466
Anal. Chem. 1967, 5 9 , 466-471
(M + H)+ and extensive fragmentation. K+IDS spectra provide molecular weight and structural information and compare most favorably with laser desorption/ionization. The K+ IDS method is preferable to techniques such as FAB, since no matrix is required. In terms of sensitivity, K+IDS is currently less sensitive than most of the other methods. Work is currently under way to increase the sensitivity of this experiment by increasing the pressure above the emitter as molecules are desorbing, which will increase the rate of cationization. We believe that the experiments described here are unique since, with one inexpensive probe, one can do K+ CI, surface ionization and K+IDS. In this work we focused only on the use of potassium ions in such experiments. Future articles will discuss variations of this technique which are based on %hermionicemission sources of other ions. Registry No. K, 7440-09-7. LITERATURE CITED (1) Beckey, H. D. Int. J. Mass. Spectrom. Ion Phys. 1989, 2 , 500. (2) Barber, M.; Bordoli, R. S.; Sedgwick, R . D.; Tyler, A. N. J. Chem. SOC.. Chem. Commun. 1981. 325. Benninghoven, A.; Sichtermann,-W. Anal. Chem. 1978, 50, 1180. Posthumus, M. A.; Kistemaker, P. G.; Meuzelaar. H. L. C.; Ten Noever de Brauw, M. C. Anal. Chem. 1078, 50,985. Macfarlane, R. D. Acc. Chem. Res. 1982, 15,268. Beuhler, R. J.; Flanigan, E.; Greene, L. J.; Friedman, L. J. Am. Chem. SOC. 1974, 96,3990. Schulten, H.-R.: Lattimer, R. P. Mass Spectrom. Rev. 1984, 3 , 231. Grassie, N. Pure Appl. Chem. 1982, 54,337. Rollgen, F. W.; Schulten, H A . Z . Naturforsch. 1975, 30A, 1685. Rd!gen, F. W.; Schulten, H.-R. Org. Mass Spectrom. 1975, 10, 660. Giessmann, U.; Rdlgen, F. W. Org. Mass Spectrom. 1978, 11, 1094. Borchers, F.; Giessmann, U.; Rollgen, F. W. Org. Mass Spectrom. 1977, 12, 539. Rbligen, F. W.; Giessmann, U.; Borchers, F.; Levsen, K. Org. Mass Spectrom. 1978, 13, 459. Van der Peyl, G. J. Q.; Isa, K.; Haverkamp, J.; Kistemaker, P. G . Org. Mass Spectrom. 1981, 16, 416. Stoli, R.; Rdlgen, F. W. Org. Mass Spectrom. 1981, 76, 72. Cotter, R. J.; Yergey. A. L. Anal. Chem. 1981, 53, 1306. Schmelzeisen-Redeker, G.; Giessmann, U.; Rollgen, F. W. Org . Mass Spectmm. 1985, 2 0 , 305. Bombick, D.; Plnkston, J. D.; Allison, J. Anal. Chem. 1084, 56,396. Blewett, J. P.; Jones, E. J. J. Phys. Rev. 1938, 50,464. Day, R. J.; Unger, S . E.; Cooks, R . G.Anal. Chem. 1980, 52,557A.
(42) 143)
(48) (49) (50)
Spears, K. G.; Ferguson, E. E. J. Chem. Fhys. 1973, 59,4174. Woodin, R. L.; Beauchamp, J. L. Chem. Fhys. 1979, 4 1 , 1. Rollgen, F. W.; Giessmann, U.; Stoll, R. Nucl. Instrum. Methods 1982, 198,93. Stoll, R.; Rollgen, F. W. Z.Naturforsch. 1982, 37A, 9. Allison, J.; Ridge, D. P. J. Am. Chem. SOC. 1979, 101, 4998. Zandberg, E. Ya.; Rasulev, U.Kh. Russ. Chem. Rev. (Engi. Trans/.) 1982, 51,819. Kawano, H.; Page, F. M. Int. J. Mass Spectrom. Ion Phys. 1983, 50, 1. Kawano, H.; Hidaka, Y.; Suga, M.; Page, F. M. Int. J. Mass Spectrom. Ion Phys. 1083, 50,35. Kawano, H.; Kidaka, Y.; Suga, M.; Page, F. M. Int. J. Mass Spectrom. Ion Phys. 1983, 50,77. Fagerson, I. S.J. Agric. FoodChem. 1989, 17, 747. Higman, E. B.; Schmeitz, I.; Schlotzhauer, W. S. J. Agric. Food Chem. 1978, 18,636. Ohnishi, A.; Kato, K.; Takagi, E. Po/ym. J. 1975, 7 , 431. Irwin, W. J. Analyfical Pyrolysis A Comprehensive Guide; Marcel Dekker: New York, 1962; Chapter 7.3, pp 339-352. Schulten, H.-R.; Gortz, W. Anal. Chem. 1978, 50,428. Shafiradeh, F. Adv. Carbohyd. Chem. 1988, 23,419. Muller, M. D.; Selbl, J.; Simon, W. Anal. Chim. Acta 1978, 700,263. Gower, J. L.; Beaugrand, C.; Sailot, C. Homed. Mass Spectrom. 1981, 8 , 36. Barber, M.; Bordoli, R. S.;Sedgwick, R. D.; Tyler, A. N.; Greene, 8. N.; Parr, V. C.; Gower, J. L. Biomed. Mass Spectrom. 1982, 9 ,11. Heerma, W.; Kamerling, J. P.; Slotboom, A. J.; van Scharrenburg, G. J.; Greene, B. N.; Lewis, I. R. S.Biomed. Mass Spectrom. 1983, 10, 13. Westmore, J. B.; Ens, W.; Standina, K. G. Biomed. Mass SDectrom. 1982, 9 , 119. Ratcliff, M. A., Jr.; Medley, E. E.; Sirnmonds, P. G. J. Org. Chem. lQ7A ., 39 - - , 1481 . . - .. Franklin, J. L. Ind. Eng. Chem. 1049, 41, 1070. Svec, H. J.; Junk, G. A. J. Am. Chem. SOC. 1984, 86,2278. Lin, Y. Y.; Low, C . I . ; Smith, L. L. J. SteroidBbchem. 1981, 14, 563. Smith, L. L. Cholesterol Autoxidation, Plenum: New York, 1981. Krull, U. J.; Thompson, M.; Arya, A. Talanta 1984, 31, 489. Lifshitz, C.; Bergmann, E. D.; Pullman, B. Tetrahedron Lett. 1987, 46, 4583. Daves, G. D. Jr. Acc. Chem. Res. 1979, 12, 359. Kambara, H.; Hishida, S.Anal. Chem. 1081, 53,2340. Reinhold, V. N.; Carr, S. A. Mass. Spectrom. Rev. 1983, 2 , 153.
RECEIVED for review January 24, 1986. Accepted October 1, 1986. This work was made possible through the financial support of (1)the National Institutes of Health (NIH Grant No. RR00480-16) and (2) the Analytical Laboratory of the Dow Chemical Co., Midland, MI.
Selective Reagents in Chemical ionization Mass Spectrometry: Diisopropyl Ether Regina Barry and Burnaby Munson*
Department of Chemistry, University of Delaware, Newark, Delaware 19716
Dllute mlxtures of dllsopropyl ether In nltrogen, methane, or helium can serve as useful reagent gases for analytlcal problems where relatively gentle lonlzatlon like that of NH, Is deslred but for which NH, does not give saUsf8ctory resutts. Mlsopropyl ether acts as a proton transfer chemkal lonlzatlon reagent gas, ghres mlnbnal rolvatlon wlth most spectes, and gives higher relatlve sen&tlvRy than ammonia for compounds whkh undergo the basegwitchlng reactton. Some lndkatlons of the sterk environment of the &e of protonation are noted. The abundant reagent Ions of dllsopropyl ether at m / z 103, 87, and 81 may Interfere with the analysis of samples which give Ions at those masses.
A major advantage of chemical ionization (CI) mass spectrometry lies in the flexibility to choose a reagent gas system
for the specific type of analysis desired (I, 2). Exothermic proton transfer reagents are the most widely used CI systems. With methane, for example, CH5+and CzH5+react with almost all samples with near-collision efficiencies to give high and similar sensitivities, frequently with extensive fragmentation (3-6). A weakly acidic ion from a reagent gas with a high proton affinity gives several analytical advantages for specific types of analyses (2, 7). For example, NH3 as a CI reagent gas produces spectra which contain predominantly (M + H)+ and (M + NH4)+ions with very little fragmentation. Proton transfer is much less exothermic from NH4+than from CH5+ or CzH5+ and there is insufficient energy for the (M + H)+ ions from many compounds to decompose. Rapid proton transfer occurs from NH4+to compounds with proton affinities greater than 204 kcal/mol, the proton affinity of NH, (8),and provides selectivity for basic compounds such as amines and
0003-2700/67/0359-0466$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
other N-containing compounds. The extent of solvation of the (M H)+ ions from these basic compounds to give (M NH4)+ions depends on the pressure and temperature of the source as well as the proton affinities and structures of the samples (2,9-12). For compounds with proton affinities less than 204 kcal/mol ammonia CI gives predominantly (M + NH4)+ions (2,9-12). At high pressures, NH3is unreactive as a CI reagent gas with a wide range of compounds whose proton affinities are much lower than the proton affinity of ammonia (2, 9-11). Although there is little fragmentation in most NH3 CI spectra, the NH, CI spectra of some hydroxy compounds contain (M + NH4)+and (M + NH4 - HzO)+ions (9, 13). Consequently, in the absence of information about the type of molecule being analyzed, it is difficult to assign molecular weight, since the characteristic ions may be (M NH4)+,(M + NH4 - H20)+,or (M + H)+. Additional experiments using ND,, however, will establish the nature of the ionic species (14). A comparison of NH, and ND, CI spectra was used to differentiate between (M + H)+ and (M NH4)+and to determine the number of labile hydrogen in amines (15). Another potential reagent gas is diisopropyl ether which has approximately the same proton affinity (206 kcal/mol) as NH, (204 kcal/mol) (8). CI spectra using a low pressure of pure diisopropyl ether as a reagent gas have been previously reported (16). In dilute mixtures with nitrogen, helium, or methane, diisopropyl ether behaves as a proton transfer CI reagent gas, with (i-C3H,)20H+as the major reactant ion. The solvation equilibrium constant
+
467
+
+
+
R20H+ + OR2 e H(OR2)2+
(1)
is significantly smaller for protonated diisopropyl ether than for other symmetrical ethers; consequently, under comparable conditions of temperature and pressure there is significantly less (R20)2H+for diisopropyl ether than for other simple ethers (17). This smaller extent of solvation is attributed to a slightly (4 kcal/mol) less negative heat of solvation and a more negative entropy of solvation for this sterically hindered ether compared with other ethers (18, 19). It is possible, therefore, that diisopropyl ether will solvate (M + H)+from the samples much less than simple molecules like NH,. In addition, a reaction analogous to the "switching" reaction between ammonium adduct ions and NH3 which reduces the NH3 sensitivities for many compounds (12)
(M + NH4)+ + NH3
+
NHd(NH3)'
(2)
may not occur with (i-C3H7)20as the reagent gas because of the poor self-solvating ability of protonated diisopropyl ether. Certain ethers have found specific uses as CI reagent gases. Vinyl methyl ether, alone or in mixtures with CS2and N2 (20, 21), has been used to locate double bonds in olefinic samples through the fragmentation patterns of characteristic adduct ions formed between the ether ion and the samples. Dimethyl ether has been suggested as a reagent system for organic functional group analysis (22). Crown ethers mixed directly with probe samples have been used to form characteristic adduct ions with samples (23). Some of the ion chemistry and analytical applications of diisopropyl ether as a CI reagent gas will be presented in this paper. EXPERIMENTAL SECTION Mass spectrometric data were obtained on a Du Pont 492-B double focusing mass spectrometerequipped with a CI source and a Hewlett-Packard 21-MX computer and a Du Pont data system. The source pressure was measured with an MKS Baratron capacitance manometer (MKS Instruments, Burlington, MA) through a glass probe inserted directly into the source. The total source pressure was maintained at 0.50 0.02 torr, the source
A N
C E 0.1
0.0 0
50
100
150
200
250
300
350
400
450
SO0
PRFSSURF. UlORR
Flgure 1. Pressure variation of relative ionic abundances: 10% diisopropyl ether in nitrogen at 220 OC.
temperature was 200 f 10 OC except as noted. The accelerating voltage was 1570 V, the repeller was maintained at 0 V, the electron energy was 70 V, and the filament emission current was 250 mA. Mixtures of diisopropyl ether were prepared in a glass manifold using UHP nitrogen (Matheson), UHP methane (Matheson), or helium (Air Products) as the bulk gas and diisopropyl ether (Aldrich) which had been carefully degassed on a vacuum manifold. These mixtures were allowed to equilibrate for 24 h prior to use. The diisopropylether was stored over a solution of ferrous sulfate in water to prevent the formation of peroxides and disposed of regularly. Ammonia (Matheson) and 1% ",/CHI (MG Scientific Gases, North Branch, NJ) were also used as reagent gases. Reagent gases were introduced into the source through a stainless steel inlet manifold equipped with high-vacuum shut-off valves and ultrafine metering valves. The samples were obtained from several commercial sources and were used without further purification. No significant impurities were detected in gas chromatographic experiments. A majority of samples were introduced with a Varian 2740 gas chromatograph equipped with a l/g in. by 6 f t stainless steel column packed with 3% SP-2100 on 100/120 Supelcoport (Supelco). Probe samples were made as standard solutions of 2 pg/mL in suitable solvents and introduced with a well probe through the direct inlet. The solvents were evaporated from the well probe in the vacuum housing prior to the CI experiments with the samples. In the relative sensitivity experiments, equimolar multicomponent mixtures were prepared volumetrically and samples of 0.03 p L of the mixtures were injected onto the column. The relative sensitivities were determined from the integrated areas of all of the sample ion currents compared with the area of the standard, N-methylaniline, which was present in each mixture. At least five analyses were made of each mixture. Ion cyclotron resonance (ICR) experiments were performed on an instrument described previously (24,25). Double resonance and ion ejection experiments were performed on samples of diisopropyl ether injected through a septum into a chamber with torr (uncora variable leak to give a total pressure of 1 x rected). RESULTS AND DISCUSSION The reactions of ions with diisopropyl ether were investigated by mass spectrometric pressure studies on the pure ether and on mixtures of the ether. A plot of the relative ionic abundances vs. pressure at 220 "C for a mixture of 10% ether in nitrogen is shown in Figure 1. A monotonic and approximately exponential decrease with increasing pressure is observed for the relative abundance of Nz+,which reacts with the neutral ether by dissociative charge exchange to form the fragment ions, 41,43,59,61, and 87, which are the abundant ether ions at low pressures. The relative abundances of these ions (except for 61 and 87) increase to a sharp maximum a t pressures less than 50 mtorr and then decrease as the total source pressure increases, as shown for 43. The ionic product of reactions of these ions with the ether is (M + H)+ a t m / z 103. Only very small abundances of the solvated ions, [(i-
468
w.0.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987 45
A
103
Table I. High-pressure Spectra of Diisopropyl Ether Mixtures relative abundance"
m.0.
mlr
m.0-
17 27 28 29 41 43 45 59 61 87 103 205
60.0-
::w. 50.0.
40.0.
CH*
87
43
LO.0
N2
He
6
3
13 14 7 20 28 100 4
12 6 18 39 100 6
6 6 6 1 8 7 4 17 50 100 11
11
Total pressure, 0.50 torr; 220 "C: 8% ether.
00.0.
103 DRO
61 70.0-
45
€4.0-
50.0, 100.0.
90.0.
87
90.0.
DRO
m.0.
--
60.0-
is0
50.0,
190
210 TlYPERAlURt
00.07
D 90.0-
170
103
.---
230
,--*------250
2-0
(Cl
Flgure 3. Temperature variation of relative ionic abundances: 10% dlilsopropyl ether in methane at 0.4 torr; A, 103; B, 61; C,43; D, 205.
00.0.
Figure 2. (A) ICR spectrum of diisopropyl ether: ionization gauge pressure, 1 x IO" torr; ionic residence time, 2 ms; swn of 100 scans. (6) Double resonance spectrum of diisopropyl ether; double resonance oscillator (bR0) on 103. (C) Double resonance oscillator on 87. (D) Double resonance oscillator on 61.
C3H7)zO]2H+, are detectable under these conditions. The relative abundance of solvated ions from di-n-propyl ether [ (n-C3H7)z0]zH+, under comparable conditions (0.4 torr, 6% ether) is approximately 10 times the abundance shown in Figure 1. Similar experiments were done on methane/ether and helium/ether mixtures and similar patterns of reactivity were observed. The ionic reactions in diisopropyl ether were also studied under ICR conditions. A typical spectrum (Figure 2A, obtained at an ionization gauge pressure of 1 x torr and an ionic residence time of approximately 2 me) shows a large extent of conversion of reactant ions to the major production ion, the protonated ether at m / z 103. Figure 2B shows a typical double-resonance experiment (monitoring 103' and increasing the kinetic energies of each of the other ions) which clearly shows that all of the fragment ions except 87 react with diisopropyl ether to form the protonated molecule, 103. Neither the ICR nor the high-pressure experiments indicated any ion/molecule reactions of 87 with diisopropyl ether. The
increase in ion current of 87 with increasing kinetic energy of 103 (Figure 2C) is consistent with the suggestion that the small increase in the relative abundance of 87 in Figure 1 results from collisionally induced decompositions. The double resonance experiments in Figure 2D for 61 (probably protonated isopropyl alcohol) show that this ion is formed by ion/molecule reactions of 43 and 45 and may also be formed by collisionally induced decompositions of 103. Mixtures of 1%to 10% of diisopropyl ether in methane can also be used successfully as a reagent gas. For these mixtures the direct ionization of the major component gives predominantly CH4+and CH3+ which react rapidly with the large excess of methane to give CH5+and CzH5+.These ions react rapidly with the ether to give mostly (M + H)+ ions at m/z 103. A comparison of the relative abundances of the ether ions in mixtures with these two bulk gases and with helium is given in Table I. The major reactant ion is 103 for all three mixtures. Figure 3 shows the variation with temperature for the relative abundances of the major ions in a 10% ether in methane mixture at a constant pressure. The major temperature effect is the large decrease in relative abundance of the solvated ions, mlz 205, with increasing temperature. Significant increases are also noted with increasing temperature of the relative abundances of the fragment ions, presumably because of the decomposition of the protonated ether ions at m / z 103. Similar temperature effects are noted on the relative ionic abundances in mixtures of diisopropylether with nitrogen and with helium. The protonated ether, 103, is the most abundant ion in the spectra of mixtures of 3-15% ether with nitrogen, helium, or methane at pressures of 0.1-0.5 torr and temperatures of 160-260 "C.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
Table 11. Ether CI Spectra of Basic Compounds,OP A (Sample) > 206 kcal/mol
:I
% sample ion current
(M + H)'
compound 12-cr0wn-4~ 15-cr0wn-5~
100
24-cr0wn-8~ P-CH~OC~H~NH~~ (i-C3H7)2Sd ethyl acetoacetated hexamethylbenzeneb n-CSH7NHZd (n-C&)zNy i-CaH7NH2 t-C4HBNHZd n-C4HSNHze (n-C4Hg)2NHe aniline N-methylanilined N,N-dimethylanilined (HZNCH2CH2CHZ)ZNHb
100 100 100 100 100 100
(M+
pyridine
I
other
0.8-
E II 00.. 67 --
T\ l o
r
I 0.5-
8
.I8
(M+ H30)+,11 8
1)
0.0
20
(IS
200
210
205
100 8
100 100
7
100 100 100 100
216
220
225
230
235
210
PROTOM AFF I N I l l K C A L N O L I
100
3
Figure 4. Relative molar sensitivities, N-methylanlline = 1.00; 8 % diisopropyl ether In methane; 210 OC; 0.44 torr. Equimolar mixtures Introduced through the gas chromatograph with N-methylaniline as internal standard: 1, cyclopentanone; 2, oxepane; 3, dl-n-butyl ether; 4, cyclohexanone; 5, di-n-pentyl ether; 6, ethyl fert-butyl ether; 7, hexamethylbenzene; 8, diisopropyl sulfide; 9, di-sec-butyl ether; 10, pyrazlne; 11, oanisidine; 12, oaminoanlline; 13, n-propylamine; 14, n-butylamine; 15, isopropylamine; 16, N-methylaniline; 17, N,Ndiethylaniline; 18, diisopropylamine; 19, di-n -butylamine; 20, tri-n propylamine; 2 1, tri-n-butylamine.
-
diisopropyl ether in CH,;220 O C ; 0.44 torr. bProbesample. -O%, or
