The Heat of Adsorption Detector

The heat of adsorption detector, devised by Claxton, consists of a small plug of adsorbent, usually silica gel, through which the chromatographic eluent passes subsequent to leaving the column. Embedded in the silica gel is either a thermocouple or a thermistor that continuously measures the temperature of the adsorbent and mobile phase. When an eluted solute comes into contact with the silica gel, the heat evolved causes the temperature to rise. As the solute is subsequently desorbed from the silica gel, heat is absorbed and the temperature falls. The output from the temperature measuring device thus records an increase in temperature and then a decrease in temperature relative to its surroundings and an S haped curve results. 

The thermopile was mounted in a PTFE disc, and soldered to PTFE-insulated leads which were connected to the input terminals of the amplifier. The thermopile was thus contained between two further discs each having a groove that allows the mobile Aiase to flow over one set of junctions. The three PTFE columns were situated between two steel flanges containing cavities for centering the columns. 

Ca is the detector plate capacity-column plate capacity ratio, 

V is the flow of mobile phase through the detector in plate volumes of the attached column, n is the efficiency of the attached column. 

With the aid of the computer in a manner similar to that of of Smuts et al. (63), the relative values of e for v = 74 to 160 were calculated for a column having an efficiency of 100 theoretical plates, and for Cg taking values of 0.25, 0.5, 1, 2 

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Condition (4) assumes that the heat convected from the plate by the flow of gas through it is negligible compared with that conducted from the plate to the surroundings. This assumption is reasonable for a GC system but, as will be seen when the heat of adsorption detector is considered, this will certainly not be true when the mobile phase is a liquid. 

The Heat of Adsorption Detector, devised by Claxton (16) in 1958 has been Investigated by a number of workers (17,18,19) but although once commercially available, has not been extensively employed as an LC detector. One reason for this is the curious and apparently unpredictable shape of the temperature-time curve that results from the detection of the usual Gaussian or Poisson concentration peak profile. The shape of the curve changes with detector geometry, the operating conditions of the chromatograph, the retention volume of the solute and for closely eluted peaks, it produces a complex curve that is extremely difficult to interpret. 

Vm volume of mobile phase in the heat of adsorption detector cell Vs volume of adsorbent in the heat of adsorption detector cell Vff volume of the wall of the heat of adsorption detector cell Vc total column volume volume Vsi volume of silica in the column Vp pore volume of the packing Vi sample volume... 

The heat of adsorption detector, devised by Claxton, consists of a small plug of adsorbent, usually silica gel, through which the chromatographic eluent passes subsequent to leaving the column. Embedded in the silica gel is either a thermocouple or a thermistor that continuously measures the temperature of the adsorbent and mobile phase. 

A Chromatogram Showing Two Peaks Monitored by the Heat of Adsorption Detector... 

Thus, the curve relating temperature with time appears as an S-shaped curve for each solute band that is eluted. An example of a chromatogram containing two peaks monitored by the heat of adsorption detector is shown in figure 7. 

It is seen from the curves in figure 3 that the heat convected into the detector cell or plate also distorts the curves. It is apparent that unless the heat lost radially is extremely high, so that little heat is convected to the sensor, symmetrical integral peaks will not be obtained. This heat loss will be difficult to achieve in practice and for normal columns, the heat of absorption detector does not seem viable for LC. However, adequate radial heat loss might be possible with very small bore columns, and consequently the possibilities of the heat of adsorption detector might well be worth re-examining for use with microbore columns. 

The heat of adsorption detector monitors the rise in temperature which occurs when eluted solutes are adsorbed exothermically. A constant flow rate is essential, together with efficient thermostatting, and the endothermic adsorption of one solute may interfere with the detection of the exothermic adsorption of the next. 

There are a number of detectors that have either not found favour for various reasons, not become commerically available, or both. An example of these is the heat of adsorption detector. The detector cell contains a small quantity of adsorbant in which is embedded a temperature sensor such as thermocouple or thermopile. As the solute is eluted from the column it is adsorbed onto the cell contents and the heat of adsorption is liberated and the temperature rises. When the solute is eluted out of the cell the solute is desorbed, the heat of adsorption absorbed and the temperature falls. This results in an S shaped elution curve being traced by the detector which is extremely difficult to interpret particularly if two peaks are only partially resolved. 

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