been placed on simplicity of construction, and there are no critical dimensions or indeed, any fixed dimensions. It has been used in the range of $10" to -70" C. with regulation t o = t O . l " C.; there is no reason a hy this could not be extended on either end. It has proved reliable in operation, and may be left unattended for several hours. The thermostat is shown in schematic cross section in Figure 1. I t is essentially a massive copper block in a n insulated box, with a spiral channel for cooling gas. Gas flow is regulated by the solenoid valve (Automatic Switch Company, Florham Park, S.J.) 1%hich is in turn actuated by the controller (Fenwal, Inc., Ashland, Mass., Model 56052 temperature-indicating controller). The valve may be remote from the thermostat, and its vibrations isolated by a rubber tubing SECtion in the gas line. Gas is cooled by passing through the copper tubing coil immersed in the refrigerant in the large Dewar flask. Dry ice-alcohol is suitable for temperatures down to -20"; liquid nitrogen is always satisfactory, and one filling lasts several hours once the thermostat is a t the desired temperature. With a little experience, it is easy to set the pressure regulator so that for any tem-
perature gas is flowing about half the time. (The controller includes visual indication of the on-off cycles.) I n case rapid raising of the temperature is desired, warm gas can be backflushed through the line at high velocity, emerging a t the opening shown closed with a rubber stopper. The cylindrical copper block is cut from bar stock and turned down in the lathe until it Kill just slip snugly into a piece of copper pipe. Leaving a little distance a t each end, a helical groove is cut around the block. Convenient dimensions are 3/16 inch deep, 3 turns per inch. A silver-solder seal is made between the block and pipe on the ends to ensure t h a t the "coil" is gas tight. Holes are drilled through the ends of the block t o reach the end turns of the groove, and are tapped to receive the threaded polyethylene tubing. The threads are made tight with neoprene cement. The holes in the block for the alcoholfilled thermometer and the thermistor probe are as close as possible to the cavity for the sample cell, as shown. A little alcohol in the thermometer well prevents condensation of atmospheric moisture from freezing the thermometer in place. Practically no construction of auxil-
Fold-Over Zero-Suppression Circuit for
iary equipment is required. The thermistor-probe control unit is comniercially available. There is no bath liquid, no stirring, and no pumps; in fact, there are no moving parts a t all, a t least in the immediate vicinity of the thermostat. This makes the device especially suitable for use with silicaspring balances, strain gages, and other vibration-sensitive equipment. Heat leak t o the room, n-hile low, is of such magnitude that no heating circuit is necessary. Reading the temperature on the thermometer is B concession t o simplicity; a thermocouple may be used if desired. LITERATURE CITED
(1) Allsop, H. L., Gibbs, D. F., J . Sei. Instr. 35, 395 (1958). (2) Bose, A., Indian J . Phys. 21, 275 (1947). (3) Fuschillo, N., Krautkopf, D. W,, Rev. S a . Instr. 28, 1060 (1957). (4) Mayence, J., J . phys. radium 12, 74450 (1951). (5) Morrison, J. *4., Young, D. ll,,Rev. Sci. Instr. 25, 518 (1954). (6) Ross, S., Clark, H., J . Am. C h e m SOC. 76. 4291 (1954). (7) Taylor,~W.J., Smith, A. L., Johnston, H. L., J . Opt. Soc. Am. 41, 91 (1951). (8) Ubbelohde, A. R., Woodward, I., Proc. Roy. Soc. (London) A185, 448 (1946). (9) Waldman, M. H., McIntosh, R., Can. J . Chem. 33, 268 (1955).
Use with Gas Chromatography
Lyle H. Hamilton, Research Service, Wood Veterans Administration Center and Department of Physiology, Marquette University School of Medicine, Milwaukee, Wis.
T
HE ACCURSCY O f peak height measurements in gay chromatography can be increased by shifting recorder zero with a bias (bucking) voltage introduced a t the input to the rccorder ( I , 2 ) . This report describes a zerosuppression circuit n-hich applies a bias voltage in stcpnise increments and re\ m c s the input polarity a i t h each stcp, so the rrcordc.d curve is folded back and forth on the. chart paper. The unit is available commrrcially from QuinTron Instrument Co., Inc., AMauker 18, Vis. 'The principle used in the circuit is h o n n in Figure 1. Snitcah position 1 is actually the "OFF" position, a t which the instrument has no effect on the signal from the chromatograph. K h e n the input signal from the chromatograph increaqes until it equals 1 mv. (the span of the recorder), the operator rotatrs the snitch to position 2 . This action produces two effects: The polarity is reversed, so that the 1-niv. signal is shifted to a point below the base line A ; and a zero-suppression voltage of 2 inv. is applied (as indicated by the heavy arrow in Figure I), to bring the
pen back t o the top of the chart -4'. As the signal from the chromatograph increases to 2 mv. B, the pen n-ill be driven back to the bottom of the chart B' because polarity is now negative.
K h e n the switch is rotated to position 3, the polarity is positive, and the 2niv. signal from the chromatograph C is cancelled by the 2-mv. zero-suppression voltage, so the pen will remain on
I
I
I
I
c7-2mv POSITION 1 Polarlty positlve; No ruppr. voltage
POSITION 2 Polarity negative ; 2mv ruppr. voltage
r f l c 0
1 I
,
-
:I
/ I mv. Figure 1. Principle used to provide a folded record of a 3-mv. signal on a recorder with 1 -mv. capacity
',\
B'
I
\\
*E \ N
A
+-
'C POSITION 3 Polarity positive ; 2 mv. suppr. voltage
I mv.
\
\
\
E '2mv
VOL. 34. NO. 3, MARCH 1962
445
-
3 4 mv.
INPUT
-
i’\
+ Gd
9 9 9
Chromatograph outpul
Signal wldth O t half-height
1
A,B,C 8 D ore sections of a double wafer, 4-pole, 6-position rotary switch
I 1,
level
0
, , - P 3 2
2
5
1
k
“200 ohm precision resistors matched
~
I I
(OFF1 (OFF) Figure 2. Chromatogram with peak height of 3.4 mv. recorded on a 1 -mv. recorder using the fold-over zero-suppression unit
the base line C’. The pen is now ready to move upward on the chart as the signal from the chromatograph increases furthcr. The procedure can be repeated as many times as necessary by reversing polarity with each step and adding 2-mv. steps of suppression a t alternate steps. This provides a folded record of the Chromatogram until the peak of the response curve is recorded.
A chromatogram with a 3.4-mv. peak height, recorded on a I-mv. recorder by shifting recorder zero and reversing input polarity. is diagrammatically illustrated in Figure 2. Although the area under the folded curve cannot be easily measured by integration or planimetry, its area can be estimated by multiplying peak height times the width at the half-height level which can be measured from the recorded chromatogram (see Figure 2). The circuit diagram for the fold-over zero-suppression unit is shown in Figure
2
3. A 4-pole %position rotary shorting switch of high quality is used-e.g., Centralab JV-9034. The bias voltage is adjusted to the span of the recorder with a 0.25-megohm potentiometer (for 1- to 5-mv. recorders). Matched resistors are used on the switch so that the increments of zero-suppression will be the same for each step. Precision resistors, +0.05%, can be used, or a set of less expensive resistors can be matched with a potentiometer and selected so that they introduce the same increments of bias voltage when the switch is rotated. The zero-suppression unit is connected in the line between the chromatograph and its recorder so that the original polarity is maintained when the switch is in the “OFF” position. It is calibrated by producing esactly one fullscale deflection of the recorder pen (with the balance adjustment of the chromatograph), turning the zero-suppression switch to its first suppression position, and adjusting the 0.25-megohm potentiometer until the pen does not
3. 4.
446
0
3
4
Chromatograms recorded with and without zero-suppression circuit 1. 2.
20.40% 02, attenuation X 3 2 , without zero-suppression circuit 20.94% 02,attenuation X 3 2 , without zero-suppression circuit attenuation X 4 , with zero-suppression circuit 20.40% 02, 20.94% 02, attenuation X 4 , with zero-suppression circuit
ANALYTICAL CHEMISTRY
0
Figure 3. Circuit diagram for the zero-suppression and polarity reversing unit
=TI.& Figure 4.
with a potentiometer (the recorder can be used for Ihis)
move when the switch is alternated between “OFF” and its first suppression position. If the resistors have been properly matched, the unit will be calibrated for its entire range by this procedure. For the unit to operate satisfactorily, recorder zero must be the same as electrical zero for the chromatograph. If it is not, a calibration setting which is correct for the first suppression position, where polarity is negative, will not be correct for the second suppression position, where polarity is positive. Electrical zero can be checked by removing the battery from the zero-suppression unit, balancing the recorder on zero, and rotating the zero - suppression switch. If recorder pen zero does not represent electrical zero, the pen will alternately shift off the base line as the switch is rotated. Grounding problems or a x . interference in the recorder may shift the recorder base line, and these problems must be resolved before the foldover zero-suppression unit \vi11 work properly. Some recorders have rnavinium stability when one input lead is connected to ground. The zero-suppression unit can be used n-ith these recordus if the positive terminal is connected to ground nt the place indicated by the tie-in in Figure 3. The effectiveness of the zero-suppression unit is increasing the accuracy of peak height measurements in gas chromatography is demonstrated in Figure 4. A gas mixture containing 20.40% oxygen was compared with a sample containing 20.94% oxygen. When the zero-suppression unit was not used. it was necessary to operate the cbromatograph a t low sensitivity, so theie was a difference of only 1.25 recorder units (0.0125 mv.) between the t n o peaks. The sensitivity of the chromatograph was increased when the zerosuppression unit was used, and there was then a difference between the two peaks of 9.5 recorder units (0.095 mv.). Since the unit has a capacity for seven
Modification of the Photovolt Densitometer to Permit Monitoring Ultraviolet Absorption of Column Effluents W. D. Conway and E. J. Lethco, Environmental Cancer Section, National Cancer Institute, Bethesda 14, Md. common laboratory practice to I use ultraviolet light absorption as a T 15
locating device for aromatic substances eluted from chromatographic columns. Several commercial devices have recently been offered to monitor the column effluent automatically. This paper describes a flow-cell holder and a simple voltage attenuator and event marker which permit use of the Photovolt Model 525 Densitometer for this purpose. The device has been mentioned earlier but has not been described in detail [Conway, W. D., Lethco, E. J., Ax.4~.CHEM. 32, 838 ( 1960) 1. Construction of tlie flow-cell holder is illustrated in Figure 1. Balsa wood was used to construct the present model, which was then painted flat black. The holder accommodates a standard flow cell (Pyrocell Co., outside dimensions 1.2 x 1.2 X 4.0 em., volume 1.5 ml., light path 1.0 cm.). Standard Beckman cells will also fit. A more complicated model having an adjustable entrance slit which produced a collimated light beam was also constructed, but its performance was not INPUT
1
l TO
i
EVENT M A R K E R SWITCH 0
0
1
OUTPUT
Figure 2. Voltage attenuator and event marker
Figure 1 .
superior to the simple model described here. The voltage attenuator and eventmarker circuit is shown in Figure 2. The components are conveniently mounted in a 6 X 4 X 4 inch box. The input leads are connected in parallel with the microammeter and its series resistor in the Photovolt Densitometer, which is equivalent to attaching one lead to each cathode of the 6K6 output tubes.
A 1-volt input to the attenuator can be adjusted with the 50,000-ohm potentiometer to produce an output of 2.3 to 116 mv., which is suitable for driving most laboratory recorders. The output resistance of the circuit can be a maximum of 51,000 ohms and is usually much less in actual use. This is suitable for the Varian G-10 graphic recorder with a 10-mv. span, n hich we used, but may have t o be changed to suit other recorders. The event-marker circuit imposes a small d.c. voltage from a flashlight cell on the output voltage of the attenuator. This results in a vertical line on the chart. The event-marker voltage, and therefore the length of the line produced, can be varied from 0 to 116 mv. with the 1000-ohm potentiometer. The polarity of the voltage can be re-
Flow-cell holder
versed using the double-pole doublethrow toggle switch, permitting the event mark to be made in either direction from the recorder trace. The spring return switch is used to test the size of the event mark. The event
Figure 3. Linearity of instrument in ultraviolet and visible range Benzophenone in 9570 alcohol, ultraviolet source and 5265 filter - - - - Citrus Red No. 2 in 9 5 % alcohol, visible source and 525-mp glass filter
VOL. 34, NO. 3, MARCH 1962
447
