International Journal of Mass Spectrornetry and Zen Processes, 65 (1985) 159-168 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

159

FAST-ATOM BOMBARDMENT MASS SPECTROMETRY OF GLYCEROL-ALKALI-HALIDE MIXTURES

J.L. AUBAGNAC, B. ELAMRANI

Unite 408 CNRS, Universitk des Sciences et Techniques du Languedoc, 34060 Montpeliier Cedex

(France)

F.M. DEVIENNE, R. COMBARIEU and J.C. ROUSTAN

Luboratoire de Physique Molkulaire des Hautes knergies, BP2 06530 Peymeinade (France)

(First received 13 November 1984; in final form 7 December 1984)

ABSTRACT

It is possible to obtain several cationized ions from glycerol-alkali-halide mixtures using the FAB ionization method. The specific behaviour of each ion is studied. Formation of ions by a substitution reaction with lithium is observed. Also, the alkali ion X+ becomes more abundant in the FAB spectrum as the element becomes less electronegative. The CAD spectra of these cationized ions obtained by tandem mass spectrometry are characteristic of ion structure. Lastly, in the CAD spectra of X + XCl’+ ions, there is formation of Cl’+ ions by fragmentation with a significant release of kinetic energy.

INTRODUCTION

Soft ionization mass spectrometry methods permit the study of polar and labile compounds and the determination of high molecular weights [1,2]. Among these methods, the most powerful is the fast-atom bombardment (FAB) method [3,4]. The compound under study is deposited on a target in a matrix with an alkali halide. The matrix most often used is glycerol, CH,OH-CHOH-CH,OH (molecular weight 92). The FAB spectrum is a composite one. It shows several ions characteristic of the structure of the compound studied: M + I-P, M + Na’+, M + 2Na - I-P, M + H + G’+, . . . .

(where G is the glycerol molecule and M is the compound under study). Similar ions are obtained from glycerol. Because glycerol has a relatively low molecular weight (92) several ions corresponding to the structures n G + W+ and n G + Na’+ appear in the usual mass range. Fragment ions also appear in these FAB spectra. FAB mass spectra are extremely complex: published results very often give only parts of spectra.

0168-1176/85,‘$03.30 0 1985 Elsevier Science Publishers B.V.

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In this preliminary work, we have studied structures of ions formed from glycerol-alkali-halide (LiCl, NaCl, KCl) mixtures by ionization using the FAB method. Grade and Cooks have established that ionization of the molecule M by cationization, M + M + C?+, is a general phenomenon: cations formed from numerous elements can be used [5]. The present study is limited to three alkali metals, Li, Na and K, because the majority of cationized compounds occur with these three elements.

EXPERIMENTAL

Mass spectra were obtained using a mass spectrometer built by the authors. Its characteristics have been described previously [6]. This apparatus has the configuration EBE (see Fig. 1). The gas used for the fast-atom gun was argon. The energy of this fast-atom beam is 3 keV. The collision chamber is located in the third field-free region. The pressure of argon in this collision chamber is increased until the precursor ion abundance is reduced to 30% of its original value and the CAD spectrum is obtained by a scan of the third analyzer.

RESULTS

The FAB spectrum of glycerol alone shows n G + 11+ ions: m/z 93, 185, 277, 396,. . . and ions formed from these ions by loss of water molecules:

Charge exchange

Magnetic sector

Fig. 1. Diagram of the apparatus used.

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m/z 75 (93 - 18), 57 (93 - 18 - 18), 167 (185 - 18), 149 (185 - 18 - 18), 131 (185 - 18 - 18 - 18), 259 (277 - 18),. . . The presence of ions in the mass spectrum indicates that such isolated species exist in the gaseous state. Such species question the exact nature of bonds between glycerol molecules. They are not loosely bound because they are maintained while the glycerol molecules in such clusters fragment by successive loss of water molecules.

The study of the FAB spectrum of a glycerol-sodium-chloride mixture (Fig. 2) shows two series of ions. Firstly, there are ions of m/z 93, 185, 277,. . . corresponding to the formula n G + l’+ (see above). Secondly, there are ions formed by cationization: ions formed from Na+: Naf, m/z 23; G + Nal+, m/z 115; 2 G + Na’+, m/z 207: ions formed from an Na + NaCl’+ cluster: Na + NaCT+ m/z 81; G + Na + NaCT+, m/z 173 and 2 G + Na + NaCT+, m/z 265 {such ions are easily characterized by natural isotopic abundances of chlorine) and ions formed from other ions by substitution of hydrogen with sodium: G + 2 Na - W+, m/z 137; G + 2 Na - H + NaCT+, m/z 195; 2 G + 2 Na - W+, m/z 229. This list shows the formation of numerous ions from the glycerol-sodium-chloride mixture by the FAB ionization method. Similar spectra are obtained from

Nat

,

IG*Hf

I5

i+Na)’

iG+Na+NaCl)+

(ZG+Na+NaCI)*

Fig. 2. FAB mass spectrum of a glycerol-NaCl mixture.

162

32

i+Li)+

5 j+2Li-HI*

IG+Li+LiCI)*

141 191 (2GtLi)*

I I I (ZG+Li+LiCIl*

Fig. 3. FAB mass spectrum of a glycerol-LiCl mixture.

9 131 G+K)'

(K+KCII+

113

IG+HI+

93 IG+2K-HI+

169

[G+K+KClJ+

Fig. 4. FAB mass spectrum of a glycerol-KC1 mixture.

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glycerol-lithium-chloride and glycerol-potassium-chloride mixtures (see Figs. 3 and 4). There are however several differences. Firstly, ions with isotopes 6Li and 41K are present in the spectra. Secondly, the K+ ion is present in greater abundance than is the Na+ ion, and Li+ is not observed. This confirms a general rule for the cationization phenomenon already presented by Rollgen et al. [7] that the least electronegative alkali metal ion gives the greatest relative abundance. Thirdly, the low mass of lithium permits observation of characteristic clusters obtained by substitution of several hydrogens by the same number of lithiums.

As the present work is in the positive-ion mode, the following rule must hold for the composition of every ion: the number of lithium atoms must be equal to the number of lost hydrogen atoms plus one. In these clusters, the ions observed are: G + Li, m/z 99; G + 2 Li - H, m/z 105; G + 3 Li - H, m/z 111; G+Li+LiCl, m/z 141; G+2 Li-H+LiCl, m/z 147; 2 G + Li, m/z 191; 2 G + 2 Li - H, m/z 197; 2 G + 3 Li - 2H, m/z 203; 2 G + Li + LiCl, m/z 233; 2 G + 2 Ei + LiCl - H, m/z 239; 2 G + 3 Li + LiCl - 2 H, m/z 245.

The spectra presented above show several ions characteristic of the structures G + IP, G + x1+, . . . Fragment ions are formed from these ions and it is difficult to establish fragmentation rules for determination of structures by simple mass spectrometry. To obtain this information we used tandem mass spectrometry, also called MS/MS [S]. This technique is founded on the following principle: with the first sector (magnetic sector or quadru- pole), we choose the studied ion. This ion then fragments spontaneously in the field-free region (metastable ion spectra) or fragmentation may be induced by collision with a neutral gas [collision-activated dissociation (CAD) spectra]. The fragment ions observed are characteristic of the struc- ture of the studied ion. Tandem mass spectrometry has a bidimensional property.

The CAD spectra [8] of three ion series, G + X+, 2 G + X+ and G + 2 X - I-P have been studied where X = Li, Na and K. We have also studied the CAD spectra of G+ X + XCl’+ and 2 G+ X + XCT+ ions where X = Li and Na. Numerical values are recorded in Tables 1 and 2; (relative abundances are given in brackets). The CAD spectra of X + XCT+ ions (X = Li and Na) present in the FAB spectra of glycerol-alkali-halide rnix- tures are listed in Table 2.

Study of Table 1 permits the following comments. Two fragmentations are observed in the G + X+ ion. Firstly, fragmentation of the loosely bound metal ligand results in formation of the X+ cation. Secondly, there are also other fragmentations characteristic of the glycerol structure. During these fragmentations, the metal-ligand bond formed during ionization is pre- served. So we observed loss of H’, H,, H,O, CH,OH’ and CH,OH (for

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TABLE 1

Ion X

(G + X)” ions 99 Li

115 Na 131 K

(2 G + X)’ + ions

191 Li 207 Na 223 K

(G+ZX-H)‘+ions

105 Li

137 Na 169 K

CAD spectrum

98(11); 97(9); 93(8); 92(7); 81(19); 68(13); 67(9); 63(4); 61(l); 51(4); SO(4); 43(3); 7(8) 114(12); 113(5); 108(8); 97(13); 84(12); 83(5); 23(45) 113(l); 99(6); 68(2); 61(2); 56(2); 39(87)

160(l); 105(7); 99(91); 7(l) 194(l); 176(l); 115(94); 97(l); 841); 23(2) 131(87); 113(3); 99(l); 39(9)

99(8); 87(11); 86(6); 81(7); 73(16); 67(7); 57(7); 31(32); 14(l); 7(5) 119(4); 105(35); 89(3); 83(6); 63(25); 46(7); 23(20) 137(28); 121(4); 113(3); 108(3); 99(4); 95(31); 78(13); 39(14)

example, with X 7 Na, the ions formed are m/z 114,113,97, 84 and 83. (See also, Fig. 5, CAD spectrum of G + LP ion). In the CAD spectra of 2 G + r+ ions, two ions are observed: alkali ion X+ and an abundant ion corresponding to loss of a molecule of glycerol with formation of G + X+ ions. Such fragmentations confirm previous results by Cooks and co-workers [9] on fragmentation of an ion with a dimer structure: M, . . .X . . . Ml’+. So, for 2 G + X+ ions, the structure proposed is G . . . X . . . G’+, in which bonds between metal and glycerol molecules are loose. CAD spectra of G + 2 X - IT+ ions show that ions formed by loss of water and methanol (loss of

TABLE 2

Ion X CAD spectrum

(X + XCl)’ + ions 49 Li 81 Na

(G + X + XCI)’ + ions

141 Li 173 Na

(2G+X+XCl)‘+ions

233 Li 265 Na

42(71); 35(H); 14(7); 7(11) 58(41); 46(20); 35(4); 23(35)

123(7); 105(39); 99(36); 49(14); 31(2); 14( -c 1); 7( < 1) 40( < 1); 115(23); 81(76); 23( < 1)

197(16); 191(10); 141(34); 105(16); 99(19); 49(5) 207( < 1); 173(77); 115(9); 81(13); 23( -c 1)

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99

Fig. 5. CAD spectrum of (G+ Li)+ ions; m/z 99.

masses 18 and 32) are characteristic of the glycerol structure and, similarly, for HOX:, Xt and X+ ions. The presence of HOX; ions (respectively, m/z 31, 63 and 95) and Xl ions (respectively, m/z 14, 46 and 78) provides

lb1 (G+Li + Li Cl)’

IG +Li)+ 99 ‘O5 (G+ZLi-HI’

i Li+Li Cl)+ (G+Li+LiCI-&$I!

Li* Li; 69

123

7 lb 31

Fig. 6. CAD spectrum of (G+ Li+LiCl)+ ions; m/z 141.

166

the following information. Although any of the three hydroxyl functions of the glycerol molecule can hold the two alkali cations, the presence of HOX: in the CAD spectra establishes that the two alkali metal atoms are held on the same oxygen. Furthermore, in the first step, formation of such an ion involves hydrogen migration.

In CAD spectra as in FAB spectra, the abundance of the alkali metal ions decreases in the order K+ > Na+ > Li+. The positive ion of the less electro- negative alkali metal has the highest abundance.

Fragmentations @ and @ explain the formation of abundant ions in the CAD spectra of G + X + XCT+ ions (Table 2). 0 is more significant than 0 when X = Na and vice versa when X = Li (Figs. 6 and 7).

G . . . X . . . XCl’+

Furthermore, with lithium, two other fragmentations, observed with im- portant abundances, form G + Li + LiCl’+ ions: loss of water from the glycerol molecule and substitution of hydrogen by lithium occurs at the same time as fragmentation 0, thus explaining the formation of G + 2 Li - I-I? ions.

Differences exist between the CAD spectra of 2 G + X + XCT+ ions provided that cationization is achieved using Li+ or Na+. Collision-induced dissociation of 2 G + Na + NaCP, m/z 265, gives three abundant ions:

+ NaCI)+

(Na+ G)+

115

I

Na* 23 58 141 h I\, w/M -

Fig. 7. CAD spectrum of (G + Na + NaCl)+ ions; m/z 173.

IG +Na+NaCI)+

167

113

I

12G + Na+NaCl)’

265

Nat

(Na+NaCIl+

81 (G + Na I+

115 lZG+Na)+

93 1371~2 , * 207

Fig. 8. CAD spectrum of (2 G + Na + NaCl)+ ions; m/z 265.

m/z 173, G + $a + NaCl’+ (loss of G) (fragmentation 0); m/z 81, Na + NaCT+ (fragmentation @); and m/z 115, G + Na’+ (simultaneous loss of G and NaCl) (fragmentations @ and 0). For this last ion, we propose the structure (See Fig. 8)

G . . . G . . . Na . . . NaCP

Collision-induced dissociation of 2 G + Li + LiCl’+ ions, m/z 233, gives two ions with similar structures to those described: G + Li + LiCP, m/z

ILi +LiCl)’

49

Li Cl+

$2

I Li+ Li; Cl+

7 14 35

n A - *

Fig. 9. CAD spectrum of (Li + LiCl)+ ions; m/z 49.

168

141, and G + LP, m/z 99 (this ion being accompanied by G + 2 Li - w+, m/z 105, formed by substitution of hydrogen by lithium). In this CAD spectrum there is also another ion with an important abundance, 2 G + LP, m/z 191 (this ion also being accompanied by 2 G + 2 Li - w+, m/z 197, see above). The presence of the ion of m/z 191 indicates that fragmentation @) only occurs when 2 G + X + XCT+ is formed with lithium (X = Li).

CAD spectra of X + XCl’+ ions give unusual information. While forma- tion of XCl+ and X+ call for no comment, there are, however, two other ions whose formation is very unusual: Xt (m/z 14 and 46) and Cl+ (m/z 35). This last ion is formed in two spectra with a significant kinetic energy release. This formation involves a complex rearrangement (Fig. 9).

In Fig. 9, the peak located between m/z 49 and m/z 42 is an artefact. Also, the CAD spectra of the Cs+ ion, m/z 133, that we previously studied, shows a similar peak [lo].

The modifications brought about in these types of FAB spectra by different organic compounds in the glycerol matrix are currently being investigated.

REFERENCES

1 H.R. Morris (Ed.), Soft Ionization Biological Mass Spectrometry, Heyden, London, 1981. 2 A. Benninghoven (Ed.), Ion Formation From Organic Solids, Springer Series in Chemical

Physics, Vol. 25, Springer Verlag, Berlin, 1983. 3 F.M. Devienne and J.C. Roustan, C.R. Acad. Sci. Ser. B, 283 (1976) 392. 4 M. Barber, R.S. BordoIi, R.D. Sedgwick and A.N. Tyler, Chem. Commun. (1981) 325. 5 H. Grade and R.G. Cooks, J. Am. Chem. Sot., 100 (1978) 5615. 6 J.L. Aubagnac, F.M. Devienne and R. Combarieu, Tetrahedron Lett., (1983) 2263. 7 F.W. Rollgen, U. Giesmamr, F. Borchers and K. Levsen, Org. Mass Spectrom., 13 (1978)

459. 8 F.W. McLafferty (Ed.), Tandem Mass Spectrometry, Wiley, New York, 1983. 9 D. Zakett, A.E. Schoen, R.G. Cooks and P.M. Hemberger, J. Am. Chem. Sot., 103 (1981)

1295. 10 J.L. Aubagnac and B. Elamrani, unpublished results.