w Temperature Probe for Direct introduction of W. F. Haddon, E. M. Chait, and F. W. McLafferty, Department of Chemistry, Purdue University, Lafayette, Ind. 47907
s
are now available on most conimercial mass spectronieters €or direct introduction and vaporization of the sample in the ion source. I n addition to their applicability to samples of low volatility, a major advantage of such systems over those using conventional heated reservoir vaporization and molecular leak introduction is an enhancement in sensitivity of several orders of magnitude. However, for samples of high volatility, direct introduction is unsatisfactory because of the high rate of vaporization; most organic compounds having molecular weights below 250-300 fall into this class. An obvious way to reduce the vaporization rate is to cool the sample within the spectrometer. AIcGee (2) has designed a sophisticated apparatus for transferring highly reactive species at low temperatures. This paper describes a low temperature probe suitable for submicrogram quantities of volatile samples and interchangeable with a conventional heated probe. A common source of these submicrogram samples is the column-packing collection method o€ Amy and coworkers ( 1 ) for gas chroniatographic effluents. The cooled probe has additional utility for routinely obtaining mass spectra from compounds sensitive to USTEMS
thermal degradation or catalytic decomposition. For those samples sensitive to catalytic effects, the availability of such a probe eliminates need for a conventional all-glass reservoir introduction system for sample identification. EXPERIMENTAL
The probe body (Figure 1) is made of 0.25-inch 0.d. stainless steel tubing for compatibility with the 21-086 direct sample introduction system of the CEC 21-llOR mass spectrometer. The same probe fits a eimilar system constructed for our Bendix T-0°F mass spectrometer. Nitrogen gas, cooled through a 12-ft. coil of 0.25-inch copper tubing in a Dewar vessel filled with liquid nitrogent passes through an insulated line into the 0.125-inch stainless steel inner tube t o impinge directly on the bottom of the sample block, The block is welded to the outer tube t o produce a vacuum seal and is threaded t o receive the Ificalex insulator. This insulator is required when the mass spectrometer ion source does not operate at ground potential. The sample, contained in a melting point capillary, is cooled by conduction from the block. The temperature can be varied from
13 lXi0 I
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M I c A LE X " T I
(A)
ambient t o -140' C. by varying the rate of nitrogen flowv. X sample can be cooled to -140' in approximately 10 minutes at a flow rate of 5 liters per minute. The temperature is measured approximately by monitoring the resistance of a silicon diode mounted in the probe tip. The spent nitrogen passes through a plastic bag surrounding the probe shaft t o prevent condensation. I n operation the sample capillary is inserted into the probe tip and cooled at, atmospheric pressure under a nitrogen blanket in the vacuum lock housing. The vacuum lock eystem is then evacuated in the usual fashion, and the probe is inserted into the ion source. Reduction in the rate of nitrogen flow then allows the sample to warm until a sufficient rate of volatilization is reached; a steady ion beam current can usually be achieved with relative ease by hand control of the nitrogen flow. RESULTS
A 0,010-pg. sample of acetone was collected on column packing ( 1 ) from the gas chromatographic separation of tomato volatiles. Volatilization from the probe at -100' C. gave a well defined high-resolution spectrum using recording by either conventional scan-
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CH,CCOH
P
SAMPLE DIODE
I
1
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Figure 1. Schematic diagram of low temperature sample probe
1968
a
ANALYTICAL CHEMISTRY
1
Figure 2. Mass spectra of pyruvic acid on CEC 21 -1 1 OB spectrometer, sample temperature - 2 0 " C., source temperatures: (A) 70' C., (B] 150" C., (C)200" C. Some peaks of a few per cent relative abundance were observed up to m/e 200 in (C)
ning or photoplate techniques. A variety of similar volatile samples has been run routinely. It is not necessary to lower the ion source temperature for such samples. The 70-volt mass spectrum of pyruvic acid, obtained a t different ion source temperatures (Figure 2j, indicates the catalytic decomposition of this compound which can occur at elevated temperatures. This behavior would make a conventional metal reservoir system unsatisfactory for such a compound. The spectra shown were obtained by varying the ion source temperature while maintaining a constant sample temperature of -20’ C. and a constant
source pressure. Cooling the ion source from 200’ to 130’ C. and using the same sample restored the normal pyruvic acid spectrum (Figure 2Aj. These data show that significant transfer of thermal energy between the source walls and the molecular beam can occur, as has been noted previously (3). For samples that decompose at ambient temperatures it is necessary t o use an ion source of open construction, such as the source of the Bendix T-0-F spectrometer. The low temperature probe has general applicability for obtaining mass spectra from submicrogram volatile samples in a convenient fashion, avoiding thermal and catalytic effects on the
sample encountered with conventional inlet systems, and studying the effects of temperature on samples and on mass spectra over a wide temperature range. LITERATURE CITED
&I., Baitinger, TV. E., ICIcLafferty, F. W., ANAL.CHEM.
(1) Amy, J. W., Chait, E.
37, 1265 (1965). (2) McGee, H. A., hlalone, T. J., Martin, W. J., Rev. Sci. Instr. 37, 561 (1966). (3) Spiteller-Friedmann, hl., Eggers, S., Spiteller, G., Monatsh. Chem. 95, 1740 (1964). PRESENTED in part at the 14th Annual hIeeting on Mass Spectrometry, ASTILL E-14, Dallas, hIay 1966. Work supported by a research grant from the National Institutes of Health (GM 12’7%).
Sample Preparation for Low-Level, Alpha-Particle Spectrometry of Radium-226 Bernard Keischl and Arnold S. Levine, Nuclear Science & Engineering Corp., Pittsburgh, Pa.
PARTICLE
spectrometry with solid-state detectors is an excellent method for measuring low levels of alpha radioactivity (1j because very low backgrounds can be obtained by observing only those energies of interest. However, in order to obtain meaningful spectra, sample thickness must be kept to a minimum. In our laboratories, attempts to prepare “weightless” counting samples of radium-226 by means of removing all other solids from a solution and evaporating the solution on a counting planchet were unsuccessful. At best, small deposits of solid material weighing several milligrams were obtained, even when using ion-exchange methods to purify the solution. Because of the low levels of activity involved, reduction of solids merely by taking small aliquots was unacceptable. Subsequently, a method was developed in which the radium-226 was coprecipitated with a very small mass (-0.1 t o 0.2 mg.) of BaS04 and filtered on a membrane filter. By using this method not only were good spectra and excellent radium-226 recoveries obtained, but in addition the descendant activities could be allowed to grow in because of excellent radon retention. In effect, this resulted in quadrupling the overall efficiency of the measurements. PROCEDURE
In our case, the sample solutions contain macroquantities of lead in a dilute nitric acid solution. A series of steps to remove the lead via chloride precipitation and sulfide precipitation precedes the final steps in the preparation of the counting sample. In other Present address, hIellon Institute, Pittsburgh, Pa. 15213.
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Figure 1. Alpha-particle descendants
cases, any other metal ions capable of precipitating as the sulfate would have to be removed. The resulting leadfree, dilute nitric acid solution of about 20 ml. in volume is then evaporated to a small volume, transferred to a 5ml. beaker, and evaporated just to dryness. Two to three milliliters of 1N nitric acid are added to redissolve the material, and 0.1 mg. of barium carrier (as the nitrate) is added. The solution is warmed, a few drops of 10% sulfuric acid are added to it, and the mixture is digested for 15 niinutes at a temperature just below boiling. After cooling, the solution is filtered through a membrane filter (1.2-micron pore size) in a special holder, and the filter is then mashed with a few milliliters of 1% sulfuric acid and a few milliliters of water. The holder consists of a filter-support disk (-25 mm. in diameter), to which the edges of the filter are lightly ce-
spectrum of
Ra-226 and
its
mented, and a chimney which fits closely over the disk. The filter support and chimney are products of Control Molding Corp., Staten Island, N. Y. The all-but-invisible precipitate of Bas04 is collected on the surface of the filter, and self-absorption of the emitted alpha particles is thus minimized. After drying under a heat lamp, the filter on its support disk may be stored for a t least two weeks if it is desired to allow the ingrowth of the radon, polonium, and bismuth descendants to approach equilibrium. An alpha-particle spectrum shows the four main peaks (Figure 1): Ra-226 a t 4.8 Mev., a t 5.5 Mev., Po-218 a t 6.0 Mev., and Po-zld a t 7.7 Mev. RESULTS
The observed counting rates for each peak in the alpha spectrum of a typical low-level radium standard (prepared VOL 38, NO. 13, DECEMBER 1966 e
1969
