2. The Thermal Conductivity Detector (TCD) - Eawag-Empa Library

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40 with the concentration is obtained (see Fig. 2.1) TABLE 2.1 THE THERMAL CONDUCTIVITY OF SOME COMPOUNDS w N Compound KS x lo3 [W/cm. K] Compound KS x lo3 [W/cm. K] 1710 1470 303 240 239 24s 23 I 216 164 7 0 FIG. 2.1. Thermal conductivities of some binary mixtures. 145 86.2 154 173 152 111 190 170 For very low concentrations of the eluted substance, all the terms in the expanded Taylor series (equation (2.5)) can be neglected except for the first, thus giving a linear relationship between the concentration of the eluted substance and the change in the conductivity of the binary system. Measurement with the TCD is based on monitoring changes in the electric conductivity of the filament, caused by variations in its temperature during passage of
the gaseous mixture through the measuring cell. A temperature gradient is established due to transfer of thermal energy by the medium. Under stationary conditions, the amount of heat transferred, Q, is proportional to the thermal conductivity of the flowing gaseous mixture and to the difference in the temperatures of the filament, T,, and the cell walls, T,: During the design of practical measuring devices, it was found that the overall amount of heat transferred is not given by the thermal conductivity of the medium alone, but that the molar heat capacity and other factors also play a role. The follow- ing processes contribute to the overall heat change, i.e., to the measured signal: - the thermal conductivity of the medium; - forced convection of the gaseous mixture; - free convection and diffusion of the gas; - the thermal conductivity of leads and connections; - thermal radiation. The participation of the thermal conductivity and of forced convection of the gas in the overall heat transfer can be distinguished only with great difficulty. Forced convection represents heat transfer coupled with mass transport under the dynamic conditions present in the gas chromatograph and amounts to about 25% of the overall change. This part of the transferred heat is proportional to the volume velocity of the gas, u, and to its heat capacity, C,: Hence the TCD signal depends on the flow-rate of the carrier gas. The contribution of free convection corresponds to energy transfer in the concentration gradient and is negligible compared with the other factors. Heat transfer by the leads and electrical connections is determined by their cross- section, length and thermal conductivity. Therefore, it is desirable that the leads to the sensor should be as short as possible and have as small a cross-section as possible. It has been found that this parameter does not have a large influence on the overall change in the TCD signal in devices with heated filements, but that its importance increases when thermistors and transistors are used as sensors: Heat radiation depends on the surface area of the measuring element, on its temperature and on its material quality. The literature gives values of less than 4% for the contribution of thermal radiation to the overall heat transfer. 41
Page 1: 2. The Thermal Conductivity Detecto
Page 5 and 6: tion will become smaller with incre
Page 7 and 8: [25, 341, substantial differences a
Page 9 and 10: ate and hence the fdet/d t ratio in
Page 11 and 12: The sensor temperature, T,, changes
Page 13 and 14: the temperature by AT,. Hence it is
Page 15 and 16: exclusively through convection. A n
Page 17 and 18: time constant of the sensor and con
Page 19 and 20: 24, 33, 34, 42, 481, the excellent

2. The Thermal Conductivity Detector (TCD) - Eawag-Empa Library

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