1310
Anal. Chem. 1983, 55, 1310-1313
Doubly Charged Ions in Desorption Mass Spectrometry David N. Heiler, James Yergey, and Robert J. Cotter" Department of Pharmacology & Experlmental Therapeutlcs, The Johns Hopkins Universiw, Baltimore, Maryland 2 1205
Doubly charged Ions are rarely encountered In desorption techniques. For divalent ions, the production of singly charged specles by clustering with counterlons, decomposltlon and one-electron reductlon are generally observed. Doubly charged ions are, however, observed more frequently in field desorption than In other desorption methods, lndlcatlng the Importance of the field in their extractlon from the surface. The observatlon of doubly charged Ions mlght therefore be used as a probe of field effects in desorptlon mass spectrometry.
In all of the desorption methods, organic salts generally produce ions corresponding to the cationic or anionic portions of the compound ( I , 2). Thus, quaternary ammonium cations, R4N+,are easily observed in the positive ion mass spectra of their halide salts, using field desorption (3),thermal desorption (4,5),plasma desorption (6),secondary ion mass spectrometry (6),fast atom bombardment ( I , 7,8),and laser desorption (9, 10). The negative ion, B(C6H5)4-,from sodium tetraphenylboron is easily observed in the negative ion mass spectrum using thermal desorption (4) and fast atom bombardment (11). However, when organic salts contain divalent cations (or anions), the doubly charged cationic (or anionic) species are not so easily observed, in spite of the likely presence of these ionic species on the surface (or in the liquid matrix in the case of FAB). Singly charged ions generally predominate. Clustering of the doubly charged ion with the singly charged counterion, decomposition to remove one of the charge centers, or (in certain favorable cases) one-electron reduction produce the major singly charged species. In a simple example, the thermal desorption spectrum of CaC1, reveals a much greater abundance of Ca+ than Ca2+(12), while the FAB and laser desorption techniques produce prrdominantly Ca+ and CaCF (13). Doubly charged ions, observed in desorption methods, have of course been reported in the literature. Wood has used a technique which promotes the doubly charged species (M + Ca)% in field desorption for the extension of the effective mass range of his instrument (14). McEwen and Layton have reported doubly charged ions for diquaternary salts using field desorption (15). However, they found that for compounds of the type
-a the cluster ions C2+Br-are preferred at low emitter temperatures, while diquat CH3-'-i-CH3
2Br-
and methyl green 0003-2700/83/0355-1310$01 S O / O
t
(CH3 ) p N ( -C2H5)
N(-CH3 ) 2
N(-CH) t# 12
2c1-
C -
produce singly charged ions by thermally induced one-electron reduction (16). Cooks et al., using the SIMS technique (In, observed singly charged molecular ions (produced by oneelectron reduction) for paraquat and diquat, while doubly charged ions were observed for a number of other diquaternary salts. Decomposition reactions which remove one charge center and produce even-electron fragment ions were also observed as major processes. It has been our experience that, while doubly charged ions are rare in desorption mass spectrometry, they are more easily observed in field desorption than in fast atom bombardment, thermal desorption, or laser desorption, and that the field is important for the extraction of these ions. In this paper we present some results for several organic salts with divalent cations, using a combination FD/FAl3 ion source. While the singly charged ions resulting from decomposition and clustering with anions are observed in both methods, the appearance of doubly charged ions and one-electron reduction processes is strikingly different, so that these latter ions might be used as a probe for field effects in desorption.
EXPERIMENTAL SECTION Mass spectra were recorded on a Kratos MS-50 mass spectrometer, equipped with a DS-55 data system. A combination field desorption/fast atom bombardment source, described previously (I@, was used to provide similar conditons (i.e.,focal points and extraction optics) for both modes of ionization. The similarity in geometry is illustrated in Figure 1. Switching between the two modes is accomplished by (1)changing the probe on which the sample is deposited, and (2) connecting the counterelectrode either to the &kV potential of the probe (FAB) or to ground (FD). Thus, in the FAB configuration there is no field imposed upon the surface itself, so that as indicated in Figure 1 both positive and negative ions may be desorbed, even while only positive ions are analyzed using the 8-kV accelerating potential. In the FD configuration, the accelerating potential also provides the field for preferential desorption of positive ions. An Ion-Techfast atom gun was used to produce 5-10 keV xenon atoms. Samples were dissolved in methanol and deposited on a drop of glycerol (FAJ3) or on an activated emitter (FD). Emitter currents were generally in the range of 20 to 25 mA. Accurate mass measurements were made in the FAE3 mode at a resolution of 20000. RESULTS AND DISCUSSION q d - p-Xylenediylbis(triphenylphosphoniumbromide). Table I compares the major sample ions observed for a diphosphonium salt using fast atom bombardment and field desorption. The doubly charged ion at m / z 314 is observed in each case, but with greater relative abundance in FD. 0 1983 American Chemical Society
t
TABLE I .
Xylene b i s ( t r i p h e n y l phosphonium bromide)
t
(fi-)3P-CH2-b-CH2-P( -f$)3
Br-
m/ z
FAB
FII % R.A.
% R.A.
Br-
probable s t r u c t u r e t
314
14
365
100
36 7
95
95 1
13
-
t
(6-I3P-CH2-B-CH2-P ( -4) 3
68
-.
(8-)3i-CH#CH2 t
707, 709
1.5,
(8- )3P-CH2-#-CH3
-
t
(8- )3P-CH2-6-CH2-P( -#I2 Ct2
1100, 95
1.8
Br” A
TABLE 11.
Trimethylene bi s (pyridi nium bromide)
@-CH2-CH2-CH2-;B ~
m/z
:AB
% R.A.
100
FD I R.A.
38
-
120
100
100
122
31
-
200
45
16
12
20
279, 281
20, 19
479, 481
4, 3.5
55&, 560
accurate mass
probable s t r u c t u r e
18
106
202
2Br-
~~~
0.3,
20, 18
-
1, 0.4
562
Monovalent quaternary ammonium and phosphonium salts readily undergo decomposition reactions to produce neutral tertiary structures, and this type of cleavage forms the major pathways for removing one of the charge centers in the divalent cation. Loss of a phenyl group produces the ions at m / z 551. Cleavage between the methylene group and phosphorus accompanied by hydrogen transfer in both directions results in ions with masses of 365 and 367. Clustering of the dication with one bromide ion produces the singly charged species at 707 and 709, which are the base peaks in the field desorption spectrum.
@-CH2-CH=CH2
200.0075
@-CH - CH2 -CH2 - Br (79)
200.1308
@-CH
-0
2 -CH 2 -CH 2 L @-CH2-CH2-CH2-Br t
202.0052 Ct2
C2Br
(81)
Br” t
t
CzBr2
Trimethylenebis(pyridinium bromide). Table I1 compares the major ions observed by the two techniques for the diquaternq salt, trimethylenebis(pyridinium bromide). The doubly charged ion at m / z 100 is observed easily in field desorption and at very low (
