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Field induced disintegration of glycerol solutions under vacuum and atmospheric pressure conditions studied by optical microscopy and mass spectrometry
Surface Science 266 (1992) 197-203 North-Holland
surface science
Field induced disintegration of glycerol solutions under vacuum and atmospheric pressure conditions studied by optical microscopy and mass spectromet13., U. Liittgens, T h . Diilcks a n d F.W. R d l l g e n Institut .fiir Physikalische und Theoretische Chemie, Unicersitiit Bonn, Wegelerstr. 12, W-5300 Bonn 1, FRG Received 5 August 19q:; a c c e p t e d for publication 2 S e p t e m b e r 1991
T h e ion formation in b o t h electrohydrodynamic (EH) and electrospray (ES) mass spectrometry (MSJ is based on the e!ectrohydrodynamic disintegration of sample solutions which are passed t h r o u g h a capillary biased at high potential. Vacuum is applied in E H and a t m o s p h e r i c pressure in ES MS. F o r glycerol applied as solvent in EH MS optical studies of its disintegrat;on b e h a v i o r revealed a change from axial spray m o d e s to a rim emission mode in vacuum and a change from axial spray modes to a droplet ejection mode at a t m o s p h e r i c pressure conditions with increasing potential. E H MS investigations of the ion emission from only one or a few emission sites at the rim of the capillary showed a pulsed ion emission whose frequency increased with applied potential. The pulsed ion emission is attributed to an imbalance between the supply and loss of liquid at an emission site. By lowering the surface tension of glycerol with dodecyl sulfate sodium salt an increase of mass spectral ion intensity by more than one o r d e r of magnitude could be observed.
1, Introduction In electrohydrodynamic (EH)[!,2] and electrospray (ES) [3-6] mass spectro,aetry (MS) electrolytic sample solutions are pas~ed through a capillary biased at high potential. Electrohydrod~,namic disintegration processes at the end of the capillary lead to the formation of gaseous ions. Both methods differ in the pressure at the end of the capillary, i.e. vacuum conditions in EH and atmospheric pressure (AP) in ES MS. Glycerol, a solvent of low vapor pressure, is used in EH MS, whereas solvents of high vapor pressure, such as m e t h a n o l / w a t e r solutions are applied in ES MS. Mass spectral ions are sampled in EH MS at potentials close to that of the capillary. This provides evidence for the formation of these ions by electrohydrodynamic disintegration of the liquid at the end of the capillary. In contrast mass spectral ions in ES MS are released from charged droplets, emitted from the capillary tip, by solvent evaporation in the atmospheric gas. Fenr,
and others [5-7] have demonstrated that biomolet, ules having molecular weights from sevend to more than l{10 kDa can be ionized and mass analyzed by ES MS. SitlCC the n'lolecnl;~r ions are typicafiy tormed m a highly charged state they are amenable to the mass range of a quadrupole mass analyzer (m/z < 2000). Quite differently, EH MS yields abundant molecular ions in a lower charge state only. So far EH MS has not found widespread applications mainly because it is more difficult to handle than other mass spectrometric techniques, and in addition because E l l mas:~ spectra are typically dominated by cluster ions [8]. So far details of the ion forming proccsse~'; in EH and ES MS are largely unknown. We have performed experiments to investigate the field induced disintegration of liquids at the end of the capillary under vacuum and atmospheric pressure conditions with the objective to elucidate the role of such processes in the ion formation. Optical r~h~,~,',,.,~;,~," ' " ~.'it. ~.t. t U.1l_y.kl I o. t d .y I l a m lC" O l s i n t e . . . . . . . . . ~ , ~ , , . , o f "t~l~
0039-6(}28/92/$(}5.00 :~ 1992 - Elsevier Science Publishers 13.V. All rights r e s c u e d
198
U. Liittgens et al. / Field induced disintegration of glycerol solutions
gration of liquids at the end of capillaries have been reported before [1,3,9,1(I]. In this paper we report results of the study of the disintegration behavior of glycerol solutions both for vacuum and atmospheric pressure conditk.,ns by applying ovtieal microscopy ~ombined with video recording, and mass spectrometry for EH MS conditions. Glycerol is used as a solvent in EH but not in ES MS. However, due to its low vapor pressure it is amenable to both techniques of spraying and thus well suited for studying the disintegration processes.
2. Experimental The optical investigations were performed with an optical microscope combined with a video recordint, :,ystem with a resolution of about 5 p.m at a working distance of about 45 mm from the capillary. Liquids were sprayed from the tip of a stainless steel capillary (180 tam I.D., 360 txm O.D.). A flat grounded stainless steel plate, positioned at a distance of 5 mm for vacuum and of 1 em for atmospheric pressure conditions, served as the counter electrodc. The mass spectrometric investigations were performed with a double focusing mass spcctromclcr (A~-I MS-9) equipped with a self-cohort ,,'t,~ed EH ion soui..,.. "i'il~ ion source consisted of a P t / I r capillary (21{) tzm I.D., 410 ~ m O.D.) with a parabolically tapered tip. The sample solution was supplied with flow rates < 0.16 ~ l / m i n . The capillary was positioned at a distance of about 0.5 mm in front of a flat counter electrode with a 5 mm cylindrical hole, which was part of the ion optics. (The "capillary potential" in the text below refers to the difference between the potentials applied to the capillary tip and the counter electrode, respectively.) Micromanipulators facilitated ~hree-dimensionai adjustment ~f the position of the capillary tip. It was possible to observe the capilla~' tip with "n optical microscope through a viewing window in the ion source. All solutions used under vacuum conditions were carefully degassed by heating and by sonicaring under vacuum (< 10 -2 hPa). This was fl!~portant ~o avoid disturbances by gas evolution.
Fig. l. Vacuum conditions; emission of pure glycerol via an axial jet, flow rate: 1 ,ul/min; capillary potential: 3.2 kV.
3. Results and discussion 3.1. Vacuum conditions The disintegration behavior of pure glycerol was studied as function of applied capillary potential and for flow rates of I g l / m i n and less. The optical studies revealed a change of the spray mode with increasing potential: At very low potentials droplets were formed from the liquid at fl3e end of the capillary and werz rcn-,~,vcd with a i', cqaency increasing with applied potential. For pure glycerol above about 3 kV axial pulsing jets wer- observed. The jets emerged from the apex of a cone formed by the liquid. The pulsing was caused by an imbalance between the constant supply of liquid and its loss via the jet. By appropriate setting of the potential slightly above the onset of the axial emission mode continuous axial jets could be observed for minutes (fig. 1). With increasing potential the pulse frequency increased and in a potential range between about 5 and 6 kV the emission changed from an axial mode to one wherein emission proceeded via jets from the rfln of the capillary (fig. 2). In this rim emission mode ~.he meniscus of the liquid was concavely curved into the capillary and the liquid, wetting the wall, flowed to the rim which was exposed to high field s:rengths. The number of emission sites along the rim increased with increasing potential. For pure glycerol and capillar3~
199
U. Liittgens et al. / FieM induced disintegra#on o f gl)cerol s o h a i o n s
However, the pu!:;i;~ vf cm~:slon sites could be observed by mass spectrometr),. For this purpose the capillary tip in the E H ion source of ~he mass spectrometer was positioned by micromanipulators in such a way that only one or a few emission sites were in the acceptance area of the mass analyzer. Monitoring the (2M + Na) + glycerol signal with a fast recorder revealed pulse frequencies between a few Hz and several kHz and the contribution of several emission sites of
Fig. 2. Vacuum conditions; emission of pure glycerol from the rim of the capillary; flow rate: 1 #l/min, capillary potential: ~.5 kV.
a ) ut~ 0.8
o.6 O,t~
potentials >_ 9 kV no emission of mass spectral ions could be observed by E H MS. Doping of the glycerol with NaI did not change the basic p h e n o m e n a observed with pure glycerol but it led to the formation of finer jets in both the axial and the rim emission mode. For E H MS conditions, i.e. capillary potential ~ 0 kV and a flow rate of < 0.1 btl/min, a vet), dispersive fine spray was f,,rmed with ihe g i y c e r o l / N a l solution (fig. 3). The spraying appeared to be fairly uniformly distributed along the rim of the capillary. Within the time resolution of the video recording system no pulsing of emission could be observed.
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Fig. 4. Sequence of pulsed ion signals o[ m / z = 207 t [ O : + Na]" ) with different pulse frequencies, obtained flora various emission sites with a solution of ]0 wt.C~ of Na~ in glycerol: the capillary potentials were set between 7.5 and ; [ kV. Pulse frequencies: (a) 3.2 Hz, (b) 76.2 Hz, (c) Snperposi~ or. of three pulse frequencies: 6.3, 21 and 33.7 Hz, (d) Superposifion of a number of unresolved pulse frcquencie:.
L: Lilugens et al. / FieM induced disintegration of glycerol solutions
200
indicated by an irregular change in the emission pattern which is optically observable at higher flow rates and lower potentials as well as by mass spectrometry. Since mass spectral ions should be extracted from the liquid by a cooperative mechanism, as discussed for field desorption mass spectrometry (FD MS) [12,13], particular conditions are required for the ion formation in EH MS regarding flow rate, size of protrusion and field strength. It is hardly possible that these conditions can be maintained for the lifetime of a jet. Therefore, it is probable that higher-frequency intensity fluctuations also exist. For LMIS pulsed emission with frequencies up to the l0 s Hz range have been reported and attributed to tip oscillations [14-19]. A high viscosib' and a low surface tension should favor the desolvation of ions by electrohydrodynamic disintegration of liquids. Since the viscosity of glycerol is already high we have reduced the surface tension by adding a surfactant, i.e. 0.1 wt.% dodecyl sulfate sodium salt, to a glycerol/!0 wt.% Nal solution. The additon of the surfactant lowered the potential for the onset of the axial emission mode from 3 to about 1.5 kV and for the transition from the axial to the rim emission mode by about 2 kV to about 3.5 kV. A remarkaJle increase of the emission cur-
different pulse frequencies to the intensity of ion signals. This effect is shown in fig. 4. A pulsed ion emission under EH MS conditions was reported before for liquid metal ion sources (LMIS) only [11]. The emission sites of mass spectral ions may not be identical with the optically observable emission sites. Scanning of the emission sites along the rim of a capillary revealed a much smaller number of emission sites of mass spectral ions than emission sites observed optically. The low and rather constant frequencies of pulsed ion emission suggest that an imbalance between the loss of liquid during the emission of mass spectral ions and the supply of liquid to the emission sites caused the observed discontinuity in the ion emission. On the more macroscopic scale such an imbalance between loss and supply of liquids leads to the pulsing of axial jets as discussed above. This explanation of the pulsed ion emission is supported by the mass spectral observation that the repetition rate of the ion emission increases with increasing capillary potential (fig. 5). For the pulsing axial jet the same observation was made (see above). It has to be added that large intensity fluctuations may also be caused by an unstable flow of liquid to the various emission sites at the rim. This effect is
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of glycerol solutions
U. Liittgens et al. / Field induced disintegration
rent of mass spectra! ions by one order of magnitude was observed by mass spectrometry upon addition of the surfactant. The relative intensity of the ion signals, however, did not show any significant difference. Notably, the surfactant did not appear in the spectra within the examined mass range, i.e. m/z = 50-750. In fig. 6 the mass spectra of sucrose dissolved in the glycerol/Nal solution with and without the surfactant added are compared. The ion signals are less fluc;:mting with the surfactant present. This probably indicates an increase in the number of sites at the rim emitting mass spectral ions. The effect of surfactants on ion emission has not been systematically explored yet.
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3.2. Atmospheric pressure conditions For lower capillary potentials (3-4.5 kV) the disintegration behavior of pure glycerol and of the glycerol/Nal ~olution at atmospheric pressure was qualitatively similar to that observed under vacuum conditions, i.e. the emission occurred via pulsing and continuous axial jets above a threshold potential. For a supply of liquid larger than the loss of liquid by the jet the emission was interrupted by ejection of the droplet extending out of the capillary. At higher potentials ( > 4.5 kV), the disintegration behavior of glycerol at AP differed from that under vacuum conditions: with increasing potential the emission via an axial jet
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202
U. Li#tgens et al. / Field induced disintegration of glycerol solutions
This effect of the external pressure is not completely understood yet. It has been attributed to a stabilization of the meniscus of the liquid by the external pressure in leaving the capillary tube before Jt becomes exposed to the high field stress resulting in a droplet ejection proces~ [20]. Without the external pressure the flow of glycerol is determined by the loss of liquid from the rim and its supply along the capillary wall as mentioned above. In contrast, the droplet ejection process, which is observed under AP conditions, depends on the exposure of a liquid meniscus (or cone) to a field stress high enough to tear up a larger part of the surface. In contrast to a previous interpretation the appearance of the droplet ejection mode does not depend on viscosity [20]. This is supported by the observation that under AP conditions water showed a similar behavior as glycerol [21]. On the other hand, a low surface tension should be in favor of a rim emission mode. Thus in contrast to glycerol, a glycerol/methanol solution having a lower surface tension changed from an axial to a rim emission mode with increasing potential (fig. 8). ES MS conditions depend on a fine dispersive spraying o | sample solutions. This is achieved by spraying in the rim emission mode but can also be achieved by spraying in axial emission modes. The capillary potential applied in ES MS is limited by the onset of a corona discharge [3,6] but the available potential range is still high enough
Fig. 7. Atmospheric pressure conditions: sequence of photographs of pure glycerol showing the droplet ejection mode (~l) at t = 0 ms, (h) at t = 42 ms, (c) at t = 84 ms: flow rate: 1 gl/min, capillary potenlial: 6 kV.
changed to a droplet ejection mode in which the liquid leaving the capillary was ejected as large droplets (fig. 7a-c). No emission from the rim of the capillary was observed.
Fig. 8. A t m o s p h e r i c pressure conditions; rim emission mode of a g l y c e r o l / m e t h a n o l solution ( 2 5 : 7 5 vol.%); flow rate: I # l / r a i n , capillaD' potential: 5 kV.
U. Liittgens et al. / Field htduced di, 're, ration of kh'cerol sohaions
for the use of the rim emission mode with a n u m b e r of solvents. These and other clectrohydrodynamic phen o m e n a have to be studied in more dctail to elucidate the role of electrohydrodynamic processes in EH and ES MS.
Acknowledgements The authors are grateful to K.D. Cook for stimulating discussions and to the Deutsche Forschungsgeme.inschaft for financial support of this work.
References [1] D.S. Simons, B.N. Colby and C.A. Evans, Int. J. Mass Spectrom. Ion Phys. 15 (1974) 291. [2] K.D. Cook, Mass Spectrom. Rev. 5 (1986) 467. [3] M.Y. Yamashita and J.B. Fenn, J. Phys. Chem. 88 (1984) 445 I. [4] M.L. Alexandrov, L N Gall, N.V. Krasnov and V.I. Nikolacy, Int. J. Mass Spectrom. Ion Proc. 54 (1983) 231. [5] J.B. Fcnn, M. Mann, ('.K. Meng, S.F. Wong and C,M. Whitehouse, Mass Spectrom. Rev. 9 (1990) 37. [6] R.D. Smith, J.A. Loo, C.G Edmonds, C.J, Barinaga and tl.R. Udscth, Amd. Chem. 62 (1990) 882.
2/t3
i7] J.A. Loo, it.R, Udscth and R.D. Smith. Anal. Biochem. 59 (1989) 2642. [8] B.P. Stimpson, D.S. Simons and ('.A. E~ans, J. Phx~. Chem. 82 (1978) 6611. [9] J.j. ttogan, R.S. Carson, J.M. Schneider and ('.I3. [[ct~ dricks, AIAA J. 2 (1964) 1460. [10] D. Michelson, Electrostatic Atomization (Adam Hilger, Bristol and New York, 1990) and references cited therein. [I1] C.A. Evans and C.D, He~dricks, Rev, Sci. lnstrum. 43 (1972) 1527. [12] U. Giessmann and F.W. R611gen, Int. J. Mass Spectrom. ion Phys. 38 (1981) 267. [13] S.S, Wong, U. Giessmann, M. Karas and F.W. R611gen, int. J. Mass Spectrom. Ion Proc. 56 (1984) 139. [14] D.R. Kingham and L.W. Swanson, Vacuum 34 ~lqXa~ 945. [15] V.G. Dudnikov and A.L. Shabalin, Soy. Phys. Techn. Phys. 30 (1985) 462. [Its] D.L. Barr, W.L. Brown and D.J. Thomson, J. Phys. (Paris) 49 (1988) C6-177. [17] G.L.R. Mair, J. Phys. (Paris) 50 (1989) C8-171, [18] R.I. Hornsey, J. Phys. (Paris) 50 (1989) C8-197. [19] N.M. Miskovsk3', J He, P.H. Cutler and M. Chung, J. Appl. Phys. 69 (1991) 1956. [20] U. Liittgen~, F.W. R611gen and K.D. Cook, in: Methods and Mechanisms for Producing Ions from Large Molecules. eds. K.G. Standing and W. Ens (Pergamon Press), in press. [21] U. Liittgens. F.W. R611gen and K.D. Cook, Proc. 38th ASMS Conf. on Mass Spectrom. :rod AI;icd Topics, Tuc;n ~!t~UO~p. 132.
surface science
Field induced disintegration of glycerol solutions under vacuum and atmospheric pressure conditions studied by optical microscopy and mass spectromet13., U. Liittgens, T h . Diilcks a n d F.W. R d l l g e n Institut .fiir Physikalische und Theoretische Chemie, Unicersitiit Bonn, Wegelerstr. 12, W-5300 Bonn 1, FRG Received 5 August 19q:; a c c e p t e d for publication 2 S e p t e m b e r 1991
T h e ion formation in b o t h electrohydrodynamic (EH) and electrospray (ES) mass spectrometry (MSJ is based on the e!ectrohydrodynamic disintegration of sample solutions which are passed t h r o u g h a capillary biased at high potential. Vacuum is applied in E H and a t m o s p h e r i c pressure in ES MS. F o r glycerol applied as solvent in EH MS optical studies of its disintegrat;on b e h a v i o r revealed a change from axial spray m o d e s to a rim emission mode in vacuum and a change from axial spray modes to a droplet ejection mode at a t m o s p h e r i c pressure conditions with increasing potential. E H MS investigations of the ion emission from only one or a few emission sites at the rim of the capillary showed a pulsed ion emission whose frequency increased with applied potential. The pulsed ion emission is attributed to an imbalance between the supply and loss of liquid at an emission site. By lowering the surface tension of glycerol with dodecyl sulfate sodium salt an increase of mass spectral ion intensity by more than one o r d e r of magnitude could be observed.
1, Introduction In electrohydrodynamic (EH)[!,2] and electrospray (ES) [3-6] mass spectro,aetry (MS) electrolytic sample solutions are pas~ed through a capillary biased at high potential. Electrohydrod~,namic disintegration processes at the end of the capillary lead to the formation of gaseous ions. Both methods differ in the pressure at the end of the capillary, i.e. vacuum conditions in EH and atmospheric pressure (AP) in ES MS. Glycerol, a solvent of low vapor pressure, is used in EH MS, whereas solvents of high vapor pressure, such as m e t h a n o l / w a t e r solutions are applied in ES MS. Mass spectral ions are sampled in EH MS at potentials close to that of the capillary. This provides evidence for the formation of these ions by electrohydrodynamic disintegration of the liquid at the end of the capillary. In contrast mass spectral ions in ES MS are released from charged droplets, emitted from the capillary tip, by solvent evaporation in the atmospheric gas. Fenr,
and others [5-7] have demonstrated that biomolet, ules having molecular weights from sevend to more than l{10 kDa can be ionized and mass analyzed by ES MS. SitlCC the n'lolecnl;~r ions are typicafiy tormed m a highly charged state they are amenable to the mass range of a quadrupole mass analyzer (m/z < 2000). Quite differently, EH MS yields abundant molecular ions in a lower charge state only. So far EH MS has not found widespread applications mainly because it is more difficult to handle than other mass spectrometric techniques, and in addition because E l l mas:~ spectra are typically dominated by cluster ions [8]. So far details of the ion forming proccsse~'; in EH and ES MS are largely unknown. We have performed experiments to investigate the field induced disintegration of liquids at the end of the capillary under vacuum and atmospheric pressure conditions with the objective to elucidate the role of such processes in the ion formation. Optical r~h~,~,',,.,~;,~," ' " ~.'it. ~.t. t U.1l_y.kl I o. t d .y I l a m lC" O l s i n t e . . . . . . . . . ~ , ~ , , . , o f "t~l~
0039-6(}28/92/$(}5.00 :~ 1992 - Elsevier Science Publishers 13.V. All rights r e s c u e d
198
U. Liittgens et al. / Field induced disintegration of glycerol solutions
gration of liquids at the end of capillaries have been reported before [1,3,9,1(I]. In this paper we report results of the study of the disintegration behavior of glycerol solutions both for vacuum and atmospheric pressure conditk.,ns by applying ovtieal microscopy ~ombined with video recording, and mass spectrometry for EH MS conditions. Glycerol is used as a solvent in EH but not in ES MS. However, due to its low vapor pressure it is amenable to both techniques of spraying and thus well suited for studying the disintegration processes.
2. Experimental The optical investigations were performed with an optical microscope combined with a video recordint, :,ystem with a resolution of about 5 p.m at a working distance of about 45 mm from the capillary. Liquids were sprayed from the tip of a stainless steel capillary (180 tam I.D., 360 txm O.D.). A flat grounded stainless steel plate, positioned at a distance of 5 mm for vacuum and of 1 em for atmospheric pressure conditions, served as the counter electrodc. The mass spectrometric investigations were performed with a double focusing mass spcctromclcr (A~-I MS-9) equipped with a self-cohort ,,'t,~ed EH ion soui..,.. "i'il~ ion source consisted of a P t / I r capillary (21{) tzm I.D., 410 ~ m O.D.) with a parabolically tapered tip. The sample solution was supplied with flow rates < 0.16 ~ l / m i n . The capillary was positioned at a distance of about 0.5 mm in front of a flat counter electrode with a 5 mm cylindrical hole, which was part of the ion optics. (The "capillary potential" in the text below refers to the difference between the potentials applied to the capillary tip and the counter electrode, respectively.) Micromanipulators facilitated ~hree-dimensionai adjustment ~f the position of the capillary tip. It was possible to observe the capilla~' tip with "n optical microscope through a viewing window in the ion source. All solutions used under vacuum conditions were carefully degassed by heating and by sonicaring under vacuum (< 10 -2 hPa). This was fl!~portant ~o avoid disturbances by gas evolution.
Fig. l. Vacuum conditions; emission of pure glycerol via an axial jet, flow rate: 1 ,ul/min; capillary potential: 3.2 kV.
3. Results and discussion 3.1. Vacuum conditions The disintegration behavior of pure glycerol was studied as function of applied capillary potential and for flow rates of I g l / m i n and less. The optical studies revealed a change of the spray mode with increasing potential: At very low potentials droplets were formed from the liquid at fl3e end of the capillary and werz rcn-,~,vcd with a i', cqaency increasing with applied potential. For pure glycerol above about 3 kV axial pulsing jets wer- observed. The jets emerged from the apex of a cone formed by the liquid. The pulsing was caused by an imbalance between the constant supply of liquid and its loss via the jet. By appropriate setting of the potential slightly above the onset of the axial emission mode continuous axial jets could be observed for minutes (fig. 1). With increasing potential the pulse frequency increased and in a potential range between about 5 and 6 kV the emission changed from an axial mode to one wherein emission proceeded via jets from the rfln of the capillary (fig. 2). In this rim emission mode ~.he meniscus of the liquid was concavely curved into the capillary and the liquid, wetting the wall, flowed to the rim which was exposed to high field s:rengths. The number of emission sites along the rim increased with increasing potential. For pure glycerol and capillar3~
199
U. Liittgens et al. / FieM induced disintegra#on o f gl)cerol s o h a i o n s
However, the pu!:;i;~ vf cm~:slon sites could be observed by mass spectrometr),. For this purpose the capillary tip in the E H ion source of ~he mass spectrometer was positioned by micromanipulators in such a way that only one or a few emission sites were in the acceptance area of the mass analyzer. Monitoring the (2M + Na) + glycerol signal with a fast recorder revealed pulse frequencies between a few Hz and several kHz and the contribution of several emission sites of
Fig. 2. Vacuum conditions; emission of pure glycerol from the rim of the capillary; flow rate: 1 #l/min, capillary potential: ~.5 kV.
a ) ut~ 0.8
o.6 O,t~
potentials >_ 9 kV no emission of mass spectral ions could be observed by E H MS. Doping of the glycerol with NaI did not change the basic p h e n o m e n a observed with pure glycerol but it led to the formation of finer jets in both the axial and the rim emission mode. For E H MS conditions, i.e. capillary potential ~ 0 kV and a flow rate of < 0.1 btl/min, a vet), dispersive fine spray was f,,rmed with ihe g i y c e r o l / N a l solution (fig. 3). The spraying appeared to be fairly uniformly distributed along the rim of the capillary. Within the time resolution of the video recording system no pulsing of emission could be observed.
,I
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i
i
i f
llln
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!!!I
I! !ill
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~ %!i ,i!i k e HIL.J~
0,1
0,2
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*
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/
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Fig. 3. Vacuum conditions: rim emission mode of glycerol doped with Na]: flow rate: 0.02 #l/rain, capillavs potential: 9 kV.
,' ,3
Fig. 4. Sequence of pulsed ion signals o[ m / z = 207 t [ O : + Na]" ) with different pulse frequencies, obtained flora various emission sites with a solution of ]0 wt.C~ of Na~ in glycerol: the capillary potentials were set between 7.5 and ; [ kV. Pulse frequencies: (a) 3.2 Hz, (b) 76.2 Hz, (c) Snperposi~ or. of three pulse frequencies: 6.3, 21 and 33.7 Hz, (d) Superposifion of a number of unresolved pulse frcquencie:.
L: Lilugens et al. / FieM induced disintegration of glycerol solutions
200
indicated by an irregular change in the emission pattern which is optically observable at higher flow rates and lower potentials as well as by mass spectrometry. Since mass spectral ions should be extracted from the liquid by a cooperative mechanism, as discussed for field desorption mass spectrometry (FD MS) [12,13], particular conditions are required for the ion formation in EH MS regarding flow rate, size of protrusion and field strength. It is hardly possible that these conditions can be maintained for the lifetime of a jet. Therefore, it is probable that higher-frequency intensity fluctuations also exist. For LMIS pulsed emission with frequencies up to the l0 s Hz range have been reported and attributed to tip oscillations [14-19]. A high viscosib' and a low surface tension should favor the desolvation of ions by electrohydrodynamic disintegration of liquids. Since the viscosity of glycerol is already high we have reduced the surface tension by adding a surfactant, i.e. 0.1 wt.% dodecyl sulfate sodium salt, to a glycerol/!0 wt.% Nal solution. The additon of the surfactant lowered the potential for the onset of the axial emission mode from 3 to about 1.5 kV and for the transition from the axial to the rim emission mode by about 2 kV to about 3.5 kV. A remarkaJle increase of the emission cur-
different pulse frequencies to the intensity of ion signals. This effect is shown in fig. 4. A pulsed ion emission under EH MS conditions was reported before for liquid metal ion sources (LMIS) only [11]. The emission sites of mass spectral ions may not be identical with the optically observable emission sites. Scanning of the emission sites along the rim of a capillary revealed a much smaller number of emission sites of mass spectral ions than emission sites observed optically. The low and rather constant frequencies of pulsed ion emission suggest that an imbalance between the loss of liquid during the emission of mass spectral ions and the supply of liquid to the emission sites caused the observed discontinuity in the ion emission. On the more macroscopic scale such an imbalance between loss and supply of liquids leads to the pulsing of axial jets as discussed above. This explanation of the pulsed ion emission is supported by the mass spectral observation that the repetition rate of the ion emission increases with increasing capillary potential (fig. 5). For the pulsing axial jet the same observation was made (see above). It has to be added that large intensity fluctuations may also be caused by an unstable flow of liquid to the various emission sites at the rim. This effect is
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U. Liittgens et al. / Field induced disintegration
rent of mass spectra! ions by one order of magnitude was observed by mass spectrometry upon addition of the surfactant. The relative intensity of the ion signals, however, did not show any significant difference. Notably, the surfactant did not appear in the spectra within the examined mass range, i.e. m/z = 50-750. In fig. 6 the mass spectra of sucrose dissolved in the glycerol/Nal solution with and without the surfactant added are compared. The ion signals are less fluc;:mting with the surfactant present. This probably indicates an increase in the number of sites at the rim emitting mass spectral ions. The effect of surfactants on ion emission has not been systematically explored yet.
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3.2. Atmospheric pressure conditions For lower capillary potentials (3-4.5 kV) the disintegration behavior of pure glycerol and of the glycerol/Nal ~olution at atmospheric pressure was qualitatively similar to that observed under vacuum conditions, i.e. the emission occurred via pulsing and continuous axial jets above a threshold potential. For a supply of liquid larger than the loss of liquid by the jet the emission was interrupted by ejection of the droplet extending out of the capillary. At higher potentials ( > 4.5 kV), the disintegration behavior of glycerol at AP differed from that under vacuum conditions: with increasing potential the emission via an axial jet
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202
U. Li#tgens et al. / Field induced disintegration of glycerol solutions
This effect of the external pressure is not completely understood yet. It has been attributed to a stabilization of the meniscus of the liquid by the external pressure in leaving the capillary tube before Jt becomes exposed to the high field stress resulting in a droplet ejection proces~ [20]. Without the external pressure the flow of glycerol is determined by the loss of liquid from the rim and its supply along the capillary wall as mentioned above. In contrast, the droplet ejection process, which is observed under AP conditions, depends on the exposure of a liquid meniscus (or cone) to a field stress high enough to tear up a larger part of the surface. In contrast to a previous interpretation the appearance of the droplet ejection mode does not depend on viscosity [20]. This is supported by the observation that under AP conditions water showed a similar behavior as glycerol [21]. On the other hand, a low surface tension should be in favor of a rim emission mode. Thus in contrast to glycerol, a glycerol/methanol solution having a lower surface tension changed from an axial to a rim emission mode with increasing potential (fig. 8). ES MS conditions depend on a fine dispersive spraying o | sample solutions. This is achieved by spraying in the rim emission mode but can also be achieved by spraying in axial emission modes. The capillary potential applied in ES MS is limited by the onset of a corona discharge [3,6] but the available potential range is still high enough
Fig. 7. Atmospheric pressure conditions: sequence of photographs of pure glycerol showing the droplet ejection mode (~l) at t = 0 ms, (h) at t = 42 ms, (c) at t = 84 ms: flow rate: 1 gl/min, capillary potenlial: 6 kV.
changed to a droplet ejection mode in which the liquid leaving the capillary was ejected as large droplets (fig. 7a-c). No emission from the rim of the capillary was observed.
Fig. 8. A t m o s p h e r i c pressure conditions; rim emission mode of a g l y c e r o l / m e t h a n o l solution ( 2 5 : 7 5 vol.%); flow rate: I # l / r a i n , capillaD' potential: 5 kV.
U. Liittgens et al. / Field htduced di, 're, ration of kh'cerol sohaions
for the use of the rim emission mode with a n u m b e r of solvents. These and other clectrohydrodynamic phen o m e n a have to be studied in more dctail to elucidate the role of electrohydrodynamic processes in EH and ES MS.
Acknowledgements The authors are grateful to K.D. Cook for stimulating discussions and to the Deutsche Forschungsgeme.inschaft for financial support of this work.
References [1] D.S. Simons, B.N. Colby and C.A. Evans, Int. J. Mass Spectrom. Ion Phys. 15 (1974) 291. [2] K.D. Cook, Mass Spectrom. Rev. 5 (1986) 467. [3] M.Y. Yamashita and J.B. Fenn, J. Phys. Chem. 88 (1984) 445 I. [4] M.L. Alexandrov, L N Gall, N.V. Krasnov and V.I. Nikolacy, Int. J. Mass Spectrom. Ion Proc. 54 (1983) 231. [5] J.B. Fcnn, M. Mann, ('.K. Meng, S.F. Wong and C,M. Whitehouse, Mass Spectrom. Rev. 9 (1990) 37. [6] R.D. Smith, J.A. Loo, C.G Edmonds, C.J, Barinaga and tl.R. Udscth, Amd. Chem. 62 (1990) 882.
2/t3
i7] J.A. Loo, it.R, Udscth and R.D. Smith. Anal. Biochem. 59 (1989) 2642. [8] B.P. Stimpson, D.S. Simons and ('.A. E~ans, J. Phx~. Chem. 82 (1978) 6611. [9] J.j. ttogan, R.S. Carson, J.M. Schneider and ('.I3. [[ct~ dricks, AIAA J. 2 (1964) 1460. [10] D. Michelson, Electrostatic Atomization (Adam Hilger, Bristol and New York, 1990) and references cited therein. [I1] C.A. Evans and C.D, He~dricks, Rev, Sci. lnstrum. 43 (1972) 1527. [12] U. Giessmann and F.W. R611gen, Int. J. Mass Spectrom. ion Phys. 38 (1981) 267. [13] S.S, Wong, U. Giessmann, M. Karas and F.W. R611gen, int. J. Mass Spectrom. Ion Proc. 56 (1984) 139. [14] D.R. Kingham and L.W. Swanson, Vacuum 34 ~lqXa~ 945. [15] V.G. Dudnikov and A.L. Shabalin, Soy. Phys. Techn. Phys. 30 (1985) 462. [Its] D.L. Barr, W.L. Brown and D.J. Thomson, J. Phys. (Paris) 49 (1988) C6-177. [17] G.L.R. Mair, J. Phys. (Paris) 50 (1989) C8-171, [18] R.I. Hornsey, J. Phys. (Paris) 50 (1989) C8-197. [19] N.M. Miskovsk3', J He, P.H. Cutler and M. Chung, J. Appl. Phys. 69 (1991) 1956. [20] U. Liittgen~, F.W. R611gen and K.D. Cook, in: Methods and Mechanisms for Producing Ions from Large Molecules. eds. K.G. Standing and W. Ens (Pergamon Press), in press. [21] U. Liittgens. F.W. R611gen and K.D. Cook, Proc. 38th ASMS Conf. on Mass Spectrom. :rod AI;icd Topics, Tuc;n ~!t~UO~p. 132.