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 selectively to 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-fillingprocesses 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
1994
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Table I. Comparative Analytical Results for Nitrate and Sulfate in Tsoical Water Samdes Usine Mass Spectrometry-(MS)and Ion Chromatogiaphy (IC)
sample
5’
MASS ( daltons 1
100
Figure 1. Mass spectrum obtained for rain 1 in Table I . i5N0, (mass 63) and H34S04(mass 99) were added to the sample at 1.4 and 3.3 ppm, respectively. The labeled nltrate signal is less than 0.4 of the labeled sulfate signal because nitrate binds less strongly to the monolayer.
than 3 min and the entire analysis takes less than 15 min. It has not escaped our notice that anions can be extracted from water into a volatile organic solvent using the same surfactant as a phase transfer reagent. This approach has been investigated using methylene chloride. In this case sample concentration is easier because of the rapid evaporation of the organic solvent relative to water. This approach was ultimately deemed less desirable, however, because it requires additional manipulations which afford great opportunity for sample contamination and because it has the potential to be isotope selective. Reagents. Common grades of glycerol have been found to contain unacceptable levels of inorganic salts. An acceptable source is the “Gold Label” spectrophotometric grade offered by Aldrich Chemical Co., Milwaukee, WI. Note that great care must be taken to avoid contamination of any and all reagents with sulfate-bearing particulates from the laboratory atmosphere. Potassium 15N03(99 atom %) was obtained from KOR Isotopes Inc., Cambridge, MA. Elemental %S (93 atom %) was obtained from the Monsanto Research Corp., Miamisburg, OH. This sulfur was combusted in oxygen in a gas train yielding SOzwhich was trapped in aqueous hydrogen peroxide. The resulting sulfuric acid solution was analyzed for sulfate content by stable isotope dilution mass spectrometry using natural sulfate as the internal standard. The yield was about 95%. Based on this analysis, the solution was neutralized gravimetricallyto hydrogen sulfate using KOH and used in that form for the experimenta described herein. Oak Ridge National Laboratories has recently reported the preparation of 90 kg of enriched 34Swhich is intended for acid rain research (13). Tetramethylammonium hydroxide was obtained from Alfa Products, Danvers, MA, as a 25% solution in methanol (about 2.2 M). 1-Hexadecylpyridinium chloride was obtained from Eastman Organic Chemicals, Rochester, NY. This was converted to the acetate form using Dowex SBR, Nuclear Grade, hydroxide form, anion exchange resin which was obtained from Sigma Chemical Co., St. Louis, MO. The resin was loaded into a column and converted to the acetate form using dilute acetic acid. The pyridinium salt was loaded onto the column and eluted with 50% methanol in lOI5-Q deionized water. The ratio of available ion exchange sites to moles of pyridinium salt was 101. The acetate anion was chosen because it binds very weakly to the monolayer and does not compete effectively with nitrate or sulfate for sites at the surface. The hydroxide anion which was used in our earlier work involving tetradecyltrimethylammonium ion (9) could not be used here because its pyridinium salt was found to be chemically unstable.
DISCUSSION SECTION Potassium K15N03and KHNS04were added to each water sample at the 1.4 ppm level and 3.3 ppm level, respectively, to allow quantification by the stable isotope dilution technique. Figure 1shows the mass spectrum obtained for a typical rain sample. The signals a t mass 62 and 63 are the natural and labeled nitrates, respectively. The signals at mass 97 and 99
rain 1” rain 2” rain 3” city water condensateb std 3.1 ppm nitrate std 4.8 ppm sulfate
amt of sulfate, ppm MS IC
amt of nitrate, ppm MS IC
4.9
4.4
0.36
0.5
1.2
0.9
0.1
2.0 22.0
1.8
0.29
23.0
1.1
0.7
1.1 0.1 3.1
0.1 0.2 1.3
4.9
4.8
0.08
3.1
“The rain samples were collected in Schenectady, NY, on November 5, 1984. Rain 1 was collected between 7:15 and 8:40 am, rain 2 was collected between 840 and 11:20 am, and rain 3 was collected between 1120 am and 230 pin. bCondensate (from a cold finger) of outdoor urban air was provided by R. Castillo, Atmospheric Sciences Research Center, State University of New York at Albany. are the natural and labeled sulfates, respectively. In these experiments only singly charged anions are observed. In the case of sulfate, hydrogen sulfate is observed independent of the particular salt (sulfate or hydrogen sulfate) which was used to prepare the solution. We have not determined if conversion of sulfate to hydrogen sulfate occurs a t the monolayer or whether it is an artifact of the sputtering process. The response of the natural materials relative to the internal standards is linear between 100 ppb and 50 ppm. Accordingly, this particular rain sample can be quantified a t 0.36 ppm nitrate and 4.9 ppm sulfate (as hydrogen sulfate). Table I shows the results of our analyses of natural waters from a variety of sources. Each of these samples has also been analyzed by ion chromatography to confirm the mass spectrometry result and these data are also provided in the table. We feel that the almost trivial amount of sample preparation required ,to obtain these results is truly remarkable. Such simple procedures are favored not only because they are “easy” but also because they afford minimal opportunity for sample contamination and do not involve derivatizations which may be isotope selective. These results are especially significant because they provide the necessary analytical methodology to allow long range tracer studies using stable isotopes. Such experiments are generally considered to represent the only truly unequivocal method for demonstrating source-receptor relationships in those cases where multiple sources (e.g., ,natural and man-made) are present. Work to date has provided parametrization of conversion rates for sulfur dioxide (14) and nitrogen oxides (15) under some typical atmospheric conditions. Further, the long range trajectories of air masses have been mapped in many cases using stable fluorinated tracers such as SFB. Such studies provide useful models for long range transport phenomenon but invariably involve extrapolations when attempts are made to extend our understanding to a more general case. These models provide data which are suggestive but not definitive. Stable isotope tracer studies can provide a much more generalized understanding of the interconversions and ultimate destinations of these species. Such experiments are now possible. It should be mentioned that the practical limits of detection in tracer experiments will not be determined by the base sensitivity of the mass spectrometer. Both 15Nand 34Soccur naturally and it is these natural background levels (which are well within the detection limits of the mass spectrometer) that will set the practical limits. The tracer cannot be considered to have been detected unless its level is found to be at least a factor of 2 greater than the largest known natural positive
1995
Anal. Chem. 1985, 57, 1995-1998
excursion in the abundance of that isotope. The natural abundance of 34Sis on the order of 4.0% and that of I5N is about 0.37%. It will, therefore, be possible to follow the dilution of 16N down to approximately a factor of 10 lower abundance than will be possible with Calculations based on these estimates suggest that 1kg of %SO4would label about 1mile3 of air a t the detection limit given a background level of about 8 pg/m3 sulfate. Registry No. NO3-, 14797-55-8; SO?-, 14808-79-8; H@, 7732-18-5; tetramethylammonium hydroxide, 75-59-2;glycerol, 56-81-5; cetylpyridinium acetate, 7439-73-8.
(6) Barber, M.; Bordoli, R. S.; Elliott, G. H.; Sedgewich, R. D.; Tyler, A. N. Anal. Chem. 1882, 54, 645A. (7) Rlnehart, K. L. Science 1982, 218, 247. (8) Javanaud, C.; Eagles, J. Org. Mass Specfrom. 1883, 18. 93. (9) Ligon, W. V.; Dorn, S. B. Int. J . Mass Spectrom. Ion Proc. 1985, 62, 315. (10) Ligon, W. V.; Dorn, S. B. Int. J . Mass Specfrom. Ion Proc. 1984, 57, 75. (1 I ) Ligon, W. V. I n t . J . Mass Specfrom. Ion Phys. 1982, 4 1 , 205. (12) Fitch, R. K.; Ali, K. S.; Inmann, M. J . Phys. E : Sci. Insfrum. 1984, 17, 939. (13) Chem. Eng. News 1984, 62(No. 51), 28. (14) Gillani, N. V.; Kohii, S.; Wilson, W. E. Atmos. Envlron. 1981, 15, 2293. (15) Spicer, C. W. Science 1882, 215, 1095.
Woodfin V. Ligon, Jr.* Steven B. Dorn
LITERATURE CITED (1) Ligon, W. V. Science 1879, 205, 151. (2) DePauw, E.; Marien, J. Inf. J . Mass Specfrom. Ion Phys. 1981, 38, 11. (3) Heumann, K. 0.;Unger, M. Fresenius’ 2. Anal. Chem. 1983, 315, 454. (4) Wagner, 0 . H.; Steeie, K. F. Am. Lab. (Fairfield, Conn.) 1982, (July), 12. (5) Fritz, J. S.; Gherde, D. T.; Pohlandt, C. “Ion Chromatography”; Huthig: Heidelberg, Basel, New York, 1982.
General Electric Company Corporate Research and Development Schenectady, New York 12309
RECEIVEDfor review April 4, 1985. Accepted May 3, 1985.
Response of Ion-Selective Field Effect Transistors to Carbon Dioxide and Organic Acids Sir: In recent years, the fabrication and study of various miniaturized versions of ion-selective electrodes (ISEs) have been vigorously pursued (1). Once fully developed, it is hoped that such electrochemical devices could be routinely used for continuous in vivo monitoring of blood electrolytes (e.g., Kt, Nat, C1-, etc.) during surgical procedures or at the bedside of patients in critical care units. Band and co-workers have reported considerable success in preliminary animal studies with simple scaled down versions of conventional ion-exchanger or neutral carrier based polymer membrane type ISEs (with internal electrolyte solutions) (2,3). Others have described the fabrication and performance of ion-selective field effect transistors (ISFETs) which are, in effect, solid-state devices (no internal reference electrolyte) (4-8).The ISFET has attracted considerable attention because it is envisioned that a single miniaturized solid-state chip could contain multiple gates and be used to sense several ions simultaneously, ISFETs which are not covered with polymeric ion-selective membranes can be used directly as pH sensors. The insulating gate materials are typically silicon nitride and silicon oxide which, by their own intrinsic surface properties, develop phase boundary potentials proportional to the logarithm of hydrogen ion activity of any solution they are in contact with. However, potassium ISFETs, for example, are usually prepared by coating the gate region of these pH devices with the same polymeric ion-selective membranes used in conventional potassium ISEs. Thus, once the gate region is covered with the polymeric membrane material, two potentials are generated: a Donnan potential at the interface of the membrane and the sample, dependent on the analyte ion activity in the sample, and an unknown potential a t the interface of the membrane and the gate which is dependent on the activity of hydrogen ions present in this region. It is our belief that, since the hydrogen ion activity is not fixed at this membrane-gate interface, species which can enter this region by passing through the membrane can alter the p H and therefore interfere with the measurement of the analyte ions. In this correspondence, we provide preliminary data which strongly supports this view. Indeed, i t is clearly shown that 0003-2700/85/0357-1995$01.50/0
ISFETs based on polymer ion-selective membranes are subject to positive interference by carbon dioxide and organic acids in the sample solution. As a model, we describe experimental results obtained with potassium ISFETs and nonselective ISFETs coated with polymer membranes containing valinomycin and no exchanger or ionophore, respectively. We further postulate a mechanism which partly explains the observations made with these devices and speculate as to possible implications of these findings with regard to the feasibility of in vivo measurements with such sensors. EXPERIMENTAL SECTION Apparatus. n-Type pH ISFETs, Models pH-3035 or pH-1015, were obtained from Kuraray Co., Ltd. (Osaka, Japan). Measurements were made in a constant current mode at 100 pA source-drain current. Thus, if the gate potential increases, the reference electrode potential decreases so tw to maintain a constant source-drain current (4). The drain-source voltage ( VDS)was maintained at 4.0 V. All measurements were made in conjunction with a separate double junction reference electrode in which the outer chamber was filled with 1 M NaN03 (NaN03 was used rather than equitransferent KNOBor KC1 so that leakage of K+ would not be a problem). This was true regardless of whether the devices had their own built-in reference element (Model 3035) or not (Model 1015). Changes in reference half cell potentials were recorded on a Kipp Zonen (Model BD-40) strip chart recorder. Preparation of ISFETs. All pH ISFETs obtained from Kuraray displayed appropriate pH response using standard buffers (e.g., 52-60 mV/pH unit) and were used in subsequent studies. The potassium ISFETs were prepared by dipping these devices into either of two polymeric ion-selective membrane solutions: (1)0.6 g of poly(viny1chloride) (PVC), 1.2 g of dipentyl phthalate (DPP), 10 mg of valinomycin, and 1 mg of potassium tetraphenylborate (KTPB) dissolved in 5 mL of tetrahydrofuran (THF); or (2) a 2% w/w solution of Dow Silicone Medical Adhesive dissolved in xylene containing 0.5% valinomycin. For the potassium-selective PVC membranes, KTPB was added to minimize lipophilic anion response whereas such an additive is not required for the silicone rubber membranes. Generally, the ISFETs were dipped two to three times at 1-min intervals. The resulting potassium ISFETs were then allowed to dry for 1 day at room temperature before testing. The strong bond between 0 I985 American Chemical Society
