754
Anal. Chem. 1983. 55. 754-757
6 orders of magnitude from the low picogram to the low microgram range. The development of the thermospray technique has reached the point that it appears to be a useful analytical tool both for producing spectra of nonvolatile, thermally labile molecules and for use in an on-line LC/MS system. Results similar to those presented here have been obtained for a variety of other classes of compounds including amino acids, small peptides, nucleosides, and antibiotics. Despite these successes, some additional research is required to develop a system which is routinely applicable to a broad range of analytical problems. Many details of the mechanism of ionization are not yet well understood and the optimum configuration of the vaporizer and ion sampling have not yet been established. Major limitations on the technique at present are the difficulties with day-to-day reproducibility of ionization efficiency and fragmentation patterns and the rather severe dependence of the ion intensities on the liquid flow.
ACKNOWLEDGMENT We thank C. R. Hsieh for preparing the figures for publi-
cation and H.-Y. Kim for assistance in obtaining some of the data presented. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11) (12) (13)
Arplno, P. J.; Gulchon, G. Anal. Chem. 1979, 51, 682A. McFadden, W. H. J. Chromafogr. Sci. 1979, 17, 2. Arplno, P. J. Trends Anal. Chem. 1982, 1 , 154. Arplno, P. J. Blomed. Mass Specfrom. 1982, 9 , 176. Blakley, C. R.; McAdams, M. J.; Vestal, M. L. J. Chromafogr. 1978, 158, 261. Blakley, C. R.; Carmody, J. J.; Vestal, M. L. Anal. Chem. 1980, 52, 1636. Blakley, C. R.; Carmcdy, J. J.; Vestal, M. L. J. Am. Chem. Soc. 1980, 102, 5931. Sissom, L. E.; Pltts, D. R. "Elements of Transport Phenomena"; McGraw-HIII: New York, 1972; pp 488-496 and 673-675. Vestal, M. L. Int. J . Mass Specfrum. Ion Phys. 1983, 4 6 , 193. Dcdd, E. E. J. Appl. Phys. 1953, 2 4 , 73. Katakuse, I.; Matsuo, T.; Wollnlk, H.; Matsuda. H. Org. Mass Specfrom. 1979, 14, 457. Cotter, R. J. Anal. Chem. 1980, 52, 1589A. Carroll, D. I.; Nowlln, J. G.; Stlllwell, R. N.; Hornlng, E. C. Anal. Chem. 1981, 53, 2007.
RECEIVED for review October 12, 1982. Accepted December 13, 1982. This work was supported by the Institute of the General Medical Sciences (NIH) under Grant GM 24031.
Nitrous Oxide as Reagent Gas for Positive Ion Chemical Ionization Mass Spectrometry Charles W. Polley, Jr., and Burnaby Munson" Department of Chemistry, Universiw of Delaware, Newark, Delaware
Nitrous oxide may be an analytkaily useful chemlcal ionization reagent gas for positive Ion as well as negative ion analyses. The low-energy NO' ion frequently gives M+ or (M NO)' ions for molecular welght determination. The higher energy N,O+ ion gives signiflcant amounts of fragmentation from dissoclative charge exchange with allphatlc systems.
+
Nitrous oxide, NzO, has the potential of being a very useful reagent gas in chemical ionization mass spectrometry (CIMS). Its use in mixtures with hydrocarbons to produce OH-for negative CIMS has been demonstrated (1-4). Nitrous oxide is somewhat unusual as a reagent gas in that reactions occur between negative ions from the sample and N20 (1). Reactions of 0- ions from N 2 0 have also been studied with organic compounds (5, 6). Little work has been reported on the positive ion chemistry of N20with complex organic molecules. It has been reported that NO+ and NzO+were stable in NzO at usual CI pressures, although some clustering is observed at high pressures (7,8).Since the ionization energy of N 2 0 is 12.9 eV and IE(N0) = 9.27 eV (9) there is an energetic reactant ion, N20+,which can produce structurally useful fragment ions and a low energy ion, NO+, which may give ions in the molecular weight range from hydride or electron transfer reactions. Nitrous oxide, then, may give CI spectra similar to those obtained previously with N2/N0 mixtures ( 1 0 , I l ) without the complications of handling NO mixtures. In addition, these may be reactions of positive ions with N20 analogous to the reactions with negative ion reported previously (1). 0003-2700/83/0355-0754$01.50/0
1971 1
EXPERIMENTAL SECTION The majority of these spectra were obtained with a CEC (Du Pont) 21-llOB mass spectrometer which has been described previously (12,13). The compounds used in these experiments were obtained from several commercial sources and were used without further purification. The nitrow oxide and nitrogen/lO% nitric oxide mixtures were obtained from Matheson, East Rutherford, NJ. The volatile liquids were introduced into the hot (200-250 "C) mass spectrometersource from a conventional heated oven or from a gas chromatograph. Solid samples were vaporized into the source from a glass capillary in the well of a separately heatable glass probe. NzO and Nz/lO% NO CI spectra were obtained at 0.5 h 0.1 ton:and an electron energy of approximately 600 eV. The precision in the relative abundances of ions in the spectra is f20%. Electron ionization (EI) spectra were obtained with conventional low pressure sources. The major fragment ions of NzO (Nz+,N+,and 0') react rapidly with NzO. The dominant ions in the high-pressurespectra of NzO are NO+ (33% of ionization)and NzO+ (63% of ionization). The relative abundances of these two ions are essentially constant from about 0.1 torr to the highest pressure used,0.6 torr. Small, variable abundanceswere noted for an ion of m/z = 45, presumably HNzO+ formed from reaction of NzO+with ever present traces of water or with the saniples. The association ions, NOf.NzO and Nz0+.N20were not detected at the high temperatures of these experiments. These observations are in agreement with earlier reports for large negative temperature coefficients for the formation of these cluster ions (8). The ratio, NO+/Nz+, in the Nz/lO% NO mixtures was approximately 3. RESULTS AND DISCUSSION Spectra were obtained for several classes of compounds with NzO and N2/10% NO as the reagent gases for comparison with 0 1983 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983 M/Z
4p
z
i
60
8p
12p
100
1po
755
Table I. N,O CI Spectra of Isomeric Octenes
110
N20 C I
% samrde ionization
r: v
c=c-c-c-c
2.0 2.2 3.9 69.4 0.8
41 55 56 57 C,H,+ 58 69 70 97 111 ( M - H)+ 112 M+ 130 nlz
Figure 1. A comparlson of the E I , N,/10% NO, and N,O C I mass spectra of camphene.
their electron ionization spectra. The N20 and N2/10% NO spectra of n-octadecane contained almost exclusively alkyl ions, but the extent of fragmentation was large: (M - H)+ = 2% of sample ionization and 8% of the base peak, C4H9+,with N 2 0 and 15% sample ionization and 83% of the base peak, CSH1l, with N2/10% NO. This abundance of (M - H)+ is much lower than the abundance of (M - H)+in the NO CI spectra of alkanes (70-80% of the total ionization) (14) and also lower than the relative abundance of (M - H)+ ions in the CHI CI spectra of n-alkanes (25-40% of total ionization) (15). For both N 2 0 and Nz/lO% NO the abundance of (M - H)+ ions is sufficiently high that the ion can be readily recognized. Significant amounts of (M - 3)+ and (M - 2 + NO)+ were not observed with either of these reagent gases. The relative abundances of these ions have been reported to decrease with increasing temperature with pure NO as a reagent gas and also to decrease as the NO is diluted with Nz (14). The absence of these ions in the N20 spectrum supports the idea that they are formed by reactions with NO. There are no produds which suggest reactions of alkyl ions with NzO. Figure 1shows the EI, Nz/lO% NO CI, and N20 CI spectra of camphene, C10H14. The N 2 0 spectrum contains no unexpected ions when compared with the E1 and N2/10% NO CI spectra. However, the adduct ion (M + NO)+ is not the dominant ion of the spectrum in contrast with the NO CI spectra of terminal olefins (16). Perhaps this smaller (M NO)+/M+ ratio results from the higher temperatures of these experiments than those reported previously. The presence of small amounts of (M - CH3 NO)+ ions (m/z = 151) in the NzO CI spectrum (as well as in the N2/10% NO) suggests that some of these ions are formed by decomposition of (M + NO)+ ions. The larger abundance of these and related ions in the N2/10% NO spectra suggests reactions of sample ions with NO. Similar spectra were observed for the isomeric compound d-limonene, for which the most abundant ion is M+, not (M - H)+ or (M + NO)+. Table I shows the N20 CI spectra of two isomeric octenes. The compounds are easily differentiable. Again, M+ is the abundant species, not (M - H)+ or (M + NO)+ in contrast to the NO CI spectra of these two compounds (16).The very abundant M+ ions in these spectra are perhaps produced by electron transfer of NzO+in which some of the excited molecular ions, M+*, are stabilized by collision. Alternatively, it may be that at these higher temperatures, the (M + NO)+ ions dissociate to give M+ ions. No products are observed that suggest reactions of sample ions with NzO. Table I1 shows partial mass spectra of three polynuclear aromatic hydrocarbons. The NO CI spectra of aromatic hy-
2.1
1.6 2.0 35.6 0.3 1.6 4.0 17.9
0.5
2.0 1.5 17.1 0.7
36.3
Table 11. Partial Spectra of Some Polynuclear Hydrocarbons % sample ionization
N,OCI
N,/10% NOCI
Anthracene, C,,H,, 0.7 1.0
167 176 177 178 Mt 179 180 194 (M + O)+ 195
74.7 20.4 2.1 0.6 0.2
1.7 82.7 13.8 0.8
9-Methylanthracene, C,,H,, 189 190 191 9.8 69.5 75.3 192 M+ 193 19.0 13.4 194 2.6 1.4 0.7 208 (M + O)+
E1 0.5 5.8 3.0 40.4 6.1 0.4
6.8 2.6 16.9 30.8 4.7 4.7
1-Methylphenanthrene,C,SH,, 189 190 191 192 M t 193 194
71.2 23.9 2.0
9.9 74.6 14.1 1.4
7.6 3.7 14.4 34.7 5.3 0.4
+
+
drocarbons and their derivatives contain predominantly M+ and (M NO)+ ions (17,18). It has also been noted that the ratio, (M NO)+/M+, decreases with decreasing ionization energy of the aromatic species: (M + NO)+ is the dominant species for benzene and M+ is the dominant species for naphthalene (18). Since the ionization energies of anthracene and phenanthrene are lower than the ionization energy of naphthalene, one would expect electron transfer reactions to NO+ to be rapid and essentially no adduct formation will be observed. The ratios of ionic abundances, 179/178 and 193/192, for the N20 CI spectra are significantly higher than the ratio expected for 13Conly. These ratios for the E1 spectra are approximately the correct values for I3C isotopes. Consequently, some proton transfer reactions must be occurring, probably from the hydrogen-containingimpurity ions among CI reactant ions. One observation that is potentially very important analytically is the observation of the small amount of (M + 0)+ions for the two anthracenes and the absence of this ion for methylphenanthrene. One short experiment showed that the (M + O)+/M+ratio for anthracene was essentially constant as the entire sample was evaporated from a probe. This experiment suggests that the two ions originate
+
+
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
from anthracene. This (M + 0)" ion may result from reactions of M+ from anthracene with NzO or from an 0" transfer reaction of NzO+. One notes here that N20 would be better than Nz/lO% NO (and NO) for quantitation of complex mixtures of polynuclear aromatic hydrocarbonsbecause of the lower abundance of the (M - H)+ ions. The spectra of long chain halides contain mostly hydrocarbon fragment ions. These spectra are, however, readily differentiable from the spectra of alkanes by the presence of significant concentrations of even mass olefin ions at each carbon number. Cyclic halonium ions, C4H&+ and C,H&+, are also observed in these charge exchange spectra. (M - X)+ ions are present of 1-2% of total ionization, possibly as the result of X- abstraction reactions of NO+. (M - HX + NO)+ ions are also observed at approximately 1% of t ~ t aionization, l perhaps from dissociation of (M NO)+ or perhaps from addition of NO+ to the small amounts of neutral olefins produced by thermal decomposition of the alkyl halides. These spectra contain only very small amounts of (M - H)+ ions, 0.2-0.3% of total ionization. Characterization of the halides from these NzO CI spectra, however, is difficult. Spectra of alcohols can be more readily used for identification. The N20 CI spectrum of 1-octadecanol contains the (M - H)+ ion as the base peak, 14% of total ionization, with another major ion at (M - OH)+, 6%. The (M - HzO)+ion is only present at 0.7% of the total ionization. Additional ions occur at each carbon number with masses nominally assigned as CnH2n-2+,CnH2n-l+,CnHPn+,and CnHzn+lwith the most abundant ion in each cluster being and the most abundant cluster at Cg. One unusual ion is observed at m/z = 241 which is also observed in the N2/10% NO CI spectrum but not in the E1 spectrum. Precise mass measurements have not been made for any of these ions. For comparison, the E1 spectrum of 1-octadecanolcontains essentially no M+ and only a low abundance of (M - H20)+ions, 0.5%. The N20 CI spectrum of the unsaturated steroidal alcohol, cholesterol is a simple one: M+, 30% of the total ionization as the base peak; (M - H20)+,10%; (M - OH)+, 10%. The next most significant fragment ion is at mlz = 301, (M C6H13)+,which is also abundant in the E1 mass spectrum. There is virtually no fragmentation of the steroidal nucleus. The abundant M+ ion for this alcohol results from ionization of the double bond. The much greater abundance of the (M - H20)+ion in this spectrum compared with the spectrum of 1-octadecanol is another reflection of the stability of the steroidal nucleus. It should be mentioned that the E1 spectrum of cholesterol also contains a very abundant M+ ion for molecular weight identification. The N20 CI spectra of 1- and 2-adamantanol shown in Figure 2 (like their E1 spectra) are readily differentiable. The most abundant ion in the spectrum of 1-adamantanol is the (M - OH)+ ion, 61% of sample ionization. This ion is presumably formed by abstraction of the tertiary hydroxyl group by NO+ since it is abundant in the Nz/lO% NO CI spectrum and minor in E1 spectrum. The (M - H)+ion is a minor one, about 0.7% of the total ionization. For 2-adamantanol, the (M - OH)+ ion is still the base peak, 28% of the total ionization, but the (M - H)+ ion is now a major one, 25% of total ionization. This difference is readily explained in terms of a greater ease of abstraction of a tertiary than of a secondary hydroxyl group and of a strong preference for abstraction of the hydrogen of the substituted carbon in preference to all others. These observations for the reactions of NO+ are consistent with early conclusions from the NO CI spectra of alcohols (19,20). Dissociative charge transfer from NzO+ to 1-adamantanol produces C6H70+,18%, the major ion in the E1 spectrum. Dissociative charge transfer from NzO+to 2adamantanol produces (M - H20)+,9%, and the c6 and c7
1
40
,
MI2
lQ0
,
170
, 190 ,
lF0
,
IF0 I
I
+
m m-
(BiNO)+
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
757
NzO as a reagent gas for positive ion CIMS has advantages over NO or N2/NO mixtures in ease of handling. The spectra appear to be reasonably useful for characterization and N20 may replace N2/N0 mixtures. NzO may be particularly useful for instruments for which positive and negative ion spectra may be easily obtained. Additional experiments are planned to see if the suggested reactions with polynuclear aromatic hydrocarbons are analytically useful.
ACKNOWLEDGMENT SAbiPLE I O N I Z A T I O N ,
%
OF
TOTAL
lONlZATlON
Flgure 3. Sample lon/sample molecule reactlons in N,O C I spectra of 1,I-octanediol.
ions obviously reacts with or is produced by reaction of another ion with N20. For some simple nitrogen-containing aromatic compounds (benzamide, carbazole, p-dimethylaminobenzaldehyde,Nphenylglycine), M+ is always one of the major ions in the mass spectra. The fragmentation products can generally be rationalized readily in terms of abstraction by NO+ of small groups to give stable ions or by decomposition of excited M+ ions according to mechanisms that are well established from low-pressure electron ionization studies. These N20 CI spectra of polar organic compounds have an artifact that is characteristic of charge exchange or electron transfer spectra obtained with other reagent gases. The abundances of what should be the 13Cisotope of M+ ions are almost always too high. These anomalously high isotope ratios result from ion/molecule reactions of hydrogen-containing ions in the reagent gas plus sample. With sufficiently small samples, sample ion/sample molecule reactions can be reduced to essentially zero. However, in routine spectra it is likely that isotope ratios of M+ ions will be high. Figure 3 illustrates this point dramatically for 1,8-octanediol. The major high mass ions, (M - H)+, (M - H - HzO)+,and (M - H - 2H20)+change only slightly with increasing sample size. The relative abundance of the (M H)+ions increases significantly with increasing sample size. The increasing gas-phase concentration of sample is indicated by the increase in the ratio of the sum of all sample ions to the s u m of sample and reagent ions. The relative abundance of the propyl ion shows a marked decrease from a proton transfer reaction with l,&octanediol. These reactions can be very useful for confirmation of molecular weights, but they can create significant confusion in molecular weight and structure determination if their occurrence is not considered.
+
The authors are grateful to Patrick Rudewicz for obtaining some of these spectra. Registry No. NzO, 10024-97-2;NzO+, 12269-46-4;2,4,4-tri107-40-4; methylpentene, 25167-70-8; 2,4,4-trimethyl-2-pentene, anthracene, 120-12-7;9-methylanthracene,779-02-2; l-methylphenanthrene, 832-69-9;2,2-dimethyl-3-methylenenorbornane, 79-92-5; 1-adamantanol, 768-95-6; 2-adamantanol, 700-57-2; 1,7,7-trimethyl-2-norbornanone, 76-22-2.
LITERATURE CITED (1) Smit, A. L. C.; Field, F. H. J . Am. Chem. SOC.1077, 99, 6471-6483. (2) Bruins, A. P. Anal. Chem. 1070, 5 1 , 967-972. (3) Sieck, L. W.; Jennlngs, K. R.; Burke, P. D. Anal. Chem. 1070, 5 1 , 2232-2235. (4) Hunt, D. F.; Shabanowltz, J.; Glorganl, A. B. Anal. Chem. 1080, 52, 386-390. ( 5 ) Harrison, A. 0.; Jennlngs. K. R. J . Chem. Soc., Faraday Trans. 1 1076, 72, 1601. (6) Gregor, 1. K.; Jennlngs, K. R.; Sharma, D. K. S. Org. Mass Specffom. 1077, 12, 93-97. (7) Derwlsh, G. A.; Galll, A.; Glardlnl-Guldonl, A.; Volpl, G. G. J . Chem. PhyS. 1064, 40, 3450-3451. (8) Sleck, L. W.; Gorden, R., Jr.; Ausloos, P.;Lias, S. G.; Field, F. Rad/at. Res. 1078, 56. 441-459. (9) Rosenstock, H. M.; Draxl, K.; Stelner, B. W.; Herron, J. T. J . Phys. Chem. Ref. Data 1077, 6, Suppl. No. 1. (10) Jelus, B. L.; Munson, B.; Fenselau. C. Blamed. Msss Spectrom. 1074.
.
7. . QS-102. .. . .-.
(11) Jelus, B. L.; Munson, B.; Fenselau, C. Anal. Chem. 1074, 4 6 , 729-730. (12) Hatch, F.; Munson, B. Anal. Chem. 1077, 49, 169-174. (13) Hatch, F.; Munson, 8. Anal. Chem. 1077, 4 9 , 731-733. (14) Hunt, D. F.; Harvey, T. M. Anal. Chem. 1075, 47, 1965-1969. (15) Field. F. H.; Munson, M. S. 8.; Becker, D. A. Adv. Chem. Sef. 1066, NO. 58, 167-192. (16) Hunt, D. F.; Harvey, T. M. Anal. Chem. 1075, 4 7 , 2136-2141. (17) Hunt, D. F.; McEwen, C. N.; Harvey, T. M. Anal. Chem. 1075, 47, 1730-1734.
(16) Elnoif, N.f Munson, B. Int. J . Mass Spectrom. Ion Phys. 1072, 9 , 141-1 60. (19) Hunt, D. F.; Ryan, J. F. J . Chem. Soc.. Chem. Commun. 1072, 620-621. (20) Jelus, 8. L.; Munson, B.; Bablak, K. A.; Murray, R. K., Jr. J . Org. Chem. 1074, 39, 3250-3254.
RECEIVED for review October 12,1982. Accepted January 13, 1983.
