Anal. Chem. 1984, 56,2295-2297
s. A recent paper goes into this subject in very good detail (31)* CONCLUSION The tandem quadrupole time-of-flight instrument has the potential to be a very useful and simple means to perform MS/MS experiments. It has a unique advantage in obtaining daughter ion MS/MS spectra on transient species such as those produced by pulsed laser ionization methods and has the potential to allow MS/MS to be combined on-line with chromatographic methods. By the addition of an electric sector in the TOF portion, this instrument could be very flexible in that access to both low- and high-energy CAD MS/MS would be possible. ACKNOWLEDGMENT We gratefully acknowledge W. B. Whitten and J. M. Ramsey for their helpful discussions and the use of their electronics for the detection circuit. LITERATURE CITED Cooks, R. G.; Gllsh, G. L. Chem. Eng. News 1081, 59 (Nov 30), 40-52. McLafferly, F. W.. Ed. ”Tandem Mass Spectrometry”; Wlley: New York, 1983. Beynon, J. H.; Cooks, R. G.; Amy, J. W.; Baitinger. W. E.; RMley, T. Y. Anal. Chem. 1073. 45, 1023A-1031A. Cooks, R. G.; Beynon, J. H.; Caprloll, R. M.; Lester, G. R. “Metastable Ions”; Elsevler: New York. 1973. Crow, F. W.; Tomer, K. 8.; Gross, M. L. Mass Spectrom. Rev. 1983, 2,47-76. Yost, R. A.; Enke, C. G. Anal. Chem. 1070, 51, 1251A-1264A. Yost. R. A.; Enke, C. G.; McGilvery, D. C.; Smlth, D.; Morrlson, J. D. Int. J . Mass Spectrom. Ion Phys. 1079, 30. 127-136. Maquestlau, A.; Van Haverbeke, Y.; Flammang, R.; Abrassant, M.; Flnet, D. Bull. SOC. Chlm. &/g. 1078, 87, 765-770. Russell, D. H.; Smith, D. H.; Warmack, R. J.; Bertram, L. K. Int. J . Mass Spectrom. Ion Phys. 1080, 35, 381-391. McLafferly, F. W.; Todd, P. J.; McGihrery, D. C.; Baldwin, M. A. J . Am. Chem. SOC. lS80, 102,3360-3363.
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(11) Gllsh, G. L.; McLuckey, S. A,; Rldley, T. Y.; Cooks, R. G. Inf. J . Mass Spectrom. Ion Phys. 1082, 4 1 , 157-177. (12) Louter, 0. J.; Boerboom, A. J. H.; Staimeier, P. F. M.; Tulthof, H. H.; Klstemaker, J. Int. J . M s s Spectrom. Ion Phys. 1980, 33, 335-347. (13) Cody, R. B.; Frelser, B. S. Int. J . Mass Spectrom. Ion Phys. 1982, 4 1 , 199-204. (14) Stults, J. T.; Enke, C. G.; Holland, J. F. Anal. Chem. 1083, 55, 1323- 1330. (15) Enke, C. G.; Stub, J. T.; Holland, J. F.; Plnkston, J. D.; Allison, J.; Watson, J. T. Int. J . MassSpectrom. Ion Phys. 1983, 46,229-232. (16) McKown. H. S.; Smlth, D. H.; Sherman, R. L. Int. J . Mass Spectrom. Ion Phys. 1083, 51,39-46. (17) Smith, D. H.; Waiton, J. R.; McKown, H. S.; Walker, R. L.; Carter, J. A. Anal. Chlm. Acta 1982, 142,355-359. (18) Schade, U.; Stoll, R.; Rollgen; F. W. Org. Mass Spectrom. 1961, IO, 441-443. (19) Gllsh, G. L.; Smith, D. H. Inf. J . Mass Spectrom. Ion Phys. 1983, 50, 143-149. (20) McLuckey, S.A.; Glish. G. L.; Cooks, R. G. Int. J. Mass Spectrom. Ion Phys. 1081, 39, 219-230. (21) Douglas, D. J. J . Phys. Chem. 1982, 86, 185-191. (22) Brlcker, D. L.; Adams, T. A,; Russell, D. H. Anal. Chem. 1083, 55, 2417-2418. (23) Glish, G. L.; Todd, P. J. Anal. Chem. 1082, 54,842-043. (24) Wlley, W. C.; McLaren, T. H. Rev. Scl. Instrum. 1955, 26, 1150-1 157. (25) Muga, M. L., paper presented at the Proceedlngs of the 31st Annual Conference on Mass Spectrometry and Allied Toplcs, Boston, MA, 1983. (26) Mamyrin, B. A.; Shmikk, D. V. Sov. Phys .-JETP (Engl. Transl.) 1070, 49,762-772. (27) Turko, B. T.; Macfarlane, R. D.; McNeal, C. J. Int. J . Mass Spectrom. Ion Phys. 1983, 53, 353-362. (28) Zakett, D.; Schoen, A. E.; Cooks, R. G.; Hemberger, P. H. J . Am. Chem. SOC. 1081, 103,1295-1297. (29) Davis, D. V.; Cooks, R. 0.; Meyer, B. N.; McLaughlin, J. L. Anal. Chem. 1083, 55, 1302-1305. (30) Perchalski, R. J.; Yost, R. A.; Wilder, B. J. Anal. Chem. 1083, 55, 2002-2005. (31) Holland, J. F.; Enke. C. G.; Allison, J.; Stults, J. T.; Plnkston, J. D.; Newcome, J. T. Anal. Chem. 1083, 55, 997A-1012A.
RECEIVED for review February 23,1984. Accepted June 22, 1984. This work was sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract W-7405-eng-26with the Union Carbide Corporation.
Field Desorption Mass Spectra of Pyridinium Oxime Salts with Rapidly Heated Emitter Arabinda Bhattacharya* and Durgesh N. Tripathi Analytical Services Wing, Defence Research & Development Establishment, Gwalior 474002, India
Mass spectral data of a few pyridlnlum oxime salts are reported. These thermally labile salts did not give consistent mass peaks in the molecular ion region with slowly heated FD technique. Rapid heating of the activated emltter at a rate of 15-25 mA/s in the field desorptlon/field ionization mode gave rise to cationized molecular ion peaks along wHh other peaks that facllltaie Identification of molecular ions of these salts In the concentration range 1-10 pg/mL.
Mass spectral analyses of mono- and diquarternary ammonium and phosponium salts are reported in the literature using field desorption techniques (I,2). More recently secondary ion mass spectrometry (3) and FAB (4) have also been used with success. In all the above cases the salts have been desorbed and ionized either as di- or monocations. Our ex0003-2700/64/0356-2295$01.50/0
periments with a few pyridinium oxime salts in conventional FD technique could yield the monocation species. In a bid to see if the salt molecule as a whole can be desorbed and made available for giving mass spectral peaks, we utilized rapid heating of the emitter as a means of volatility enhancement, as explained by Beuhler et al. (5) and followed by many later (6) including P. J. Derrick et al. (7), who employed very rapid heating in the field desorption (FD) mode to obtain mass spectra of high molecular weight saccharides. Because heating of the FD emitter is only for activation and facilitating migration of molecules, normal FD is generally run with slow heating of the emitter to find best anode temperature/ best emitter temperature. With pyridinium oximes slow heating of the emitter (at the rate of 1-5 mA/min) mainly gave rise to fragments with monocationic species in some cases. In this paper we describe the results obtained by rapid heating (at the rate of 15-20 mA/s) of a activated emitter in a con0 1984 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1984
Table I compd no.
common nomenclature
mp, "C
molecular structure
155
I1
Toxogonin
225
G
O
H
N-CHzOCHz-
I11
4-PAM chloride
215-225
IV
TMB-4
240
V
HS-6
150
Table 11. compd no I
I1
I11 IV
V
% re1
mle
abundance
remarks
119 136 137 272 323 325 357 380 136 137 194 134 292
7 24.8 62.3 5.8 44.1 13.7 5.3 30.2 30.5 4.9 63.1 17.3 35.5
366 468 282
22.6 24.6 42.6
322 380
30.2 22.4
(137- HzO)' (M - acid)+ (M - anion)+ (M + K)+ (M - 35c1)+ (M - 37c1)+ (M - H)+ (M + Na)+ (M - acid)+ (M - anion)+ (M + Na)+ (Diionic species - H20)+ (M - [4-cyano-N-methylpyridinium ion + 2Hz0])+,a decomposition product (M - anion)+ (M Na)+ (M - [CH=NCH + HCl])", a decomposition product (M - acid)+ (M + Na)+
+
ventional FD ion chamber for obtaining the mass spectra of a few pyridinium oxime salts which are commonly used as potent antidotes against poisoning by anticholinesterase compounds. EXPERIMENTAL SECTION Instrument. All mass spectra were obtained in a JEOL JMS DX-300 double focusing mass spectrometer with an EI/FI/FD combination ion source attached. This instrument is combined with a JMA 2000 on-line computer for data collection, storage, and retrieval in a magnetic cassette tape system. Ion source chambertemperatures were maintained at 100-110 O C in all cases. Method of Sampling. The solid salts were dissolved in methanol with slight warming where necessary. The concentration used was 1-10 pg/mL. Sampling was done by dipping in the solution an activated 10 pm thick tungesten wire (carbon emitter) which was fitted in the FD probe housing. To avoid contamination of emitter, the experiment on each salt was carried out with fresh emitter. Chemicals. All the salts were procured from M/s Fluka, USA, except I1 and V which were synthesized in our laboratory. Procedure for Recording Mass Spectra. Calibration with perfluorokerosene was done by using the E1 mode of the combination ion source. The FD source was adjusted and kept ready for best focusing at a cathode voltage of 4.5/5 kV. Accelerating voltage was set at 2.3 kV. The computer was kept ready for scanning mms range from 1 to 500 amu in 2.8 s with 1.2 s between
CHENOH
C G O H
emitter mol wt current, mA 232
20
358
20
172
15
446
23
ZCI-
two scans. The best emitter current was assessed by trial and error by stepwise heating and analyzing the reconstructed ion chromatogram. This current is given in Table I. Afterward fresh sampling was done and the emitter was rapidly heated (controlled manually) from 0 to that previously determined current value within the scan interval of 1.2 s. The heating was then kept for a few more seconds at the rate of 10 mA/min. Scanning was continued. Mass spectra were obtained from the reconstructed ion chromatogram which showed complete desorption of sample within the previously determined current value and had an approximately triangular characteristic as reported by Holland et al. (8). RESULTS AND DISCUSSION The chemical structure and the molecular weight of the compounds studied are shown in Table I while the FD mass spectral data are given in Table 11. The spectra of each compound contain one cluster ion peak characteristic of FD and rapidly heated mass spectra for certain classes of compounds (9-11). Each spectrum also contains one peak with the loss of an anion or acid component from the sample molecule. The absence of isotope peaks in almost all the spectra is attributed to the low intensity of the peaks because of limited availability of neutral molecules participating in ion formation and also to the limited availability of cations (Na+ or K+) as no addition of sodium or potassium salts was done. Higher concentration of sample on the emitter increased the decomposition products without even giving the cluster ion peaks. We also did the experiments in comparatively low concentration to make sampleaurface interaction predominant, thus avoiding agglomeration of molecules on the filament surface to the limit to obtain meaningful spectra. The only isotope peak of chlorine in I1 (323 and 325) along (M - 1)' has raw intensity of 6.1 in this spectrometer computer system. All other intensities are within 2 units including TIM34 at IV. Our results also show an uncertainty of fl mass unit, as observed by authors in ref 7. Since obtaining isotope peaks in a mass spectrum has some bearing on probability in natural abundance, this along with the ion optical abberation is the probable cause of their absence. The results suggest that this method can be adopted to get useful supportive information on molecular ions of such solid salts as a complement to the conventional FD technique. ACKNOWLEDGMENT We thank P. K. Ramachandran, Director, Defence Research & Development Establishment, Gwalior, for guidance and helpful suggestions and S. N. Asthana, Scientist specially, for synthesizing the compound HS6 for our work.
Anal. Chem. lQ04, 56,2297-2303
Registry No. I, 154-97-2;11,114-90-9;111,51-15-0;IV,56-97-3;
V, 53370-49-3. LITERATURE CITED (1) Wood, G. W.; McIntosh, J. M.; Law, P. Y. J . Org. Chem. 1975, 4 0 , 636-636. (2) Brent, D. A,; Rouse, D. J. Tehahedron Lett. 1973, 42, 4127-4130. (3) Ryan, Y. M.; Day, R. J.; Cooks, R. 0. Anal. Chem. 1980, 52. 2054-2057. (4) Heller. D. N.; Yergey, J.; Cotter, R. J. Anal. Chem. 1983, 55, 1310-1313.
2297
Beuhler, R. J.; Flanlgan, F. E.; Greene, L. J.; Friedman, L. J . Am. Chem. SOC. 1974, 96, 3990-3999. Cotter, R. J. Anal. Chem. 1980, 5 2 , 1589A. Cullls, P. 0.; Neumann, G. M.; Rogers, D. E.; Derrick, P. J. Adv. Mass Spectrom. 1980, BB, 1729. Soltmann, 6.; Sweeley, C. C.; Holland, J. F. Anal. Chem. 1977, 49, 1164- 1 166. (9) Fukushlma, K.; Aral, T. Mass Spectrom. 1978, 2 6 , 197. (10) JEOL FD-MS Application Data Voi. 102, 103. (11) Stoll, R.; Rollgen, F. W. Org. Mass Spectrom. 1979, 14, 642.
RECEIVED for review January 31,1984. Accepted May 24,1984.
Nonsupervised Numerical Component Extraction from Pyrolysis Mass Spectra of Complex Mixtures Willem Windig* and Henk L. C. Meuzelaar Biomaterials Profiling Center, University of Utah, Salt Lake City, Utah 84108
Pyrolysis mass spectral data of complex organic mixtures can be expressed in subpatterns describing the pure Components of the mixtures and their relative concentrations. The approach described involves factor and discriminant analysis and does not require the presence of pure component spectra in the data set. I t Is based on a representation of correlations in a data set in the form of a “variance diagram”. Applications of the procedure are discussed for various sets of sampies: biopolymers, lignites (brown coals), and grass leaves.
Recently, Windig e t al. described a mixture analysis procedure for pyrolysis mass spectrometry (Py-MS) data seta by graphical rotation (1-3). This visually assisted approach was chosen over mathematical procedures because pyrolysis mass spectra of complex mixtures are not necessarily exact linear combinations of the spectra of the individual components. Furthermore, reference spectra of pure biochemical components are often not available. This severely limits the applicability of library search systems, such as used in GC/MS (4), or of target rotation methods, such as developed by Malinowski (5, 6), for Py-MS data sets. Other methods for mixture analysis, often based on factor analysis, do not require reference spectra. Knorr and Futrell developed a technique to extract information on the pure components from mixture data (7). This procedure relies on a number of assumptions with regard to the structure of the data set: (a) a “pure mass” is present for every component (i.e., a mass value with a finite intensity for one component and zero intensity for all other components), and (b) the mathematically extracted spectrum contains only positive intensities (7). The procedure for finding pure masses or a key set of variables has been refined by Malinowski (8), thereby making this method applicable for mixtures with more than three components. Successful applications of this nuclear technique have been reported for mass spectral (7,8), and gas-liquid chromatography (8) magnetic resonance (8,9), data. However, this type of technique is not suited for Py-MS data, since the complexity of most Py-MS spectra (10) makes fulfillment of assumption (a) generally impossible. Due to the normalization procedures applied to the peak intensities, mathematically extracted spectra from Py-MS data sets show 0003-2700/84/0356-2297$01.50/0
positive and negative intensities (1-3), therefore assumption (b) is not fulfilled either. Although the graphical rotation of Py-MS data has proven to give valuable information about chemical components (1-3, 11-14) as well as accurate quantitative results (15),the v i s d y assisted graphical rotation approach has some definite disadvantages. These disadvantages are (a) a highly experienced person is necessary to judge the results of graphical rotations, (b) the presence of unknown components may be overlooked, and (c) other important “tendencies” than changes in the concentration of a component in a set of mixture spectra (e.g., a p H change, changes caused by linkage difference in polymers, matrix effects or other physicochemical reactions) may not be recognized visually. For these reasons, an automated mathematical approach to graphical rotation capable of extracting major component patterns from a Py-MS data set will be a valuable research tool. This paper describes such a mathematical approach for mixture analysis which, compared to graphical rotation, is a time saving aid for the visual evaluation of the results of factor/discriminant analysis.
MATERIALS AND METHODS Simulated Mixtures. Three pyrolysis mass spectra of biopolymers, viz., the protein bovine serum albumin (BSA), the polyhexose glycogen (GLY), and the peptidoglycan of Bacillus subtilis (PG),form the basis for a set of 10 simulated mixture spectra. Although PG contains aliphatic amino acid residues in addition to the N-acetylamino-sugar chain, the latter units dominate in the pyrolysis mass spectrum. The spectra were taken from literature (I). A matrix of random numbers was generated by computer for constructing the mixtures. In matrix notation where [SI is the data matrix, a 100 X 10 matrix of the intensities of the 100 masses in the 10 different mixtures. The matrix [C], containing the spectra of the three components,is a 100 x 3 matrix and [F] is the matrix (3 X 10) with the mixture composition. As the factor analysis program used can handle a maximum of 100 variables, the 100 most intense mass peaks out of the 150 present were chosen from the averaged spectrum of the 10 mixtures. The sum of the elements of each column of [C] is 1, resulting in normalized spectra in [SI. Real Mixtures. Ten mixtures of three water-soluble biopolymers, gelatin (protein), corn starch (hexose polymer), both 0 1984 American Chemical Soclety
