Field Effects in Molecular Ion Formation by the Thermospray Technique G

FIELD EFFECTS IN MOLECULAR ION FORMATION BY THE THERMOSPRAY TECHNIQUE G. Schmelzeisen-Redeker, U. Giessmann, F. Röllgen

To cite this version:

G. Schmelzeisen-Redeker, U. Giessmann, F. Röllgen. FIELD EFFECTS IN MOLECULAR ION FORMATION BY THE THERMOSPRAY TECHNIQUE. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-297-C6-302. 10.1051/jphyscol:1984950. jpa-00224431

HAL Id: jpa-00224431 https://hal.archives-ouvertes.fr/jpa-00224431 Submitted on 1 Jan 1984

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FIELD EFFECTS IN MOLECULAR ION FORMATION BY THE THERMOSPRAY TECHNIQUE

G. Schmelzeisen-Redeker, U. Giessmaim and F.W. Rollgen Institute of 'Physical Chemistry, University of Bonn, Wegelerstr. 12, D-S200 Bonn, F.R.G.

Résumé - Il semble que le champ électrique soit pour partie responsable de la formation des ions par la technique thermospray (TSP). Quelques phénomènes d'ionisation observés dans les spectres -TSP des saccharides et des sels d'ammonium diquaternai- res sont rapportés et discutés. Seuls les signaux di-cationiques de ces spectres sont indicatifs de 1'évaporation ionique de champ à partir de gouttelettes chargées.

Abstract - Field effects thought to be involved in the forma tion of ions by the thermospray (TSP) techniques are outlined. Some ionization phenomena observed in the TSP spectra of sac charides and diquaternary ammonium salts are reported and dis cussed. Only the dication signals in the spectra of the latter are indicative of field induced ion evaporation from charged droplets.

The application of high electric fields to the surfaces of liquid or solid samples provides a means of soft ionization of thermally labile or nonvolatile compounds. Field effects in the formation of gaseous molecular ions with reduced thermal stress, compared to thermal eva poration of molecules and ionization in the gas phase, is well estab lished for field desorption mass spectrometry /1,2/, electrohydro- dynamic ionization mass spectrometry /3/ and electrospray techniques /4/. Recently a new ionization technique based on thermospray (TSP) phenomena was introduced by Vestal /5,6/ in which ion formation was attributed in part to field emission from charged droplets /7/. The present work is concerned with the elucidation of the role of field effects in ion formation by the TSP technique.

I - PROPOSED IONIZATION MECHANISMS IN TSP /I/ In the TSP technique a jet of randomly charged droplets is formed by passing an electrolytic solution through a heated capillary into a "jet chamber". The jet chamber is heated to facilitate the desolvation of ions by evaporation of solvent molecules. Desolvated ions are extracted through an orifice for mass analysis.

Statistically the mean charge of a droplet of either sign is given by /8/ «„.<«*,'" with V = volume of the droplet, and n = charge density (n = n = n_). The thermal evaporation of solvent molecules from charged droplets

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decreases the radius of the droplets and hence increases the surface charge density and the field strength acting on the droplets. The maximum field strength acting on a charged droplet is limited by the onset of macroscopic and microscopic disintegration processes. At field strengths above the Rayleigh limit /9/ the field stress exceeds the capillary stress caused by the surface tension y of the liquid resulting in the formation of smaller droplets. The Rayleigh limit in terms of the maximum number of charges NR on a droplet as a function of droplet radius R is given by /7/

with e = electron charge and E = vacuum permittivity. Ion evaporation processes are induced at fieldOstrengths which are difficult to esti- mate /lo/. For partially solvated molecular ions with a number of solvent molecules attached, the evaporation field strengths are lower than those required for the extraction of desolvated ions from a surface nearly free of solvent molecules, as in FD MS /11/. Thus field strengths EI of the order of lo8 V/m are probably a reasonable assump- tion for the onset of ion evaporation processes. Taking EI as a maximum field strength, the limit for the number of charges NE on a droplet is given by

Both limits indicate the contribution of field effects in the process of formation of desolvated ions. However, only when the number N of charges on a droplet exceeds NE, does a significant reduction of thermal stress become possible, compared to that occuring in conventional techniques of sample molecule evaporation for gas phase ionization. Since NE < NR for droplet radii in the submicron range /7/ ion evaporation has been considered as a major mechanism for the production of molecular ions under appropriate TSP conditions. The removal of solvent molecules from molecular ions is facilitated by the elevated temperature and pressure of the jet chamber. The formation of molecular ions independent of field effects is accom- plished by ;chemical ionization" i.e. reactions of molecules with ions such as NH4 in the gas phase. Gas phase ion chemistry may dominate the mass spectra at least under TSP operating conditions that give rise to long residence times for desolvated ions in the jet chamber. Furthermore, desolvated molecular ions can be formed in a soft mode by a purely thermal volatilization of singly charged droplets. So far it has not been considered that singly charged droplets may be produced to a significant extent fvr the formation of molecular ions by reactions of ions such as NH4 with neutral droplets and clusters in the jet. The extent to which field effects play a role in ion formation and determine the character of TSP mass spectra is still unknown. We report below some ionization phenomena, obtained with a self-constructed TSP system, that are in part indicative of a field effect in TSP ion formation. - ION OPTICS

-c PUMP

JET - CHAMBER :A1 LKB CU - BLOCK

Fig. 1 -Thermospray ion source

I1 - EXPERIMENTAL The self constructed TSP system is shown in Fig. 1. The end of the capillary (80 pm i.d.) was typically at 250 OC and the jet chamber at 180 OC. Aqueous sample solutions were introduced through the capillary. For the ionization of glucose and sucrose ammonium acetate was added to the solution. The quadrupole mass filter (Finnigan 400) employed had a mass range of 1 - 420 a.m.u.

I11 - RESULTS AND DISCUSSION A series of experiments were performed with sucrose and glucose to examine the influence of various parameters on molecular ion formation and fragmentation. In the case of sucrose (Fig. 2) abundant fragment ions were formed under all experimental conditions. The high level of fragmentation is not in support of a reduction of thermal stress in the ionization mechanism. The fragmentation is even higher than expec- ted. For example in the TSP spectrum of glucose an (M - H20 + NH4)+ peak is present whereas the elimination of water from glucose is not observed in mass spectra obtained by conventional thermal evaporation of glucose and subsequent soft ionization in the gas phase /12/. Al- though no protonated fragments are present in the TSP spectra, fragmentation of molecules via protonation in solution (promoted by a field generated charging of a liquid surface /2/) or in the gas phase is very probable. The relative fragment ion intensities in the TSP spectra were found to be raised by addition of HC1 to the aqueous solution of glucose and ammonium acetate, without the appearance of new peaks due to the formation of protonated frqgments. The observed increase of the intensity ratio (M - H 0 + NH4) /(M + NH~)' with in- creasing concentration of ammonium acegate suggests a connection be- tween the water elimination reaction and the formation of solid particles detectable via a raised noise level at the detector of the quadrupole. The involvement of solid particles in molecular and fragment ion formation is also supported by the observation of a strong de- crease in the molecular and fragment ion yield when instead of the thermally l-abile ammonium salts alkali salts are employed for ionization. Under these conditions water elimination from glucose gave a C9-300 JOURNAL DE PHYSIQUE

Fig. 2 - TSP mass spectrum of sucrose obtained by applying a 0.1 molar aqueous solution of ammonium acetate. The concentration of sucrose was 5- 10-4 mol dm-3. The abundant fragment ions are exclusively formed by NH~+attachment.

Fig. 3 - TSP spectrum of a diquaternary ammonium salt obtained by appl ing an aqueous sample solution of about 2.10-~moldm-3. M2' = dication weak signal. The details of the ion chemistry associated with the de- composition of solid particles are still unknown. So far ion formation with reduced stress has not been detected in a number of experiments, probably, because of the long residence time of ions in the jet chamber and the many collisions desolvated ions experience within a heated gas during that time. The appearance of doubly and multiply charged peptides /13/ has been taken as providing direct evidence for the contribution of field effects in ion formation because multiply charged ions are typically formed in FD MS. We have obtained TSP spectra of diquaternary salts /14/ in which doubly charged cations invariably form the base peaks (Fig. 3). Gaseous dications are obviously stable at the elevated gas temperatures (g 180 OC) encountered in TSP MS. In these experiments aqueous sample solutions of about 2.10~~mol dm-3 were analyzed without the use of further electrolyte. Similar spectra of diquaternary salts have been obtained by a field induced extraction of ions from a solution prepared by dissolving the salts in glycerol /15/. The formation of-gaseous dications requires a separation from the counter ions (e.9. I ) in the desolvation process. A field assisted ion evaporation mechanism accounts for the retention of the counter ions in the liquid and thus explains the observed low level of fragmentation in the TSP spectra more easily. Gaseous dications cannot be formed by thermal de- composition of salt molecules or particles as fragmentation reactions are induced under these conditions. The ion signals of the diquaternary salts became very weak when high concentrations of ammonium acetate were included in the sample solution. An unambiguous assignment of ionization phenomena in TSP MS to field effects is difficult. However, the detection of multiply charged ions is highly indicative of a mechanism of field induced ion evaporation from charged droplets under TSP conditions.

ACKNOWLEDGEMENT - Support of this work by the Deutsche Forschungs- gemeinschaft is gratefully acknowledged.

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9 Lord Rayleiqh, Phil. Mag. 14, 184 (1882)

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11 U. Giessmann and F.W. Rollqen, Int. J. Mass Spectrom. Ion Phys. -38, 267 (1981); S.S. Wong, U. Giessmann, M. Karas and P.W. Rollqen Int. J. Mass Spectrom. Ion Processes 56, 139 (1984) 12 for example: G. Schmelzeisen-Redeker, U. Giessmann and F.W.RBllqen Proc. 32nd Ann. Conf. Mass Spectrom. All. Topics, San Antonio 1984

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