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Cationization in laser desorption mass spectrometry
Nuclear Instruments and Methods 198 (1982) 125-130 North-Holland Publishing C o m p a n y
125
CATIONIZATION IN LASER D E S O R P T I O N M A S S S P E C T R O M E T R Y
Gerard J.Q. VAN DER PEYL, Kimio ISA,* Johan H A V E R K A M P and Piet G. K I S T E M A K E R FOM-lnstitute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
Laser desorption results of sucrose can be explained by a gas phase complexation: Alkali ions from hot parts ( T > 7 0 0 K) of the surface combine in the gas phase with sucrose molecules which desorb from cooler places on the surface. This ionization process seems to be applicable to other "non-volatile" compounds too. Gas phase ionization contributions to other soft ionization techniques are also discussed.
1. Introduction
Several techniques have been developed to produce quasi-molecular ions of organic compounds which cannot be obtained with classical sample introduction and ionization techniques like direct probe introduction and electron impact ionization [1-5]. General features of the new techniques are the introduction of the sample inside the ion source of the mass spectrometer and the application of ionization methods based on thermal ion-molecule reactions rather than on energetic electronmolecule interactions. As a result of the ion-molecule reactions, the mass spectra obtained generally display no molecular (radical) ion peaks but socalled quasi-molecular ion peaks. A quasi-molecular ion consists of a complex of the neutral organic molecule and the reagent ion. Examples are protonated and alkali ion cationized molecules. The main advantage of the ionization by ion-molecule reactions is the relatively low energy deposition in the resulting ion which enhances considerably the stability of the complex. Therefore, the term soft ionization techniques has been introduced. Undoubtedly the most succesful technique is field desorption mass spectrometry (FDMS) where ion-molecule complexes are produced under the influence of strong electric fields ( > 10 7 V / m ) [1]. Other techniques like direct chemical ionization mass spectrometry [2], laser desorption mass spec-
trometry (LDMS) [3] and ion induced desorption mass spectrometry [4,5] have been developed as very useful alternatives [6]. Although it is clear that in all these techniques ionization proceeds via ion-molecule reactions, the details of these processes have not been clarified up till now. Ionization in the solid, liquid and gas phase have been postulated. It has been assumed for a long time that the thermally labile organic molecules analyzed could not be evaporated, thus excluding a gas phase ionization. Therefore, complexation in the solid was assumed to preceed desorption and models have been developed on this basis [7,8]. Contrasting with this point of view we shall demonstrate in this article that in one of the techniques, LDMS, gas phase complexation of neutral molecules and alkali ions is a feasible process to explain the results observed. To do this we shall discuss data, obtained in our group using various experimental set-ups. Both a high power pulsed laser and a low power continuous wave (cw) laser have been used as well as resistive heating procedures to generate quasi-molecular ions of sucrose. Because part of the experimental results have been published elsewhere [9,10] only the most interesting details of these results are discussed in conjunction with some recently obtained data. This article will be concluded by a discussion about the possibility of applying a gas phase complexation model to results obtained with other soft ionization techniques.
* Guest scientist from Fukui University, Fukui, Japan.
0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
III. MECHANISMS
G.J.Q. van der Peyl et al. / Laser desorption mass spectrometry
126
2. Experimental In all experimental set-ups sucrose (MW 342) has been used as sample material. Some other sample compounds have been used only in one set-up; LD-valine (MW 117), sodium acetate (MW 82), amygdalin (MW 457), ergosterol (MW 397), cellobiose (MW 342) and 4-methylumbelliferyl-jRD-galactoside (MW 338). All compounds were dissolved in water-methanol mixtures (1 m g / m l ) . Of these solutions 1-10 ~1 drops were deposited on the substrate. Substrate materials which have been used are: stainless steel, quartz, N i C r - N i and tungsten. No alkali salts have been added to the sample in the experiments described in this article. Experimental set-ups used are schematically presented in fig. 1 and a short description of each experiment will be given below. A pulsed TEA CO 2 laser ()t = 10.6/~m, i" ~ 150 ns, maximum laser energy 1 J) was focused on a substrate surface covered with the sample material ( ~ 300 monolayers) in experiment I [9]. Laser produced ions were mass analyzed in a magnetic sector mass spectrometer with simultaneous ion detection capability. The results are obtained with only a small fraction of the laser energy (0.6-21 m J) focused down to a spot of 0.24 m m diameter. Calculated energy densities lie in the range of 1.3-45 J / c m 2, corresponding to mean power densities of 8.7 × 106-3.1 N 108 W / c m 2. As substrate materials both stainless steel and quartz were used. LDMS results have also been obtained for a set-up where a cw CO2-1aser (3 W) was focused in
TEACO2
I PULSEDLASER LDM II CWCO2LASERLDMS [[I CWCO2LASERH
RESISTIVE HEATIN~)I~)TIVE HEATIN~I
HEATINGL~
Fig. 1. A schematic presentation of the different experimental set-ups described in this article. Roman numerals are used to refer to the experiments in the text.
the ion source of a Finnigan 3000 quadrupole mass analyzer in experiment II [10]. The sample was deposited at the junction of a N i C r - N i thermocouple (diameter 50 /~m), positioned in the ion-source. This enabled a measurement of maxim u m substrate temperatures to be made during the laser desorption process. Power densities up to 10 k W / c m 2 were obtained in the laser focus of 0.2 m m diameter. Normally, the electron beam in the ion source was switched off but in a few experiments electron impact ionization was used to monitor the neutral particle desorption during laser irradiation. In subsequent experiments other desorption and ionization configurations were used in the ion source of the quadrupole mass spectrometer. In experiment III a thin W-wire (diameter 60 ~m) was heated resistively (0.8 A) to produce alkali ions. This wire was placed close ( ~ 3 mm) to the thermocouple wire, which was irradiated by the laser. In this way sample heating and alkali ion generation could be performed independently from each other. Resistive heating of both the sample substrate and the K+-source was performed in experiment IV. In this experiment a W-ribbon (30 # m thick) was used as substrate for the sample. This ribbon was heated resistively by an electric current of about 1 A while K + -ions were emitted from a thin W-wire. In some experiments an electrostatic potential difference of 0 - 3 V was applied between the two wires to investigate quasi-molecular ion generation as function of the potential applied. Controlled heating of the sample substrate was applied in experiment V [10]. A thin thermocouple was spotwelded to the W-ribbon to measure the temperature of the substrate. A commercial type of K +-source (Spectramat [11]) was used to obtain an intense "monoenergetic" ion beam. The ion source was mounted relatively remote from the sample ribbon and was shielded mechanically to reduce exposure of the sucrose sample to K +-ions. In this way also exposure of the alkali ion source to evaporated sucrose was reduced. In experiment V I a simplified thermal desorption probe for alkali cationization was used [10]. An electric current (0.8A) passed through a Wfilament (diameter 60/~m) and stainless steel wires (diameter 1 mm), which were spotwelded together in series. The sample was deposited on one of the thick wires, a thermocouple was spotwelded to this
G.J.Q. van der Peyl et a L / Laser desorption mass spectrometry
wire to give a temperature indication of the sample substrate.
3. Compilation of experimental results In this section some new results will be presented from investigations on the basic mechanism in LDMS. For a coherent survey some results, which we have published recently are also discussed here briefly. As will be shown below complexation with either Na+-ions (I) or K+-ions (II-VI) is predominant. In our opinion this reflects the different abundances of these ions in the substrate materials. An indication for this can be found in the experimental result that if e.g. [M + K] + -ions are more abundant than [M + Na] + -ions also K +-ions are more abundant relative to Na +ions and vice versa. 3.1. Pulsed CO 2 laser L D M S
The influence of laser pulse energy on laser produced ion currents has been investigated in this experiment [9]. Minimum laser energies of 2 m J and 0.6 mJ were required for stainless steel and quartz substrates, respectively, to produce measurable currents of Na +-ions and quasi-molecular ions of sucrose. The ion intensity curves versus
.
,10 z
m l z 23
J
////I v
laser energy for one ion mass but different substrates showed a clearly different behaviour. With increasing laser energy, the alkali ion and organic ion currents increased up to a maximum value. The increase in the alkali ion current appeared to be stronger than the increase in the quasi-molecular ion current. For a thermal interpretation, a laser heating model as described by Ready [12] was used to calculate substrate surface temperatures as function of laser pulse energy. The surface temperatures obtained by laser irradiation were characterized by the maximum temperature Tm reached on the substrate surface during the pulse. In this way ion intensities could be plotted as function of Tm (see fig. 2). The most interesting conclusions extracted from these curves are: 1) No quasi-molecular ions or Na +-ions have been observed at temperatures below about 700 K. 2) The ion intensities for one mass value but different substrates show a similar behaviour as function of substrate temperature. 3.2. Cw CO 2 laser L D M S
Ion intensitie., and thermocouple temperatures T~ were measured simultaneously as a function of irradiation time [10]. Using this information the laser produced ion intensities have been plotted as a function of Tc in fig. 3. In a few experiments the electron beam in the ion source was switched on during the laser pulse. The typical sucrose fragment at m / z = 126 has been monitored in this way and the variation of this ion intensity with sub-
//
!'°I' -
,"', m / z 126
>, ¢J
10o
750 CALCULATED
127
1250 1750 ' / SURFACE TEMPERATURE Tm (K)
Fig. 2. Ion intensities as function of the c a l c u l a t e d m a x i m u m surface t e m p e r a t u r e Tm d u r i n g a laser pulse. The curves corres p o n d to a t h e r m a l d e s o r p t i o n m o d e l [9] b u t are used o n l y to g u i d e the eye. The ion intensities have been n o r m a l i z e d to unit i n t e n s i t y at the lowest m e a s u r e d v a l u e s . Q - N a + - i o n s ( m / z = 23) from a q u a r t z substrate; 27 - N a + - i o n s ( m / z = 23) from a stainless steel substrate; O - [sucrose + Na] +-ions ( m / z = 365) from a q u a r t z substrate; /x - [sucrose + Na] + ( m / z = 365) from a stainless steel substrate.
ZK÷]
\
,!
o
5
,
300
,,
,
m / z 381
600 900 1205 THERMOCOUPLE TEMPERATURE Tc (K)
Fig. 3. C w laser p r o d u c e d ion currents as function of the t h e r m o c o u p l e t e m p e r a t u r e Tc. The sucrose fragment ion m / z = 126) was m e a s u r e d b y electron i m p a c t ionization. III. M E C H A N I S M S
128
G.J.Q. van der Peyl et aL / Laser desorption mass spectrometry
strate temperature is shown in fig. 3. Two important conclusions can be extracted from fig. 3: 1) No quasi-molecular ions or K + -ions have been observed at temperatures below about 700 K. 2) Desorption of neutral particles, as represented by m / z - - 126, took place already in the temperature range 400-600 K. If the laser beam was interrupted during quasimolecular ion production, these ions could still be observed until some tens of milliseconds afterwards. Effectively, during this time quasi-molecular ions and K +-ions were observed at zero laser intensity. The role of the sample temperatures in the L D M S process was investigated in more detail in the following experiments where the influence of laser heating was partly or totally replaced by direct resistive heating of the substrate.
3.3. Cw CO 2 laser with independent K + -source In this experiment a K + -source was introduced close to the thermocouple wire. The thermocouple was heated by the laser to relatively low temperatures ( < 700 K). If during the laser heating of the sample the K+-source was on, quasi-molecular ions could be observed at low thermocouple temperatures T~ in the range of 400-500 K (see fig. 4). In this temperature range also desorption of neutrals occurred as deduced from experiments where additional EI was used (not shown, but see also fig. 3).
m,23
~,
,
!
K*
3.4. Resistive heating of the sample substrate Quasi-molecular ions could be obtained in this set-up, indicating that laser heating is not a prerequisite for quasi-molecular ion formation. To make sure that no K +-ions could impinge on the sucrose substrate the K+-source was set at a voltage of 3 V relative to the sucrose substrate. Also under these conditions quasi-molecular ions were observed. -
3.5. Controlled resistive heating of the sample In a more defined set-up the temperatures of the W-ribbon could be monitored during the desorption process. In fig. 5 resulting partial mass spectra ( m / z = 200-400) for sucrose are presented for ribbon temperatures of 460 K and 475 K. These spectra are obtained by signal averaging of low ion signals for about 1 rain. Clearly it is demonstrated by these results that a mere temperature rise of 15 K of the sample already results in extensive fragmentation of the sucrose molecules.
3.6. Simplified cationization probe The best practical results, i.e. intense quasimolecular ion currents for a relatively long time (30s), were obtained in these experiments [10]. Conditions in the laser spot in LDMS were approximated as good as possible in this set-up. The thin W-filament is heated to temperatures o f 1000K needed for alkali ion emission while the
:yl 381
w 363
300
500
700
";'
THERMOCOUPLE TEMPERATURE
Tc (K)
Fig. 4. Q u a s i - m o l e c u l a r ion current of sucrose ([M + K] +, m / z = 381) as function of t h e r m o c o u p l e t e m p e r a t u r e Tc.
200
300
~00 - - - ~
rn /z
Fig. 5. Partial mass spectra for sucrose at r i b b o n t e m p e r a t u r e s of 460 K (a) a n d 475 K (b).
G.J.Q. van der Peyl et a L / Laser desorption mass spectrometry
stainless steel wires remain at relatively low temperatures (400-600 K). Besides intense quasimolecular ion currents for sucrose as reported previously [10], also quasi-molecular ions for some other polar organic molecules were observed, notably LD-valine, sodium acetate, amygdalin, ergosterol, cellobiose and 4-methylumbelliferyl-flD-galactoside. Furthermore, it has to be stated that in first approximation optimal quasi-molecular ion currents were obtained at sample temperatures close to the melting point of the sample under investigation.
4. Discussion
Up till now models [7,8] have been proposed to explain results of soft ionization on the basis of a process where first complexation takes place on the surface, followed by desorption of quasimolecular ions. However, the L D M S and thermal desorption results reported in this article can be explained by a gas phase complexation model: Both neutral molecules and alkali ions desorb from the substrate surface independently and combine in the gas phase. Alkali ions are emitted by a thermoionic process at surface temperatures higher than about 700 K, neutral molecules are desorbed from areas with a much lower temperature (400600 K) probably via a simple evaporation process. The function of the laser seems only to be to create these two temperature areas close to each other in space and time. In such a model the high surface temperatures required for quasi-molecular ion formation in the experiments I and II can be understood as in these experiments the alkali ion production is the limiting factor. Quasi-molecular ions are indeed formed at much lower sample temperatures if alkali ions are produced in a separate hot source independent from the sample substrate as in experiments III, IV and V. Further evidence for a gas phase model can be extracted from experiments III, IV and V where the alkali ions were only supplied from an external source via the gas phase. Together with the result that quasi-molecular ions are observed at sample temperatures required for desorption of neutral particles, this points strongly to a gas phase complexation. However, surface complexation cannot be excluded at this point. The possibility that
129
alkali ions from the external source can impinge on the sucrose surface and form complexes has been reduced experimentally. First, the alkali source in experiment V was placed far from the sucrose sample and was shielded so that alkali ions could reach the sucrose substrate only via multiple collisions. A second inhibition which excludes the possibility that alkali ions reach the sample surface was a potential difference of a few volts between the sample surface and the alkali ion source in experiment IV. Surface complexation on the sucrose surface is excluded in this way. Furthermore surface complexation on the hot surface of the alkali ion source is not likely to occur because of results reported by Stoll and R611gen [13]. Therefore, the results of experiments IV and V can only be explained by a gas phase reaction. Complexation reactions between polar gas phase molecules and alkali ions are already well known [14] and have been studied in quite some detail [15]. Proof of a gas phase reaction has been given for a set-up without any laser (experiments IV and V). F r o m these experiments it can be concluded that sufficiently high gas phase sucrose densities can be generated by thermal heating of the sample substrate. Of course the minimum sucrose density required for measurable quasi-molecular ion currents depends on the alkali ion density and the sensitivity of the mass spectrometer. The proposition made in this paper is that under laser irradiation conditions also sufficiently high gas phase sucrose and alkali ion densities can be produced. This implies that in L D M S at least part of the quasi-molecular ions are formed by gas phase cationization. Because the results of the laser experiments I, II and III can be adequately interpreted using this model we conclude that gas phase complexation of alkali ions and neutral evaporated sucrose molecules is a major process in LDMS. F r o m the fact that also for other "involatile" compounds quasi-molecular ions have been obtained in the thermal experiments, it might be concluded that gas phase alkali cationization is not restricted to sucrose. In this paper only alkali cationization in thermal desorption and laser desorption experiments has been discussed. However, in m a n y L D M S spectra also other types of molecular ions are observed, e.g. metal ion cationized molecules, protonated and deprotonated molecules and sometimes radical molecular ions. These types of ions III. M E C H A N I S M S
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G.J.Q. van der Peyl et al. / Laser desorption mass spectrometry
are well k n o w n products from gas phase i o n molecule a n d e l e c t r o n - m o l e c u l e reactions of vaporizable c o m p o u n d s . Since various "involatile" c o m p o u n d s analyzed in L D M S seem to be more vaporizable than has been assumed before, also analogous gas phase ionization processes can be active in L D M S . It has to be noted that these reactions have n o t been studied in this research project.
5. Gas phase complexation in other soft ionization techniques Of course, it is very interesting to investigate the possible c o n t r i b u t i o n of gas phase reactions in other soft ionization techniques. F o r Field D e s o r p t i o n gas phase c o m p l e x a t i o n can be excluded for the following reasons. Firstly, because of the strong electric fields ( > 107 V / m ) which are used in this technique alkali ions are accelerated very rapidly after leaving the surface. At distances of only 1/~m from the surface the kinetic energy exceeds already 10 eV. If the complex is created via a bimolecular collision of a thermal sucrose molecule a n d a 10 eV K + - i o n almost the total kinetic energy has to be converted into internal energy of the complex, this leads to highly unstable complexes. Furthermore, the cross section for c o m p l e x a t i o n decreases rapidly with increasing energy of the K + - i o n , thus the gas volume available for c o m p l e x a t i o n is extremely small in FD. Secondly, in some F D M S experim e n t s quasi-molecular ions have been observed while the alkali ions could not be detected [16]. Direct chemical ionization can also be exp l a i n e d by gas phase complexation. The m a i n p o i n t made here is that m a n y more polar molecules can be desorbed intactly from a heated substrate than has been assumed before. At this m o m e n t we c a n n o t say whether ion i n d u c e d desorption should be explained by a gas phase or by a surface complexation model. If a gas phase c o m p l e x a t i o n process is responsible for the results, the c o m p l e x a t i o n has to take place very close to the surface. This follows from the fact that only a small n u m b e r of secondary particles are desorbed per impact ( < 1000 [17,18]) and conseq u e n t l y the chance for b i n a r y collisions decreases strongly with distance from the surface. Possibly
the i o n - m o l e c u l e reactions proceed in the transition region of the solid a n d gas phase [4] which would h a m p e r the identification of gas phase a n d surface reactions in ion i n d u c e d desorption. The authors are i n d e b t e d to Prof. Dr. J. K i s t e m a k e r for stimulating discussions. This work is part of the research program of the Stichting voor F u n d a m e n t e e l Onderzoek der Materie ( F o u n d a t i o n for F u n d a m e n t a l Research o n Matter) a n d was made possible by financial support from the N e d e r l a n d s e Organisatie voor Zuiver Wetenschappelijk Onderzoek ( N e t h e r l a n d s O r g a n i z a t i o n for the A d v a n c e m e n t of Pure Research).
References [1] H.R. Schulten, Int. J. Mass Spectrom. Ion Phys. 32 (1979) 97. [2] D.F. Hunt, J. Shabanowitz, F.K. Botz and D.A. Brent, Anal. Chem. 49 (1977) 1160. [3] P.G. Kistemaker, G.J.Q. van der Peyl and J. Haverkamp in Soft Ionization BiologicalMass Spectrometry, ed., H.R. Morris (Heyden&Son, London, 1981) p. 120. [4] R.J. Day, S.E. Unger and R.G. Cooks, Anal. Chem. 52 (1980) 557A. [5] R.D. Macfarlane and D.F. Torgerson, Int. J. Mass Spectrom. Ion Phys. 21 (1976) 81. [6] R.D. Macfarlane, Nucl. Instr. and Meth. this Issue. [7] F.R. Krueger, Surf. Science 86 (1979) 246. [8] R.D. Macfarlane, Proc. of 26th Annual Conf. on Mass Spectrometry and Allied Topics, St. Louis (1978), p. 490. [9] G.J.Q. van der Peyl, J. Haverkamp and P.G. Kistemaker, Int. J. Mass Spectrom. Ion Phys., in press. [10] G.J.Q. van der Peyl, K. Isa, J. Haverkamp and P.G. Kistemaker, to be published in Org. Mass Spectrom., 16 (1981) 416. [11] O. Heinz and R.T. Reaves, Rev. Sci. Instr. 39 (1968) 1229. [12] J.F. Ready, Effects of High Power Laser Irradiation (Academic Press, New York, 1971) p. 70 and following pages. [13] R. Stoll and F.W. R6llgen, Z. Naturforsch. 370 (1982) 9. [14] R.V. Hodges and J.L. Beauchamp, Anal. Chem. 48 (1976) 825. [15] (a) W.R. Davidson and P. Kebarle, J. Am. Chem. Soc. 98 (1976) 6125; (b) W.R. Davidson and P. Kebarle, J. Am. Chem. Soc. 98 (1976) 6133. [16] N.M.M. Nibbering, private communication. [17] R. Pedrys, R.A. Hating, A. Haring, F.W. Saris and A.E. de Vries, Phys. Lett. 82A (1981) 371. [18] W.1. Brown, W.M. Augustyniak, E. Brody, B. Cooper, L.J. Lanzerotti, A. Ramirez, R. Evatt and R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321.
125
CATIONIZATION IN LASER D E S O R P T I O N M A S S S P E C T R O M E T R Y
Gerard J.Q. VAN DER PEYL, Kimio ISA,* Johan H A V E R K A M P and Piet G. K I S T E M A K E R FOM-lnstitute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
Laser desorption results of sucrose can be explained by a gas phase complexation: Alkali ions from hot parts ( T > 7 0 0 K) of the surface combine in the gas phase with sucrose molecules which desorb from cooler places on the surface. This ionization process seems to be applicable to other "non-volatile" compounds too. Gas phase ionization contributions to other soft ionization techniques are also discussed.
1. Introduction
Several techniques have been developed to produce quasi-molecular ions of organic compounds which cannot be obtained with classical sample introduction and ionization techniques like direct probe introduction and electron impact ionization [1-5]. General features of the new techniques are the introduction of the sample inside the ion source of the mass spectrometer and the application of ionization methods based on thermal ion-molecule reactions rather than on energetic electronmolecule interactions. As a result of the ion-molecule reactions, the mass spectra obtained generally display no molecular (radical) ion peaks but socalled quasi-molecular ion peaks. A quasi-molecular ion consists of a complex of the neutral organic molecule and the reagent ion. Examples are protonated and alkali ion cationized molecules. The main advantage of the ionization by ion-molecule reactions is the relatively low energy deposition in the resulting ion which enhances considerably the stability of the complex. Therefore, the term soft ionization techniques has been introduced. Undoubtedly the most succesful technique is field desorption mass spectrometry (FDMS) where ion-molecule complexes are produced under the influence of strong electric fields ( > 10 7 V / m ) [1]. Other techniques like direct chemical ionization mass spectrometry [2], laser desorption mass spec-
trometry (LDMS) [3] and ion induced desorption mass spectrometry [4,5] have been developed as very useful alternatives [6]. Although it is clear that in all these techniques ionization proceeds via ion-molecule reactions, the details of these processes have not been clarified up till now. Ionization in the solid, liquid and gas phase have been postulated. It has been assumed for a long time that the thermally labile organic molecules analyzed could not be evaporated, thus excluding a gas phase ionization. Therefore, complexation in the solid was assumed to preceed desorption and models have been developed on this basis [7,8]. Contrasting with this point of view we shall demonstrate in this article that in one of the techniques, LDMS, gas phase complexation of neutral molecules and alkali ions is a feasible process to explain the results observed. To do this we shall discuss data, obtained in our group using various experimental set-ups. Both a high power pulsed laser and a low power continuous wave (cw) laser have been used as well as resistive heating procedures to generate quasi-molecular ions of sucrose. Because part of the experimental results have been published elsewhere [9,10] only the most interesting details of these results are discussed in conjunction with some recently obtained data. This article will be concluded by a discussion about the possibility of applying a gas phase complexation model to results obtained with other soft ionization techniques.
* Guest scientist from Fukui University, Fukui, Japan.
0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
III. MECHANISMS
G.J.Q. van der Peyl et al. / Laser desorption mass spectrometry
126
2. Experimental In all experimental set-ups sucrose (MW 342) has been used as sample material. Some other sample compounds have been used only in one set-up; LD-valine (MW 117), sodium acetate (MW 82), amygdalin (MW 457), ergosterol (MW 397), cellobiose (MW 342) and 4-methylumbelliferyl-jRD-galactoside (MW 338). All compounds were dissolved in water-methanol mixtures (1 m g / m l ) . Of these solutions 1-10 ~1 drops were deposited on the substrate. Substrate materials which have been used are: stainless steel, quartz, N i C r - N i and tungsten. No alkali salts have been added to the sample in the experiments described in this article. Experimental set-ups used are schematically presented in fig. 1 and a short description of each experiment will be given below. A pulsed TEA CO 2 laser ()t = 10.6/~m, i" ~ 150 ns, maximum laser energy 1 J) was focused on a substrate surface covered with the sample material ( ~ 300 monolayers) in experiment I [9]. Laser produced ions were mass analyzed in a magnetic sector mass spectrometer with simultaneous ion detection capability. The results are obtained with only a small fraction of the laser energy (0.6-21 m J) focused down to a spot of 0.24 m m diameter. Calculated energy densities lie in the range of 1.3-45 J / c m 2, corresponding to mean power densities of 8.7 × 106-3.1 N 108 W / c m 2. As substrate materials both stainless steel and quartz were used. LDMS results have also been obtained for a set-up where a cw CO2-1aser (3 W) was focused in
TEACO2
I PULSEDLASER LDM II CWCO2LASERLDMS [[I CWCO2LASERH
RESISTIVE HEATIN~)I~)TIVE HEATIN~I
HEATINGL~
Fig. 1. A schematic presentation of the different experimental set-ups described in this article. Roman numerals are used to refer to the experiments in the text.
the ion source of a Finnigan 3000 quadrupole mass analyzer in experiment II [10]. The sample was deposited at the junction of a N i C r - N i thermocouple (diameter 50 /~m), positioned in the ion-source. This enabled a measurement of maxim u m substrate temperatures to be made during the laser desorption process. Power densities up to 10 k W / c m 2 were obtained in the laser focus of 0.2 m m diameter. Normally, the electron beam in the ion source was switched off but in a few experiments electron impact ionization was used to monitor the neutral particle desorption during laser irradiation. In subsequent experiments other desorption and ionization configurations were used in the ion source of the quadrupole mass spectrometer. In experiment III a thin W-wire (diameter 60 ~m) was heated resistively (0.8 A) to produce alkali ions. This wire was placed close ( ~ 3 mm) to the thermocouple wire, which was irradiated by the laser. In this way sample heating and alkali ion generation could be performed independently from each other. Resistive heating of both the sample substrate and the K+-source was performed in experiment IV. In this experiment a W-ribbon (30 # m thick) was used as substrate for the sample. This ribbon was heated resistively by an electric current of about 1 A while K + -ions were emitted from a thin W-wire. In some experiments an electrostatic potential difference of 0 - 3 V was applied between the two wires to investigate quasi-molecular ion generation as function of the potential applied. Controlled heating of the sample substrate was applied in experiment V [10]. A thin thermocouple was spotwelded to the W-ribbon to measure the temperature of the substrate. A commercial type of K +-source (Spectramat [11]) was used to obtain an intense "monoenergetic" ion beam. The ion source was mounted relatively remote from the sample ribbon and was shielded mechanically to reduce exposure of the sucrose sample to K +-ions. In this way also exposure of the alkali ion source to evaporated sucrose was reduced. In experiment V I a simplified thermal desorption probe for alkali cationization was used [10]. An electric current (0.8A) passed through a Wfilament (diameter 60/~m) and stainless steel wires (diameter 1 mm), which were spotwelded together in series. The sample was deposited on one of the thick wires, a thermocouple was spotwelded to this
G.J.Q. van der Peyl et a L / Laser desorption mass spectrometry
wire to give a temperature indication of the sample substrate.
3. Compilation of experimental results In this section some new results will be presented from investigations on the basic mechanism in LDMS. For a coherent survey some results, which we have published recently are also discussed here briefly. As will be shown below complexation with either Na+-ions (I) or K+-ions (II-VI) is predominant. In our opinion this reflects the different abundances of these ions in the substrate materials. An indication for this can be found in the experimental result that if e.g. [M + K] + -ions are more abundant than [M + Na] + -ions also K +-ions are more abundant relative to Na +ions and vice versa. 3.1. Pulsed CO 2 laser L D M S
The influence of laser pulse energy on laser produced ion currents has been investigated in this experiment [9]. Minimum laser energies of 2 m J and 0.6 mJ were required for stainless steel and quartz substrates, respectively, to produce measurable currents of Na +-ions and quasi-molecular ions of sucrose. The ion intensity curves versus
.
,10 z
m l z 23
J
////I v
laser energy for one ion mass but different substrates showed a clearly different behaviour. With increasing laser energy, the alkali ion and organic ion currents increased up to a maximum value. The increase in the alkali ion current appeared to be stronger than the increase in the quasi-molecular ion current. For a thermal interpretation, a laser heating model as described by Ready [12] was used to calculate substrate surface temperatures as function of laser pulse energy. The surface temperatures obtained by laser irradiation were characterized by the maximum temperature Tm reached on the substrate surface during the pulse. In this way ion intensities could be plotted as function of Tm (see fig. 2). The most interesting conclusions extracted from these curves are: 1) No quasi-molecular ions or Na +-ions have been observed at temperatures below about 700 K. 2) The ion intensities for one mass value but different substrates show a similar behaviour as function of substrate temperature. 3.2. Cw CO 2 laser L D M S
Ion intensitie., and thermocouple temperatures T~ were measured simultaneously as a function of irradiation time [10]. Using this information the laser produced ion intensities have been plotted as a function of Tc in fig. 3. In a few experiments the electron beam in the ion source was switched on during the laser pulse. The typical sucrose fragment at m / z = 126 has been monitored in this way and the variation of this ion intensity with sub-
//
!'°I' -
,"', m / z 126
>, ¢J
10o
750 CALCULATED
127
1250 1750 ' / SURFACE TEMPERATURE Tm (K)
Fig. 2. Ion intensities as function of the c a l c u l a t e d m a x i m u m surface t e m p e r a t u r e Tm d u r i n g a laser pulse. The curves corres p o n d to a t h e r m a l d e s o r p t i o n m o d e l [9] b u t are used o n l y to g u i d e the eye. The ion intensities have been n o r m a l i z e d to unit i n t e n s i t y at the lowest m e a s u r e d v a l u e s . Q - N a + - i o n s ( m / z = 23) from a q u a r t z substrate; 27 - N a + - i o n s ( m / z = 23) from a stainless steel substrate; O - [sucrose + Na] +-ions ( m / z = 365) from a q u a r t z substrate; /x - [sucrose + Na] + ( m / z = 365) from a stainless steel substrate.
ZK÷]
\
,!
o
5
,
300
,,
,
m / z 381
600 900 1205 THERMOCOUPLE TEMPERATURE Tc (K)
Fig. 3. C w laser p r o d u c e d ion currents as function of the t h e r m o c o u p l e t e m p e r a t u r e Tc. The sucrose fragment ion m / z = 126) was m e a s u r e d b y electron i m p a c t ionization. III. M E C H A N I S M S
128
G.J.Q. van der Peyl et aL / Laser desorption mass spectrometry
strate temperature is shown in fig. 3. Two important conclusions can be extracted from fig. 3: 1) No quasi-molecular ions or K + -ions have been observed at temperatures below about 700 K. 2) Desorption of neutral particles, as represented by m / z - - 126, took place already in the temperature range 400-600 K. If the laser beam was interrupted during quasimolecular ion production, these ions could still be observed until some tens of milliseconds afterwards. Effectively, during this time quasi-molecular ions and K +-ions were observed at zero laser intensity. The role of the sample temperatures in the L D M S process was investigated in more detail in the following experiments where the influence of laser heating was partly or totally replaced by direct resistive heating of the substrate.
3.3. Cw CO 2 laser with independent K + -source In this experiment a K + -source was introduced close to the thermocouple wire. The thermocouple was heated by the laser to relatively low temperatures ( < 700 K). If during the laser heating of the sample the K+-source was on, quasi-molecular ions could be observed at low thermocouple temperatures T~ in the range of 400-500 K (see fig. 4). In this temperature range also desorption of neutrals occurred as deduced from experiments where additional EI was used (not shown, but see also fig. 3).
m,23
~,
,
!
K*
3.4. Resistive heating of the sample substrate Quasi-molecular ions could be obtained in this set-up, indicating that laser heating is not a prerequisite for quasi-molecular ion formation. To make sure that no K +-ions could impinge on the sucrose substrate the K+-source was set at a voltage of 3 V relative to the sucrose substrate. Also under these conditions quasi-molecular ions were observed. -
3.5. Controlled resistive heating of the sample In a more defined set-up the temperatures of the W-ribbon could be monitored during the desorption process. In fig. 5 resulting partial mass spectra ( m / z = 200-400) for sucrose are presented for ribbon temperatures of 460 K and 475 K. These spectra are obtained by signal averaging of low ion signals for about 1 rain. Clearly it is demonstrated by these results that a mere temperature rise of 15 K of the sample already results in extensive fragmentation of the sucrose molecules.
3.6. Simplified cationization probe The best practical results, i.e. intense quasimolecular ion currents for a relatively long time (30s), were obtained in these experiments [10]. Conditions in the laser spot in LDMS were approximated as good as possible in this set-up. The thin W-filament is heated to temperatures o f 1000K needed for alkali ion emission while the
:yl 381
w 363
300
500
700
";'
THERMOCOUPLE TEMPERATURE
Tc (K)
Fig. 4. Q u a s i - m o l e c u l a r ion current of sucrose ([M + K] +, m / z = 381) as function of t h e r m o c o u p l e t e m p e r a t u r e Tc.
200
300
~00 - - - ~
rn /z
Fig. 5. Partial mass spectra for sucrose at r i b b o n t e m p e r a t u r e s of 460 K (a) a n d 475 K (b).
G.J.Q. van der Peyl et a L / Laser desorption mass spectrometry
stainless steel wires remain at relatively low temperatures (400-600 K). Besides intense quasimolecular ion currents for sucrose as reported previously [10], also quasi-molecular ions for some other polar organic molecules were observed, notably LD-valine, sodium acetate, amygdalin, ergosterol, cellobiose and 4-methylumbelliferyl-flD-galactoside. Furthermore, it has to be stated that in first approximation optimal quasi-molecular ion currents were obtained at sample temperatures close to the melting point of the sample under investigation.
4. Discussion
Up till now models [7,8] have been proposed to explain results of soft ionization on the basis of a process where first complexation takes place on the surface, followed by desorption of quasimolecular ions. However, the L D M S and thermal desorption results reported in this article can be explained by a gas phase complexation model: Both neutral molecules and alkali ions desorb from the substrate surface independently and combine in the gas phase. Alkali ions are emitted by a thermoionic process at surface temperatures higher than about 700 K, neutral molecules are desorbed from areas with a much lower temperature (400600 K) probably via a simple evaporation process. The function of the laser seems only to be to create these two temperature areas close to each other in space and time. In such a model the high surface temperatures required for quasi-molecular ion formation in the experiments I and II can be understood as in these experiments the alkali ion production is the limiting factor. Quasi-molecular ions are indeed formed at much lower sample temperatures if alkali ions are produced in a separate hot source independent from the sample substrate as in experiments III, IV and V. Further evidence for a gas phase model can be extracted from experiments III, IV and V where the alkali ions were only supplied from an external source via the gas phase. Together with the result that quasi-molecular ions are observed at sample temperatures required for desorption of neutral particles, this points strongly to a gas phase complexation. However, surface complexation cannot be excluded at this point. The possibility that
129
alkali ions from the external source can impinge on the sucrose surface and form complexes has been reduced experimentally. First, the alkali source in experiment V was placed far from the sucrose sample and was shielded so that alkali ions could reach the sucrose substrate only via multiple collisions. A second inhibition which excludes the possibility that alkali ions reach the sample surface was a potential difference of a few volts between the sample surface and the alkali ion source in experiment IV. Surface complexation on the sucrose surface is excluded in this way. Furthermore surface complexation on the hot surface of the alkali ion source is not likely to occur because of results reported by Stoll and R611gen [13]. Therefore, the results of experiments IV and V can only be explained by a gas phase reaction. Complexation reactions between polar gas phase molecules and alkali ions are already well known [14] and have been studied in quite some detail [15]. Proof of a gas phase reaction has been given for a set-up without any laser (experiments IV and V). F r o m these experiments it can be concluded that sufficiently high gas phase sucrose densities can be generated by thermal heating of the sample substrate. Of course the minimum sucrose density required for measurable quasi-molecular ion currents depends on the alkali ion density and the sensitivity of the mass spectrometer. The proposition made in this paper is that under laser irradiation conditions also sufficiently high gas phase sucrose and alkali ion densities can be produced. This implies that in L D M S at least part of the quasi-molecular ions are formed by gas phase cationization. Because the results of the laser experiments I, II and III can be adequately interpreted using this model we conclude that gas phase complexation of alkali ions and neutral evaporated sucrose molecules is a major process in LDMS. F r o m the fact that also for other "involatile" compounds quasi-molecular ions have been obtained in the thermal experiments, it might be concluded that gas phase alkali cationization is not restricted to sucrose. In this paper only alkali cationization in thermal desorption and laser desorption experiments has been discussed. However, in m a n y L D M S spectra also other types of molecular ions are observed, e.g. metal ion cationized molecules, protonated and deprotonated molecules and sometimes radical molecular ions. These types of ions III. M E C H A N I S M S
130
G.J.Q. van der Peyl et al. / Laser desorption mass spectrometry
are well k n o w n products from gas phase i o n molecule a n d e l e c t r o n - m o l e c u l e reactions of vaporizable c o m p o u n d s . Since various "involatile" c o m p o u n d s analyzed in L D M S seem to be more vaporizable than has been assumed before, also analogous gas phase ionization processes can be active in L D M S . It has to be noted that these reactions have n o t been studied in this research project.
5. Gas phase complexation in other soft ionization techniques Of course, it is very interesting to investigate the possible c o n t r i b u t i o n of gas phase reactions in other soft ionization techniques. F o r Field D e s o r p t i o n gas phase c o m p l e x a t i o n can be excluded for the following reasons. Firstly, because of the strong electric fields ( > 107 V / m ) which are used in this technique alkali ions are accelerated very rapidly after leaving the surface. At distances of only 1/~m from the surface the kinetic energy exceeds already 10 eV. If the complex is created via a bimolecular collision of a thermal sucrose molecule a n d a 10 eV K + - i o n almost the total kinetic energy has to be converted into internal energy of the complex, this leads to highly unstable complexes. Furthermore, the cross section for c o m p l e x a t i o n decreases rapidly with increasing energy of the K + - i o n , thus the gas volume available for c o m p l e x a t i o n is extremely small in FD. Secondly, in some F D M S experim e n t s quasi-molecular ions have been observed while the alkali ions could not be detected [16]. Direct chemical ionization can also be exp l a i n e d by gas phase complexation. The m a i n p o i n t made here is that m a n y more polar molecules can be desorbed intactly from a heated substrate than has been assumed before. At this m o m e n t we c a n n o t say whether ion i n d u c e d desorption should be explained by a gas phase or by a surface complexation model. If a gas phase c o m p l e x a t i o n process is responsible for the results, the c o m p l e x a t i o n has to take place very close to the surface. This follows from the fact that only a small n u m b e r of secondary particles are desorbed per impact ( < 1000 [17,18]) and conseq u e n t l y the chance for b i n a r y collisions decreases strongly with distance from the surface. Possibly
the i o n - m o l e c u l e reactions proceed in the transition region of the solid a n d gas phase [4] which would h a m p e r the identification of gas phase a n d surface reactions in ion i n d u c e d desorption. The authors are i n d e b t e d to Prof. Dr. J. K i s t e m a k e r for stimulating discussions. This work is part of the research program of the Stichting voor F u n d a m e n t e e l Onderzoek der Materie ( F o u n d a t i o n for F u n d a m e n t a l Research o n Matter) a n d was made possible by financial support from the N e d e r l a n d s e Organisatie voor Zuiver Wetenschappelijk Onderzoek ( N e t h e r l a n d s O r g a n i z a t i o n for the A d v a n c e m e n t of Pure Research).
References [1] H.R. Schulten, Int. J. Mass Spectrom. Ion Phys. 32 (1979) 97. [2] D.F. Hunt, J. Shabanowitz, F.K. Botz and D.A. Brent, Anal. Chem. 49 (1977) 1160. [3] P.G. Kistemaker, G.J.Q. van der Peyl and J. Haverkamp in Soft Ionization BiologicalMass Spectrometry, ed., H.R. Morris (Heyden&Son, London, 1981) p. 120. [4] R.J. Day, S.E. Unger and R.G. Cooks, Anal. Chem. 52 (1980) 557A. [5] R.D. Macfarlane and D.F. Torgerson, Int. J. Mass Spectrom. Ion Phys. 21 (1976) 81. [6] R.D. Macfarlane, Nucl. Instr. and Meth. this Issue. [7] F.R. Krueger, Surf. Science 86 (1979) 246. [8] R.D. Macfarlane, Proc. of 26th Annual Conf. on Mass Spectrometry and Allied Topics, St. Louis (1978), p. 490. [9] G.J.Q. van der Peyl, J. Haverkamp and P.G. Kistemaker, Int. J. Mass Spectrom. Ion Phys., in press. [10] G.J.Q. van der Peyl, K. Isa, J. Haverkamp and P.G. Kistemaker, to be published in Org. Mass Spectrom., 16 (1981) 416. [11] O. Heinz and R.T. Reaves, Rev. Sci. Instr. 39 (1968) 1229. [12] J.F. Ready, Effects of High Power Laser Irradiation (Academic Press, New York, 1971) p. 70 and following pages. [13] R. Stoll and F.W. R6llgen, Z. Naturforsch. 370 (1982) 9. [14] R.V. Hodges and J.L. Beauchamp, Anal. Chem. 48 (1976) 825. [15] (a) W.R. Davidson and P. Kebarle, J. Am. Chem. Soc. 98 (1976) 6125; (b) W.R. Davidson and P. Kebarle, J. Am. Chem. Soc. 98 (1976) 6133. [16] N.M.M. Nibbering, private communication. [17] R. Pedrys, R.A. Hating, A. Haring, F.W. Saris and A.E. de Vries, Phys. Lett. 82A (1981) 371. [18] W.1. Brown, W.M. Augustyniak, E. Brody, B. Cooper, L.J. Lanzerotti, A. Ramirez, R. Evatt and R.E. Johnson, Nucl. Instr. and Meth. 170 (1980) 321.