outlets for some important signals: 1-MHz oscillator, PG clock out, DM clock out, and PG and DM overflow flag signals can be accessed through the front panel.
I
I
0.1I e
eI
Flgure 3. Front panel display showing frequency LEDs (DM frequency = lo5Hz, PG frequency = 10' Hz), status display (count is Down, mode is Sef, PG flag is Clear, and DM overflow is Set), count output (5876),
and banana jack outputs
positions for: (1)loading the PG counter preset, (2) setting the PG frequency, (3) setting the DM frequency, and (4) setting the PG conditions of Mode and Count Direction. BCD thumbwheel switches S2-S5 provide the data for frequency and preset controls, and spst switches S6 and S7 provide Mode and Count Direction data. The circuit design is such that all control data are presented to the main clock at once, but only the data chosen by the load select is accepted. As mentioned previously, each function is controllable through the input bus. In manual control, however, it is not possible to initialize both DM and PG clocks and to clear all flags synchronously using the individual functions, so a Start function unique to the manual control system joins these functions under control of PB2 and switches S8 and S9 (Figure 2B). The switches are to select whether the PG or DM clock is to be started, or both simultaneously. Pushing the Start pushbutton carries out the reset operation for the chosen clock(s). The front panel (Figure 3) has a visual output of the clock states, showing the count in the output latch through four 7 segment LEDs. (A circuit similar to that of Figure 2B is provided for manual updating of the output latch.) The frequency for each clock is shown by the on state of one of nine LEDs per clock. PG status of Count Direction, Mode, and PG and DM flag states are also indicated by individual LEDs. The front panel also provides convenient banana jack
CONCLUSION The lab clock described can provide considerable flexibility in choice of timer-computer-experiment interactions. The input bus and control design is not dedicated to a specific computer, but can be easily interfaced to any computer or microprocessor. The basic design for the heart of the clock is not unique, but is based on previous designs (largely unpublished) for computer mainframe clocks (2-4). It is the versatility of interaction which makes our clock unique and which amplifies its utility. The provision for human interaction is a step forward in clock design, not backward. In laboratories involved in development of new instrumentation, manual intervention is often highly desirable, especially during initial stages of development or during testing. Our design allows manual control and observation of every clock function. Experimental interaction allows things not possible with operation of mainframe clocks-for example 1-ps and 10-ps time scales are useless on a mainframe clock because of computer cycle time considerations. Most experiments, however, make direct use of the Start and Stop commands, and thus the high frequency selections can be used advantageously. If the experiment is sufficiently complex, data lines can also be controlled by the experiment. With a total parts cost including power supply of approximately $300, the circuit described is within the budget of any laboratory. Detailed circuit diagrams are available from the authors. LITERATURE CITED (1) B. K. Hahn and C. G. Enke, Anal. Chem., 45, 651A (1973). (2) B. K. Hahn, Ph.D. thesis, Michigan State University, East Lansing, Mich., 1974. (3) E. J. Darland, R.D. thesis, Michigan State University, East Lansing, Mich., 1977. (4) T. Last, W.D. thesis, Michigan State University, East Lansing, Mich., 1977.
RECEIVED for review March 11,1977. Accepted May 9, 1977. This work was supported, in part, by a grant from Research Corporation.
Unheated External Column and Inlet Modification of a Gas Chromatograph for Phosphine Determinations Jerry R. Bean" and Robert E. White2 U S . Fish & Wildlife Service, Wildlife Research Center, Denver, Colorado 80225
The gas chromatographic detection and measurement of phosphine (PHJ due to hydrolized zinc phosphide (Zn3P2) has been carried out in this laboratory for a number of years, serially using the methods of the Food and Drug Administration ( I ) , Robinson ( 2 ) ,Hilton (3),and Okuno (4). A recent modification to the gas chromatograph used in three of these procedures has increased its utility as a dual-detector instrument (flame photometric and microcoulometric). Before this modification, the column oven temperature was lowered from 190 "C to about 40 OC for phosphine analyses which precluded the use of the columns in the microcoulometric system for chlorinated hydrocarbon analysis and the flame Present address, US.Fish & Wildlife Service, 238 E. Dillon St., Pocatello, Idaho 83201. Present address, International Joint Commission, 100 Ouellette Avenue, Windsor, Ontario, Canada N9A 6T3. 1468
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
photometric detector columns for other organophosphorous insecticide detection. This drastic temperature change frequently resulted in leaks (especially with glass columns) due to expansion and contraction of ferrules used to seal the columns a t the column inlet, long waits for instrument equilibration, and inability to utilize both detectors concurrently. By simply adding another injection inlet and a 6-mm X 46-cm aluminum column outside the column oven, the phosphine measurements can still be made while column oven temperatures for microcoulometry and flame photometric detection of the commonly occurring insecticides are maintained. MODIFICATIONS The gas chromatograph used in our laboratory is a Microtek MT-220 (Tracor Instruments, Austin, Texas) with two detectors and four columns in a common oven. Two column
fa
dJ
-
PYROLYSIS FURNACE
1
MICROCOULOMETER TITRATION CELL
FLAME PHOTOMETRIC ROTAMETERS
4 - PORT VALVE
-GC-2
INLET
-ROTAMETER
I
1-
I -FLOW
CONTROLLER
-EXTERNAL
Figure 1.
COLUMN
Carrier flow arrangement after modification
04 yr
02
Figure 2. Phosphine chromatogram obtained with 6-mm aluminum column packed with Chromosorb 102
X 46
cm
effluents are attached through a combiner to a Dohrmann pyrolysis furnace and microcoulometer system terminating a t a titration cell. This system is used oxidatively for the detection of chlorinated hydrocarbons. The other two column effluents (before modification) were not combined but each was attached to a separate port of high temperature, four-port valve. The third port was used as a vent to atmosphere and the fourth connected to the flame photometric detector. The valve helped select the column to be used in the analysis and vent solvent which could extinguish the flame in the detector. The diagram in Figure 1shows the carrier flow arrangement after the modification. In modifying, we combined the heated column effluents going to the flame photometric detector and attached this combiner to the four-port valve. The second and third ports were left vented to atmosphere and attached to the detector, respectively. The fourth port was attached to another column which is the major modification described here. A tee was placed in the carrier gas (nitrogen) line to the instrument and connected in series with l/s-inch copper tubing to a rotameter and a flow controller. The rotameter and controller were attached to an external panel beside the instrument’s column oven. An injection port from another instrument (Beckman model GC-2) was then attached to this
panel and the inlet line connected to the controller. This injection port was attached to and provided support for a chromatographic column placed in the space between the column oven and the external panel. The fourth port of the four-port valve was then connected to this column. When the heated columns to the flame detector are needed, the flow in the ambient temperature column is vented with the 4-way valve. Selection of one heated column over another is accomplished by reducing flow to one and increasing it appropriately to the other. Since column temperatures for microcoulometric and flame photometric detection are similar, as we use them, no conflict of use occurs and these systems now can be used simultaneously.
DISCUSSION Figure 2 illustrates a phosphine chromatogram obtained with a 6-mm X 46-cm aluminum column packed with Chromosorb 102. Phosphine will successfully chromatograph at ambient temperatures. This allows us to have a fifth column outside the heated oven with an unheated inlet and utilize two detectors concurrently without costly time interruptions. This modification should be feasible for most other models of gas chromatographs. When analyzing compounds like phosphine that can be readily detected and chromatographed without heating, such an addition can provide more efficient instrument use. LITERATURE CITED (1) Hazleton Labs, Inc., in “US. Food and Drug Administration, Pesticide Analytical Manual”, Vol. 11, Pesticides Reg. Sec. 120.235 Aluminum Phosphide Method A, 1968. (2) W. H. Robison and H. W Hilton, J. Agrlc. Food Chem., 19, 875-878
(1971). (3) . . H. W. Hilton and W. H. Robison. J . Aarlc. FoodChem.. 20, 1209-1213 (1972). (4) I. Okunc, R. A. Wilson, and R. E. White, Bull. Envlron. Contam. Toxicol., 13, 392-396 (1975).
RECEIVED for review April 15, 1977. Accepted June 6, 1977.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 9, AUGUST 1977
1489
