Hauser 2 1 Centre for Environmental Technology and Sustainable Development CETASD, Hanoi University of Science, Hanoi, Vietnam 2 Department of Chemistry, University of Basel, Basel, Swit
Trang 1Huong Thi Anh Nguyen 1, 2
Pavel Kubánˇ 2, 3
Viet Hung Pham 1
Peter C Hauser 2
1 Centre for Environmental
Technology and Sustainable
Development (CETASD),
Hanoi University of Science,
Hanoi, Vietnam
2 Department of Chemistry,
University of Basel,
Basel, Switzerland
3 Institute of Analytical Chemistry,
Academy of Sciences
of the Czech Republic,
Brno, Czech Republic
Received February 2, 2007
Revised April 4, 2007
Accepted April 4, 2007
Research Article
Study of the determination of inorganic arsenic species by CE with capacitively coupled contactless conductivity detection
The determination of arsenic(III) and arsenic(V), as inorganic arsenite and arsenate, was investigated by CE with capacitively coupled contactless conductivity detection (CE-C4D)
It was found necessary to determine the two inorganic arsenic species separately employing two different electrolyte systems Electrolyte solutions consisting of 50 mM CAPS/2 mML-arginine (Arg) (pH 9.0) and of 45 mM acetic acid (pH 3.2) were used for arsenic(III) and arsenic(V) determinations, respectively Detection limits of 0.29 and 0.15 mM were achieved for As(III) and As(V), respectively by using large-volume injection
to maximize the sensitivity The analysis of contaminated well water samples from Viet-nam is demonstrated
Keywords:
Capacitively coupled contactless conductivity detection / CE / Inorganic arsenic ions / Large-volume injection DOI 10.1002/elps.200700069
1 Introduction
CE is a very useful tool in heavy metal analysis due to the low
consumption of reagents and consumables and to the fact
that short analysis times, high separation efficiencies, and
low operating costs are achieved In particular, for speciation
the method is significantly less expensive then the
often-used combination of chromatography with inductively
cou-pled plasma (ICP) spectroscopy Several review articles on
the applications of CE in metal speciation have been
pub-lished in the last few years [1–5] as well as specific reviews
dealing with the determination of arsenic [6–8]
The two inorganic forms of arsenic, arsenite, and
arsenate, have been found to be more toxic than the organic
arsenic compounds The widespread occurrence of these
species has created a strong demand for their monitoring
Toxic levels of arsenic in groundwater samples have been
found not only in developing countries of South and
South-east Asia but also in South America, the United States of
America, and Europe [9] The maximum tolerated
contami-nant level of all arsenic species in drinking water had been
set to 50 mg/L by the EPA for the time period between 1975 and 2006, but it was lowered to 10 mg/L (0.13 mM) in January
2006 [8]
The first analysis of arsenic species (inorganic As(III)
and As(V)) by CE was described by Wildman et al [10] using
indirect UV detection Standard solutions containing a set of common inorganic anions and the two arsenic species were used for the optimization of the BGE solution and for the subsequent determination of As(V) and As(III) in a spiked urine sample Note, however, that the sensitivity of the method was relatively poor, reaching LOD in the mg/L range and could, therefore, not be used for the analysis of real samples Several other methods based on indirect UV-detec-tion, which all had low detection sensitivity, were reported in subsequent years [11–14] Alternative detection methods were investigated in order to achieve higher sensitivities, and X-ray [15], indirect LIF [16, 17], and conductivity [18] detec-tion modes were shown to be suitable analytical tools for arsenic speciation The sensitivity was, however, in the same range as for indirect UV detection with the exception of con-ductivity measurements where LODs as low as 60 mg/L were achieved for As(V) Mass spectrometric detection was also applied for the CE determination of arsenic species [14, 19]; the sensitivity, in comparison with other detection methods, was slightly higher for organic species, however, extremely poor LODs were achieved for inorganic arsenic A slightly better sensitivity was obtained when direct UV-detection was applied Arsenic species absorb strongly in low UV-range and detection at wavelengths between 190 and 210 nm has been
used for speciation purposes [20–26] Interfering ions (e.g.,
Correspondence: Professor Peter C Hauser, Department of
Chemistry, University of Basel, Spitalstrasse 51, CH-4004 Basel,
Switzerland
E-mail: Peter.Hauser@unibas.ch
Fax: 141-61-267-1013
Abbreviations: Arg,L-arginine; C4 D, capacitively coupled
contact-less conductivity detection
Trang 2inorganic anions and cations in water samples) do not
absorb in this region (except for nitrate and nitrite) and
matrix effects of samples containing high concentrations of
inorganic ions (e.g., Cl–, SO42–, Ca21and Na1) can thus be
minimized The determination of several arsenic species in
real samples was demonstrated [22–27], although
pre-concentration techniques, such as large-volume injection
and field-amplified sample stacking, had to be applied in
most cases
For the determination of arsenic species, CE has also
been hyphenated to the ICP, however, the sensitivity of the
optical detection method [28, 29] did not allow to reach the
mg/L concentration levels which are required for real sample
analyses So far, the most sensitive detection for arsenic
spe-ciation in CE has been achieved with the ICP-MS (see, e.g.,
the following reviews [30–32]), which unfortunately is not
suitable for wide use due to its high cost
In the past decade, yet another detection scheme for CE
was presented, namely capacitively coupled contactless
con-ductivity detection (C4D) [33–36] This method is simpler
than the common UV-detection, and significantly less
com-plex and expensive than ICP-MS Good sensitivity has been
achieved for determination via C4D for most inorganic
spe-cies Several comprehensive reviews on C4D in CE have been
presented in recent years [37–39] and the theoretical
princi-ples of C4D have been described in detail, for example, in the
following articles [40, 41]
In this contribution, new methods were developed for
the CE-C4D determination of inorganic arsenic species in
water samples from Vietnam Serious pollution of ground
and drinking water is found for areas in North Vietnam with
arsenic concentrations significantly exceeding the EPA
lim-its of 10 mg/L, the most abundant species being inorganic
arsenic As(III) and As(V) Large-volume injection was used
to achieve LODs close to the regulatory limits and to
deter-mine inorganic arsenic pollutants in real water samples
2 Materials and methods
2.1 Instrumentation
Separations were carried out on a purpose-made instrument
which is based on a high-voltage power supply with 630 kV
interchangeable polarity (CZE 2000R) from Start Spellman
(Pulborough, UK) The contactless conductivity detector
consists of two tubular electrodes of 4 mm length separated
by a gap of 1 mm and a Faradaic shield [35, 36] Cell
excita-tion was carried out using a sine wave with a frequency of
200 kHz generated by an external function generator
(GFG-8216A from GW Instek, Tucheng City, Taiwan) and was
boosted to a peak-to-peak amplitude of 300 Vp–p The
result-ing current signal was converted to voltage usresult-ing an OPA655
operational amplifier (Texas Instruments, Dallas, TX, USA),
amplified, rectified, and low pass filtered with a circuitry
described elsewhere [36] before passing to a MacLab/4e data
acquisition system (AD Instruments, Castle Hill, Australia) for recording of the electropherograms pH measurements were carried out with a model 744 pH meter from Metrohm (Herisau, Switzerland)
2.2 Reagents and methods
All chemicals were of analytical reagent grade and deionized water was used throughout Stock solutions (10 mM) of As(III) and As(V) were prepared from sodium arsenite (Fluka, Buchs, Switzerland) and disodium hydrogen arsenate (Merck, Darmstadt, Germany), respectively Stock solutions of inorganic anions (10 mM) were prepared from the corresponding sodium or potassium salts (Fluka or Merck) All multi-ion standard and calibration solutions were prepared from these stock solutions MES, TAPS, CHES, MOPS, CAPS, L-arginine (Arg), CTAB, and acetic acid (99.8%) were obtained from Fluka or Merck Fused-silica capillaries of 50 mm id and 375 mm od (purchased from Polymicro Technologies, Phoenix, AZ, USA) were used for the electrophoretic separations The total length of the
separation capillaries was 50 cm (Leff= 43 cm) and 75 cm
(Leff= 68 cm) and the capillaries were preconditioned with
1 M NaOH for 10 min, deionized water for 5 min, 1 M HCl for 10 min, deionized water for 10 min, and finally with electrolyte solution for 15 min Electrolyte solutions were prepared daily from the corresponding pure chemicals
10 mM CTAB was prepared as stock solution CAPS and Arg were weighed into a 25 mL volumetric flask and dissolved in
10 mL of deionized water CTAB (30 mM) was added to the volumetric flask from a corresponding stock solution and the flask was filled to the mark with deionized water The elec-trolyte solution was equilibrated for 15 min Elecelec-trolyte solutions containing acetic acid were prepared directly from concentrated acid by dilution with deionized water Electro-lyte solutions were degassed in an ultrasonic bath for 5 min and filtered through a 0.20 mm nylon syringe filter (BGB
Analytik, Böckten, Switzerland) Resolution values, R, were calculated according to the standard formula (see, e.g., [42]).
3 Results and discussion
3.1 Analytical procedure for the determination of As(III)
3.1.1 Electrolyte system
The pKa1-value of arsenous acid (H3AsO3), the form in which inorganic As(III) is present in aqueous solutions, is 9.2 Thus, the pH value of an electrolyte solution used for the electrophoretic determination of arsenic(III) should be at least 9.0 or higher in order to render a substantial fraction of the analyte in the charged form Arg was chosen as basic component for the preparation of electrolyte solutions
be-cause of its high pKa-value (12.5) and low conductivity in
Trang 3aqueous solutions As acidic counter-ions five different
spe-cies were selected, namely MES, CHES, MOPS, TAPS,
CAPS, and the effect of electrolyte solutions consisting of
either of these and Arg in each case on the electrophoretic
determination of 100 mM arsenic(III) was investigated All
electrolyte solutions had a pH value of pH 9.0 To enable the
determination of As(III) as the arsenous anion concurrent
with the EOF, CTAB, which was found to be compatible with
conductivity detection [43], was added to the electrolyte
solu-tions
The resulting electropherograms for the determination
of arsenic(III) in these electrolyte solutions are shown in
Fig 1 Clearly, the composition of the electrolyte solution has
a pronounced effect on the sensitivity The direction of the
peak deviation from the baseline is dependent on whether
the ion of the same charge contained in the buffer has a
lower or higher molar conductivity than the analyte ion, and
both orientations are acceptable in conductivity detection
Note that fractional charges due to partial dissociation also
have a bearing on the effective conductivity of the species
The system peak found in all electropherograms was
asso-ciated with the presence of CTAB and the carbonate peak
(mostly due to bicarbonate) is due to absorption of ambient
CO2from air into the solutions
The S/N ratios were evaluated for the electrophoretic
determination of As(III) in the electrolyte solutions
de-scribed above by comparing the peak heights with the level of
noise present on the baseline Values of 300, 20, 8, 35, and 60
were obtained for CAPS/Arg, CHES/Arg, TAPS/Arg, MES/
Arg, and MOPS/Arg, respectively Best results were thus
achieved for the CAPS/Arg solution even though the peak for
the analyte in this solution was not quite the tallest
Electro-lyte solutions based on CAPS and Arg were therefore used
for a more detailed investigation of As(III) determination
Next, the molar ratios of CAPS and Arg were varied to
examine the effect of changes of the pH value of the
electro-lyte solution on the determination
It is shown by the results given in Fig 2 that with an
increase in the pH value of the electrolyte solution the peak
heights increase (presumably due a higher degree of
depro-tonation of the arsenous acid) However, it was found that the
baseline noise was also increasing with the pH value S/N
ratios of 300, 210, 190, 195, 185, 100, and 75 were thus
determined for the solutions with pH values of 9.0, 9.2, 9.4,
9.6, 9.7, 9.9, and 10.0, respectively, so that the best S/N was in
fact obtained for the lowest pH value The short-term
base-line noise (as opposed to longer term instabilities due to
Joule heating), was found to show a correlation with the
conductivity of the electrolyte solution, which increased with
the pH value from 55 mS/cm (pH 9.0) to 135 mS/cm
(pH 10.0) Furthermore, the separation from the carbonate
peak is more pronounced for the electrolyte solutions with
the lower pH values, a feature which is important for the
large volume injection method discussed below It was also
found that the baseline stability in the region of the
carbon-ate peak deteriorcarbon-ated with time and a stepwise profile of the
Figure 1 Electropherograms for a standard solution of 100 mM
As(III) in different electrolyte solutions at pH 9.0 (30 mM CTAB was added to all buffers for EOF modification) Sample injection: hydrodynamic, 20 cm for 10 s Detection parameters: 300 V p–p ,
200 kHz Separation potential: –20 kV Capillary: fused silica,
50 mm id, Lt= 50 cm (Leff= 43 cm) (1) 50 mM CAPS/2 mM Arg; (2)
50 mM CHES/30 mM Arg; (3) 15 mM TAPS/20 mM Arg; (4) 12 mM MES/20 mM Arg; (5) 12 mM MOPS/20 mM Arg.
Figure 2 Electropherograms for a standard solution of 100 mM
As(III) in electrolyte solutions composed of CAPS/Arg at different
pH values pH 9.0: 50 mM CAPS/2 mM Arg; pH 9.2: 20 mM CAPS/
2 mM Arg; pH 9.4: 20 mM CAPS/5 mM Arg; pH 9.6: 20 mM CAPS/
8 mM Arg; pH 9.7: 20 mM CAPS/10 mM Arg; pH 9.9: 20 mM CAPS/20 mM Arg; pH 10.0: 10 mM CAPS/20 mM Arg Separation and detection parameters as for Fig 1.
Trang 4baseline was observed after several electrophoretic runs This
effect was more pronounced for the higher pH values of the
electrolyte solution, and it is assumed that this is related to
the fact that the higher pH values are closer to the pKa2value
of carbonate (10.3), so that the fraction of doubly charged
carbonate present is larger The reason for the pronounced
shift in the elution times for carbonate and As(III) between
the solutions of pH 9.9 and 10.0 is not clear, but was found to
be reproducible An electrolyte solution with a pH value of
9.0 (corresponding to a ratio of 50/2 mM (CAPS/Arg)) was
thus chosen for the determination of As(III) as overall the
best performance was achieved with this electrolyte solution
In order to obtain best baseline stability, the electrolyte
solu-tion in the two containers at the capillary ends was replaced
before each run and the capillary itself rinsed with fresh
electrolyte solution
3.1.2 Large-volume sample injection
Sample injection was carried out in the hydrodynamic mode
by elevating one end of the separation capillary to a given
height for a specific time The injection height (20 cm) was
kept constant in all experiments and the injection time was
varied between 10 and 120 s Longer injection times resulted
in higher peaks for As(III), note however, that, as would be
expected, reduced peak resolution was observed for larger
injected volumes, especially when real samples with high
concentrations of other anions were injected An injection
time of 60 s was chosen for CE measurements as this
gen-erally provided sufficient resolution and enabled also high
sensitivity
3.1.3 Separations
A calibration for As(III) was carried out in the range of 1–
100 mM and a correlation coefficient (r2) of 0.9975 was
achieved (based on five points) The LOD value was
deter-mined as the As(III) concentration giving a peak height
cor-responding to three times the baseline noise and was found
to be 0.29 mM The reproducibility of peak areas, given as
RSD values for three consecutive injections of a standard
so-lution, ranged between 0.1 and 6.8% (n = 3) in the
con-centration range of 1–100 mM of arsenic(III)
Groundwater samples were taken from contaminated
areas in Vietnam where a high occurrence of arsenic species
was expected as evidenced by AAS measurements (samples
VN1 and VN2 were taken in the Ha Nam provinces and VN3
to VN5 were taken in Ha Noi City (Thanh Tri district)) [44–
47] The water was collected in plastic bottles (which had
been washed with a detergent solution, deionized water,
0.1 M nitric acid, deionized water, and finally three times
with appropriate water sample) directly from the well or tap
The supply tubing was flushed for 15 min before the
sam-pling took place
As(III) is unstable at ambient conditions and is rapidly
oxidized by oxygen in air to As(V) Moreover, when samples
contain iron, the arsenic species can be adsorbed by and/or coprecipitated with iron(III) hydroxide As(III) was, there-fore, not detected in any of the samples which had been brought to Switzerland from Vietnam but it was demon-strated that after spiking with As(III) solution, the determi-nation of low concentrations of As(III) was possible in the matrix of these samples
Electropherograms for one of the samples, before and after spiking with 1.5 mM As(III), are shown in Fig 3 The analysis was carried out using the optimized electrophoretic conditions (50 mM CAPS/2 mM Arg, and 30 mM CTAB,
pH 9.0) The pronounced elongations from the baseline observed for both samples (see Fig 3) are due to the major anions Even though the sample matrices are quite different,
as evidenced by the electropherograms, the determination of arsenic at much lower concentrations is still possible in both backgrounds In Fig 3B the relevant sections of the electro-pherograms are shown at an enlarged scale for illustration of
Figure 3 (A) Determination of As(III) in a groundwater sample
(VN4) from Vietnam The spiked concentration of As(III) is 1.5 mM (B) Enlarged sections of the electropherograms Electrolyte solu-tion: CAPS 50 mM /Arg 2 mM, 30 mM CTAB, pH 9.0 Sample injection: hydrodynamic, 20 cm for 60 s Detection parameters:
300 Vp–p, 200 kHz Separation potential: –20 kV Capillary:
fused-silica, 50 mm id, Lt= 75 cm (Leff= 68 cm).
Trang 5this point The unknown peak visible in the
electro-pherograms next to that of the arsenic species, was also
found to be present in tap water taken in the laboratory at the
University of Basel, Rhine river water sampled in Basel and
bottled water purchased in a local shop, but could not be
identified
Recovery values for the water samples taken in Basel, and
for the well water samples from Vietnam, spiked with 1.5 mM
As(III) are given in Table 1 The results show that it is
possi-ble to determine As(III) reliably in real samples at this level
3.2 Analytical procedure for the determination of
As(V)
3.2.1 Electrolyte system
It was initially attempted to develop a method that would
allow the determination of As(III) and As(V) concurrently in
the same electrolyte solution The first two pKa’s of arsenic
acid (H3AsO4), the species in which inorganic As(V) is
pres-ent in aqueous solutions, are 2.2 and 7.1, therefore, at a pH
value of 9 as used for the determination of As(III) it is also
possible to detect As(V) in anionic form Several of the
elec-trolyte solutions that had been examined for the analysis for
As(III) were thus also tested for As(V) Although high
sensi-tivity could be achieved for As(V), it was found that a serious
interference from other inorganic anions occurred
Phos-phate and carbonate, which are present in most samples,
comigrated partially with As(V) and only marginal
improve-ments of the separation resolution was achieved by varying
the composition and/or concentration of the electrolyte
solutions The closeness of the electrophoretic behavior of
arsenate and phosphate (the size and pKa’s of the species are
very similar) precluded the use of high pH electrolyte
solu-tions for As(V) and, therefore, electrolyte solusolu-tions of low pH
were investigated for the separation of these two species
A preliminary investigation of the separation was
per-formed using the modeling software PeakMaster 5.1 (http://
www.natur.cuni.cz/,gas) and acetic acid was chosen as
elec-trolyte for the determination of As(V) Several reports have
been published on the successful determination of inorganic anions in low pH electrolyte solution using CE-C4D (see, for example the recent reviews [37–39]) An EOF modifier was not used with this buffer solution, as the silanol groups of the separation capillary are protonated to a high degree at low pH values and the EOF is thus significantly reduced
The dependence of the peak height for As(V), and the resolution between As(V) and phosphate, on the concentra-tion of acetic acid is illustrated in Fig 4A According to these results, 45 mM acetic acid was chosen as optimal electrolyte concentration since good sensitivity and excellent resolution between As(V) and phosphate could be achieved An electro-pherogram for the two substances separated at these condi-tions is given in Fig 4B Moreover, no interference from car-bonate was observed in the low pH electrolyte solutions The
pKavalues of carbonate are 6.4 and 10.3 and no analytical signal can be measured in acetic acid electrolyte solutions
3.2.2 Sample injection
Injection parameters were adopted from the conditions reported for As(III) for which optimization of hydrodynamic injection was performed with respect to analyses of real samples and an injection time of 60 s for the capillary end elevated to a height of 20 cm was used
3.2.3 Separations
Calibration for As(V) was carried out in the range of 0.5–
100 mM (based on six points) and a correlation coefficient (r2)
of 0.9998 was achieved The LOD value was calculated based
on the 36S/N criteria (by comparing peak height with base-line noise) and was determined to be 0.15 mM The RSD values for peak areas in the concentration range of 0.5–
100 mM were between 1.3 and 2.3% (n = 3).
The ground water samples taken in Vietnam were also analyzed for As(V) As(V) was present in detectable con-centrations in three of the five samples The electro-pherograms for the positive samples are given in Fig 5 Dif-ferences in the matrix composition are also evident from the
Table 1 Determination of As(III) in spiked samples using an electrolyte solution of 50 mM CAPS/2 mM Arg and
30 mM CTAB as EOF modifier, a fused-silica capillary of 50 mm id, 75 cm total, and 68 cm effective lengths,
and a separation voltage of –20 kV
Sample Spiked concentration (mM) Concentration determined (mM) Recovery (%)
Trang 6Figure 4 (A) Optimization of the acetic acid concentration for
peak height of As(V) and separation between As(V) and
phos-phate (n) Peak height for 5 mM As(V) (m) Resolution R
Separa-tion and detecSepara-tion parameters as for Fig 3 (B) Electropherogram
illustrating the separation of phosphate and As(V) for a
con-centration of 5 mM in 50 mM acetic acid Separation and detection
parameters as for Fig 3.
electropherograms, but were of no concern for the current
project The concentrations were determined as 0.16 6 0.01,
1.81 6 0.06, and 0.18 6 0.01 mM in the three samples VN2,
VN3, and VN5, respectively Note that two of these values are
close to the detection limit value given above The peak area
integration employed for quantification of the samples
(rather then peak heights) led to still acceptable precision
values for these low concentration levels For verification of
Figure 5 Determination of As(V) in ground water samples from
different places in Vietnam Electrolyte solution: 45 mM acetic acid, pH 3.2 Separation and detection parameters as for Fig 3.
the method, the two samples from Vietnam without meas-urable As(V) as well as tap, Rhine river, and bottled water were spiked with standard and the recovery was calculated The results summarized in Table 2 show that the quantifica-tion is reliable
4 Concluding remarks
The study presented is one of few to date to investigate the determination of trace levels of inorganic analytes in the presence of high levels of major ions using CE with contact-less conductivity detection The determination of the inor-ganic anionic arsenic species of As(III) and As(V) was found possible in natural water samples with this simple and inex-pensive method by using individually optimized methods These have potential in environmental applications and may
be used for fast screening of groundwater contaminated with arsenic CE with contactless conductivity detection is a much simpler and less expensive approach then using ion-chro-matography or atomic spectrometry methods, which also has
Table 2 Determination of As(V) in spiked samples using an electrolyte solution of 45 mM acetic acid, pH 3.2, a
fused-silica capillary of 50 mm id, 75 cm total and 68 cm effective lengths, and a separation voltage
of220 kV
Sample Spiked concentration (mM) Concentration determined (mM) Recovery (%)
Trang 7the potential to be implemented in field portable
instru-mentation [48] Please note, however, that the detection
lim-its achieved, expressed in conventional unlim-its 22 and 11 mg/L
for As(III) and As(V), respectively, while adequate for the
previous limits in potable water of 50 mg/L, are not quite
adequate for all samples of interest, in particular considering
the current guideline for drinking water which states a
threshold level of 10 mg/L
The authors would like to thank the Swiss Federal
Com-mission for Scholarships for Foreign Students (ESKAS) to
enable the postgraduate study for Huong Thi Anh Nguyen (Ref.
No.: 2005.0339/Vietnam/OP), the Swiss National Science
Foundation (grant No 200020-105176/1 for partial financial
support, as well as Michael Berg (Swiss Federal Institute of
Aquatic Science and Technology (EAWAG), Switzerland), and
Pham Thi Kim Trang from the Centre for Environmental
Tech-nology and Sustainable Development (CETASD), Hanoi
Uni-versity of Science, Vietnam) for supplying the groundwater
samples.
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