Anal. Chem. 1999, 71, 4603-4608
Voltammetric and Amperometric Investigations of Azide Oxidation at the Basal Plane of Highly Oriented Pyrolytic Graphite Jishou Xu and Greg M. Swain*
Department of Chemistry & Biochemistry, Utah State University, Logan, Utah 84322-0300
The electrochemical oxidation of dissolved azide anion has been investigated at the basal plane of highly oriented pyrolytic graphite. Cyclic and linear sweep voltammetry and differential pulse voltammetry were employed to study the oxidation reaction mechanism in neutral pH as a function of the potential sweep rate, analyte concentration, and electrolyte composition and in the presence of adsorbed anthraquinone-2,6-disulfonate (2,6-AQDS). The linear dynamic range in the differential pulse voltammetric measurements was 4 orders of magnitude and the estimated limit of detection (SNR ) 3) was ∼2.3 × 10-7 M (9.7 ppb). The electron-transfer kinetics for azide oxidation appear rapid at this surface, and the voltammetric features are independent of the fraction of exposed edge plane, the presence of surface oxides, the electrolyte composition, and the adsorption of 2,6-AQDS. The reaction is shown to proceed by an EC(dim) mechanism. Amperometric detection results for flow injection analysis (FIA) and ion chromatography are also presented. A linear dynamic range of nearly 5 orders of magnitude, an estimated detection limit (SNR ) 3) of 3.7 nM (0.16 ppb or 74 fmol injected), and a response variability of 2% or less were observed in the FIA measurements. Sodium azide is reactive and used commercially, particularly as an inflator in automotive airbags. The azide anion is highly toxic and can present a health hazard. Therefore, industries producing or using azide generally have tight controls on the levels of the anion allowed in wastewater effluent. Azide is being phased out as an inflator for airbags and is not a common pollutant in water supplies. However, as current model cars containing azide-based airbags are retired to salvage yards, the likelihood of azide contamination of groundwater and soil will increase significantly in future years. To the best of our knowledge, there exists no EPA recommended protocol for the detection of dissolved azide, nor is there a mandated limit of exposure. The Occupational Safety and Health Administration has set exposure limits of 0.1 ppm for gaseous HN3 and of 0.3 ppb for solid NaN3.1-3 Generally, gas chromatography (detection of HN3), ion chromatography, and (1) Dalmia, A.; Wasmus, S.; Savinell, R. F.; Liu, C. C. J. Electrochem. Soc. 1996, 143, 556. (2) Dalmia, A.; Savinell, R. F.; Liu, C. C. J. Electrochem. Soc. 1996, 143, 1827. (3) Dalmia, A.; Wasmus, S.; Savinell, R. F.; Liu, C. C. J. Electrochem. Soc. 1995, 142, 3735. 10.1021/ac9814501 CCC: $18.00 Published on Web 09/08/1999
© 1999 American Chemical Society
capillary electrophoresis are used to analyze for azide anion with detection limits in the high-ppb to low-ppm range.1-3 New methods are needed to detect azide stably and reproducibly in the lowppb range. Electrochemical methods of analysis are inexpensive and can provide high sensitivity, long-term response stability, and good assay reproducibility for analytes that are amenable to such detection. It turns out that azide is electrochemically active at boron-doped diamond, glassy carbon, Pt, and Au electrodes.1-10 In general, however, there have been only a few comprehensive investigations of this anion’s electrochemistry. Roscoe and Conway6 reported that azide anion can be oxidized to nitrogen, nitric oxide, nitrous oxide, and nitrogen dioxide, as well as reduced to NH3 at platinum depending on the applied potential. Most recently, Savinell and co-workers1-3 have reported on azide oxidation and reduction at platinum, gold, and glassy carbon. The elegance of these studies was the combined use of electrochemistry and in situ mass spectrometry to monitor the reaction products generated. The products of azide oxidation at platinum and gold are influenced by the oxygen coverage as the reaction occurs at potentials where the metal surface oxides are formed. Nitrogen gas and various nitrogen oxides were detected at platinum and gold electrodes, but nitrogen was the primary product produced at glassy carbon. Nitrogen is the principal product at glassy carbon because of the relative absence of surface oxides, as compared to the noble metal electrodes. The oxidation of azide at borondoped diamond thin films and at glassy carbon was investigated using linear sweep and differential pulse voltammetry.9 The results indicated that diamond provides superior response performance compared with glassy carbon in terms of several detection figures of merit. Specifically, diamond thin films exhibited a low and stable voltammetric background current in neutral phosphate buffer, a linear dynamic range of 4 orders of magnitude for azide oxidation, and a limit of quantitation (i.e., actual concentration detected) of 1.0 µM (SNR ) 3), which corresponds to 4.3 ppb or 2.0 pmol of (4) Miyama, H.; Nosaka, Y.; Fukushima, T. J. Electrochem. Soc. 1986, 133, 336. (5) DeFelippis, M. R.; Faraggi, M.; Klaper, M. H. J. Phys. Chem. 1990, 94, 2420. (6) Roscoe, S. G.; Conway, B. E. J. Electroanal. Chem. 1988, 249, 217. (7) Alfassi, Z.; Harriman, A.; Huie, R.; Mosseri, S.; Neta, P. J. Phys. Chem. 1987, 91, 2120. (8) Alfassi, Z.; Schuler, R. J. Phys. Chem. 1985, 89, 3359. (9) Xu, J.; Swain, G. Anal. Chem. 1998, 70, 1502. (10) Granger, M. C.; Xu, J.; Strojek, J. W.; Swain, G. M. Anal. Chim. Acta, in press.
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azide. Amperometric detection data for azide using flow injection analysis (FIA) and ion chromatography have also been presented.9,10 The dynamic range was nearly 5 orders of magnitude with FIA, the limit of quantitation (SNR ) 3) was 8.0 nM, which corresponds to 0.33 ppb or 160 fmol injected, and a stable response was observed over a 12-h period of continuous use in FIA. The results demonstrated that diamond can be used to reproducibly and stably detect sub-ppb levels of azide anion and provides superior detection figures of merit compared to polished glassy carbon. To the best of our knowledge, there have been no reports describing the electrochemistry of azide at the basal plane of highly oriented pyrolytic graphite (HOPG). HOPG is of interest as an electrode for two reasons: (i) the material exhibits low voltammetric background current and double-layer capacitance, much like boron-doped diamond thin films,9-14 and (ii) it is the most microstructurally ordered sp2 carbon material available, making it useful for structure-reactivity studies. The study of this well-defined sp2 electrode complements our earlier work with diamond electrodes. We report here on the voltammetric investigation of the azide oxidation mechanism at the basal plane of HOPG. The reaction was studied by cyclic and linear sweep voltammetry and differential pulse voltammetry in neutral pH as a function of the potential sweep rate, analyte concentration, and electrolyte composition and in the presence of adsorbed anthraquinone-2,6-disulfonate (2,6-AQDS). Amperometric detection data for azide using FIA and ion chromatography are also presented. The results demonstrate that, like boron-doped diamond thin-films, HOPG functions as a sensitive anode for the detection of this analyte. The electron-transfer kinetics appear rapid and are insensitive to the fraction of exposed edge plane, the presence of surface oxides, the electrolyte composition, and the adsorption of 2,6-AQDS. It is shown that the reaction proceeds by an EC(dim) mechanism. EXPERIMENTAL SECTION The cyclic and linear sweep voltammetric, differential pulse voltammetric, and chronoamperometric measurements were conducted in a single-compartment glass cell at room temperature. The working electrode was placed against a smooth ground joint at the bottom of the cell, isolated by a viton O-ring. The O-ring defined the area of the working electrode exposed to the solution (∼0.2 cm2). A platinum wire was used as the counter electrode, and a saturated calomel electrode (SCE) served as the reference. All potentials are quoted with respect to this reference. The electrochemical measurements were performed using a CYSY1090 digital or an Omni-90 analog potentiostat (Cypress Systems, Inc., Lawrence, KS). Unless otherwise stated, the voltammetric measurements were made without any iR compensation. The HOPG samples (a mixture of grades) were a generous gift from Dr. Arthur Moore (Advanced Ceramics Corp., Cleveland, OH). Three electrodes (HOPG 1, 2, and 3) were used and prepared by simply cleaving off the topmost layers with Scotch (11) Jolley, S.; Koppang, M.; Jackson, T.; Swain, G. M. Anal. Chem. 1997, 69, 4099. (12) Xu, J.; Granger, M. C.; Chen, Q.; Strojek, J. W.; Lister, T. E.; Swain, G. M. Anal. Chem. 1997, 69, 591A. (13) Swain, G. M.; Anderson, A. B.; Angus, J. C. MRS Bull. 1998, 9, 56. (14) Tenne, R.; Le´vy-Cle´ment, C. Isr. J. Chem. 1998, 38, 57.
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tape. The freshly exposed surface was immediately immersed into the electrolyte to preserve a high degree of cleanliness. One of these electrodes (HOPG 2) was also anodically polarized by potential cycling (40 cycles) between -0.5 and +1.5 V versus SCE at 50 mV/s. Ultrapure water (>17 MΩ) from a Barnstead E-pure system was used to prepare all the solutions. All glassware was cleaned by rinsing with ultrapure water after a short soak in a warm 1:1 nitric acid/water bath. Potassium dibasic and monobasic phosphate salts (Mallinckrodt) were mixed equally at 0.1 M to prepare the pH 7.2 buffer solution which served as the supporting electrolyte, hereafter referred to as PBpH7.2. Reagent grade sodium azide (Sigma) and potassium ferrocyanide (Aldrich) were used as supplied. The concentration of the azide solutions ranged from 10 nM to 1 mM. In the experiments with the co-anions, 10 mM KF, KCl, KBr, and KNO3 (Fisher Scientific) were mixed individually with the phosphate buffer. In the experiments involving the adsorption of anthraquinone 2,6-disulfate (Aldrich), the electrode surface was first exposed to a 0.2 mM solution of 2,6AQDS in 0.1 M HClO4 for 15 min. This solution was then removed, and the cell was rinsed repeatedly with ultrapure water. The electrode was then exposed to the sodium azide solution. The amount of 2,6-AQDS adsorbed was determined from chronocoulometric measurements, as described previously.15 The FIA system and the thin-layer flow cell have been described elsewhere.9-11,16 The carrier solution flow was regulated with a Constametric III metering pump (Milton Roy), through a model 7125 injector valve (Rheodyne) with a 20-µL injection loop and into a home-built thin-layer flow cell. The system possessed inert gas purge capability, and the carrier solution was constantly deaerated with nitrogen (99%) during the analyses. A homemade pulse dampener was used in series to reduce the pump noise. The data were recorded on an analog stripchart recorder. The entire setup was electrically grounded, and the flow cell was housed in a Faraday cage to reduce the electrical noise pick-up. The thin-layer flow cell was constructed with Kel-F in two pieces.9-11,16 The top piece contained the entrance and exit ports for the fluid flow and a place for the Ag/AgCl (3 M NaCl) reference electrode. The exit port was fitted with a short piece of stainless steel tubing (∼6 cm in length) which served as the counter electrode. The bottom piece supported the working electrode. Electrical contact was made to the backside of the HOPG sample. A 0.1-cm-thick neoprene rubber gasket separated the surface of the working electrode (∼0.11 cm2) from the top piece of the cell. A rectangular groove (∼1.1 × 0.1 × 0.1 cm) was cut in the gasket that defined the flow channel. Assuming a 25% compression of the gasket when the two pieces of the cell were clamped together, the cell volume was estimated to be ∼8 µL. An Omni-90 analog potentiostat (Cypress Systems, Inc.) was employed to control the potential applied to the working electrode. The carrier solution was 0.1 M PBpH7.2. The ion chromatographic separations were conducted with a QIC analyzer (Dionex) using a regenerated anion-exchange column (AS4, Dionex). Several pore volumes of mobile phase were flowed through the column at room temperature prior to perform(15) Xu, J.; Chen, Q.; Swain, G. M. Anal. Chem. 1998, 15, 3146. (16) Koppang, M. D.; Witek, M.; Blau, J.; Swain, G. M. Anal. Chem. 1999, 71, 1188.
Figure 1. Cyclic voltammetric i-E curve for 0.10 mM N3- + PBpH7.2 at the basal plane of HOPG. Scan rate, 50 mV/s.
ing a separation. Conductivity detection was performed with the assistance of an anion micromembrane suppressor. The regenerant for the suppressor was 25 mM H2SO4 at a flow rate of 2.0 mL/min. Amperometric detection was accomplished using the homemade thin-layer flow cell. A phosphate buffer of 5 mM Na2HPO4 and NaH2PO4, pH 7.2, was used as the mobile phase in the amperometric detection mode, and a carbonate buffer of 1.8 mM Na2CO3 + 1.7 mM NaHCO3, pH 10, was used in the conductivity detection mode. The injection volume was 50 µL. RESULTS AND DISCUSSION Voltammetric Studies of Azide Oxidation. Several voltammetric methods of analysis were used to investigate the azide oxidation reaction mechanism at HOPG. Figure 1 shows a cyclic voltammetric i-E curve for 0.10 mM NaN3 + PBpH7.2 at the basal plane of HOPG. The background voltammetric i-E curve in the absence of azide is also shown for comparison. It can be seen the reaction is electrochemically irreversible as only a well-defined, oxidation peak is present. The supposed overall reaction, based on work by Savinell and co-workers,1 is
2N3- f 3N2 + 2e-
The peak potential, Epox, is 1011 mV and the peak current, Ipox, is 8.1 µA. The Epox is very near the value obtained at glassy carbon and diamond.9 The signal-to-background ratio (SBR, itot - ibkg/ ibkg) is 28, close to the value observed for diamond but significantly higher than the ratio seen for glassy carbon.9 The low background current and high SBR are desirable features of HOPG, very similar to diamond.9-16 The oxidation peak current in the 0.10 mM NaN3 solution varied linearly with the square root of the scan rate from 10 to 150 mV/s. Linear regression statistical analysis (y ) mx + b) of a plot of Ipox versus (scan rate)1/2 yielded coefficients of m ) 33 µA‚s1/2/V1/2, b ) -0.093 µA and r2 ) 0.9999. The linearity confirms that the current is limited by semi-infinite linear diffusion of azide to the interfacial reaction zone. The linearity and near-origin intercept also suggest that there are no rate-limiting adsorption steps or specific surface interactions in the oxidation reaction mechanism at HOPG. The low background current and high SBR of HOPG render the material a good substrate to use for the detection of dissolved azide; hence, the relationship between the Ipox and the azide concentration was examined. Figure 2A shows a background-
Figure 2. (A) Background-corrected linear sweep voltammetric i-E curve in 1.0 µM N3- + PBpH7.2 at the basal plane of HOPG. Scan rate, 50 mV/s. (B) Plot of the Ipox versus the azide concentration determined by linear sweep voltammetry at a scan rate of 50 mV/s.
corrected linear sweep voltammetric i-E curve for 1.0 µM NaN3 + PBpH7.2. The oxidation peak is well-resolved with a signal-to-noise ratio (SNR, itot/inoise) greater than 3. The SNR, rather than the SBR, is a more appropriate evaluation parameter for low-current, background-corrected signals. The Epox is 1011 mV and the Ipox is 53 nA. A log-log plot of Ipox versus the azide concentration is shown in Figure 2B. Ipox varies linearly from 1 µM to 1 mM. Linear regression statistical analysis of a plot of Ipox versus concentration yielded coefficients of m ) 71 nA/µM, b ) -0.006 µA, and r2 ) 0.9999. Differential pulse voltammetry was also used to examine the azide oxidation response as this method better discriminates against double layer charging current, compared to linear sweep and cyclic voltammetry, by using a timed sample-and-hold current measurement protocol. Generally, the method provides improved SBR and SNR values and, thus, lower limits of quantitation. Figure 3A shows a background-corrected differential pulse voltammetric i-E curve for 0.30 µM NaN3 + PBpH7.2. A well-defined peak is observed with an Epox of 975 mV and an Ipox of 63 nA. The SNR is ∼4, and the limit of quantitation, 0.30 µM, is very near the estimated detection limit (SNR ) 3) of 0.23 µM, which corresponds to 9.7 ppb. This compares favorably to the limit of quantitation at diamond of 4.3 ppb.9 Figure 3B shows a log-log plot of Ipox versus the azide concentration. A linear response is observed over 4 orders of magnitude. Linear regression statistical analysis of a plot of Ipox versus concentration yielded coefficients of m ) 27 nA/µM, b ) 0.83 µA, and r2 ) 0.9972. Due to the nature of the measurement and the reaction mechanism, differential pulse voltammetry provides a lower sensitivity, compared to cyclic voltammetry, by a factor of ∼3. Mechanistic Studies of the Azide Oxidation Reaction. The manner in which the linear sweep voltammetric Epox and Ep/2 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
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Table 1. Linear Sweep Voltammetric Oxidation Peak Potential Data at HOPG as a Function of Potential Sweep Rate and Azide Concentration concn (mM)
potential sweep rate (V/s) 0.010
0.020
0.040
0.080
0.150
0.10 0.20 0.40 0.80
1008 1003 998 991
1013 1010 1002 998
1021 1018 1011 1005
1027 1023 1018 1008
1034 1031 1024 1015
dEpox/d log C
-18.5
-17.5
-18.1
-20.4
-21.1
dEpox/ d log ν 22.3 23.3 22.9 19.6
Table 2. Effect of the HOPG Defect Density and the Adsorption of 2,6-AQDS on the Linear Sweep Voltammetric Oxidation Peak Potential and Peak Current for Azide Epox (mV) HOPG HOPG HOPG 1 2 3 HOPGa
Figure 3. (A) Background-corrected differential pulse voltammetric i-E curve in 0.30 µM N3- + PBpH7.2 at the basal plane of HOPG. Pulse height, 50 mV; step height, 2 mV; cycle period, 200 ms, pulse width, 40 ms; sample time, 35 ms. (B) log-log plot of the Ipox versus the azide concentration determined by differential pulse voltammetry.
values vary as a function of the scan rate and azide concentration are consistent with an EC(dim) reaction mechanism.17,18 The reaction is supposed to proceed in the following way:
2(N3- f N3• + e-) N3• + N3• f 3N2
(E) (Cdim)
The electron-transfer reaction is described by a heterogeneous electron-transfer rate constant, ket, and the follow-up chemical reaction is characterized by a second-order rate constant, kch. It is assumed that kch is very large compared to ket. A kch value of 8.8 × 109 M-1 s-1 has been reported for the dimerization reaction.8 ESR spin-trapping studies in aqueous media by Amadelli have demonstrated that indeed azide is oxidized to a radical.19 Parker has shown, based on work by Nicholson, that for an EC(dim) reaction with the electron transfer reversible, plots of dEpox/d log ν and -dEpox/d log Ca should have slopes 19.7 mV/decade.17 Table 1 shows a summary of linear sweep voltammetric Epox values as a function of the concentration and the scan rate. These data are corrected for iR loss as we unexpectedly observed that the magnitude of such effects for the HOPG samples used were significant at the highest concentration used (∼10-15 mV). It can be seen that the average values of dEpox/d log ν (22.1 ( 1.7) and dEpox/d log Ca (-19.1 ( 1.5) are in good agreement with the theoretical predictions. Tafel plots of i-η data at HOPG had a (17) Parker, V. In Topics in Organic Electrochemistry; Fry, A. J., Britton, W. E., Eds.; Plenum Press: New York, 1986; Chapter 2, pp 35-79. (18) Nicholson, R. S. Anal. Chem. 1965, 37, 667. (19) Amadelli, R.; Maldotti, A.; Bartocci, C.; Carassiti, V. J. Phys. Chem. 1989, 93, 6448.
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∆Ep for 1 mM Fe(CN)6-3/-4 Epox (mV) for 0.10 mM N3coverage of 2,6-AQDS (mol/cm2) Ipox (µA) for 0.10 mM N3Epox (mV) for 0.10 mM N3- at 2,6-AQDS covered surface Ipox (µA) for 0.10 mM N3-
525 1004 7.5 8.2 1007
410 1007 11.9 7.9 1013
235 1003 38.1 8.3 1007
190 1003 55.7 8.4 1010
8.3
7.8
8.3
8.3
a HOPG 2 anodically polarized by 40 cycles between -0.5 and +1.5 V versus SCE at 50 mV/s. The electrolyte was PBpH7.2. Calculation of the coverage assumes a molecular size of 126 Å2 and a flat adsorption orientation.23
slope of ∼52 mV/decade which indicates that the anodic transfer coefficient, R, is close to 1.1 For comparison, Tafel slopes for diamond and glassy carbon were 63 and 74 mV/decade, respectively. Also, the Epox - Ep/2 value for the cyclic voltammetric i-E curve shown in Figure 1 is 39 mV, which is exactly the value predicted for a second-order follow-up chemical reaction.18 HOPG is a very useful sp2 carbon material for structurereactivity studies because of its atomic order and well-defined microstructure, low propensity for adsorption, and surface cleanliness. The electrochemical reactivity of HOPG toward some redox systems is known to be dependent on the defect density. For instance, McCreery and co-workers have shown that the ∆Ep for Fe(CN)63-/4- tracks the exposed edge plane density,20-24 with the ∆Ep decreasing (i.e., faster electron-transfer kinetics) with increasing edge plane density. The dependence of the defect density on the azide oxidation response (Epox and Ipox) was investigated, and the resulting data are reported in Table 2. An increase in the defect density on the HOPG samples, as evidenced by the decreasing ∆Ep values for Fe(CN)63-/4- from 525 to 190 mV, has little effect on the azide Epox, which changes by only 4 mV, or on the Ipox. This demonstrates that azide oxidation is insensitive to the defect density or the fraction of edge plane exposed. The data in the (20) Bowling, R.; Packard, R T.; McCreery, R L. Langmuir 1989, 5, 683. (21) Rice, R. J.; McCreery, R L. Anal. Chem. 1989, 61, 1637. (22) McDermott, M. T.; Kneten, K.; McCreery, R. L. J. Phys. Chem. 1992, 96, 3124. (23) McDermott, M. T.; McCreery, R. L. Langmuir 1994, 10, 4307. (24) Kneten-Cline, K.; McDermott, M. T.; McCreery, R. L. J. Phys. Chem. 1994, 98, 5314.
Table 3. The Effect of Coadded Anions on the Linear Sweep Voltammetric Response for Azide Oxidationa original Epox for 0.10 mM N3- (mV) Ipox for 0.10 mM N3- (µA) change in Ibkg at Epox (µA) a
1007 8.7 0
10 mM 10 mM 10 mM 10 mM FClBrNO31008 8.8 0.03
1008 8.6 0.02
1005 7.7 6.4
1008 8.7 0.04
Scan rate, 50 mV/s.
right-most column of the table are for the anodically polarized HOPG 2. The unchanging Epox and Ipox indicate that the azide oxidation reaction is insensitive to the presence of surface oxides, which form as a result of the treatment. The effect of 2,6-AQDS adsorption on the azide oxidation was also studied. 2,6-AQDS adsorbs weakly on the basal plane of HOPG and strongly near the edge plane sites, with multilayer coverages at the latter.22-24 The coverage tracks the defect density. It has been shown by McDermott and co-workers that the only electrochemically active AQDS is that near the edge plane sites.25 The data shown in Table 2 reveal that the adsorption of 2,6-AQDS increases with defect density, as expected, but the coverage has little influence on the azide Epox or Ipox values. Therefore, the azide oxidation reaction appears to be occurring at the basal plane sites and not exclusively at the edge plane sites where AQDS is strongly adsorbed. If the reaction were occurring exclusively at the edge plane sites, one would expect the adsorbed AQDS to increase the tunneling distance for azide electron transfer, thereby slowing the kinetics. A series of experiments were performed to determine how the electrolyte composition influences azide oxidation at HOPG. A summary of some linear sweep voltammetric data for azide oxidation in the presence of different added halide anions and nitrate is presented in Table 3. In the presence of 10 mM KF, KCl, and KNO3, there is no influence on the Epox and Ipox values. The addition of KBr did, however, affect the azide oxidation response.9 The background current at the azide oxidation potential increases by 1 order of magnitude in the presence of KBr due to the bromide oxidation reaction to bromine. The azide Ipox value decreases by ∼12% because of the competition between the azide and bromide oxidation reactions. Bromine generated at the interfacial reaction zone may be homogeneously oxidizing some of the azide nearby. The results indicate that as long as there is no direct electrolysis of the coadded anion, the addition of the electrolytes has little influence on the azide response at HOPG, much like the response at diamond. Flow Injection Analysis with Amperometric Detection. FIA with amperometric detection was used to quantify the dissolved azide in aqueous solutions. Figure 4A shows a hydrodynamic curve (20-µL injection) for 0.10 mM N3- + PBpH7.2 at HOPG. A mass-transport-limited current of 1200 nA is reached at +1.20 V. A detection potential of +1.20 V was therefore used for all subsequent measurements. The SBR at this detection potential is 85. The response for multiple injections of 0.10 mM N3- was reproducible with a standard deviation of less than 2% for five injections. This compares favorably with the 0.5-1% values observed for diamond.9 The SBR for HOPG is also similar to the (25) T. C.; Ta, Kanda, V.; McDermott, M. T. J. Phys. Chem. B 1999, 103, 1295.
Figure 4. (A) Hydrodynamic voltammetric i-E curve for 20-µL injections of 0.10 mM N3- + PBpH7.2 at the basal plane of HOPG. Each datum represents the average of five injections. The carrier solution was PBpH7.2 at 1.0 mL/min. (B) log-log plot of the oxidation response at +1.20 V versus the injected (20 µL) azide concentration.
ratio for diamond but more than 10 times higher than the ratio for glassy carbon.9 The enhanced SBR at HOPG, and diamond, results from the lower and more stable background current which is attributed to a reduced activity for surface oxidation and adsorption. Figure 4B shows a log-log plot of the detector response versus the azide concentration from 1 × 10-7 to 1 × 10-3 M. The response increases linearly with the injected concentration from 3 × 10-7 to 1 × 10-3 M. The response for the 3 × 10-7 M azide solution (i.e., the limit of quantitation, SNR > 3) deviates slightly from the linear response. Linear regression statistical analysis of a plot of the response versus the concentration yielded coefficients of m ) 3.0 nA/µM, b ) 3.6 nA, and r2 ) 0.9999. The estimated limit of detection (SNR ) 3) is 3.7 nM, which corresponds to 0.16 ppb or 74 fmol injected. This is slightly lower than the limit of quantitation at diamond of 0.3 ppb.9 The response variability (RSD) at any concentration was 2% or less for a minimum of three injections. The long-term response stability was not investigated in any detail. Ion Chromatography with Amperometric Detection. Gas chromatography (detection of HN3), ion chromatography, and capillary electrophoresis are the methods commonly used to detect azide, with quantifiable limits in the high-ppb to low-ppm range. Parts A and B of Figure 5 show ion chromatograms in the amperometric detection mode at +1.20 V using HOPG. They show the response for 50-µL injections of 0.10 mM N3- and 30 nM N3(1.5 pmol) + 0.10 mM NO3-, respectively, both in 5 mM PBpH7.2. The peak at ∼3.3 min (Figure 5A) is due to azide, and even at the 30 nM concentration level (1.3 ppb or 1.5 pmol injected), it is clear that a quantifiable response is observed with a SNR of 4.5 (Figure 5B). Nitrate is a common interference in the ion chromatographic analysis of azide when conductivity detection is used because the capacity factor is similar to that for azide.10 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
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Figure 5. (A) Ion chromatogram for a 50-µL injection of 0.10 mM N3- + PBpH7.2 at the basal plane of HOPG. The carrier solution was 5 mM PBpH7.2, the detector potential was +1.20 V, and the flow rate was 2.0 mL/min. (B) Ion chromatogram for a 50-µL injection of 30 nM N3- + 0.10 mM NO3- in 5 mM PBpH7.2 at the basal plane of HOPG. The separation conditions were the same as in (A).
Therefore, it can be difficult to quantitate low levels of azide in the presence of large excesses of nitrate. Figure 5B shows an ion chromatogram for azide in the presence of a large excess of nitrate. The problem of nitrate interference is alleviated by using amperometric rather than conductivity detection. Azide anion is oxidized at the detection potential, while nitrate is electroinactive under these conditions. Therefore, the azide peak at 3.6 min is the main signal detected. The response reproducibility (RSD) was 3% or less with a minimum of three injections. A signal for nitrate is detected at 4.3 min, but we suppose that this response results from the anodic current associated with intercalation of this anion into HOPG at the detection potential.26 The small signal at 1.7 min is due to the anodic detection of residual nitrite in the solution.10 (26) Alsmeyer, D. C.; McCreery, R. L. Anal. Chem. 1992, 64, 1528.
4608 Analytical Chemistry, Vol. 71, No. 20, October 15, 1999
CONCLUSION The oxidation of azide anion in aqueous media has been investigated at the basal plane of HOPG by voltammetric methods of analysis. The azide oxidation reaction follows a EC(dim) mechanism, and the Epox and Ipox values are insensitive to the defect density, surface oxide coverage, electrolyte composition, and adsorption of 2,6-AQDS. This suggests that the reaction is outer-sphere in nature on HOPG and is not mediated by the defect or edge plane sites. This electrode material exhibits an analytically useful response for azide oxidation in both the FIA and ion chromatographic modes and yields performance characteristics for the detection of this toxic anion similar to those for diamond and much superior to those for glassy carbon. A linear dynamic range of nearly 5 orders of magnitude, an estimated limit of detection of 3.7 nM (0.16 ppb or 74 fmol injected), and a response variability of 2% were determined from the FIA measurements. Both HOPG and diamond are useful electrodes for the detection of azide, giving similar figures of merit, at least for short periods of time. The response precision is slightly better at diamond while the limit of detection is slightly better at HOPG. The real advantage of diamond over HOPG will likely be in the response stability. At the detection potential of +1.2 V, HOPG can intercalate anions and undergo oxidation reactions that produce changes in the surface chemistry and structure. Over time, these physicochemical changes lead to increasing and more variable background signals and reduced SBR values. Diamond is resistive to such structural changes leading to excellent response stability. Future work will examine the detection of azide in industrial water samples using HOPG and diamond with the response stability being a key detection figure of merit to investigate. ACKNOWLEDGMENT The research was generously supported by the National Science Foundation (Grant CHE-9505683) and the Utah State University Faculty Research Grant Program. The informative discussions with Dr. Ivan Mendenhall and co-workers at Autoliv ASP, Inc. (Ogden, UT) were grately appreciated. We thank Professor Mark T. McDermott (University of Alberta-Edmonton) for providing us the preprint of ref 25.
Received for review December 31, 1998. Accepted July 31, 1999. AC9814501
