ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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Oscillating Slit Mechanism for the Determination of Hydrogen Isotope Ratios in a Microwave Induced Plasma Frederick P. Schwarz,' Walter Braun, and Stanley P. Wasik National Bureau of Standards, Washington, D.C. 20234
A low pressure microwave discharge through deuterated and hydrogenated hydrocarbon mixtures eluting from a gas chromatograph results in fragmentation of the hydrocarbons and generation of intense atomic hydrogen emission at 6562.8 A and atomic deuterium emission at 6561.0 A. By replacing the exit slit mechanism of the monochromator viewing the emissions with an oscillating slit mechanism (OSM), the hydrogen emission is measured alternately with the deuterium emission. The operation of the OSM is based on oscillating mechanically the exit slit aperture twice across the hydrogen line and once across the deuterium line during one cycle of oscillation. Two lock-in amplifiers resolve the modulated emissions into a hydrogen and a deuterium signal. The ratio of the two signals is a linear function of the atomic hydrogen isotope ratio in the isotope hydrocarbon mixtures over one order of magnitude.
Quantitative measurements of hydrogen to deuterium isotope ratios are important in water pollution analysis. The amount of a pollutant in a water sample can be determined by adding a known amount of the perdeuterated analogue to the water sample and measuring the hydrogen to deuterium isotope ratio. The concentration of the pollutant is determined by multiplying the known concentration of the perdeuterated analogue by the isotope ratio. Mass spectrometry and gasliquid chromatography (GLC) are currently employed in measuring the hydrogen isotope ratio ( I ) . Mass spectrometric analysis is limited by expense and GLC analysis by possible overlap of the separated isotope peaks with other hydrocarbon peaks and poor separation of the isotope peaks. An alternative method of determining hydrogen isotope ratios based on spectroscopic analysis of the atomic hydrogen and deuterium emissions generated in a radio frequency discharge through flowing gas mixtures was fist reported by Broida and Morgan ( 2 ) . They found that the isotope ratio in hydrogen, deuterium, and air mixtures could be accurately determined under carefully controlled flow conditions. Later Luippold and Beauchamp spectroscopically determined hydrogen isotope ratios in isotopically mixed hydrocarbons eluting from a GLC (3). They analyzed the CH and CD emissions generated in a low pressure microwave discharge through the flowing elutants. Recently, Schwarz analyzed the hydrogen and deuterium atomic emissions generated in a low pressure microwave discharge through isotopically mixed octane vapors eluting from a GLC and found the emission ratio to be proportional to the hydrogen isotope ratio ( 4 ) . This method was found to be quantitative over a t least one order of magnitude and showed promise of being applicable to other hydrogen isotope mixtures. In this report, we describe and evaluate in more detail the method of determining hydrogen isotope ratios based on measuring the deuterium and hydrogen atomic emissions generated in a microwave discharge through hydrocarbons eluting from a GLC. This investigation was facilitated by a novel instrumental modification of the monochromator
viewing the emissions. By replacing the monochromator exit slit mechanism with an oscillating slit mechanism (OSM),the hydrogen emission at 6562.8 A was measured alternately with the deuterium emission a t 6561.0 PI.. T h e OSM modulated the hydrogen emission at twice the frequency of the deuterium emission. Two lock-in amplifiers iresolved the modulated emissions into a deuterium signal and a hydrogen signal of an isotopically diluted elutant. T h e analytical range and response were investigated for different classes of hydrocarbons and partially deuterated hydrocarbons. Application of the method to isotope dilution analysis of a water sample was also investigated.
EXPERIMENTAL The microwave plasma detection apparatus was a modified version of one described previously ( 4 ) . This modification consisted of replacing the exit slit mechanism of the detection monochromator by an oscillating slit mechanism (OSM) to measure alternately the hydrogen emission at 6562.8 A and the deuterium emission at 6561.0 A. The operation of the OSM is shown schematically in Figure 1. A 1 cm x 0.1 cm vane (Bulova Watch Co.) with a 0.5 cm X 0.03 cm opening was driven by a mechanical resonanting circuit to oscillate at 200 Hz ( u in Figure 1)with a spectral sweep of 3.4 A. (The monochromator dispersion is 16.7 A/min and the vane oscillated aith an amplitude of hO.01 cm). At rest the vane holder was positioned so that the vane opening transmitted light at the wavelength reading of the monochromator dial. During one cycle of oscillation and with the monochromator set at 6562.8 A, the vane opening crossed the hydrogen line (6562.8 A) twice and the deuterium line (6561.0 A) once, thereby modulating the hydrogen signal at 2 Y and the deuterium signal at 1 Y . The photomul1:iplier output was fed into two lock-in amplifiers operated in the external reference mode with the reference signal supplied by thle mechanical resonanting circuit. One lock-in amplifier was referenced at 2 u to monitor the hydrogen emission and the other a t 1 u to monitor the deuterium emission. Prior to the experiment, the vane opening was conveniently centered on the background hydrogen emission line by turning the wavelength dial of the monochromator until the 1 - v signal registered zero and the 2-0 signal was maximized. The entrance slit of the monochromator was set at 0.03 cm. For direct measurements of the emissions, the OSM was turned off and the photomultiplier output was connected to a picoammeter. The microwave plasma was maintained at the optimum sensitivity conditions of 0.5 to 1.0% 013 concentration in helium carrier gas and at a pressure of 5 Torr ( 4 ) . The stainless steel GLC columns were 3 m X 3 mm containing 3% Dexsil 300 on 80/100 chromosorb WAW and 8 m X 3 mm containing Squalane on 80/100 chromosorb WAW. The helium carrier gas had a stated purity of 99.999% and was used without further purification. The hydrocarbon solutions were made up by weight on a microgram electrobalance and injected into the GLC by a 10-pL syringe. The naphthalene-benzene-water sample was prepared by mixing aliquots of saturated aqueous solutions of benzene and naphthalene. The saturated water solutions were prepared by stirring an excess of the solute in distilled water for a t least 24 h. The hydrocarbon vapor mixtures in 760 Torr helium were prepared on a separate gas handling system and contained in 1-L stainless steel cylinders. Samples of 10 pL of the mixtures were injected into the GLC by a gas sampling valve. The butylbenzene and 1-methylnaphthalene were 99 mol% pure. The octane, isooctane, cyclohexa.ne, toluene, cyclohexene,
This article not subject to U S . Copyright. Published 1978 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978 I
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Figure 1. Schematic diagram of operation of the OSM
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Figure 2. OSM hydrogen (2 u ) and deuterium (1 u ) and FID chromatograms of a 0.2-pL injected sample of 0 33 wt YO each of benzene, butylbenzene, naphthalene, methylnaphthalene, and 0 10 wt YO perdelrterated benzene in hexadecane The Dexsil column was operated at 165 OC
benzene, and naphthalene and their perdeuterated isomers were all 99 mol% pure. The partially deuterated toluene-ad3, benzene-d,, butane-l,l,l-cl,, and ethylene-l,l-d, were also 99 mol% pure.
RESULTS AND DISCUSSION In Figure 2 are shown OSM hydrogen (2 v) and deuterium (1v) chromatograms of a 0.2-pL injected sample of 0.33 ut% each of benzene, butylbenzene, naphthalene, methylnaphthalene, and 0.10 w t 7 ~perdeuterated benzene in hexadecane. Despite the high hexadecane concentration, lO0OX greater than the CsD6 concentration, the hexadecane peak is absent in the OSM deuterium (1 u ) chromatogram. The OSM chromatograms are similar in peak shape and resolution to the FID chromatogram of the sample in Figure 2. As verified by earlier results, the FID-to-OSM hydrogen peak height ratio is proportional to the C/H composition ratio for each solute.
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
Table I. Naphthalene H/D Ratios of Benzene and Naphthalene in Organic Solvent Extracts Following Treatment by Various Analytical Procedures FID FID naphthabenzene lene/concn rel. benzene naphthalene ratios OSM ratio treatment units 1 mL extract extract after standing 24 h extract aerated for 5 min 0.05 mL fresh organic solvent extract from AgNO, extract
1258 192
1.5 2.1
4 . 3 ?: 0 . 3 4.3 f 0.3
350
4.8
4 . 3 f 0.3
302
1.4
4.6 f 0.3
the picoammeter which is analogous to reducing the duty cycle of the OSM, the deuterium signal-to-noise ratio improved by a t least a factor of 3. At the lower limit of the curve, i.e., where the hydrogen concentration = 0, a hydrogen (2 v) peak was observed simultaneously upon the elution of perdeuterated octane or naphthalene through the microwave plasma. A hydrogen peak was also observed for the perdeuterated analogues of cyclohexane, cyclohexene, isooctane, toluene, and benzene. However, a corresponding deuterium peak was not observed upon the elution of the protonated hydrocarbon through the microwave plasma. T h e intercept in the least squares fit of the data points resulted from the constant contribution (the deuterium concentration was kept constant below H / D = 1) of this hydrogen (2 u ) peak t o the hydrocarbon hydrogen (2 u ) peak. In Figure 3, the OSM signal ratios fall on the same calibration curve for all of the isotope mixtures and for the partially deuterated hydrocarbons. The toluene/toluene-d8, the isooctane/isooctane-dI8 and the cyclohexene/cyclohexene-dlo mixtures were made up by weight on the tenths of the weight 70 level in hexadecane so that the 0.5-pL injected sample was below the microwave plasma saturation level. The OSM measurements on the partially deuterated species, toluene-ad3, benzene-dl,butane-l,l,l-d3, and ethylene-1,l-d,, were performed on a 10-*-mL injected sample of a mixture of 1 Torr each of the partially deuterated hydrocarbon vapor in 760 Torr of helium. The independence of the calibration curve from elutant structure suggests either complete or constant fragmentation of the perdeuterated and partially deuterated hydrocarbons in the microwave helium plasma. Similar results were reported for the protonated hydrocarbons ( 4 ) . T h e calibration curve in Figure 3 can thus be prepared from structurally known partially deuterated hydrocarbons, thereby avoiding the necessity of preparing accurately calibrated isotope mixtures. A typical application of isotope dilution analysis by the OSM was performed on a water sample containing approximately 20 ppm naphthalene and 18 ppm benzene. Approximately 15 mg of perdeuterated naphthalene was added to a 1-L portion of the sample. The 1-L portion was then extracted with 1 mL of an organic solvent, a typical method for concentrating contaminants in water ( I ) . The extract was treated by additional operations of the type that normally would be performed in water pollution analysis. Effects of
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excessive handling of the extract were simulated by letting the extract stand for 24 h and by bubbling air through the sample. A 0.5-mL aliquot of the extract was extracted with 10 mL saturated AgN03 solution and the 10 mL AgNO, extract was reextracted with 0.5 mL of a fresh organic solvent. T h e AgNO, extraction is analogous to the case where the contaminant is distributed between the bulk solution and the surfaces of the sediment and container. The results of these operations are presented in Table I along with the split flow FID integrated areas for benzene and naphthalene. In all the operations, the OSM-determined naphthalene hydrogen isotope ratios remain constant within the experimental error of 8%. On the calibration curve in Figure 3, this ratio yields a H / D ratio of 1.33 which is equal to the naphthalene/ naphthalene-d, ratio in the isotopically diluted protion of the water sample. The principal advantage of the isotope dilution method is the constancy of the isotope ratio in the sample during the concentration of the sample by traditional analytical methods. From the standpoint of cost and simplicity, the OSM isotope ratio determinations are preferable to mass spectrometric isotope ratio determinations. Furthermore, some of the hydrocarbons would be difficult to analyze by mass spectrometric techniques because of complex fragmentation patterns. The OSM isotope ratio determinations are, however, independent of the fragmentation kinetics of the hydrocarbon in the plasma and the hydrocarbon structure. This independence permits the calibration of the isotope ratio measurement curve by known partially deuterated hydrocarbons, thereby eliminating the necessity of weighing known mixtures of the isotopes. From the standpoint of resolution of the isotopes, the OSM method is preferable to the GLC method which does not provide complete separation of the isotopes ( 1 ) . Furthermore, the GLC isotope peaks may overlap other hydrocarbon peaks in the chromatogram. One disadvantage of the OSM method is the poor S / N of the isotope ratio measurements which results in an inaccuracy in the isotope ratio measurements of almost 10%~and restricts the measurement range to almost one order of magitude. However, the S / N can be improved by reducing the duty cycle of the OSM, which would also extend the measurement range to a t least two orders of magnitude.
ACKNOWLEDGMENT The authors wish to acknowledge the help of Brenton L. Baugher in the design and fabrication of the OSM.
LITERATURE CITED (1) (2) (3) (4)
S. P. Wasik and W. Tsang, Anal. Chert., 42, 1649 (1970). H. P. Broida and J. H. Morgan, Anal. Chem., 24, 799 (1952). D. A. Luippold and J. L. Beauchamp, Anal. Chem., 42, 1374 (1970). F. P. Schwarz. Anal. Chem., 5 0 , 1006 (1978).
RECEIVED for review March 24, 1978. Accepted August 23, 1978. This work has been supported by the Office of Air and Water Measurement a t the National Bureau of Standards, Washington, D.C. Certain commercial equipment, instruments and materials are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material, instruments, or equipment identified is necessarily the best available for the purpose.
