1989
Anal. Chem. 1985, 57, 1989-1993
Cyclic Voltammetry and Anodic Stripping Voltammetry with Mercury Ultramicroelectrodes Kenneth R. Wehmeyer and R. Mark Wightman*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A method for the preparatlon of mercury microvoitammetrlc electrodes of hemlspherlcal geometry wlth r a d of 2.3-7.3 /.un has been developed. Mercury Is eiectrodeposlted from solutlons of Hg(1) onto a microvoltammetric platinum disk electrode at a constant potential sufflclent to ensure diffusion limited conditions. The radius of the deposlted mercury electrode Is a function of the square root of the deposltlon t h e and was experlmentally evaluated by applying the equation for steady-state llmltlng current at a hemlspherlcal electrode to the reduction of Ru(NH,),'+ at the mercury electrode. The mercury mlcrovoltammetrlc electrode has been employed In several unique appilcatlons. Anodic stripping voltammetry wlth these electrodes can be performed wlth a quiescent solution durlng deposltlon due to the enhanced mass transfer resulting from nonlinear diffusion. The strlpplng peaks are as narrow as those expected for thln fllms, and the peak current for the stripping of lead was found to be linear over the concentration range of 7 X IO-" M to 1 X lo-' M (5-mln preconcentratlon interval) and to have hlgher precision than conventional stripping technlques. Mercury mlcrovoltammetric electrodes also are demonstrated to be of value in fast scan cyclic voltammetry in aqueous solution. A well-deflned wave can be obtained for the oxldlzed form of ascorblc acid at pH 7.0 at a scan rate greater than 1 kV s-'.
Microvoltammetric electrodes have been shown to possess several unique properties which have found application in the areas of neurochemistry (1,2),spectroelectrochemistry (3), liquid chromatography ( 4 , 5 ) ,and electrode reaction mechanisms and electrode kinetics (6, 7). The microvoltammetric electrodes used in these studies were fabricated from carbon, platinum, or gold wires sealed in glass or epoxy and used in geometries of disk or cylindrical shape. Relatively few reports have appeared which involved the use of mercury microvoltammetric electrodes. Anderson (8)has reported the use of a mercury-coated carbon fiber electrode of cylindrical geometry for anodic stripping voltammetry; however, the distribution of the mercury on the surface of the electrode was not evaluated. Hills and co-workers (9-11) have shown that the deposition of single mercury nucleus on platinum and carbon disk microvoltammetric electrodes can be achieved. Microvoltammetric electrodes facilitate studies of the nucleation processes associated with the growth of a metallic phase on a substrate electrode. To date, however, no reports have appeared demonstrating the formation of stable mercury microvoltammetric electrodes of known radius for use in routine electroanalytical applications. Mercury has enjoyed a long and fruitful history as an electrode material and the development of mercury microvoltammetric electrodes should allow the favorable electrochemical properties of this material to be used in applications unique to microvoltammetric electrodes. For example, heterogeneous electron transfer rates are often more rapid a t mercury surfaces. Thus, the advantages of ultrafast voltammetry (6)can be explored more readily. In addition, mercury microvoltammetric electrodes should exhibit uniform acces-
sibility for the diffusion of molecules to the interface (12). Mercury deposition at a microvoltammetric electrode has been shown to follow behavior predicted for a hemispherical deposit (10, 13). Under diffusion control and at large overpotential, the radius (r) of the hemisphere is related to the deposition time by
r = [2MDCt/p]'i2 where M is the atomic weight of mercury, D is the diffusion coefficient of the mercurous ion, C is the concentration of the mercurous ion, p is the density of mercury, and t is the deposition time in seconds (24). The steady-state limiting current for electrolysis of a solution species at a hemispherical microvoltammetric electrode is given by
i
= 2nnFDCr
(2)
where r is the radius of the hemispherical electrode and the other terms have their usual meaning (15). This expression is useful for values of Dt/r2 > 100. Thus, the limiting steady-state current from cyclic voltammograms provides an alternate means of evaluating the radius of the electrode. If agreement between the two methods occurs, it can be assumed that the deposited mercury remains hemispherical after deposition. In this work we demonstrate that this is the case for mercury deposited on platinum disk microvoltammetric electrodes. It was found that mercury electrodes produced by constant potential deposition were easily made, durable, and reproducible. The mercury microvoltammetric electrodes were shown to have several unique applications. The mercury electrodes were found to be useful in fast scan cyclic voltammetry in aqueous solutions. For anodic stripping voltammetry the use of the mercury microvoltammetric electrode allowed the preconcentration to be accomplished without forced convection.
EXPERIMENTAL SECTION Reagents. Ruthenium hexaammine trichloride and mercurous nitrate monohydrate were from ICN Pharmaceuticals, Plainview, NY, and Aldrich Chemical, Milwaukee, WI, and were used as received. All inorganic salta used in electrolyte preparation were analytical grade. Alumina for electrode polishing was obtained from Buehler, Ltd., Evanston, IL. Lead and cadmium reference standards were purchased from Fisher Scientific, Cincinnati, OH. Electrodes. Platinum microvoltammetric electrodes were prepared by sealing platinum wires of 10,2, and 0.6 pm diameter (Goodfellow Metals, Cambridge, UK) in soft glass tubing, which had been predrawn to an inner diameter of approximately 1.0 mm or less, in an air/gas flame (6). The 2.0- and 0.6-pm platinum wire was received in the form of Wollaston wire which has a protective silver overcoating. The silver was removed with concentrated nitric acid before the wires were sealed in glass. Disk-shaped electrodes were prepared by abrasion with 1000 grit carborundum and successive polishing with 5.0,0.3, and 0.5-pm alumina. Electrodes were repolished with 0.05 pm alumina before use in each experiment. ElectrochemicalProcedure. A deoxygenated solution of 1.0 M KNOBcontaining 5.7 mM mercurous ion and 0.5% concentrated nitric acid was used for mercury deposition. Mercury deposition on platinum microvoltammetric electrodes was carried out by the application of a constant potential of 0.0 V vs. saturated calomel reference (SCE) using a BAS Model LC-2A potentiostat
0003-2700/85/0357-19~9$01.50/0 0 1985 American Chemical Society
1990
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
d
I I
I
I
2.0 nA
20 nA
I
1
I
I
0.0 -0.2 -0.4 2.5 5.0 Time, (min) E ( V v s SCE) Figure 1. Current during deposition of mercury onto (a) 0.3 pm radius Pt disk and (b) 1.0 pm radius Pt disk from a deoxygenated 1.0 M KNO, solution containing 5.7 mM mercurous ion at 0.0 V vs. SCE. Cyclic voltammograms of RU(NH,)~+ (20mV s-’) at (c) 1.0 pm radius Pt disk and (d) 5.3 pm radius mercury drop grown on the same platinum disk. 0
(Bioanalytical Systems, West Lafayette, IN) for various time intervals (-30 s to 7 min). The reference electrode was isolated from the deposition solution by the use of a Vycor frit sealed in heat shrink Teflon to prevent precipitation of the mercurous chloride. Following deposition the electrodes were washed with doubly distilled water before further use. The diffusion coefficient for Hg(1) (D = 0.96 X loa5cm2 8-l) was obtained from ref 10. Cyclic voltammograms of ruthenium hexaammine in deoxygenated 0.1 M phosphate buffer (pH 7.0) were obtained with a PARC Model 174A polarographic analyzer (Princeton Applied Research Corp., Princeton, NJ) and a flat bed X-Y recorder (Houston Instruments, Austin, TX). The diffusion coefficient of ruthenium hexaammine in this buffer was 6.0 X lo4 cm2 s-l as determined from the limiting steady-state current at a 10-pm carbon disk electrode (6).Fast scan cyclic voltammograms of ascorbic acid in 0.1 M phosphate were obtained using a locally constructed “fast” potentiostat (6)and a 10-MHz storage oscilloscope (Tektronix, Inc., Beaverton, OR). Linear scan anodic stripping voltammetry (LSASV) employed a locally constructed potentiostat with a 47-ms time constant (6). The supporting electrolyte (0.1 M KN03, pH 3.0) was preelectrolyzed for 96 h at -1.0 V vs. SCE over a mercury pool electrode. The stripping analysis conditions involved the application of a constant potential (-800 mV vs. SCE) for 5 min in an unstirred solution. Immediately after the deposition interval the potential was scanned linearly in the positive direction at a rate of 50 mV 6’. Lead solutions for trace analysis were prepared by successive additions of appropriate aliquots of a 1.9 X lo4 M lead standard solution to 25 mL of the supporting electrolyte solution. In all electrochemical experiments a saturated calomel electrode and a platinum wire were used as the reference and auxiliary electrodes, respectively. Deoxygenation of solutions was accomplished with argon which had been passed through an ammonium vanadate scrubber and doubly distilled water. RESULTS AND DISCUSSION Characterization of Mercury Microvoltammetric Electrodes. The initial step in the deposition of a metallic
phase onto a substrate involves the formation of nuclei of critical size from subcritical clusters of adatoms in a process referred to as nucleation. Following nucleation the growth of the nuclei occurs spontaneously (IO). If a sufficiently large overpotential is applied, nucleation occurs immediately and only the growth of the nuclei is observed. It has been shown that the current-time plot for three-dimensional growth of a mercury nucleus should follow a t t 1 / 2relationship (IO). In Figure 1,the current during mercury deposition is shown for both a 0.6- and 2.0-pm diameter platinum disk electrode. The curves are independent of the diameter of the substrate and the current increases with the square root of time. For example, linear regression of the current against t1/2for a 333-9 deposition gives a correlation coefficient of 0.9997 and a slope within 3% of that expected for a hemispherical deposit. Voltammograms for the reduction of Ru(NHJe3+ at a 1.0 pm radius platinum disk electrode and a t the same disk electrode following mercury deposition are sigmoidal in shape (Figure 1). The steady-state response arises from an increased mass transport due to nonlinear diffusion at electrodes of these dimensions. The current is larger following mercury deposition indicative of i k increased size. The limiting steady-state current from these voltammograms has also been used to evaluate the radius of the mercury hemisphere. These values have been compared to the radius of the deposited hemispherical mercury drop calculated from the known deposition time (eq 1). Values of the radius of the mercury deposit obtained from each method are compared in Table I for various deposition time intervals. The first voltammogram with a freshly prepared mercury electrode often resulted in a limiting steady state current that was 2-1070 smaller than that obtained on subsequent scans. Therefore, data from the subsequent scans are presented. Excellent agreement is found between the
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Table I. Comparison of Electrode Radius Calculated from the Deposition Time (rdsp) with Experimentally Determined Radius (rh,)Found from the Limiting Current of Voltammograms from the Reduction of Ru(NHg)63+ (3.2 mM) at 20 mV s-l Scan Rateo
IO
1991
1000 vs"
vs-j
IP-
rfar, Irm
1.0 pm
5 pm
time, s
rdep. pm
Pt disk
Pt disk
Pt disk
37 70 95
2.5 3.4 3.9 4.9 5.4 6.4 7.4
2.3 f 0.1 3.2 f 0.03
5.3 f 0.07
2.2 f 0.02 3.2 f 0.06 3.7 f 0.03 4.8 f 0.07 5.3 f 0.03
7.3 f 0.06
7.3 f 0.06
150 180 250 333
0.3 pn
4.9 f 0.10 6.0 f 0.2 7.1 f 0.2
a n = 3 for each radius. ~
calculated radius from each method for mercury drops ranging from 2.3- to 7.3-pm radius. The currents at the mercury electrode grown at the 0.3 and 1.0 pm radius platinum disks are reproducible with a relative standard deviation of 1-4% for a wide range of deposition times. In contrast, the experimentally determined radii obtained with the 5-pm radius disk are less than expected. This may be due to incomplete coverage of the platinum surface, Since the exact location of the initial mercury nucleus is not known, it would appear prudent to deposit a mercury drop whose radius is at least equivalent to the diameter of the disk to ensure complete coverage. The mercury microvoltammetric electrodes prepared in this fashion are relatively durable and rugged. The electrode can normally be washed repeatedly with distilled water or left in solution while deoxygenating with a nitrogen stream without loss of the mercury drop. The mercury electrodes were found to be usable when transferred to nonaqueous solvents (acetonitrile, dimethylformamide, and dimethyl sulfoxide). For the study of electrode kinetics the purity of the deposited mercury is crucial since as little as 5 X lod mol fraction of an impurity can result in a 25 mV shift in the PZC of mercury (16). Platinum is somewhat soluble in mercury (0.1 at. %) but the dissolution process is slow due to the presence of surface oxides (17). Also the high mercury volume to platinum surface area ratio should help maintain the purity of the deposited mercury. A totally inert substrate or a substrate with a lower solubility in mercury would, however, be preferable. Toward this end attempts were made to deposit mercury on carbon disk electrodes by the same procedure. In our hands, the deposition process was found to be very irreproducible and usually resulted in a failure of the drop to adhere to the carbon. In a few cases, mercury drop electrodes on carbon were prepared and were viable for voltammetry in aqueous solution. However, the electrodes could not be transferred to nonaqueous solvents without loss of the drop. The results obtained with carbon are probably due to poor adhesion of the mercury to the carbon surface. As the radius of the substrate electrode is decreased it would be expected that homogeneous rather than heterogeneous nucleation is preferred because of surface tension considerations (10). Unless the adhesion of the mercury to the substrate is sufficiently strong, an unstable deposit results. Cyclic Voltammetry at Fast Scan Rates. Electron transfer reactions accompanied by chemical reactions have been studied frequently. A large number of diagnostic criteria have been developed to distinguish different possibilities through the use of cyclic voltammetry (18). We have previously demonstrated that microvoltammetric electrodes allow cyclic volammetric results to be obtained at scan rates up to 20 000 V with minimal distortion (6). Microvoltammetric
I
I
I
0 2
00
-02
I -04
I
I
I
02
00
I -02
I -04
E ( V v s SCE)
Figure 2. Cyclic voltammograms of ascorbic acid (6.8 mM) in 0.1 M phosphate (pH 7.0) at a mercury microvoltammetricelectrode (r = 5.0 pm) at (a) 10 V s-' and (b) 1000 V SI.
electrodes facilitate this type of measurement because iR distortion is greatly reduced. Therefore microvoltammetric electrodes can be used for voltammetry on a microsecond time scale and many reactions that were considered too fast for measurement by electrochemical methods should be accessible with microelectrodes. At scan rates above 200 V s-' diffusion is essentially linear, resulting in conventional peak shaped voltammograms, and thus existing theory can readily be employed. Our previous work has been carried out in nonaqueous solvents with noble metal or carbon electrodes (6). Preliminary investigations in aqueous solutions with platinum and gold microvoltammetric electrodes revealed the presence of large surface waves presumably resulting from surface oxides. These reduce the useful potential window for cyclic voltammograms. Mercury is free of surface oxides and should be more suitable for fast scan cyclic voltammetry in aqueous solution at potentials negative of that for dissolution of the mercury. Voltammograms for the oxidation of ascorbic acid in aqueous solution demonstrate that mercury microvoltammetric electrodes are useful for this type of application (Figure 2). Ascorbic acid is known to be oxidized in a two-electron process and the initial oxidized product undergoes a fast, irreversible hydration reaction (19). Thus, at a scan rate of 10 V s-' a reverse wave is not observed after scan reversal at any size electrode. However, at a scan rate of 1000 V s-' the wave for the reduction of the initial product is clearly observed. The first-order rate constant for this reaction has been reported to be 1.4 X lo3 s-' (19),and the data at the fast sweep rate are in accord with this value. The waves indicate a quasi-reversible system since the separation of the peak current is -100 mV. Anodic Stripping Voltammetry. Anodic stripping voltammetry (ASV) is one of the most sensitive electroanalytical techniques for the trace analysis of metal ions with detection limits reported in the M to M range (17). The excellent detection limits result from a preconcentration of the metal ion of interest into a mercury film. The preconcentration is aided by convection induced by mechanical stirring, by use of a rotating disk/mercury film electrode, and more recently by the combination of ASV with flow injection analysis methodology (20). The reproducibility of mass transport is a major source of error in the ASV technique (21). The steady-state response observed in Figure 1 for the reduction of Ru(NH3):+ demonstrates that microvoltammetric electrodes do not require forced convection because of the enhanced mass transport by radial diffusion. Thus, the use of mercury microvoltammetric electrodes should eliminate this source of error in the ASV experiment.
1092
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Table 11. Peak Stripping Current and RSD for Various Lead Concentrationsn [Pb2+],M blank
7.9 x 3.2 x 1.1 x 1.0x
istrip,
PA
(TI
= 3)
2.6 10-10
10-9 10-8 10-7
5.2 13.0 35.0 314.0
RSD, % 1.13 0.83 0.65 0.72 0.18
" T h e mercury microvoltammetric electrode had a radius of 7
wn.
I
100 pA
Linear scan stripping voltammograms obtained at mercury microvoltammetric electrodes following deposition in a quiescent solution containing cadmium and lead show welldefiied stripping peaks (Figure 3). (The peak at the potential for copper oxidation is a solution impurity.) The ratio of the stripping peak current to the calculated steady state current is on the order of lo3 indicating an efficient preconcentration. The amount of preconcentration was found to be linearly dependent on the deposition time. Typical values for concentration enhancement with forced convection methods are on the order of lo2 to lo5 depending on the method of convection and the deposition time. However, the current enhancement is less than the calculated concentration enhancement in the mercury (18000 times greater than the solution concentration, based on Faraday's law and the volume of the mercury). This is to be expected since the radial diffusion which provides the enhanced mass transport during deposition also results in a dimunition of the stripping current
different mercury drops deposited for identical times. The RSD obtained with the mercury microvoltammetric electrode at the lo-'' M to M level are well below the values (6-12 %) obtained with conventional mercury drop or film electrodes ( 2 4 , 2 5 ) . The relatively constant value of the RSD over the concentration range examined is the result of the increased reproducibility of mass transport by radial diffusion in a quiescent solution. Thus, the data indicate that ASV at mercury microvoltammetric electrodes is sensitive and is superior to conventional ASV techniques which use mercury drop or film electrodes with forced convection in terms of the RSD for trace determinations (26). Radial diffusion contributes to the narrowness of the stripping peaks as well as improved deposition reproducibility. Since the substrate material is completely covered by the mercury, anomalous peak splitting (8) or broadening (24) is not observed. Other advantages of mercury microvoltammetric electrodes that have not yet been investigated include the use in very small volumes of solution and the use of low supporting electrolyte concentration to reduce the effect of impurities. Mercury microvoltammetric electrodes with single mercury nucleus of radii from 2.3 to 7.3 pm can be prepared on platinum microdisks. They are found to be durable and are suitable for work in aqueous or nonaqueous solvents at slow and fast scan rates. Due to the enhanced mass transfer at microvoltammetric electrodes, preconcentration of metal ions into mercury microvoltammetric electrodes can be accomplished in quiescent solutions and without the need for forced convection. The precision obtained for lead is in the 0.5-1.6% range over the entire concentration range examined (10-7-10-10 M) and, at the lower concentrations, is much superior to that reported with conventionally sized electrodes. The use of mercury microvoltammetric electrodes in ASV simplifies the methodology and results in more reproducible results. Registry No. Hg, 7439-97-6; Pb, 7439-92-1; ascorbic acid,
(22).
50-81-7.
I
I
02
00
I -02
I
i
i
- 0 4 -06 -08
E ( V v s SCE) Figure 3. Linear scan anodic stripping voltammogram at a mercury drop electrode (r = 7.0 pm) following a 10-mln deposition In a quiescent solution containing lead (8.7 X M) and cadmium ions (1.6 X lo-') at -800 mV vs. SCE. Scan rate was 50 mV s-'.
The width at half height of the stripping peaks for both lead and cadmium is 38 mV, and this width is that predicted for thin film behavior (23). At thin mercury films concentration depletion in the mercury occurs during the voltammogram. With an electrode of spherical geometry and a microscopic radius, both concentration depletion and radial diffusion inside the drop lead to a decrease in the current (22). Thus, very sharp stripping voltammograms are obtained. Stripping voltammograms obtained at a mercury electrode with a nominal radius as large as 20 Km also show sharp peaks (half width of 40 mV) because of these effects. The calibration curve for lead was examined in detail and found to be linear as a function of concentration over the range 7X M to 1 X lo-' M using a deposition time of 5 min (r = 0.9999). The detection of lower concentrations was limited by the presence of trace lead in the electrolyte solution. The relative standard deviations (RSD) for replicate determinations of the same solution with the same electrode were found to vary from 0.5 to 1.6% (Table 11). An RSD of 1-3% was found for determinations on the same solution with
LITERATURE CITED (1) Wightman, R. M. Anal. Chem. 1981, 53, 1125A-1134A. (2) Ewing, A,; Dayton, M. A,; Wightman, R. M. Anal. Chem. 1981, 53,
1842-1 847. (3) \ , Robinson. R. S.:McCurdy. C. W.; McCreery, R. L. Anal. Chem. 1982. 54, 2356-236 1. (41 Caudill. W. L.: Howell, J. 0.: Wightman, R. M. Anal. Chem. 1982, 54, 2532-2535. (5) Knecht, L. A.; Guthrie, F. J.; Jorgenson. J. W. Anal. Chem. 1984, 56, 479-482. (6) Howell, J. 0.; Wightman, R. M. Anal. Chem. 1984, 5 6 , 524-529. (7) Fleischmann, M.; Lasserre, F.;Robinson, J.; Swan, D. J . Electroanal. Chem. 1984, 777, 97-114. I
.
(8) Cushman, M. R.; Bennett, 6. G.; Anderson, C. W. Anal. Chim. Acta 1981, 130, 323-327. (9) Hllls, G.; Pour, A. K.; Scharifker, 6. Electrochim. Acta 1983, 28,
891-898. (IO) Sharifker, 6.; Hills, G. J . Electroanal. Chem. 1981, 730, 81-97. (11) Gunawardena, G.; Hills, G.; Scharifker, 6. J . Electroanal. Chem. 1981, 130,99-112. (12) Albery, J.; Bruckenstein, S . J . Electroanal. Chem. 1983, 144,
105-112. (13) Hills, G. J.; Schiffrin, D. J.; Thompson, J. Electrochim. Acta 1974, 79, 657-670. (14) Gunawardena, G. A.; Hills, G. J.; Montenegro, I. Electrochim. Acta 1978, 23, 693-697.
1993
Anal, Chem. 1985, 57, 1993-1995 (15) Dayton, M. A,; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 50,946-950. (16) Delahay, P. "Double Layer and Electrode Kinetics"; Interscience: New York, 1965; p 125. (17) Vydra, F.; Stulik, K.; Julakova, E. "Electrochemical Stripping Analysis"; Wiley: New York, 1976. 118) Galus. Z. "Fundamentals of Electrochemical Analysis": E. Harwood, Ltd.: Chlchester, 1976. (19) Perone, S. P.; Kretlow, W. J. Anal. Chem. 1966, 38, 1761-1763. (20) Wise, J. A,; Heineman, W. R.; Klssinger, P. T. Anal. Chim. Acta, in press. (21) Barendrecht, E. I n "Electroanalytlcal Chemistry"; Bard, A. J., Ed.: Marcel Dekker: New York, 1967: Vol. 2. (22) Reinmuth, W. H. Anal. Chem. 1961, 33, 185-187. ,
(23) De Vries, W. T.; Van Dalen, E. J . Electroanal. Chem. 1967, 14, 315-327. (24) Florence, T. M. J. Electroanal. Chem. 1970, 27, 273-281. (25) Underkofler, W. L.; Shain, I. Anal. Chem. 1965, 37, 218-222. (26) Copeland, T. R.; Skogerboe, R. K. Anal. Chem. 1974, 4 6 , 1257A1267A.
I
RECEIVED for review February 28, 1985. Accepted April 29, 1985. This research was supported by the United States Army Research Office and the National Science Foundation. R.M.W. is an Alfred P. Sloan Fellow.
CORRESPONDENCE Determination of Sulfate and Nitrate Anions in Rainwater by Mass Spectrometry Sir: In recent years mass spectrometry has emerged as one of the most powerful trace analysis techniques. Mass spectrometry is characterized by exceptional sensitivity g) combined with the ability to distinguish elemental and isotopic compositions. For a general review, consult ref 1. The advantages of this technique have been, however, largely limited to organic analysis and to elemental analysis. Attempts to analyze complex anions such as the sulfate and nitrate involved in acid precipitation processes have invariably produced results which were unsatisfactory for routine analytical use. Electron impact ionization methods fail because most salts cannot be volatilized. Particle-induced ionization methods such as secondary ion mass spectrometry (SIMS) generally fail because they result in loss of the original molecular structure due to extensive fragmentation (2). Measurements of the isotopic composition of atmospheric nitrogen species have been reported using thermal ionization mass spectrometry methods (3);however, sensitivities were relatively poor and sample preparation was extensive. Because of these difficulties, anion analysis in natural precipitation is generally carried out by wet chemical methods ( 4 ) or by ion chromatography (5). These methods do not provide information regarding isotopic composition and they do not allow stable isotope tracer experiments. Barber et al. (6) have described particle-induced ionization mass spectrometry methods which allow the sputtering of fragile organic structures as large as 9OOO daltons without loss of molecular structure. These methods involve sputtering from liquid solution using 8-10 keV particle beams. The most commonly used solvent is glycerol. For a general review, see ref 7. In this method, the yield of ionic species evaporated from the liquid surface by the sputtering process (sensitivity) is controlled in part by the surface activity of the analyte in the liquid solvent. Molecules with high surface activity present a high density of analyte molecules to the incident particle beam and, in general, show higher sensitivity than similar molecules which lack surface activity. Small inorganic anions can be analyzed by this method without loss of molecular structure (8). However, these materials generally lack surface activity and can be detected only a t high concentrations in the liquid solution. Ligon and Dorn (9) have reported that small inorganic anions can be analyzed from glycerol solution with very high sensitivity if a cationic surfactant is added to the solution prior
to analysis. The surfactant covers the surface of the analyte droplet and behaves much like an anion exchange resin in that it can bind certain anions selectivelyto the surface. Quantities of nitrate as small as 10" M in the analyte solution are readily detected. In this paper we report the application of this methodology to the analysis of natural precipitation.
EXPERIMENTAL SECTION The mass spectrometer and ita operating parameters have been described previously (10). It should be noted that this instrument utilizes a relatively large target droplet and that only a small fraction of the droplet surface is sampled by the primary beam. This arrangement ensures that the first monolayer cannot be removed from the entire droplet at once. The sampled region can be renewed, therefore, by side-filling processes resulting from surface tension differences generated by the sputtering process itself (10). Rainwater was collected using standard precautions to avoid contamination. One-milliliter aliquots of water were combined with about 20 mg of glycerol. Aqueous tetramethylammonium hydroxide (10 pL, 0.04 M) was added to prevent evaporative loss of nitrate. At least 90% of the nitrate present was lost by evaporation if the base treatment was avoided. This may indicate that much of the nitrate which falls as rain simply reevaporates. were added as an aqueous soSufficient KI5NO3and KH34S04 lution to produce concentrations of 1.4 ppm and 3.3 ppm, respectively. Water was removed from the sample slowly (ca. 3 h) at room temperature using a stream of filtered dry nitrogen. The residual glycerol was then treated with sufficient aqueous acetic acid (10 pL, 0.04 M) to neutralize the tetramethylammonium hydroxide which was added earlier and with sufficient methanolic cetylpyridinium acetate (4 pL, 0.1 M) to produce a concentration of 0.02 M in this reagent. The glycerol droplet was then spread on the target stage and introduced into the mass spectrometer. The droplet was sputtered with an 8-keV xenon particle beam consisting of both ions and neutrals. The output of the primary gun used (Ion Tech B11N)has been described by two groups (11, 12). Fitch (12) found that under typical operating conditions the output was about 75% ions and 25% neutrals. Quantification was obtained from the ratio of negative ion currents observed for the natural and labeled isotopes with respect to previously established calibration curves. We have found that a very quick analysis for sulfate alone is possible at the 1ppm level using only 1.0 pL of water. The water sample is merely combined with an equal volume of glycerol containing the surfactant, placed on the target and analyzed. The water is removed by the action of the forevacuum pumps in less
0003-2700/85/0357-1993$01.50/0 0 1985 American Chemical Society
