has its maximum a t a lower frequency than the nominal center of the band of frequencies emitted by the excitation monochromator. In consequence, spectral features occur a t higher frequencies than expected. The error may be reduced by using smaller bandwidths, but because this is achieved a t the cost of the signal-noise ratio, the possible improvement is limited. I n assessing the possible usefulness of measuring the excitation spectra of totally absorbing solutions one must bear in mind that equivalent data may
be obtained directly by using the thin film method. With the apparatus and fluorescent species used above, the two methods are roughly comparable in their manipulative demands : this may not be so with other fluorescent compounds, particularly if their quantum yields are low. The main advantage of the fluorescence method is that it allows the form of a spectrum spanning as much as two decades to be obtained without the dilution necessary to preserve equal resolution a t widely different densities.
LITERATURE CITED
(1) Ainsworth, S., Winter, E., A p p l . Opt. 3, 371 (1964). (. 2,) Bowen. E. J.. Proc. Rou. SOC.Ser. A 154,349'(1936j. (3) Parker, C. A., Analyst 85, 587 (1960). ( 4 ) Parker, C. A., Nature 182,1002 (1958). (5) Lnicam SP 800 Recording Spectro-
photometer, Unicam Instruments, Ltd., Cambridge, England. ( 6 ) Weber, G., Teale, F. W. J., Trans. Faraday SOC.54, 640 (1958). (7) Weber, G., Biochem. J . 75,335 (1960).
RECEIVED for review October 19, 1964. Accepted January 11, 1965.
Flash-Vaporization of Solid Materials for Mass Spectrometry by Intense Thermal Radiation KENNETH A. LINCOLN
U. S.
Naval Radiological Defense laboratory, Son Francisco, Calif.
Flash-vaporization mass spectrometry offers a new approach to obtaining mass spectra from a diversity of solid materials, such as polymers and inorganic compounds. Single, thermal pulses from a xenon flashtube vaporize the samples directly into the ion source of a time-of-flight mass spectrometer; the mass spectra appear momentarily on the screen of an oscilloscope and are recorded photographically. This technique, which also renders some short-lived species amenable to mass spectrometric studies, and the apparatus (including vacuum lock for introducing solid samples into the instrument) are described.
I
N MORE conventional mass spectrom-
etry where fairly volatile materials are analyzed, the sample is vaporized in an external oven and the vapors are piped into the ion source via a calibrated leak or valve. On the other hand, materials having low vapor pressures are commonly vaporized in some type of small furnace within the ion source housing. Usually this is either in the form of a miniature furnace positioned close to the electron beam or a Knudsen cell from which the vapors effuse out as a molecular beam into the ion gun ( 4 ) . The cell is situated usually within the vacuum of the ion source and is heated by a resistance wire or by bombardment from high energy electrons. These techniques for studying solid samples suffer from several drawbacks which include the limited upper temperature available (1500' t o 2200' c.)q reactions of some vapors with the hot metal structures, and frequent degassing problems.
94 7 35.
Without reviewing the subject of mass spectrometric analysis of solids, it may be noted that the prevalent method of analyzing inorganic materials employs the radiofrequency spark ion source which ionizes the sample as it is vaporized from an electrode ( 2 ) . Unfortunately, the unsteadiness and very large energy spread of the ion beam often detract from the usefulness of this technique. A less commonly used method for inorganic materials is thermal ionization of the solid from the surface of a heated filament (IS); however, the ionization efficiencies with this technique vary with the chemical elements over several orders of magnitude. I n this paper attention is directed to a different heating method which has proved to be very successful for rapidly heating many types and kinds of solid materials to high temperatures. This technique utilizes the intense thermal radiation obtained from capacitor discharges through xenon flashtubes (8). Nelson and Kuebler (9) have vaporized such refractory materials as tungsten by this technique, and have applied it to the vaporization of elements for atomic absorption spectroscopy (10). Described here is a similar singlepulse method for vaporizing solid samples within the ion source housing of a time-of-flight mws spectrometer; also detailed is the solid-sample inlet which is an integral part of the system. We have utilized this technique of flashvaporization mass spectrometry to study high-temperature decomposition of cellulose and have applied it to obtain mass spectra of several other low vapor pressure organic materials as well as some inorganic compounds and meteoric fragments.
It should be pointed out that a sample must possess certain physical characteristics t o make this type of flashheating feasible. For maximum temperature rise, a material must have a high optical absorptance in the visible and near infrared plus a large surfaceto-volume ratio-i.e., the greater the exposed surface the more energy absorbed by the material. Nelson (8) has shown that for spheres and rodshaped particles a diameter of about 10 microns or less is advantageous; he has found that very thin metal foils are also readily vaporized in this manner. Where temperature requirements are not so high-e.g., vaporization of organic materials-the surface-to-volume ratio is not so critical. We have found that 2-mil sheets of cellulose made black by inclusion of 2% carbon black can be almost completely vaporized by radiant exposure as low as 2.0 cal./cm.* incident on both surfaces. On the other hand, the technique is not restricted to finely divided or thin samples; if the material has a low heat diffusivity and a reasonably good absorptance, each flash will vaporize enough of the surface layer from thick samples to provide a spectrum. Moreover, this has the advantage of not requiring sample reloading for each spectrum obtained. One of the more striking features of the radiant flashvaporization method is that the intense pulse of light heats only the sample; all of the surroundings remain cool and thus reactions with hot ion-source structures are obviated. Although the most difficult parameter to obtain is the temperature rise of the flash-heated material, an approximate upper temperature sometimes can be calculated (If). On the other hand, VOL. 37, NO. 4, APRIL 1965
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B A C K I N G PLATE
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1; I
'ELECTRON
BEAM
Figure 2. Oscilloscope presentation of flash-vaporization mass spectrum of cellulose Upper: 15-e.v. ionizing potential Lower: 70-e.v. ionizing potential
Figure 1 . Ion source of mass spectrometer with attached flash-heating inlet system
the amount of energy per flash, and the peak intensity incident on the samples, has been carefully measured for several flashtubes and it is easily varied by adjustment of the electrical parameters in the power supply of the flashtube (6). EXPERIMENTAL
The principal component of the flashvaporization ma,ss spectrometry system is a Bendix Model 14-107 time-offlight mass spectrometer. Coupled to this is a flashtube thermal-radiation unit) which has been fully described elsewhere (6); it is comprised of a General Electric type FT-625 xenon flashtube surrounded by an aluminum reflector and mounted in a compact enclosure which is readily attached to the mass spectrometer. Adaptation of the flash-vaporization technique and solid-inlet system to the mass spectroniet,cr required no niodificrttions in the Bendix instrument itself; but rather, it required an appendage which included the following features : sample positioned where it can be irradiated from an external source of light (in this case it had to be located within a glass tube which inserts into the helix of the flashtube), line-of-sight path between sample and ionizing electron beam, close proximity between sample and electron beam-significantly less than a mean-free path, sample chamber contiguous with high vacuum of ion source, and vacuuni lock to expedite the introduction of samples into the instrument. In order to introduce a solid sample into the vacuum of the ion source, 542
ANALYTICAL CHEMISTRY
the instrument must be shut off electrically and the system brought up to atmospheric pressure during the transfer unless some kind of vacuum lock is employed. For this reason a combination sample tube and vacuum lock was incorporated onto the Bendix mass spectrometer by replacing the lower blank-off header on the ion source housing with a 2-inch gate valve and the glass sample tube attachment as shown in Figure 1. The 3/4-inch quickconnect vacuum coupling (Titon O-ring) permits easy removal of the glass tube to simplify sample reloading, and lipless glass test tubes serve as convenient sample tubes. The sample rests on the end cf a vertical, slender quartz rod so that when the flashtube is positioned coaxially around the glass tube, the sample is irradiated from all sides. Before the gate valve is opened, the body of the valve and the sample tube are evacuated by an auxiliary pumping system which is comprised of a mechanical pump with a molecular sieve foreline trap and a 5-liter per second getter-ion pump. The approximately half-millisecond thermal pulse vaporizes the sample directly into the ionizing beam from whence the resulting ions are immediately mass analyzed About 10 yg, (of cellulose) is the maximum quantity of sample which can be instantaneously vaporized into the Bendix model 14 without raising the internal pressure of the mass spectrometer above its operating limit. Incidental to this work the vacuunilock inlet has been of considerable value for introducing samples in other than flash-heating applications-i.e,, with materials which can be vaporized at lower temperatures. For this ap-
plication the sample is placed a t the bottom of the glass tube which is then evacuated, the gate valve is opened, and the tube is warmed by an external heater to a temperature a t which the vapor pressure of the material is adequate to obtain a mass spectrum. By attaching a thermocouple to the sample tube, we have extended the usefulness of the instrument to include slow, low-temperature pyrolysis studies and mass spectrometric thermal analysis (?\ITA) (5) where the gaseous products are individually identified and recorded as a function of temperature. These results can give much more information than thermal gravimetric analysis, especially for polymer decompo-itions. For the recording of flash mass spectra, oscilloscope presentation of the spectrometer output is almost always required, because of the short interval of time that the sample gases are in the ion source. This is easily recorded by a scope camera equipped with a flash-synchronized shutter. While the mass spectrometer is running continuously and the scope is displaying any spectra present (this, incidentally, monitors background spectra during degassing of the sample), the camera shutter is actuated, thereby triggering the flashtube power supply which, in turn, fires the flashtube. Thus, during the time the shutter is open the mass spectrum of the vaporized sample appears on the scope and is photographically recorded. DISCUSSION
-4 typical mass spectrum of a flashpyrolyzed polymer obtained by this technique is shown in Figure 2. This is, of course, time-integrated over the brief interval of the event and is quite adequate for most applications of this type. Although oscilloscopic presentation is not as quantitative as the more typical strip chart presentation. it is evident' from the photograph that
relative peak heights can be established easily within a few per cent. Figure 3 is an expanded portion of the mass spectrum from flash-heated CdS showing the Cd isot'opes. I n so far as is discernible, the resolution of the mass spectrometer is not affected by the abrupt heating of the sample within the ion source. Flash heating for mass spectrometry is, of course, not 1imit.ed to flashtube thermal sources or to the particular ionsource configuration outlined here. Other possible radiant thermal sources include shuttered carbon arcs and highintensity incandescent lamps, but the most promising extension of this work lies with pulsed lasers for flash heating. The coherent beam from a laser can be focused to concentrat,e the energy on a small area; therefore, the sample need not be prepared to a particular size, shape, or color to reach even higher temperat'ure in a small spot and readily vaporize enough material to produce a spectrum. Thus, a unique method is practicable for performing analysis on minute areas of comparatively large pieces of materials. This concept has been carried out by Honig and Woolstron (3) who have incorporated a laser with the ion source of a mass spectrograph to produce spectra recorded on photographic plates and by Berkowitz and Chupka (I) who have used a laser with the same type of instrument except for an electron multiplier out'put which permits them to record one mass peak
detecting thermally-produced shortlived products of solid materials. The subject of time-resolved mass spectrometry (wherein the time sequence in which the various m a b b peaks occur is manifested) and the methods for achieving it have been delineated previously ( 7 , f 2 ) . LITERATURE CITED
Figure 3. Portion of mass spectrum of flash-heated CdS displaying relative intensities of cadmium isotopes
(1) Berkowitz, J., Chupka, W. A, J . Chem. Phys. 40, 2735--6 (1964).
U. 9. Dept. bf Commerce, 1'346. ( 3 ) Honig, It. E., IVoolstron, J. I
