Anal. Chem. lQQ2,64, 1205-1211
1205
AC RESEARCH
Direct Detection and Identification of Volatile Organic Compounds Dissolved in Organic Solvents by Reversed-Phase Membrane Introduction Tandem Mass Spectrometry Frants R. Lauritsen? Tapio Kotiaho,t Tarun K. Choudhury, and R. Graham Cooks* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
A new technlque Is descrlbed for the dlrect detectlon and Identlticatknof volatileorganlc compoundsdlroolved In organk solvents. The mothod uses a porous membrane lntroductlon system to a mass spectrometer and lo capable of contlnuous on-line monltorlng. The porous polypropylene membrane chosen for thls work allows sufflclent amounts of the solvent to penetrate Into the Ion source for chemlcal lonlzatlon (CI) to occur wlth the vaporlzed solvent actlng as the reagent gas. Solute molecules permeate the membrane together wlth the solvent and can be analyzed by CI/MS and tandem mass spectrometry (MS/MS). I n caws In whlch the solute can not be chemlcally lonlzed by the vaporized solvent CIplasma, lt k porrible to co-Introduce a conventlonalC I reagent gas to achkve addltlonalcontrolof the lonlzatlonprocess. WHh thls technlque, a broad range of compounds dlmolved In organlc solvents can be detected In the rub-ppm range wHh rlse and fall tlmes faster than 10 8. Detedlon llmlts depend on the efflclency of lonlzatlon, not on the rate of pastage through the membrane, and they can often be Improved by udng tandem rather than dngle-stage mass spectrometry. As an example of the appllcatlon of the technlque, the detectlon and Identllcatlonof addltlvesIn gasollne samples Is demonstrated.
INTRODUCTION Membrane introduction mass spectrometry (MIMS) can be considered as a well-established technique for the on-line monitoring of gases dissolved in water,1,2 and it is under rapid development for the monitoring of volatile organic compounds in aqueous s ~ l u t i o n .Applications ~ to the monitoring of bioprocess sol~tions~-~0 and environmental samplesl1-l6have been pursued vigorously. The development of direct-insertion t Permanent address: Institute of Biochemistry, Odense University, Denmark. 1 Permanent address: The Chemical Laboratory of the Technical Research Center of Finland. (1) Degn, H.; Cox, R. P.; Lloyd, D. Methods Biochem. Anal. 1985,31, 165-194. (2) Lloyd, D.; Bohatka, S.; Szilagyi, J. Biosensors 1985, I, 179-212. (3) Kotiaho,T.;Lauritaen,F.R.;Choudhury,T. K.;Cooks,R. G.;Tsao, G. T. Anal. Chem. l991,63,875A-883Ae (4) Heinzle, E.; Reuss, M. Mass Spectrometry in Biotechnological Process Analysis and Control; Plenum Press: New York, 1987. (5) Jsrgensen, L.; Degn, H. Biotech. Lett. 1987, 9, 71-76. (6) Jensen, B. B.; Cox, R. P. Methods Enzymol. 1988, 167, 467-474.
0003-2700/92/0364-1205$03.00/0
membrane probes17-19 has been particularly important in allowing organic compounds to be detected with rapid response times and low detection limits in an on-line fashion. Such probes, used together with flow injection analysis procedures, have made on-line quantification possib1e.Q Recently, detection limits in the ppt range for some nonpolar compounds were achieved by using a helium purge gas to flush the interior of the membrane.20 The use of chemical ionization and tandem mass spectrometry has facilitated easy identification of volatile organic compounds in complex media in these e~periments.9.~~ Whereas MIMS has been extensively used for the detection of compounds dissolved in water, the use of the technique for the detection of compounds dissolved in organic liquids has received little a t t e n t i ~ n .However, ~ ~ ~ ~ ~a technique which allows on-line monitoring of organic compounds dissolved in organic liquids can be expected to have great value in the chemical and in particular the petrochemical industry. This paper describes the extension of MIMS to cover such (7) Hayward, M. J.; Kotiaho, T.; Lister, A. K.; Cooks, R. G.; Austin, G. D.; Narayan, R.; Tsao, G. T. Anal. Chem. 1990,62, 1798-1804. (8) Lauritsen, F. R.; Bohatka, S.; Degn, H. Rapid Commun.Mass Spectrom. 1990, 4 , 401-403. (9) Hayward, M. J.; Reiderer, D. E.; Kotiaho, T.; Cooks, R. G.; Austin, G. D.; Syu, M.-J.; Tsao, G. T. BOC. Control Qual. 1991, 1, 105-116. (10) Carlsen, H. N.; Jsrgensen, L.; Degn, H. Appl. Microbiol. Biotechnol. 1991, 35, 124-127. (11) Harland, B. J.; Nicholson, P. J. D.; Gillings, E. Water Res. 1987, 21, 107-113. (12) Lister, A. K.; Wood, K. V.; Cooks, R. G.; Noon, K. R. Biomed. Enuiron. Mass Spectrom. 1989, 18, 1063-1069. (13) LaPack, M. A.; Tou, J. C.; Enke, C. G. Anal. Chem. 1990, 62, 1265-1211. (14) Kotiaho, T.; Hayward, M. J.; Cooks, R. G. Anal. Chem. 1991,63, 1794-1801. (15) Sturaro, A.; Doretti, C.; Parvoli, G.; Lecchinata, F.;Frison, G.; Traldi, P. Biomed. Enuiron. Mass Spectrom. 1989, 18, 707-712. (16) Choudhury, T. K.; Kotiaho, T.; Cooks, R. G. Talanta, in press. (17) Bier, M. E.; Cooks, R. G.; Brodbelt, J. S.; Tou, J. C.; Westover, L. B. Capillary Membrane Interface for a Maaa Spectrometer. US. Patent 4 791 292, 1989. (18) Lauritsen, F. R. Int. J.Mass Spectrom. Ion Proc. 1990,95, 259268. (19) Bier, M. E.; Kotiaho, T.; Cooks, R. G. Anal. Chim. Acta 1990,231, 175-190. (20) Slivon, L. E.; Bauer, M. R.; Ho, J. S.; Budde, W. L. Anal. Chem. 1991, €3, 1335-1340. (21) Lauritaen, F. R.; Nielsen, L. T.; Degn, H.; Lloyd, D.; Bohatka, S. B i d . Mass Spectrom. 1990,20, 253-258. (22) Jones, P. R.; Shen, K. Y. Anal. Chem. 1975, 47, 1000-1003. (23) Bohatka, S.; Degn, H. Rapid Commun. Mass Spectrom. 1991,5, 433-436. 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
applications. By analogy with terminology used in liquid chromatography, we refer to these as reversed-phase MIMS experiments, the conventional phase in this case being aqueous. The membranes used successfully for the monitoring of organic and simple inorganic compounds in aqueous solutions are all hydrophobic; they include silicone,24Teflon, polyethylene, and polypropylene25polymers. Such membranes have the great advantage that they discriminate against passage of water, resulting in an enrichment of the compounds of interest. It is therefore appropriate to use a hydrophilic membrane for the monitoring of polar compounds in a nonpolar (e.g., hydrocarbon) matrix, and indeed such membranes have been used successfully for the measurement of water activity in organic solvents.23 Hydrophilic membranes, however, have drawbacks such as the fact that transport properties are strongly modified by water and in some cases polar compounds permeate the membrane to such a degree that vacuum in the mass spectrometer cannot be maintained.23 In this paper we describe an alternative method for monitoring organic solvents by MIMS. Instead of using a membrane that discriminates against the solvent, a porous membrane is used to increase the amount of solvent flowing into the ion source of the mass spectrometer. By choosing the appropriate membrane and solvent conditions, the flow into the ion source can be optimized to create optimal conditions for chemical ionization with the vaporized solvent acting as the reagent gas. In this way compounds dissolved in the organic solvent can be chemically ionized and analyzed by MS and MS/MS. A related idea was used earlier12 to characterize organic compounds dissolved in water with a nonporous silicone membrane and an ion trap detector. The water flux through the siliconemembrane was used to generate protonated water, which served as the chemical ionization reagent. However, the partial pressure of water was too low for chemical ionization on instruments other than the ion trap. The use of a porous membrane for the introduction of both the sample and CI reagent gas also has some similarities to the novel frit LC/MS interfaces.26 In this experiment the eluent from a HPLC column is vaporized from a heated frit and the mobile phase is used as a reagent gas for CI. However, the two techniques differ fundamentally in the way vacuum is maintained. The membrane with its small pores supports the vacuum interface between the liquid analyte and the mass spectrometer, whereas the frit, with its much larger pores, is less capable of holding the vacuum. In this study we will demonstrate how membrane introduction mass spectrometry with a porous membrane can be used with single-stage and tandem mass spectrometry to identify and quantitate additives and minor impurities in gasoline samples. EXPERIMENTAL SECTION Membrane Probe. The membrane probe used is a directinsertion membrane probe originally developed for on-line monitoring of fermentation processes and described in detail e1se~here.l~ The probe employs a sheet membrane and connects directly to the ion source in such a way that the membrane effectively constitutes one wall of the ion source (Figure 1). Although the membrane probe can be heated, no heating was used in these experiments since the solvent flux through the membrane gave sufficient solvent pressure for it to be used as the CI gas. The sample solution was pumped through the probe (24) Westover, L. B.; Tou, J. C.; Mark, J. H. Anal. Chem. 1974, 46, 568-571. (25) Hoch, G.; Kok, B. Arch. Biochem. Biophys. 1963,101, 160-170. (26) Matsuura, K.; Otsuka, K.; Kobayashi, T.; Kubota, E.; Itagaki, Y.; Musselman, B. D.; Higuchi, T. Presented a t the 36th Annual Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, June 1988.
a
b
C
d
I e
Flgure 1. Placement of membrane probe in relation to ion source. (a) Membrane;(b) support; (c) ion volume; (d) filament; (e) extractionlenses.
at a rate of 1mL/min by a peristaltic pump and could either be returned to the sample solution vessel or be sent to waste after passage through the probe. The membrane chosen was a microporous polypropylene membrane (CELGARD2402)from Hoechst Celanese,Charlotte, NC. It is 50 pm thick, has an effective pore size of 0.02 pm, and a 38% porosity. The pores appear as rectangular slots under high magnification, and each pore forms part of a set of tortuous interconnected channels running through the membrane. The molecular mass cutoff value of the membrane is severalhundred thousand d a l t ~ n All . ~ ~experiments were carried out with two superimposed membranes supported by a 0.125-mm-thickstainless steel sheet with 0.3-mm-diameter apertures covering the exposed area of the membrane. The area of the apertures constitutes approximately 5 % of the total membrane area, which was 27 mm2. Mass Spectrometry. The measurements were performed using a triple quadrupole mass spectrometer (Finnigan MAT, San Jose, CA, Model TSQ 4500) operated via an INCOS data system. The ion source and the analyzer are differentially pumped by 270 L/s turbo molecular pumps (Balzers,Liechtenstein). Product ion (MS/MS) spectra were recorded with argon as collision gas at 1mTorr, which correspondsto multiple-collision conditions. The collision energy was typically set at 20 eV. Depending on the viscosity of the solvent and its interactions with the membrane, relativelylarge amounts of solvent penetrate the membrane pores and pass into the ion source. The optimal pressure for chemical ionizationwith the vaporized solvent acting as reagent gas was obtained by adjusting the tightness of the connection between the membrane probe and the ion source (Figure 1). In cases where the flux through the membrane is too small for CI conditions to be established, it can be increased by heating the membrane probe, although this was not necessary in the experiments described here. The ion source was thermostated to 190 "C. Other Conditions. The solvents used were hexane (actually a mixture of hexanes, predominantly n-hexane and methylcyclopentane, from Mallinckrodt, Inc., St. Louis, MO), reagentgrade toluene (J.T. Baker Inc., Phillipsburgh, NJ), benzene, chlorobenzene (Mallinckrodt), acetone (Fisher Scientific, Fair Lawn, NJ), and ethanol (Midwest Grain Products, Pekin, IL). Stock solutions of the analytes were prepared by using commercially availablereagents, and gasoline samples were obtained from nearby gas stations. Detection limits were measured using solutions prepared by serial dilution of stock solutions with the solvent. The measurements of detection limits and response times, as well as tests of linearity, were carried out by pumping successively through the probe the pure solvent and solutions containing well-defined amounts of the analyte of interest in the same solvent. Detection limits were defined at a signal-to-noise ratio of 3. Responsetimes were measured by making a step change in analyte concentration and were defined as 10-90% signal. The quantitative measurements of additives in gasoline were carried out either by comparingproduct ion abundances obtained from the gasoline samples and standard solutionsof the additives dissolved in hexane or by comparing product ion abundances for gasoline samples with abundances for gasoline samples doped with the analyte of interest. Since the ionization efficiency of (27) Master Your Universe with Celgard Microporous Membranes; Hoechst Celanese Corporation: Charlotte, NC.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992 73
7
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100 110 120130 140 150
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Flgurr 9. Hexane CI mass spectrum obtained from a mixture of 500 ppm each of ethanol,acetone, 2,3-butanedlol,and 2hexanoidissolved
in hexane.
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the CI-plasma created depends on the solvent used, the complex gasoline mixtures were measured with methane CI in order to allow a quantitative comparison of ion abundance5 between spectra obtained from the gasoline samples and from standard solutions.
RESULTS AND DISCUSSION Chemical Ionization Conditions. Figure 2a shows the ions extractedfrom the CI-plasma created when hexane passes through the membrane inlet. The spectrum is characteristic of the CI mass spectra of alkanes with [M - HI+ at mlz 85 as the dominant ion and the alkyl cations [CsH111+ (mlz 711, [C4H91+ (mlz 57), [C3H,l+ (mlz 43), and [C2H5If (mlz 29) as major secondary ions. The proton affinity of the CsH12 neutral analogue of the [M - HI+ ion from n-hexane was estimated
by extrapolation of proton affinity values28 from the linear C2H4-C4H8 series to be 180 kcallmol. With the exception of linear hydocarbons, most organic compounds have proton affinities larger than 180 kcal/mol, and they should therefore be protonated in the hexane CI-plasma. Indeed all compounds used in this study were protonated in the hexane CI-plasma. Figure 2b shows the ions extracted from the CI-plasma createdwhen toluene is pumped through the membrane probe. The major ion in this plasma is the toluene radical cation [MI.+ (m/z92) with [M-HI+ ( m / z91), [M + 13]+( m / z105), and [2M - HI+ (mlz 183) as the most abundant secondary ions. This mixture of ions was found not to be effective at protonation of all organic compounds tested. As a consequence, conditions for chemical ionization were established according to the following procedure: The membrane probe was slightly retracted from the ion source in order to decrease the solvent pressure in the source, and methane was then introduced to establish conditions for methane CI. This alternative method of protonation,however, has the drawback of yielding a more complicated background spectrum. Figure 2c shows the ions extracted in presence of methane. Note that the major ion from toluene is the protonated molecular ion at mlz 93 and not the radical cation at m/z92 as observed in the CI-plasma obtained from pure toluene (Figure 2b). Detection Limits and Response Times. Figure 3 shows the hexane CI mass spectrum obtained when hexane doped with 500 ppm each of ethanol, acetone, 2,3-butanediol, and 2-hexanol is passed through the membrane probe and the hexane background is subtracted. Three of the major ions observed are mlz 47,59, and 91, and they correspond to protonated ethanol, acetone, and 2,3-butanediol, respectively. The fourth mlz 73, is created by protonation of 2,3-butanediol followed by loss of water. Although the protonated molecular ion of 2-hexanol is observable at mlz 103, normally 2-hexanol gives rise to an intense m/z 85 ion formed by protonation of the molecule followed by loss of water. However, this ion coincides with a lo00 times more intense [M - HI+ reagent ion from n-hexane, and it is removed from the spectrum when the hexane background is subtracted. The ions observed at mlz ratios above 103 are created by gasphase reactions between the analyte ions and neutral analyte molecules. For example,the proton-bound dimer of acetone is observed at m/z 117, and mixed proton-bound dimer ions occur at mlz 105 (ethanol and acetone), at mlz 137 (ethanol (28) Lias, S.G.;Liebman, J. F.;Rhoda, D.L. J . Phys. Chem. Ref. Data 1984,13,695-808.
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
Table I. Measured Detection Limits and Response Times in Hexane Solvent compd
measured ion
mlz
proton affinity,' kcal/mol
detection limit, ppm
response time 10-90%,s
dimethylsulfoxide methanol ethanol 2,3-butanediol acetone ethylacetate benzene chlorobenzene hexanoic acid
[M + HI+ [M + HI+ [M + HI+ [M + H - H20]+ [M + HI+ [M + HI+ [M + HI+ [M + H - HCll+ [M + HI+
79 33 47 73 59 89 79 77 117
211 182 188 212 197 201 181 182 194b
0.2 0.2 0.1 0.25 0.15 0.5 5 20 1
25 5 5 15
n
47
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30
HCOOH-C2H5COOH.
Data from ref 28. This value is estimated by extrapolation of proton affinitiesZ8from the series m /z
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Flgurr 4. MuMple-Ion monltorlng data of successlve Injections of 500 ppm of 2hexanol, acetone, ethanol, and 2,bbutanedlol.
and butanediol), and at mlz 149 (acetone and butanediol). Other ionlmolecule reaction product ions that can be easily identified are the ions at mlz 119 and mlz 131. These ions are formed in reactions of the ion at mlz 73 with neutral ethanol and acetone, respectively. None of the high-mass ions were observed in a similar spectrum obtained from a solution containing 50 ppm of each of the solutes. We expect that ions created by gas-phase reactions will exist whenever the solvent contains more than lo00 ppm of solutes. This complicates the spectra obtained and makes the analysis of such high-level samples more difficult. Multiple-ion monitoring data obtained when a solution containing 500 ppm of either 2-hexanol, acetone, ethanol, or 2,3-butanediol in hexane is passed through the membrane probe are shown in Figure 4. 2-Hexanol, acetone, and ethanol were measured using the abundance of the protonated molecule, whereas 2,3-butanediol was measured using the abundance of the mlz 73 [M + H - HzOl+. As can be seen from Figure 4, the rise and fall times are very fast and almost identical for the different analytes. Table I shows the measured response times for a variety of compounds dissolved in hexane. It can be seen that, with the exception of 2,3butanediol, dimethyl sulfoxide, and hexanoic acid, all the compounds have identical response times. This situation is different from that observed with MIMS when nonporous membranes are used. With nonporous membranes the transport through the membrane is a diffusion process2gand large differences in response times occur depending on analyte molecular size and other experimental parameters. The microporous membrane used in this study has relatively large pores as compared to the dimensions of the small organic compounds studied, and the pores are i n t e r a x " through~
(29)Tsai, G.-J.; Austin, G. D.; Syu,M. J.; Tsao, G. T.; Hayward, M. J.; Kotiaho, T.;Cooks, R. G . Anal. Chem. 1991,63,2460-2465.
W
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-
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(min)
Chemical noise evldent In single-Ion monltorlng data for injections of (a) 250 ppb of dimethyl sulfoxide In hexane and (b) 200 ppb of acetone in toluene. out the membrane. We therefore believe that the flow through the membrane can be regarded as laminar flow, and this is consistent with the fact that all compounds have similar response times. The longer response times observed for 2,3butanediol, dimethyl sulfoxide, and hexanoic acid are explained by the very polar nature of these compounds, which results in strong interactions between sample molecules and surfaces in vacuum. This same kind of effect has already been reported when polar compounds at low concentrations are analyzed from aqueous solutions.'* Since the flow through the membrane is to be regarded as laminar flow and not a diffusion process, no enrichment of the sample as compared to the solvent can be expected. The detection limits should therefore be independent of transport through the membrane and depend primarily on the effectiveness of the ionization (here protonation) process. Table I includesmeasured detection limits together with the proton affinities (PA) for different compounds dissolved in hexane. All compounds measured have PA values larger than 180 kcallmol and were easily protonated in the hexane CI-plasma. With the exception of benzene and chlorobenzene, the detection limits were all 1 ppm or lower. It is noteworthy that these two analyteshave the lowest proton affinitiesamong all the compounds listed in Table I. In many cases the Flgure 5.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
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Table 11. Measured Detection Limits in Toluene compd acetone 2,3-butanediol dimethyl sulfoxide ethyl acetate methanol ethanol hexanoic acid benzene chlorobenzene
method toluene CI toluene CI toluene CI toluene CI MSIMS methane CI methane CI methane CI methane CI methane CI
measured ion [M + HI+ [M + H - HzOl+
mlz 59 73 79
[M + HI+ [M + HI+ - CzH4
61
[M + HI+ [M + HI+ [M + HI+ [M + HI+ [M + H - HCll+
33 47 117 79 77
detection limit, ppm 0.2
proton affinity) kcallmol
0.2
197 212 211
10
201
0.5
10 10 25 100
100
182 188 194 181 182
By tandem mass spectrometry (see text). * Data from ref 28. IC
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Flgvr 6. Successbe injectionsof increasing concentratlons of acetone in hexane: mlz 59 is monitored. 57
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m/z Flgurr 7. Methane C I mass spectrum of gasoline introduced via the
polypropylene membrane probe.
detection limits were limited by a chemical background from ions in the hexane CI-plasma. This is demonstrated in Figure 5a, which shows single-ion monitoring data of protonated dimethyl sulfoxide for two successive injections of 250 ppb of dimethyl sulfoxide in hexane. Table I1 shows the measured detection limits when the analytes were dissolved in toluene. The four compounds (acetone, 2,3-butanediol, dimethyl sulfoxide, and ethyl acetate) with proton affinities of 197 kcallmol or larger were easily protonated in the toluene CI-plasma,whereas the others could not be effectively protonated in this CI-plasma. This finding is in good agreement with the PA value of C6H&H2* (199.1kcaltmol~), the precursor for the toluene radical cation. The detection limits for acetone, 2,3-butanediol, and dimethyl sulfoxide were in the ppb range, and the actual values
are comparable to those observed for the same compounds dissolved in hexane. As an example of this, Figure 5b shows the result of three successiveinjections of 200 ppb of acetone in toluene. Protonated ethyl acetate, mlz 89, coincided with a very strong [C,H6]+ ion from the toluene CI-plasma, and it was not possible to detect amounts less than 200 ppm in toluene using single-stage mass spectrometry. However, by the use of tandem mass spectrometry the two different ions at mlz 89 could easily be distinguished. A collision-induced dissociation product spectrum of mlz 89 obtained from a solution of 0.1 7% ethyl acetate in toluene consisted of mlz 39 (7) [C3H31+ and 63 (10) [C6H31+ from toluene and 29 (28) [CH3CH2]+,43(7) [CH3CO]+,and61(43)[CH&OOH+ HI+ from ethyl acetate (relative abundances in parentheses, mlz 89 = 100%1. These assignments were confirmed by recording M S I M S product ion mass spectra of pure toluene and of ethyl acetate dissolved in hexane. Hexane was used as solvent to obtain the standard ethyl acetate spectrum since hexane has no mlz 89 ion in its CI-plasma. By monitoring the mlz 61 product ion of the mlz 89 precursor ion, the detection limit for ethyl acetate in toluene could be reduced to 1ppm. The detection limits for the remaining compounds listed in Table I1 were measured by using single-stage mass spectrometry and methane CI according to the procedure described in the section on chemical ionization conditions. In this way detection limits in the low-ppm range were obtained. The limits are roughly 50 times higher than the detection limits observed for the same compounds dissolved in hexane, where they were ionized directly in the hexane CI-plasma. For quantitative purposes, a wide linear dynamic range is desirable for any analytical method. In this case the linearity of the system was tested with ethanol, acetone, hexanoic acid, 2,3-butanediol, and ethyl acetate dissolved in hexane by injecting standard solutions with different concentrations. Figure 6 shows the result of these measurements for acetone. In the cases of all the analytes a linear relationship between concentration and signal intensity was observed in the ppm range. However, at higher concentrations, we expect that the response of one compound might be affected by the presence of other compounds with higher PA in the anaiyte.19 In order to test the general validity of the reversed-phase MIMS technique, the ability to measure dimethyl sulfoxide dissolved in ethanol, acetone, benzene, and chlorobenzene was established. The major ions extracted from the CI-plasma created by these solvents were the proton-bound dimers in the cases of ethanol and acetone and the molecular radical cation in the aromatic solvents benzene and chlorobenzene. The protonated molecular ion of dimethyl sulfoxide (mlz 79) was easily measured a t the low-ppm level in the ethanol and acetone CI-plasma using single-stage mass spectrometry. A high background level a t mlz 79 in the aromatic CI-plasmas made it necessary to use MSIMS in order to obtain detection
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
Table 111. Product Ions from m / z 85
hexane 0.1% thiophene in hexane 0.1 % thiophene in toluene gaeoline a gasoline b
27 16 20 0.2 14 13
29 15 16 0 11 12
39 0.4 0.5 5 1 0.9
96 relative abundance, at mlz 41 43 19 100 100 23 100 6 16 100 15 100
57
45 0 0.1 38 0 0
57 1 1 0 2 2
1
85 13 10 84 8 9
total counta 1.57 x 107 1.44x 107 2.45 X 106 1.17 X 106 1.36 X 106
I
(min)
b
57
'0°1
I
m/z
Flgwe 8. Colllslon-Induced dissociation product spectra of mlz 89 obtained from (a)gasoline and (b) 100 ppm of tertautyl methyl ether In hexane.
limits in the ppm levels. Therefore, it appears that the technique describedhere can be used with almost any solvent that passes the membrane in sufficient amounts to create conditions for chemical ionization. However, the detection limits achieved depend strongly on whether the analyte has a proton affmity higher than the solvent or not and on whether the chemically ionized analyte ion coincides with any ion in the chemical background from the solvent CI-plasma. If the proton affinity is higher, the detection limits will typically be in the sub-ppm range, whereas the limits will be in the ppm range whenever methane has to be used as reagent gas. The reason for the lower limits is that the total flow of analyte (solvent) into the ion source has to be reduced before proper conditions for methane CI can be set up. Better gas handling in the CI source may make it possible to improve this restriction in the future. On the other hand, response times can be expected to depend only slightly on the solvent, and typically they will be shorter than 10 s. Gasoline Samples. In order to demonstrate the practical usefulness of this technique, we have used it to analyze some minor compounds present in gasoline. Gasoline is a very complex mixture consisting primarily of alkanes and methylated benzenes and naphthalenes. This is clearly demonstrated by the measured methane CI mass spectrum (Figure 7) obtained when a sample of gasoline was passed through the membrane probe. The major ions in the spectrum
Flgure 9. Slnglareactlon monitoring data of the reactlon m/z 89+45 product lon during a screenlng of three gasollnesamples for thlophene. (a)Toluene: (b) gasoline: (c) toluene; (d) gasoline doped with 100 ppm of thiophene.
observed at mlz 43,57,71,85, and 99 are typical of the alkanes, whereas the ions observed at mlz 93,107,121,135, and 149 are characteristic of the methylated benzenes. Although relative abundances varied from sample to sample, this pattern of ions was found in all the gasoline samples examined. tert-Butyl methyl ether is an important additive in gasoline that is used to ensure smooth combustion. We have therefore studied the possibilities of measuring this compound with the reversed-phase MIMS technique. Figure 8a shows the collision-induced dissociation product spectrum of mlz 89 obtained from a gasoline sample, and Figure 8b shows the collision-induced dissociation spectrum of the protonated molecule of tert-butyl methyl ether. The standard spectrum (Figure 8b) of tert-butyl methyl ether was obtained from a solution of 100 ppm of tert-butyl methyl ether dissolved in hexane. We used hexane as solvent for the standard solution since hexane has no CI ions at the mlz 89 parent. The product ion spectrum of the [M+ HI+ ion of tert-butyl methyl ether shows product ions at mlz 57,41,and 29. The presence of the same ions with the same relative abundances in the spectrum obtained from gasoline confirms the presence of tert-butyl methyl ether in gasoline. The two other ions at mlz 39 and 63 in the gasoline spectrum are derived from toluene, a major component in gasoline which has a relatively intense [C,Hs]+ ion in the CI spectrum. Several gasoline samples obtained from two different gas stations were measured, and the concentration of tert-butyl methyl ether were estimated by comparison with standard solutions of tertbutyl methyl ether dissolved in hexane. With the exception of one sample, which had a concentration of approximately 7 % ,the concentrations were found to be in the range of 1G 100 ppm. These findings are consistent with literature values,30 which show that gasoline samples with added tertbutyl methyl ether contain between 0.4% and 10% tert-butyl methyl ether. Even a sample containing only 10 ppm of tert-butyl methyl ether gave a product ion spectrum that allowed a positive identification of the compound. (30)Pauls, R.E.J. Chromatogr. Sci. 1986,23,437-441.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
Thiophene is a possible impurity in gasoline and other fuels; we have therefore studied the possibility of measuring this compound. The measurement of thiophene in gasoline is complicated by the fact that the major CI ion (mlz 85) from hexane [M - HI+ coincides with the protonated molecular ion [M + HI+ from thiophene. Therefore, a characteristic product ion from the protonated thiophene must be found in order to measure the concentration of thiophene in gasoline. Table I11summarizesthe resulta of recording collision-induced dissociation product spectra of the ion a t mlz 85 from hexane, 0.1 % thiophene in hexane, 0.1 % thiophene in toluene, and two gasoline samples. Toluene was used as a solvent for the standard spectrum of thiophene, since toluene has no CI fragment ion at mlz 85. Only the spectra with added thiophene has a product ion at mlz 45, although this ion can hardly be seen on the large hexane background in the spectrum obtained from the solution of 0.1 % thiophene in hexane. The spectra from the two gasoline samples are almost identical and very similar to the spectrum obtained from hexane. Since mlz 45 was only observed in the product ion spectra with thiophene added, this ion was used in single-reaction monitoring to screen gasoline samples for thiophene. Figure 9 shows the result of such a screening experiment where the membrane was flushed successively with toluene, gasoline, toluene, and gasoline doped with 100 ppm of thiophene. The sequence was repeated for three different gasoline samples.
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Whereas the gasoline samples doped with 100 ppm of thiophene show a large signal, the pure gasoline samples only show a very small noisy signal. However, since toluene gives a zero baseline, there is definitely some mlz 45 product ions from the pure gasoline samples. The experiment suggests that the concentration of thiophene in the gasoline samples was a few ppm or less. The measurements of tert-butylmethyl ether and thiophene in gasoline samples demonstrate that reversed-phase membrane introduction tandem mass spectrometry may have applicability for the measurement and identification of target compounds in complex matrixes such as gasoline.
ACKNOWLEDGMENT The work was supported by the National Science Foundation (EET 87-12867) and by BP America. F.R.L. acknowledges the support from the Danish Center for Process Biotechnology and the Danish Natural Science Research Council, and T.K. acknowledges support from the Emil Aaltonen Foundation. RECEIVED for review December 16, 1991. Accepted March 9, 1992.
