86
Anal. Chem. 1989, 6 1 , 86-88
TECHNICAL NOTES Indium-Alkene Complex Ions as Reagents for Selective Chemical Ionization A. Kasem Chowdhury, John R. Cooper, and Charles L. Wilkins* Department of Chemistry, University of California, Riverside, Riverside, California 92521 A major analytical goal of chemical ionization (CI) mass spectrometric analysis of mixtures is to selectively ionize components while minimizing fragmentation and chemical interferences. For such analyses to be feasible, suitably specific chemical ionization reagents are necessary and should provide both unambiguous molecular weight information and the capability for quantitative target compound analysis. Selective chemical ionization also can serve as the first step in tandem mass spectrometric analyses (MS/MS) of mixtures where structure elucidation is the goal. Although there has been a good deal of progress in development of selective reagents for , applications to direct mixture chemical ionization ( l ) their analysis are still relatively unexplored. Use of metals or metal complex ions as CI reagents is emerging as an area of active current research interest (2-9). We recently suggested (10) that indium-alkene complex ions might be promising as reagents for selective chemical ionization mass spectrometry. Here, we demonstrate the potential of this method by applying it to analysis of a model mixture and to an unleaded gasoline sample using InC3H6+generated via laser desorption Fourier transform mass spectrometry (LD-FTMS) as a chemical ionization reagent.
EXPERIMENTAL SECTION Reagents and Inlet System. All chemicals were purchased from Aldrich Chemical Co., except unleaded gasoline which was obtained from a local gas station. A stock solution of model mixture was prepared by mixing 5-10 pL of the reagents listed in Table I. About 1 mg of naphthalene was added and shaken well to dissolve in the stock solution. The sample inlet system (Nicolet FTMS-2000) allows introduction of volatile samples through an expansion volume with a fixed molecular leak. The analyte mixture (10 pL) was taken in a clean sample tube and connected to one of the compression fittings. After freezepumpthaw cycles, the mixture was allowed to expand and introduced into the mass spectrometer through a leak valve. 2-Chloropropane was introduced into the system through a separate inlet. Instrumentationand Procedures. Experiments were carried out by using a Nicolet FTMS-2000 Fourier transform mass spectrometer equipped with a dual 4.78-cm3cell and a 7-T superconducting magnet. Indium ions were generated by focusing the beam of a Tachisto 215G pulsed TEA C02 laser onto an indium foil target mounted upon a stainless steel probe tip. The laser delivered approximately 0.3-0.4 J per 40-11s pulse at 10.6 pm. Following the ion formation laser pulse, positive indium ions were trapped in the source cell for variable times (from 300 to 1000 ms) to allow ion-molecule reactions with 2-chloropropane, which was present with a pressure of 1 X lo4 Torr, producing the indium-propene reagent ion (eq I). When an analyte mixture is also present in the FTMS cell, at similar pressure (ca. 2 X lo4 Torr), the In complex ion subsequently cationizes mixture components, yielding In+(M) ions (eq 2). Such ions are formed for those components with Do(In+(M))> Do(In+(C3H6)), provided they are present with sufficient vapor pressure to react during the delay period prior to spectral measurement. In+ + CH3CHC1CH3 hC&+ + HC1 (1)
-
InC3H6+ + M
+
In+(M) + C3H6
(2)
Table I. Mass Spectrum of Known Mixture nominal mass, amu 115 175 193 201
In+ (In + CH30H)+ (In + PrOH)+ (In + C&)' (In + C2H6COC2H6
203
(In + C2H,&02CH3
147
5 pL 5 pL 5 pL 10 p L
methanol propanol benzene 3-pentanone
10 p L
methyl propionate
5 pL 5 pL
toluene ethylbenzene
1 mg
naphthalene p-bromobenzaldehyde
)+
I+
(In + C6H&H3)+ 221 (In + CJ%CzW+ not detected (In + CloH,)+ not detected (In + C7H5BrO)+ 207
mixture compound
ion composition quantity
5 pL
Table 11. Mass Spectrum of Unleaded Gasoline
nominal mass, amu 115
175 185 193 199 207 213 221
235 249 263
ion
composition In+ (In + PrOH)+ (In + C5Hro)' (In + C6H6)+ (In + CsHlZ)+ (In + C7Hl0)' (In + C7HJ (In + CBH$
component
propanol pentene benzene hexene toluene heptene ethylbenzene or xylenes (In + C,H,,)+ propylbenzene or isomers (In + C10H14)+butylbenzene or isomers (In + CllH16)' pentylbenzene or isomers
component verified by GC/MSD
pentene isomers benzene hexene isomers toluene heptene isomers ethylbenzene and xylenes propylbenzene and isomers butylbenzene and isomers pentylbenzene isomers
To verify the identity of the cationized molecular ions listed in Table 11, the unleaded gasoline sample was separated by capillary gas chromatography and analyzed with a Hewlett-Packard 5970 mass selective detector (GC/MSD). A Varian 3700 GC was used and programmed from 45 to 230 "C at 6 'C/min. after a hold time of 5 min. A volume of 0.2 p L of gasoline was introduced (with on-column injection) into a 60 m X 0.32 pm i.d. capillary column, with a 1 pM thick 5% phenyl methyl polysiloxane stationary phase (DB-5). Helium carrier gas at 23 psi provided a 3 mL/min flow rate at 50 'C. An effluent flow tee provided a split ratio of 95:5, resulting in an MSD background pressure of 2 X lo4 Torr.
RESULTS AND DISCUSSION Figure 1is the Fourier transform mass spectrum of a known mixture of alcohols, esters, alkenes, and aromatic compounds comprised of the components listed in Table I. Seven of the nine are cationized and are readily observed in the spectrum. Two of the components (naphthalene and bromobenz-
0003-2700/89/0361-0086$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 1, JANUARY 1, 1989
87
lox ORDINATE EXPANSION 1
I
\,,j
i
.
(
,
,
,
11,,;,J MINUTES
0
200 MASS IN A . M . U .
150
100
250
300
Fourier transform mass spectrum of the nine-component model mixture.
Flgure 1.
175
0
I . . ,
160
Flgure 2.
I
,
.
.
.
.
,
150
.
.
1
..I. MASS
260
IN
A.M.U.
,I!,
.I
Total ion chromatogram of gasoline sample analyzed by GClMSD. Indicated peaks ( * ) are butylbenzene isomers, corresponding to m l r 249 in Figure 2. Flgure 3.
249
263 , . I , .
250
,
, , ,
3d0
Fourier transform mass spectrum of an unleaded gasoline
sample. aldehyde) were not detected because their partial pressures were too low. Cationization with In+ was established by separate experiments with the two pure compounds, establishing that both yield abundant In+(M) ions. As an application of the method to an unknown, a sample of unleaded gasoline was analyzed, with the results summarized in Table 11. From the spectrum, Figure 2, it was possible to identify not only a number of the unsaturated hydrocarbons present but an alcohol additive which was present in low concentration. Selective chemical ionization of alcohol additives in unleaded gasoline using GC/CI-MS recently was reported (11). The identity of the compounds corresponding to the cationized molecular ions were further verified by the GC/MSD analysis. Note that all ions recorded in Figure 2 are indium-attachment ions. Thus, m / z 249, after subtracting the mass of indium (115 daltons) corresponds to the C10H14 isomers discussed next. The MSD reconstructed chromatogram from the total ion current (TIC) is shown in Figure 3. The axis has been separated to better illustrate the complexity. An ordinate expansion has also been applied to reveal the lower concentration components. The maximum TIC signal for this sample is 1.85 x lo6counts for a peak near 3.7 min. The chemical dynamic range is therefore a t least a factor of 200, assuming a linear response at the upper range. Selected ion chromatograms (SIC) were produced for each compound or compound class in Table I1 to aid in determining retention times and possible number of isomers present. SIC’S for m / z 78 and 92 each revealed only a single component, corresponding to benzene and toluene, respectively. For the aliphatic alkenes, SIC‘Swith m / z 70 (C5), 69 (C6), and 98 (C,)were utilized. For the alkyl-substituted aromatics, the molecular ions with m / z 106,
20
Flgure 4.
MINUTES
22
24
26
(A) Mass-selective detector mass spectrum of a butyl-
benzene isomer from gasoline. (B) Selected ion chromatogram of unleaded gasoline generated by monitoring m l z 134 with the MSD. Indicated peaks (*) are butylbenzene isomers. (C) Total ion chromatogram trace for time range in B. 120, and 134 were used. For the pentylbenzene isomers, the (M - 15) ion with m / z 133 provided a more reliable SIC than the less abundant m / z 148 molecular ion. In all cases, manual inspection of each peak in each SIC (near the estimated elution times) was performed due to the overlap in detection possible when using certain ions. Library searches with HewlettPackard PBM software were also used where required. A mass spectrum typical of eight of the C10H14 isomers is shown in Figure 4A. With the relatively abundant ion at m/z 134 monitored, the SIC shown in Figure 4B was generated. The indicated peaks revealed 11 components, which were identified as CloH1, isomers. Two of the eleven isomers, occurring with retention times of 22.12 and 22.67 min in the reconstructed chromatogram,gave ions of m/z 105 as the base peak rather than m / z 119 as shown in Figure 4A. An additional isomer with a retention time of 20.77 min had m / z 91 as the base peak, indicating it was an n-alkyl-substituted benzene. For comparison, the same section of the TIC is shown in Figure 4C. The group of peaks preceding these isomers includes the propylbenzenes. Although many of the components detected by using the SIC are also discernible in the TIC, selected ion monitoring was imperative for the location of the alkenes by conventional GC/MS due to the significant interference from aliphatic components in the early part of the separation. Retention times for selected compounds found in this sample are listed in Table 111. The present chemical ionization method, not requiring a prior separation step, can provide complementary information for selective analysis of mixtures. As is seen from the present results (Table 11),the indium reagent ion is selective for al-
Anal. Chem. 1989, 6 1 , 88-90
88 T a b l e 111. R e t e n t i o n T i m e s (minutes): pentene isomers (6) 3.500 3.824 3.900 4.056 4.167 4.238 benzene 8.070 hexene isomers (7) 5.207 5.651 5.965 6.086 6.158 6.269 6.908 toluene 12.214 heptene isomers (3) 8.771 8.996 8.914 ethylbenzene; xylenes (3) 15.731 16.148 16.972
Gasoline Sample propylbenzenes (6) 18.971 19.312 19.509 19.915 20.459 21.378 butylbenzenes (11) 20.774 21.172 22.122 22.312 22.665 22.950 23.022 23.216 23.911 24.174 24.339 pentylbenzenes (8) 23.615 24.655 24.849 25.105 25.792 26.485 26.613 28.896
kenes and aromatic species and discriminates against alkanes present in a complex mixture. Although the indium-alkene reagent ion reacts with some monohalogenated organic compounds, it is totally inert toward polyhalogenated species.
Thus, the method shows promise for selective analysis of mixtures containing alkanes and polyhalogenated species, together with unsaturated analytes of interest. Specific potential applications include qualitative analysis, where compound or compound class confirmation may be performed without prior separation of the mixture. As is also clear from the present results, this method is complementary to conventional GC/MS in that it yields a single response for each group of isomers. We are currently investigating the use of probe distillation of polymer additives using the indiumpropene reagent ion for qualitative and quantitative analysis.
LITERATURE CITED Harrison, A. G. Chemical Ionization Mass Spectrometry; CRC Press: Boca Raton, FL, 1983. Rollgen, F. W.; Schuiten, H A . Org. Mass Spectrom. 1075, IO, 660. Hodges, R. V.: Beauchamp, J. L. Anal. Chern. 1076, 48. 825. Burnier, R . C.; Byrd, G. D.; Freiser, B. S. Anal. Chem. 1980, 52, 1641. Bombick, D.; Pinkston, J. D.; Allison, J. Anal. Chem. 1084, 56, 396. Peake, D. A.; Gross, M. L. Anal. Chem. 1085, 57, 115. Forbes, R. A.; Tews, E. C.; Frelser, B. S.; Wise, M. B.; Perone, S. P. Anal. Chem. 1088, 58, 684. Schmelzeisen-Redecker, G.; Geissmann, U.; Rollgen, F. W. Org. Mass Spectrom. 1085, 20, 303. Peake, D. A.; Huang, S. K.; Gross, M. L. Anal. Chem. 1987, 59, 1557. Chowdhury, A. K.; Wilkins, C. L. Int. J. Mass Spectrom. Ion Pfocesses 1088, 82, 163. Orlando, R.; Munson, B. Anal. Chem. 1086, 58, 2788.
RECEIVED for review April 20, 1988. Accepted October 11, 1988. We gratefully acknowledge support from the National Institutes of Health (GM-30604) and the National Science Foundation (CHE-85-19087).
Extrapolating Gas Chromatographic Retention Indices at Two Temperatures to a Third Temperature Stephen J. Hawkes Department of Chemistry, Oregon State University, Coruallis, Oregon 97331 Published collections of retention indices seldom record the indices at more than two temperatures. To interpolate or extrapolate from these to compare with an experimental value at another temperature, some linear relation must be assumed, because any other relation would require a fit to at least three values. The relation has been investigated a number of times (1-5) and fortunately the retention index is found to be nearly linear with temperature. This note explores the reason for the linearity, the circumstances under which an assumption of linearity must be treated cautiously, when the assumption is valid, and how much uncertainty the assumption of linearity adds to the extrapolated value. We shall show that under any real chromatographic conditions it is never wholly unreliable but will often give error beyond the limits of experimental error. In some extreme cases the error may be as high as 0.1 index unit per degree. The relationship has become important with the publication of precise retention indices in ref 6, which are usually given at only two temperatures.
THEORY Since this note explores the limitations of linearity, it is necessary to derive a ielationship and expose and discuss the assumptions that are needed to reduce it to linearity. The first step in this derivation is to redefine the retention index I in terms of the free energy of partition AGOCH~ asso-
ciated with a methylene group. The definition of I is the starting point log t’- log t’,
I = 100log t ;+I - log t;
+ lOOn
(1)
where t’is the corrected retention time (the total retention time minus the retention time of an unretained substance) and t’” and t’,+l are the corrected retention times of two successive n-alkanes with carbon numbers n and n 1 bracketing the analyte. Since t’is linear with the partition coefficient K and log K is linear with the free energy of partition AGO, it follows that
+
I = 100
AGO - AGO,
AGO,+, - AGO,
+ lOOn
I t would be easiest to assume here that AGO,, = nAGcH, but we avoid assuming that AG is linear with carbon number by putting AGO, = (n
+ G)AGOCH~
(3)
where 6 represents the error in the assumption of linearity and is probably negligible. We further note that AGOn+, - AGO, = AGOCH~
0003-2700/89/0361-0088501.50/00 1988 American Chemical Society
(4)
