Anal. Chern. 1983, 55, 2195-2196
2195
Ion Promotion in Fast-Atom Bombardment Mass Spectrometry by Charge Transfer Complexation Sir: In fast atom bombardment and secondary ion mass spectrometry (F.AB-SIMLS) of organic molecules, the proportion of ions in the total sputtered material is rather low (below W ) (I). Among the ions, molecular radical ions Mi’. are rarely observed. To increase the sensitivity of the method, one can optimize the momentum of the primary particles (2, 3) and increase, for a given sputtering yield, the ionization efficiency by in situ derivatization reactions, chiefly by achdo-basic reactions or cation attachment (4, 5 ) . A promotion reaction designed to selectively enhance the M+. ion signal is presented here together with possible applications and consideration concerning the ion formation mechanism. The selected property that will be used. in this promotion reaction is the possibility that polar and nonpolar molecules undergo charge transfer complexation leading to an irreversible one-electron transfer in polar solutions (6). donor -t acceptor
%
(DA)
-
D+.+ A-e
(1)
Comparison between spectra of D with or without complexation by A will indicate how the change in the charge state of the target molecule is reflected in its FAB spectrum. Furthermore, contrary to cation attachment that can be produced both in gas phase or in solution, this electron transfer reaction can only occur with the help of the solvation energy of the ions (6). The presence, in this system, of an unambigous ion preforming effect will thus be tested.
EXPERIMENTAL SECTION N,NN’W’-Tetramethyl-1,4-benzenediamine (TMPD) was chosen as the D molecule, not as a typical FAB compound but for its charge transfer complexation properties and the stability of its D+. ion in solution (6). Quinones (iaubstituted p-benzoquinones) were used as acceptors. The spectra were taken on a AEIMSS double focusing instrument adapted for FALBand operated in the low resolution mode (Amlrn = 1.000). Typical spectra were recorded with a 7-keV, 10-rA Xe beam produced by a saddle field type gun. Samples were prepared by dissolving 0.1 mg of D and A in 1 mL of glycerol. Under those conditions, the dominant species in solution are El+., A-e, and the protonated forms of A - a . The equilibrium constants of reaction 1 are not available for glycerol. In solvents with dielectric constants above 30 (methanol, water, and thus glycerol), the complex (DA) is almost completely dissociated into ions. Spectra were recorded 5 min after introduction of the sample in the ion source.
RESULTS FAB Spectra in Glycerol. The FAB spectrum of TMPD in glycerol is similar to the spectra of other amino compounds. Intense (ID + H)+,D+., and (D - H)+ signals are detected as well as those produced by the successive imethyl losses. Peaks at +14 aniu are observed, corresponding to H substitution by CHB. At higher masses, those ions solvated by glycerol are also detected. Quinone spectra exhibit only weak signals of the (A + 2H)+ ion for p-benzoquinone arid the (A 3H)+ ion for C1 and CN substituted ones. The (I)+ H)+ peak can be explained by protonation of D by glycerol (pK of glycerol in water = 14.15). D+. and (D - H)+ ions can be formed by oxidation and imine formation during the cascade process or by gas phase dissociation of (D + €I)+ (Figure la). As bombardment proceeds, (D + H)+ and (D - H)+ signals increase as compared to 1)’s. Time dependence of the spectra could be correlated to radiation damages on the sample (7).
+
However factors like preferential sputtering and evaporative losses of the sample cannot be excluded. In the present case, however, the time dependence of spectra is different for various ionic species originating from the same molecule, which favors a radiation damage mechanism, but others could be evoked like gas phase ionization of the evaporated neutrals. Addition of acidic compounds (tartaric acid, NH,Br) increases the (D + H)+ signal and, to a lesser extent, the D+- signal (Figure lb,c). The latter can be at least partially produced by dissociation of (D + H)+ during the desorption process. On the contrary, the (M - H)+ ion signal is not correlated to others and decreases. Addition of quinone produces a selective increase of the D+. peak height (Figure Id). For an excess of D, after bombardment, an increase of the (D + H)+ signal is also observed, correlated with those of (A 2,3H)+ ions. Undissuciated complexes are detected as (DA)H+ and (DAH2)H+ions. The (D H)+ signal increase is explained by formation of an acidic compound, the 1,4-dihydroxy compound, as checked by addition of hydroquinone in solution. Different reactions (Hscavenging, acid-basic reactions with the matrix) can explain the formation of phenolic molecules but the first necessary step is the one-electron transfer from D, the quinone being only weakly detected in its absence. The classical solution chemistry of DA complexes is well reflected in their FAB spectra. FAB in Dimethyl Sulfoxide (Me2SO). In order to lower the available amount of hydrogen ions, the FAB spectra were taken in Me2SQ. Spectra are similar to those observed in glycerol. Upon quinone addition, the increase of the D+. signal is more important. Two parameters can be involved: the decrease of protonation of D and the increase of the solvent evaporation rate leading to the precipitation of the stable (U+A-.) salt.
+
+
DISCUSSION Charge transfer complexation is a simple, selective promotion reaction that can be used when molecules with low ionization energy, polar or nonpolar, form dissociated complexes in solution. Besides the ion signal, the solubility of nonpolar compounds in glycerol is increased by organic salt formation. Other molecules have been submitted with success to this promotion reaction: various leuco species of aromatic dyes, methoxy-substituted benzenes, and neuroleptic drugs derived from phenothiazine. Promotion of the AH+ ion from quinones (anthraquinone, vitamin K3) can also be produced by reducing these acceptors by TMPD. The selective increase of the molecular radical ion signal is explained by the formation in solution of the ionic I)+. species after a one-electron transfer according to reaction 1, the FAB spectra matching the solution chemistry. Gas phase dissociation of (DH+) producing D+. can be discarded in view of the spectra in acidic solutions; linked scan experiments not available at the moment could be performed on the system. Concerning the ion formation mechanism, one can infer that molecular radical ion ejection is possible in the sputtering process once those are preformed in the sample. This, as well as ejection of intact DA complexes, leads to a view of the ejection process as a rapid, out of equilibrium desolvation process allowing weakly bound DA species to survive. A direct extension for this charge transfer concept can be foreseen and is under investigation in our laboratory. C T
0003-2i’00/83/0355-2195$0 1.50/0 0 1983 American Chemical Society
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Anal, Chem. 1983, 55, 2196-2199
A-.
+ DI+., (AD),, (AD)s,...
Tuning the photon source a t the wavelength of a specific complex could achieve the selectivity of the promotion.
ACKNOWLEDGMENT The author thanks F. W. Roellgen, U. Giessman, and S. S. Wong from Bonn University for helpful discussions. Registry No. N,N,N',N'-Tetramethyl-l,4-benzenediamine, 100-22-1.
LITERATURE CITED
165 165 165 165 m/e Flgure 1. Molecular region of the FAB-MS spectrum of TMPD: (a) solution in glycerol, (b) solution in glycerol-NH,Br, (c) solution In gly-
cerol-quinone, (d) solution in Me,SO-quinone. Relative recorder settings are as follows: (a) 1; (b) 0.2; (c) 0.6; (d) 0.5. complexes not dissociated in solution can be brought to an excited dissociated state by irradiation in the CT absorption band. This band is located in the visible-near-UV range and thus is easily accessible. Moreover its position is directly related to the ionization energy of D. Simultaneous photoexcitation of the glycerol drop during FAB can lead t o a selective promotion reaction according to the reaction
(1) Benninghoven, A.; Slchtermann, W. Anal. Chem. 1978, 50, 1 180- 1 184. (2) Kambara, H. Org. Mass. Spectrom. 1982, 17, 29-33. (3) Martin, S. A.; Costello, C. E.; Biemann, K. Anal. Chem. 1982, 5 4 , 2362-2368. (4) Busch, K. L.; Cooks, R. G. Science 1982, 278, 247-254. (5) Day, R. J.; Unger, S. E.; Cooks, R. G. Anal. Chem. 1980, 52, 353-354. (6) Foster, R. "Organic Charge Transfer Complexes"; Academic Press: London and New York. 1969; Oraanic Chemistry, a Series of Monographs, Voi. 15. (7) Field, F. H. J . fhys. Chem. 1982, 86, 5115-5123.
E. De Pauw Institut de Chimie Universit6 de LiBge, B6 B 4000 Liege I, Belgium RECEIVED for review April 14, 1983. Accepted July 11, 1983.
AIDS FOR ANALYTICAL CHEMISTS Time-Optimized Thin-Layer Chromatography in a Chamber with Fixed Plate Lengths Ronald E. Tecklenburg, Jr., Rose M. Becker,' Eric K. Johnson, and David Nurok* Department of Chemistry, Indiana University-Purdue Indianapolis, Indiana 46223
University at Indianapolis, P.O. Box 647,
Thin-layer chromatography (TLC) has many advantages over high-performance liquid chromatography (HPLC) for the separation of simple mixtures. These include the fact that multiple samples can be run simultaneously, that solvent properties do not interfere with solute detection, and that the volume of solvent used per sample is generally 1t o 2 orders of magnitude less than in HPLC provided that a suitable development chamber is used. A limitation of TLC is that solvent path length is limited t o about 20 cm with conventional TLC plates and to about 10 cm with high-performance plates. These path lengths are often not long enough for difficult separations. Such separations can often by accomplished by continuous development whereby solvent is allowed to evaporate off the end of the TLC plate. It has been recommended that binary solvents comprising a weak solvent such as hexane and a stronger solvent such as acetone be used for these separations (1). Apparatus for such developments-the Regis Short Bed/Continuous Development (SB/CD) chamber-has been available for several years and has been used for a variety of separations. It has recently been shown that continuous developments with binary solvents can be time-optimized (2). The suitability of 'Present address: Dow Chemical Co., Midland, MI 48640.
using the Regis SB/CD chamber for time-optimized separations is evaluated below.
EXPERIMENTAL SECTION The Regis SB/CD chamber wm used for chromatography. The chamber is rectangular with four ridges along the floor,all parallel to its long axis. This allows a TLC plate to be inserted silica gel coated side up in five positions with the base of the plate resting against a ridge (or in position 5 on the wall) and the upper end of the plate propped against the opposite wall. The line of contact between the TLC plate and the glass cover of the chamber defines the line where solvent starts to evaporate. It is essential to allow about 1.5 cm of plate to extend beyond this contact line as this is the area from which solvent evaporates. The rate of evaporation can be roughly optimized by placing the chamber in a hood and raising the front to an appropriate level. In our laboratory an opening height of 18 in. was found satisfactory. With 26 mL of solvent in the chamber, the path lengths for positions 1through 5, from solvent source to the initial point of solvent evaporation, are 2.35 cm, 3.85 cm, 5.40 cm, 7.00 cm, and 8.30 cm. Analtech, Inc. (Newark, DE) silica gel plates, catalogue no. 47011, were cut into appropriate sections 10 cm wide before use. Plates were stored in the laboratory atmosphere overnight before use. The humidity reported ahead refers to the ambient laboratory humidity.
0003-2700/83/0355-2196$01.50/00 1983 American Chemical Society
