A Solids Inlet System for a Mass Spectrometer H. E. Lumpkin and G. R. Taylor, Research and Development Division, Humble Oil and Refining Co., Baytown, Tex. of analytical mass spectrometry samples were received primarily in the gaseous state and were transferred from the glass or metal bomb to the instrument through an evacuated connecting line. The need for analyzing materials in the normally liquid state led to the use of capillary pipets (4) and hypodermic syringes with the associated orifice systems, fritted disks, and rubber caps. Melting of very viscous or solid samples and entry as a liquid with pipets through gallium-covered fritted disks into heated inlet systems (3) was, and still is, the common practice. However, high melting point materials tend to resolidify in the pipet during the transfer operation, making sample charging difficult, time-consuming, and nonreproducible. In addition, there are some solid materials subject to mass spectrometer analysis which either sublime or decompose on heating in attempts to charge them as a liquid. Although solids can be charged with the inlet system described by Caldecourt ( I ) , the presence of Teflon limits the maximum temperature to 250’ C. To overcome these difficulties, an all-glass configuration, which can be heated to about 500’ C., and which allows low vapor pressure solid materials to be introduced into the mass spectrometer without breaking vacuum, has been designed and used in these laboratories for over a year. N THE EARLY DAYS
niques are required to make the glass sample holders. The 14/a5 joint i s used occasionally for the introduction of gas samples when it is not desirahle to use the regular gas inlet system. The only servicing of the equipment necessary is occasional cleaning or replacement of the gallium. This is required about once each week or when the gallium becomes contaminated or dirty. In the fabrication of the dry ball valve, the complete assembly shown on the left side of Figure 1 is made prior to initiating any grinding operations. The outer valve body is then cut or sawed about midway between the lower part of the male 28/12 joint and the ring seal. Hand grinding and polishing with successively finer grades of grinding compounds, ending with rouge or cerium oxide, lead to a lustrous polish. In a properly polished valve, concentric colored interference fringes can be seen in white light when the joints are mated. After the top and bottom portions of the valve body are butt-sealed, the valve may be tested for leakage by attachment to any convenient vacuum system. With an atmosphere across this valve a pressure of less than 1 micron in the reservoir is maintained by the pumping system. Applications. An example of the use of the solids inlet system is the analysis of aromatic oxidation products for dicarboxylic acids and
EXPERIMENTAL
Apparatus. A scale drawing of the solids inlet system is shown in Figure 1. The right part of the apparatus is attached to the side of the inlet cabinet and the 28//12 ball and socket valve is in a thermostated heated oven with the reservoir and pump valve. The line from the oven and the exterior apparatus is wrapped with heating tape and its temperature can be controlled independently. In charging a sample the z8//12 ball joint is closed magnetically and the exterior portion is vented. The 36/25 greased ball joint is opened and a preweighed sample (ca. 1 to 2 mg.) in a small glass cup is placed in its holder. The system is closed, and evacuated with house vacuum, and the dry ball valve is raised to complete evacuation. Raising the sample into the heated zone concurrently makes an annular gallium seal around the sample holder and, as the sample is heated, the vapors expand into the reservoir. No special tech476
ANALYTICAL CHEMISTRY
Figure 1.
associated materials. These compounds either have high melting points or sublime on heating prior to melting. The spectra of terephthalaldehyde, terephthalaldehydic acid, and terephthalic acid, given in Table I and obtained using the solids inlet system (250’ C.), have been used in the analysis of oxidation products. The spectrum of terephthalic acid will bear additional comment. We had been unable previously to secure a spectrum
Table 1. Partial Mass Spectra of Oxygenated Compounds
mle
74 91 93 104 105 121 133 134 149 150 166
S/STolc
TereTerephthalTerephthal- aldehydic, phthalic aldehyde, Acid, Acid, Aa Am K‘ 18.3 0.61 4.14 55.7 84.5 100
1.05
20.0 1.10 3.48 5.66 21.9 26.5 11.8 3.96 100 81.7 0.89
18.9 5 04 4.31 4.84 3.63 27.3 0.43 0.61 100 9.72 86.2 0.38
-4= Aldrich Chemical Co. b K = K and K Laboratories. c Sensitivity of base peak in divisions per milligram referred to m/e 92 of toluene = 2384 div./mg. a
of this material in attempting to charge it as a melt or in various solvents. Our spectrum differs considerably from that of McLafferty and Gohlke ( 2 ) . Whereas they obtained a sensitivity for the base peak, m/e 149, of only 3% of that of the m/e 92 from toluene, our value is 38%. The sensitivity of the parent in our spectrum is 86% of that of the base, while McLafferty obtained 18%. McLafferty employed the glass, Teflon, and stainless steel inlet system described by Caldecourt (1) which operates a t about 1-mm. pressure. It appears probable that the low sensitivity values they obtain for iso-
and terephthalic acids result from thermal decomposition and/or incomplete sample vaporization. As they indicated (Z), terephthalic acid does decompose a t 2.50’ C. R e find the products of decomposition to be benzoic acid and COe, proceeding a t a rate of about 1% per minute.
We thank J. L. Taylor and Louis LeBlanc for their general assistance, suggestions, and testing work during the design and fabrication of the glass equipment.
Additional applications of the use of the solids inlet system include direct pyrolysis studies and the determination of more volatile components deposited on or in admixture with more refractory materials.
(1) Caldecourt, V. J., ANAL. CHEM. 27,1670 (1955). ( 2 ) McLafferty, F. W., Gohlke, R. S., Ibid., 31,2076 (1959). (3) O’hseal, M. J., Jr., Wier, T. P., Jr., Ibid., 23,830 (1951). (4) Purdy, K. M., Harris, R. J., /bid., 22, 1337 (1950).
ACKNOWLEDGMENT
LITERATURE CITED
Construction of Thermopiles from Fine Wire Clyde A. Glover and Ruth R. Stanley, Research Laboratories, Tennessee Eastman Co., Division of Eastman Kodak Co., Kingsport, Tenn.
A
paper on an apparatus for the ebulliometric determination of the number-average molecular weights of polymers (3) described the use of an 80-junction thermopile as a temperature-sensing device. The development of the apparatus required construction of a suitable thermopile which could be installed in the ebulliometer. Although several methods have been described for construction of thermocouples (1, 2, 4, 8) and of thermopiles for special purposes (5-7), none were applicable t o this problem. The method described here not only met the requirements of the immediate problem but has been used to make thermopiles containing more than 80 junctions, and is applicable in general to the construction of thermopiles from fine wire. RECEKT
APPARATUS
Only tivo devices which might be considered special w r e used in the construction of the thermopile. The first consisted of a small screw driver, mounted horizontally in a short section of glass tubing, and a small magnet.
A Guide
B
Figure 1. struction
‘copper
Form for thermopile con-
This device was used to twist the pairs of wires prior to welding them. The second device, used for welding the twisted wires, consisted of two sharpened carbon electrodes (ordinary lead pencils) mounted vertically so as to give a spark gap of approximately ‘/8 inch. A current of 7600 volts a.c. a t 18 ma. was supplied to the electrodes and controlled by means of a tap key in the primary circuit. It was convenient in practice to mount the electrodes, along with a 20-power magnifier, on a micromanipulator. PROCEDURE
In the diagrams pertaining to construction of the thermopile, the scale is distorted to show detail better. First, a form was made from three pieces of thin cardboard, cut as shown in Figure 1,A. The width of the outer pieces was about 1 em. less than the desired length of the thermopile. The inner piece was about 2 em. wider than the outer pieces. All pieces were the same length. The length can be varied according to the number of junctions desired. The three pieces were placed together and taped, with ordinarj- cellophane tape, as shonn in Figure 1,B. Copper wire (0.0015 inch in diameter) was wound in a continuous helix around the form, with sufficient escess nire a t the ends to serve as lead wires. Kext, guides of some suitable material, usually a fine wire, were placed over the copper wire on both sides of the form and taped as shown in Figure 1,B. Constantan wire (0.0015 inch in diameter) was then wound in a continuous helix on the form over the guides, crossing the copper 11-ire in all cases a t the edge of the outer boards of the form (Figure 2,:4). Each crossover point of the wire was taped securely to the board on both sides of the form. All wires, except the leads, were clipped at the outer edge of the form. The pieces of the form were then separated and the inner piece was discarded. This left essentially two thermopiles in the stage shown in Figure 2,B.
The form was then mounted on a rigid support, and each pair of vires was twisted by placing the screw driver blade below the wires, which were then clamped mechanically to its surface by the small magnet. Actual twisting was accomplished by rotating the screw driver. Each twisted pair of wires was welded autogeneously; thus, simultaneously, the insulation was removed and a junction was formed. Welding could be done with a micro gas flame, but it was preferable to weld with a highvoltage electric spark of very short duration by using the device previously described. Two precautions ivere necessary in welding: Good contact had to be made between the metals; and the heating had to be sufficiently rapid to minimize oxidation of the !vires adjacent to the weld. Use of nitrogen, which was allowed to flow from a tube near the electrodes and surround the junction during welding, was helpful for this purpose. Continuity and resistance could be checked from either lead to any junction by making contact a t the junction through platinum-tipped forceps used to grasp the weld just tightly enough to establish electrical contact. All junctions were insulated with a Glyptal resin and either baked or air-dried until the resin became hard, The thermopile was then freed from
-\
-..-Copper
L’Conrtonton
A
B
Figure 2. Wire layout for thermopile construction VOL. 33, NO. 3, MARCH 1961
477
