J . Phys. Chem. 1992, 96, 6166-6169
6166
Glow Discharge Ionization of a Seeded Supersonic Jet George A. Miller School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received: March 16, 1992; In Final Form: April 20, 1992)
A study has been made of a dc glow discharge along a supersonic jet formed by expansion of carbon dioxide through a nozzle from a reservoir pressure of 34 atm. Seeding the jet with ethanol implied the predominance of charge exchange as the mechanism of seed ionization, and seeding with benzene showed the relatively favorable ionization efficiency of this approach at low seed concentration.
Introduction Among the impressive advances in mass spectrometry over the years are the many new methods of ionization. The glow discharge has played a role here in the sampling of solids,’ in the plasma spray interface for liquid chromatography/mass spectrometry? as a source of ions for chemical ionization? and as an atmospheric pressure ionization ( M I ) so~rce!~~ In the glow discharge version of MI a relatively cold plasma is produced consisting principally of electrons and positive ions that have been produced by electron impact (EI). Ionization of the minor constituent (the seed) can occur by E1 as well or alternately through charge exchange (CE) or chemical ionization. The advantages of this kind of ion source are a possibly higher ionization efficiency than conventional E1 sources6and the elimination of the contribution of contaminants inside the mass spectrometer to the mass spectrum. The present study was undertaken to see if a stable, weak glow discharge could be maintained along a supersonicjet formed from a relatively high pressure source and to learn about its properties as an ionizing source. Stable glow discharges are limited normally to gas densities corresponding to pressures of a few Torr (but see ref 5 ) . In the present setup the background pressure was held at a value that could support a discharge and at the same time allow the jet to have reasonable dimensions. A further advantage of a relatively high background pressure is the reduced pumping speed required.’ High-pressurecarbon dioxide was chosen because its vapor pressure could be controlled at high values conveniently and because, in the supercritical state, it provides a way of seeding the jet with compounds of low vapor pressure.8 Although the positive ions in a glow discharge are impinging on the cathode, the cathode fall region produces ion kinetic energies too high for a quadrupole mass spectrometer (QMS). Previous API results indicate that positive ions can be detected at the anode! The present study therefore was limited to anode sampling. The expansion nozzle served as the cathode and the skimmer as the anode. The normal function of the skimmer is to deflect the shock wave while sampling the jet into a low-pressure region where the resulting beam maintains its jetcooled properties. In the present setup the protruding shape of the skimmer helped concentrate the negative glow along the jet axis. Experimental Section Figure 1 shows the setup for the anode sampling of the glow discharge by a QMS. The 150-mL stainless steel reservoir was kept at 0 O C , giving a stagnation pressure of 34 atm behind the nozzle. The linear velocity of the C 0 2gas through the sample U-tube was calculated from the nozzle flux formula (see below) to be 0.75 cm s-l, slow enough to allow for rough equilibration with the vapor of the liquid seed. The U-tube was ‘/4-in. stainless steel, 4-mm i.d., and was shallow enough to hold a 0.1-mL liquid sample without blockage. The nozzle pinhole was an Ealing utility type, 25-pm diameter, made from 40-pm-thick nickel. This pinhole was clamped to the end of the thick walled, stainless steel, I/&. supply tube with a standard compression fitting end plug. The plug had a conical opening bored out, 0.5-mm diameter at the inside surface where it contacted the pinhole disk and 5 mm 0022-365419212096-6166$03.00/0
at the outside surface. Inside the glass glow discharge chamber the nozzle assembly was insulated against secondary glow discharge paths, leaving only a 5-mm-diameter area around the pinhole exposed. An adjustable dc supply with an adjustable series ballast resistor served to excite the glow discharge with the nozzle negative and the skimmer positive. Due to the restricted area of the cathode, the discharge was of the adnormal type,gas confirmed by a steady rise in the maintenance voltage as the discharge current was increased from 10 to 50 PA. Otherwise, all measurements were confined to a current of 20-30 PA. The background pressure and the jet was controlled between 0.2 and 0.35 Torr by adjusting a butterfly valve leading to the first pumping system. A maximum speed of about 25 L s-l was achieved with a liquid nitrogen trap, 5-cm-i.d. and 20 cm deep, leading to a roughing pump of 1 L s-I speed. Beyond the skimmer a small diffusion pump also provided about 25 L s-I to keep the pressure at Torr, so that the skimmed beam could pass through an exit hole to the QMS. A potential of up to 50 V between skimmer mount and exit plate in this differential pumping setup helped focus the positive ions through the exit hole and remove plasma electrons. The diameter of the skimmer opening was 0.3 mm. The positive ion beam was mass analyzed with an EA1 Quad 230 QMS. The dual filament system and faraday cage were grounded for these experiments. A third diffusion pump maintained the pressure at Torr. Measurement of the ion and electron beam currents through the skimmer was done by removing the mass spectrometer and the exit plate beyond the skimmer, providing a glass housing for this second pumping section, and mounting a current collecting plate whose position could be adjusted axially. By carefully removing sources of leakage, beam currents to 1 nA could be detected across a 1000-Q load resistor with a microvoltmeter. The seed compounds were U. S. Industrial absolute alcohol and Fisher Certified ACS benzene; the carbon dioxide was Holox bone dry. The lower concentrationsof benzene were achieved by adding small amounts directly to the sample cylinder. The solubility of benzene in liquid carbon dioxide has been measured at low temperatures up to -15 O C . ’ O A plot of log (g mL-l) versus inverse absolute temperature gave a straight line, which extrapolated to 1.3 g of benzene per milliliter of solution at 0 OC, which is much higher than the concentrations used in the present study. Since the Henry’s law constant is unknown for this system, it was necessary to assume Raoult’s law to find the composition of the vapor in the sample cylinder. At 0 OC the vapor pressures are 24 Torr for benzene and 34 atm for C 0 2 . The density of benzene at room temperature is 0.88 g mL-’. Thus, the mole fraction of benzene in the equilibrium vapor was, by Raoult’s law
Y, = (4.61
X
10-4)(mL of benzene/g of C02(1))
The weight of C02(l) in the cylinder was determined immediately after each successful scan. Since, by fractionation, most of the benzene remained in the liquid phase, there was no need to correct for loss to the jet before scanning. 0 1992 American Chemical Society
0 t o +50 V
~ - - 5 5 0V
i
GRND I
\
1'
The Journal of Physical Chemistry, Vol. 96, No. 15, 1992 6167
Ionization of a Seeded Supersonic Jet
QMS
L--/
1 1
I
0
O O
N A 20
5
25 L/s
25 L/s
0 0 0
- 40
i
1
50 L/s
00 0 0
-601
I liq
-*O
I 3
t
-200
Figure 1. Experimental setup for anode sampling of a supersonic jet/ glow discharge with a 7 - ~ mpore filter (F), a U-tube for seeding the carrier gas (U), a nozzle (N), a skimmer (S),a quadrupole mass spectrometer (QMS), and an electron multiplier (EM).
l o
700
0
O0
-100
0 VOLTS
0
0
200
100
Figure 3. Retarding voltage curve for the supersonicjet/glow discharge plasma of Figure 2 as sampled through the anode-skimmer.
the point where the velocity is Mach 1, V, is the jet velocity at that point, and A, is the area of the nozzle opening. Letting do be the density and Uothe velocity of sound in the reservoir behind the nozzle, we have the relations Il(7-1)
L,
d, = do( Y + l
VOLTS I
0
6001
0
'
0
2
I
3
CM
Figure 2. Glow discharge maintenance voltage at 20 pA versus nozzleto-skimmer distance for a high-pressurejet with a reservoir pressure of 34 atm, a nozzle diameter of 25 pm, and a background pressure of 0.2 Torr (full circles) and for a virtually uniform gas at 0.2 Torr (open circles). The nozzle was the cathode.
Result9 1. Mainteuance Voltage Curves. The glow discharge (both dc and rf) has been used as a method of flow visualization of strong supersonic jets including the Mach disk." At the Mach disk the density of the expanding jet has approached that of the surrounding gas, allowing the latter to invade and destroy the supersonic flow. In the present experiments, involving a relatively weak flow, no Mach disk was observed. The discharge contained only the negative glow; no positive column was observed. The negative glow appeared to be concentrated close to what was probably the boundary shock wave of the jet. If this interpretation is correct, the jet could by described as an elongated ellipsoid, one end close to the exposed part of the nozzle and separated by a very narrow cathode dark space, the other end joined to the skimmer tip. Figure 2 shows the maintenance glow discharge voltage as a function of nozzle-skimmer separation for a typical set of conditions. The curve is not unlike those observed many years ago in a uniform gas between plane-parallel electrodes.12 For comparison, the curve with the same background pressure but with the reservoir pressure at only 1 atm is shown. Here, the jet is a minor feature of the discharge. The fact that the two curves are close to one another supports the idea that the discharge is mostly outside the jet. 2 cpIculrtioaoftheFractioaski"ed. The available formulas describing supersonic nozzle e~pansions'~ are for ideal monatomic gases. Applying them to high-pressure C 0 2 introduces some uncertainty. However, the aim here is only to arrive at a rough description of the glow discharge plasma. The flux through the nozzle is given by the relation Fn = dnVJn (1) where d, is the density of the gas emerging from the nozzle at
where y is the ratio of specific heats, R is the gas constant, T i s the absolute temperature in the reservoir, and M is the molecular weight of the gas. The value of y of C 0 2 is 1.29 at room temperature and 1.40 at low temperatures where vibration does not contribute. With an average value of y, eq 1 becomes F,, = 0.58d0UJ, The molar volume of C02(g) at 34 atm and 300 K (reservoir conditions) is 600 cm3mol-', based on the van der Waals equation .'~ with a = 3.592 L2 atm mol-2 and b = 0.04267 L m ~ l - ~ The velocity of sound in C02(g) at 300 K is 27000 cm s-I. The pinhole area is 4.91 X 10" cm2. Thus, the nozzle flux calculates out to be
F, = (4.75 X 10-9)U0(cm s-l) = 1.3 X
mol s-l
The final jet velocity is given by Vf = U0(2/(y - 1))'l2 = 2.44U,
(2)
and the flux through the skimmer is F, = d,VfA, where d, is the jet density at the skimmer and A, is the area of the skimmer opening. As explained above, d, is estimated to be the density of the background gas, which, at a pressure of 0.2 Torr, is 1.07 X lo-* mol cm-I, and A, is 7.07 X lo4 cm2, which gives F, = (1.85 X 10-ll)Uo = 5.00 X lo-' mol
Thus, the fraction skimmed is F,/F, = 0.0039. 3. Beam Current and Degree of Ionization. The glow discharge current through the skimmer was recorded, with the collector plate in place, as a function of voltage difference between the plate and the skimmer. Figure 3 shows the result for a discharge current of 20 PA and the same conditions as in the previous calculation of the flux through the skimmer. At 0 V the net current was negative, as is normal at the anode of a glow discharge. At a negative plate potential sufficient to retard the electrons, the positive ion current leveled off at 10 nA, whereas at positive potentials the electron current leveled off at 90 nA. Thus, nine times as many electrons as positive ions make it through the
6168 The Journal of Physical Chemistry, Vol. 96, No. I S , 1992
Miller
TABLE I: Ethanol Fragmentation Patterns by SSJ/CD io CO,, Cbarge Exchange with CO,'? aod Electron Impncp relative intensity mass SSJ/GD CE E1 27 0.2 3.0 24 28 2.1 3.6 9.5 24 29 6.1 30 3.7 3.6 6 31 100.0 100.0 100 43 0.6 1.4 45 14.2 24.1 39 46 1.4 1 .o 17 47 14.3 Reference 18. bReference 19.
skimmer. Since the current in the negative glow is carried almost entirely by the electrons because of their high mobility, the discharge current of 20 pA is the electron flux to the skimmer. Dividing the skimmed electron flux of 90 nA by 20 pA gives 0.0045, in close agreement with the previously calculated fraction of the jet skimmed. The mechanism by which the positive ions make it through the skimmer is not clear. The mobility of C02+in C02is 840 cm2 TOITV-' s-l.15 A rough guess at the electric field in the negative glow is 1-10 V cm-1.16 With the pressure at 0.2 Torr, the drift velocity of C02+toward the nozzle would fall in the range 4.2 X lo3 to 4.2 X 104 cm s-', The final jet velocity from eq 2 is 6.6 X 104 cm s-I, which is only somewhat larger than the estimated drift velocity. This is before the jet is invaded by the surrounding gas. Nevertheless, it is possible that the momentum of the jet is responsible for carrying positive ions through the skimmer. The possibility of electric field reversals complicates this ana1y~is.l~ A skimmed electron beam of 90 nA converts to 9.33 X mol of electrons per second. Dividing by the skimmed beam flux (F,= 5.00 X lo-') gives an upper bound to the degree of ionization of 1.9 X lod, since the drift velocity of the electrons is added to the velocity of the jet. By the same argument the IO-nA positive ion flux gives a lower bound of 2 X lo-' to the degree of ionization. 4. Mechanism of Seed Ionization. The negative glow plasma consists of approximately equal numbers of electrons and positive ions. In the present supersonic jet/glow discharge (SSJ/GD) setup, relative intensities in the pure C02 mass spectrum were 100 at 44 Da, 22 at 32 Da, and 13 at 88 Da. A peak at 18 Da (H20+)was always present in variable intensity but usually about half that of the 44 peak. The electrons in the plasma must be energetic enough on the average to maintain the glow discharge by ionizing C02molecules fast enough to counterbalance losses by recombination. However, since the ionization potential of C02 (13.8 eV) is appreciably higher than that of the typical organic molecule, the means of ionization of the seed molecules could be CE as well as EI. Furthermore, the presence of H20+made proton transfer a possible mode of ionization. Measurements by von Koch and Lindholm'* show the CE fractionation pattern of ethanol by eo2+.In the present SSJ/GD study, mass spectra were obtained with the QMS by passing C02(g) at 34 atm over liquid ethanol at 0 "C(vapor pressure = 11.7 Torr) to give a seed mole fraction of 4.5 X 10-4, The results, normalized to the most intense peak at 3 1 Da, are given in Table I. Comparison with the 70-eV E1 spectrum'9 and the CE spectrum of von Koch and Lindholm suggests the predominance of CE with C02+as the mode of seed ionization in the present setup. In particular, peaks at 27,29, and 46 Da, which are strong in the E1 spectrum, are weak in the CE spectrum. Nevertheless, the presence of a peak at 47 Da, which presumably represents the protonated molecular ion, C2HSOH2+,formed by the exothermic reactionmof neutral ethanol with the appreciable amounts of H20+in the discharge (AH= 46.6 kcal mol-'), shows that water may be a factor in the fractionation pattern. 5. Efficiency dSeed Ionizatktn. The SSJ/GD mass spectrum with benzene as the seed gave only the molecular ion (78 Da), and was convenient, therefore, to study the effect of seed concentration. The C02spectrum included minor peaks mrresponding
N
I
10-3
4 IO-^ IO-^ 10-6
YB
Figure 4. Ratio, R, of the benzene ion current to the CO, ion current versus the mole fraction of benzene in the jet.
to 02+ and (C02)2+,as well as C02+(see above). Figure 4 compares the QMS ion current of the benzene seed at 78 Da to the summed QMS ion current of the C02carrier. In these runs the jet background pressure was set at 0.35 Torr to maximize the was signal. The highest mole fraction of benzene (YB= achieved by putting benzene in the sample U tube at 0 "C. A least-squares fit of the data in Figure 4 gives the following for the ratio, R, of seed to carrier ion currents: log R = 0.478 log YB 0.697
+
The important result here is the relatively high efficiency of charge exchange ionization of benzene at low benzene concentration. Thus, at mole fraction one has R = 2 X and R/YB= 2 X lo4. Taking an average value of 10" for the degree of ionization of the plasma as calculated above, the degree of ionization of the benzene is (R/YB) X lo4 = 0.02. Although this value is not very reliable, it is clear that the degree of ionization of the seed is orders of magnitude greater than that of the carrier. The excitation of a cold glow discharge in an organic vapor can result in the deposition of a polymer coating on exposed surfaces along with higher molecular weight ions in the mass spectrum. In the present SSJ/GD study of benzene, no deposition was observed on the walls of the discharge chamber, apparently due to the short residence time of the seed molecules in the discharge and their low concentration. However, in preliminary experiments with 1-bromopropane as seed, a thin, brownish film had formed on the skimmer and on the wall close to the skimmer after about 6 h of discharge time. The seed concentrations here are estimated to mole fraction. to have been in the range 4 X
Acknowledgment. The author is indebted to W. K. Williams for help in the design of certain components of the apparatus used in the present study and to R. V. Smith for carrying out the btam current measurements. Registry No. Carbon dioxide, 124-38-9; ethanol, 64-17-5; benzene, 71-43-2.
References and Notes (1) Harrison, W. W.; Hess, K. R.; Marcus, R. K.; King, F. L. AMZ. Chem. 1986,58, 341A. (2) Odham, G.; Valeur, A,; Michelsen, P.; Aronsson, E.; McDowall, M. J . Chromatogr. 1988, 434, 3 1. (3) Matsumoto, K.; Kojima, H.; Yasuda, K.; Tsuge, S. Org. Mass Spectrom. 1985, 20, 243. (4) McLuckey, S.A,; Glish, G. L.; Asano, K. G.; Grant, B. C. Anal. Chem. 1988,60, 2220. (5) Sofer, I.; Zhu, J.; Lee, H A ; Antos, W.; Lubman, D. M. Appl. Spectrosc. 1988, 44, 1391. (6) McKeown, M.; Siegel, M. W. Am. h b . 1975, Nou, 89. (7) Campargue, R. J . Phys. Chem. 1984, 88, 4467. (8) Sin,C. H.; Linford, M. R.; Goates, S.R. Anal. Chem. 1992,64, 233. (9) Raizer, Y. P. Gas Discharge Physics; Springer-Verlag: Berlin, 1991. (IO) Gouw, T. H. J . Chem. Eng. Data 1969, 14,473. (11) Fisher, S. S.;Bharathan, D. J. Spacecr. Rockets 1973, 10, 658. (12) Gilntherschulze,A. Z.Phys. 1928, 49, 358.
J. Phys. Chem. 1992,96,6169-6172 (13) Balle, T. J.; Flygare, W. H. Rev. Sci. Instrum. 1981, 52, 33. (14) H a n d h k of Chemistry and Physics; Weast, R. C., Ed.; CRC Rcss: Boca Raton, FL, 1989. (IS) Von Engel, A. Ionized Gases, 2nd ed.;Clarendon: Oxford, 1965. (16) Davies, A. J.; Evans, J. G. J. Phys. D 1980,13, L161. (17) Gottscho, R. A,; Mitchell, A,; Scheller,G. R.; Chan, Y.-Y.; Graves, D. B. Phys. Rev. A 1989,40, 6407.
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(18) Von Koch,H.; Lindholm, E. Ark. Fys. 1961, 19, 123. (19) Eight Peak Index of M a s Spectra, 2nd ai.;Mass Spectrometry Data Centre; AWRE Aldermaston, Reading, UK, 1974. (20) Lias, S.G.; Bart", J. E.; Liebman, J. F.;Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Suppl. 1. (21) Weston, R. E., Jr.; Schwarz, H. A. Chemical Kinetics; Rentice-Hall: Englewood Cliffs, 1972.
Polarlzabllltles of Hollow Spherical Organic Molecules with Encapsulated Cations R. S . Ross,* Physics Department, University of California. Santa Barbara, California 931 06
P.Pincus, Materials and Physics Departments, University of California. Santa Barbara, California 93106
and F.Wudl Physics and Chemistry Departments, University of California, Santa Barbara, California 931 06 (Received: August 30, 1991; In Final Form: January 27, 1992)
Electric dipole moments and electric and thermal polarizabilities are calculated for so called "heterospherophane" molecules encapsulating single cations. These are hollow spherical shell molecules comprised of eight benzene rings linked each to three others by single heteroatoms. Two cases are considered in which the anion (electron) is either localized to a single benzene site or delocalized throughout the r and lone pair molecular orbitals. A classical continuum approach is employed to reveal the essential physics operative.
Introduction Over the years we have considered the challenge of the prep aration of a truly three dimensional organic metal; Le., one composed of essentially close-packed spheres. An example of such a "sphere" is the much talked about truncated icosahedral Cbo carbon cluster Buckminsterfullerene,'-' which was recently converted to a superconductor5g6and a ferromagnet.' While these are all fascinatingreaults, we believe that even more unusual results may be obtained if the counterions to the charged spherical molecules were encapsulated. We were driven to consider the charged heterospherophanes* as possible candidates for a three dimensional organic metal by the constant and sometimes unpredictable counterion effect on the structure and properties of molecular solids. If the counterion of a radical ion salt could be moved from anywhere around the periphery of the D or A molecule, most of our design problems related to the counterions would be solved. One aspect of a potential solution, described in this paper, is to move the " i o n from the exterior to the interior of a donor or acceptor molecule, as shown in Figure 1. As can be seen, the globular molecule can be a monovalent or multivalent "atom" depending on (i) the number of charged counterions it encapsulates or (ii) the valence of the engulfed species. Note also that the word charged heterospherophane has a special meaning; whereas the entity itself is electroneutral, its surface exhibits a negative (positive) charge to the environment. As a result of these potential intermolecular surface Coulomb repulsions and with the assumption that there is delocalization of charge throughout the sphere, most likely for thiaspherophune,where delocalization through sulfur is possible, one could predict widely varying properties for solids made up of such molecules depending on the composition (valency) of the included species. A "Dreiding model" of a spherophane molecule is shown in Figure 2. The cavity diameter of the spherophanes depends on the covalent radius of the heteroatom and is approximately 5 A for "oxaspherophane" in which the covalent radius of oxy en is about the same as that for carbon, and approximately 7 for
fb,
"thiaspherophane" in which the covalent radius of sulfur is a bit larger due to its additional electronic shell. The outer diameter of these two spherands is approximately 12 and 14 k respectively. These numbers are based on Allinger MM2 molecular model simulations, corrected to include the van der Waals radii of the relevant atoms. The cavity is not large enough to trap any solvent or salt; however, it can accommodate individual ions which can even be used as templates in synthesizing heterospherophanes. A qualitative distinction can then be made between "neutral" heterosphemphanes, which have empty central cavities, and "charged" heterospherophanes, which encapsulate charged ions. Charged spherands are of course electrically neutral, the neutralizing charge residing on the continuous tr and lone pair electron orbitals which make up the outer surface of the molecule. They therefore present the external environment with a charged surface and should generally possess sizeable permanent dipole or quadrupole moments. This should make for extremely interesting electronic properties of the liquid and solid structures arising from these molecules. In this paper, we present the results of calculations of the static electric dipole moments and polarizabilities for individual charged oxaspherophane and thiaspherophane molecules encapsulating a singly ionized cation. The carbon based cage is modeled by a spherical shell of uniform static dielectric constant e. On the basii of the chemical structure simulations, the outer radius of the shell, denoted b, is taken to be roughly twice that of the inner radius, d. The trapped ion is confined within the inner radius of the shell, and its associated electron is taken in this model to be confined to the outer surface of the shell. This electron is actually delocalized among the various tr orbitals of the benzene rings, but to avoid complications we restrict ourselves to the simplest model as presented. For the oxaspherophane molecule, the donated electron is localized on trioxybenzene sites. It is not able to delocalize through the heteroatoms and hence is confined to a single benzene ring. This electron is modeled for these calculations by a negative charge fixed to the outer surface of the shell. For the thiaspherophane molecule, where delocalization through the
0022-3654/92/2096-6169$03.00/0 Q 1992 American Chemical Society
