Anal. Chem. 1987, 5 9 , 1239-1240
1239
AIDS FOR ANALYTICAL CHEMISTS ~~
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Derivatization Procedure for Gas Chromatographic Determination of Hydroxylamine Robert L. Pesselman,' Michael J. Foral, a n d Stanley H. Langer* Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706 Hydroxylamine has been of special interest recently because of the possibilities of manufacturing it by using electrochemical methods in which ammonia might also be formed (1-3). Its general chemistry and applications have been described elsewhere (4-6). The majority of the analytical methods for hydroxylamine determination (7) incorporate titration procedures. The colorimetric procedure ( I , 8) for determination of hydroxylamine in perchloric acid was unsuitable for similar analyses in sulfuric acid electrolytes while titrimetric techniques have proven unreliable in our laboratory. Presently, there are two gas chromatographic methods reported for the determination of hydroxylamine in aqueous media. The first involves reacting the hydroxylamine to form acetone oxime and subsequently injecting the aqueous solution directly into a gas chromatograph equipped with nitrogen and flame ionization detectors (9). The second involves oxidizing hydroxylamine to nitrous oxide and trapping the gas for determination in an electron capture equipped gas chromatograph (IO). Limits of detection are in the micromolar range for the former and the nanomolar range for the latter. Here, an alternate gas chromatographic procedure for determining hydroxylamine as the acetone oxime is described. Neither direct aqueous injection nor elaborate instrumentation is required. Extraction of the oxime into an ether phase allows for concentrating the analyte and facilitates the use of an internal standard. The limitations and inconveniences of direct aqueous injection are avoided including column degradation and filament burnout in mass spectrometry detectors. THEORY Low molecular weight aldehydes and ketones react readily with hydroxylamine to form corresponding oximes in acidic solutions according to
",OH
+
R-C-H
I'
0
H+
1
I
R-C-NHOH bH
1
R-C=NOH
I H
",OH
sample ID
oxime/heptanol peak height ratio
N010; 0.299 V N010; 0.206 V N010; 0.131 V
0.077 i 0.006 0.377 f 0.015 0.594 i 0.025
concn, mg/mL (from calibration plot)
0.178 f 0.012 0.771 i 0.030 1.201 f 0.050
EXPERIMENTAL SECTION Reagents and Chemicals. A stock solution of hydroxylamine sulfate was prepared by dissolving 0.820 g of the salt into 100 mL of 3 M sulfuric acid to produce a 0.100 M ",OH solution. Subsequent dilutions for standards were all prepared with aqueous 3 M HZSOh Procedure. One-milliliter aqueous acidic samples were mixed with 1 mL of 2 M sodium acetate in a 150 mm X 20 mm Teflon-lined screen top test tube (Fisher Scientific). The samples were then reacted with 1 mL of reagent grade acetone for 30 min in a 50 "C heater block (American Scientific Products). After the sample was neutralized with a saturated Na2C03solution, the acetone oxime was extracted with two 6-mL portions of anhydrous ethyl ether, dried with sodium sulfate, and removed. After ether wash of the sodium sulfate, the extract was concentrated under nitrogen to 1 mL. Five microliters of 1-heptanol was then added as an internal standard. A Carle 85009 basic gas chromatograph equipped with a thermal conductivity detector and a 6 ft X 1/8 in. stainless steel column packed with 10% Carbowax 1540 (on Chromosorb W) was used for routine analysis. The carrier gas was helium at 10.0 cm3/min, and the oven temperature was 120 "C. Calibration. Known solutions of hydroxylamine sulfate were processed together with unknowns. A calibration plot of the ratio of acetone oxime peak height relative to heptanol peak height vs. milligrams of ",OH was generated with the knowns to determine concentrations. Calibration standard concentrations bracketed the expected unknown sample range; plots were linear over the entire range. A typical chromatogram is shown in Figure 1.
+
H20 ( 1 )
The optimum pH for this reaction has been reported to fall in the range of 4 to 6 ( I I ) , although pH does not seem to be critical in the present method. The relative stability and high volatility of acetone oxime provide a convenient retention time for analysis and eliminate problems of thermal decomposition reported in the gas chromatographic determination of other oximes (12). The absence of cis-trans isomerization, which occurs with the aldoximes, results in a single product peak. Present address: Hazleton Laboratories America, Inc., Madison,
WI 53707.
Table I
R E S U L T S AND DISCUSSION Representative results from triplicate experiments on three reactor product electrolytes are presented in Table I. Relative standard deviation of the peak height ratios was about 4% a t the higher concentrations and increased to about 7% a t the lower concentration. For hydroxylamine levels down to about 0.1 mg/mL, 1 mL of aqueous electrolyte has been found to be an adequate sample size. If very dilute samples are to be analyzed, or if detector sensitivity probles arise, more concentrated ether solutions may be prepared. This may be done by using larger sample sizes for derivatization or evaporating ether extracts below the 1 mL prescribed in this procedure. In one instance, evaporation to -0.05 mL allowed accurate analysis of a 0.21 WMhydroxylamine solution with the thermal conductivity detector. With larger sample sizes and/or a flame ionization detector, quantitative analysis for lower concentrations was found feasible.
0003-2700/87/0359-1239$01.50/0 0 1987 American Chemical Society
Anal. Chem. 1907, 59, 1240-1242
1240
Registry No. Hydroxylamine, 7803-49-8; sodium acetate, 127-09-3;acetone, 67-64-1.
(1)
1
2
4
6
TIME frnln)
Figure 1. Sample gas chromatogram for analysis of hydroxylamine as acetone oxime. Column is 10% Carbowax 1540 on Chromosorb W at condiiions outlined in text. Key: I, injection; A, ether and acetone peak; 6,residual water peak; C, acetone oxime; D, acetone impurity; E, 1-heptanol internal standard. Injection size was 1.4 gL, thermal conductivity detector. Sample is 1.0 mg of NH,OH/mL.
This improved procedure is an accurate, forgiving gas chromatographic approach for routine hydroxylamine determination over a range of concentrations in a reasonable time period.
LITERATURE CITED Langer, S . H.; Pate, K. T. Ind. Eng. Chem. Process Des. Dev. 1983,
22, 264. (2) Pate, K. T.; Langer, S. H. Environ. Sci. Techno/. 1985, 19, 371. (3) Bathias, M. L.; Watkinson, A. P. Can. J . Chem. Eng. 1979, 5 7 , 631. (4) Kirk-Othmer Encycbpsdia of Chemical Technolow; _. Wiley: New York. 1966; Vol. 11, p 4 9 3 . (5) Cason, J.; Harris, E. R. J . Org. Chem. 1959, 2 4 , 676. (6) Vogh, J. Anal. Chem. 1971,-43, 1616. ( 7 ) Streuli, C. A.; Averell, P. R. The Analytical Chemistry of Nitrogen and Its Compounds; Wiley: New York, 1970; pp 72-76. (8) Snell, F. D.; Snell, C. T. Cobrimetric Methods of Analysis; Van Nostrand: New York, 1954; Vol. IV, pp 54-55. (9) Darke, D. J. J . Chromatogr. 1980, 181, 449-462. (10) von Breymann, M. T.; de Angeleis, M. A.; Gordon, L. I. Anal. Chem. 1982, 54, 1209-1210. (11) Jencks, W. P. J . Am. Chem. SOC. 1959, 81, 475. (12) Lohr, L. J.; Warren, R. W. J . Chromatogr. 1962, 8 , 127.
RECEIVED for review October 6,1986. Accepted December 16, 1986. This work was supported by the National Science Foundation and the University of Wisconsin.
Inductively Coupled Helium Plasma as an Ion Source for Mass Spectrometry Akbar Montaser* and Shi-Kit Chan Department of Chemistry, T h e George Washington University, Washington, D.C. 20052 David W. Koppenaal Mineral Studies Laboratory, Bureau of Economic Geology, T h e University of Texas at Austin, Austin, Texas 78712 In earlier communications (1,2), we reported the generation of various types of helium inductively coupled plasmas (He (ICPs) operated at atmospheric pressure. Preliminary studies (1-4) showed that the annular He ICP was an efficient excitation source for atomic emission spectrometry (AES). In addition, observation of prominent emission lines of chlorine ion at 479.54,481.00, and 481.95 nm revealed (4) the potential of the annular He ICP as an ion source for mass spectrometry
(MS). The origin, development, and analytical applications of atmospheric-pressure plasmas as ion sources for MS have been reviewed (5-7). Gray (8-10) demonstrated that useful mass spectra of elemental constituents in solutions could be obtained from a capillary arc Ar plasma, while Douglas and French (11) reported the analytical performance of a microwave induced Ar plasma as an ion source for MS. Analytical mass spectra obtained from an Ar ICP were first reported by Houk et al. (12). Among the plasmas cited above (8-12), the Ar ICP offers the best analytical performance due to its relatively high gas and ionization temperatures and its annular configuration. Because the ionization energy of He (24.6 eV) is higher than that of Ar (15.8 eV), the use of a He ICP as an ion source for MS should, in principle, enhance the degree of ionization for every element, in particular for the non-metals possessing high ionization energies. Also, certain mass spectral interferences arising from the presence of Ar and various polyatomic species in the Ar ICP could be avoided if a He ICP is used as an ion source. To cite a specific example, the 40Ar35C1+ ion presents a major problem in the trace measurement of 75As+because arsenic is monoisotopic and the 40Ar35C1+is quite intense in the Ar ICP when HCl is present in the sample (13).
In this report, we describe the first successful coupling of an atmospheric-pressure He ICP to a commercial ICP-MS system. Minor modifications of the load coil and the plasma impedance matching network were made to sustain the annular He ICP in a low-gas-flow torch (2) at a forward power of 500-900 W and a total gas flow of 8 L/min. The objectives of this work were to investigate the feasibility of using a mass spectrometer for fundamental studies of He ICPs and to explore the analytical capabilities of the He ICP-MS system for the determination of halogens and sulfur. Except for this report on the He ICP and an earlier study with an Ar-N2 ICP (14),all previous investigations of ICP mass spectrometry have been concerned with Ar discharges.
EXPERIMENTAL SECTION The PlasmaQuad ICP-MS (VG Isotopes, Ltd., Winsford, Chesire, England) was used for these experiments. The other commercial instrument available,the ELAN system (SCTEX, Inc., Thornhill, Ontario, Canada) was unsuitable for these studies because the high vacuum of the mass analyzer chamber is maintained by a He-cooled cryogenic pump. The sampling depth of the plasma, defined as the distance between the tip of the sampler cone and the top turn of the load coil, was fiied at 18 mm. The sampler cone and the skimmer cone of the ion extraction interface were made from titanium nitride coated nickel and had orifices with diameter of 300 and 750 km, respectively. Under these conditions, typical pressure readings (calibrated for N2gas) of the ion extraction interface, the ion focusing stage, and the mass analyzing stage corresponded to and 6 X torr, respectively. Orifice approximately 6, 1 X diameters greater than 300 pm were not used for the sampler cone in this preliminary study because the high apparent pressure in the ion extraction interface activated the vacuum safety interlocks, thus preventing ion sampling by the mass spectrometer. No other
0003-2700/87/0359-1240$01.50/0 0 1987 American
Chemical Society
