Anal. Chem. 1990, 62, 2692-2697
2892
Liquid Chromatography-Electrochemical Detection of Inorganic Arsenic Using a Wall Jet Cell with Conventional and Microsized Platinum Disk Electrodes R. S. Stojanovic and A. M. Bond* Department of Chemical and Analytical Sciences, Deakin University, Geelong 3217, Victoria, Australia
E. C. V. Butler CSIRO Division of Oceanography, Marine Laboratories, Hobart 7001, Tasmania, Australia
A critical evaluation of a highly sensitive and spectflc llquld chromatography amperometric detection method (wall jet cell) for the detennlnatlon of arsenlc has been undertaken with plathum disk electrodes of variable &e. The limit of detection is conshfably iower with plaUnum dldc electrodss of convmtbnal size (mi#imaer radius) than Is the case with microelectrodes (radius In micrometer range) because of a more favorable M a l to ndso ratio. Thus, the determkratlon of arsenic Is readily achieved in a bottled mineral water sample with a conventional slre electrode, whereas the arsenic levels are too low to be detected precisely wlth a microelectrode. However, wlth respect to the deslrable properties of a lower dependence on flow rate and a shorter t h e taken to achieve a stable baseline, and In cases where sensitlvlty is not a problem, the microelectrode method is advantageous. The method for detection of arsenic Involves a surface-based oxidation process In which As( I I I ) is conThis work suggests that advantages and verted to A@). disadvantages offered by using microelectrodes as an alternative to conventionally sized electrodes are critically dependent on the mechanism of the electrode process.
INTRODUCTION Modern liquid chromatography has relied heavily on the development of suitable detection systems that are capable of determining the separated compounds rapidly, reliably, and with a minimum sample volume (1,2). The main criteria of an ideal detector for liquid chromatography are that they be suitably selective for the problem being examined, have a low limit of detection, have a minimal effect on peak width, and are as independent as possible of chromatographic parameters such as the rate of eluent flow (1-3). While no universally accepted detector satisfies all of the requirements for highperformance liquid chromatography (LC), electrochemical (EC) detectors have found widespread application and a variety of indicator electrodes and flow-cell designs have been developed for use in the combined LCEC method ( 4 ) . Presently, considerable effort in the field of LCEC is being devoted to the development of more selective, sensitive, and stable detection systems to supplement the rapid advances taking place in separation methods. For example, chemically modifying the surface of an electrode has been used to improve the specificity of electrochemical detection for certain classes of compounds (5, 6). Recently, the unique properties of microelectrodes (7-22) (typically disks or fibers of radius 1-100 pm) have been exploited for use as detectors in liquid chromatography (13-20) and have been found to offer advantages over conventionally sized electrodes in LCEC work such as small size (small volume required), time independent current response (steady state), small diffusion layer (analytical re-
sponse is significantly less dependent on flow rate), less ohmic iR drop, little or no supporting electrolyte needed, and a more rapid response time for the detector. While voltammetric microelectrodes have been shown to possess certain desirable features for detection in flowing streams, there still appears to be a reluctance to use these electrodes in analytical applications and studies in the practical sense have been restricted to catecholamines, their derivatives, and a few other applications (see refs 13-20 for example). Additionally, except for the determination of copper in urine (19),and the determination of tocopherol (20), and some more fundamental studies (16,281,only a microelectrode of a single size (radius) has been examined with respect to improving the analytical sensitivity. In view of the paucity of information available on the electrode size as a factor in electrochemical detection, and since this is likely to be one of the more important aspects of the use of microelectrodes, further examination is required to ascertain the exact usefulness of these electrodes in flowing streams. The determination of arsenic has always presented problems for the analytical chemist since not only is it present in natural waters at extremely low levels but it is also present in different chemical forms (21-25). It is now widely acknowledged that the various chemical species of arsenic have different levels of toxicity. The chemical forms or species of dissolved arsenic in natural waters are usually limited to arsenate (HAsOd2-), arsenite (H3As03),monomethylarsonic acid (MMA) (CH3AsO(OH)J, and dimethylarsinic acid (DMA) ((CH3)~sOOH). Arsenic is generally present in the pentavalent state, As(V), although the As(II1) form is significant in poorly oxygenated waters. The relative toxicity has been found to be in the order ASH, > As(II1) > As(V) > C H ~ A S O ( O H>) ~(CHJ~ASOOH. Since As(II1) has been reported to be some 50 times more toxic than As(V) and several hundred times more toxic than MMA or DMA, it is important in terms of the degree of toxicity to distinguish between the inorganic species in an analytical procedure. A wide variety of methods have been used to determine arsenic such as hydride generation atomic absorption spectrometry, inductively coupled plasma atomic emmision spectrometry, and neutron activation analysis (26-30). All of these methods require expensive instrumentation and are essentially sensitive to total arsenic. In contrast, electrochemical detection methods are instrumentally inexpensive and species sensitive, with methods being available to specifically detect the highly toxic As(II1) in flowing streams (31-33). While electrochemical detection can often lack the ability to descriminate between species with similar redox Characteristics, the resolving power of liquid chromatography permits the removal of other chemical species from interfering with As(III), as well as providing additional information on the sample matrix. Ion exclusion chromatography has been used successfully to separate weak inorganic and organic acids
0003-2700/90/0362-2692$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 24, DECEMBER 15, 1990
such as As(II1) (arsenite) from As(V) (arsenate) with no interference from most other inorganic ions (33, 34). In the present study, the LCEC detection of inorganic arsenic using conventional and microsized platinum disk electrodes of various radii are compared in a wall jet flow cell so as to better understand the advantages and disadvantages of using microelectrodes as detectors for LC. Although carbon paste and glassy carbon electrodes have proven themselves to be highly useful in LCEC (351, they are unsuitable for use in the detection of As(III), since As(II1) cannot be oxidized at a bare carbon electrode (36,37). An application to a bottled mineral water sample is presented and the advantages of both microsized and conventional sized electrodes are discussed in detail on the basis of our findings. EXPERIMENTAL SECTION Reagents and Standard Solutions. All solutions were prepared from analytical reagent grade chemicals. Distilleddeionized water was obtained by passing distilled water through a Barnstead Sybron Nanopure water purification system (Barnstead Sybron Corp., Boston, MA) and then through a Millipore Organex-Q cartridge to remove any trace organics. A stock solution of 5.00 mM As(II1) was prepared by dissolving 0.1237 g of arsenic trioxide (As203)(BDH Chemicals) in 10 mL of a 10% potassium hydroxide solution,after which 1.0 mL of concentrated sulfuric acid was added and diluted to 250 mL. All working standards were then prepared by serial dilution with either water or eluent. Orthophosphoric acid (0.01 M H3PO4) (Ajax Chemicals) was used as the eluent and was filtered through a Millipore (Type HA, 0.45 pm) filter and degassed under vacuum in an ultrasonic bath. Sample. The arsenic content of a commercially available bottled mineral water (Volvic), imported from France, was examined directly for As(II1). Total inorganic arsenic was measured after chemical reduction of As(V) to As(II1) with sulfur dioxide. The reduction procedure of Bodewig et al. (38)was used to reduce As(V) to As(II1) with some modification. These modifications involve using concentrated orthophosphoric acid in place of concentrated sulfuric acid and removing residual sulfur dioxide with nitrogen and then air at 90 “C for 45 min. The total arsenic concentration was determined by using the method of standard additions with individual data points being the average of three determinations. Chromatographic System. The chromatographic system was constructed from individual components and consisted of a separator column (either a Waters Fast Fruit Juice column (polymeric base 10 pm, 150 mm length X 7.8 mm diameter) operated at a flow rate of 1.0 mL min-’ or a Brownlee Polypore H column (10 pm 250 mm length X 4.6 mm diameter) operated at a flow rate of 0.6 mL min-’, a guard cartridge (Brownlee Polypore H 10 pm, 30 X 4.6 mm), Waters 6000A solvent delivery system, a Rheodyne 7126 sample injector with a 200-pL loop, and a 0.2-pm in-line filter. To deal with external sources of noise such as pump pulsation, a pulse dampener (Waters) was placed between the solvent delivery system and the sample injector. All chromatograms were recorded at 20 OC. Since the electrochemical detector is specific for As(II1) and does not measure As(V), a variable-wavelength ultraviolet/visible detector operated at 200 nm (Linear Instruments, UVIS-200) was used to confirm that chromatographic separation of As(II1) and As(V) occurred. Electrochemical System. Two Electrode Format. The following two-electrode instrumental configuration was used to measure the small currents obtained at a microdisk electrode under LCEC conditions. A Metrohm 656 wall jet electrochemical cell (Metrohm, Herisau, Switzerland), a Keithley 614 digital electrometer (Keithley Instruments, Inc.) as a current measuring device, and a variable battery-operated voltage source (16) was used to apply a constant dc potential. A Waters 740 data module or an IC1 Instruments (Victoria,Australia) DP600 dual pen chart recorder were used as the recording device. Voltammetric measurements obtained by using a two-electrode arrangement were undertaken with a platinum disk microelectrode as the working electrode and a Ag/AgCl (3 M KCl) reference electrode. To provide the lowest possible level of electronic noise, all detector
2693
cables were shielded and properly grounded so as to avoid ground loops. To deal with external sources of noise, all measurements were carried out in a Faraday cage and optimum results were achieved by placing the waste bottle within the Faraday cage. Three Electrode Format. Conventional three-electrode POtentiostatic instrumentation was used for LCEC measurements with conventionallysized electrodes and consisted of a BAS LC-4B amperometric detector (Bioanalytical Systems, Inc.) which served as a potentiostat and a current measuring device coupled to the same Metrohm 656 wall jet electrochemical cell as used in the two-electrode format. A time constant of 1s was used to measure the current. Voltammetric measurements with a three-electrode arrangement were made with a conventional platinum disk electrode, which has a radius of 0.8 mm as the working electrode, a Ag/AgC1(3 M KCl) reference electrode, and a platinum auxiliary electrode. Electrodes. Platinum microdisk electrodes were fabricated by sealing platinum wire in glass as described elsewhere (39). Cyclic voltammetric measurements in a static solution provided a useful way of checking and assessing the quality of a microelectrode for use in flowing streams (40,41). The radius of each microelectrode was determined from the steady-state voltamM ferrocene mograms obtained for the oxidation of a 1 X solutin in acetonitrile (0.1 M Et4NC104)and using the equation (9, 16)
iL = 4nFDCr
where iL is the diffusion controlled limiting current, n is the number of electrons transferred (one for ferrocene),F is Faraday’s constant, C is the bulk concentration, D is the diffusion coefficient (2.3 X 10” cm2s-l for ferrocene at 22 “C (9,16)),and r is the radius of the microelectrode. Surface treatment of the conventional circular disk electrodes (0.8 mm radius) and microdisk electrodes involved successive polishing with 0.5- and 0.05-pm alumina slurries, immersing in hot concentrated perchloric acid, and thoroughly rinsing with distilled water. Electrodes prepared in this manner were termed “clean” if a reproducible profile was obtained while cycling the potential between 0.0 and 1.0 V vs Ag/AgCl in 0.01 M orthophosphoric acid (eluent). RESULTS AND DISCUSSION Stationary Cell Voltammetry. The unique surface properties of noble-metal electrodes, in particular platinum, have been used advantageously to dramatically electrocatalyze the oxidation of a number of species, including As(II1) to As(V). The standard redox potential of the As(V)/As(III) couple in acidic media is 0.573 V vs the standard hydrogen electrode (42),although the oxidation of As(II1) and reduction of As(V) are highly irreversible processes under voltammetric conditions. The electrochemical oxidation of As(OH), a t a platinum electrode in acidic solutions has been proposed to be an irreversible two-electron process with the transfer of an oxygen atom from water and has been reported to proceed only when a surface oxide is present on the electrode surface (43-46).
As(OH),
+ H20
-
OAs(OH),
+ 2H+ + 2e-
(2)
Figure 1shows a comparison of the cyclic voltammograms obtained a t a 5-pm Pt microdisk electrode in a stationary solution for (a) the oxidation of 1 X M ferrocene (Fc) in acetonitrile (0.1 M Et4NC104)and (b) the oxidation of 5 X M As(II1) acid in 0.1 M H3P04. While a well-defined near-steady-state (sigmoidal-shaped) voltammogram is obtained at a microelectrode for the electrochemically reversible and diffusion-controlled one-electron oxidation of Fc F c s Fc+
+ e-
(3) the peak shape of the wave for the oxidation of As(OH), is consistent with a surface rather than diffusion-controlled mechanism in which the oxidation is catalyzed by the formation of adsorbed hydroxy radicals at the surface of the
2894
ANALYTICAL CHEMISTRY. VOL. 62. NO. 24, DECEMBER 15. 1990
Table 1. Voltammetric Data in a Stationary Solution for the Oxidation of Arsenie(II1) and Ferrocene as a Function of Electrode Size ratio of peak
As(III)/As(V) radius,' pm 5.0 12.5
50 800
i, A 6.8 X 3.2 x 3.6 X 3.8 X
IO-'' 10-9 10" 10"
Fc/Fc+
i J r , A/pm
i p / r l ,A/pmz
i, A
1.4 X 2.5 x 10-10 7.1 X 4.1 X 10.'
2.7 X 10." 2.0 x lo-" 1.4 X IO-" 5.9 x 10-12
4.9 X lodb 1.7 X 10"' 7.0 X 2.2 x 10-5
iJr, A/pm 9.8 1.4
X X X
iD/rz,A/pm2 2.0 X 1.1 x 10-10 2.8 X 10." 3.4 x 10-1'
lo4
IO4 2.8 X 10" 1.4
currents i,(As)/i,(Fe) 0.14
0.19 0.51 0.17
'Scan rate = 50 mV s-I. Arsenic(II1) concentration = 5.0 x lo-' M in aqueous 0.1 M H,PO,. Ferrocene concentration = 1.0 x M in acetonitrile (0.10 M Et4NC104). bNear-steady-state voltammagram for which a limiting current rather than peak current is measured.
i 0.7
00
0
10
23
:3S
4C
50
60
Detection Limit (10-EM As(ll1)) Figure 2. Effect of varying ttm electrode radius on the limR of detection for the oxidation of As(II1) in a wail jet amperometric detectw: applied potential. 0.8 V vs AgIAgCI: flow rate, 1.0 mL min-'.
Potential (V) vs AgIAgCI(3M NaCi) Figure 1. Voltammograms at a 5 pm radius R dish electrode in a Stationary solution of (a) 1 X M Fc in acetonitrile (0.1 M Et,NCIO,) and (b) 5 X IO-' M AS(OH)~(0.1 M H,PO,): scan rate. 50 mV s-'. electrode (43). Voltammetric data for the oxidation of both processes in a stationary solution are summarized in Table I. The concentration of ferrocene (n = 1) used to obtain data for this tahle was M, whereas for arsenic (n = 2) a 5 X M solution was employed. The difference in concentration takes into account the 1:2 ratio of electrons transferred in the charge transfer step. The ratio i , / r (i, is the peak current) for the oxidation of As(OH), using conditions described in Table I decreases markedly with decreasing radius for all sizes of platinum electrode as expected for a surfacebased process where the electrode area (9)rather than radius ( r ) is the more important electrode dimension. In contrast, for the oxidation of ferrocene the value of i,/r ( i L / r )is much less dependent on the radius of the microelectrodes as expected for a diffusion-controlled process (eq 1). In particular the different electrode process dependence on electrode size means that the ratio of peak (or limiting) currents for As:Fc becomes very small a t the small-sized microelectrodes. The different electrode size dependence for a complex surfacebased process relative to a diffusion-controlled process has implications with regard to the limit of detection of arsenic a t a microelectrode. Detection of Inorganic Arsenic Using Liquid Chromatography w i t h Electrochemical Detection. (i) Limit of Detection. The limit of detection (twice the standard deviation of the noise (3))at the conventionally sized 0.8 mm radius platinum electrode under LCEC conditions was determined to he 5 X M. Platinum microdisk electrodes ranging in size from 2.5 to 50 pm radius also were used to determine the limit of detection of As(II1) in a wall jet flow cell a t an applied potential of 0.8 V vs Ag/AgCI. As can he
seen in Figure 2, the limit of detection for the determination of As(III), also calculated as twice the standard deviation of the noise (3), deteriorates rapidly as the size of the electrode is reduced. This behavior appears to limit the usefulness of extremely small microelectrodes as highly sensitive measuring devices when the electrochemical process is surface controlled. In terms of detection limit, there is no doubt that the conventional sized electrode is to he preferred for the determination of arsenic. Unlike the case with some voltammetric measuring systems in stationary solutions, the limit of detection in the flow cell commonly is governed hy the faradaic to background system noise rather than faradaic to charging current ratio. The inherent noise level (47-49) present is of the order of 100 fA with our system, and this extraneous system noise places a restriction on the limit of detection that can he achieved and is therefore reached a t a higher concentration for the smaller electrodes. The response index ( r ) is a measure of detector linearity and is defined by the following power function (50) y = AC'
(4)
where y is the detector output, C is the concentration, and A is a constant. A log-log plot of peak current @) versus concentration (C)gave an r value (slope) and linear correlation coefficient of 0.99 and 0.98 respectively for the conventional size electrode (radius 0.8 mm) and 0.98 and 0.97 respectively for the microdisk electrode (radius 12.5 ,". The linear dynamic ranges of both calibration curves were 0.01-50 pM for the conventional electrode and 0.05-50 pM for the 12.5 fim radius Pt microdisk electrode. For a truly linear detector r = 1, hut the practical hounds for response linearity are usually between 0.98 and 1.02 (50).The linearity of the response is therefore excellent for both conventional and microsized electrodes. (ii) Stability. The maintenance and long-term reproducibility of solid electrode detectors have always presented problems since they are prone to gradual fouling of the surface
ANALYTICAL CHEMISTRY, VOL. 62, NO. 24, DECEMBER 15, 1990
2605
3
c
C
E L
3 0 Y (0
a
a
-..- - m L
1
1
0
10
'
1
'
1
20
'
30
1
40
'
j 1
50
Time (hours) Figure 3. Comparison of the stability of a wall jet amperometric detector for the oxidation of 1 p M As(II1) at a conventionally sized 0.8 mm radius Pt disk electrode (m) and a 12.5 pm radius pt disk electrode (El): applied potential, 0.8 V vs Ag/AgCI; flow rate, 1.0 mL min-'.
lo
c *.O
1
1.8 c
1.6
C
2 ~
L
3
1.4
0
0 * C
? L 3 0
Y
Y
m
03
a
'1.2
a
a
a
1 .o 0
1
-
2
Flow Rate (ml/min) Figure 4. Dependence of flow rate on detector response for the
oxidation of 1 pM As(III), at a conventionally sized 0.8 mm radius Pt disk electrode (m) and a 12.5 pm radius Pt disk electrode (El): applied potential, 0.8 V vs Ag/AgCI.
due to adsorption, film formation, and other related phenomena, which result in a loss of long-term stability. Consequently, an important criterion for detectors in liquid chromatography is that the measuring device give a reproducible response over an extended period of time. Figure 3 shows a comparison of the long-term performance of a Pt macrodisk and a microdisk electrode for the oxidation of 1 X IO+ M As(II1). The formation of adsorbed hydroxyl groups (PtOH) on the surface of Pt electrodes initiates the oxidation of As(II1) to As(V) (43),with the longer term conversion of PtOH to PtO being responsible for the loss in detector response. The performance of the wall jet amperometric detector exhibits better stability over an extended period of time when a conventional rather than a microsized electrode is used in the flow cell. Light polishing of the platinum surface restored both electrodes to their initial current response. A further comparison that needs to be made between conventional sized disk electrodes and microdisk electrodes concerns the time to achieve a constant background response. A fast background equilibration time is an advantage in LCEC since it enables measurements to be made almost immediately after the application of an applied potential. With conventional size electrodes, the background current in the electrolyte used to determine As(II1) requires between 2 and 3 h to equilibrate. With microelectrodes, the background current stabilizes within 15 min. The ability to rapidly achieve background equilibration via the use of microelectrodes has been observed for other systems (51, 52) and is a very considerable advantage with respect to the determination of arsenic.
4.0
8.0
TIME (min.) Flgure 5. Separation of 0.5 mM As(II1) and As(V) by ion exclusion
chromatography (Waters Fast Fruit Juice column) with UV detection at 200 nm: flow rate, 1.0 mL min-'; peak [l] anions, [2]As(V), [3] As(II1).
(iii) Flow-Rate Dependence. Figure 4 shows a comparison of the flow-rate dependence a t both a conventional size electrode and a microelectrode for the oxidation of As(II1) approximately 24 h after the application of an applied potential of 0.8 V vs Ag JAgCl. After this time of operation, the wall jet amperometric detector still exhibits a linear response over the concentration range 0.1-50 pM for both conventional size and microdisk electrodes. However, while the conventional disk electrode still shows an exponential flow rate dependence of 0.80, which is close to the theoretical value of 0.75 predicted for sample flow into a wall jet detector (53, 5 4 ) , essentially flow rate independent data are obtained at a microelectrode. This is an advantage in LCEC since a decreased flow rate dependence on the response can lead to reduced noise levels caused by fluctuations in eluent flow and, therefore, to a higher reproducibility. (iv) Electrode Position. The position of both a conventional and microdisk sized electrode in a wall jet flow cell were found to effect the performance of the electrochemical detector. The optimum current response was obtained when both electrodes were positioned as close as possible to the impinging jet, while a decrease in the current response was observed as both electrodes were positioned further away from the orifice of the jet. This is a critical parameter if a low limit of detection is to be obtained a t both conventional and microdisk electrodes. (v) Application. Hydride generation atomic absorption spectrometry has proven to be a very popular method of determining arsenic in natural waters and has recently been used to determine the arsenic content of a range of bottled mineral waters (55). An advantage with using electrochemical methods is that they are able to distinguish between As(II1) and As(V) in an analytical procedure since the latter is not electrochemically active. The usefulness of ion exclusion chromatography for the separation of As(II1) and As(V) is shown in Figure 5 where both species are detected with an ultraviolet (UV) detector a t 200 nm. The electrochemical method enables As(III), if present, to be determined directly by injection of a sample of mineral water. Total inorganic arsenic can be determined as As(II1) after chemical reduction
2696
ANALYTICAL CHEMISTRY, VOL. 62, NO. 24,DECEMBER 15, 1990
Table 11. P r e c i s i o n and R e c o v e r y Data for T o t a l A r s e n i c D e t e r m i n a t i o n
amt of As(V) added, pg L-’
sample
Volvic mineral water
4.0
8.0
% recovery
14.9 70.3
0.64 0.81
93.7
CONCLUSIONS
n
, i
1
LITERATURE CITED
4.0
std dev
While microelectrodes may provide greater analytical sensitivity than macrosized electrodes for detection of a species exhibiting reversible processes, this is not necessarily true for more complex electrode processes such as oxidation of arsenic(III), which utilize adsorption or related surface phenomena. The rapid time response for the background current to reach an equilibrium value with a microelectrode is useful for studies where the concentrations of the sample are sufficiently high to be detected. Additionally, flow rate independent data can be obtained at a microelectrode for the detection of As(III), which may be an advantage. Application to a bottled mineral water sample revealed that conventionally sized Pt disk electrodes can be used to detect the total arsenic concentration,whereas the undesirable level of noise associated with measurement of small currents restricts the analytical usefulness of microelectrodes at low concentrations.
TIME (min.)
0
average, pg L-’
for ten replicate determinations (Table 11)and is in agreement with previously published results (55) which gave a value of 18 pg L-l. Figure 6a shows a chromatogram obtained for Volvic bottled mineral water at a wall jet amperometric detector with a conventional size electrode after chemical reduction of As(V) with SOz. The peak due to the oxidation of As(II1) is adequately resolved from the remaining peaks and easily quantified. The effectiveness of the reduction of As(V) by SOz was determined by standard addition of 1.0 pM As(V) to distilled water. The average of five duplicate determinations (Table 11) proved the reduction procedure effective, with 93.7% of the added As(V) recovered as As(II1). Figure 6b shows a chromatogram obtained by using a 25 pm radius platinum disk electrode as a detector. While the peaks are adequately resolved, the undesirable level of noise present restricts the analytical usefulness of microelectrodes at the low levels of arsenic present in mineral water, and conventionally sized electrodes must be used for an accurate determination.
-
0
L-I
14.8, 15.3, 14.4, 14.2, 15.6, 16.1, 14.6, 14.9, 14.1, 15.2 69.2, 70.4, 71.3, 70.9, 69.9
none 75.0
distilled water
total As found, pg
8.0
TIME (min.)
Figure 6. Determination of total inorganic arsenic (As(V)) in Vohric bottled mineral water after reduction with SO, by ion exclusion chro-
matography (Waters Fast Fruit Juice column) with amperomehic detection in a wall jet flow cell using (a) a conventionally sized 0.8 mm radius platinum disk electrode and (b) a 25 pm radius platinum disk microelectrode: applied potential, 0.8 V vs Ag/AgCI; flow rate, 1.0 mL min-’; peak [l] system peak, [2]system peak, [3] sulfite, [4] total inorganic arsenic, 14.9 ppb. of As(V) to As(II1) with sulfur dioxide (see Experimental Section) and the concentration of As(V) can then be evaluated by difference. Previous reports appear to indicate that arsenic is generally present in the pentavalent state (As(V)) (55,56) in mineral water. This is confirmed by the present study since the As(II1) concentration was below the detection limit in the Volvic mineral water sample. The concentration of total inorganic arsenic in the sample was determined to be 14.9 0.6 pg L-l
*
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RECEIVED for review June 19,1990. Accepted August 30,1990. This work was supported by a CSIRO/Deakin University Collaborative Research Award.
Anodic Stripping Voltammetry at Mercury Films Deposited on Ultrasmall Carbon-Ring Electrodes Danny K. Y. Wong and Andrew G. Ewing* Department of Chemistry, 152 Davey Laboratory, Penn State University, University Park, Pennsylvania 16802
Anodic strlpplng voltammetry of lead and cadmlum without deliberately added electrolytes has been studied at uitrasmaii carborrrlng electrodes following In sltu deposttion of mercury. The stripping of lead has been studled in detail to investigate the dependence of strlpplng peak current on experlmentai parameters such as potential scan rate, preconcentration duration, deposition potential, concentratlon of Hg' during deposition, and Concentration of Pb2+. Anodlc strlpplng voltammetry In solutions without deilberatety added supporting electrolyte avolds problems associated with knpurlties Introduced when electrolyte Is added. These Impurities appear to be highly anportant when Pb2+analysis Is carried out in dlute solutions. I n addition, a unique effect Is observed when relatlvely low concentrations of Hg' (1.0 X lod M) are used for the in situ deposition step. When low Hg+ concentrations are used, the stripping current does not decrease as rapidly as expected as the concentration of Pb2+ is reduced. The slope of the ~ps-iogplot of peak current M m2+concentration is sigrtlfkantly leils than unlty, but the callbratkn plot is ilnear, and the resuitlng enhanced peak currents at low Pb2' concentrations make strippkrg analysls possible at extremely low concentrations. Concentrations as low as 3.2 X lo-'' M of Pb2+ have been examlned.
INTRODUCTION Anodic stripping voltammetry (ASV) has always been regarded as one of the most sensitive techniques for trace-metal analysis. In general, this technique is a nondestructive method
* To whom correspondence should be addressed. 0003-2700/90/0362-2697$02.50/0
and is applicable to multielemental analysis. The part per billion detection limit (1) of stripping analysis is attributable to the preconcentration that takes place during the deposition step. ASV is often carried out with a mercury-thin-film electrode (21, which provides a large surface area to volume ratio, and this yields superior sensitivity during voltammetry. There has also been intense study in the fabrication and development of micro voltammetric electrodes (3-5) during the past decade. Microelectrodes offer a number of attractive features suitable for many electroanalytical applications over conventional-sized electrodes. Small double-layer capacitance owing to the small electrode surface area results in diminished electrode charging current. This produces a greater ratio of faradaic to nonfaradaic current and can lead to enhanced detection limits (6). Additionally, the small currents passed by microelectrodes result in negligible ohmic losses and this allows electroanalysis to be carried out in poorly conductive media. This permits the use of systems without any deliberately added supporting electrolyte. Impurities introduced with electrolyte are thus eliminated, and the range of potentials accessible for electrochemical measurements can be extended (7). The combination of small capacitive charging current and small potential drop across the uncompensated cell resistance provides a system where high scan rate cyclic voltammetry is possible. In fact, scan rates above 1000000 V s-l have been demonstrated (8). In contrast, at low scan rates (
