Desolvation of ions and molecules in thermospray mass spectrometry

International Journal of Mass Spectrometty and Zon Processes, 90 (1989) 139-150 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

DESOLVATION OF IONS AND MOLECULES MASS SPECTROMETRY

G. SCHMELZEISEN-REDEKER

*, L. BUTFERING

IN THERMOSPRAY

and F.W. RGLLGEN

Institute of Physical Chemistry, University of Bonn, Wegelerstrasse D-5300 Bonn (Federal Republic of Germany) (First received

24 June 1987; in final form 1 December

139

**

12,

1988)

ABSTRACT Different mechanisms of desolvation of ions by thermospray (TSP) vaporization of liquids are critically discussed. It is concluded that field effects cannot play a major role in the desolvation of ions and molecules. This conclusion is supported by results of experiments with dication salts and alkaline-earth metal salts. It was found that at low solute concentrations (
INTRODUCTION

In the thermospray (TSP) technique pioneered by Vestal [l-8] for on-line liquid chromatography-mass spectrometry (LC-MS) coupling, a sample solution is vaporized into the ion source of a mass spectrometer by passing it through a heated capillary. For solutions containing an electrolyte as analyte or additive the TSP vaporizer combines the favourable properties of an LC-MS interface with a soft ionization technique. The ionization of polar non-ionic compounds is achieved by addition of an electrolyte such as ammonium acetate to the mobile phase in order to form molecular ions by protonation, cation attachment, deprotonation or anion attachment. For compounds forming solvated ions in the mobile phase no extra TSP electrolyte is needed for the production of gaseous ions. * Present address: Boehringer-Mannheim GmbH, 31, Federal Republic of Germany. * * To whom correspondence should be addressed. 0168-1176/89/$03.50

0 1989 Elsevier

Sandhoferstrasse

Science Publishers

B.V.

116, D-6800 Mannheim

140

Soft ionization of non-volatile polar molecules by the TSP technique has been demonstrated for a variety of compounds, in particular for “larger” peptides [9-111. Molecular ions of glucagon [lo] and also of melittin [ll] have been obtained. (M + 2H)2+ to (M + 41-Q4+ions have been reported for peptides [9,10]. The level of fragmentation is typically low. The TSP technique has also been successfully applied to complex salts with doubly to quadruply charged cations [12-171 and doubly to triply charged anions [l&20]. In these experiments abundant intact cations and anions, respectively, with typically a low level of fragmentation were obtained, provided no TSP electrolyte such as ammonium acetate was added to the sample solution. Soft ionization of molecules by TSP vaporization of an electrolyte containing sample solution has been explained by Vestal [6,7,21-231 and attributed to the following steps. (i) Nebulization of the liquid: an aerosol of randomly charged droplets of either charge sign and with zero mean charge is formed [24]. (ii) Vaporization of neutral solvent molecules (and volatile solute molecules) from the droplet: the radius of charged droplets decreases and the field strength at the surface of the charged droplets increases. (iii) Field induced ion evaporation from charged droplets: the high surface field strength of charged droplets gives rise to the evaporation of ions, such as molecular ions of sample molecules and/or electrolyte ions with solvent molecules attached. The ion evaporation occurs in competition with the evaporation of solvent molecules and requires a sufficient reduction of the desolvation energy of ions by the surface field compared to the heat of vaporization of neutrals. A further requirement is the stability of the droplets under field stress with respect to macroscopic disintegration processes, as given by the Rayleigh limit [25]. The mechanism of ion evaporation from charged droplets as well as its prerequisites have been discussed and investigated by Iribarne and Thomson [26,27]. Vestal has attributed the softness of ionization of molecules by the TSP technique mainly to this ion evaporation mechanism. (iv) Gas phase ion/molecule and anion/cation recombination reactions: desolvated ions and molecules react in the gas phase according to differences in proton, cation or anion affinities. In this model of TSP ionization only the first step, i.e., the initial formation of an aerosol of charged droplets, and the fourth step, i.e., the dependence of TSP mass spectra on gas phase ion chemistry [23-301, are well established. In contrast the assumptions underlying the second and third steps are not obvious. In particular, the question of the role of “macroscopic” disintegration processes compared to solvent evaporation in decreasing the size of initially formed uncharged droplets and of charged droplets below the Rayleigh limit, has not yet been addressed. Furthermore,

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the energetics and kinetics of evaporation of organic molecular ions from charged droplets have not yet been discussed in sufficient detail to allow conclusions to be drawn about the possibility of such a mechanism for TSP conditions. This paper is concerned with the desolvation of ions and molecules in the TSP technique. In the first part different mechanisms of soft desolvation of ions and molecules by TSP vaporization of liquids are discussed. The mechanism of field induced ion evaporation is shown to depend on some rather unrealistic assumptions and should not contribute to the desolvation of ions. The second part of this paper is an extension of previous studies [15,16,31,32] and reports results of experiments which provide evidence for the desolvation of ions and molecules by solvent evaporation from very small droplets without field effects involved. DESOLVATION

MECHANISMS

FOR IONS AND MOLECULES

FROM DROPLETS

Field induced ion evaporation from charged droplets as discussed by Iribarne and Thomson [26,27] is widely considered as the principal mechanism of soft desolvation of ions in TSP. The possibility of this mechanism is based on model calculations, showing that the high electric field on the surface of a small charged water droplet, whilst still too low to induce macroscopic disintegration processes, can be high enough to reduce the energy barrier for evaporation of partially solvated ions. Thus ion emission becomes possible in competition with solvent evaporation. However, as has been discussed in a separate paper [33], not all the assumptions of the model are acceptable. They lead to calculated field strengths for ion evaporation which are much too low. The strong lowering of the energy barrier for ion evaporation is achieved by the assumption of a field penetration into the surface of the charged droplet neglecting the strong screening of the field by polarized water molecules. Therefore, it is very unlikely that the field strength for ion evaporation becomes smaller than the critical field strength for disintegration of charged droplets given by the Rayleigh limit. A further critical point of the ion evaporation mechanism is the disregard of the onset of an electrohydrodynamic disintegration induced by the removal of an ion with part of its solvation sphere from the highly charged surface [33]. In view of the fluidity of liquids such as water, it seems hardly possible that the generation of a surface deformation as required for the release of an ion from the liquid under field stress does not develop into a jet of the liquid, thus removing a number of charges from the droplet. Therefore, this surface deformation is related to the onset of an electrohydrodynamic instability of the surface under field stress rather than to the evaporation of an ion with subsequent surface relaxation.

142

The onset field strength ( ER) of electrohydrodynamic by the Rayleigh limit

instabilities is given

E, = 6.6 - 10p4( T/R)I’~ (V m-‘) where R is the droplet radius in m and y is the surface tension in N m-r. The Rayleigh limit is derived for disintegration processes, starting with elliptical deformations of the droplet [25]. For other deformations of higher order, i.e., surface deformations of smaller radii of curvature, the onset field strengths are higher. However, even surface deformations of larger radii of curvature at the beginning of a disintegration process can develop to a jet of small droplets [34]. Under appropriate conditions regarding the viscosity of the fluid and high field strength at which the disintegration starts, even partially solvated ions might be formed by this mechanism. The required surface field strengths, significantly higher than those given by the Rayleigh limit, may be achieved prior to the onset of disintegration processes by fast evaporation of solvent molecules from charged droplets. The formation of partially solvated and even free ions in addition to cluster ions of solute molecules and droplets via electrohydrodynamic disintegration of liquids with surfaces of larger radii of curvature is known for electrohydrodynamic (EH) [35,36] and electrospray (ES) [37] mass spectrometry. The TSP conditions are characterized by fast heating of liquid droplets in a rather turbulent gas flow through a small capillary vaporizer and by a pressure drop along the capillary. These conditions facilitate the breaking up of droplets via the excitation of mechanical vibrations and bubble formation within the heated droplets. Since the derivation of the Rayleigh limit is based on the assumption of an incompressible liquid and amplitudes of surface deformations, which are small compared with the droplet radius, it is very unlikely that the field strength of the Rayleigh limit is reached. Field effects may support the mechanical decomposition of droplets, but should not play a major role in the desolvation of ions from droplets. Soft desolvation of ions and molecules is most easily achieved by solvent evaporation from small droplets, containing one solute molecule or ion only. Since small droplets are generated by mechanical disintegration processes in the capillary vaporizer, in the end decomposing by solvent evaporation only, this mode of desolvation of ions and molecules should dominate at sufficiently low solute concentrations. The range of this solute concentration is determined by the mean size of the droplets which subsequently decompose by solvent evaporation only. Applying higher solute concentrations either by increasing the concentration of the analyte and/or by addition of a TSP electrolyte, neutral or ionized clusters of solute molecules are formed by solvent evaporation from droplets. These clusters or solid particles may decompose into molecules

143

and/or fragments, depending on the ambient gas temperature. Extensive fragmentation is to be expected for thermally labile and non-volatile molecules if they are volatilized via the formation and decomposition of solid particles. These considerations of different possibilities of soft desolvation of ions by TSP do not support a significant contribution of a field effect in the ion formation. They rather point to the desolvation of ions and molecules from very small droplets only by thermal evaporation of the solvent. EXPERIMENTAL

The self-constructed TSP assembly used in this study is described in refs. 12 and 15. The vaporizer consists of a stainless-steel capillary of 80-pm i.d. which is brazed at one end into an electrically heated copper block. The length of the copper block was 3 cm. In all the experiments water as solvent with a flow rate of 1 ml mm-’ was applied. Typical temperatures applied were about 250 o C for the copper block (vaporizer) and about 200 o C for the jet chamber. Mass analysis was performed with a Finnigan 400 quadrupole mass filter with a mass range of l-420 u. The spectra were obtained by signal accumulation employing a Tracer NS 570 A multichannel analyzer. RESULTS AND DISCUSSION

TSP at low

solute concentration

The analysis of complex salts with multiply charged cations such as diquaternary salts has shown that at analyte concentrations of low3 M or less and without a TSP electrolyte such as 0.1 M ammonium acetate applied to the solution, the intact cation forms the base peak in the mass spectra [13,15,16]. Since singly charged ions are formed when a dication and an anion combine by solvent evaporation, the high abundance of dications compared to singly charged ions in the spectra provides evidence for the predominant desolvation of ions apart from counter ions. Similar conclusions can be drawn from the appearance of abundant dianions in TSP mass spectra of complex salts [18-201. In these experiments again low solute concentrations without TSP electrolyte were applied. In Fig. 1 a further example of desolvation of doubly charged cations by TSP is shown. The spectrum of Sr(NO,), exhibits Sr(H,O)i+ ions with n = 3-12. No singly charged SrNOc or SrNO,(H,O),+ ions could be detected at 10e3 M and below, although these ions gave peaks at higher salt concentrations. Similar results were obtained for Mg(OAc), and BaCl, in the concentration range between 10e4 and 10e3 M. At low concentrations

144

r

100

I 0 0

1 50

-

[Sr+nH,Ol" ,n=3..

100

150

12,

200

m/z

Fig. 1. TSP mass spectrum of Sr(NO,), obtained from a 10d3 M aqueous solution of the salt. The ion signals not indicated are due to impurities, i.e., to Na(H,O)i, K(H,O)T and Mg(H,O)i’ ions.

only the doubly charged metal ions were formed, whereas at higher concentrations of about low3 M small peaks due to singly charged ions of these salts could also be recorded. For the range of low salt concentrations the spectra of the metal salts again demonstrate that ions are predominantly desolvated apart from counter ions and the contribution to mass spectral ions from desolvation of “cluster” ions such as SrNOz is low or negligible. The abundant doubly charged metal ions and the absence or weak intensity of these “cluster” ions in the spectra leads to the conclusion that the heat of solvation of ions does not play a major role in ion formation. This is in contrast to the ion evaporation mechanism. This conclusion is also supported by the recently measured relative ion yield of Ba2+ to Cs+ of about 0.5, which was obtained by applying an equimolar mixture of BaBr, and CsBr to a directly heated capillary vaporizer in the limit of low concentrations [38]. (The slightly smaller yield for Ba2+ compared to Cs+ can be explained by an easier loss of Ba2+ on the wall of the capillary.) Furthermore, the statistical formation of highly charged droplets [24], leading to high salt concentrations with strongly interacting cations and anions in the droplet, is not in favour of electrohydrodynamic disintegration processes, in which the desolvation of doubly charged metal ions is preferred over the singly charged “cluster” ions. Experiments with BaCl, and a directly heated capillary vaporizer revealed that the concentration range in which the BaCl(H,O)i ion signals become significant compared to the Ba(H,O)z+ ions, depends very much on the type of vaporizer applied. This critical concentration range was found to be about one order of magnitude lower for a directly heated 18-cm capillary vaporizer (0.1~mm i.d. and 0.5-mm o.d.; temperature 180°C) than for the indirectly heated vaporizer applied in the above experiments. The ap-

145

pearance of singly charged ions in the TSP spectrum of this salt gives an estimate of the concentration range in which molecules or molecular ions are desolvated apart from each other without significant clustering effects. Without field effects no principal difference exists between ions and neutrals in the desolvation by TSP. At low solute concentrations molecules and molecular ions, if preformed in solution, are desolvated as single species by solvent evaporation from small droplets. The close similarity between the mass spectra of thermally labile compounds obtained by the single-beam and by the dual-beam mode of TSP ionization [32] furthermore supports a desolvation mechanism which is independent of the charge state of the solute. In the dual-beam technique one capillary vaporizer is used for the desolvation of the analyte molecule, while the other provides the reagent ions, by vaporization of an electrolyte solution, for chemical ionization of the analyte molecules in the gas phase. TSP at high solute concentration High solute concentrations are provided by the TSP electrolyte which is typically applied to the mobile phase at a concentration of about 0.1 M for ionization of analyte molecules. Under these conditions of TSP ionization the formation and decomposition of small solid particles dominate in the “desolvation” of molecules and ions [31] and give rise to enhanced fragmentation in TSP mass spectra [15]. This effect of an increase of fragmentation by ammonium acetate applied as TSP electrolyte is shown in Fig. 2 for arginine. Since ammonium acetate, which is most widely used as TSP electrolyte, has a very low decomposition and volatilization temperature, the formation of solid particles or cluster ions of the salt remains “invisible” in TSP spectra. However, salts of higher volatilization temperature such as NaCl (Fig. 3) and NaOAc (Fig. 4) form cluster ions by solvent evaporation from droplets which decompose only weakly or not at all at the applied vaporizer temperatures. Similar results have been obtained for ammonium halides [39]. As shown in Fig. 5 for sucrose, the level of fragmentation in TSP spectra increases with the concentration and volatilization temperature of the salt. For analyte concentrations G 10d3 M it is reasonable to assume that predominantly only one molecule is attached to a neutral or ionized salt cluster in the desolvation process. This is supported by the fact that dimer ions of analyte molecules in the spectra are absent or of very low abundance. However, they become abundant at analyte concentrations > 10V3 M [30]. The ammonium acetate clusters or particles can be considered to decompose already during solvent evaporation. The size distribution of these clusters or the number of salt molecules participating in the desolvation of molecules

146 100

la)

IM+Hl'

NH

1

175

H,NKNH'-fcooH NH,

IS/N:\

0

IOO-

(bl A

I

I 150

I&

1:33

I If

I 1

0

100

200

m/z

IS/N:5001

lM+HI+ 175 I

i8

I--4-

I

150

,

,

I 200

m/z

Fig. 2. Influence of ammonium acetate on the mass spectral fragmentation of arginine. The arginine spectrum in (a) was obtained without TSP electrolyte while in (b) NH,OAc of concentration 0.1 M was applied to the aqueous sample solution. Concentration of arginine in water: about lo- 3 M. The intensity of the (M + H)+ ion is much smaller in (a) than in (b). S/N = signal to noise ratio.

depends on the type and operating conditions of the TSP vaporizer as could be established by a comparison of directly and indirectly heated TSP vaporizers, regarding the formation of cluster ions from non-volatile salts. An interesting question is that of the mean size of droplets from which molecules or molecular ions are desolvated by solvent evaporation without further disintegration of the droplets. In Fig. 6 the ion currents for glucose and its fragments are shown as a function of concentration of NaOAc [31]. A maximum ion current is obtained at about 5 - 10e3 M of NaOAc. This

147

0 -1 ,=

-2 -3

m 0 -

-4

i

-5 1

1l-i-l 2

3

4

5

6”

”

Fig. 3. Abundance distribution of (NaCl),Na+ cluster ions in the TSP mass spectrum NaCl obtained from a 0.1 M aqueous solution of the salt. (Mass range < m/z 420).

-’

of

0

2

-1

-1

-1 -

Gl 0

-2

IL 2

3

4”

Fig. 4. Abundance distribution of (NaOAc),Na+ cluster ions in the TSP mass spectrum of sodium acetate obtained from a 0.1 M aqueous solution of the salt. (Mass range < m/z 420).

NH40Ac

NH4CI

NH4Br

molecular ion of sucrose to its fragments for Fig. 5. Intensity ratio of the (M+NH,)+ and different ammonium salts applied as TSP electrolytes at concentrations of 0.1 M ( -) ) respectively to the aqueous sample solution. Concentration of sucrose low3 0.05 M (Z=E M.

148

0

1 d

I

IV3 cont. NaOAc

I

w2

I

lo-’

imol .dm"l

Fig. 6. Sum of molecular and fragment ions of glucose as function of NaOAc concentration. Glucose was applied to the aqueous solution at a concentration of about 10e3 M.

maximum should arise from the competition between the formation of isolated Na+ ions and nondecomposing salt cluster ions with increasing concentration of the salt. Thus, the position of the m~mum is related to the size of the droplets from which single Naf ions are desolvated. Taking the mean distance of ions in solution at the salt concentration of the maximum in Fig. 6, the diameter of the droplet produced by the TSP vaporizer is about 8 nm. This is a very rough estimate of the droplet size from which solute molecules and ions are desolvated by solvent evaporation. It is independent from the TSP electrolyte applied, but dependent on the type and operating conditions of the vaporizer. For NH,OAc no maximum but a change in the slope of the analyte ion current as function of concentration is observed [6,40]. The low temperature at which clusters of this salt already decompose, makes an attribution of the change of slope to the onset of a predominant desolvation of clusters of the salt more difficult than in the above case of a maximum in the ion current. CONCLUSIONS

Soft ionization by the TSP technique can be attributed to a desolvation mechanism in which single neutral and/or ionized molecules are released from very small droplets by solvent evaporation. ~ompo~ds which do not form solvated ions require the application of a TSP electrolyte such as ammonium acetate for ionization either by desolvation of molecular ions from charged droplets or by ion/molecule reactions of desolvated molecules with electrolyte ions in the gas phase. The application of an electrolyte of high concentration has the disadvantage of forming salt clusters and cluster ions by solvent evaporation from droplets. These clusters, even though they

149

in the mass spectrum of ammonium acetate due to their are “invisible” thermal instability, lead to an increase of the level of fragmentation and give rise to a decrease of mass spectral sensitivity for ionic compounds [15]. There is no experimental evidence for the contribution of a field effect to the ion formation by TSP. This is in support of the above considerations that neither the ion evaporation mechanism nor the electrohydrodynamic disintegration of charged droplets should play a role in the desolvation of ions. However, field effects might play a part in the mechanical break-up of charged droplets, leading to the formation of smaller droplets at field strengths below the Rayleigh limit, ACKNOWLEDGEMENTS

We should like to thank U. Giessmann, B. Thomson and M. Vestal for stimulating discussions. The authors are also grateful for financial support of this work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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