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spectrum of the interferogram for the rn-methylphenetole entry is shown in Figure 4a. Figure 4b is the spectrum of acetone, and the spectrum of rn-methylphenetole is presented in Figure 4c. In this case, an error in the data set was located, rather than a failure of the search system.
CONCLUSIONS A functional interferogram search system was clearly demonstrated. Several assumptions were made in the demonstration of the system. The primary assumption was that dependent features of the data do not interfere with the search. These features include, sampling interval, instrumental band shape, bandwidth, inherent dynamic range, and detector responsiveness. This assumption is not precisely true. The data base spectra have a bandwidth of approximately 4000 wavenumbers (Le., 450-4500 cm-') because the spectra were collected with a TGS detector. In practical GC/FT-IR applications, an MCT detector that covers the bandwidth of approximately 750-2800 wavenumbers is used. Interferogram patterns from these two detectors are incompatible due to the different bandwidths of the detectors. Work is continuing to correct this incompatibility and demonstrate the system with GC/FT-IR data. Nevertheless, a viable, workable interferometric search system has been demonstrated and the
need for instrument-independent interferometric data has been clearly defined.
LITERATURE CITED (1) Fellgett, P. 6.J. Phys. Radium 1958, 19, 187-237. (2) de Haseth, J. A.; Isenhour, T. L. Anal. Chem. 1977, 49, 1977-1981. (3) de Haseth, James A. Ph.D. Dissertation, Universlty of North Carollna at Chapel Hill, 1977. (4) Small, G. W.; Rasmussen, G. T.; Isenhour, T. L. Appl. Spectrosc. 1979, 33, 444-450. (5) Schroder, 6.;Geick, R. Infrared Phys. 1978, 18, 595-605. (6) Mertz, L. Infrared Phys. 1967, 7, 17-23. (7) Forman, Michael L.; Steel, W. Howard; Vanasse, George A. J. Opt. SOC.Am. 1966, 56, 59-63. (8) Griffiths, Peter R.; Azarraga, Leo V.; de Haseth, James; Hannah, Robert W.; Jakobsen, Robert J.; Ennis, Margaret M. Appl. Spectrosc. 1979, 33, 543-5148,
RECEIVED for review July 17, 1981. Accepted September 18, 1981. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. Environmental Protection Agency. Financial support for J.A.deH. by the U S . Environmental Protection Agency under Cooperative Agreement CR807302010 is greatly appreciated. This work was presented in part a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1981.
Some Cation and Anion Attachment Reactions in Laser Desorption Mass Spectrometry K. Balasanmugam, Tuan Anh Dang, R. J. Day, and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Positive- and negatlve-ion laser desorption mass spectra reveal cationlration by metals and anlonizatlon by chloride. Cationized molecules can be detected in the absence of protonated molecules and metal-containing fragment ions are sometlmes observed. Chloride ion attachment to pyrldoxamlne and pyridoxine occurs when thelr hydrochloride salts are sublected to laser Irradiation. These Ion attachment processes are analogous to those observed in other forms of desorption lonlration.
In recent years significant progress has been made toward obtaining mass spectra of involatile organic compounds. Crucial to this effort has been the exploitation of new ionization methods such as field desorption (FD) (1, 2) laser desorption (LD) (3-8), secondary ion mass spectrometry (SIMS) (9-13) and the related fast, atom bombardment (FAB) techniques (14), electrohydrodynamic ionization (EHD) (15-17),and 252Cf-plasmadesorption (PD) (18-20). A common characteristic of these techniques is the generation of cationized species in which a metal cation, usually Na or K, becomes attached to the intact sample molecule. Although alkali cations present as impurities are often sufficient to cationize samples using these ionization methods, results are sometimes obtained after the addition of a metal salt to the sample (10, 13, 21, 22). Cationization reactions occurring during SIMS have been studied and are known to be general in nature; alkali, transition, and main-group metals have all been observed to cationize organic molecules (10,22).
Cation attachment reactions are also initiated by laser desorption, as evidenced by the detection of (M Na)+ ions for a variety of samples including the carbohydrate stachyose (3). Indeed, in many cases cationization is critical to the observation of intact molecular species for involatile and thermally labile samples, (M + H)+ typically being absent. Recent experiments have shown that alkali cationized molecules can be generated simply by heating (23,24). Studies of (M Na)+ formation in LD have suggested that cationized species arise by attachment of thermally emitted Na+ ions to desorbed organic molecules (6). In addition, silver attachment to sucrose has been reported by LD (7). Heavy metals such as Sb can attach to organics in EHD. The observation of (M + C)' ions, C = cation, for these various techniques suggests that the ion attachment processes initiated by all the desorption ionization methods are similar. In fact, the same mechanism has been proposed to account for cation attachment in several of these methods: metal ion formation followed by formation and emission of the metal-molecule complex (6, 10, 13). The present report describes further results on metal attachment reactions in LD. The analogous anion attachment process, anionization, has been reported for field desorption (2),electrohydrodynamic ionization (15-1 9, and P D (19). We have also observed attachment of chloride ions in LD. The purpose of the present communication is to show that attachment reactions are general in LD both with regard to organic types and for different metal ions. EXPERIMENTAL SECTION LD mass spectra were obtained by using commercially available instrumentation (Leybold-Heraeus LAMMA 500). The output
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0003-2700/81/0353-2296$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY. VOL. 53. NO. 14. DECEMBER 1981 2297 SUCROSE+U CI P O S I T I V E I O N LOMS 01
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SUCROSE+Aa2CQ POSITIVE ION konsl
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2. A portrar of lha LD mass specbum of sucrose mixed wiih sllver carbonate.
of a frequency quadrupled Q-switched Nd-YAG laser (265 nm, 15 NI pulse width) is focused onto the sample using one of three micrcscopic objectives: lox, 32X, 1OOX. In most eases the 32X objective was used. Changes in laser spot size had little effect on the mass spectra. Pulse power was varied with a set of filters and was adjusted to give the lowest power density needed to obtain a mass spectrum, -1Oe W/cm2. Ions were accelerated (3 keV) into the drift tube of a time-of-flight mass spectrometer. The output from a 17-stage electron multiplier was coupled to a transient recorder after appropriate amplification. The transient recorder functioned as a storage buffer for selected portions of the mass speetnun. Its timing sequenee was triggered by the laser pulse, and in all cases mass spectra were obtained for single laser firings. Spectra were displayed on an oscilloscope; hard-copy output was via a strip chart recorder. Mass assignments were determined by measurement along the chart paper and hand calculation. Mass spectra were linear in time, giving a mass scale proportional to (rn/z)''2. Compounds were obtained commercially. The powdered samples were dissolved in ethanol, acetone, or methanol and evaporated onto filmed copper grids. The grids are identical with those used in electron microscopy. The Formvar (polyvinyl formal) films used to support the sample were sufficiently thin that they did not contribute significantlyto the LD mass spectra. In some cases a small crystal of NaCl was added to the sample solution to enhance (M + Na)+ abundances. Spectra were also obtained after deposition of the solid on the copper grid in the absence of solvent.
RESULTS AND DISCUSSION Table I summarizes the LD results for a variety of compounds including amides, carbohydrates, and ketonea. Silver, copper, cobalt, lead, and cadmium all attach to organic compounds during irradiation. In some caws cationized fragments are also observed. Although these ions may also arise by cationization of decomposition products, recent mass spectrometry experiments (7) have shown that (M + C)+ ions, C = cation, do dissociate to yield metal-containing ions. For example, ions corre-
Table 1. Metals Observed to Cationize Organic Compound by LD cationized ( M + H)' frag detected ments no no
compound metals acetamideAs benzaldehyde p-toluamide Ag, Cu yes no no no p-nitroAg benzamide yes(very no terphthalic acid Pb diamide small) N-acetyl-pAg no no toluidine yes no p-acetanisidine Cu, Co Na. Ag, Li, Cd no Yes sucrose Cu yes no glucosamine Cu, As, Na yes no galactosamine glucuronamide Cu. Ag, Na, K no yes anthraquinone Cu yes no naphthyl phenyl Cu yesb no ketone bend Cu no no uridine Cu no yes a In the LD mass spectrum (glucose + Cd)' was observed M + was more abundant than but not (sucrose + Cd)'.
(M
+ HI'.
+
sponding to (glucose metal)+ are observed in LD masa spectra of sucrose (Table I) (3-5,7). Similar results were reported for SIMS analysis of silver-supported sucrose (12). A particularly striking occurrence of a cationized fragment was observed in the LD mass spectrum of sucrose mixed with CdCI2. Although cationized sucrose was not detected, (glucose + Cd)+ was an abundant ion. This can be contrasted to the results obtained when LiCl is the cationizing reagent. Figure 1 shows the emission of (sucrose Li)+at higher abundance
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ANALYTICAL CHEMISTRY. VOL. 53, NO. 14. DECEMBER 1981
. P Y R I D O X A M I N E 2HCL
contrasted to FD and SIMS speetra of amine salts,which show abundant C&+ and CAz- ions (C = cation, A = anion). On the other hand, loss of HCI from the cluster ion MHCI; (which is of the form CAz-) would yield the anionized free amine. It is not known whether (M + C1)- arises by dissociation of the cluster or by chloride addition to thermally desorbed free amine molecules.
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LITERATURE CITED (1) schunen. H.-R.; Schlebel, H. M. Nahr&semchalten
I
, , , , , .
_ 1
1978, 65. 223-230. (2) Ott, K. H.: R6Ilgen. F. W.; Zwhselman. J.: Fokkens. R. H.; N i M n p . N. M. N. Org. Mass Specborn. 1980. 15,419-422. (3) Posmumus. M. A,: KbtemBkBr. P. G.; Meure!aaaar. H. L. C.; Ten Nde Brauw. M. C. Anal. Chem. 1978. 50,985-991. (4) Stoll. R.: RBllgen. F. W. Org. Mass. Smtrom. 1979, 14,642445. (5) Heresch. F.; Schmki. E. R.; Huber. J. F. K. Anal. Chem. 1980. 52. 1803-1607.
F b n 9. A portion of the u)m a s spectnm of pyrldoxamine2HCI.
Note anionization by CI.
than (glume + LOc. Similar LLI results for (sucrose + Na)+ have been reported (3). In the presence of silver, cationized glucose is more abundant than (M Ag)*, hut the latter is still a major ion in the LD mass spectrum (7)(Figure 2). It is interesting to consider the thennochemical implications of these results. Increased fragmentation of (M C)* ions would be expected for the more exothermic ion attachment reactions. Thus, the relative abundance of (glucose C)+ may be a crude measure of the enthalpy of cationization. More sophisticated mass spectrometric experiments such as those reported for volatile compounds (25) will he needed to suhstantiate this hypothesis. The negative ion counterpart of cationization, anion attachment to yield (M + A)- (A = anion) is illustrated by the appearance of (M CW, m l z 2031205, in the negative ion LD spectrum of pyridoxamine dihydrochloride (M = free pyridoxamine) as shown in Figure 3. The spectrum also contains the quasi-molecular anion, (M - H)- m/z 167, and several fragments, including (M - H - NH3)-, mlz 150, (M - H - HzO)-, mlz 149, and (M - H - CH30H)-, mlz 135. Anionized molecules (M C1)- were also detected in the LD spectrum of pyridoxine hydrochloride, mlz 2041206, which again shows emission of (M - H)., m/z 168, as well as fragments of this parent ion that can he rationalized as losses of ammonia, mlz 151, water, m/z 153, earbon monoxide, formaldehyde, and methanol, m/z 140, 138, and 136, respectively. It is striking that laser irradiation of these compounds generates what can be described as the anionized free amine rather than cluster ions such as MHCI;. These data can he
+
+
+
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+
(6) Colter. R. J. Anal. chem. 1981. 53. 719-720. (7) Zaken, D.: Sehoen. A. E.; W s , R. G.; Hemberger, P. H. J. Am. Chem. Soc.1981, 103, 1295-1297. (8) Heinen. H. J.; Mew, S.: Vcgl. H.; Weboung. R. A&. Mas specbrvn. 1980, 8. 942-953. (9) Benninghaven, A,; ~asph.s.D.; skhtermann. w. nppr. mys. 1978, 11.35-39. (10) Day, R. J.; U m , S. E.: W a , R. G. Anal. chem.1980. 52. 557 A-572 A. (11) Eicke. A,; SlcMermann. W.; Bennin@tmen. A. W.Mas spaamm. 1980. 15. 289-294. (12) Llu. L. K.: Susch. K. L.: Caaks. R. 0. AMI. chem. 1981, 53. 4,0
.4.,
,"I I t". -
(13) Grade, H.: W s . R. G. J. Am. Chem. Soc.1978, 100. 5615-5621. (14) Barber. M.; Bardall. R. S.: Sedgwick. R. D.: Tyler. A. N. J. chem. Soc..chem.Commun. 1981,325. (15) Simons, D. S.;W b y . B. N.: Evans. C. A., Jr. Int. J. Mas SpeCaom. Ion pmn. 1974, 15, 291-302. (16) Stimpson. B. P.: Simons. D. S.: Evans. C. A,. Jr. J. phyg. chem. 1978, 82. 660-6783. (17) Stimpson. B. P.: Evans, C. A,, Jr. Blamed. M a s S p c b m . 1978. 5, 52-63. (16) Macfarhne. R. D.: Togerson. D. F. Sc!ence 1978. lSl. 920-925. (19) Krueger. F. R.; Wien, K. A&. Mas Spectmm. 1978. 7, 1429-1432. (20) McNeaI. C. J.; Macfarlane. R. D. J. Am. chem. Soc. 1981. 103, 1609-1810. (21) Rollgen, F. W.; Schunen, H.4. Og. Mas specborn. 1975. 10, 660-666.
(22) Day. R. J.; Unger. S. E.: Cwks. R. G. Ansl Chem. 1980. 52, *r;, ,E" "*"-""-.
(23) Stall, R.; Wlgen, F. W. Org. M a s specborn. 1981, 16. 72-75. (24) Colter, R. J.: Y q e y , A. L. AMI. chem. 1981. 53, 1306-1307. (25) Cody. R. 8.; Burnier. R. C.; Reents. W. D.. Jr.; Cadin, T. J.; Mccrery. D. A,: Lengel, R. K.: Freiser, 0. S. Int. J. Mas Specnom. Ion mw.
1980. 33. 37-43.
RE~EIVED for review June 26,1981. Accepted September 11, 1981. This work was supported by the National Science Foundation under Grant CHE78-00876. Partial funding for purchase of the LAMMA-500 was provided by the National Science Foundation Chemical Instrumentation Program.
