JOURNAL OF MASS SPECTROMETRY, VOL. 31, 383-388 (1996)

Field-induced Ion Chemistry Leading to the Formation of (M - 2nH)+' and (2M - 2mH)+' Ions in Field Desorption -Mass Spectrometry of Saturated Hydrocarbons

G. Klesper and F. W. Rollgen Institut fur Physikalische und Theoretische Chemie, Universitat Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany

The formation of [ M - 2HI" ions has been reported in the field desorption mass spectrometry of saturated hydrocarbons. It is shown that these ions predominantly have an alkene structure and that a field-induced ion chemistry in multimolecular or condensed layers produce [M - 2nHI +' and [2M - 2rnH]+' ions with n and m = 1, 2, . . . , from saturated hydrocarbons. For the primary reaction of the dehydrogenation chemistry, a field- induced proton transfer from a molecular ion to a neighbouring molecule is suggested to produce an I M - H] + ion and an IM - HI' radical after elimination of molecular hydrogen, which in secondary reactions form an alkene ion or a dimer ion. Multiple dehydrogenation occurs by repeating this reaction sequence with other parts of mol- ecules having long alkyl chains. The primary reaction is inhibited by the admixture of molecules with lower ioniza- tion energies than those of the alkanes.

KEYWORDS : field desorption mass spectrometry ; hydrocarbons; field-induced dehydrogenation; ion-molecule reactions

INTRODUCTION

Field ionization (FI) mass spectra of hydrocarbons typi- cally show abundant molecular ions and a low level of fragmentation. Accordingly, the analysis of low-boiling hydrocarbon mixtures was one of the early applications of F1MS.l After the introduction of field desorption mass spectrometry (FDMS), high-boiling hydrocarbon mixtures also became amenable to FI processes by forming radical M" ions. Today, FDMS has become an important tool for the analysis and characterization of high-boiling fractions of oil, waxes, etc.'

In the FIMS of alkanes, fragment ions are produced by field dissociation processes in which the field- dependent charge localization at a carbon atom causes the rupture of a neighbouring bond.' Products of field dissociation appear with higher abundance in the lower mass range and are typically weak or absent in FD mass spectra.

[M - HI+ and [M - 2H]" ions are also formed in FIMS and FDMS of alkanes. [M - H I + ions are known to be formed by hydride ion transfer to carbo- cations on the emitter surface, which may be provided by the surface of activated emitters themselves or by field-induced ionic adsorption of unsaturated com- p o u n d ~ . ~ The origin of the [M - H I + ions was also attributed to FI processes combined with hydrogen transfer from molecules to radical sites of the However, in the early studies with low molecular mass alkanes the question of the dehydrogenation reactions leading to the [M - 2H]+' ions in FI mass spectra remained unsolved. While the relative abundance of the [M - 2H]+' ions is very weak in the FI mass spectra of

low molecular mass alkanes, it may become high in the FD mass spectra of higher molecular mass alkanes6 and accordingly may affect the interpretation of FD spectra in mixture analysis regarding the abundances of alkenes and/or of cyclic alkanes.

More recently, Heine and Geddes7 have published interesting results of a systematic study on the depen- dence of the [M - 2H]+' ion formation in the FDMS of hydrocarbons on the experimental conditions and the structure of the molecules. For saturated hydrocarbons they found an increase in the [M - 2H]+'/M+ intensity ratio with increasing chain length and with branching of the chains. This intensity ratio also increased with the applied potential between the FD emitter and the counter electrode, i.e. field strength, and with the amount of sample loaded on to the emitter. However, the presence of aromatic rings in molecules was found to inhibit the formation of the [M - 2H]+' ions. No dependence of the intensity ratio on the emitter temperature was observed.

These results prompted us to perform further experi- ments to elucidate the field-induced ion chemistry leading to the formation of the [M - 2H)" radical ions. The main questions left were those of an olefinic or cyclic structure of the [M - 2H]+' ion, of the role of the emitter surface in the dehydrogenation chemistry and of the reaction mechanism.

EXPERIMENTAL

The FD mass spectra were obtained with a modified double-focusing MS-9 mass spectrometer equipped with an FD ion source. At the applied emitter potential of 6

CCC 1076-5174/96/040383-06 0 1996 by John Wiley & Sons, Ltd.

Received 9 October 1995 Accepted 19 December 1995

384 G. KLESPER AND F. W. ROLLGEN

n-octacosane

squalane

yo

squalene

Figure 1. Structures of the hydrocarbons used in the experi- ments.

kV the mass range was 1670 u. The counter electrode was at -4 kV, i.e. 10 kV was applied between emitter and counter electrode. The mass resolution was set to about 1000. The F D source was equipped with viewing windows and a stereo microscope, which allow the exact positioning of the loaded part of the emitter with respect to the counter electrode and the ion optics of the mass analyser without the need for an ion signal.8

F D emitters prepared by activation of 10 pm tung- sten wires with indene' were used. The needle length was about 30 pm. The samples were loaded on to the emitter by dipping the emitter into a solution of the sample. The concentration of the sample solution was between 0.2 and 50 g dmp3. The solvents were n-hexane and n-heptane. The mass spectra were recorded with a three-channel strip-chart recorder covering a dynamic range of up to five decades. For full mass spectra, a mass scan of 20-100 s per decade and steps of 2 mA emitter heating currents were applied. In addition, the molecular ion range was repeatedly recorded with a slow mass scan of 1500 s per decade.

The hydrocarbons used in this study were octacosane (98%) from Fluka, n-eicosane (99%) from Lancaster and squalane (99Y0) and squalene (97%) from Aldrich. The structures of these compounds are shown in Fig.1.

Coating of the surface of activated emitters with a thin layer of gold in one experiment was performed by vapour deposition of sputtered gold atoms and clusters in an argon atmosphere at 0.1 mbar (1 bar = lo5 Pa) (Edwards Coating System E 306A).

RESULTS

Alkene formation versus cyclization

The FD mass spectra of long-chain saturated hydrocar- bons showed the formation of [M - 2nH]+' ions with n = 1. Weak signals of n 2 2 were also found in mass spectra of saturated hydrocarbons. As shown in the FD mass spectrum of octacosane in Fig. 2, which covers a

3op

m/z -->

L Figure 2. Molecular ion group in the FD mass spectrum of octa- cosane. Emitter heating current: 4 mA.

dynamic range of three decades, molecules are dehydro- genated up to n = 3. In the spectrum of squalane even an ion with n = 4 was detected with a relative abun- dance of 0.01% of the molecular ion.

Considering the state of the sample molecules in a condensed or adsorbed layer on the emitter surface, it is hardly possible that dehydrogenation of molecules via multiple cyclization reactions occurs during FD. Since in addition no unimolecular elimination of H, from M + ' in the gas phase could be detected, the conclusion can be drawn that the [M - 2nH-J'' ion predominantly have an alkene structure.

Influence of the emitter surface and of sample loading

The emitter surface may influence the formation of alkene ions by dehydrogenation of molecules prior to ionization or by participation in the field-induced ion chemistry producing these ions. The chemical reactivity of the surface of the carbon needles of activated emitters under FI and FD conditions should mainly arise from surface charging of the needles leading to the formation of carbocations, the ionization of double bonds, etc. In order to probe the chemical influence of the emitter surface on the desorption of the dehydrogenation pro- ducts, the surface of activated emitters was treated in various ways, such as by heating to -2000 K, resulting in the formation of a graphite layer, and by oxidation via FI of water. No significant effect on the abundance distribution of the dehydrogenation products was observed. In a final experiment, the whole surface of an activated emitter was coated with a thin gold film (about 50 atomic layers) by vapour deposition of gold in an argon atmosphere. Again no significant effect on the dehydrogenation products was observed, i.e. the [M - 2H]+'/M+' abundance ratio increased from about 25% to 35%.

These results allow the conclusion to be drawn that the dehydrogenation of molecules is caused by field- induced reactions in multimolecular or condensed layers without chemical surface interaction. This con- clusion is supported by the observed increase in the [M - 2H]+'/M+' abundance ratio with sample loading of the emitter.7 In addition, we found that the state of the deposited layer also plays a role. In an experiment with

385 DEHYDROGENATION OF HYDROCARBONS IN FDMS

l a -

a-

c m - .- m i - a- ! :;

a-

m -

b)

(M-ZH)"

10 ( M - ~ H $ '

o . . . . . . . . ! ? . . . - 410

I , , , . , 410

d z -> 4 1

. (b) tm - a-

a-

. g m - c s L o - - '

41E am m/z -->

UD

Figure 3. Molecular ion group of squalane in the FD mass spec- trum of (a) pure squalane and (b) a 1 : 1 mixture of squalane and squalene. Emitter heating current: 0 mA.

octacosane, the [M - 2H]+'/M+' abundance ratio was 6% after sample loading on to the emitter and at a 7 mA emitter heating current. However, after heating of the emitter at 14 mA to about 100°C for a few seconds at zero field strength, i.e. above the melting point of octacosane, the abundance ratio increased to 106%. We believe that the liquid paraffin forms a more uniform layer in the tip area of the field-enhancing microneedles by wetting the hydrophobic surface of the carbon needles. Furthermore, a layer thus treated by melting has a more rigid structure with respect to disintegration processes caused by field forces and therefore may allow exposure to higher field strengths. The dependence of the yield of the dehydrogenation products on the applied field has already been reported7 and was con- firmed in the present work.

In FIMS of alkanes an increase in the [M - 2H]+'/M+' abundance ratio was observed with increasing gas pressure of the sample in the ion source. For example, with n-decane an increase in pressure from to loT3 mbar led to an increase in the abun- dance ratio from 0.2% to 0.8%. This effect can be related to the increase in the [M - 2H]+'/M+' ratio with sample loading in FDMS and points to the depen- dence of the field-induced dehydrogenating ion chem- istry on the formation of multimolecular layers on the microneedle tips.

Effects of additives

Aromatic rings in molecules with alkyl chains inhibit

the formation of [M - 2H]+' ;om7 The same effect was observed if an aromatic compound or a compound with many double bonds was admixed with an alkane sample. In a 1 : 1 mixture of squalane with squalene, the [M - 2H]+'/M+' abundance ratio of squalane was about 11% whereas with pure squalane the ratio was 60% (Fig. 3). In these experiments with squalane and a mixture of squalane and squalene, the F D emitter was loaded by dipping it into the liquid samples without the use of solvents. For naphthalene admixed with alkane samples this effect was much smaller, probably because naphthalene was not homogeneously distributed in the layer deposited from solution.

Assuming that the [M - 2H]+' ions are the result of a field-induced ion chemistry in multimolecular or condensed layers, the inhibition effect can be attributed to a partial screening of field ionization of alkane mol- ecules within the layer by the additives having a lower ionization energy than the alkanes.

Dimerization of hydrocarbons

Inspection of the higher mass range of pure compounds revealed the formation of [2M - 2nH]+' dimer ions. For squalane even a weak signal of a [3M - 4H]+' trimer ion could be observed. In the F D spectrum of binary mixtures mixed dimers appeared in addition to the pure dimers of the components. This is shown in Fig. 4 for a mixture of eicosane and squalane. The molecular ion groups of the monomers are also shown for comparison. It is most probable that the formation of both types of dehydrogenation products are related by the field-induced ion chemistry in the adsorption layers.

The formation of such dimer ions has not yet been reported. In an F D mass spectrum of squalane6 an ion signal was attributed to a [2M]+' ion instead of a [2M -- 2H]+' ion, probably because of a mistake in mass assignment. The dimer ions are not easily observed in the F D mass spectra of real mixtures because the many components of such mixtures only lead to very weak ion signals of dimers. Dimer peaks may have also been attributed to ion signals from alkenes or cyclic alkanes.

ION CHEMISTRY INDUCED BY THE HIGH EXTERNAL FIELD

Primary reaction

The experimental results show that [M - 2H] +' alkene ions are formed by a field-dependent ion chemistry in thick adsorption layers of alkane molecules. Consider- ing the formation of an alkane molecular ion in such a layer by FI, double bond formation by monomolecular elimination of H, from this ion can be excluded for energetic and steric reasons. Hence the question of the dependence of the dehydrogenation chemistry on ion- molecule reactions remains.

386 G. KLESPER AND F. W. ROLLGEN

1 I

l01

... m/z -->

840 I mlz -> mi2 -->

Figure 4. Molecular and dimer ion groups in the FD mass spectrum of a mixture of eicosane (Ma) and squalane ( M b ) . (a) Molecular ion group of eicosane: (b) molecular ion group of squalane; (c) dimer ion group of eicosane; (d) dimer ion group of squalane; (e) mixed dimer ions of eicosane and squalane. Emitter heating current: 4 mA.

Disregarding charge-transfer reactions to neighbour- ing molecules, the following proton transfer reaction should become possible in a high external field:

M + ' + M + [ M - H ] ' + [ M - H ] + + H , (1)

In this reaction (Fig. 5), a charge shift of about 0.3 nm is involved, which leads to a field strength dependence of the thermochemistry of this reaction. Estimates show that without an external field applied this reaction is endothermic by about +63 kJ mol-' for secondary carbon atoms in the alkyl chains and by about +4.2 kJ mol- for tertiary carbon atoms. This reaction becomes exothermic in an external field higher than -2.2 and -0.02 V nm-', respectively. These estimates do not

consider the effect of polarization of a reactant molecu- lar ion in a high field, which further decreases the field- dependent heat of the reaction. The estimates also do not imply activation energies. However, the activation energies should be small or negligible if the effect of proton tunnelling is considered in the H, formation. The width of the barrier is less than 0.1 nm and lies in the range where fast proton tunnelling is observed.' O

We believe that the proton transfer reaction (1) is the primary reaction of the field-induced ion chemistry leading to the formation of the dehydrogenation pro- ducts. A prerequisite of this reaction is that no charge transfer by electron transition from neighbouring mol- ecules occurs in the range of field strengths required for

DEHYDROGENATION OF HYDROCARBONS IN FDMS 387

Figure 5. Mechanism of the primary field-induced proton transfer reaction. The direction of the field is from left to right.

the proton transfer reaction. Considering the strong effect of Franck-Condon factors on the ionization of alkanes, i.e. significantly higher vertical ionization ener- gies than the adiabatic ionization energies, and in addi- tion the effect of polarization of molecular ions in an external field on the Franck-Condon factors, as indi- cated by the measured high-field ion appearance ener- gies of molecular ions of alkanes," it is reasonable to assume that the threshold field strength for charge transfer is frequently higher than the field strength needed for the proton transfer reaction.

Secondary reactions

Since the products of the primary reaction in a multi- molecular or condensed layer are not directly removed, two field-independent exothermic ion-molecule reac- tions are possible between the products of the primary reaction. In the first reaction an alkene ion and an alkane molecule are formed by hydrogen transfer:

[M - H] + + [M - HI' + M + [M ~ 2H]+' (2) The reaction is exothermic by about -80 kJ mol- '. In the second reaction, which is exothermic by about - 115 kJ mol-l, a dimer ion is formed:

[M - HI+ + [M - HI' + [2M - 2H]+' (3) In both reactions, the ionization of the neutral reac-

tant radical is prevented by the strong Coulombic attraction of the electron to the neighbouring reactant ion in the ionization process. This Coulombic inter- action raises the ionization energy of the radical by several eV. At large distances from ions, the radicals are preferentially field ionized owing to their low ionization energy. [M - HI+ ions apart from radicals can react with alkane molecules by hydride ion transfer, thereby forming [M - HI+ ions again.

Reaction (1) may also occur between two parts of a folded long-chain hydrocarbon molecule. Then both types of secondary reactions lead to [M - 2H]+' ions having either an alkene or a cyclic structure. In our opinion, the reaction sequence (1) to (3-analogous) is the only possiblity of forming cyclic [M - 2H]+' ions in FD of saturated hydrocarbons.

In thick layers, field-induced charge transfer from reaction products of secondary reactions to alkane mol- ecules can lead again to the formation of dehydroge- nation products by the reaction sequences (1) to (2) and (1) to (3). Accordingly, the movement of a single charge

through a layer can result in the formation of several dehydrogenation products. Considering space-charge effects in the layer from ionized double bonds which keeps mobile charges apart from these double bonds, multiple dehydrogenation of molecules with long alkyl chains is facilitated.

If the [M - HI+ product ion of the primary reaction (1) is neutralized by a field-induced charge-transfer reac- tion, two radicals are left. The following exothermic recombination reactions of these radicals are analogous to the secondary ion-molecule reactions and lead to the formation of [M - 2H] and [2M - 2H] molecules:

[M - HI' + [M - HI. + M + [M - 2H] (4) [M - HI. + [M - HI' + [2M - 2H] ( 5 )

Subsequent FI and desorption of these molecules con- tribute to the signal intensity of the dehydrogenation products in the F D mass spectra. However, the forma- tion of alkenes and of dimers by recombination of neutral radicals from reaction (l), after diffusion of the radicals in the layer, is rather improbable compared to the ionization of these radicals.

The field-induced primary and secondary ion- molecule reactions explain the formation of the dehy- drogenation products appearing in FD mass spectra of saturated hydrocarbons. This ion chemistry is also con- sistent with the observed dependence of the [M - 2H]+'/M+' abundance ratio on the sample loading of the emitter, the applied emitter/counter electrode potential, the length and branching of alkyl chains and the effect of additives with aromatic rings or double bonds of low ionization energy. The dependence of dehydrogenation chemistry on the length and branching of alkyl chains should be related to the thermochemis- try of the field-induced primary reaction (1) and to the stability of the hydrocarbon layer in a state near the melting point, i.e. to a low vapour pressure which decreases with increasing chain length and to a high vis- cosity which inhibits rapid disintegration of the layers under field stress.

CONCLUSIONS

Products of single and multiple dehydrogenation reac- tions which appear in the FD mass spectra of high- boiling saturated hydrocarbons are [M - 2nH] +'

monomer ions with one or several double bonds and [2M - 2mH] +. dimer ions with and without double bonds. Even a trimer ion was found. Field-induced proton transfer reactions between alkane ions and alkane molecules and secondary ion-molecule reactions between the products of the primary reactions in a multimolecular or condensed layer are suggested to produce these ions. The formation of [M - 2H]+' ions with a cyclic structure from long-chain hydrocarbons may also occur by this ion chemistry. The [M - kH]+ carbocations of the FD spectra with k = 1, 3, . . . should be products of proton transfer and hydride ion transfer reactions.

388 G. KLESPER AND F. W. ROLLGEN

The field-induced proton transfer reaction in an tion energies than those of the alkanes, as demonstrated alkane layer has not been considered before and needs further studies regarding its dependence on field strength, chemical structure of the alkane molecules and

with a mixture of squalane with squalene.

Acknowledgement proton tunnelling. The inhibition of this Droton transfer reaction. which

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