Temperature detector sensitivity

Fig. 4. Sensitivity as a function of detector temperature showing (—) experimental results for HgCdTe (H and A) Hg CdTe, x = 0.185 and x = 0.280,...

Fig. 9. Spectral sensitivity of detectors where the detector temperatures in K are in parentheses, and the dashed line represents the theoretical limit at 300 K for a 180° field of view, (a) Detectors from near uv to short wavelength infrared (b) lead salt family of detectors and platinum siUcide (c) detectors used for detection in the mid- and long wavelength infrared. The Hg CdTe, InSb, and PbSnTe operate intrinsically, the doped siUcon is photoconductive, and the GaAs/AlGaAs is a stmctured supedattice and (d) extrinsic germanium detectors showing the six most popular dopants.

Temperature detectors embedded in the motor winding give close, accurate indication of motor temperature. Both conventional resistance temperature detec tors (RTD) and special thermistors (highly temperature-sensitive nonlinear resistors) are used. With appropriate auxiliaries these devices can indicate or record motor temperature, alarm, and/or shut down the motor. 

Quantitative analysis using the internal standard method. The height and area of chromatographic peaks are affected not only by the amount of sample but also by fluctuations of the carrier gas flow rate, the column and detector temperatures, etc., i.e. by variations of those factors which influence the sensitivity and response of the detector. The effect of such variations can be eliminated by use of the internal standard method in which a known amount of a reference substance is added to the sample to be analysed before injection into the column. The requirements for an effective internal standard (Section 4.5) may be summarised as follows ... 

Detector Sensitivity, or Minimum Detectable Concentration Pressure Sensitivity Flow Sensitivity Temperature Sensitivity... 

Both the sensing device of the LC detector and the associated electronics can be temperature sensitive and cause the detector output to drift as the ambient temperature changes. Consequently, the detecting system should be designed to reduce this drift to a minimum. In practice the drift should be less than 1% of FSD at the maximum sensitivity for 1°C change in ambient temperature. 

Detector Type" Maximum sensitivity Flow-rate sensitivity Temperature sensitivity Gradient compatibility... 

To overcome such a problem, a silicon heater of negligible heat capacity was added to each detector to trim its sensitivity by a slight change of the (detector) temperature around the working temperature (see Section 16.6). Due to the steep dependence on T of the R and C parameters, changes in detector temperatures of the order of 1 mK are needed for the equalization of the detector response. 

One temperature-sensitive resistor as compensator and another one as detector are integrated into adjoining strings of a Wheatstone bridge circuit the voltage can be measured. Since both resistors are exposed to the test gas flow, disturbances caused by changes in temperature and humidity are compensated. 

These detectors respond to UV/visible absorbing species in the range 190-800 nm and their response is linear with concentration, obeying the Beer-Lambert law (p. 357). They are not appreciably flow or temperature sensitive, have a wide linear range and good but variable sensitivity. 

Detector basis Type Maximum sensitivity Flow rate sensitive Temperature sensitivity Useful with gradient ... 

There are several types of RI detector, all of which monitor the difference between a reference stream of mobile phase and the column effluent. Any solute whose presence alters the refractive index of the pure solvent will be detected, but sensitivity is directly proportional to the difference between the refractive index of the solute and that of the solvent. At best they are two orders of magnitude less sensitive than UV/visible detectors. All RI detectors are highly temperature-sensitive, and some designs incorporate heat exchangers between column and detector to optimize performance. They cannot be used for gradient elution because of the difficulty in matching the refractive indices of reference and sample streams. 

The refractive index detector operates by comparing the refractive index of the mobile phase prior to the column with the refractive index of the column eluate. This detector responds to nearly all solutes but it is highly temperature-sensitive (Skoog et al., 1998). This type ofdetector can be used for sugars and fatty acids. 

The assay was carried out using a Varian gas chromatograph (model 5000 LC) under the following experimental condition. The oven injector and flame ionization detector temperatures were 125°C and 225°C respectively. A Porapak column was used, the eluent was N2 at a flow rate of 30 ml/min and the injected volume 2 pi. Various concentrations of purified methylene chloride in purified methanol were injected (both solvents were distilled to discard any impurity which might interfere with the sensitive assay). Calibration curves were linear in the range 50-500 ppm (the limit of detection was 10 ppm). Methylene chloride detection in the microspheres was performed by dissolving various amounts (20-200 mg) of microspheres in 220 ml of purified methanol prior to the injection. 

Refractive index detectors are useless in gradient elution because it is impossible to match exactly the sample and the reference while the solvent composition is changing. Refractive index detectors are sensitive to changes in pressure and temperature (—0.01 °C). Because of their low sensitivity, refractive index detectors are not useful for trace analysis. They also have a small linear range, spanning only a factor of 500 in solute concentration. The primary appeal of this detector is its nearly universal response to all solutes, including those that have little ultraviolet absorption. 

Thermistors, which are metal oxide beads used as temperature-sensitive resistors, have been used in thermal conductivity detectors since the mid-1950s. They offer several advantages. 

The large value of the filament temperature sensitivity shows how important temperature control of the detector is. Problems in this area appear most frequently with temperature controllers which oscillate about their setpoint, with a period of a few minutes. If the cells of the detector respond to this change with different delay times, then the oscillation will appear in the baseline. 

Set the detector temperature just slightly above the column temperature (or maximum temperature, for temperature-programming). Higher setpoints sacrifice sensitivity. 

The capture process of the electron capture detector can be very temperature-sensitive. The sensitivity may either increase or decrease with an increase in temperature, depending on the compound involved, as illustrated in Figure 6.24 for three benzene derivatives. Since detector temperature may affect sensitivity it is sometimes possible to improve the analysis by operating at a different temperature. The radioactive source determines the maximum temperature limit for the detector which is listed in Table 6.6. Exact values vary with manufacturer. 

Flow-through conductivity sensors suitable for insertion in pipelines (see Fig. 6.47a) are now available for use at temperatures up to 480 K. and pressures up to 1700 kN/m2(64). As conductivity is temperature sensitive, a thermistor is usually included in the detector circuit as part of a temperature compensator. Screw-in cells (Fig. 6.476) will withstand higher pressures. More recently, electrodeless methods of measuring conductivity have become available. In this case the solution is placed between two energised toroids. The output voltage of the instrument (from the output toroid circuit) is proportional to the conductivity of the solution provided that the input voltage remains constant. This type of conductivity meter can be used under much more severe conditions, e.g. with highly corrosive or dirty systems 43 . 

Carrier gas, detector current and detector temperature arc selected for maximum sensitivity. 

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