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Field desorption of sodium acetate studied by combined optical microscopy and mass spectrometry
Zntemational humal
of Mass Specfrornetry and Zon Processes, 82 (1988) 55-60
55
Elsevier Science Pubfshers B.V., Amsterdam - Printed in The Netherlands
F. OKUYAMA
*, S.S. WONG and F.W. R6LLGEN
Institute of Physicaal Chemistry, University of Bonn, D-5300 Bonn (F, R.G.) (First received 18 June 1987; in final form 11 September 1987)
ABSTRACT Optical microscopy combined with field desorption mass spectrometry revealed that NaOAc polycrystals exposed to high positive electric fields dis&egrate below the melting point while emitting cations and cluster ions. The sample disintegration started with a field-induced deformation of the original sample, which was repeatedly followed by extraction of parts of the sample material. Such a discontinuok sample removal is in contrast to a uniform sample depletion process being characteristic for ion desorption from NaCi and KCl crystals.
INTRODUCTiON
Ion generation in field desorption (FD) mass spectrometry (MS) has been divided into two categories, field ionization (FI) and desolvation mechanism [I]. In the FI mechanism, molecules are ionized close to high-field points by electron tunnelling, giving rise to, for example, molecular ions. Depending on the ionization potential of the molecules and on surface conditions, necessary field strengths for thii process are in the range from 2 to 6 x 109 V m-r. In the desolvation mechanism, on the other hand, ions preformed through an electrochemical process are extracted directly from the condensed layer at field strengths typically lower than those required for FI. Ions formed by this mechanism are quasi-molecular ions (e.g. molecules cationized by proton or alkali ion attachment). Thus far, three different modes of ion desolvation have been identified. The first of them is the desorption of quasi-molecular ions from a viscous solution of organic samples doped with an ekctrolyte [2-41. This type of ion * Permanent address: Department of Systems Engineering, Nagoya Institute of Technology, Nagoya 466, Japan. 0X8-1176/88/!fiO350
0 1988 Ekvier
!kience Publishers B.V.
56
desorption involves the following sequential processes: (a) the formation of field-enhancing protrusions arising from an electrohydrodynamic sample disintegration, (b) the solidification of the protrusions into a glassy state, subsequent to solvent evaporation, and (c) the extraction of ions from the tips of these glassy protrusions. This sequence proceeds at temperatures lower than the melting point of the solvent-free sample material. The second mode is the ion extraction from a liquid phase. When exposed to a high electric field, a droplet of glycerol or molten salt, for example, is instantaneously pulled off from the substrate while abundantly emitting ions, including cluster ions distributed over a wide mass range. Ion emission from a liquid phase is the basis of electrohydrodynamic mass spectrometry [S] but its relation to FDMS has not yet been fully understood. The third mode of ion desorption, which is from inorganic salts such as NaCl and KC1 [6], is in striking contrast to the above two cases, insomuch as no hydrodynamic sample deformation is involved. KC1 crystals exposed to a positive field, intensely emit cations and cluster ions below the melting point, while by sublimation the size of the crystals is diminished continuously [6]. Like inorganic salts, organic salts have been known to field-emit cations and cluster ions, but their FD processes have remained largely unknown. Compared with alkali halides, organic salt crystals are typically less rigid and frequently less stable thermally, so samples prepared for FD may suffer from plastic deformation under field stress. It can therefore be assumed that FD processes of organic salts are not identical with those established for the two alkali halides. In this study, sodium acetate (NaOAc) was chosen as the test substance for determining the FD behaviour of organic salts. EXPERIMENTAL
The details of the experimental set-up have already been described elsewhere [3,6]. The FD ion source of a single-focusing magnetic mass spectrometer was combined with an optical microscope through a viewing window, enabling us to observe the sample depletion process at an optical resolution of about 0.8 pm while monitoring mass-selected ion signals. The operating conditions of the ion source were the same as employed previously. With the aid of a microsyringe, a droplet of saturated aqueous solution of sodium acetate was deposited in the middle of a supported tungsten wire 5 mm in length and 10 pm in diameter, or onto the end of a 10 pm W wire spot-welded to a 0.15 mm Ta wire loop. The deposit was then placed at the focus of both the mass spectrometer and the microscope. The solvent was evaporated during evacuation of the ion source. The samples thus prepared
57
were polycrystalline. For samples deposited on supported wires, heating was done resistively and the temperature was estimated from calibration curves relating the heating current to the wire mean temperature [3]. The temperature of the samples deposited on wire ends was determined similarly after calibration of the heating current through the Ta loop. RESULTS AND DISCUSSION
Figure 1 shows the morphological variation of an NaOAc deposit on a 10 pm wire anode, associated with ion desorption from the deposit. Also presented in Fig. 1 are the intensities of Na,OAc+ (MNa+) ions corresponding to the respective stages of sample disintegration. Note that the sample began to deform at 260°C about 60” C lower than the melting point of NaOAc (324OC). At this temperature, the detected ions were of very low intensity. By a slight increase of the temperature above 260 OC, the sample disintegration, along with the MNa+ emission, was drastically enhanced [Fig. l(c)-(e)]. When the depletion of the sample was completed [Fig. l(f)] the ion intensity fell to zero. In the stage of sample depletion, MNa+ ions were still emitted intensely when only small deposits were left on the anode [Fig. l(e)]. No doubt sample deposits undergoing strong dynamical deformation were the more effective ion sources. At the onset of the ion emission, MNa+ signals were rather broad and were shifted towards lower masses because of poor electrical conductivity of the sample deposit at the beginning of FD. As the ion emission continued, however, the signals were gradually sharpened and moved to the expected mass positions. Obviously, the electrical conductivity of the sample was improved during the ion emission, probably due to enhanced ion mobility in the decomposing sample. For alkali halides KC1 and NaCl, just below the temperature at which the sample sublimation became optically observable, cluster ions started to appear with a long time lag [6]. This delayed onset is thought to be due to the formation of “microscopic” surface protrusions through a slow surface diffusion process. In the present case, no similar phenomenon was recognized for MNaf ions. Hence, sharp protrusions responsible for the generation of MNa+ ions must have been created by a rapid process (see below). Along with the MNa+ emission, the Na+ emission increased during the disintegration of the sample. However, Naf ions could be detected even before the sample deposit began to disintegrate. Furthermore, Na+ remained at a detectable intensity after the sample had vanished. In addition to the sample surface, therefore, the sample/filament interface and/or optically undetectable microdeposits served as the origin of Na+ ions.
90. -
(a)
Id)
e)
f)
Fig. 1. Disintegration behaviour of an NaOAc deposit on a 10 pm wire anode observed by optical microscopy. Listed in the table attached are heating current, time, and intensity of MNa+ corresponding to the respective images.
59
(a) f(s) i(a.u.)
~
20 5
Fig. 2. Disintegration successively recorded
I
(b)
(c)
(d)
73
125
152
15
120
0
of an NaOAc sample deposited on the tip of a broken 10 pm wire, after setting the temperature at a constant value ( - 300 o C).
It was mentioned earlier that ion desorption from the alkali halides involves a uniform reduction in sample volume by sublimation. In contrast, the sample depletion process of NaOAc disclosed in Fig. 1 was far from being uniform; a volume of sample material was pulled off at certain places until the sample was exhausted. Such a discontinuous sample removal could be seen more clearly for NaOAc deposits placed on the end of the wire. A typical example is shown in Fig. 2, where a rather thick protrusion, indicated by an arrow in Fig. 2(b), appeared. The protrusion was further pulled out by the applied field and torn off from the deposit without developing into a “needle-like” protrusion. This removal process was too fast to be recorded photographically. The removal of the protrusion was quickly followed by the formation of a new protrusion [see arrowed portion in Fig. 2(c)] and the sample continued to diminish in this way. As seen from the table given in Fig. 2, MNa+ ions with enhanced intensity could be observed during this cyclic process. From the above observations, MNa+’ ions, as well as the majority of Na+ ions, originated from the disintegrating sample. The onset of the MNa+ emission was always preceded by an appreciable sample deformation. It should be emphasized, however, that, in agreement with the positive ion emission from the alkali halides, the sample disintegration, and accordingly the MNaf emission, take place below the sample melting point. A cooperative mechanism 131 has been proposed for the first mode of field-induced ion desolvation. In this mechanism, the ion extraction occurs at the tips of growing projections by successive rupture of intermolecular bonds. The stepwise desolvation of ions is due to the formation of very fine transient protrusions of molecular dimensions.
60
This mechanism was devised to explain the ion desorption from viscous organic solutions. The present observation seems to confirm that it also holds for ion extraction from NaOAc polycrystals. But the difference between the former [3] and the present case is the sample disintegration process. As has been noted repeatedly, the sample disintegration is induced electrohydrodynamicahy for viscous solutions but is not permitted for NaOAc deposits below the melting point. It is therefore supposed that a field-induced plastic deformation of the original sample deposit is the beginning of the ion-emission process. Such deformation prompts an extraction of parts of the sample material, which in~turn produces irregularities on the surface of the remaining sample. The surface irregularities so created may develop into projections having tips sharp enough to emit field ions. The ion extraction from the tips and the resulting disintegration of the projections result in the formation of new surface irregularities and thus this process repeats itself until the sample is exhausted. CONCLUSION
The present investigation showed that an NaOAc polycrystal exposed to a positive high field emits cations and cluster ions during its disintegration below the melting point. The disintegration is due to plastic deformation of the sample and the removal of parts of the sample material, thus forming field-enhancing and ion-emitting protrusions. This FD behaviour differs from that of the alkali halides NaCl and KC1 and from that of viscous solutions of organic samples such as sucrose. Therefore, field ion emission from NaOAc polycrystals may represent a fourth mode of “ion desolvation”. ACKNOWLEDGEMENT
Financial support of this work by the Wissenschaftsministerium Landes Nordrhein-Westfalen is gratefully acknowledged.
des
REFERENCES 1 F.W. Rollgen, Springer Ser. Chem. Phys., 25 (1983) 2. 2 U. Giessmann and F.W. Riillgen, Int. J. Mass Spectrom. Ion Phys., 38 (1981) 267. 3 S.S. Wong, U. Giessmann, M. Karas and F.W. RBllgen, Int. J. Mass Spectrom. Ion Processes, 56 (1984) 139. 4 E. Bramer-Weger, S.S. Wong, S. Subhan and F.W. Riillgen, J. Phys. (Paris), 47 (1986) C7. 5 See, for example, K.D. Cook, Mass Spectrom. Rev., 5 (1986) 467. 6 F. Okuyama, S.S. Wong and F.W. Rollgen, Surf. Sci., 151 (1985) L131.
of Mass Specfrornetry and Zon Processes, 82 (1988) 55-60
55
Elsevier Science Pubfshers B.V., Amsterdam - Printed in The Netherlands
F. OKUYAMA
*, S.S. WONG and F.W. R6LLGEN
Institute of Physicaal Chemistry, University of Bonn, D-5300 Bonn (F, R.G.) (First received 18 June 1987; in final form 11 September 1987)
ABSTRACT Optical microscopy combined with field desorption mass spectrometry revealed that NaOAc polycrystals exposed to high positive electric fields dis&egrate below the melting point while emitting cations and cluster ions. The sample disintegration started with a field-induced deformation of the original sample, which was repeatedly followed by extraction of parts of the sample material. Such a discontinuok sample removal is in contrast to a uniform sample depletion process being characteristic for ion desorption from NaCi and KCl crystals.
INTRODUCTiON
Ion generation in field desorption (FD) mass spectrometry (MS) has been divided into two categories, field ionization (FI) and desolvation mechanism [I]. In the FI mechanism, molecules are ionized close to high-field points by electron tunnelling, giving rise to, for example, molecular ions. Depending on the ionization potential of the molecules and on surface conditions, necessary field strengths for thii process are in the range from 2 to 6 x 109 V m-r. In the desolvation mechanism, on the other hand, ions preformed through an electrochemical process are extracted directly from the condensed layer at field strengths typically lower than those required for FI. Ions formed by this mechanism are quasi-molecular ions (e.g. molecules cationized by proton or alkali ion attachment). Thus far, three different modes of ion desolvation have been identified. The first of them is the desorption of quasi-molecular ions from a viscous solution of organic samples doped with an ekctrolyte [2-41. This type of ion * Permanent address: Department of Systems Engineering, Nagoya Institute of Technology, Nagoya 466, Japan. 0X8-1176/88/!fiO350
0 1988 Ekvier
!kience Publishers B.V.
56
desorption involves the following sequential processes: (a) the formation of field-enhancing protrusions arising from an electrohydrodynamic sample disintegration, (b) the solidification of the protrusions into a glassy state, subsequent to solvent evaporation, and (c) the extraction of ions from the tips of these glassy protrusions. This sequence proceeds at temperatures lower than the melting point of the solvent-free sample material. The second mode is the ion extraction from a liquid phase. When exposed to a high electric field, a droplet of glycerol or molten salt, for example, is instantaneously pulled off from the substrate while abundantly emitting ions, including cluster ions distributed over a wide mass range. Ion emission from a liquid phase is the basis of electrohydrodynamic mass spectrometry [S] but its relation to FDMS has not yet been fully understood. The third mode of ion desorption, which is from inorganic salts such as NaCl and KC1 [6], is in striking contrast to the above two cases, insomuch as no hydrodynamic sample deformation is involved. KC1 crystals exposed to a positive field, intensely emit cations and cluster ions below the melting point, while by sublimation the size of the crystals is diminished continuously [6]. Like inorganic salts, organic salts have been known to field-emit cations and cluster ions, but their FD processes have remained largely unknown. Compared with alkali halides, organic salt crystals are typically less rigid and frequently less stable thermally, so samples prepared for FD may suffer from plastic deformation under field stress. It can therefore be assumed that FD processes of organic salts are not identical with those established for the two alkali halides. In this study, sodium acetate (NaOAc) was chosen as the test substance for determining the FD behaviour of organic salts. EXPERIMENTAL
The details of the experimental set-up have already been described elsewhere [3,6]. The FD ion source of a single-focusing magnetic mass spectrometer was combined with an optical microscope through a viewing window, enabling us to observe the sample depletion process at an optical resolution of about 0.8 pm while monitoring mass-selected ion signals. The operating conditions of the ion source were the same as employed previously. With the aid of a microsyringe, a droplet of saturated aqueous solution of sodium acetate was deposited in the middle of a supported tungsten wire 5 mm in length and 10 pm in diameter, or onto the end of a 10 pm W wire spot-welded to a 0.15 mm Ta wire loop. The deposit was then placed at the focus of both the mass spectrometer and the microscope. The solvent was evaporated during evacuation of the ion source. The samples thus prepared
57
were polycrystalline. For samples deposited on supported wires, heating was done resistively and the temperature was estimated from calibration curves relating the heating current to the wire mean temperature [3]. The temperature of the samples deposited on wire ends was determined similarly after calibration of the heating current through the Ta loop. RESULTS AND DISCUSSION
Figure 1 shows the morphological variation of an NaOAc deposit on a 10 pm wire anode, associated with ion desorption from the deposit. Also presented in Fig. 1 are the intensities of Na,OAc+ (MNa+) ions corresponding to the respective stages of sample disintegration. Note that the sample began to deform at 260°C about 60” C lower than the melting point of NaOAc (324OC). At this temperature, the detected ions were of very low intensity. By a slight increase of the temperature above 260 OC, the sample disintegration, along with the MNa+ emission, was drastically enhanced [Fig. l(c)-(e)]. When the depletion of the sample was completed [Fig. l(f)] the ion intensity fell to zero. In the stage of sample depletion, MNa+ ions were still emitted intensely when only small deposits were left on the anode [Fig. l(e)]. No doubt sample deposits undergoing strong dynamical deformation were the more effective ion sources. At the onset of the ion emission, MNa+ signals were rather broad and were shifted towards lower masses because of poor electrical conductivity of the sample deposit at the beginning of FD. As the ion emission continued, however, the signals were gradually sharpened and moved to the expected mass positions. Obviously, the electrical conductivity of the sample was improved during the ion emission, probably due to enhanced ion mobility in the decomposing sample. For alkali halides KC1 and NaCl, just below the temperature at which the sample sublimation became optically observable, cluster ions started to appear with a long time lag [6]. This delayed onset is thought to be due to the formation of “microscopic” surface protrusions through a slow surface diffusion process. In the present case, no similar phenomenon was recognized for MNaf ions. Hence, sharp protrusions responsible for the generation of MNa+ ions must have been created by a rapid process (see below). Along with the MNa+ emission, the Na+ emission increased during the disintegration of the sample. However, Naf ions could be detected even before the sample deposit began to disintegrate. Furthermore, Na+ remained at a detectable intensity after the sample had vanished. In addition to the sample surface, therefore, the sample/filament interface and/or optically undetectable microdeposits served as the origin of Na+ ions.
90. -
(a)
Id)
e)
f)
Fig. 1. Disintegration behaviour of an NaOAc deposit on a 10 pm wire anode observed by optical microscopy. Listed in the table attached are heating current, time, and intensity of MNa+ corresponding to the respective images.
59
(a) f(s) i(a.u.)
~
20 5
Fig. 2. Disintegration successively recorded
I
(b)
(c)
(d)
73
125
152
15
120
0
of an NaOAc sample deposited on the tip of a broken 10 pm wire, after setting the temperature at a constant value ( - 300 o C).
It was mentioned earlier that ion desorption from the alkali halides involves a uniform reduction in sample volume by sublimation. In contrast, the sample depletion process of NaOAc disclosed in Fig. 1 was far from being uniform; a volume of sample material was pulled off at certain places until the sample was exhausted. Such a discontinuous sample removal could be seen more clearly for NaOAc deposits placed on the end of the wire. A typical example is shown in Fig. 2, where a rather thick protrusion, indicated by an arrow in Fig. 2(b), appeared. The protrusion was further pulled out by the applied field and torn off from the deposit without developing into a “needle-like” protrusion. This removal process was too fast to be recorded photographically. The removal of the protrusion was quickly followed by the formation of a new protrusion [see arrowed portion in Fig. 2(c)] and the sample continued to diminish in this way. As seen from the table given in Fig. 2, MNa+ ions with enhanced intensity could be observed during this cyclic process. From the above observations, MNa+’ ions, as well as the majority of Na+ ions, originated from the disintegrating sample. The onset of the MNa+ emission was always preceded by an appreciable sample deformation. It should be emphasized, however, that, in agreement with the positive ion emission from the alkali halides, the sample disintegration, and accordingly the MNaf emission, take place below the sample melting point. A cooperative mechanism 131 has been proposed for the first mode of field-induced ion desolvation. In this mechanism, the ion extraction occurs at the tips of growing projections by successive rupture of intermolecular bonds. The stepwise desolvation of ions is due to the formation of very fine transient protrusions of molecular dimensions.
60
This mechanism was devised to explain the ion desorption from viscous organic solutions. The present observation seems to confirm that it also holds for ion extraction from NaOAc polycrystals. But the difference between the former [3] and the present case is the sample disintegration process. As has been noted repeatedly, the sample disintegration is induced electrohydrodynamicahy for viscous solutions but is not permitted for NaOAc deposits below the melting point. It is therefore supposed that a field-induced plastic deformation of the original sample deposit is the beginning of the ion-emission process. Such deformation prompts an extraction of parts of the sample material, which in~turn produces irregularities on the surface of the remaining sample. The surface irregularities so created may develop into projections having tips sharp enough to emit field ions. The ion extraction from the tips and the resulting disintegration of the projections result in the formation of new surface irregularities and thus this process repeats itself until the sample is exhausted. CONCLUSION
The present investigation showed that an NaOAc polycrystal exposed to a positive high field emits cations and cluster ions during its disintegration below the melting point. The disintegration is due to plastic deformation of the sample and the removal of parts of the sample material, thus forming field-enhancing and ion-emitting protrusions. This FD behaviour differs from that of the alkali halides NaCl and KC1 and from that of viscous solutions of organic samples such as sucrose. Therefore, field ion emission from NaOAc polycrystals may represent a fourth mode of “ion desolvation”. ACKNOWLEDGEMENT
Financial support of this work by the Wissenschaftsministerium Landes Nordrhein-Westfalen is gratefully acknowledged.
des
REFERENCES 1 F.W. Rollgen, Springer Ser. Chem. Phys., 25 (1983) 2. 2 U. Giessmann and F.W. Riillgen, Int. J. Mass Spectrom. Ion Phys., 38 (1981) 267. 3 S.S. Wong, U. Giessmann, M. Karas and F.W. RBllgen, Int. J. Mass Spectrom. Ion Processes, 56 (1984) 139. 4 E. Bramer-Weger, S.S. Wong, S. Subhan and F.W. Riillgen, J. Phys. (Paris), 47 (1986) C7. 5 See, for example, K.D. Cook, Mass Spectrom. Rev., 5 (1986) 467. 6 F. Okuyama, S.S. Wong and F.W. Rollgen, Surf. Sci., 151 (1985) L131.