DYNAMIC SIMS OF SUPERSATURATED SOLUTIONS

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DYNAMIC SIMS OF SUPERSATURATED SOLUTIONS

E. Junker, K. Wirth, F. Röllgen

To cite this version:

E. Junker, K. Wirth, F. Röllgen. DYNAMIC SIMS OF SUPERSATURATED SOLUTIONS. Journal

de Physique Colloques, 1989, 50 (C2), pp.C2-53-C2-58. �10.1051/jphyscol:1989210�. �jpa-00229406�

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DYNAMIC SIMS OF SUPERSATURATED SOLUTIONS

E. JUNKER, K.P. WIRTH and F.W. ROLLGEN

Institut fiir Physikalische Chemie, Universitdt Bonn, Wegelerstr. 12, 0-5300 Bonn 1, F.R.G.

R e s u m e

E n S I M S l i q u i d e u n B c h a n t i l l o n e s t s o u m i s a u f a i s c e a u d e p a r t i c u l e s ( k e V ) e t l e s i o n s m o l e c u l a i r e s s o n t e n r e g i s t r e s p e n d a n t l ' e r o s i o n d e l a t o u c h e . O n s ' e s t i n t e r e s s e a u m o d e d e f o r m a t i o n d e s i o n s m o l e c u l a i r e s d a n s d e s c o n d i t i o n s d y n a m i q u e s d e b o r n b a r d e m e n t e n f o n c t i o n d e d i v e r s p a r a m e t r e s d e l a s o l u t i o n . L e s e x p e r i e n c e s o n t e t e r e a l i s e e s a v e c d e s p e p t i d e s e t s e l s o r g a n i q u e s e n u t i l i s a n t l a spectrometric d e m a s s e e t l a microscopic o p t i q u e .

Abstract

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In liquid SIMS (or fast atom bombardment mass spectrometry) a liquid sample solution is exposed to an incident keV particle beam and molecular ions are recorded during continuous erosion of the layer. We have investigated this mode of molecular ion formation under dynamic bombarding conditions regarding the effect of concentration, solid particle formation by cristallization of solutes from supersaturated solutions and the formation of a closed solid surface layer prior to the onset of particle bombardment. The experiments were performed with pep- tides and organic salts applying mass spectrometry and optical microscopy. They showed that molecular ions and/or cluster ions are obtained under dynamic bombarding conditions without accumulation of radiation damage from supersaturated solutions, even in the presence of small precipitated particles and after erosion of thin solid surface layers from the remaining solution. Furthermore it was found that the incident particle beam hampers crystallization of the solute from a supersaturated solution.

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INTRODUCTION

Secondary ion mass spectrometry (SIMS) applying an incident high flux primary particle beam in the keV energy range to sputtering and ionization of molecules from liquid matrices is called fast atom bombardment mass spectrometry (FABMS) or liquid SIMS /1,2/. This method is widely used in analytical chemistry since abundant and long lasting molecular ion signals are obtained from thermally labile and nonvolatile sample molecules. Continuous molecular ion signals are recorded under dynamic bombarding conditions (i.e. high primary flux densjties) in FABMS as a result of a renewal of the surface layer by the sputtering process itself, thus avoiding accumulation of radiation damage / 3 , 4 / .

The liquid matrix (typically glycerol is used) has to fulfil three major requirements to provide intense and long lasting secondary molecular ion signals /5/. First it needs a low vapour pressure pv to keep the sample solution in a liquid state during the time needed for recording a mass spectrum. Sec- ondly it has to provide good solubility for the sample and it needs an elec- trical conductivity to avoid charging of the sample layer. These conditions explain the following two sample preparation techniques, which are mostly employed in routine FABMS applications:

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For samples soluble in glycerol, a concentrated homogeneous solution in glycerol is applied.

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Provided the sample is not or only poorly soluble in glycerol a suitable cosolvent has to be found. It may have a high vapour pressure, has to dissolve the sample, and must be soluble in glycerol. Usually first the sample is dj.ssolved before glycerol is added. Then this three component Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989210

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C2-54 JOURNAL DE PHYSIQUE

solution is inserted into the mass spectrometer. This method is routine- ly employed in analytical applications because glycerol is not a good solvent for many compounds.

The vapour pressures of common cosolvents for glycerol are listed in table 1. They show that after inserting a cosolvent containing sample solution into vacuum, a supersaturated solution of the sample in glycerol is formed by evaporation of the cosolvent. Supersaturation can even be reached by employing very highly concentrated solutions of samples in pure glycerol.

This paper deals with the role of highly volatile cosolvents in FABMS. By evaluation of erosion and evaporation rates it is shown how long the cosolvent can stand the vacuum conditions in the ion source. Furthermore it is shown how secondary ion emission is affected by evaporation of the cosolvent, which results in supersaturation of the sample solution or even precipitation of the solute. In this work the tetrapeptide Ala-Ala-Ala-Ala (= Ala4) dissolved in water (5 g/l corresponding to a weight ratio of 1:210) was used as test compound. Glycerol was added in the following ratios:

Ala4/H20/gl. (1:210:180 weight ratio) Ala4/H20/gl. (1:210:380 weight ratio).

In comparison neat glycerol and glycerol/water (weight ratio 1:1.6) was ex- amined.

Table 1

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Equilibrium vapour pressures of some widely

applied cosolvents for glycerol in FABMS

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EXPERIMENTAL METHODS

cosolvent methanol ethanol water

dimethylformamide (DMF) dimethylsulfoxide (DMSO) compare: glycerol

In this study optical microscopy of the sample solution was combined with FAB mass spectrometry. The mass spectra were obtained with a modified AEI MS902 double focusing mass spectrometer (for details see ref. /4/). The primary beam of Xe atoms was produced in a self-constructed saddle field dis- charge source which was operated at 6 kV anode potential and 1 mA discharge current. Due to high energetic xen+ ions /6,7/ and electrons / 8 / in the xe0/xen+-beam it has not been possible yet to determine the particle flux density of the atom beam at the target with sufficient accuracy. The current

pv (25 OC) / h ~ a 150

73 32 5.8 0.6 2

density is roughly estimated to be about 3. 1013 particles/ (cm2 . s) at the target, corresponding to about 5 pA/cmZ /4/. The target-surface (that is the stainless steal metal support on which the sample solution was inserted into the spectrometer), was in a horizontal position allowing the deposition of thick layers (300-800 pm). With a target potential of +4 kV the mass range of the spectrometer was 2300 u. The resolution was about 1200. The ion extrac- tion voltage was between 9 and 14 V.

Evaporation and erosion rates were measured with the aid of a microscope in a separate experimental set-up, allowing to simulate the ion source conditions. The microscope (Olympus VMT-4F) was placed within a working distance of 43 mm from the target (Fig. 1). The erosion and evaporation rates were determinated with a magnification between 80 and 120 times corresponding to a resolution of about 5 Gm.

The sample solutions were prepared by mixing a highly concentrated solution of the peptide in water as cosolvent with glycerol. Then a droplet of this homogeneous solution was deposited on the target and inserted into the mass spectrometer by keeping the sample solution about half a minute in the vacuum lock system.

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MICROSCOPICAL OBSERVATIONS

Figure 2 illustrates the height loss due to solvent evaporation from four different solutions deposited on the target. The Ala4 sample solutions are compared with pure glycerol and H20/glycerol. It can be seen that water as a cosolvent takes about 12 to 15 minutes to evaporate almost completely from the solution. Since the sample is not soluble in glycerol a highly supersaturated solution of the peptide in glycerol is left. After a residence time of about tv = 25 min in vacuum only (i.e. without particle bombardment), the rates of solvent evaporation of pure glycerol and of the solutions H20/glycerol (initial weight ratio 1:1.6) and Ala4/H20/glycerol (1:210:380) are the same. In contrast, as shown in Fig. 2, the curve of the solution with higher sample concentration (Ala4/HZO/glycerol 1:210:180) levels off after about tv = 20 min. Microscopy of the state of the solution clearly reveals that a closed "skinw of a solid surface layer is formed under these conditions by precipitation of sample molecules out of the supersaturated solution. This llskinfr hampers further glycerol evaporation; (particle bombardment destroys this "skinw and intense molecular ion currents are obtained as explained in section 4)

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Fiqure 2

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Decrease of layer thick- Fiaure 3

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Decrease of layer thick- ness h with residence time tv in ness h with bombarding time tb due vacuum due to solvent evaporation to sputtering and solvent evapor- only (i.e. without particle bombard- ation for two solutions and pure ment) for three solutions and for glycerol (weight ratios: H20/gly- pure glycerol; the initial composi- cerol : 1 6 , Ala4/H20/glycerol tion of the solutions is given in 1:210:180).

weight ratios (for H O/gl. 1: 1.6)

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C2-56 JOURNAL DE PHYSIQUE

In Fig. 3 height loss due to evaporation and sputtering is shown for pure glycerol and the two solutions H20/glycerol (1:1.6) and Ala4/H20/glycerol (1:210:380). After a bombarding time of about tb = 8 rnin the curves are near- ly parallel thus showing that a supersaturated solution of Ala4 is formed by the evaporation of the cosolvent water. The formation of a crystalline surface layer is hampered under bombardment as no skin on the droplet is observed within 20 minutes bombarding time. But the state of the sample solution changes such as small island like precipitated particles are observed in a highly viscous liquid. Figure 3 also shows that the erosion rate by sputtering which is proportional to the slope of the graph, of HZO/glycerol (1:1.6) and the concentrated sample solution of Ala4/H2.0/glycerol (1:210:180) is the same at tb = 8 rnin and levels off afterwards, probzbly caused by pre- cipipated crystals.

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EFFECTS ON SECONDARY ION FOFWATION

In order to study the effect of supersaturated sample solutions on secondary ion formation mass spectra were recorded as function of the residence time tv of the sample solution in vacuum and the bombardment time tb.

In Fig. 4 the intensity of Ala4 molecular ions obtained by sputtering of a Ala4/H20/glycerol solution is displayed as function of bombarding time tb. In this experiment the residence time of the sample solution in vacuum (tv = 1 min) prior to the onset of bombardment was determined by the time needed to insert the sample solution into the ion source of the mass spectrometer.

During the first eight to ten minutes the cosolvent evaporates from the solution as shown before (Fig. 3). The resulting rise in sample concentration causes an increase of the molecular ion signal for about the first seven minutes. The same effect is observed for the [G?+H]+ glycerol-cluster ion signal with bombarding time. The [G~+H]+ intensity parallels that of the

[M+H]+ ions.

A different dependence of the Ala4 molecular ion intensity is observed for a Ala4/H20/glycerol solution inserted into vacuum tv = 50 rnin prior to the onset of bombardment as shown in Fig. 5. Since a solid surface layer is formed by precipitation of the sample after about tv = 20 min of solvent evaporation, the strong increase of molecular ion emission within the first 30 s of particle bombardment can be attributed to the removal or destruction

t,=SO'

2

^1,o-

\ + A

.- _m

w C

%0,5-

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b

0 5 10 15 20 1 3 5 7 9 1 1

tb/ min tb/mi n

Fisure 4

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Abundance of molecular Ficrure 5

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Abundance of molecular ions versus bombarding time tb after ions versus bombarding time t~ after a residence time tv = 1 rnin in vac- a residence time tv = 50 rnin In vac-

uum. uum

.

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mass spectra obtained from the remaining solution during continuous erosion do not show a memory effect of radiation damage from sputtering of the solid layer before. This allows the conclusion to be drawn, that the radiation damage accumulated during sputtering of the solid layer is quickly removed within a few seconds of sputtering of the underlying liquid (supersaturated) solution. These (and other experiments with the aid of the microscope) also clearly reveal that the formation of a solid surface layer by crystallization of sample molecules from supersaturated solutions is hampered by particle bombardment.

The effect of solid layer formation by precipitation of sample molecules on the surface of supersaturated solutions is also seen in the dependence of the molecular ion intensity on concentration as shown in Fig. 6 (bottom).

Measurements of the molecular ion signal just after introduction of the sample solution into vacuum gives a linear relationship between ion intensity and concentration. However, performing the measurement delayed after tv = 30 min waiting time in vacuum a curve with a maximum as displayed in Fig. 6 (top) is obtained. The decrease in intensity at higher concentration is caused by precipitation of sample molecules and solid layer formation on the surface of the sample solution after solvent evaporation as dicussed above.

Similar observations were made for other solute/cosolvent systems such as quaternary ammonium salts dissolved in glycerol with the aid of cosolvents like DMSO. Changes in the abundance distribution of cluster ions of salts during bombardment of concentrated and even supersaturated solutions hsve already been reported /4,9,10,11/.

Fisure 6

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Abundance of molecular ions versus sample concentration in glycerol after tv = 30 rnin (top) and tv = 1 rnin (bottom).

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CONCLUSIONS

As a result of evaporation of matrix molecules from a concentrated sample solution in FABMS a supersaturated solution is formed. If cosolvents with high vapour pressures are used, supersaturation is favoured. Under bombarding conditions water as cosolvent takes about 8 minutes to evaporate almost completely from the sample solution. Without particle bombardment the formation of a solid surface layer of sample crystals is observed. This "skin1' is not formed under bombarding conditions, at least not within typical measuring times. Only small precipitated single crystals can be observed with a, microscope. They do not disturb the registration of intense molecular ion signals in the mass spectrometer. If a thin solid layer on the samnple solution surface is formed by pre-evaporation of matrix molecules ; e , g . in the vacuum lock system) it can be removed (or destroyed) by the primary particle beam and the ion signals show no memory effect of this skin. Rarely it is observed that this layer is too thick to be destroyed by particle bombardment and no molecular ions are obtained.

Acknowledsement: The authors are grateful to the Deutsche Forschungsgemein- schaft for financial support of this work.

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REFERENCES

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