New Drift-Tube Source for Use in Chemical Ionization Mass Spectrometry P. C. Price,‘ H. S. Swofford, Jr., and S. E. Buttrill, 51’: Department of Chemistty, University of Minnesota, Minneapolis, Minnesota 55455
A new ion source incorporating an Integral drift tube allows control over the extent of fragrnentatlon of the quasknolecular ions produced in chemical ionlratlon mass spectrometry. Increasing the electric fleid strength in the drift-tube region causes fragmentation slmllar to that observed at higher source temperatures or with more energetic reagent gases. Effectlve /on temperatures in excess of 1000 K are available at the highest drifl fields. Slnce a slngle drlft voltage settlng controls the extent of fragmentatlon, It is possible to obtain both molecular ion spectra and extenslve fragmentation on successlve scans of a single sample.
Chemical ionization mass spectrometry (CIMS) has gained very wide acceptance as an analytical tool in the decade since its initial development ( I ) . One of the greatest advantages of CIMS over electron impact ionization is the great reduction or elimination of fragment ions, simplifying the spectrum and usually producing an intense quasi-molecular ion, i.e., either MH+ or (M - l)+. CI conditions under which only quasimolecular ions are present are ideal for quantitation of a compound, and they greatly facilitate the determination of the number of components or the purity of an unknown sample. However, the molecular weight alone is not sufficient to identify a compound; considerable additional information, such as is available from the masses and intensities of fragment ions, is almost always required. The extent of fragmentation of the quasi-molecular ions in CIMS may be increased by increasing the source temperature (Z),or by using a more energetic reagent gas (3). Both of these methods are relatively slow and experimentally inconvenient and would certainly require the introduction of additional samples with conventional instrumentation. Early in the development of CIMS, it was recognized that increasing the electric field inside the ion source increased the relative intensities of fragment ions (4-6). This effect was attributed to unimolecular fragmentation of the MH’ ions activated by energetic collisions with neutral reagent gas molecules in the source. This report describes a drift-tube chemical ionization source which uses this effect to provide a variable degree of fragmentation of the quasi-molecular ions in the CI spectrum. A drift tube is a cylindrical chamber in which a uniform longitudinal electric field is maintained by a series of ring shaped electrodes. Ions introduced into one end of the tube by an ion source may be accelerated through a neutral buffer gas by applying a voltage gradient down the tube. The theory of ion motion in drift tubes has been reviewed by several authors (7-11). Briefly, ions entering a drift tube will acquire energy from the electric field. It has been shown that this energy is related to a parameter EIP, where E = electric field strength, and P = neutral gas pressure in the tube. E / P has lPresent address, Union Carbide 770-120, P.O. Box 8361, South Charleston, W. Va. 25303. Present address, Stanford Research Institute, Menlo Park, Calif. 94025.
the units V/cm Torr. This quantity may also be expressed as E I N or V cm2 where N = molecules/mL, the conversion being E / N = (E/P)(2.83X lo-”). Often E I N is expressed in Townsends, where T d = (V ~ m ’ ) ( l O ~ ~ ) . The mean kinetic energy of the ion distribution may be approximated (12) by
KE = m v 2 / 2+ M v 2 / 2 + 3 k T / 2
(1)
where m = mass of the ion, M = mass of the neutral gas, u = drift velocity, and T = gas temperature. The additional ion kinetic energy from the electric field may be described in terms of an “effective ion temperature,” which will be equal to T a t very low E / P , but may be several thousand Kelvins as E I P enters the high field region (13,14). This “effective temperature” is defined by Mason (11) as:
Teff=T(l t M v 2 / 3 k T )
(2)
Qualitatively, both the ion velocity and internal energy increase with EIP. The average translational energy of the ions is determined by the steady-state balance of the energy gained from the electric field and the energy lost to the neutrals as a result of collisions. These collisions with the reagent gas molecules also increase the internal energy of the ions, resulting in an effective “vibrational temperature” which is comparable to the “translational temperature.” If ions are formed by the CI process at the rear of a drift tube, one should be able to raise the “effective temperature” of those ions by collisions with neutral gas molecules to very high values: This should simulate the operation of a conventional CIMS source a t temperatures not normally attainable, and should result in increased fragmentation. Since the crucial parameter, E / P , may be rapidly changed by varying E , by merely changing the voltage of the drift electrodes, one should be able to rapidly change the “effective source temperature”.
EXPERIMENTAL The basic mass spectrometer used in this work is a DuPont 21-490B modified for CIMS as described previously (15). A cross-sectionalview of the drift tube chemical ionization source is shown in Figure 1. The source is cylindrically symmetrical about the direction of ion motion. The electron beam enters at the rear of the source at an angle of approximately 45O with respect to the axis. The filament and rear electrode of the Pierce style (16)electron gun are about 2.5 cm from the 0.79-mm electron beam entrance hole. Under the usual operating conditions, the filament and rear electrode are at zero potential, the front electrode is at 300-700 V, and the source is at accelerating potential (approximately 1300 V). From observation of the burned discoloration of the polished flat surface at the rear of the source housing, it appears that the image of the filament on the source is 100% to 125% of the actual filament area (0.75 X 7.5 mm). The filament is made of 0.75 mm X 0.025 mm rhenium ribbon and lasts at least 4 months in normal operation. The metal parts of the source are machined entirely from stainless steel. The insulators are made of MACOR Machinable Glass-Ceramic (Corning Glass Works) and are shown shaded in Figure 1. Beginning at the left in Figure 1, part A is the ion source itself which may be biased by up to 45 V with respect to the drift tube base, part B. In addition to serving as the union between ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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n
Table I. Measurement of Approximate Effective Ion Temperature of MH' Ions from 1-Octene in Water Reagent Gasa as a Function of E / P
h i t for an injected aqueous solution of 1-octenewas 5 to 10 ppm. CAP 135 b
it is sufficient to demonstrate potential analytical utility.
RESULTS AND DISCUSSION 69.5 K
69.5 K
DRIFT
RINGS
69.5 K
It was of immediate interest to demonstrate that, under CI conditions, large amounts of energy could be imparted to ions in the drift tube. This measurement was made by utilizing a basic property of a magnetic sector mass spectrometer; ions are focused a t the detector according to the following formula:
rnle = k H 2 r 2 / V 69.5 K
69.5 K
4
SOURCE (Repeller)
Figure 2. Drift tube C I source circuitry
the source and drift tube, the base plate, B, contains the gas inlets and a thermocouple well, which are not shown. Part C and the next four metal parts are the drift rings which are designed t o completely shield the insulators from the ions moving through the drift tube. Part D is the drift tube cap and contains a second thermocouple well and mounting holes for an Einzel lens assembly (not shown). The ion exit aperture is a 1.0-mm hole in a 9.5-mm diameter disk 0.05 mm thick which is clamped to the cap. To make the source relatively gas tight without the use of numerous gaskets, both the insulating rings and the mating surfaces on the metal parts were machined flat and highly polished. The entire stack is clamped tightly together with four 0-80 rods running through the eight metal parts. The voltages for the drift tube source are provided by the circuit in Figure 2. The voltage divider, consisting of seven 69.5-kOhm 0.5% resistors, is mounted inside the source housing. The batteries and controls are enclosed in a box floated at accelerating potential. With this particular circuit, both the repeller field and the drift field may be varied independently over a wide range without appreciably changing the energy of the ions reaching the magnetic sector (but see below). The electron gun could be biased at any potential up to 1000 V relative to the ion source. However, with the source pressure at 0.1 Torr, the ion current increased steadily with increasing electron energy. Thus, maximum sensitivity was obtained by running the filament at ground potential giving an electron energy of approximately 1300 eV. Using water as reagent gas, repeller settings of greater than 6 V gave less than 10% increase in sensitivity, even with low drift fields. Hence the repeller was usually run at 5 V or less. As in previous work with a more conventional CI source (17-19),maximum total ion current and 1498
ANALYTICAL CHEMISTRY, VOL. 49, NO.
11, SEPTEMBER 1977
(3)
where k is a constant, H i s the strength of the magnetic field, and V is the voltage through which ions are accelerated. If a given ion is focused at the collector, it would be observed to shift position if the accelerating voltage was changed. Thus, at constant accelerating voltage, any changes in the energy (eV) of ions leaving the source will appear as a slight shift of V in Equation 3. There will then be a shift of the peak position as observed at the collector. If the accelerating voltage is changed an amount corresponding in voltage to the additional energy accumulated by the ion in the source, the peak should return to its previous position as observed at the collector. First, to determine the accuracy of this method, an experiment was performed with the source operating in the electron impact mode. The source pressure was less than 1 Hm, the temperature was 30 "C, and the repeller was set a t 0 V, the only gas present was air. With the accelerating voltage set at 1270.6 V, m l e 28 was focused on the collector. An increase of the drift voltage from 0 to 3.3 V necessitated a decrease of 3.2 V of the accelerating voltage to 1267.4 V to refocus m / e 28. Thus, the application of 3.3-V drift voltage increased the ion energy by 3.2 eV. Similarly, 24.3 V applied to the drift tube contributed 24.5 eV to the ion energy, and 95.8 V contributed 95.4 eV of energy. From these data, it can be seen that by keeping an ion peak focused on the collector by varying the accelerating voltage, changes in energy of ions leaving the source can be measured. Slight errors are the result of the problems associated with measuring 0.1-V changes a t the kilovolt level. The experiment was repeated with the drift-tube source operated in the CI mode with water as the reagent gas and 1-octene as the sample. The source pressure was 0.217 Torr, the temperature was 30 "C, and the repeller voltage was 0. The MH' ion (mle 113) was focused at the collector with the accelerating voltage at 1268.4 V. The results of increasing the drift voltage under these conditions are shown in Table 1.
100
I
‘t19
I
:H I
I
z 100
1
I\
“‘t 60
z
P 0
100
200
300
400
500
600
700
800
900
TEWPERATURE I’CI
10
Flgure 5. H20 clusters
c 002
0
006
004
008
010
Table 11. Methane Reagent Gas Ions (Percent Total Ion Current) Observed in the Drift-Tube Source at 30 “ C and 0.10 Torr
PRESSURE trorrl
Figure 3. H20 cluster ions vs.
pressure: drift voltage = 15 V, 115
OC 100
I
I
I
I
I
I
30 ‘C
15 17 19 27 29 39 41
0 105 torr
100
150
200
250
300
E P I V c m torr1
Flgure 4.
Water cluster ions: 30 OC, 0.105 Torr
As the drift voltage (or E / P value) was increased, the corresponding accelerating voltage necessary to center mass 113 at the collector decreased. At an E / P of 73.4 V/cm Torr, the ions exiting the source have accumulated an excess of 0.6 eV over their energy when the drift field is zero. This excess kinetic energy of motion along the axis of the drift tube (eAV) is just the quantity represented by the first term (11) in Equation 1, so that we have a crude measure of the drift velocity. Combining this fact with Equation 2 yields an expression for the “effective temperature” which can be evaluated directly:
Teff = TI1 + ( 2 M e A V)/(3rnhT)]
(4)
As shown in Table I, the effective temperature of the ions estimated in this way exceeds 1000 K a t E / P = 73.4 V/cm Torr. It is of interest to compare the water reagent ion intensities obtained with this source with those observed in the more conventional CI source (17) and those expected under equilibrium conditions (22). Figure 3 shows the variation of water cluster ion intensities at 115 “C and constant 3.45 V/cm drift field. The larger clusters at m/e 55 and 73 begin to dominate the spectra at much lower pressures than in the modified DuPont source (17). This is undoubtedly due to the much longer distance which the ions must now travel within the source to reach the source exit aperture. Figure 4 shows the variation of water cluster ion intensities with E / P a t 0.105 Torr and 30 “C. The effects of increasing E I P are qualitatively very similar to those expected from raising the source temperature. Figure 5 is a plot of the relative equilibrium concentrations of the various ions in water calculated from Kebarle’s thermochemical data for these ions
EIP values, V/cm Torr 66.7 133 200
0
33.3
6 28 11 0.9 45
0.3 15 32 0.3 37 0.1 15
0.0
9.3
0.3 26 22 3.2 34 0.4 14
1.9 26 6.4 22 19 10 15
6.3 18 1.9 28 27 12 7.0
267 12 14 0.6 29 30 9.8 3.9
(22). Comparing Figures 4 and 5 shows that at low values of E / P in water, the ion intensities are very close to those expected at equilibrium. The small signals a t mle 55 and 37
1 50
vs. temperature at 0.10 Torr
are possibly due to collisional dissociation of a small fraction of the ions as they leave the source. At higher values of EIP, the behavior of the relative intensities of the cluster ions deviates from that expected at any particular temperature. At any given E / P value between 100 and 200 V/cm Torr, three different ions are always present in significant (>15%) amounts, whereas under true thermal equilibrium, there is no temperature at which more than two ions are significant. This observation is totally consistent with the fact that only the ion “temperature” is increased by the drift field. Also, the ion speeds and vibrational energies are not accurately described by the Boltzmann distribution. Since methane was the first reagent gas used in CIMS and is probably still the most common, it was tried in the new source. Table I1 shows the intensities of the major peaks observed in methane with the drift-tube source a t a variety of E / P values. At zero field, the spectrum is similar to that obtained by Field (1) and others under conventional CI conditions. As E / P is increased, there do appear to be increases in fragmentation; the mle 15, 27, and 39 peaks increase, while mle 17, 29, and 41 decrease. Plots of the intensity of ions in methane reagent gas vs. pressure were similar to Field’s work (at low E / P ) , except it appeared that a “steady-state” population was reached at approximately 0.1 Torr. Notable exceptions were m/e 39 and 27, which continued to decrease in intensity up to 0.2 Torr. In terms of pressure behavior, methane is analogous to water reagent gas in that CI conditions are attained a t 0.07 to 0.10 Torr in the drift-tube source (as compared to 0.7 to 1.0 Torr in many conventional CI sources). It should be noted that, in contrast to water, methane reagent gas is markedly affected by the repeller voltage. Above 10 V, the spectrum is “distorted”, and m/e 41 becomes very intense. Because of the complex nature of the changes in the methane reagent ions with drift field, methane will probably be of limited value as a reagent gas in the drift tube CI source. In contrast to methane, isobutane is almost ideal as a reagent gas for the drift tube CI source. Figure 6 shows the
PrNALYTICAL CHEMISTRY, VOL. 49, NO. 1 1 , SEPTEMBER 1977
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Table 111. Comparison of Decane CI Spectra (Percent Total Ion Current Above m / e 5 7 ) Methane CI at 110 C Methane CI (Ref. 23) source Isobutane CI at 110 C temperature, C EIP, V/cm Torr EIP, V/cm Torr O
mle 71 85 99 113 127 141
35 12 10 4.2 1.2 2.8 70
100
200 25 25 12 1.2 0.6 36
100 15 15 6.1 1.2 2.1 60
I
I
I
0 23 23 12 2.9 0.3 39 I
60
-
I
8.7
100 32 35 7.4 3.2 22
m/e 71
200 42 33 12 -
85 99 113 127 141
-
14
1
4
30 C
00066 torr
43 1200
T
0 50
100
150 E P IV
Figure 6. Major
CIP
r 250
I. 1
300
50
10.11
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
1 00
150
200
250
E P I V cm torr)
isobutane ions: 30 “C,0.066 Torr
major ions in isobutane as a function of E I P at 0.066 Torr and 30 “C. Over most of the available range of EIP, the C4Hg+ ion at m l e 57 accounts for 90% of the total reagent ion current. At low values of EIP, there were some contributions from m l e 41 and 42 which are omitted from Figure 6 for clarity. The spectra obtained in the drift tube source at 0.06 to 0.09 Torr are similar to those obtained by Field ( 4 ) at somewhat higher pressures. Both methane and isobutane CI spectra of decane were obtained at three different values of E / P in order to compare the variation in fragmentation available with the temperature effects reported by Hunt and McEwen (23). Since the C4Hg+ reagent ion from isobutane is identical with one of the decane fragment ions, only the peaks above m l e 57 are included in Table 111. Hunt’s data were obtained a t 1.0 Torr methane pressure while the drift-tube source data were taken with 0.10 Torr of isobutane or methane. Using methane, it is obvious that, even at zero drift field, the drift-tube CI data show more fragmentation than Hunt found at 100 “C. This is probably the result of the lower source pressure, although, as noted above, the reagent ion distribution is also somewhat different. Increasing EIP causes fragmentation even more extensive than was observed by Hunt at 200 “C. The isobutane data show the extent of control over the degree of fragmentation available with the drift-tube CI source. With no drift field, the fragmentation of the (M - 1)’ ion at m / e 141 is essentially the same as that found by Hunt at 1.0 Torr methane and 35 “C. With E I P = 100 V/cm Torr, the extent is substantially greater than that which can be obtained by heating the source to 200 oc. The ability to control and rapidly change the extent of fragmentation under CI conditions has tremendous analytical utility. As an example, Figure 7 shows the isobutane CI spectrum of ethyl butyrate as a function of E / P . Below 50 V/cm Torr, the MH’ ion a t m / e 117 is 95% of the total sample ionization. At 200 V/cm Torr, the MH+ is 70% fragmented. Furthermore, the fragments are just those which would be obtained from ethyl butyrate with methane CI reagent gas (2). Thus the additional information which could be obtained by switching from isobutane to methane reagent gas is available with the drift-tube CI source by simply 1500
-
9.4
0 10 8.4 5.3 2.4 2.4 71
57
I 5
200 43 36 13 -
100 46 36 7.6 1.0
Flgure 7. Isobutane CI spectrum of ethyl butyrate as a function of E I P . Isobutane pressure = 0.070 Torr; source temperature = 115
“C changing a single voltage. This unique feature makes it possible to determine the molecular weight and purity of a material with one mass spectrometer scan and obtain structural information on the next scan of the same sample.
LITERATURE CITED (1) (2) (3) (4) (5) (6)
M. S. B. Munson and F. H. Field, J . Am. Chem. Soc., 88, 2621 (1966). M. S. E. Munson and F. H. Field, J . Am. Chem. Soc., 88, 4337 (1966). C. W. Tsang and A. G. Harrlson, J. Am. Chem. Soc., 88, 1301 (1976). F. H. Field, J. Am. Chem. Soc., 81, 2827 (1969). J. Michnowicz and E. Munson, Org. Mass Spectrom., 4, 481 (1971). F. H. FleM, in “Ion Molecule Reactions”, J. L. Franklin, Ed., Plenum Press,
New York, N.Y., 1972, pp 261-313. (7) E. W. McDaniel et al., “Ion-Molecule Reactions”, John Wiley, New York, N.Y.. 1970. ChaDter 2. (8) E. W. McDaniel, “Colllsion Phenomena in Ionized Gasses”, John Wlley, New York, N.Y., 1964, Chapter 9. (9) I. R. Gatland, Case Stud. At. Colllslon Phys., 4, 369 (1974). (10) D. R. James, E. Graham, G. R. Akrldge, and E. W. McDanlel, J . Chem. Phys., 82, 1702 (1975). H. E. Revercomb and E. A. Mason, Anal. Chem., 47, 970 (1975). M. McFarland, D. L. Albritton, F. C. Fehsenfeld, E. E. Ferguson, and A. L. Schmeltekopf, J . Chem. Phys., 50, 6620 (1973). E. A. Mason, L. A. Vlehland, H. W. Ellis, D. R. James, and E. W. McDanlel, Phys. Fluids, 18, 1970 (1975). E. W. McDanlel, E. A. Mason, et el., J . Chem. Phys., 83, 2238 (1975). I. C. Wang, H. S. Swofford, Jr., P. C. Prlce, D. P. Martinsen, and S. E. Buttrlll. Jr.. Anal. Chem.. 48. 491 (19761. J. R. Pierce, J . Appl. Phys.,‘ 1I, 548 (1940). P. C. Price, H. S. Swofford, Jr., and S. E. Buttrlll, Jr., Anal. Chem., 48, 494 (1976). P. C. Prlce, D. P. Martlnsen, R. P. Upham, H. S. Swofford, Jr., and S. E. Buttrlll, Jr., Anal. Chem., 47, I90 (1975). D. P. Martinsen and S. E. Buttrlll,Jr., O g . Mess Spectrum.,11, 762 (1976). PhlliD C.Prlce. Ph.D. Thesls. Universitv of Mlnnesota. MlnneaDOllS. Minn.. December 1976. Davld P. Martlnsen, Ph.D. Thesis, Unlverslty of Mlnnesota, Mlnneapolls, Mlnn.. December - -. . - . 1976. P. Kebarle, In “Modern Aspects of Electrochemlstry”, G. Conway and J. O’M. Bockrls, Ed., Plenum Press, New York, N.Y., 1974, Vol. 9, pp 1-46. D. F. Hunt and C. N. McEwen, Org. Mass Spectrom., 7, 441 (1973).
.
RECEIVED for review April 15,1977. Accepted June 16,1977. Presented in part at the 24th Annual Conference on Mass Spectrometry and Allied Topics, May 9-13, 1976, San Diego, Calif. This work was supported by NSF Grants GP-38764X, MPS-7510940, and CHE76-20096.
