Anal. Chem. 1994,66, 1704-1707
Membrane Ion Source for Mass Spectrometry B. S. Yakovlevt and V. L. Talrose
Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow Region 142432, Russia Catherine Fenselau' Department of Chemistry and Biochemistry, University of Maryland Baltimore County, Baltimore, Maryland 2 1228
A membrane consisting of polymeric film densely perforated with channels is used as the interface between a solution at atmospheric pressure and the vacuum chamber of a mass spectrometer. It is shown that an electric field can stimulate ion emission from the liquid sample directly into the vacuum. Emitted ions were investigated with a time-of-flight mass spectrometer. The method of evaporation of ions from molecular liquids directly into the vacuum chamber of a mass spectrometer under the stimulation of an electrical field (electrohydrodynamic mass spectrometry, EHDI) dates back to the early 1970s, when it was introduced by Simons, Colby, and Evans.' The attractive feature of this method is that it allows analysis in situ of ions in liquids and continuous sample introduction without breaking the vacuum. However, the method has not been as well developed as thermospray2 or electrospray3 met hods. The major limitations of EHDI have been the following: instability of emitted ion current, requirement of high electroconductivityof the solution, significant flow of the liquid into the vacuum chamber. These appear not to be due to inherent properties of field evaporation of ions but rather to the instability of the liquid surface during the electrohydrodynamic process. For EHDI, a single metallic capillary needle is used to supply a liquid sample to a region of ion emission at the needle tip placed in the vacuum chamber. The smallest needle aperture available has a radius larger than 10pma4This radius is too large to prevent destruction of the surface of a molecular liquid by the strong electric field (>lo6 V/cm) applied at atmospheric pressure for effective ion evaporation. For a liquid with adequate conductivity, the stability of the surface in an external electric field E is determined by the Rayleigh condition5 shown as t Deceased May 19, 1993. ( I ) Simons, D. S.; Colby, B. N.; Evans, C. A. Int. J. Mass Spectrom. Ion Phys. 1974, 15, 291-302. (2) Iribarne, J. V.; Dziedzic, P. J.; Thomson, B. A. Int. J. Mass Spectrom. Ion Phys. 1983, 50, 331-347. ( 3 ) Whitehouse, C. M.; Dreyer, R. N.; Yamashita, M.; Fenn, J. B. Anal. Chem. 1985, 57, 675-679. (4) Murawski, S. L.; Cook, K. D. Anal. Chem. 1984.56, 1015-1020. ( 5 ) Lord Rayleigh, Philos. Mag. 1882, 14, 184-190.
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where y is the liquid surface tension, r is the radius of curvature, and p is the pressure in the liquid. Under equilibrium conditions r > ro,where ro is the radius of the capillary needle tip. Thus surface stability will be higher when ro is smaller. For molecular liquids in the usual range of interest, y N 60 mN/m. Therefore, we need to use ro < 0.1 mm to provide stability for the liquid surface at p = 1 atm. The membrane used in the present work allows us to employ capillaries with much lower radii than those used in EHDI and to compensate for the decrease in surface area for emission by increasing the number of capillaries.
EXPER I MENTAL SECT1ON Material and Methods. Figure 1 illustrates the membrane ion source. It consists of an insulating polymer film with multiple channels. The density of the channels is about lo7 cm-*. The film is mounted on a metal diaphragm with an orifice 3 mm in diameter. A liquid sample is placed over the film on the atmospheric side. An electric field is formed by two electrodes, one of which is sunk in the liquid and the other placed on the vacuum side. The field draws ions from the solution to the surface between the liquid and the vacuum at the tips of the channels and stimulates their transition into the vacuum. The membrane consists of poly(ethy1ene terephthalate) 10 pm thick, irradiated by xenon or cobalt ions and treated ~ h e m i c a l l y . ~ The . ~ radius of the channels, ro, is made sufficiently small (250-500 A) that the liquid does not flow through the channel under atmospheric pressure. Microscopic measurements indicated that this is the case when ro is less than 0.1 pm, as expected from eq 1. The sample is applied as a droplet with an area of about 0.1 cm2and a thickness of 0.1 cm. To change the sample the membrane was blotted dry. Nicotinic acid and acetylsalicylic acid were purchased from Sigma Chemical Co., St. Louis, MO. (6) Beck, R. E.; Schuitz, L. S . Eiochim. Eiophys. Acta 1972, 255, 273-278. (7) Flerov, G. N.; Barashenkov, V.S. Usp. Phys. Nauk (USSR)1974,114,351313.
0003-270019410366-I704$04.50/0
@ 1994 American Chemlcal Society
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Figure 1. Schematic diagram of the membrane ion source: (1) liquid sample; (2) polymeric fllm wlth channels; (3) metalllc diaphragm; (4) electrode.
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k Figure 2. Schematlc diagram of the timeof-flight mass spectrometer wlth the membrane ion source In place: (a) membrane;(b) deceleration lens; (c, d) electrodes In the Ion accumulation region;8 (e) extraction electrode; (f) parallel electrodes; (h) electrostatic mirror; (k), detector; (x, yj components of Ion velocity.
Mass spectrometry was performed using a time-of-flight mass spectrometer designed by Dodonov et a1.* Figure 2 shows the general scheme with the membrane ion source in place. Ions emitted by the membrane source a, with kinetic energy of 1-2 keV were retarded to 0-300 eV in a lens assembly, b. After accumulation in the region between the electrodes, c and d, ions were expelled into the region between grids d and e, where they were accelerated up to about 2 keV in the perpendicular direction of they vector of their initial velocity. Two parallel electrodes, f, 5 cm in length and 5 cm apart, allowed selection of ions with different y velocity components for detection after reflection in the electrostatic mirror. h.
RESULTS AND DISCUSSION In contrast to electrohydrodynamic ionization, membrane ion emission does not require solutions with high electrical conductivity. Ion emission with total ion currents as high as 10 nA can be obtained for pure glycerol and glycerol/water solutions, with conductance of 10-70-1cm-l. (For ion emission in EHDI, addition of an electrolyte is required to increase the conductance up to lo4 0-l cm-l.) The current in the present experiments was stable to f10% through more than 4 h. Another feature of membrane ion emission is the absence of liquid flow into the vacuum chamber of the mass spectrometer. The flow of liquid through the channels on the vacuum side of the membrane was checked with a microscope, (8) Dodonov, A. F.;Chernuschevich, I. V.; Laiko, V. V. Electrospray Ionization on a Reflecting Time-of-flight Mass Spectrometer. In Time of Flight Mass Spectrometry, Cotter, R. J., Ed.; American Chemical Society, Washington, DC. 1994.
Figure 3. Mass spectra obtained by direct analysis from (a) 20/80 mol % glycerol (G)/water solution and (b) 0.1 M nicotlnlc acld (N) In 20/80 mol % glyceroVwater.
and the occasional formation of drops on the vacuum surface was considered symptomatic of defective membranes. The measured air flow through a clean dry membrane is no more than 1017molecules; for membranes without defects the total flux of neutral molecules, e.g., glycerol, through the membrane was estimated as 1010-1014mol/s from the rate of increase of pressure in the vacuum chamber in the absence of pumping. This flux is much less than the lo1*mol/s that is typical for EHDI. Mass Spectra. Figure 3 shows mass spectra of anions that are emitted from the porous membrane. Small cluster ions and their fragments are detected. Because the mass spectrometer used offered the capability to select ions with different y velocity components, two different strategies were developed to obtain spectra. Spectra of Emitted Ions. In the first case, the voltage Uf on the electrodes f (Figure 2) was adjusted to obtain the maximum intensity of cluster ions. The value of Ufapproximately corresponds to they component of the translation energy of ions, entering the region between grids c and d, which can be estimated for singly charged ions as
where e is the electron charge, Ua is the potential supplied to the sample, U, is the potential between grids c and d, 6U is a voltage drop within the channels of the membrane. According to our estimation, based on the channel sizes and ion mobility, 6U is several tens of volts for glycerol solutions and several volts for 20/80 mol 7'% glycerol/water solutions. Figure 3 shows examples of such mass spectra obtained from solutions of 20/80 mol 5% glycerol (G)/water and of 0.1 M nicotinic acid (N) in 20/80 mol 5% glycerol/water. Ions are observed that correspond to two series of cluster anions [G, - HI- and [G, + N - HI-. The most abundant ions have n = 2 or 3. The first series is qualitatively similar to that observed from solutions of KI in glycerol by E H D I . ' T ~The AnaiyticalChemisfry, Vol. 66, No. 10, May 15, 1994
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Time of F l i g h t , ps Figure 4. Mass spectra obtalned with surface-Induced dissoclatbn from (a) 20180 mol % glycerol/water solution, (b) 0.1 M nlcotlnic acid In 20/80 mol % glycerol/water, and (c) 0.1 M acetylsalicylic acid (A) in 20180 mol % glycerol/water.
assumption is made that the ions observed did not undergo dissociation after emission from the membrane and prior to acceleration/expulsion into the analyser. . Dissociated Ions. If the voltage on electrode f Spectra of is held near to zero, very different spectra can be obtained from those shown in Figure 3. Ions can be observed that correspond to declustered ions, as shown in Figure 4. Fragmentation is also observed, producing ions of mass 78 (Figure 4b) by elimination of COz. The contribution of ions with low m / z values increases when the kinetic energy of ions entering the area between grids c and d is increased. This energy was changed by changing the potential U a and evaluated from eq 2.
The gas pressure between grids c and d is estimated to be too low to promote dissociation by gas-phase collisions. Consequently, the dissociation observed is mainly attributed to collisions of cluster ions with surfaces in the region. The low probability expected for theprocess9 might becompensated by two factors: (1) effecient collisions of a high proportion of the ionic clusters and (2) conditions that favor accumulation of ions with lower energies. This could explain why the total ion intensities of the activated spectra are not lower than those of spectra recorded directly. Figure 5 demonstrates that direct ions (A) and dissociated ions (B) can be readily distinguished by their different dependence on Uf,the voltage between electrodes f. The dependence of the relative intensities of directly detected ions on Ufwas approximately the same at different values of the voltage difference U, - U,. In contrast, the energy for maximum abundances of dissociated ions was proportional to the value of Ua - U,. These observations suggest that ions with different energies in the region between grids c and d contribute to these peaks. The energy, shown in the upper scale in Figure 5,was estimated from the voltage on electrodes f (9) Dekrey, M. J.; Kenttamaa, H. I.; Wysocki, V. H.; Cook,R. G.Org. Mass Specfrom. 1986, 21, 193-195.
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Ion Energy E ( e V )
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V o l t a g e Uf ( V I Flguro 5. Dependence of relathre Intensities on the voltage 4 of electrodes f (Figure 2): (A) the cluster Ion (glycerols H)-: (B) fragment Ions of mass 59. The upper scale shows the energy of these Ions between grids c and d.
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where eUo is the energy of ions in the drift region between elements e and k (Figure 2), le is the effective length of electrodes f, and d is the distance between electrodes f. Taking into account the boundary electrode effects, it was accepted that the strength of the electric field between electrodes f equaled Uf/d at a distance I, I, where the effective length I, is practically the same as the length I. According to this estimation, most of the ions detected in the dissociation spectra after dissociation between grids c and d have energies lowered by 3-5 eV relative to ions entering the region.
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CONCLUSIONS Among the possibilities for this new membrane ion source for analysis of liquids, several seem to be worth exploring further. (1) Becauseof the effective exclusion of neutral molecules, the membrane ionization source could be useful for production of portable devices for analysis of liquids. (2) Cluster ions emitted from the membrane can be fragmented to provide structural as well as molecular weight
information. Studies are being extended to heavier molecules, including peptides and nucleotides, to cation analysis, and to specific environmental targets. (3) The small volume of liquid employed is compatible as an interface for liquid chromatography and electrophoresis and might be utilized in monitoring chemical or physiologic reactions.
ACKNOWLEDGMENT The authors are grateful to Dr. A. F. Dodonov for valuable advice and to L. I. Novikova for assistance in membrane preparation. The research was supported in part by cooperative grants from the Russian Academy of Sciences and the United States National science Foundation, Received for review November 3, 1993. Accepted March 3, 1994.' *Abstract published in Advunce ACS Absrrucfs, April 1, 1994.
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