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Anal. Chem. 1986, 58,671-672
Table I. Elemental Compositions Calculated for Ions Producing Folded-Back Peaks apparent mass, amu
true mass, amu
calcd mass, amu
deviation,"
kHz 795.651 646 781.189830 766.200 324 734.712 766 700.318891 682.184 055 508.229 795 481.416570 423.631 839
57.694 301 58.762 285 59.911 783 62.496 214 65.547 393 67.289 713 90.318468 95.348 243 108.352 137
57.070 961 56.062 952 55.054 866 53.039 321 51.023 612 50.015 521 42.046 887 41.039 050 39.023 265
57.070 425 56.062 600 55.054 775 53.039 125 51.023 475 50.015 650 42.046 950 41.039 125 39.023 475
-9.40 -6.30 -1.65 -3.69 -2.68 1.98 1.50 1.83 5.39
apparent freq,
a
PPm
Average deviation: 3.82 m m .
Nine fragment ions in the mass range 39-57 amu were identified with an average error of 3.82 ppm. These results show that it is possible to calculate the elemental composition for an ion outside of the detection bandwidth that is folded back into the spectrum, based on the detection bandwidth and the apparent frequency of the folded-back peak.
(2) Wilkins, C. L.: Gross, M. L. Anal. Chem. 1981, 53, 1661A. (3) Johlman, C. L.; White, R . L.: Wilkins, C. L. Mass Specfrom. Rev. 1983, 2, 389-415. (4) Gross, M. L.; Rempel, D. L. Science 1984, 226, 261-268. (5) Horlick, G.; Hall, R. H.: Yuen, W. K. I n "Fourier Transform Infrared Spectroscopy": Ferraro, J. R., Basile, L. J., Eds.: Academic Press: New York, 1982. (6) Ledford, E. B., Jr.; Rempel, D. L.; Gross, M. L. Anal. Chem. 1984, 56, 744-748.
LITERATURE CITED (1) Comlsarow, M. B.; Marshall, A. G. Chem. Phys. Lett. 1974, 25, 282.
RECEIVED for review May 30,1985. Accepted October 22,1985.
Miniature Mercury Contact Switch for Chromatographic Applications Thomas J. Bruno* and J e r r y G. Shepherd Thermophysics Division, Center for Chemical Engineering, National Bureau of Standards, Boulder, Colorado 80303 Temperature control for the components of gas chromatographic instrumentation is achieved in a variety of ways. Depending on the level of accuracy required, contact switches, thermocouples, thermistors, and resistance devices are commonly used to control the power applied to heaters (1, 2). Thermistors and resistance devices are well suited to proportional control of the power input. Contact switches and thermocouples are easily applied to bipolar (on/off) control of heating elements. All of these devices have their inherent limitations. For example, thermistors are usually limited to a maximum working temperature of 150 "C (423 K). Thermocouples are subject to aging effects and become brittle and difficult to handle after operation a t high temperature. The control circuitry associated with the use of thermistors and resistance devices is relatively expensive, especially considering that many chromatographic applications require upward of a half dozen or more separate temperature environments or zones. Mercury contact switches (contact thermometers) have been used extensively for the control of chromatographic column temperature, especially in physicochemical measurement work (3). The circuitry required for these on/off switches is inexpensive relay devices. For these applications, the column thermostate usually consists of a vigorously stirred liquid bath, controlled by the contact switch (and solid-state relay circuit) to within fO.01 K. The large size and heat capacity of commercial mercury contact switches have limited their use in most other applications. In order to overcome some of these limitations and apply the contact switch to the 400 to 525 K region, we have designed and fabricated a miniature, high sensitivity fixed contact (nonadjustable) switch, as shown in Figure 1. The switch was
TUNGSTEN -CONTACT
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MERCURY
I
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I
-.- _ _ - _ - - _ _ _ _
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7
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L ' I L
Figure 1. Schematic diagram of the minature mercury contact switch.
made from borosilicate glass tubing (of 1.0 mm o.d., 0.3 mm i.d., and 5 cm length) and incorporates tungsten contacts. The bulbs at each contact were made of uranium glass to grade the seal to the contact. These bulbs also provide for the expansion of the mercury, with the upper bulb serving as an overflow. This overflow volume prevents the unit from shattering in the unlikely event of a large temperature overshoot or circuit failure. Each switch is constructed so that it will complete the contact circuit a t a specific temperature. This control temperature can be set (to within 2 K) by heating the unit in a silicone oil bath to the desired set point. The excess mercury in the upper bulb is then removed, and the unit is evacuated and sealed. Alternatively, for low-temperature operation, a porous sintered glass frit may be sealed in place. The switch will then control at the set point to within f0.05 K. In practice, the contact switch is potted in a small pellet of a
This article not subjed to US. Copyright. Published 1986 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 3, MARCH 1986
specially formulated aluminum/silicon oxide ceramic ( 4 ) . This pot provides good heat conduction and makes the switch more durable. This is desirable since the glass tubing is quite fragile. The contact switch has been used to control the temperature of a chromatographic sampling system at 400 K ( 4 , 5 ) . In this temperature region, the performance of thermistorbased temperature controllers can be marginal. The circuit (which responds to the contact switch) required to supply power to the heating elements is a simple solid-state relay constructed for this application (6). The principal limitation of the switch in its present configuration is the inability to adjust the set point once the upper bulb is evacuated and sealed. Thus, a separate switch is needed for each desired set point. This is not a serious problem for operations involving long-term isothermal operation, but an adjustable unit would be useful. Modifications
along these lines are currently being pursued. Registry No. Hg,7439-97-6.
LITERATURE CITED Vassos, B. H.; Ewing, G. W. "Analog and Digital Electronics for Scientists", 2nd ed.; Wiley: New York, 1980. Quin, T. J. "Temperature"; Academic Press: London, 1983. Bruno. T. J.; Martire, D. E.: Harbison, M. W. P.: Nikoiic, A.; Hammer, C. F. J . Phys. Chem. 1983, 8 7 , 2425. (4) Bruno, T. J. J . Chromafogr. Sci. 1985, 2 3 , 325. (5) Bruno, T. J. J . Res. Naf/. Bur. S t a n d ( U . S . )1985, 90, 127. (6) Frederick, N. V., McFarlane, J., private communication, 1985.
RECEIVED for review September 11, 1985. Accepted October 14, 1985. The financial support of the Gas Research Institute and the U S . Department of Energy, Office of Basic Energy Sciences, is gratefully acknowledged.
