FOR ANALYTICAL CHEMISTS I
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Simple, Inexpensive Diverter Valve for GWMS Systems R. L. Wolen and H. E. Pierson Lilly Research Laboratories, Indianapolis, Ind. 46206
Increasing utilization of chemical ionization (CI) gas chromatographylmass spectrometry (GC/MS) in which the entire effluent of the gas chromatograph is routed to the ion source has led to the need for the development of an effective diverter valve. This device would permit the diversion of the gas stream during the solvent front elution and, a t other times, when substances that are of no interest to the operator are being eluted. The inability to vent during these periods results in rapid deposition of residue on the surfaces of the source and associated structures, such as quadrupoles, and results in excessive downtime for cleaning and loss of sensitivity. Commercial high temperature valves are available and a design has been reported by McFadden (1) but these do not adapt themselves readily to automation. They require materials which must withstand the high temperatures of a transfer oven and, because of construction, increase the potential contact of gas phase materials with hot surfaces. The latter may result in degradative processes, thus producing unsatisfactory performance of the analytical procedure. We would like to describe a simple mechanism which permits effective carrier venting, is compatible with automation, and can be built in less than half a day by skilled shop craftsmen. The device, shown in Figure 1, consists of a modified bulkhead union (Swagelok ss400-61) designed to accept Ih-in. tubing. The fitting is modified, as shown, to provide a narrow (I/S in.) internal shoulder under the body hex portion. The body hex area is drilled to accept a Yg-in. steel vent tube which has been formed to permit the free end to exit from the oven conveniently with the device in place in the transfer line. The hole in the body hex for the tubing is drilled into the central chamber as shown. The vent tube is welded into position on the body hex. All materials in our diverter are 316 stainless steel. Installation in a CI system is shown in Figure 2 with the lines leading from the GC and to the MS being firmly seated against the inner shoulder prior to tightening the ferrule nuts. The vent line is provided with either a manually (Whitey Research Tool Co.) or electrically controlled (ITTGrinnell Corp.) shutoff valve which in turn is connected by way of a Ik-in. tube to a vacuum source of at least 50 l./min. capacity. With the venting connection in place, it is possible, merely by opening the valve, to divert virtually all the effluent to the vacuum source without any additional valves or devices between the fitting and the mass spectrometer. A working analyzer vacuum of 1 X Torr will drop to 1 X lo-' upon opening the valve. The diverter permits transfer of virtually all effluent from the GC to atmosphere via the vacuum system without requiring any heat resistant valve parts or any moving parts within the heated transfer oven. Operation of the valve over several months has markedly decreased the need for source cleaning, has indicated no decrease in sensitivity, and has shown no operational problems. We have automated the opening and closing of the electrical valve so that a timer controls routing of the gas to the vacuum for a pre2068
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$ 5 7 ODT L,06913 125 BE
Figure 1. Vacuum diverter construction details The basic structure is that of a Swagelok union Tube Fitting (55400-61). Ail parts and tubing are 316 stainless steel
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MECHANICAL
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I Figure 2. Installation details of vacuum diverter Variations for adaptation to the particular instrument design will be required in many cases
set period following injection to vent the solvent peak and to again vent the carrier gas after peaks of interest have eluted. The modification further has permitted the examination of trace constituents by simplifying the bypassing of massive peaks which one would not wish to have enter the source while permitting MS examination of the smaller peaks upon injection of a larger than usual sample. Additional testing of the device by W. H. McFadden (Finnigan Instruments Corp.) has indicated its utility in instruments fitted with a jet separator and in GCIMS systems employing capillary columns in the gas chromatograph. Its usefulness in permitting examination of peaks normally obscured by solvent peaks and the minimal effect on peak spreading with carrier flow rates above 2 ml/min has also been demonstrated.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
During the period of development and testing of the present Kuehl et (2) described an automatic vent system utilizing a similar principle but consisting O f all-glass construction. We have not examined their system in the chemical ionization mass spectrometer and thus can offer no comparative data.
LITERATURE CITED (1) W. H. McFadden, R. Terinishi. D. R. Black, and J. C. Day, J. Food Sci., 28, 316 (1963). (2) D. W. Kuel, G. E. Glass, and F. A. Puglisi, Anal. Chem., 48, 804 (1974).
RECEIVEDfor review May 27,1975. Accepted July 3 , 1975.
Inexpensive Device for the Determination of Photomultiplier Current Gains N. W. Bower and J. D. Ingle, Jr.’ Department of Chemistry, Oregon State University, Corvallis, Ore. 9733 1
Since its advent, the photomultiplier tube (PMT) has been an unsurpassed transducer for UV-visible radiation with relatively noiseless amplification. If characteristics of the P M T such as the current gain, collection efficiency, and photocathodic responsivity are known, it is possible to perform absolute calculations so that signals can be reported in units of watts or photons per second instead of the somewhat less meaningful photoanodic current or voltage. This would greatly aid the intercomparison of work of different researchers. Knowledge of the current gain also makes it possible to calculate the fundamental noise in the photocurrent signal and ascertain if other noise sources are present. Although typical values of PMT parameters such as the gain are given by the manufacturer, there is considerable variability among individual tubes, and calibration by the user is mandatory for exacting work. Previously, procedures for measurement of the P M T gain have been somewhat tedious or require special instrumentation. In this paper, we describe the construction of a simple apparatus for evaluation of the P M T gain. The materials to construct the apparatus cost less than fifteen dollars and the procedure for current gain determination takes less than five minutes.
BACKGROUND AND THEORY The basic equations that relate the measurable photoanodic current, photocathodic current, and photoanodic pulse rate in the P M T to the radiant power incident on the photocathode are shown in Table I. The equations and variables in Table I are discussed in more detail in another paper ( I ) . The incident radiant power ( P ) can be calculated from the current gain ( m ) ,the collection efficiency of the first dynode ( T ) , and the cathodic responsivity ( Q A ) . Equations are also presented for the anodic dark current (iad) and the shot noise in the dark current and photocurrent. Note that the fundamental shot noise can be determined if the P M T gain, collection efficiency, and the bandwidth constant is evaluated from knowledge of /If (from the electronic time constant) and an estimate of (Y (from the dynode statistics and dynode gains, 6, ( 1 - 3 ) ) . For these equations, it is assumed that photoemission occurs only from the photocathode and not the dynodes, and that photoelectrons that are not collected by the first dynode cause negligible secondary emission a t other dynodes. It is also assumed that the collection efficiency for secondary electrons between dynodes is one or is incorporated into the dynode gains (&), and that m and Q A are independent of the incident radiant power and constant over the incident wavelength interval. Finally, the dark current is taken to be independent of the incident radiant power and Author to whom correspondence should be addressed.
to originate solely from thermionic emission at the photocathode. Equations 8 and 9 show the relationship between the current gain and dynode gains (Si), and in Equation 9, all dynode gains are assumed equal. Usually, the PMT specification sheets give only typical and maximum and minimum values of the cathodic responsivity (sensitivity) ( Q A ) and the anodic responsivity (mQ,J. Since the anodic responsivity depends on the gain, it is specified a t one P M T voltage, or a plot of responsivity vs. P M T voltage is reported. The cathodic and anodic responsivities are evaluated with a calibrated light source such that the radiant power incident on the photocathodic surface is known ( I , 2). Usually the responsivities are measured at relatively high radiant powers and are assumed to be independent of the radiant power. Manufacturer’s data indicate that cathodic responsivity can vary approximately f100% from the typical value. The standard procedure ( I , 2, 4 ) for calculating the gain involves taking the ratio of the photoanodic current to the cathodic current measured under equivalent radiant power and biasing conditions. The photoanodic current is first measured with the P M T wired in its typical configuration. Then the dynode chain is rewired or the P M T is transferred to another housing and socket so that all the dynodes are tied together to the anode and used as the “new anode”. Then the voltage is adjusted until the cathodeanode potenti’al is the same as the cathode-first dynode potential in the normal PMT configuration. There are a number of disadvantages and assumptions to this standard technique ( 4 ) . First, it assumes a collection efficiency of one. Second, the incident radiant power cannot be too large or the photoanodic current will be in the nonlinear region (i.e,, the photoanodic current will not be proportional to the incident radiant power). Under conditions of high P M T gain, the photocathodic current may be too small and difficult to measure because of noise limitations. Third, elaborate and time consuming changes to the biasing network are often required. Fourth, if neutral density filters are used to attenuate the radiant power by a known amount for photoanodic current measurements so that the photocathodic current will be larger and easier to measure, then the accuracy depends upon the accuracy of the filter characteristics. Fifth, the two currents are not measured under equivalent conditions because of a time factor (Le., drifts in the calibration light source) or because the photocathode may be moved (differences in cathode responsivity with respect to position). One assumption of the standard procedure which was stated above is that the collection efficiency is one. Note that the ratio of photoanodic to photocathodic current as defined by Equations 1 and 2 is f m and, hence, the calcu-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 12, OCTOBER 1975
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