Thermometric Monitor for Chromatographic Streams MAX BLUMER' Exploration and Production Division, Shell Development Co., Houston, Tex.
b The movement of concentration gradients through chromatographic columns is always associated with thermal gradients caused by the adsorption and desorption of solutes. These thermal gradients are sensed by a minute thermistor probe, and the resulting signal is amplified and then differentiated. The output from the differentiator is read on a vacuumtube voltmeter and can b e simultaneously recorded. A pronounced signal correlates with the appearance of a concentration gradient a t the thermistor probe in elution, displacement, and frontal analysis separations.
solvent, the heat liberated by the advancing front in each of the repeated adsorption steps is not readily dissipated and a considerable over-all heating a t the boundary can be noticed. I n extreme cases, this causes undesirable convection currents, degassing of solvents and adsorbents, and destruction of heat-sensitive materials. The smaller effects, however, which are present nhen solutes move over adsorbents are rarely noticed. Still, it seemed probable that the movement of very dilute solutions through adsorbent beds would be associated with thermal gradients of sufficient magnitude to permit their detection with temperature-sensitive probes.
A
and desorption of niolecules on the surface of a solid lead to thermal effects. The heat of wetting is liberated n-hen the solid is brought in contact with a liquid. On the other hand, energy is required to remove the liquid from the surface. In chromatography, the movement of a concentration gradient over the adsorbent always produces such thermal effects. If the solute moves a t a rate comparable to that of the inert carrier DSORPTIOS
THERMOMETRIC CHROMATOGRAPHY
Temperature us. T i m e Curves. Because of their small size and high sensitivity, thermistor probes are well suited for the detection of small temperature effects. I n the initial experiments, a simple Wheatstone bridge circuit with a pen recorder as null detector was used t o measure and record t h e resistance changes of a thermistor in the chromatographic column. The temperature-time curve (Figure
Present address, \Voods Hole Oceanographic Institution, Woods Hole, Mass.
1) is typical of the thermal effects in graded-elution chromatography. The thermistor was located a t the bottom of a IO-ml. column of silica gel in n-pentane. After the flow of the column was started and the recorder was connected, a mixture of pentane ivith benzene (3 to 2 ) was added to the top of the bed to displace the pentane. After all the solvent had entered the column, it was displaced by a benzene-acetone mixture (3 to 2 ) . The temperature, which had remained constant during the pentane elution, rose very quickly as the pentane-benzene front advanced over the thermistor. It slowly dropped after the front had passed and rose again upon the approach of the benzene-acetone front. The temperature rises were considerable-3' C. at the first interface, and 11' C. a t the second interface. I n this example, the emergence of solvent fronts from the chromatographic bed was monitored. Figure 2 shows a record of the much smaller effects connected n ith the elution of a small quantity of a polar 50lute. Ten milligrams of benzene was added to a bed of 14 ml. of alumina in pentane and was eluted with n-pentane. Again. the movement of the benzene zone was associated with a (much smaller) temperature rise at the front. (Measured upon emergence from the bed, the rise n as 0.45' C.)
L __
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E L U T I O N OF P E N T A N E - B E N Z E N E 13 21
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--BENZENE
I
IC
85
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Figure 1.
772
ANALYTICAL CHEMISTRY
25
30
35
40
Break-through of graded eluents
45
50
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2
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Figure 2.
-HEPUbL
CRIFT I.
14
16
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20
22
24
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Elution of 10 mg. of benzene
- I E Q M L L D E 4 K ASSCCIATEO I T H C C b, C E h ? A T I O N G R A 0 I E N
bed volume operated a t flow rates of 4 ml. per minute. This time constant is adequate for columns of between 5and 100-ml. capacity operated a t correspondingly different flow rates. Larger ~ recolumns and much slower f l o rates quire a, longer time constant. THERMOMETRIC MONITOR
An instrument was designed which produces a signal proportional to the first derivative of the temperature as sensed by the thermistor probc. For greater sensitivity, it incorporates a twostage direct current amplifier, a differentiator, and a vacuum-tube voltmeter. .4n output is ai-ailable for the connection of a recorder or accessories which might, for instance, utilize the output signal to actuate a fraction collector. The instrument circuit is shown in Figure 4.
Figure 3.
Temperature vs. volume and temperature derivative vs. volume curves
To measure such small temperature changes, the chromatographic column must be insulated, and the adsorbent and eluents must be brought into thermal equilibrium. Without these precautions the thermal peaks may be superimposed over an erratic base line. Equilibration of adsorbent bed and eluent is time-consuming and means mere sought to discriminate between slow, erratic temperature changes due to thermal nonequilibrium and the rapid changes associated with adsorption and desorption processes. Temperature Derivative os. Time Curves. If the derivative temperature function is plotted instead of the temperature function itself (Figure 3), the slow and nearly constant drift due to warming or cooling of the bed by outside influences is differen-
tiated into a constant displacement of the base line. Thermal-elution peaks, on the other hand, are differentiated into twin peaks-a positive one for the heating of the thermistor probe, and a negative one for the consecutive cooling of the probe. Elcctronically, the signal from the thermistor circuit is easily differentiated in a resistance-capacitance circuit. The time constant selected should be sufficiently short to provide an almost instantaneous indication of the rate of temperature change and to produce a minimum signal for the slow, erratic temperature drifts in the column. However, for sensitivity and signal-noise ratio, the time constant should not be too short. As a compromise, a time constant of 1 second was chosen, which is optimum for columns of about 15-ml.
The thermistor (Veco KM 1830 F hypodermic needle, approximately 100 kiloohms a t 25' C.) and RJ form a voltage divider across the B-minus supply. The voltage a t their junction, which is a function of the temperature of the thermistor, is applied to the grid of either VI (high sensitivity) or V z (low sensitivity). I n the first case, the signal is amplified by V1 and V Zbefore it is differentiated by CS and Rs. In the second case, the signal is amplified only by V2. The derivative signal is applied to the grid of the VTVM tube, Vs, while the grid potential of the reference tube, Vd, is held constant. Meter and recorder output are connected in parallel in the cathode circuit of V S and V4. The meter reading is not affected by the shunting effect of the recorder if its input resistance is a t least 100 kiloohms. The noise level of the instrument is significantly reduced by capacitors Cl and Cz in the plate circuits of the amplifier and Cnin the grid circuit of the VTVM. Switch Sla (ganged with the on-off switch of the power supply) shorts the meter terminals when the instrument is VOL. 32, NO. 7, JUNE 1960
773
RECORDER
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Figure 4.
Circuit of differential amplifier and VTVM VI, VZ: 1 2 AY 7 V3, V4:12 AU 7 M. 50-0-50Ha. meter, 2000.ohm Internal reristaTce
These resistors are selected.
not in use. Switch Sza disables VI by applying a cutoff voltage to its grid while the range switch, SB, is in the lowsensitivity position. The t\To positions of switch SOb permit reading on the vacuum-tube voltmeter of either the derivative thermistor signal or the voltage on the grid of Vz. (Within a limited range, this voltage is also a linear function of the temperature of the thermistor; in the high-sensitivity position, it changes approximately 1 volt for 0.5' C.) This voltage is adjusted into the working range of V I by potentiometer Re before a chromatogram is started with the instrument in the highsensitivity position. The line voltage t o the amplifier is stabilized by a magnetic voltage stabilizer. The power supply is conventional and provides the filament voltage and well-filtered and well-regulated voltages of +300, $150, and -150 volts. SENSITIVITY
OF THERMOMETRIC
Figure 5. Column for thermometric chromatography
ie cm
So 1 v e n t
I D 20 rnm
Reservoir
MONITOR
The sensitivity of the thermometric monitor is determined as follon s: The thermistor is immersed in distilled water in a Dewar flask. The water is stirred well with a small magnetic stirrer, and a constant current is passed through a resistance wire completely immersed in the water. The temperature is measured with a sensitive thermometer before and after the experiment, and the duration of heating is recorded. Absolute sensitivities are calculated from the heating rate and from the reading of the thermometric instrument a t different settings of its sensitivity control.
I n this manner, the maximum sensitivity of the prototype instrument was determined as 2 X l o a 3 ( " C. per sec774
ANALYTICAL CHEMISTRY
2 1 crn
ID 106 mm
.
Separat i r g Sect i or
Figure 6. Break-through of pentane plus 1% benzene after pentane
Figure 7.
Peaks of similar height were associated with the consecutive breakthrough of pentane containing 2, 5, 10, 20, and 40% benzene
ond)/(divisionj in the low-sensitivity position and 1.3 X ( " C. per second)/(division) in the high-sensitivity position. Since t,he completion of our investigation Claxton ( I ) has described a chromatographic detector using the same thermal effects. However, his technique utilizes a separate detector instead of direct monitoring of the column effluent. Also, by using a thermocouple instead of the thermistor and by avoiding further amplification and differentiation of the signal, his method is limited in sensitivity and apt to be disturbed by erratic thermal drifts unless column and solvents are very carefully equilibrated. APPLICATIONS OF THERMOMETRIC MONITOR
For the experiments described below,
a column (Figure 5) is used which
permits insertion of the thermistor needle in a Teflon plug through a short side arm. The tip of the thermistor needle penetrates the adsorbent bed approximately 1 cm. above the bottom. The separating section of the column is insulated with about 1 cm. of glass ~ 0 0 1 , but no special precautions are taken to bring the column into thermal equilibrium with the surrounding air and eluents. For recording the derivative thermometric curve, the output of the instrument is fed through a calibrated attenuator to a Varian Model G-10 graphic recorder. Break-through of Graded Eluents. In graded elution, a small amount of a complex sample is charged to a column and is fractionally eluted with solvents, or solvent mixtures, of increasing polarity. For maximum resolution, the fractions should be cut just before the break-through of each eluent. Upon migration of the solvent mixtures through the column, frontal analysis occurs and the breakthrough is delayed by the volume of solvent adsorbed on the bed.
Figure 8.
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Elution of 0.24 mg. of benzene with pentane
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Elution
of monoaromatics from crude oil distillate
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The thermometric instrument is useful for monitoring the break-through of the eluents, and proper positioning of the thermistor in the bed results in sufficient advance warning of the appearance of a none. For a typical elution chromatogram, a column was packed with 15 ml. of alumina (Harshaw, aluminum oxide, 0102-P) and was eluted consecutively with 25 ml. each of pentane and mixtures of pentane with 1, 2, 5, 10, 20, and 40y0 of benzene. I n every instance, a peak was associated with the movement of the solvent front (Figure 61, and there was sufficient delay between the appearance of the thermal peak a t the thermistor and the break-through of the solvent in the eluate (appearance of schlieren lines in the eluate, S, Figure 6). For maximum resolution, the fractions should be cut just as the signal starts to drop from its peak value. Elution of Chromatographic Zones. The thermistor is also a sensitive monitor for detecting the elution of solutes during development with a solvent of constant composition. As a typical example, the elution of benzene from an alumina bed by npentane was followed. Figure 7
shoms the temperature-derivative curve for elution of 0.24 mg. of benzene from a 15-ml. bed of alumina. The signal is pronounced and the signal-noise ratio of the original recording is approximately 20 to 1. Elution curves haye as a peculiar feature a second positive peak, which can be explained as folloas: Desorption of the benzene by the pentane a t the trailing edge of the zone is an endothermic process, and, as a consequence, the pentane n hich follows the peak is cooled below its original temperature. Pentane of the original temperature then reaches the thermistor, and a second peak is registered in the derivative curve, indicating a temperature rise. A negativegoing section of the curve after the second peak indicates a slow cooling which msy be associated nith a gradual cooling of the column wall, nhich has high heat capacity and small conductance. It is heated during the passage of the first positive peak and cools very s10\idy. Figure 8 shons the differential chromatogram of the elution of monoaromatics from a crude oil distillate. For this chromatogram, 100 pl. of a 250" to 325" C. distillate with 287, monoaroVOL. 32, NO. 7, JUNE 1960
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