied to the fully-automated and semi-automated determination of radioactive
isotopes in environmental samples, including automatic procedures for separation and pre-concentration of radioisotopes. We
discuss in detail advantages and drawbacks of automatic protocols exploiting various generations of flow techniques [e.g., flowinjection analysis (FIA), sequential-injection analysis (SIA), multi-syringe FIA (MSFIA), multi-pumping flow systems (MPFS) and
lab-on-valve (LOV)] using a variety of detection systems, including scintillation counting, a-spectrometers, proportional counters,
mass spectrometry and spectrophotometry.
ª 2010 Published by Elsevier Ltd.
Keywords: Flow-analysis technique; Flow-injection analysis (FIA); Lab-on-valve (LOV); Multi-pumping flow system (MPFS); Multi-syringe flowinjection analysis (MSFIA); Radiochemical separation; Radioisotope detection; Radioisotope determination; Radioisotope pre-concentration;
Sequential-injection analysis (SIA)
1. Introduction
Radionuclides should be pre-concentrated
and separated before measurement because of their low activities in environmental samples and the presence of other
radionuclides and interfering elements.
Traditionally, separation and purification
processes and radioisotope analysis are
carried out manually, results being obtained after application of one or several
separation protocols with many steps.
These radiochemical separations are carried out using a variety of classical and
chromatographic methods, including precipitation, co-precipitation, liquid-liquid
extraction, and ion exchange.
Flow-analysis techniques have been
applied to the determination of many
analytes, including radioisotopes. The
combination of classical methods with
modern instrumentation makes possible
the total or partial automation of many
conventional analytical methods in the
radiochemical field, which, in practice,
leads to simplicity, reliability, significant
decrease in time of analysis, reduction of
sample and reagent consumption and
minimal handling of samples and standards, improving the safety of the analyst.
Moreover, automation of the analytical
method allows precise control of sample
and reagent volumes and flow rates,
which lead to improvement in reproducibility [1].
The origin of current flow techniques
was in the 1950s, with segmented flow
analysis (SFA). Flow analysis has since
evolved, especially with the advent of
flow-injection analysis (FIA), sequentialinjection analysis (SIA), multicommutation FIA (MCFIA), multi-syringe FIA
(MSFIA), lab-on-valve (LOV) and multipumping flow systems (MPFSs). Although
they exploit different flow strategies, these
flow systems make use of similar components (i.e. devices for inserting and
propelling solutions, and commutation
units). Linking these components allows
0165-9936/$ - see front matter ª 2010 Published by Elsevier Ltd. doi:10.1016/j.trac.2010.07.018
1
Yamila Fajardo,
Jessica Avivar,
Laura Ferrer,
Enrique Gómez,
Vı́ctor Cerdà*,
Department of Chemistry,
University of the Balearic
Islands, E-07122 Palma de
Mallorca, Spain
Montserrat Casas
Department of Physics, IFISC,
University of the Balearic
Islands, E-07122 Palma de
Mallorca, Spain
*
Corresponding author.
Tel.: +34 971 173261;
Fax: +34 971 173426.;
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Trends in Analytical Chemistry, Vol. xxx, No. x, 2010
us to benefit by designing more versatile methodologies
adapted to a wide range of analytical situations.
Analytical methods developed using a flow-analysis
approach for determination of radioactive isotopes
comprise pre-treatment, separation and pre-concentration steps, and sometimes also include a detection step.
Even though development and application of automated
methods based on flow techniques involve a great advance in radiological control, at present there are a few
fully automated systems. The main impediment to
achieving full automation of radiometric methods for
analysis of environmental samples is the low activity
found in these kinds of sample. These elements are found
in the environment at trace and ultra-trace levels, which
need much time of counting, usually close to one day
using radiometric detection. For this reason, there are
many partly automated methods, in which detection is
off-line. Methods developed for samples with high
activities (e.g., nuclear waste) can be coupled on-line
with the radiometric detector system and results are
obtained immediately. However, mass spectrometry (MS)
detection coupled to flow techniques has increased the
development of fully automated methodologies for
environmental radioactivity analysis. Fig. 1 shows different approaches used in automating the methodologies
reviewed in this article.
Besides, some detectors are unable to discriminate
radioisotopes, so previous separation procedures have to
be applied. Probably, chromatography is the most versatile separation technique used in recent decades,
especially reversed-phase partition chromatography,
which has been subject to significant development. This
technique combines the selectivity of extraction with
organic solvents with the simplicity of ion-exchange
resins. The employment of resins allows selective isolation of the analyte and sequential extraction of diverse
analytes [2]. As shown below, the use of these chromatographic materials in automated separation
processes based on flow systems is very extensive.
This article reviews relevant works concerning the
automation of radioisotope determination using flow
systems, as described in the literature from early 1970s.
We aim to discuss the advantages and the drawbacks of
different generations of flow analysis (FIA, SIA, MSFIA,
LOV and MPFS) and their combination with various
detection systems. We give particular attention to the
potential applicability of flow manifolds for pre-concentrating samples on-line.
2. Detection systems
Apart from a limited number of radionuclides that can be
analyzed by high-resolution gamma spectrometry (e.g.,
134
Cs, 137Cs, 154Eu, 155Eu, 60Co, 54Mn and 125Sb), direct
analysis is impossible if the sample is not previously
chemically processed with separation and pre-concentration methodologies.
There is little use of direct radiometric detection of beta
emitters by a low-background proportional counter or
Radionuclide
sensors*
Liquid
scintillation*
Beta emitters
Radioactive
Isotopes
ICP-MS*
Alpha emitters
Environmental
sample
Gas proportional
counter
Isolation and
preconcentration
α-Spectrometer
Gas proportional
counter
ICP-OES*
Radioactive
Element
Automatedsteps
Spectrophotometer*
* fully automation capability
Figure 1. Strategies used for automating radiochemical analysis.
2
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Trends in Analytical Chemistry, Vol. xxx, No. x, 2010
liquid scintillation because of its low capacity to discriminate between different beta particles. This is because beta particles from a single isotope are emitted
with a range of energies, giving a spectrum that overlaps
with those of other beta-emitting radioisotopes. So, beta
emitters normally have to be isolated.
Alpha spectroscopy is a more selective technique,
since alpha particles are emitted with characteristic
energies, which facilitate simultaneous determination of
alpha isotopes with good-quality energy resolution.
Nevertheless, various alpha emitters can be affected by
the presence of spectral interferences (e.g., 241Am/238Pu
and 237Np/234U) and by auto-absorption effects, which
require careful sample preparation.
Gas proportional counters (for alpha and beta emitters) and alpha spectroscopy have been used as off-line
detectors in partly automated methodologies, which require previous isolation of the radioisotope of interest.
However, liquid-scintillation counters (for beta emitters) have been coupled to flow-based methods, allowing
full automation to analyze samples with high activities.
While radiation counting is a very sensitive detection
technique for radioisotopes with short lives (T½ <10
year) and is used in analytical techniques for the determination of most long-lived radionuclides in environmental samples, MS is more sensitive for the
determination of isotopes with very long lives. Thus,
atomic emission spectroscopy with a plasma source
coupled to a mass detector (ICP-MS) is an option for
determining various radioisotopes with limits of detection (LODs) (we use LOD without distinction to refer to
both limit of detection and lower limit of detection)
comparable to radiometric methods in a shorter time [3].
Besides, the easy connection with flow-analysis systems
has allowed development of fully automated methodologies [4–6]. However, low-resolution quadrupole
ICP-MS is susceptible to isobaric interferences (e.g.,
241
Pu /241Am, 99Tc/99Ru, 238U/238Pu), molecular
interferences (e.g., 239Pu/238UH+) and spectral interferences (e.g., 237Np/238U) [7]. These inconveniences may
be solved by isolating the analyte of interest and eliminating the sample matrix.
If the objective is determination of a total element
(e.g., total-uranium determination), spectrophotometry
appears an alternative to the above-mentioned techniques, since it has good precision and accuracy, and
lower cost. Moreover, it is possible to automate its
methods fully. However, due to the low sensitivity of
spectrophotometric determinations of radioisotopes, it is
difficult to carry out a direct determination without preconcentrating the analyte. Furthermore, to enhance its
sensitivity, it can be coupled with long path-length
waveguide capillary cells.
Other radiometric detectors have been exclusively
developed for FIA based on solid scintillation counting,
reaching LODs in the range 5–10 Bq/mL for alpha
Trends
emitters, but they are not sensitive enough to analyze
environmental samples [8,9].
3. Radiochemical separation
Usually, the above-mentioned detection methods are
used for determination of radioactive isotopes, but they
are not very selective, so previous separation and purification of the analyte of interest is necessary. There are
various chemical and physical factors to consider in
order to standardize a radiochemical methodology,
including:
type and composition of the matrix;
type of radiation emitted by the radionuclide and its
daughters;
disintegration time; and,
type of available detectors.
Besides, the time between separation and determination of the radioisotope could be of particular importance.
For example, in the case of 90Sr and 90Y, it can differentiate isotopes whose parents and daughters disintegrate and emit the same type of radiation. Then, not only
are chemical relationships between the analyte and the
extraction system important, but also radiological characteristics of the radionuclide or group of radionuclides of
interest and the technique used for its determination.
Most commonly used separation protocols exploiting
for radioisotope isolation are based on:
precipitation or co-precipitation processes [10];
adsorption processes [11];
ion exchange using anionic and cationic resins
[12,13];
liquid-liquid extraction (LLE) [14,15]; and,
chromatographic separations [16,17].
Separation by extraction chromatography appears as
the most suitable technique to develop automated
methodologies. In the 1990s, development of new
chromatographic materials by Horwitz et al. [2,16,18]
improved classical separation methods. At present, there
are a large number of specific resins (for determination
e.g., Ra, Ni, Pb, Th, U, Np, Pu, Am, Cm, Sr, Tc, 3H, Fe
and Pa) [19], which have been included in protocols for
automated separation [20–23].
Another kind of separation exploiting flow systems
and carried out by LLE involves the formation of complexes between several organic compounds and radioisotopes. Development of automated methods allowed
the separation of 90Sr using a wetting-film extraction
method [15] and of 90Y with a selective extractant adsorbed on a C18 support [24].
Capillary electrophoresis has also been used for the
separation of Pu and Np, providing advantages (e.g., short
separation time and a high separating efficiency) [25].
Recently, electrochemically-modulated separation was
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mated method for determination of plutonium isotope
ratios and exploiting an FIA technique [26].
4. Automated radioisotope determination based on
flow techniques
Automation of radioisotope separation using flow analysis achieves more reproducible results, reduces labor
costs, cuts analysis time, and minimizes secondary waste
generation through miniaturization of the process. In
the radioanalytical field, this advance implies high
operational simplification, rapidity, reliability, saving of
reagents and, mainly, minimization of sample handling,
thereby safeguarding the health of analysts.
In the beginning, application of flow techniques to
radioisotope analysis was limited by the requirements of
methodologies with respect to separation and purification steps. Nowadays, the trend is to develop fully
automated methods, especially including ICP-MS as
detector in the analysis of environmental samples.
Table 1 summarizes the most representative papers of
each flow technique, in order to discuss their main features. The LODs achieved depend on both the detector
used and the capability of the method to extract and to
pre-concentrate the radionuclide of interest. Sample and
reagent volumes and sample throughputs depend most
on the flow technique used.
4.1. FIA methods
FIA was introduced as an innovative, non-segmented
flow technique to overcome the inconvenience of laborious batch-mode operations. Fig. 2 shows a general FIA
system. Several features make flow injection a very
attractive tool for automation of radionuclide separations (e.g., high injection throughput, easy implementation, precise automated fluid handling and minor cost
of instrumentation). Moreover, it encompasses a large
number of applications.
Nevertheless, comparing applications of FIA in radiochemical analysis with its development in other fields
(e.g., environmental, pharmaceutical, biological or clinical analyses) shows that they are scarce. This could be
due to the lack of versatility of the FIA technique that
makes difficult adequate automation of the whole process in radiochemical separations. Other drawbacks of
FIA are:
instability due to Tygon tubing;
high consumption of reagent and sample volumes;
and,
difficulty in designing multi-component analysis.
The introduction of flow-injection inductively coupled plasma MS (FI-ICP-MS) in 1986 [27] opened the
way to automating classical radiometric methods using
the FIA technique [28,29], so some authors have used
this technique coupled with chromatographic systems.
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Trends in Analytical Chemistry, Vol. xxx, No. x, 2010
Dadfarnia and McLeod [30] employed an activated
alumina column to pre-concentrate U in surface and
seawaters. Hollenbach et al. [4] developed a method
for separation and pre-concentration of ultra traces of
U, Tc and Th in soil samples. Aldstadt et al. [5]
determined U in surface waters using a column with
TRU resin, achieving a very low LOD (0.30 ng/L). It is
also worth mentioning the high sample throughput
(10/h) achieved as an inherent advantage of FIA. In
all these cases, an FIA system was coupled on-line
with ICP-MS, achieving full automation that included
separation and pre-concentration steps and on-line
determination, improving considerably the LOD of the
radioisotopes. However, high reagent consumption
inherent to FIA and the elevated cost of the detector
were disadvantages of these methods.
Later, Egorov et al. [20] developed a rapid method for
selective separation of Am and Pu using a column filled
with TRU resin. In this work, an FIA system was used to
introduce reagents, sample and eluent fractions to the
column. Grate et al. [31] optimized the previous method
by adjusting redox reactions to allow selective separation
of both isotopes. In these works, detection was carried
out on-line by coupling separation to a liquid-scintillation counter – possible due to the high activity of the
analyzed samples.
Despite the efforts to design and automate radiochemical separation in FIA systems, the high complexity
of procedures has meant that, in recent years, this
technique has been supplanted by other flow techniques,
especially SIA.
4.2. SIA methods
SIA appears as an alternative to FIA, and has proved to
offer more possibilities, since SIA is more adaptable than
FIA, and that makes SIA especially suitable for the
radiochemical field [32]. Besides, each experimental
parameter in SIA is directly controlled by computer and
it is possible to change it without reconfiguring the
system. Also, it is possible to develop methods in stopped
flow that allow longer counting times [33]. Fig. 3 shows
a typical SIA system.
The first fully automated methodology for radiochemical separation using SIA was proposed by Grate
et al. in 1996 [34]. They employed an SIA system for
separation and determination of 90Sr in aged nuclear
waste. Separation was achieved using a sorbent-extraction minicolumn containing Sr-Spec resin and the
detection was carried out on-line by liquid scintillation,
obtaining a sample throughput of 1.5/h and an LOD of
2.6 Bq. In a later work [35], they optimized this method
and increased the injection throughput, to improve
recovery and eliminate carry over. The main limitation
of this method is the requirement to stop the system to
renew the stationary phase.
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Table 1. Analytical characteristics of flow systems for determination of radioisotopes
Flow
technique
FIA
Isotope
On-line/ICP-MS
Soil
230
TRU resin
On-line/ICP-MS
Soil
99
TEVA resin
On-line/ICP-MS
Soil
TRU resin
TRU resin
On-line / ICP-MS
On-line/liquidscintillation detector
On-line/liquidscintillation detector
238
U
Am
241
239
TRU resin
90
1-octanol film with
4,4 0 (5 0 )-bi(tertbutylcyclohexano)18-crown-6 (BCHC)
TEVA resin
Pu
Sr
99
Tc
90
Sr
Sr-Spec resin
226
Ra
Manganese dioxide
formed on cotton
fibers
Manganese dioxide
formed on cotton
fibers
Manganese dioxide
formed on cotton
fibers
226
Ra
90
Sr
90
HDEHP adsorbed on
C18
90
Sr-Spec resin
Y
Sr
MSFIA-MPFS
226
Ra
Manganese dioxide
formed on cotton
fibers
TRU resin
241
Am
239+240
TRU resin
U(VI)
TRU resin
U(VI)
UTEVA resin
Pu
LOV-MSFIA
Sample matrix
TRU resin
U
Tc
MSFIA
Detection method
234
Th
SIA
Separation method
Detection
limit
Sample
volume
(mL)
Sample
throughput
(/h)
Ref.
10
9
[4]
10
9
[4]
10
9
[4]
Groundwater
Standard samples
0.74 mBq/g
(0.003 ng/g)
3.7 mBq/g
(0.005 ng/g)
11 mBq/g
(0.02 ng/g)
0.30 ng/L
-
5
0.25
10
4
[5]
[31]
Standard samples
-
0.25
4
[31]
Off-line/a-b lowbackground
proportional counter
Mineral water,
groundwater, seawater,
powdered milk and soil
0.07 Bq
2
2.4
[15]
On-line-stopped
flow/liquidscintillation detector
On-line/liquidscintillation detector
Off-line/a-b lowbackground
proportional counter
Off-line/a-b lowbackground
proportional counter
Off-line/a-b lowbackground
proportional counter
Nuclear waste
2 ng
1
3
[33]
Nuclear waste
2.62 Bq
0.1
1.5
[34]
Drinking and thermal
water
0.012 Bq
40
3
[37]
Water
0.15 Bq/L
40
3
[39]
Water
1.0 Bq/L
40
3
[39]
Off-line/a-b lowbackground
proportional counter
Off-line/a-b lowbackground
proportional counter
Mineral and tap water,
and biological samples
(urine and human blood)
Drinking and tap water,
seawater, powdered
milk and soil
0.05 Bq
2
1.2
[24]
0.01 Bq
2
2
[43]
Off-line/a-b lowbackground
proportional counter
Off-line/a-b lowbackground
proportional counter
Off-line/a-b lowbackground
proportional counter
On-line/
spectrophotometer
Mineral and tap water,
and seawater
0.05 Bq/L
40–200
4
[11]
Soil, vegetable ashes,
synthetic urine and
synthetic blood
Soil, vegetable ashes,
synthetic urine and
synthetic blood
Mineral, fresh and tap
water and seawater,
vegetable ashes and
phosphogypsum
4 Bq/L
1–5
2
[23]
4 Bq/L
1–5
2
[23]
12.6 ng/L
0.1–100
0.8–5
[44]
On-line/
spectrophotometer
Mineral, fresh and tap
water and seawater,
vegetable ashes and
phosphogypsum
10.2 ng/L
0.1–30
1.2–5.5
[45]
FIA, Flow-injection analysis; SIA, Sequential-injection analysis; MSFIA, Multi-syringe flow-injection analysis; MPFS, Multi-pumping flow system;
LOV, Lab-on-valve.
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Trends in Analytical Chemistry, Vol. xxx, No. x, 2010
V1
R
W
S
W
C
Peristaltic
Pump
D
W
R
V3
V2
RC
R
Peristaltic
Pump
Figure 2. Flow-injection analysis (FIA) system. C: Carrier; D: Detector; R: Reagents; RC: Reaction coil; S: Sample; V: Commutation valve;
W: Waste.
S
C
HC
R1
R2
W
R3
Syringe
Pump
Selection
Valve
D
Figure 3. Sequential-injection analysis (SIA) system. C: Carrier; D: Detector; HC: Holding coil; R: Reagents; S: Sample; W: Waste.
In order to overcome this disadvantage, Miró et al.
developed an SIA system with LLE based on a flowreversal, wet-film extraction method [15]. It is worth
emphasizing the automated renewal of the film, which
avoids both carry over and decrease of resin efficiency
caused by irreversible interferences. Drawbacks are low
reproducibility of film formation in different tubes and
the absence of full automation of the system.
Another SIA system was developed by Egorov et al.
[33] for 99Tc separation using TEVA resin as the
extracting material. Radiometric detection was carried
out on-line by liquid-scintillation counter in a stoppedflow approach to increase the analytical signal. This
method was also applied to aged nuclear waste. This
system was further optimized and improved by the same
authors [36], with a fully automated microwave-assisted
sample treatment method for analyzing 99Tc in different
6
matrixes (environmental samples and caustic aged nuclear waste samples).
Taking up again the results obtained with the FIA
system, Grate et al. designed an optimized procedure for
actinide separation using an SIA technique [22]. Separation was based on the different behaviors of nitrate and
chloride complexes of actinides of valence III, IV and VI
with respect to the extracting material. On-line detection
was carried out by liquid-scintillation counter, with an
integrated counting time of 6 s. A similar system using
an FIA approach was later coupled to ICP-MS to achieve
the same separation and response times [5]. Both
methods were applied to nuclear waste with high
activities, so detection could be continuous, even using a
liquid-scintillation counter.
Other SIA systems were developed in order to determine low-activity environmental samples, which forced
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detection off-line, resulting in a semi-automated methodology. Caldés et al. described a separation method for
226
Ra in thermal and mineral water, based on the
absorption of radium on the surface of manganese
dioxide formed on cotton fibers [37]. Detection was
carried out before daughters reached equilibrium.
Exploiting the absorbent properties of the manganese
dioxide to separate the yttrium (III) ions, Mateos et al.
determined 90Sr and 90Y simultaneously [38]. Yttrium,
retained on the surface of manganese dioxide, was eluted
with hydroxylamine. Total beta activity was determined
by an alpha-beta proportional counter, assuming that
90
Sr and 90Y were in secular equilibrium, and resolving
the Bateman equations, which allowed the initial activities of 90Sr and 90Y to be known. This method was
applied to mineral and thermal waters and achieved
recoveries greater than 80%.
Later, Mateos et al. designed a method for simultaneous determination of 226Ra and 90Sr [39]. 226Ra
activity was obtained by direct determination of the
precipitate of Ba(Ra)SO4, while 90Sr activity was determined through its descendant 90Y co-precipitating with
Fe(III) hydroxide. Full separation took 20 min.
Pu isotopes have been determined by an SIA system
using a TEVA-extraction chromatographic method in
environmental samples with on-line detection by ICP-MS
[40,41]. These methods were successfully applied to soil,
seaweed and seawater samples.
Another application of an SIA method was developed
by Kim et al. [42] for the separation of Pu, 210Po and
210
Pb from environmental samples. The proposed
method included fully automated separation although
detection was carried out off-line.
The effectiveness of flow techniques, especially SIA,
has been proved in many applications of high complexity. This allowed the design of novel analytical methodologies, with efficiently and automatically integrated
procedures of surface or stationary-phase renewal,
which subsequently led to the LOV technique. LOV has
been employed in not only homogeneous solution-based
assays, but also heterogeneous assays, and, due to its
flexibility in fluid manipulation, it is also suitable for
delivering beads in flow-based manifolds. Thus, Egorov
et al. [21] developed a sequential-injection and renewal
separation-column (SI-RSC) methodology to isolate some
radioisotopes chromatographically. They used some
selective commercial resins (Sr-Spec resin for 90Sr, TRU
resin for transuranides and TEVA resin for 99Tc and U),
which were automatically replaced without changing
the system configuration. This methodology offers many
advantages, all shared with the LOV technique,
including:
elimination of carry over;
avoidance of irreversible interferences;
possibility of multi-component analysis; and
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a significant decrease of both time and cost per analysis, since it is not necessary to stop the method to
change the extractant phase.
4.3. MSFIA methods
MSFIA was developed with the aim of incorporating
and combining the advantages of FIA, SIA and MCFIA
in just one technique. This technique uses a conventional automatic burette, modified in order to handle
simultaneously four syringes, whose pistons are connected to the same transmission bar. The module has
the functionality of four burettes working in parallel,
which increases sampling frequency. Due to the use of
three-way solenoid valves placed on the head of each
syringe, liquids can be returned to reservoirs when
they are not required, minimizing sample and reagent
consumption and waste generation. Communication
between the multi-syringe module and the computer
provides constant, precise control of the hydrodynamic
variables of each sample and reagent. The system has
great versatility and flexibility that are enhanced by
the simultaneous injection mode, which makes the
frequency of analysis of MSFIA comparable with that
of FIA.
This technique has been successfully applied to
radioisotope analysis in environmental samples. Fajardo et al. [43] developed a semi-automatic method for
strontium separation using the selective Sr-Spec solid
extraction. In 30 min, Sr is isolated and eluted, which
contrasts dramatically with the routine methodology
employed at radiological control laboratories. Concentration of the stable Sr is determined off-line by ICPAES, while beta activity of 90Sr is measured with a
low-background proportional counter. To measure
activity of 0.02 Bq, beta counting time is 1000 min,
with recovery of both species being over 90%. The
methodology was applied to samples of environmental
interest (water, milk and soil), and managed to reduce
the cost per analysis, reusing the resin for up to 30
analyses.
Another MSFIA method was developed by the same
authors to separate and determine stable and radioactive
yttrium [24]. This time, LLE was used to isolate the
isotope. The extraction system was established inside a
column that contained di-2 ethylhexylphosphoric acid
carefully adsorbed on the C18 inert support. As in the
previous work, the concentration determination and the
beta-activity detection of the stable and radioactive
yttrium were carried out off-line using an ICP-AES and a
low-background proportional gas counter, respectively.
LOD in concentration units of the stable isotope was
0.5 mg/L and the LOD of 90Y was 0.05 Bq. The semiautomatic procedure was applied to water and biological
samples with a recovery of 97% and a relative standard
deviation (RSD) of 3%.
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Trends in Analytical Chemistry, Vol. xxx, No. x, 2010
The multiconmutated techniques (i.e. SIA and
MSFIA), although slower, as shown in Table 1, consume
less reagents, since they are dispensed to the system only
when needed.
4.4. Hyphenated methods
Flow-technique hyphenation improves their individual
benefits and also enables the development of some
applications, so MPFSs have been used combined with
other flow techniques. An MPFS involves solenoid
micropumps, which simultaneously work as liquid
drivers and as commutation valves, allowing the introduction of large volumes in a short time. This unit is
responsible of dispensing and redirecting liquids in the
flow system. This double function and the simplicity of
its electronic operation make the MPFS versatile,
allowing it to be easily coupled with other techniques
(e.g., MSFIA).
With the aim of getting the maximum benefit of these
techniques, Fajardo et al. [11] developed MSFIA-MPFS to
determine 226Ra in different kinds of water (mineral, tap
and sea). Combining both techniques allowed use of
large sample volumes (40 mL) and increased the
throughput rate (4/h) by the continuously pumping
liquids through the MPFS. The volume of reagent or
sample dispensed is controlled by varying the number of
pulses. Flow rate is controlled according to the frequency
and the volume dispensed in each pulse. Another
advantage of an MPFS is that reagents are propelled
through the system only when necessary. Moreover,
compared with other flow techniques, the pulsed flow of
micro-pumps is better and faster at homogenizing the
reaction zone and that improves the analytical perfor-
mance (recovery greater than 90% and 0.4% RSD). The
methodology was satisfactorily applied in the working
range 0.25–50 Bq/L.
Fajardo et al. [23] also developed MSFIA-MPFS to
isolate and to pre-concentrate 241Am and 239+240Pu
using a column filled with TRU resin. Isolation of both
species was carried out by a selective on-column
oxidation/reduction of Pu and elution of Am, achieving an injection frequency of 2/h. Once the separation
step was finished, eluted fractions were prepared to
analyze activities of isotopes in a batch with a lowbackground proportional counter. With an LOD of
0.004 Bq/mL, recovery was greater than 90% with an
RSD of 3%. The procedure was successfully applied to
complex-matrix samples (soils, vegetal ashes, urine
and blood).
Later, Avivar et al. [44] developed a fully automated
method based on MSFIA-MPFS techniques for determination of uranium at ultra-trace levels with on-line
detection. The method was based on on-line uranium
isolation and pre-concentration with TRU resin, followed
by complex formation with arsenazo-III and spectrophotometric detection through a long path-lengthwaveguide capillary cell.
Fig. 4 shows this kind of system. Analytical data obtained with this method enabled its results to be comparable with those involving much more expensive
instrumentation. This method is an alternative to classical detection methods for determination of uranium(VI) and achieves a similar LOD (12.6 ng/L).
Moreover, this method has several advantages (e.g.,
simplicity, sensitivity, selectivity, low operational cost,
versatility, repeatability and robustness).
S
W
R3
V1
R4
W
HC
RC
V2
Multipumping
D
C R1 R2 C
Multisyringe
module
Figure 4. Hyphenated system: multi-syringe-flow-injection analysis (MSFIA) and multi-pumping flow system (MPFS). C: Carrier; D: Detector;
HC: Holding coil; R: Reagents; RC: Reaction coil; S: Sample; V: Commutation valve; W: Waste.
8
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Trends
For uranium determination, LOV-MSFIA coupled to
spectrophotometric detection was proposed by Avivar
et al. [45]. This method had an LOD of 10 ng U/L, using
a small amount of UTEVA resin. The injection
throughput was around 6/h. Moreover, LOV enabled
resin renewal (bead injection), and its coupling to MSFIA
fully automated uranium determination.
5. Radionuclide sensors for environmental
monitoring
Pre-concentrating minicolumn sensors integrate radionuclide separation and detection steps within a single
device for alpha and beta emitters [46]. The sensors are
based on the use of scintillating microspheres, which are
loaded into a renewable minicolumn that serves to separate and to pre-concentrate radionuclides within a
detector of well-defined geometry and emits a photometric signal. Since these sensors are built in flow mode,
the sensing material can be regenerated or renewed online. Fig. 5 shows an example of these sensors.
Egorov et al. [47] developed a sensor-based procedure
for 99Tc, achieving an LOD of 9.8 lg/L with an injection
throughput of 2/h. The method was based on SIA and
bead injection.
Later, Egorov et al. [48] designed an equilibrium-based
sensor using a pre-concentrating minicolumn for 99Tc
and 90Sr determinations. In this approach, a solution
with a given analyte concentration was delivered
through the minicolumn until resin breakthrough was
complete. Nevertheless, if the analyte concentration is
low, the column is not saturated, it is equilibrated, and
the amount captured is proportional to the sample
concentration. The main advantage lies in a long-term
application, since the precise volume delivery of the
pump over time is not a factor of maintaining calibration, as occurs when using the FIA technique, which
requires periodic recalibration of flow rates.
The features of these sensors are advantageous for
environmental monitoring applications because the extent of pre-concentration is maximized for a given chromatographic resin and geometry column, and because of
its high sensitivity and operational simplicity [46].
6. Conclusions
Undoubtedly, flow techniques provided a major step
forward in totally or partly automating radioanalytical
methods. They also considerably improved classical
procedures used in analysis of radioisotopes, implementing new automatic methodologies of separation and
pre-concentration. Automation provides each methodology with accurate control of reagents and sample
volumes dispensed, their flow rate, as well as of injection
throughputs, and results in simplicity, savings, effectiveness and safety. Design and implementation perspectives are extended, due to the easy miniaturization of
these techniques, which makes them more versatile,
simpler and more efficient than classical methodologies,
which are tedious, time consuming and labor intensive.
The possibility of coupling a detection system to a
manifold, in which the separation and pre-concentration
steps have been carried out, allows radioisotope determination to be fully automated. Sometimes, it is impossible to
Sample in
PMT
PMT
Bead delivery
Bead waste
Sample out
Figure 5. Radionuclide sensor. PMT: Photomultiplier tubes.
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Trends in Analytical Chemistry, Vol. xxx, No. x, 2010
automate radioisotope analysis fully, due to the low
activity encountered in environmental samples. Nonetheless, despite the off-line detection, the steps susceptible
to automation are the most laborious, with a higher degree of manipulation by the analyst, the highest reagent
consumption and the longest execution times.
Compared with classical methodologies, sample
throughput is significantly increased in flow techniques.
In all cases, isolation procedures are performed in
minutes in contrast with longer times required by
conventional methodologies applied in most official laboratories undertaking radioactivity monitoring.
The main advantages of a flow-injection system
compared to the traditional separation methods can be
summarized as:
(1) rapid separation;
(2) on-line sample pre-concentration;
(3) possibility of on-line detection;
(4) minimization of reagent consumption and waste
generation;
(5) reduction of cost per analysis;
(6) minimization of cross contamination;
(7) minimization of sample and reagent handling; and,
(8) improvement in analyst safety.
Acknowledgements
This work was funded by the SpainÕs Ministry of Education and Science (Project CTQ2010-15541) and the
SpainÕs Ministry of Science and Innovation (Project
FIS2008-00781). J.A. thanks the Balearic Islands University for funding her research.
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