2940
Anal. Chem. 1985, 57, 2940-2944
Laser Desorption Fourier Transform Ion Cyclotron Resonance Mass Spectrometry vs. Fast Atom Bombardment Magnetic Sector Mass Spectrometry for Drug Analysis Ronald E. Shomo, 11,' A l a n G. Marshall,*lm3and C.
R. Weisenberger3
Department of Chemistry, Department of Biochemistry, and Chemical Instrument Center, The Ohio State University, 140 West 18th Avenue, Columbus, Ohio 43210
Mass spectra of several cllnlcally Important drugs of low volatility (amoxiclllln, mol wt 365; daunorublcin, mol wt 527; erythromycln, mol wt 733; digoxln, mol wt 780) have been obtained via fast atom bombardment wlth a double-focuslng magnetic sector Instrument and vla pulsed CO, laser desorption lonlzatlon Fourler transform Ion cyclotron resonance mass spectrometry. Compared to FAB/MS, the LD/FT/ICR spectrum resutllng from a slngle laser pulse produces a much more promlnent molecular or pseudomolecular ion wlth little fragmentatlon. Additlonal maJorfragment peaks (If deslred) can be produced by electron ionization of the neutrals produced by a second pulse from the same laser (LD/EI/FT/ ICR). LD/FT/ICR mass callbration for a four-component mixture ylelds a mass accuracy of better than 5 ppm over a mass range of 404 < m / z < 819.
Until relatively recently, mass spectra of involatile substances were obtained by first vaporizing the sample and then ionizing it (usually via electron ionization (EI) with an electron beam) in two distinct steps. Several newer schemes achieve desorption and ionization in a single procedure: field desorption ( I ) , plasma desorption ( 2 ) , secondary ion mass spectrometry ( 3 , 4 ) ,laser desorption (5),and (for liquids) fast atom bombardment (FAB) (6) and thermospray (7). Of these, FAB has become the most generally popular because of the ease of sample preparation and simplicity of operation. Soon after Kistemaker e t al. (5) demonstrated that laser desorption/ionization could produce molecular or pseudomolecular ions from large organic molecules, various laser/ mass spectrometer configurations were tested. From a number of such experiments, it now appears that virtually any pulsed laser able to generate ca. 10 MW/cm2 of power incident on a solid or solvent-evaporated sample on a metal probe can yield qualitatively similar results (8-10). The most common mass spectrometer choice has been the time-of-flight design ( I O ) , because most other mass spectrometers cannot scan the necessary mass range in the time available following the laser pulse. Unfortunately, even the best time-of-flight instruments offer relatively poor mass resolution in the mass range (500 < m/q < 5000) for which the laser source is most needed (11). Fourier transform ion cyclotron resonance mass spectrometry (FT/ICR or FT/MS) (12,13)offers high mass resolution and rapid data acquisition of the whole spectrum a t once (14, 15) and is inherently a pulsed experiment. I t is thus ideally suited for laser desorption mass spectrometry (LD/MS). Once the feasibility of the experiment had been demonstrated (8-IO), the LD/FT/ICR technique was developed t o its present state by Hein and Cody (16),and promptly applied by others (14,15,17,18). In this paper, we present the first 'Department of Chemistry. Department of Biochemistry. Chemical Instrument Center.
direct comparisons between LD/FT/ICR and FAB/MS, for several underivatized drugs in current clinical use. Identical compounds have been run on a double-focusing E / B mass spectrometer equipped with a xenon FAJ3 source and a Fourier transform ion cyclotron resonance mass spectrometer equipped with a pulsed COz laser operating a t 10.6 km. EXPERIMENTAL S E C T I O N Sample Preparation. All compounds were obtained from Sigma Chemical Co. and used without further purification. Solvents and KBr were reagent grade. Samples for FAB/MS were prepared as a 1:l mixture of glycerol and a saturated solution of the sample in either chloroform/methanol (daunorubicin), dimethyl sulfoxide (daunorubicin),or methanol (amoxicillin, digoxin, erythromycin). Samples for laser desorption FT/ICR were prepared by dissolving approximately 100 wg of sample in an appropriate solvent (e.g., methanol, tetrahydrofuran, etc.), followed by addition of 1 wg of KBr to give a total volume of about 0.1 cm3. This solution was then placed on a stainless steel probe tip, and the solvent allowed to evaporate. FAB/MS. A fast-atom beam was generated with an Ion-Tech ion gun interfaced to a Kratos MS-30 double-focusing E/B mass spectrometer, with Xe as the collision gas. The acceleratingvoltage in the mass analyzer was either 4 kV (amoxicillin,daunorubicin) or 3 kV (digoxin, erythromycin). LD/FT/ICR and LD/EI/FT/ICR. FT/ICR spectra were obtained with a Nicolet FTMS-1000 spectrometer, operating at a magnetic field strength of 3.019 T. The laser beam focused at the probe tip produces both neutrals and ions in the gas phase: approximately 1 in 1000 of the desorbed neutrals are ionized (19). Four distinct types of mass spectra can be produced by the laser/FT/ICR combination. First, one may observe positive ions produced directly by the action of the laser (LD/FT/ICR). Second, the neutrals desorbed by the laser may be bombarded by an electron beam to form positive ions via electron ionization (LD/EI/FT/ICR). Two additional mass spectra are obtained when the instrument is adjusted for negative-ion detection of ions or electron-ionized neutrals formed by the laser. For all of the laser experiments, ions and neutrals are formed by a pulse (ca. 1 J in 50 ns to give ca. 20 MW) from a Tachisto 215G C 0 2 laser. The laser beam passes through a Zn/Se window into the high-vacuum chamber and is focused by a 4-in. focal length Zn/Se lens (mounted on the support for the trapped-ion cell) onto a 1 mm diameter spot on a stainless steel probe tip. The laser interface was supplied by Nicolet and modified as follows. The grating in the laser was replaced by a mirror; the 3 in. focal length lens was replaced by a 4 in. focal length lens; and the laser power was attenuated by passing the beam through a 3 mm diameter hole in a piece of cardboard, in order to prevent laser damage t o the lens. The experimental event sequence is shown in Figure 1. Each experiment begins with a laser pulse, after which the background pressure in the vacuum system increases from about 2 X lo-' torr torr. Since high-resolution FT/ICR performance to 5 x requires low pressure torr), an adjustable delay time, DL3, of 1-10 s is introduced after the laser pulse in order to allow most of the neutrals to be pumped away before ion excitation/detection begins. The ions remain trapped by a voltage of 0.34-0.50 V on the end plates of the 1-in. trapped-ion cell (20, 21) during this delay period. The delay period also provides sufficient time for the combination of initially formed K+ ions (22) t o combine with
0003-2700/85/0357-2940$01.50/01-b 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985 LASER
L D MODE OR L D / E I MODE
e - BEAM ~
+
DL43,
AM OXlClL LIN
ION
EXCITATION
ION
I R E C E I V E R “ON” I
Chart I
~~
EXCITE DL3)
2941
DETECTION
I +
QUENCH
DAUNORUBICIN
QUENCH DELAY
ERYTHROMYCIN
OCH,
n a c A , . . a a
T I M E -+
Flgure 1. Event sequence for Ion formation, excitation, detectlon, and removal: LD/FT/ICR, with electron beam off, so that ions formed directly by the laser are observed: LD/EI/FT/ICR, with electron beam on, so that Ions formed by electron ionization of laser-produced neutrals are observed.
C
na
-
a
OR
na
LD/FT/ICR ERYTHROMYCIN
D
+
to give (M K)+ at m / z 772.4. Loss of H20 from that ion is clearly visible. Although several small fragment peaks are assignable (see Figure 2), the more exciting feature of this spectrum is that the (M K)+ ion intensity is so large that the parent molecule is unmistakeable. That feature appears to be general, at least for more than 2 dozen organic molecules with molecular weight 300 to 1200 that we have examined so far. Although addition of KBr enhances the intensity of the (M K)+ peak, potassium (or sodium) ion attachment can occur whether or not salt is added to the solution in which the sample is dissolved. Substitution of NaCl for KBr in the sample solution leads to (M Na)+ rather than (M K)+ as the major peak, confirming its assignment. LD/EI/FT-ICR. Once the parent chemical formula has been identified from an LD/FT/ICR spectrum such as that in Figure 2, a second laser pulse followed by electron ionization (LD/EI/FT/ICR) can be used to generate a variety of fragment ions from which molecular structural features can be inferred. For example, the LD/EI/FT/ICR spectrum of erythromycin (Figure 3) contains a prominent peak a t m / z 158, arising from cleavage at the hemiacetal linkage to give the amino sugar (compare to the fast-atom bombardment spectrum shown at the bottom of Figure 3). It is worth noting that switching from LD/FT/ICR to LD/EI/FT/ICR involves simply turning on an electron beam, so that the two experiments can be run successively in less than 10 s. The spectra shown in Figures 2 and 3 each resulted from a single laser pulse, showing that extraordinarily good signal-to-noise ratio can be obtained without signal averaging of many scans. It is also interesting to note that the LD/EI/FT/ICR yields a strong molecular ion peak at (M + H)+,because this sample was not doped with KBr. Even so, both (M + K)+ and (M + Na)+ peaks are evident. Combination of the LD/FT/ICR and LD/EI/FT/ICR methods provides both the parent molecular weight as well as several useful fragments for aid in structure determination. Finally, it should be noted that all the FT/ICR spectra have been scaled to the height of the largest peak in that spectrum. On the same vertical scale, the (M + H)+peak obtained by LD/EI/FT/ICR in Figure 3 would be only 20% as intense as the (M + K)+ peak observed by LD/FT/ICR in Figure 2. LD/FT/ICR vs. FAB/MS. In the FAB/MS experiment (61, ions are produced by directing energetic xenon atoms at a sample dissolved in a glycerol matrix. FAB has come to be
; I +
DC
(M+K-H20)
, @-3H20l
0 I00
200 300
400 500
600 700 8oC
9CO 1000
VASS IN AMU
Flgure 2. LD/FT/ICR spectrum of erythromycin, based on one laser pulse. Nominal mass calibratlon was based on perfluorotrlbutylamlne. 32K data points were zero-fllled once, for a mass range of 100-1200 amu.
laser-desorbed neutrals to give (M + K)+ pseudomolecular ions. Although some K+ attachment is usually observed, the intensity of the (M + K)+ peak can be significantly enhanced by addition of a small amount (ca. 1% w/v) of KBr to the sample solution before depositing it on the probe. For LD/FT/ICR, the electron beam is off; for LD/EI/FT/ICR, the electron beam is on, with a filament current of about 3 A to give an emission current of about 500 nA measured by a collector at the same end of the cell as the sample. Ions are then excited to ca. 10 mm diameter orbits by ca. 10 V/m rf electric field for an “excite” period of 400-900 ps. A short time (DL4 = 200 ps) later, the oscillating charge induced in the detector plates is observed in heterodyne mode for ca. 100 ms at a digitizing rate of ca. 250 kHz according to the lowest mass in the mass range (say, 100 amu). Pressure during detection ranged from 1.9 to 7.0 X torr. Data sets were typically 32K or 64K with either zero or one zero-filling. The laser typically removes most of the sample from the probe tip after a single laser shot. Although not necessary in the present examples, signal averaging can be achieved by physically rotating the probe tip between scans-the diameter of the probe tip (ca. 1 cm) and the small size of the focused laser spot (1mm) combined to provide 20-30 scans without removing the probe.
RESULTS AND DISCUSSION The structures of the four compounds described in this paper are shown in Chart I. LD/FT/ICR. Figure 2 shows a laser desorption Fourier transform ion cyclotron resonance mass spectrum (LD/ FT/ICR) of erythromycin (an antibacterial agent which was the fifth most prescribed drug of 1983 (23)),in which ions observed are formed as a direct result of the action of the laser. The largest peak in the spectrum is the pseudomolecular ion, formed by attachment of potassium ion to the parent neutral
+
+
+
2942
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
'"q (113
LD/E I / FT/I C R
ERYTHROMYCIN
734
FA 6
fM+".'
0 100
1 600 200 300 93,7 .373 400 500
M A S S IN A M U
FAB
100-
DAUNORUBICIN LD/ FT/ IC R
(M+K)+ 566
Flgure 3. LD/EI/FT/ICR spectrum (top) and FAB/magnetic sector spectrum (bottom) of erythromycin. Conditions are given in Flgure 2, except that an electron beam (50 V at 500 nA for 100 ms) was applied after the laser pulse.
regarded as the simplest and most reliable means for producing pseudomolecular ions (e.g., (M H)+, (M - H)-, (M Na)+,etc.) from high molecular weight involatile molecules. Direct comparisons of LD/FT/ICR and FAB/MS spectra are offered in Figures 2 and 3 (erythromycin), Figure 4 (daunorubicin), Figure 5 (digoxin), and Figure 6 (amoxicillin). Daunorubicin is a close homologue of doxorubicin (adriamycin), one of the most important drugs in the treatment of human cancers (24). In this case, no parent ion is visible in the FAB/MS spectrum obtained with a glycerol matrix (Figure 4, top), due mainly to the poor solubility of daunorubicin in glycerol. The only peaks in the FAB/MS spectrum originate from glycerol. No improvement in the FAB/MS result was obtained by prior doping with NaCl nor by substitution of thioglycerol for glycerol. A much improved FAB/MS result was produced when the sample was first dissolved in Me2S0 and then mixed with glycerol (Figure 4, middle). This sort of manipulation is typical of FAB/MS experiments-it is frequently necessary to try out several different solvent combinations, sample derivatization, pH variation, or other additives in order to obtain a satisfactory FAB mass spectrum. In contrast, LD/FT/ICR of the same compound (Figure 4, bottom) yields an intense pseudomolecular ion at m / z 566.1, corresponding to (M + K)'. Useful fragment ions corresponding to loss (from (M + K)+) of K+ and the daunosamine sugar at m / z 437 (or daunosamine t HzO) at m/z 419 are also seen. For digoxin, a cardiotonic drug which was the third most prescribed retail drug of 1984 (25),FAB/MS with glycerol matrix gave a weak (M Na)+ peak (Figure 5, bottom), and several other peaks arising from glycerol. Again, however, LD/FT/ICR (Figure 5 , top) generates an intense pseudomolecular (M + K)+ ion at m / z 819.4, with a second strong peak at m/z 801 arising from loss of H20from the potassium adduct. Amoxicillin, an antibiotic, was the most-prescribed drug of 1983 (23). The FAB/MS spectrum in glycerol matrix (Figure
+
+
+
'700
200
300
400
500
600
M A S S IN A W U
Flgure 4. Comparison of FABImagnetic sector mass spectra of daunorubicin in 1: 1 mixture of glycerol with chioroformhethanol (top) or Me,SO (middle) and LD/FT/ICR mass spectrum (bottom). Conditions are given in Figure 2.
6, bottom) shows a weak molecular ion a t m / z 366 which is nearly obscured by a peak due to a protonated glycerol trimer at m/z 369. The most intense peak in the FAB/MS spectrum if m / z 277, corresponding to two glycerols plus a proton. These results point up another major problem with FABIMS, namely, the confusion that can result from the presence of chemical background peaks from the glycerol matrix, particularly a t intermediate masses (say, 300-500 amu). The LD/FT/MS experiment (Figure 6, top) again produces an unambiguous pseudomolecular (M + K)' ion at m / z 404.1 as well as a peak corresponding to addition of two K+ with loss of a proton. Mass Calibration: FAB/MS. The Kratos MS-30 instrument is a dual-beam design, in which the sample and reference beam follow parallel paths through the electric and magnetic sectors. Mass calibration for FAB/MS was based on introduction of perfluorokerosene (PFK) spiked with tris(perfluoroheptyl)triazine,for the reference beam. Calibration at nominal mass accuracy was feasible up to approximately 1000 amu. Mass Calibration: LD/FT/ICR. Nominal mass calibration for LD/FT/ICR was achieved via best fit of perfluorotributylamine (PFTBA) peaks ( m / z 69,131,219, 264, 402, 514) to a mass/frequency relation (26, 27). This calibration could then be extended to give nominal mass accuracy (in subsequent experiments) to m/z 51000, for samples in the absence of PFTBA. However, because so much of the LD/FT/ICR ion intensity for a typical sample (Figures 2 and 3 and 4-6) is concentrated into one peak, a mixture of several compounds still gives a very simple spectrum and offers a convenient and more accurate mass calibration without the use of perfluorinated
ANALYTICAL CHEMISTRY, VOL. 57, NO, 14, DECEMBER 1985 T a b l e I. L D / F T / I C R Mass C a l i b r a t i o n for F o u r Drugsa
C
2943
a Mixture of
compound
calcd mass, amu
measd mass, amu
error in amu
error in PPm
digoxin erythromycin daunorubicin amoxicillin
819.392 71 772.416 84 566.142 27 404.067 67
819.393 98 772.416 83 566.147 89 404.066 54
+0.00126 +0.00176 -0.002 64 +O.OOO 56
+1.5 +2.2 -4.6 +1.3
In each case. the mass is meaeured and computed for the (M +
IB+K? -'
0
400
i(
1
'
500
, .
,
rr CI
I
L
600
700
800
900
1
LD/ FT/ IC R 4-Component Mixture
772
D 819
FA B
+W
B
566
1
+
U
(M + Na)
-I W 0:
0
0
2
Figure 5. LD/FT/ICR spectrum (top) and FAB/magnetic sector spectrum (bottom) of digoxin. Conditions are given in Figure 2.
81
L
w
1 I
'
l
100
200
300
M A S S IN A
500
400
I L L1, 400
450 500 550 600 650 700 750 800 850
Figure 7. LD/FT/ICR spectrum of a mixture of amoxicillin (A), daunorubicin (B), erythromycin (C), and digoxin (D). Conditions are given in Figure 2.
+
,I404
AMOXICILLIN LD/FTKR
M U.
M+Hf f
.. 200 300 400 Figure 6. LD/FT/ICR spectrum (top) and FAB/magnetic sector spectrum (bottom) of amoxicillin. Conditions are given in Flgure 5.
hydrocarbon standards. LD/FT/ICR of a mixture of four large organic molecules (A, amoxicillin; B, daunorubicin; C, erythromycin; and D, digoxin) yields a remarkable mass spectrum in which the four largest peaks ( m / z 404,566, 772,
1
and 819) correspond to pseudomolecular (M K)' ions for each of the components of the mixture. For this mixture (Figure 7) , mass calibration (Table I) yielded a mass accuracy to within 3 millimass units-sufficient to confirm the molecular formula with good reliability. In conclusion, the present results demonstrate that the use of a laser with a Fourier transform ion cyclotron resonance mass spectrometer offers an attractive alternative to fast atom bombardment mass spectrometry. For the four compounds presented here, LD/FT/ICR of the ions formed as a direct result of the laser pulse produced a much more prominent pseudomolecular ion than did FAB/MS with glycerol as the liquid matrix, with an order of magnitude less sample. Although better FAB/MS results may well be possible with different solvents (as illustrated for the daunorubicin example), the main point is that LD/FT/ICR appears to work equally well with whatever solvent dissolvesthe sample. Thus, because the sample need not be soluble in glycerol or other special FAB solvents, a much wider range of compounds is accessible to LD/FT/ICR. The chemical background peaks arising from the glycerol matrix in FAB/MS are absent in LD/FT/ICR. Fragment ions useful for structure determination can be generated by LD/EI/FT/ICR, in which electron ionization is applied to the neutrals desorbed by the laser. Moreover, the extent of fragmentation in LD/EI/FT/ICR can be controlled by the electron beam voltage and current. Sample introduction in LD/FT/ICR is noncritical: we have even seen a spectrum of a solid sample stuck onto double-sided adhesive tape affixed to the metal probe tip. Since LD/ FT/ICR is conducted on evaporated samples at low pressure (ca. torr), the sample chamber is not so quickly contaminated as in the FAB experiment which requires a liquid sample and much higher ion source pressure. Although high mass resolution with FAB/MS is difficult, high mass accuracy is routinely possible with LD/FT/ICR, based on a mass
2944
Anal. Chem. 1985, 57,2944-2951
+
calibration for several (M K)' ions from a mixture of large molecules in a single sample.
ACKNOWLEDGMENT The authors thank D. Horton, S. L. Mullen, W. Priebe, and L. W. Robertson for helpful discussions. Finally, the guidance of R. E. Hein and R. B. Cody is gratefully acknowledged. Registry No. Amoxicillin, 26787-78-0; daunorubicin, 2083081-3; erythromycin, 114-07-8; digoxin, 20830-75-5. LITERATURE CITED Beckey, H. D. Int. J . Mass Spectrom. Ion Phys. 1969, 12, 500-503. Torgerson, D. F.; Skowronski, R. P.; Macfarlane, R. D. Blochem. Biophys. Res. Commun. 1974, 6 0 , 616. Benninghoven, A.; Sichtermann, W. Org. Mass Spectrom. 1977, 12,
595. Grade, H.; Winograd, N.; Cooks, R. G. J . Am. Chem. SOC. 1977, 99, 7725-7726. Posthumus, M. A.; Kistemaker, P. G.; Meuzelaar, H. L. C.; Ten Noever de Brauw, M. C. Anal. Chem. 1978, 50, 985-991. Barber, M.; Bordoli, R. S.;Sedgwick, R. D.; Tyler, A. N. J . Chem. SOC.,Chem. Commun. 1981, 325-327. Blakely, C. R.; Carmody, J. J.; Vestal, M. L. J . Am. Chem. SOC. 1980, 102, 5931-5933. McCrery, D. A,; Ledford, E. B., Jr.; Gross, M. L. Anal. Chem. 1982, 5 4 , 1435-1437. Burnier, R. C.; Cariin, T. J.; Reents, W. D., Jr.; Cody, R. B.; Lengei, R. K.; Freiser, B. S.J . Am. Chem. SOC. 1979, 101, 7127-7129. Cotter, R. J.; Tabet. J.-C. Anal. Chem. 1984, 56, 1662-1667.
(11) Neusser, H. J.; Bosei, U.; Weinkauf, R.; Schlag, E. W. I n t . J . Mass Spectrom. Ion Proc. 1984, 60, 147-1513, (12) Marshall, A. G.; Comisarow, M. B. Chem. Phys. Lett. 1974, 25,282. (13) Marshall, A. G.; Comlsarow, M. B. Chem. Phys. Lett. 1974, 26,486. (14) Marshall, A. G.Acc. Chem. Res., in press. (15) Marshall, A. G. "Proceedings of the International Symposlum on Mass Spectrometry in the Health and Life Sciences"; Burlingame, A. L., Ed.; Eisevier Science Publishers B. V.: Amsterdam, in press. (16) Hein, R. E.; Cody, R. B. Anal. Chem., in press. (17) Marshall, A. G.; Wang, T.4. L.; Mullen, S. L.; Santos, I.32nd Annual Conference on Mass Spectrometry and Allied Topics, San Antonio, TX, 1984; pp 589-600. (18) Wilkins, C. L.; Well, D. A.; Yang, C. L. C.; Ijames, C. F. Anal. Chem. ianr. .""",
57
*on
I ,I, S " .
(19) (20) (21) (22) (23) (24)
Vestal, M. L. Mass Spectrom. Rev. 1983, 2 ,447. McIver, R. T., Jr. Rev. Sci. Instrum. 1970, 4 1 , 5 5 5 . Comisarow, M. B. Int. J . Mass Psectrom. Ion Phys. 1981, 3 7 , 251. Cotter, R. J.; Yergey, A. L. J . Am. Chem. SOC. 1981, 103, 1596. Pharm. Times 1984, 5 0 , 31. Wiernik, P. "Anthracyclines: Current Status and New Developments"; Crooke, S. T., Reich, S.D., Eds.; Academic Press: New York, 1980; pp 273-294. (25) American Drugglsf (1985), 191, 30. (26) Jeffries, J. 8.; Barlow, S. E.; Dunn, G. H. Int. J . Mass Spectrom. Ion Proc. 1983, 5 4 , 169-187. (27) Ledford, E. B., Jr.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1984, 56, 2744-2748.
RECEIVED for review April 29, 1985. Accepted July 9,1985. This work was supported by grants (to A.G.M.) from the National Institutes of Health (GM-31683)and The Ohio State University.
Approach for Structural Interpretation of Laser Microprobe Mass Spectra of Organic Compounds Luc Van Vaeck,* Jan Claereboudt, Johan De Waele, Eddy Esmans, and Renaat Gijbels Department of Chemistry, University of Antwerp, Universiteitsplein 1, B 2610 Wilrijk, Belgium
Organk compounds from different classes were measured by uslng the laser mlcroprobe mass analyzer (LAMMA) In the podtlve and negative ion detection mode. Examples, selected for dlscusslon, Include polycycllc aromatic hydrocarbons, the correspondlng aza heterocycllc and oxygenated analogues, and several polyfunctlonal molecules wlth phenolic groups. High mass resolution electron impact mass spectrometry (EI-MS) wlth dlrect probe Introduction was applled to the same samples. A model for Interpretation of the LAMMA mass spectra has been developed to allow for structural assignment of the ions, though It stlll remains rather tentatlve in nature. As to the poslthre Ions, a strlklng slmllartty between LAMMA and EI-MS was observed. Hence, a major role Is attributed to the formatlon and subsequent fragmentatlon of odd-electron molecular Ions upon laser microbeam irradlation of solids. I n the negative Ion detection mode, LAMMA mass spectra revealed that nonionic organic compounds readily undergo dlslntegratlon: the malor slgnals are due to carbon cluster-type Ions (C,- and C,H-), which do not contain molecular information.
The laser microprobe mass analyzer (LAMMA, LeyboldHeraeus) has been revealed to represent a significant breakthrough in the field of microanalysis. On the one hand, LAMMA allows for highly sensitive elemental determinations in (non)conducting samples (1-4). It originally aimed at biomedical research, but it soon became used in almost all
scientific disciplines (5-9). On the other hand, a major asset of the technique concerns the potential benefits for the measurement of organic compounds. As a mass spectrometer (MS), providing laser irradiation to induce desorption ionization (DI) in solid samples, LAMMA offers the possibility of coping with high-molecular weight, nonvolatile, and/or thermolabile products (10-14). Moreover, with its introduction, the advantages of microprobe analysis became available for organic applications. Promising results have been reported from some feasibility studies (15-18). The major limitation to the use of LAMMA for organic compounds remains the interpretation of the mass spectra; indeed, the actual ion formation mechanisms, involved in the DI processes upon laser microbeam irradiation, are still to be studied in detail (10,19,20). Meanwhile, a tentative structural assignment of the detected ions can be attempted by fairly extrapolating the common knowledge from other MS methods. A basic feature, on which the MS behavior of ionized organic molecules depends, concerns the odd- or even-electron nature of the initially generated parent ions. A s w e y of the literature data on LAMMA analysis reveals that the formation of even-electron type parent species is commonly accepted. Frequently encountered processes include, e.g., proton transfer, alkali attachment, desorption of intact preformed ions from salts, gas-phase reactions between neutrals and codesorbed alkali ions (10, 19-24). This observation is consistent with the general idea about laser DI-MS as a soft ionization method. It has been stated explicitly that the generation of odd-electron ions is not a common process in LAMMA (10, 19). An ex-
0003-2700/85/0357-2944$01.50/00 1985 American Chemical Society
