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Anal. Chern. 1985, 57,786-789
Neutral Reactions in Gas Chromatography/Chemical Ionization Mass Spectrometry Patrick Rudewicz and Burnaby Munson*
Department of Chemistry, University of Delaware, Newark, Delaware 19716
The use of NH, as a carrier gas and as a reagent gas produces neutral reactlons between NH, and carbonyl compounds to glve bask products which are detected by NH, chemical loniratlon (CI)In the mass spectrometer. Reactions are observed with aldehydes and ketones but not alcohols, ethers, or esters. Postcolumn addltlon of NH, or CH,NH, also gives neutral products which can be detected by NH, CI. On-column reactlons are observed with carbonyl compounds by the Injection of an amine lmmedlalely after the lntroductlon of the sample. On-column reactlons are also observed by adding acetone or acetic anhydride lo a mlxture of amines.
The ammonia chemical ionization (NH3 CI) mass spectra of certain aldehydes and ketones contain ions corresponding to protonated Schiff bases or imines which are also isobaric with the molecular ions. The mechanism of formation of the imines has not been clearly established. An early report indicated that aldehydes but not ketones formed protonated imines in a two-step process involving a neutral gas phase reaction between ammonia and the aldehyde to form the imine and subsequent protonation of the imine by NH4+ (1). In a later study, however, the ammonia CI spectrum of cyclohexanone was reported to contain the protonated imine formed not by a two-step mechanism involving a neutral reaction but by a single-step ion/molecule reaction ( 2 ) . Finally, it has been suggested that protonated imines in the NH3 CI mass spectra of 3-keto bile acid derivatives are formed by the two-step process of neutral reaction with NH3 followed by protonation by NH4+ (3). Protonated imines were not observed with 7-keto or 12-keto bile acid derivatives. We have observed neutral reactions between ammonia and various ketones and aldehydes on packed GC columns, using ammonia as the GC carrier gas. The products of these oncolumn reactions elute from the gas chromatograph into the source of the mass spectrometer and are protonated under ammonia GI conditions. Alcohols, ethers, and esters do not react with ammonia in neutral on-column reactions to give detectable products under ammonia CI conditions. These on-column reactions with NH3 as the carrier gas are very similar to experiments in reaction chromatography which have been reported previously (4, 5 ) . In reaction chromatography, compounds having a particular functional group may be completely removed from the chromatogram (subtractive chromatography) (6) or converted to volatile derivatives (peak shifting) (7) by precolumn, on-column, or postcolumn reactions. Pyrolytic and catalytic reactions have also been employed ( 4 9 ) . On-column hydrogen/deuterium exchange reactions have been carried out using Carbowax columns pretreated with deuterium oxide (10). Somewhat more recently, reaction chromatography has been combined with mass spectrometry for the conversion of selected classes of compounds to derivatives whose electron ionization mass spectra are more informative (11). EXPERIMENTAL SECTION These experiments were done with a Du Pont 21-492B mass spectrometer (Hewlett-Packard 21MX computer, Du Pont data
system) and a Varian 2740 gas chromatograph. The source pressure was measured with a MKS Baratron capacitance manometer (MKS Instruments, Burlington, MA) connected to the source through the probe inlet. The reagent gas pressure was 0.5 torr and the source temperature was kept between 160 "C and 180 "C. The ammonia, used for both the reagent gas and the GC carrier gas, was obtained from Matheson (anhydrous 99.99% min). The electron energy was 75 eV and the emission current was 250 fiA. The repeller voltage was set to zero and the accelerating voltage was approximately 1750 V. The majority of the experiments were done with a 6 ft X 1 / 4 in. glass column, packed with 3% SP-2100on 80/lOO mesh Supelcoport. The performance of the column before and after aproximately 80 h of use with ammonia as the carrier gas was checked with a standard polarity mixture. No significant degradation in column performance was noted. The resolution for the separation of 2,4-dimethylaniline and naphthalene in a programmed temperature experiment was 1.2 before and 1.2 after 80 h of use with ammonia as the carrier gas. Chromatographic efficiency appeared to be about the same, but full characterization studies were not performed. For the postcolumn derivatization experiments, He was used as the GC carrier gas and the reactive gas was introduced via a Swagelok tee placed directly behind the GC column. RESULTS AND DISCUSSION The ammonia CI mass spectrum of acetophenone is shown in Figure 1. This spectrum was obtained from a GC/CIMS experiment with He as the carrier gas in the gas chromatograph and ammonia as the CI reagent gas in the mass spectrometer source: P(He) = 0.07 torr; P(NH3) = 0.46 torr; t = 175 "C. In this spectrum there are essentially no ions at m / z 120, neither C&&+ from acetophenone nor CsHloNf as the protonated imine; I(120) < 0.1% of the total sample ionization. The spectrum is a very simple one: predominantly the (M + NH4)+adduct with a small amount of solvated adduct, (M NzH7)+,and small traces of (M H)+ ions. This spectrum agrees with earlier work (1). The reaction time for neutral acetophenone with ammonia within the source of the mass spectrometer is not known; however, experiments under similar conditions with another instrument (CEC-110) suggest that the residence times of neutral molecules within the source of the mass spectrometer in these experiments are only a few tenths of a second (12). Comparisons of peak widths obtained with a flame ionization detector (FID) and with the mass spectrometer give no indications of significant peak broadening due to retention within the source of the mass spectrometer. With ammonia as a GC carrier gas, acetophenone forms an imine in a condensation reaction in the gas chromatograph C6H,COCH, + NH3 CGHBC(NH)CH, + HzO (1)
+
+
+
The imine elutes from the column and is protonated in the ion source to give m / z 120 as the major ion in the spectrum NHd(NHs),+
+ CeH&(NH)CH3
+
C6H5C(NH2)CH,++ ( x
+ l)NH3 (2)
Precise mass measurements show that the ion at m / z 120 in these experiments is C8HloN+,not the molecular ion for acetophenone, C&&+. Not all of the acetophenone is converted to the imine and the unreacted acetophenone elutes and forms an adduct ion, (M + NH4)+,at m / z 138. Selected
0003-2700/85/0357-0788$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
(M+NHs* "4)
+
7606 1 '
li
m/z 155
I00
160
787
2
3
800
M/Z
Figure 1. Ammonia GC CI mass spectrum of acetophenone using He as a carrier gas and ammonia as a reagent gas: source pressure, 0.07 torr He, 0.046 torr NH,; source temperature, 175 O C .
(b'
.
1
Scan Number
100
+ NH$ adduct Ions of (1) cyclohexanone, (2) acetophenone, and (3) 2-octanone. He is the GC carrier gas with postcolumn introduction of ammonia for reaction and as the CI reagent gas. (b) Single ion traces for the (M -I-H)+ ions of imine derivatives wPh NH3 of (1) cyclohexanone, (2) acetophenone, and (3) P-octanone. He is the GC carrier gas with postcolumn introduction of ammonia for reaction and as the CI reagent gas. Flgure 3. (a) Single Ion traces for the (M
+ NHJ+ is about 15, whereas in the conventional CI experiment, with He as the carrier gas and ammonia as the reagent gas, virtually no (M + NH4 - HzO)+ ions are detected.
i 1
+ I ,
26
34
43
Si
59
SCAN NUMBER
Figure 2. Single ion traces for the protonated imine of acetophenone, m l z 120, and the adduct ion for the unreacted acetophenone, m l z 138, using ammonia as the GC carrier gas and the CI reagent gas.
ion monitoring shows that the imine and the unreacted acetophenone have different chromatographic peak profiles, Figure 2. The ratio of the areas of the two peaks is an indication of the extensive conversion of acetophenone to the imine. However, the ratio of the peak areas is not the same as the mole ratio since the CI sensitivity for acetophenone with ammonia is much less than the sensitivities for amines and by inference the sensitivity of the imine. Imine formation is observed with several aliphatic and aromatic ketones and aldehydes and appears to be a general process under the relatively mild conditions of 100-150 "C and on-column times of a few minutes. For many of the compounds the imines elute as well-defined peaks which partially overlap the peak for the unreacted ketones and have widths a t half-height that are significantly wider than the half-widths of the peaks for the unreacted ketones and which have retention times that are slightly longer than the retention times of the corresponding unreacted ketones. A decrease in the flow of ammonia through the gas chromatograph increases the retention and reaction time of the carbonyl compounds and increases the extent of conversion to the imines. Slow flow rates and long reaction times induce extensive tailing in the peak profile for the imine without a similar effect on the peak profile for the unreacted ketone. Peak profiles for the imine depend on the chemical nature of the carbonyl compound as well as the physical parameters of the separation. Comparisons were made for several ketones between experiments with ammonia as the GC carrier gas and the GI reagent gas and with He as the GC carrier gas and ammonia as the CI reagent gas. For all these compounds trace amounts or none of the protonated imines was observed unless ammonia was the GC carrier gas and the CI reagent gas. With 5-nonanone, for example, with ammonia as the GC carrier gas and the GI reagent gas, the ratio of (M + NH,, - H,O)+/(M
Consequently, under our conditions, the overwhelming amount of (M NH4 - H20)+ions are protonated imines formed by a two-step process with the neutral reaction occurring prior to entry of the sample into the source of the mass spectrometer. The sometimes surprisingly well-shaped chromatographic peaks for the imines suggested formation in a narrow region or short time. Consequently, experiment,s were performed at different temperatures of the GC injection port and the same temperature for the GC oven and column. No significant and systematic changes were noted in the ratios of concentrations of unreacted carbonyl compounds to imines, as indicated by the ratios of the ion currents, (M + NH,)'/(M + NH, - H,O)+. Consequently, only a very small extent of reaction can be occurring in the injection port. Postcolumn reaction with ammonia (through the flame ionization detector oven at approximately 300 "C and the transfer line at approximately 125 "C for less than 7 s) gave small extents of conversion of carbonyl compounds to imines (Figure 3). Retention times for the imines and unreacted ketones are the same, although the half-widths of the peaks for the imines are somewhat larger than the half-widths of the peaks for the unreacted ketones. The peak broadening and tailing, shown in Figure 3, probably result from adsorption of the imines on the unsilanized glass transfer line betwen the gas chromatograph and the mass spectrometer and are not dependent on the reaction. The ratio of the ion currents for the protonated imine to that for the ammonium ion adduct of the ketone is roughly proportional to the extent of conversion. For a few ketones, this ratio was about 50 times larger (range of 15-70) for the on-column experiments than for the postcolumn experiments. Consequently, we consider that the reactions with ammonia as the GC carrier gas are occurring almost entirely within the chromatographic column. Figure 3 also shows differences in the extents of reaction of ammonia with three ketones. Previous experiments indicate that the sensitivities with ammonia for strongly basic secondary amines are essentially constant; therefore, we assume that the sensitivities of these imines are roughly the same and
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
that the areas under the peaks for the protonated imine ions are directly proportional to the extents of reaction with the same proportionality constant. Since there are equimolar amounts of cyclohexanone, 2-octanone, and acetophenone in the mixture and, in the postcolumn experiment, the reaction times are the same for all three compounds, we say that the neutral reaction of ammonia with cyclohexanone is much faster (approximately a factor of 10) than the reaction of neutral ammonia with 2-octanone or with acetophenone. The data from the on-column experiments do not provide so direct a comparison, but they also indicate a greater reactivity of ammonia with cyclohexanone than with 2-octanone or acetophenone. One cannot determine the extent of conversion for aldehydes because the ammonia CI sensitivity for the unreacted aldehyde is too low to provide reliable data. However, postcolumn experiments, like those of Figure 3, indicate a slightly higher reactivity for ammonia with hexanal than with cyclohexanone. The postcolumn experiments clearly indicate that some of the reaction can be occurring in the gas phase in the chromatographic column. However, we have not determined if the dominant process occurs in the gas phase or in the liquid coating of the support. One preliminary experiment indicated an increase in the extent of conversion of acetophenone to the imine for on-column reactions using a Carbowax rather than a silicone column. The order of reactivity of these compounds with ammonia in the gas chromatograph roughly correlates with the reported reactivity of carbonyl compounds with ammonia in solution: i.e., aldehydes are more reactive than ketones with sterically hindered ketones being particularly unreactive (13). Aromatic ketones are even less reactive than aliphatic ketones. In solution, the condensation of acetophenone and ammonia requires an aluminum chloride catalyst and a 4-h reaction time a t 180 "C (14). The diones, 2,4-pentanedione and 2,3-pentanedione, primarily condense with ammonia a t one carbonyl site, since essentially none (
