Anal. Chem. 1980,52, 2299-2304
2299
Field Ionization and Field Desorption for Collisional Activation Mass Spectrometric Determination of Organic Tin Compounds Raymund Weber, Frieder Visel, and Karsten Levsen Instituf fur Physikalische Chemie, 0-5300Bonn, Federal Republic of Germany
Field ionization and field desorption spectra of a variety of organic tln compounds are reported. In contrast to the electron Impact spectra, In which only very low molecular Ion abundances are observed, the field ionization spectra are dominated by the molecular ion and show llttle If any fragmentatlon. The colllsional activation spectra of these fleld ionized molecules allow a rapid and unambiguous structure elucidatlon. Several examples are reported which demonstrate that the components of a mlxture of unknown composition can be ldentlfled unequivocally by using the comblnation of field ionization and collisional actlvatlon.
Collisional activation (CA) spectra have been used extensively during the past 8 years in fundamental studies such as ion structure work ( I , 2). There is, however, a growing interest to explore t h e analytical potential of this method. Thus, it has been demonstrated t h a t CA spectra of field ionized (3) or field desorbed (4-9) molecules give important structural information which is often not available from the field ionization (FI) a n d field desorption (FD) spectra alone. In addition we have demonstrated in 1974 that the CA technique can be used for a direct mixture analysis (3). For this purpose two-stage mass spectrometers such as double focusing instruments of reversed geometry (the magnetic sector precedes the electric sector) are of particular advantage. Here the ionized components of the mixture are mass selected and thus separated by the magnetic field before they undergo a highenergy collision with a neutral target gas. T h e resulting collision-induced fragments of each component are mass analyzed by using the electric sector. These collision-induced fragments reflect the structure of the unknown component of the mixture in much the same way as the electron impact spectrum of the pure sample does. A variety of applications of this technique using electron impact (EI) (10, 1 1 ) , field ionization (3,12),or chemical ionization (CI) (13-20) have been reported and reviewed (21, 22). The most significant contributions to this field have been made by Cooks and co-workers (13-20) who used chemical ionization as primary ion source and were able to demonstrate t h a t the analysis of unknown mixtures is possible with high sensitivity and little or no pretreatment of the sample. This and several subsequent papers will explore the analytical potential of a combination of field ionization or field desorption and collisional activation in more detail. Organic tin compounds have been chosen in this study as these compounds are produced industrially in large quantities, e.g., as fungicides, bactericides, and wood preservatives. As organic tin compounds are of medium toxicity, it is of interest to explore their ecological impact (23). Thus the availability of analytic techniques for their identification is of interest. EXPERIMENTAL SECTION The E1 spectra of the tin organic compounds were measured 0003-2700/80/0352-2299$01 .OO/O
on a Varian MAT 44 S instrument (electron energy 70 eV, source temperature 250 "C). The FI or FD spectra were recorded by using a double focusing instrument of reversed geometry constructed by us. Tungsten wires (10 pm) activated with benzonitrile were used as FI or FD emitters. The potential difference between emitter and counterelectrode was 1 2 kV, the translational energy of the ions 5 kV. In the FI mode the sample was introduced either via the gas inlet system or via the direct insertion probe. The CA spectra were obtained with the FI instrument constructed by us. This instrument was equipped with a collision cell near the energy resolving slit between magnetic and electric sector. Argon was used as collision gas the pressure of which was adjusted to obtain optimum fragment abundances. (This led to a decrease of the precursor ion to about one-third of its original abundance due to scattering and decomposition.) CA spectra were obtained by scanning the electric sector potential repetitively. At least 15 spectra were accumulated and averaged on a multichannel analyzer. The results are the mean of at least three independent runs. The reproducibility was better than 15%. Several samples were commercially available: (n-C,H9)3SnCl (EGA), (n-C,Hg),Sn (EGA), (CH3),SnC1 (EGA), (C6H5),Snc1 (Merck), (CH3),SnC12(Fluka), (C2H5)3SnC1(Merck). (C6Hj)(tC,Hg)SnBr2 was prepared from (C6H,,)3SnC1by reaction with bromine, where the precursor was obtained via a Grignard reaction of (C6H5)3SnC1with (t-C4Hg)MgC1. (C6H5)z(t-C4H,)SnBrand (C6H5)2(i-C3H7)SnCl were synthesized in an analogous fashion. with (C2H6),SnCl2was prepared by reaction of (C2H5)2Sn(C6H5)2 HCl and (CH3)(C2H5)SnC12 by reaction of (C2Hj),(CH3),Snwith HC1. These compounds as well as (t-C4Hg)(neo-C5H11)SnClz were ((:6H5)3Sn(OCOCH3), and gifts from Mr. Visel; (C8H17)2SnC12, (C4H9),Sn(OC0CH3) have been prepared by using literature procedures (24-26). RESULTS AND DISCUSSION E l e c t r o n I m p a c t , Field Ionization, Field Desorption, a n d Collisional Activation Spectra. The E1 spectra of the organic tin compounds investigated in this study have been published (27-29) or are available on request. In Figure 1the E1 spectrum of (C4H9)&nis shown as example and contrasted with the FI and FI-CA spectra. The E1 spectra are characterized by very low molecular ion intensities (usually below 1% relative abundance) and dominated by fragments formed by elimination of the substituent by direct bond cleavages as well as fragments of low mass formed by consecutive decomposition of the substituents (e.g., C2M5+,C3Hs+,etc.) Alkene eliminations and other rearrangement reactions are observed with low abundance. It is apparent that a n unambiguous assignment of the molecular weight by E1 mass spectrometry necessitates a careful purification of the sample. In contrast the FI spectra are dominated by the molecular ion which is the base peak in each case except for (CsH5)(t-C4H,)SnBrg where m / z 57 (the butyl ion) is the most abundant ion (see Table I). The molecular ion shows the characteristic isotope distribution of tin as shown in Figure I b for (C4H9)4Sn. Fragment ions formed by loss of a substituent are usually of very low abundance ( < l o % ) if the FI emitter is kept at room temperature (Table I). A further reduction of the fragmentation is achieved if the FD rather than the FI technique is
0 1980 American Chemical
Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
100
a
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Flgure 1. (a) E1 spectrum of (n-C,H,),SN; the relative abundance of the molecule is 0.13%. (b) FI spectrum of (n-C,H,),Sn. of (n-C4H9),'20Sn (original recorder trace).
(c) F P C A spectrum
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
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Table I. Field Ionization Mass Spectra of Tin Organic Compounds compda
ion intensity (M-X)* R+
(M-R)’
M’. 100 100 100 100
(C,H,),SnClf (CzHJ2SnC12
100 100
C,H,+ = 3 2
1 3
1
3
100 100 100
5
10 4
100 100 100
(t-C,H,)(neo-C,H,,)SnCl,
other fragments
C,H,+ = 11 C,H,+ = 7
135
7
13d 2e
40 14e
Most compounds contained impurities (Table IV). These impurities were identified by CA and are omitted from the spectra. R = alkyl. X = halogen. R = C,H,+. e R = C,H,,’. Ionization by FD. Table 11. Collisional Activation Spectra of Tin Organic Compounds R,SnXaob (precursor mass) x‘
R’
R = CH, R’ = CH,
x = c1
R,SnX+. R,SnX+ R,SnX+ R,Sn+ RSnX+.e R,Sn+.= RSn’ e RSn’ SnX’ Sn +.
R = C,H, R‘ = C,H,
(244)
510
(242) 315
100
100
100
3 6 2 8
3 13 2 6
11 2 9
11
12 3
16 6
4 =
C,H,
R ’ = t-C,H, X = c1 (366) 3 54 100 3
7 2 9
R = C,H, R’ = t-C,H, X = Br
R = C,H,
R = C,H,
R’ = C,H,
R’ = C,H, X = Br
x = c1
x = c1
(200) 6 55
R
R,SnX+, R2SnX’ R,SnX+ R,Sn RSnX+.e R,Sn+. e RSn+e RSn’ SnX’ Sn+.
R = CH, R’ = CH, X = Br
7
x = c1
R’ = i-C,H, X = Br (396) 208 100 10
(35%) 333 100 9
11
5 1 12
10
10
20
7
10 3
17
3
1 11 1
5
(386)
R = C,H,
(370) 349 100
(326) 252 100
R = C,H, R’ = C,H, x = c1
(410) 293 100 1
R = C,H, R’ = i-C,H,
R = C,H, R = C,H, R’ = C,H, R’ = C,H, X = OOCCH, X = OOCCH,
R = neo-C,H,, I R’ = t-C,H, X = C1
330 100
(410) 7 00 100
(350) 296 100
(354)
12 6 3 7
13 28 33
10
g
12
h
15 2
5
3
5 2
> 200 100 4
1
10 4
12 1
As precursor the ions with the isotopes I’C, 120Sn,35Cl,anu are selectecl. Intensities normalizea relative to the most abundant fragment. Both substituents are identical. Contains two different substituents. e In compounds with aryl and alkyl substituents R = C,H,. R = alkyl. [C,H,,SnCl]+.= 5% and [C,H,SnCl]+~= 4%. [C,H,,Sn]+= 3%. In addition [C,H,]+, [C,H,,]+., and [SnCH,]’ were observed.
used. Thus impurities can be readily detected but not identified by F I mass spectrometry. T h e CA spectrum of field ionized (C4Hg)4120Sn is shown in Figure IC. T h e spectrum is strikingly simple showing four peaks which result from the successive loss of the four substituents. As a result of the poor energy resolution and the kinetic energy released upon collision-induced decomposition, adjacent peaks are not resolved. Thus it is likely that the peaks shown in Figure ICnot only are due to alkyl losses but also may contain alkene losses, although with low abundance as observed in the E1 spectra. Moreover, although the precursor at m / z 348 contains mainly the isotopes IzoSnand 12C,
t h e isotopes II9Sn and 13C also contribute to this mass. The situation is even more complicated in the case of halogensubstituted compounds. Thus the nominal mass m / z 412 in the low-resolution FI spectrum of (C6H5)(t-C4Hg)SnBrz contains six peaks with varying isotopic composition of I15SnlZ0Sn,lZC,13C, 79Br,and 81Br which upon collision-induced loss of the substituents give fragment clusters which are not resolved under our experimental conditions. Unit mass resolution could be obtained by using the “linked scan” technique (30, 31), but in this case this technique gives no additional information of analytic significance. Moreover, artifact peaks which are frequently observed in “linked scan” spectra (32)
2302
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
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only complicate the interpretation of the spectra. In Table I1 the CA spectra of tin compounds of the general type R3SnX (R = alkyl or aryl, X = halogen or acetyl) are summarized. Depending on their volatility the molecules are either field ionized or field desorbed. The high abundance of the collision-induced fragments relative to the precursor is remarkable and reflects the low stability of the molecular ion. T h e CA spectra show only fragments due to the elimination of the various substituents ( [R2SnX]+, [RSnX]+., [SnX]', Sn+., [R3Sn]+,[R2Sn]+.,and [RSn]+) and thus allow a rapid and unambiguous assignment of the structure, except that a differentiation between branched and unbranched alkyl substituent is not possible. With a 105-g sample applied directly onto the F I emitter a good CA spectrum can be obtained (signal to noise ratio >50:1). As result of the low ionization energy of tin (IE = 7.34 eV) (33) the charge is always retained on the tin containing fragment with the exception of those compounds which have both neo-C5H11and t-C4Hgas substituents (Tables I1 and 111). T h e most prominent peak results from an elimination of an alkyl or aryl substituent, a process which leads to a stable even electron species. The same behavior is observed in the E1 spectra (27-29). In general, the second most abundant peak corresponds to the formation of [SnX]+,demonstrating that the Sn-X bond is stronger than the Sn-R bond as discussed below. (Note that the relative abundances are not corrected for the reduced multiplier response with reduced translational energy (mass) of the fragment.) T h e CA spectra of compounds of the type R2Sn X2 (R = alkyl, aryl; X = halogen, acetyl) are summarized in Table 111. All fragments are again formed by loss of the various substituents which also in this case allows an unequivocal structure assignment. A comparison of the relative fragment intensities in the CA spectra of R3Sn X and R2Sn X2 compounds reveals that the latter are more stable. This again reflects the fact that the Sn-X bond is stronger than the Sn-R bond. This conclusion is also supported by the observation that always loss of R but not loss of X leads to the base peak. Moreover, the fragment [R2Sn]+-is not observed with compounds of the type R2SnXz ( iso-C3H7-Sn; C6H5-Sn > t-C4Hg-Sn; CH3-Sn > C2H,-Sn). Earlier determinations of the dissociation energies of Sn-R bonds (23)are in agreement with these observations (CH3-Sn = 218 f 4 k J molw1;C2H5-Sn = 193 f 8 kJ mol-l; C6H5-Sn = 255 f 8 k J mol-'). The particular strength of the C&-Sn bond can be rationalized assuming a delocalization of the charge between the tin atom and the aryl ring (34-39). Mixture Analysis. The low fragmentation level of field ionized or field desorbed tin organic compounds and the simple interpretation of their CA spectra should make a combination of FI and CA particularly suitable for a direct mixture analysis. Rather than testing this approach on synthetic mixtures with known constituents, it should be of interest to analyze mixtures of unknown composition. As most available compounds listed in Table I showed major impurities as evidenced by their FI or FD spectrum, it was a challenging task to identify these impurities via their CA spectra. Figure 2 shows the E1 and FI spectrum of an unknown mixture of tin organic compounds. The E1 spectrum shows fragment
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
2303
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320
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Figure 2. (a) E1 spectrum of an unknown mixture of organic tin compounds. (b) F I spectrum of the same mixture.
clusters which reflect the isotope distribution of tin. T h e fragments [C4Hg]+,[SnCl]+, [Sn(C4H9)]+,[(C4Hg)2SnCll+,and [(C4Hg)2SnBr]+can be readily identified and point t o the presence of one or several tin organic compounds containing butyl, chlorine, and bromine as substituents. The FI spectrum (Figure 2b) reveals the presence of a mixture of tin organic compounds with two major ( m / z 326 and 370) and a minor component (mlz 348). (The available FI instrument only allowed the mass range up to mlz 400 to be studied. I t cannot be excluded that there are further components in the mixture with molecular ions above m / z 400.) The CA spectra of four ions (mlz 291, 326,348, and 370) were taken. By use of the information contained in Table 11, the CA spectra allow an easy and unambiguous assignment of the structure. The ions a t m / z 326, 348, and 370 can be identified as (C4H9)3SnC1, (C4Hg),Sn,and (C4Hg),SnBr,while the CA spectrum of the weak signal a t m / z 291 (1% relative abundance) demonstrates t h a t this is a fragment ion [(C4H9)3Sn]+. (CA spectra of fragment ions not shown in Tables I1 and I11 are available on request.) The example demonstrates that mixture components with a relative abundance as low as 1% can be readily identified. It is obvious that this information is not available from the E1 spectrum. Figure 3 shows the E1 and FI spectrum of (CSH&(t-C4Hg)SnBr. The FI spectrum reveals the presence
Table IV. Impurities in the Investigated Tin Organic Compounds impurity identified by CA (C,,H,)z(t-C,H,)SnC1 (C H 5 ) ( t-C,H,)SnCl Br (C2H 42SnC1z (CH ,), Sn Br (C,H 5 ) 2 (i-C,H7)SnBr (t-C,H,)(neo-C,H,,),SnCl
(t-C,H,)(neo-C,H,,)SnBrCl (C, H 9 ) 3 Sn Br (C,H,),Sn (C, H ,),SnCl
of a larger impurity ( m / z 366) which does not show up in the E1 spectrum. The CA spectrum readily allows one to identify Finally the weak this component as (C6H5)2(t-C4HS)SnC1. signal a t m / z 353 (1%)is identified as a fragment ion, [M C4H91'. T h e two examples demonstrate t h a t fragment ions (although very weak in both instances) can be easily distinguished from molecular ions by their CA spectrum. Moreover, the temperature dependence of the F I spectrum allows the
2304
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
I LO-
I ['A]
10 1 60.
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LITERATURE CITED
(15) (16) (17)
(18)
380
LM)
3. (a) E1 spectrum of (C6H&(f-C4Hg)SnBr. (b) F I spectrum of (CBH5),(t-C4H,)SnBr containing an impurity at m l z 366.
rapid differentiation between molecular and fragment ions. If the FI emitter is heated, fragment ions increase substantially in abundance. (The relative abundance of the fragments increases by a factor of up to 20 if the FI emitter is heated by 35 mA.) The above approach has been used to identify further impurities in other tin organic compounds as listed in Table IV.
(13) (14)
-
360
m/z
Figure
Il ,I ,
Levsen, K.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1978 75,509. McLafferty, F. W. ACS Symp. Ser. 1978, No. 70. Chapter 3. Levsen, K.; Beckey, H. D. Org. Mass Spectrom. 1974, 9 , 570. Gierlich, H. H.; Rollgen, F. W.; Borchers, F.; Levsen, K. Org. Mass Spectrom. 1977, 72, 387. Weber, R.; Borchers, F.; Levsen, K.; Rollgen, F. W. Z . Naturforsch., A 1978. 33A. 540. Veith; H. J.' Org. Mass Spectrom. 1978, 73, 280. Fischer, M.; Veith, H. J. Helv. Chim. Acta 1978, 6 7 , 3038. Veith, H. J. Adv. Mass Spectrom., in press. Straub. K. M.; Burlingame, A. L. Adv. Mass Spectrom., in press. Levsen, K.; Schulten, H.-R. Biomed. Mass Spectrom. 1978, 3, 137. McLafferty, F. W.; Bockhoff, F. M. Anal. Chem. 1978, 50, 69. McReynolds, J. H.; Anbar, M. Int. J. Mass Spectrom. Ion Phys. 1977, 2 4 . 37. Kruger. T. L.; Litton, J. F.; Cooks, R. G. Anal. Left. 1976, 9 , 533. Kondrat, R. W.; McClusky, G. A.; Cooks, R. G. Anal. Chem. 1978, 50. 2017. McClusky, G. A.; Cooks, R. G.; Knevel, A. M. Tetrahedron Lett. 1978, 4 6 , 4471. McClusky, G. A.; Kondrat, R. W.; Cooks, R. G. J. Am. Chem. SOC. 1978, 700, 6045. Kondrat, R. W.; McClusky, G. A.; Cooks, R. G. Anal. Chem. 1978, 50, 1222. Kondrat, R. W.; Cooks, R. G.; McLaughlin, J. L. Science 1978, 799, 978.
(19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (36) (39)
Schoen, A. E.; Cooks, R. G.; Wiebers, J. L. Science 1979, 203, 1249. Zakett, D.; Shaddock, V. M.; Cooks, R. G. Anal. Chem., in press. Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 5 0 , 81A. McLafferty, F. W. Adv. Mass Spectrom., in press. Neumann, W. P. "Die organische Chemie des Zinns"; Ferdinand-Enke Verlag: Stuttgart, 1967. Matsuda, S.;et al. Kogyo Kagako Zasshi 1987, 70, 1747. Frankel, M.; et al. J. Organomet. Chem. 1987. 9 , 63. Vilarem, M.; Maire, J. C. C . R . Hebd. Seances Acad. Sci., Ser. C 1988, 262. 480. Gielen, M.; Mayence, G. J. Organomet. Chem. 1988. 72, 363. Chambers, D. B.; Glocking, F.; Weston, M. J. Chem. Soc.1987, 1759. Litzow, Spalding "Mass Spectrometry of Inorganic and Organometallic Compounds"; Elsevier: Amsterdam, London, New York, 1973. Weston, A. F.; Jennings, K. R.; Evans, S.; Elliott, R. M. Znt. J. Mass Spectrom. Ion Phys. 1978, 2 0 , 317. Millington, D. S.;Smith, J. A. Org. Mass Spectrom. 1977, 72. 264. Van den Heuvel, C. G.; Nibbering, N. M. M.; Heimbach, H.; Levsen, K. Org. Mass Spectrom. 1979, 74, 550. Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data 1977, 6 . Occolowitz, J. L. Tetrahedron Lett. 1988, 5291. Bou6, G.;Gielen, M.; Nasielski, J. Bull. SOC. Chim. &/g. 1988, 77,43. Gielen, M.; Nasielski, J. Bull. SOC. Chim. Be/g. 1988, 77,5. Ridder, J. J.: Dijkstra, G. Reci. Trav. Chim. Pays-Bas 1987, 8 6 , 737. Chambers, D. B.; Glocking, F.; Light, J. R. C.; Weston. M. J. Chem. Soc.,Chem. Commun. 1988, 281. Kuivib, H. G.: Tsai. K.-H.; Kingston, D. G. I.J. Organomet. Chem. 1970, 23, 129.
RECEIVED for review May 15,1980. Accepted August 20,1980. Financial support from the Wissenschaftsministerium Dusseldorf and the Fonds der Chemischen Industrie, Frankfurt, are kindly acknowledged.
