Application Note # CA-274195
Low level speciation of chromium in drinking waters
using LC-ICP-MS
Introduction
Instrumentation
Measuring only the total concentration of a particular
element in environmental or clinical samples can often lead
to errors in assessing toxicity, mobility, and bioavailability,
as these effects can differ greatly between the various
chemical species in which an element may occur. Mercury,
arsenic, chromium, lead, selenium and tin can all be found in
numerous organic and inorganic compounds, and will have
varying effects on an ecosystem depending on their form.
The LC system used was comprised of an inert Tertiary
Solvent Delivery Module, a biocompatible autosampler
fitted with a 200 μL sample loop, and a Anion exchange
column (Hamilton PRP-X100, 4.1 mm ID x 250 mm, 10
μm) and PEEK™ guard column. When measuring Cr, it is
important to make sure the LC systems are metal free.
When using LC components, be sure to select the inert or
biocompatible versions.
Chromium (Cr) is a good example of this, although in this
case the key distinction is the ionic state of the chromium
ion. The inorganic trivalent form of chromium (Cr III) is
relatively non-toxic and is an essential micronutrient.
Hexavalent chromium (Cr VI) however, is a known
carcinogen although it is widely used in numerous industries
including plastics, metalworking and dyes/paints/inks.
Many government environmental and occupational safety
regulatory agencies have established regulations, guidelines
and exposure limits regarding the use, storage and disposal
of hexavalent chromium-containing materials.[1],[2],[3],[4]
The aurora M90 ICP-MS was used for all chromium
measurements, operated in high sensitivity mode using
the standard sample introduction system. The ICP-MS
setup allows automatic optimization of both ion optics and
gas flows (plasma and CRI gases). Before connecting the
ICP-MS to LC, it was tuned automatically by using the
auto optimization routine included in the software. The CRI
system was also optimized to maximize Cr52 sensitivity in
the presence of a high Ar40C12 background signal, resulting
in an approximate 30 times improvement in signal-to-noise
when blank eluent solution was being measured. The
ICP-MS nebulizer was then connected to the LC column
using a 20 cm length of PEEK tubing (0 25 mm ID). The
mobile phase used for this experiment, ammonium nitrate,
can also be used for the simultaneous determination of
selenium [8], [9]. Galaxie software was used to control the
operation of the ICPMS software and hardware during data
acquisition and to process all of the data. See Tables 1 to 3
for the ICP-MS and LC conditions used.
Inductively Coupled Plasma Mass Spectrometry (ICPMS) is a very powerful analytical tool for measuring trace
and ultra-trace concentrations of metals and metalloids.
When coupled to a Liquid Chromatography (LC) system,
species elute one by one from the LC column directly to the
ICP-MS so that low level detection and compound-specific
identification can be achieved simultaneously. [5], [6]
Interfacing the LC system to ICP-MS is very straightforward; the LC column output is directly connected to
the nebulizer of the ICP-MS by a piece of PEEK tubing,
and no other hardware changes are required to either the
LC or the ICP-MS. Using an aurora M90 ICP-MS with
Collision/Reaction Interface (CRI) [7] technology allows
for the elimination of major polyatomic interferences
formed in the plasma, allowing for even lower detection
limits of the analytes of interest. All instrument control
during analysis and post-run data processing used Bruker’s
Galaxie Workstation Data System, which offers full control
of the ICP-MS, LC pump and autosampler for automated,
unattended operation.
Reagents and samples
Deionized water (18MΩ/cm, Millipore MilliQ, Billerica, MA,
USA) was for mobile phase preparation.
Mobile phase (LC)
Suprapur ammonia (Merck, Kilysth, Victoria, Australia) and
AR Select Plus nitric acid (Mallinckrodt Baker, Phillipsburg,
NJ, USA) were used to produce the 60mM ammonium
nitrate mobile phase. The pH of the mobile phase was 9.3.
The mobile phase was prepared daily.
Calibration standards
Chromium (+3) and chromium (+6) aqueous standards were
obtained from Inorganic Ventures, Inc., (Lakewood, NJ,
USA).
curves shown calibrate successfully from 50 ng/L to 100
µg/L, with calibration correlations of 0.999 or better, thus
demonstrating the linearity and reproducibility of the
technique. All quantitation was performed on peak area.
Figure 3 illustrates the speciation of Cr(III) and Cr(VI).
Chromium(III) EDTA complex
To stabilize the Cr(III) species and allow the determination
of both species in a single chromatographic run, each of the
standards and samples were incubated at 70 ºC with 0.2
mM of EDTA (disodium salt, from Sigma Aldrich, Castle Hill,
NSW Australia) for 1.5 hours. When reacting with EDTA,
it is then the Cr(III)-EDTA complex that is retained on the
column.
Results and discussion
ICP-MS is well known for its excellent sensitivity. The
addition of interference reduction technology, such as CRI,
on the aurora M90 ICP-MS allows Cr to be measured with
even better sensitivity and signal to background ratios.
Using CRI it is possible to use the main chromium isotope at
m/z 52 with the removal of interferences including ArC and
ClOH. Figures 1 and 2 show that it is possible to quantitate
to ng/L (ppt) levels for both Cr(III) and Cr(VI). The calibration
Figure 1: Calibration curve for Cr(III), using standards ranging from
0.05 to 100 ppb
Table 1: aurora M90 ICP-MS operating conditions
ICP Conditions
CRI Settings
Sample Introduction
Parameters
Settings
Plasma flow (L/min)
18
Auxiliary flow (L/min)
1.65
Nebulizer flow (L/min)
0.24
Sheath flow (L/min)
0.98
Plasma RF power (kW)
1.40
Monitored ion
Cr52
Dwell time (s)
0.5
Skimmer gas
60mL/min H2
Sampler gas
OFF
Pump rate (RPM)
25
Spraychamber temp (ºC)
3
Table 2: LC operating conditions
Parameters
Settings
Mobile Phase
60 mM ammonium nitrate, pH 9.3
Flow rate
1 mL/min
Run time
10 min
Column
Anion exchange, Hamilton PRPX100,
4.1 mm ID x 250 mm, 10 µm
PEEK Guard column
1
Column temperature
Ambient
-30
Sample injection volume
200 µL
-3.9
Detection
aurora M90 ICP-MS
Ion Optics (Volts) 1st Extraction lens
-500
2nd Extraction lens
-879
3rd Extraction lens
-643
Corner lens
-763
Mirror left
40
Mirror right
37
Mirror bottom
40
Entrance lens
Entrance plate
Fringe bias
Figure 2: Calibration curve for Cr(VI), using standards ranging from
0.05 to 100ppb
Analysis of samples
To apply the chromium speciation method to real world
sample types, six different brands of both Australian and
International mineral waters were analysed. These were
analysed both unspiked and spiked at levels of 0.1 ppb for
each of the chromium species. Each of the mineral waters
were designated by a letter, A through to F. As shown
in Table 3, each of the mineral waters gave very good
percentage recoveries.
Figure 4 shows mineral water E, spiked with 0.1 µg/L Cr(III).
From this chromatogram it is also possible to observe the
stability of the Cr(VI) signal, which was not spiked.
Conclusion
This work has demonstrated that the aurora M90 ICP-MS
with CRI technology is an excellent detector when coupled
with liquid chromatography for chromium speciation. Low
ng/L detection limits can be routinely obtained and very
good recoveries were obtained on a variety of spiked
mineral waters.
Figure 3: Chromatogram showing mineral water (blue) and mineral
water spiked with 1 ppb Cr(III) and Cr(VI) (red). Samples were
stabilized with EDTA prior to analysis
Table 3: Spike recovery data for 0.1 µg/L spikes of Cr(III) and
Cr(VI) in commercial mineral waters
Brand
Cr(III) % recovery
Cr(VI) % recovery
Mineral water A
105
99
Mineral water B
98
102
Mineral water C
106
108
Mineral water D
103
95
Mineral water E
102
106
Mineral water F
104
104
Figure 4: Chromatogram showing Australian mineral water E before
and after a 0.1 µg/L Cr(III) spike.
References
[1] United States Department of Labor, Occupational Health and Safety Administration
[2] Environment Canada – Canadian Environmental Protection
Act 1999
[3] UN World Health Organization – Guidelines for Drinking
Water Quality
[4] Australian National Health and Medical Research Council
[5] H. Gurleyuk and D. Wallschager, JAAS 2001,16, 926
[6] F. Byrdy, L. Olson, N. Vela and J. Caruso, J. Chrom A 1995,
712, 311
[7]I. Kalinitchenko, Mass spectrometry apparatus and method, US Patent 7,329,863,132 B2, 12 February 2008
[8] M. Leist, The Speciation of Se(IV) and Se(VI) using the
aurora M90 ICP-MS, Bruker application note
[9] Y. Martinez-Bravo, A.F. Roig-Navarro, F.J. Lopez, F. Hernandez,
J. Chrom A 2001, 926, 265
Keywords
Instrumentation & Software
Drinking water analysis
aurora M90 ICP-MS
LC-ICP-MS
Galaxie software
Speciation of chromium
Authors
Michael Leist, Ray Leiser and Andrew Toms
Bruker Daltonik GmbH
Bruker Daltonics Inc.
Bremen · Germany
Phone +49 (0)421-2205-0
Fax +49 (0)421-2205-103
[email protected]
Billerica, MA · USA
Phone +1 (978) 663-3660
Fax +1 (978) 667-5993
[email protected]
www.bruker.com/chemicalanalysis
to change specifications without notice. © Bruker Daltonics 02-2011, #CA-274195
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For research use only. Not for use in diagnostic procedures.