918
Anal. Chem. 1986, 58, 918-923
Selective Detection of Organosulfur Compounds in High-Performance Liquid Chromatography E. A. Mishalanie and J. W. Birks* Department of Chemistry and Cooperative Institute for Research in Environmental Sciences (CZRES), University of Colorado, Boulder, Colorado 80309
A chemllumlnescence detector that is seiectlve for organosulfur compounds has been developed for use wtth mkrobore hlgh-performance ilquld chromatographlc systems. The detector was orlginally developed for packed-column gas chromatography and has slnce been coupled to reversed-phase C-18 mlcrobore columns using methanoVwater and acetonltrlie/water solvent mixtures. High selectivity over non-sulfur contalnlng species has been achieved for a number of sulfides, dlsulfldes, thiols, and suifur-contalnlng pestlcides. The organoselenium compounds dimethyl selenlde and dimethyl dlselenlde were found to respond wlth lower sensltlvlty relatlve to the analogous organosulfur compounds. The detector has a linear response to most organosulfur compounds over 3 orders of magnitude and detection llmlts ranglng typlcally from 50 pg to 3 ng of analyte. The deslgn of the detector, detection prlnclples, and performance characterlstlcs are discussed.
A gas chromatography (GC) detector with high selectivity for organosulfur compounds was previously developed in our laboratory (1). This detector proved to be highly successful for the analysis of a number of sulfides, disulfides, and thiols. The performance of the GC detector was demonstrated by the analysis of natural gas, headspace samples from beer, crushed garlic and onion, and the sulfur-containing pesticides malathion and parathion (2). We have since developed this detector for use with microbore high-performance liquid chromatography (p-HPLC) and have increased our understanding of its basic principles of operation. The p-HPLC system was chosen because of the low solvent flow rates required. Since this detector requires gas-phase reactants, the LC effluent must be vaporized prior to entering the detection cell. The high gas pressure in the chemiluminescence cell that results from high solvent flow rates is detrimental to the response of the detector. Thus, one of the benefits of using p-HPLC columns is exploited in our system. A critical comparison of microbore vs. standard columns recently has been published (3). Currently, the only other HPLC detector that is designed to be somewhat selective for sulfur-containing species is the Tracor 965 photoconductivity detector (4). This detector also responds to halogenated compounds, many nitrogen-containing compounds, and sulfur-containing compounds in which the sulfur atom is oxidized. Publications regarding the detection of organosulfur compounds in HPLC are scarce and commonly make use of derivatization techniques that are time-consuming or detection methods that are not highly selective (5-15). The detector described below does not require any pre- or postcolumn derivatization of organosulfur compounds and is based on a chemiluminescent (CL) reaction of the vaporized HPLC effluent with a reagent gas. Chemiluminescence detectors generally provide high selectivity and sensitivity, since
the emissions that result from these reactions may be filtered as desired and monitored with photomultiplier tubes (PMT’s) that can detect very low levels of light.
THEORY The basis for detection is the chemiluminescent reaction that occurs when certain sulfur-containing species are reacted with molecular fluorine
R1-S-R2
+ F2
- X*
products
+ hv
(1)
The R groups bonded to the sulfur can be H (except HzS), C,H,, or a phenyl ring, or the sulfur may be contained in a ring structure. X* is an electronically or vibrationally excited species that emits light upon returning to the ground electronic or vibrational state. Atomic fluorine reacts with numerous compounds, many of which also produce chemiluminescent products (16-18). However, few species react with enough exothermicity to produce excited-state products that emit in the visible wavelength region of the spectrum. The organosulfur and organoselenium species described below react with Fz to produce excited-state products that emit at wavelengths longer than about 450 nm. Iodinated compounds also react with Fz, producing excited-state IF in the visible wavelength region, and this forms the basis of an iodine-selective detector (19).However, there are very few analytes that contain iodine atoms, and it would be extremely rare for iodine compounds to interfere in the determination of sulfur compounds. Most of the chemiluminescent products have been identified from the reaction of various sulfides, disulfides, and thiols with molecular fluorine. The primary emitting species produced in many of these reactions in the absence of vaporized solvent is vibrationally excited H F (1,2). (These conditions would correspond to the operation of the detector in the GC mode.) The Av = 4 vibrational overtone emission bands of H F are observed between 650 and 750 nm. As mentioned above, other species are known to react with atomic fluorine and may also react with molecular fluorine to produce H F ( u ’ I 3). However, the emissions observed in F atom reactions are at wavelengths greater than 800 nm and are not monitored by the detector. In addition to vibrationally excited HF (v’ I81, electronically excited thioformaldehyde (CHzS*) is observed when dimethyl sulfide, dimethyl disulfide, and methanethiol are reacted with Fz (20). Figure 1shows the location of the main emission bands of H F (u’ I6) and CH2S* relative to the spectral characteristics of the band-pass filter used in the detector to obtain selectivity for organosulfur species. Recent spectroscopic studies have revealed that another CL product, electronically excited HCF, is formed when low concentrations of certain organosulfur compounds are reacted with FD A fourth chemiluminescent product is observed in the reaction of carbon disulfide with Fz. The tentative preferred assignment for this spectrum is emission by electronically excited FCS (21). This spectrum will be referred to below as the FCS spectrum, although it is not certain that this
0003-2700/86/0358-0~18$01.50/0 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
1
I
HFt
919
Cooled, R e d Sensitive PMT and Housing?
Transmission
60
7
I-
t
F2/ He
$ 40
I
550
600
650
700
750
800
850
X (nrn) Figure 1. Spectral characteristics of the band-pass,filter used to obtain selectivity for organosulfur compounds and the locations of the emission bands of HF (Av = 4) and CH,S*. The band intensities are not corrected for the response of the IDARSS.
triatomic molecule is responsible for the spectrum observed. The emission spectrum of selenoformaldehyde has been recorded from the reaction of dimethyl diselenide with Fz. This spectrum is qualitatively similar to the thioformaldehyde spectrum but lies to the red of the thioformaldehyde spectrum near 800 nm. The nature of the emitting species produced in this reaction is highly dependent on the concentration of dimethyl diselenide reacted with F2 A selenoformaldehyde spectrum free from major spectral contaminants was obtained with reIatively high concentrations of dimethyl diselenide. Lower dimethyl diselenide concentrations produce CH2Se* in addition to HCF* and a third, unidentified species (22). The emission spectra referred to above were recorded with an intensified diode array rapid scan spectrometer (IDARSS), which requires high levels of light input relative to a PMT. Unfortunately, it is difficult to record spectra using nanogram or picogram quantities of sulfur compound. The observations made in the spectroscopic studies must be extrapolated in order to speculate as to the nature of the CL products at the very low concentrations of analyte that the P M T normally detects.
EXPERIMENTAL SECTION Instrumentation. An illustration of the front view of the detector reaction cell is given in Figure 2. The reaction cell was constructed from a 4.5-cm length of 3.8-cm stainless steel tubing and had three 0.635-cm stainless steel ports and one 1.27-cm stainless steel flexible hose welded to the circumference at right angles. The emission resulting from the CL reaction is viewed through quartz windows (50.8-mm diameter, Quartz Scientific, Inc., Fairport Harbor, OH) located on both sides of the detector cell. The windows are mounted with flanges and sealed by O-rings. Another flange, 9.5 cm thick with a 50.8-mm square cutout, is located on the face of the PMT housing and functions as a filter holder. One of the 0.635-cm ports on the cell is the inlet port for the vaporized LC effluent. Vaporization takes place in a 10-cm length of 0.159 cm o.d., 0.0254 cm id., stainless steel tubing, commonly used for HPLC. The narrow-bore LC tubing extends beyond the reducing union, external to the reaction cell. It is wrapped with heating tape and maintained at 300 "C by a variable transformer. A second port, located at a 90° angle from the LC effluent port, is used to deliver a 10% F2/He reagent gas mixture to the mixing zone. The pressure in the reaction cell is monitored through a third port (located directly opposite the LC effluent port) by a capacitance manometer (Baratron Model 220; MKS Instruments, Burlington, MA). The foarth port leads to a dry ice/acetone trap and subsequently to a 160 L/min vacuum pump (Model 1402; Sargent-Welch, Skokie, IL). The cell is maintained at 5 2 torr when all gases are flowing to reduce the collisional quenching of the CL products. The apparatus for the chromatographic studies consists of a Kratos Spectroflow 400, dual-piston pump (Kratos Analytical
Cold Trap, Vacuum Pump
Capoci tonce Ma nome ter
Figure 2. Front view of the reaction cell. The front window and flange
containing the IDARSS eyepiece have been omitted.
Instruments, Ramsey, NJ), which is connected to a microbore injector (Model 7410, Rheodyne, Inc., Cotati, CA) with a 1-pL internal sample loop. A Brownlee microbore column (1 X 250 mm, 5-pm Spherisorb, RP-18, Brownlee Labs, Santa Clara, CA) was used in early studies. Later work utilized a microbore column packed with 3-pm Du Pont Zorbax ODS (1 X 150 mm). A 2-cm length of 0.159 cm o.d., 0.0254 cm i.d., stainless steel LC tubing is inserted between the column and the heated vaporization line. The 2-cm length of LC tubing protects the column from the hot vaporization line, which is insulated with quartz tape and aluminum foil. The CL emission is viewed through one window of the reaction cell by a cooled, red-sensitivephotomultiplier tube (PMT, 9659QB; EMI, Plainfield, NY). The reaction cell is mounted on the face of a Fact-50 PMT housing (EMI). The filter holder is located between the reaction cell and housing so that the spectral region of interest can be isolated. The filter used in this study passes wavelengths between 650 and 800 nm with a peak transmittance of 77% at 700 nm, and a full width at half maximum (fwhm) of 80 nm (03 FIB 013; Melles Griot, Irvine, CA). The PMT power unit (HP 6515A; Hewlett-Packard) supplied a positive 1200 V to PMT anode. The PMT signal can be monitored as output current with a picoammeter (Model 480, Keithley, Cleveland, OH) or by photon counting using a pulse amplifier/discriminator (built in our lab) and a timer/counter (Model 5308A, Hewlett-Packard). All of the results reported below were generated in the photon counting modelwith an integration (sample and hold) time of 0.1 s. The signal from the timer/counter is converted to analog domain by a digital-analog converter (Model 5311B, HewlettPackard) and the output is directed to a strip chart recorder (Model 1202, Linear Instruments Corp., Reno, NV). For spectroscopic studies, the window opposite the PMT is utilized. It is held in place by an O-ring and a flange that contains a removable eyepiece and slit holder. The IDARSS (TN 1710, Tracor-Northern, Middletown, WI) is positioned in front of the detector cell with this eyepiece, and simultaneous spectra are recorded from 400 to 900 nm. The spectra are stored in the memory of a multichannel analyzer and are subsequently output to the strip chart recorder. Reagents. The organosulfur and organoselenium compounds were from the following sources: 2-mercaptoethanol, o-aminobenzenethiol, 1-heptanethiol,n-butyl sulfide, and dimethyl sulfide from Eastman Kodak Co. (Rochester, NY); 1,2-ethanedithiol, 1-butanethiol, allyl sulfide, phenyl sulfide, and ethyl sulfide from Matheson, Coleman and Bell (Norwood, OH); 2-methyl-2propanethiol, 1-hexanethiol, 1-octanethiol,ethanethiol, dimethyl disulfide, tert-butyl sulfide, thiophene, and n-butyl disulfide from Aldrich (Milwaukee, WI); dimethyl selenide and dimethyl diselenide from Alfa Products (Danvers, MA); Malathion Insect Control, Dexol Industries (Torrence, CA); carbon disulfide from J. T. Baker Chemical Co. (Phillipsburg, NJ); dimethyl sulfoxide from Fisher (Fairlawn, NJ). Insecticide samples were obtained from the EPA, Pesticides and Industrial Chemicals Repository (Research Triangle Park, NC). The methanol, acetonitrile, and water used as HPLC solvents were from Burdick and Jackson (Muskegon,MI). A special mixture of 10% F,/He was prepared by Matheson (East Rutherford, NJ).
920
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
C I-Hexonethiol
c
550
650
X
750
850
0.1
(nm)
Emission spectrum obtained by reacting tert-butyl sulfide with F, in the presence of vaporized methanol. The asterisks (*) indicate the location of the HF (Av = 3, 4) emission bands. The main emission band tentatlvely has been assigned to FCS‘.
RESULTS AND DISCUSSION Spectroscopy. The emission spectra recorded in the presence of vaporized LC solvents were generally complicated and widely varied. The vaporized methanol or acetonitrile in the reaction matrix caused the following effects: (a) the relative emission intensities of HF (u’l6), CHZS*, HCF*, and FCS* are different in these solvent matrices as compared to a helium matrix (GC detector); (b) the chemiluminescent product observed in a given analyte/F2 reaction may vary with solvent matrix; and (e) the relative emission intensities of the chemiluminescent products may vary with solvent (21). Specifically, the thiol/F, reactions (1-octanethid, l-hexanethiol, ethanethiol) produce electronically excited CH2S and lesser amounts of vibrationally excited HF (u’ 5 6) in a methanol matrix. In an acetonitrile matrix, the thiol/F2 reactions produce H F (u’ 5 6) and FCS*. The short-chain sulfide/F2 reactions (dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide) produce CH2S* in methanol and in acetonitrile. The longer-chain suKde/F2 reactions (ethyl sulfide, allyl sulfide, tert-butyl sulfide, dibutyl disulfide, and the aromatic species phenyl sulfide) produce H F ( u ’ r 6) and FCS* in both solvent matrices, As an example of this emission, the spectrum obtained from the tert-butyl sulfide/Fz reaction in the presence of methanol is given in Figure 3.
0.4
IO 70F2 / He(Torr )
Flgure 3.
Procedure. Single-componentsolutions were screened by flow injection analysis to test for vaporization line plugging upon vaporization of the effluent and to assess the dead volume of the system external to the column. Selectivity ratios were also determined by flow injection analysis. Detector response as a function of analyte concentration and F2concentration was determined by injecting mixtures of thiols, sulfides, and disulfides onto the microbore column and measuring the resulting peak heights or areas from the chromatograms. Detection limits were determined analogously from the most dilute mixtures. All analyses were run in triplicate. Flow rates for both flow injection analysis and chromtographic separations ranged from 60 to 100 pL min-’ using 70 to 85% methanol/water and 70 to 85% acetonitrile/water solvent mixtures. IDARSS spectra were recorded over 10-minintegration intervals while 10 to 20% (v/v) organosulfur compound/methanol or acetonitrile mixtures were flowing from a 2-mL sample loop at 100 pL min-l. The spectra were recorded under the same conditions used in the chromatographic analyses with the exception that the organosulfur compound concentrations were 3 orders of magnitude higher than any solutions used in the chromatographic studies. The spectra of a number of pure samples were obtained by filling the 2-mL loop with undiluted sample and vaporizing the effluent in the vaporization line or by pumping on the headspace of a sample contained in a glass vessel.
0.3
0.2
Effect of increasing F,/He pressure on the response of organosulfur compounds. The relative response factor gives the multiplicative factor for the increase In slgnai with added reagent gas pressure. Flgure 4.
C I-Butonethiol D I-Hexanethiol E I-Octanethiol
8
0-9-
/.-p-.-p-_.-E
0
0.2
0.4
0.6
IO ‘10 F2/ He (Torr) F,/He optimization curves for the thiols indicating an optimum pressure of 0.4 torr of 10% F,/He. There was 2 ng pL-’ of each analyte listed above in the test mixture. Flgure 5.
No emission was detected by the IDARSS for the methanol/F2 and acetonitrile/F, reactions, although a small background signal (0.5-1.0 kHz) was detected by the PMT. Optimization. LC flow rate optimizations were based on the retention times of the mixture components and not on the response of the detector. The difference in detector cell pressure in increasing the flow rate from 60 to 100 pL min-l did not affect the response of the analytes. Similarly, the water content of the mobile phase did not affect the detector response in the range used in this study (0 to 30%). The 10-cm vaporization line used for the vaporization of the LC effluent did not contribute significantly to the broadening of analyte peaks. The peak widths a t various retention times in the chromatograms generated by this system were comparable to the peak widths obtained by the manufacturers of the microbore columns using UV absorbance detectors. Considering that 50 mL of gas at 1 torr partial pressure condenses to a liquid volume of less than 0.2 pL, the dead volume of the reaction cell and vaporization line is extremely small and is not expected to contribute significantly to band broadening. Corrosion of the vaporization line was not observed in the time frame in which each vaporization line was used. The longest use of a single vaporization line was 6 months, and the vaporization lines were changed only when plugging OCcurred from the injection of solutions containing high salt concentrations.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
Table 11. Detection Limits"
Table I. Selectivity Ratios" amt injected compound hexane 1-hexene benzene
toluene m-xylene
methylene chloride carbon tetrachloride acetonitrile formaldehyde acetone aniline pyridine trans-cinnamaldehyde tetrahydrofuran
921
SR
ng
>4800 390 >7000 >5900 118 >9700 1300 >12000
660 670 880 870 860
>6300 >8500 380 >7800
2500 >7700
1300 1600 780 300 790
1020 980 1050 890
compound
mol 7.6 X 8.0 X 1.1 X 9.4 X 8.1 X 1.6 X 1.0 X 2.0 X 1.0 X 1.4 X 1.1 X 1.2 X 8.0 X 1.2 X
ng
Pg 9-I
pg
s s-l
CH3CN/H20 Solvent Mixture
lo-'
2-mercaptoethanol ethanethiol o-aminobenzenethiol 2-methyl-2-propanethiol 1-butanethiol 1-hexanethiol 1-octanethiol dimethyl sulfide dimethyl disulfide ethyl sulfide allyl sulfide phenyl sulfide tert-butyl sulfide n-butyl sulfide n-butyl disulfide dimethyl selenide
lo-'
lo-' lo-' lo-'
lo-'
lo-' lo-'
"Reference sulfur compound, I-octanethiol (850 ng, 5.8 X mol).
0.230 0.125 1.80 0.100 0.180 0.560 2.10 0.047 0.060 0.070
0.050 0.350 1.80 0.430 0.220 1.20
27 21 200 13 23 47 78 10 11 12 8 42 170 36 13 160
11 11
51 5 8
13 17 5 8 4 2 7 37 8 5 116*
CH30H/H20Solvent Mixture
The within-run reproducibility of the system is *5% , and the between-run reproducibility is *lo%. The dependence of analyte response on F,/He concentration is illustrated in Figures 4 and 5. Figure 4 shows the relative increase in signal with increasing reagent gas pressure for a variety of organosulfur compounds. It is interesting to note the large effect that an increase in Fz concentration has on thiophene, perhaps reflecting the lower reactivity of the closed-ring structure vs. the open-chain organosulfur compounds. Figure 5 shows the F2 dependence for the thiols in more detail, and a pressure of 0.4 torr of 10% F,/He was chosen for the optimization of detector response for most of the compounds tested. The 0.4 torr is added to the pressure in the cell resulting from the vaporized LC effluent. The vaporization line temperature is set at 300 "C for the vaporization of the LC effluent so that high-boiling compounds, such as 1-octanethiol, can be vaporized as efficiently as the low boiling compounds. Lower temperatures were found to adversely affect the response of the higher molecular weight compounds, as expected. Higher temperatures were not tested because of the temperature limits of the heating tape and may prove to be beneficial when compounds that boil or decompose above 300 "C are analyzed. Compounds that decompose in the vaporization line would not pose a problem if one of the decomposition products reacts with F2 to produce a chemiluminescent product that emits in the wavelength region monitored by the detector. The detrimental consequence of thermal decomposition is the formation of residue in the vaporization line that plugs the flow of the LC effluent. This was not observed with most of the compounds analyzed; however, some salt solutions do cause this effect, and plugging was observed when a solution of cysteineHCl/H20 was injected. The use of an ammonium nitrate buffer in the mobile phase (0.005 M, 85% methanol/buffer) did not plug the vaporization line when maintained at 300 OC. This buffer forms the volatile species NH3 and HN03 when vaporized. Selectivity. The selectivity of the detector for organosulfur componds over other species is expressed by the selectivity ratio signal/mole of sulfur compound
SR = signal/mole of interfering compound
(2)
The amount of compound injected in each case and the selectivity ratios are given in Table I. The reference compound is 1-octanethiol, which gives an average response in the detector. The quantity of interfering compound injected was chosen to be in the high nanogram range, which is just ap-
dimethyl sulfide dimethyl disulfide ethyl sulfide allyl sullfide phenyl sulfide tert-butyl sulfide n-butyl sulfide n-butyl disulfide
0.070 0.090
0.100 0.080 0.540 2.90 0.500 0.290
"Detection limit = 3N/(S;/m,). b
~ Se g
11 13 13 9 40 220
35 16
6 9
5 3 7 48 8 6
s-l.
proaching the point where a microbore column can become overloaded. Most compounds did not respond at all at these concentrations, and a small response is observed from compounds such as 1-hexene, m-xylene, and aniline, which have relatively weak C-H bonds. Such compounds can produce vibrationally excited H F (u' I 4)resulting in emission from the (4,O) overtone band near 680 nm (23). Detection Limits. The detection limits for a number of organosulfur compounds are given in Table 11, and are calculated using the following equation:
3N
detection limit = Si/mi
(3)
The quantitiy N is the peak-to-peak noise, Si is the peak signal obtained from the chromatogram for species i, and mi is the mass of species i injected. The detection limits were calculated from chromatograms in which the analyte signals were 5-10 times greater than the base line peak-to-peak signal. The detection limits are reported in nanograms of compound, picograms of compound per second (to correct for retention time), and picograms of sulfur per second. There is only a slight difference in detection limits between the acetonitrile and methanol solvent mixtures, the former being somewhat smaller due to lower background signals (0.5 kHz vs. 1.0 kHz). The detection limit reported for dimethyl diselenide is rather high, and this is probably due to the location of the emission bands of the CL products relative to the band-pass filter used in this study. The signal detected for dimethyl diselenide by the P M T is not primarily due to emission by selenoformaldehyde but is a result of emission by HCF* and the unidentified species mentioned earlier. The 650 to 800 nm band-pass fiiter cuts off a large percentage of this emission band, resulting in a high detection limit. Calibration Plots. A log peak area (kHz min) vs. log concentration (ng) calibration plot for allyl sulfide is linear over 3 orders of magnitude (slope = 0.99 0.02, y intercept = 0.54 f 0.03), and this linear response is observed for the sulfides in both acetonitrile and methanol. Calibration plots
*
922
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
a
+ .VI
I
1
a
I
c
c
0.10% Malathion InsecticidaKHJOI
1 0
I 0.008% Malathion Insect
ZI
.-+
2 4 6 8 IO 12 14 16
Time (mid
icide /CHsOH
v)
(62ppm Malathion)
c
0) c
0
2
4
6
8
Minutes Chromatogram of Malathion Insect Control: 75 % methanol/water at 100 pL min-'; 1-pL sample; malathlon concentration in ppm by weight. Figure 7.
Time (min)
Chromatograms of thiol (A) and sulfide (E) mixtures. The peaks, in order of elution, are (A) 2-mercaptoethanoi (56 ne), ethanethioi (42 ne), 2-methyl-2-propanethiol (40 ng), 1-butanethiol (42 ng), and 1-octanethiol (42 ng) and (B) dimethyl sulfide (42 ng), dimethyl disulfide (53ng), ethyl sulflde(42 ng), allyl sulfide (44 ng), phenyl sulfide (55 ng), teff-butyl sulfide (82 ng), n-butyl sulfide (42 ng), and n-butyl disulfide (47 ng). Chromatographic conditions were (A) 78 % methanollwater at 60 pL min-' and (B) 7 7 % acetonitrile/water at 60 pL min-', with l-pL sample. Figure 6.
made in the 0.5 to 100 ng range also exhibit a linear dependence of signal on concentration. A slight curvature is observed in the thiol calibration plots. If one assumes linearity in the 1 to 100 ng range, analyte concentrations are overestimated by ~ 5 on % the high concentration end, underestimated by =lo% in the mid-concentration region, and badly overestimated ( ~ 4 0 % at ) the low concentration end of the plot. The curvature in the thiol plots is probably due to a variation in the nature and relative emission intensity of the CL products over this concentration range. The latter has been observed in dimethyl sulfide/F2 reactions (21) and in the dimethyl diselenide/F2 reaction described earlier. Chromatography. Chromatograms of thiol and sulfide mixtures are given in Figure 6. The concentrations of the analytes range from 40 to 82 ng wL-l. The peaks are eluted isocratically, although we foresee no problems using gradient elution. Surprisingly, analyte responses were not significantly affected by switching from methanol to acetonitrile (see Table 11) or by adding 0 to 30% water to either solvent. A chromatogram of a commercial mixture of Malathion Insect Control is given in Figure 7. The commercial solution contains 50% malathion, 42% xylenes, and 8% "inert ingredients". No signal was observed from the xylenes, and the small peaks surrounding the malathion peak have not been identified. Figure 8 shows a chromatogram obtained from beer. A 50-mL sample of beer was taken directly from a can and passed through a (2-18 Sep-PAK cartridge. The cartridge was rinsed with 5 mL of methanol, and a 1-pL sample of the methanol wash was injected onto a microbore C-18 column. The peak observed at 1.8 min in Figure 8 has a retention time equal to that of dimethyl sulfide. Beer contains many organosulfur compounds at part-per-billion and part-per-trillion levels (2). The peak at 1.8 min is probably dimethyl sulfide, and corresponds to about 1ng of this compound. The second peak in Figure 8 has not been identified. No response is
0
2
4
6
8
3 I
Time (mid
Chromatogram of a flltered beer sample. The peak at 1.8 min corresponds to about 1 ng of dimethyl sulfide. Chromatographic conditions were 7 0 % acetonitriie/water at 8 0 pL min-', with 1-pL sample. Figure 8.
Table 111. Compounds Responding in the LC-CL Detector allyl sulfide ethanethiol phenyl sulfide 1-butanethiol 2-methyl-2-propanethiol tert-butyl sulfide n-butyl sulfide 2-mercaptoethanol n-butyl disulfide o-aminobenzenethiol thiophene 1-hexanethiol heptyl sulfide" 1-octanethiol dimethyl selenide dimethyl sulfide dimethyl diselenide dimethyl disulfide dimethyl sulfoxide" ethyl sulfide
malathion ametryn parathion methyl prometryn ronnel EPTC methomyl diallate dazomet butylate
Weak response.
observed from the other beer components. A number of sulfur-containing pesticides were analyzed. A list of those that respond well is given in Table I11 in addition to the other organosulfur compounds tested. Three of the pesticides, parathion methyl, prometryn, and ronnel, were not expected to respond, since they do not contain a sulfur-carbon bond, but instead have a -P=S functional group. We have not yet recorded the emission spectrum resulting from the reactions of these compounds with Fz.
Anal. Chem. 1906. 58.923-932
CONCLUSION This chemiluminescence detector, based on the gas-phase reaction of molecular fluorine with organosulfur compounds, has proven to be highly selective and sensitive when operating in either the gas chromatography or liquid chromatography mode. Much work remains to be done to elucidate the reaction mechanisms; however, it appears that compounds containing a reduced sulfur atom give the best response in the detector. Compounds containing sulfur atoms in or adjacent to ring structures respond moderately, and highly oxidized sulfur species respond poorly. Applications work has been limited, and we anticipate that the detector will be useful for the analysis of sulfur-containing biological compounds, petroleum products, food and drug samples, pesticide residues, and atmospheric samples in both the GC and LC mode. Efforts are currently under way to adapt the detector to a capillary GC system for the analysis of dimethyl sulfide and dimethyl disulfide in atmospheric gas samples (24). The detector could probably be interfaced to a supercritical fluid chromatographic (SFC) system, since the solvents typically used in SFC do not respond in the wavelength region of interest when reacted with F2. The use of molecular fluorine as the reagent gas in the detector is the major drawback to the general use of the system. We are currently investigating the potential use of nontoxic sources such as SF6and CF4, which are known to produce F2when passed through a microwave discharge (2) and may also produce F2 when passed through an electrical discharge such as an ozonizer. ACKNOWLEDGMENT The authors wish to thank Richard A. Henry of Keystone Scientific, Inc., for much assistance with the use of microbore systems and John Larmann from Du Pont for supplying the 3-pm Zorbax. We. especially thank Udo Brinkman for suggesting the adaptation of the organosulfur GC detector to HPLC. Registry No. Fz, 7782-41-4; 2-mercaptoethanol, 60-24-2; ethanethiol, 75-08-1; o-aminobenzenethiol, 137-07-5; 2-methyl2-propanethiol, 75-66-1; 1-butanethiol, 109-79-5;1-hexanethiol, 111-31-9; 1-octanethiol, 111-88-6;dimethyl sulfide, 75-18-3; di-
923
methyl disulfide, 624-92-0; ethyl sulfide, 352-93-2; allyl sulfide, 592-88-1; phenyl sulfide, 139-66-2; tert-butyl sulfide, 107-47-1; n-butyl sulfide, 544-40-1; n-butyl disulfide, 629-45-8; dimethyl selenide, 593-79-3; thiophene, 110-02-1;2-mercaptoethanol, 6024-2; Malathion, 121-75-5. LITERATURE CITED
(21) (22) (23) (24)
Nelson, J. K.; Getty, R. H.; Birks, J. W. Anal. Chem. 1983, 55, 1767-1 770. Nelson, J. K. Ph.D. Dissertation, University of Colorado, Boulder, CO, 1984. Cooke, N. H. C.; Olsen, K.; Archer, B. G. LC Mag. 1984, 2 , 514-524. Popovlch, D. J.; Dlxon, J. 6.; Ehrllch, 8. J. J . Chromafogr. Sci. 1979, 17, 643-650. Plrkle, W. H.; House, D. W. J . Org. Chem. 1970, 4 4 , 1957-1960. Hill, D. W.; Walters, F. H.; Wilson, T. D.; Stuart, J. D. Anal. Chem. 1979. 57, 1338-1341. Reeve, J.; Kuhlenkamp, J.; Kaplowitz, N. J . Chromatogr. 1980, 794, 424-428. Reed, D. J.; Babson, J. R.; Beatty, P. W.; Brodie, A. E.; Ellis, W. W.; Potter, D. W. Anal. Eiochem. 1980, 706, 55-62. Plrkle, W. H.; House, D. W.; Flnn, J. M. J . Chromafogr. 1980. 792, 143- 158. Bishop, C. A.; Kltson, T. M.; Harding, D. R. K.; Hancock, W. S. J . ChrOtMtOgr. 1981, 208, 141-147. Ingebretsen, 0. C.; Farstad, M. J . Chromafogr. 1981, 270, 522-526. Skaaden, T.; Grelbrokk, T. J . Chromafogr. 1982, 247, 111-122. Shlmada, K.; Tanaka, M.; Nambara, T. Anal. Chim. Acta 1983, 147, 375-380. Van Langenhove, H.; Van Acker. M.; Scharnp, N. J . Chromafogr. 1983, 257, 170-173. Bossle, P. C.; Martln, J. J.; Sarver, E. W.; Sommer, H. 2. J . ChromafOgr. 1984, 283, 412-416. Duewer, W. H.; Setser, D. W. J . Chem. Phys. 1973, 58, 2310-2320. Bogan, D. J.; Setser, D. W. J . Chem. Phys. 1976, 64, 586-602. Jones, W. E.;Skolnik, E. G. Chem. Rev. 1978, 76, 563-592. Getty, R. H.; Blrks, J. W. Anal. Lett. 1979, 72, 469-476. Glinskl, R. J.; Nelson, J. K.; Birks, J. W. Chem. Phys. Lett. 1985, 177, 359. Glinskl, R. J.; Mishalanie, E. A.; Blrks, J. W., unpublished work. Glinskl, R. J.; Mlshalanle, E. A.; Birks, J. W. J . Am. Chem. Soc., In press. Bogan, D. J; Setser, D. W.; Sung, J. P. J . Phys. Chem. 1977, 87, 888-898. Tawnier, J., National Center for Atmospheric Research, personal communlcation, 1985.
RECEIVED for review August 5,1985. Accepted October, 25, 1985. This work was funded by the Environmental Protection Agency, Grant No. R-810717-01-0, and was performed in partial fulfillment of the Ph.D. degree (E.A.M.) from the Department of Chemistry, University of Colorado, Boulder,
co.
Theta Pinch Discharge Designed for Emission Spectrochemical Analysis: Design and Electrical Characterization Gregory J. Kamla and Alexander Scheeline* School of Chemical Sciences, University of Illinois, 1209 West California Avenue, 79 RAL, Box 48, Urbana, Illinois 61801 A theta plnch dlscharge was deslgned and constructed as a hlgh-temperature plasma source for appllcatlon to the emlb slon spectrochemlcal analysis of solld samples. Thls source has been operated up to 32 kV (1.33 kJ), with a peak dlscharge current of 60 kA. The peak magnetic fleld generated by thls discharge through a double helical colt Is 28 kG, resultlng In a calculated peak plasma current of 32 kA. Thorough electrical characterlzatbnand deelgn spectflcatlons are reported.
Several capacitive discharges have been used as plasma sources for analytical emission spectroscopy. These discharges 0003-2700/86/0358-0923$01.50/0
have found application in the analysis of solid materials, due to their ability to directly sample solids with minimal sample preparation. The high-voltage spark discharge has been used for many years as an analytical source for multielement emission spectrochemical analysis (1-3). Spark discharges have the ability to directly analyze solid, conductive materials. Spark generated working curves are in many instances nonlinear and dependent on sample matrix, thus requiring standards matched to the sample type. Reductions in sample matrix effects have been realized by increasing the plasma source energy. A less common analytical plasma source, which employs a higher energy discharge than is used for the high voltage spark, is the electrically vaporized thin film (4). This source does not require the sample to be one of the discharge 0 1988 Arnerlcan Chemlcal Soclety
