Differential detector

As probes must be manufactured individually for each different tube type, the probe development is an important factor for the economic use of the method. The classical procedure of probe development is a combination of experience and experiment. The new probe design is based on the experience with already manufactured probes. For an evaluation of the new design the probe must be manufactured. If the probe design is complicated, for example due to dual exciter coil arrangement or segmented differential detector coil systems, the costs of the development can be very high. Therefore a method for the pre-calculation of the probe performance is extremely useful. 

Differential detector. A detector that responds to the instantaneous difference in composition between the column effluent and the carrier gas (mobile phase). 

The response we can distinguish differential detectors, which monitor the difference in the composition of the column effluent, and integral detectors, which measure the amount of sample component passing through the detector. 

Detectors. Most detectors for liquid chromatography can also be used in FFF systems. Refractive index detectors [132] are the most popular for soluble macromolecules. Designed as differential detectors, they measure differences in refractive indices of eluate relative to pure eluent, Anr This difference is proportional to the solute concentration in the eluate through the refractive index increment dnr/dc. The major problem associated with the use of a refractometer is the dependence of the refractive index on temperature and pressure, which can cause baseline drifts and fluctuations. 

The differential form of the Gaussian function has already been discussed and is sigmoid in shape with a positive maximum at the first point of inflexion of the Gaussian curve and a minimum at the second point of inflexion. If the peaks are completely resolved in the normal chromatogram, then they can be clearly and unambiguously identifiable in their differential form. If, however, the peaks are not completely resolved, then the differential curve of the unresolved peaks are confused and extremely difficult to interpret and for this reason the differential form of the Gaussian function is rarely used. Nonetheless, if the elution profile of the solutes are not Gaussian in form, the differential detector can be extremely useful. 

V.V. Brazhnikov, D/f/erenfsifl/h/e Detektory dlya Gazovoi Khromatografii (Differential Detectors for Gas Chromatography), Nauka, Moscow, 1974. 

Differential procedures are of interest when looking for the very small differences in the polymer molar mass distributions. The differentiation can be made so that (<) The samples are simultaneously applied into two identical columns and the effluent flows through adjacent measuring cells of a differential detector ... 

Fig. 55. Chromatogram of a differential detector. The shaded area is proportional to the total mass of the substance emerging from the column in the time t2 - t ...

Integral detectors successively add all input signals together and issue a corresponding summated signal at the output. Only differential detectors play a major role in gas chromatography. 

The TCD is a differential detector that measures the thermal conductivity of the analyte in carrier gas, compared to the thermal conductivity of pure carrier gas. In a conventional detector at least two cell cavities are required, although a cell with four cavities is more common. The cavities are drilled into a metal block (usually stainless steel) and each contains a resistance wire or filament (so-called hot wires). The filaments are either mounted on holders, as shown in Figure 7.11, or are held concentrically in the cylindrical cavity, a design that permits the cell volume to be minimized. They are made of tungsten or a tungsten-rhenium alloy (so-called WX filaments) of high resistance. 

Mehran, M.T. Golkar, N. Cooper, W.T. and Vickers, A.K. Headspace analysis of some typical organic pollutants in drinking water using differential detectors Effects of columns and operational parameters. Journal of Chromatographic Science 1996, 34 (3), 122-129. 

Refractive index (RI) monitors are the closest to being universal HPLC detectors, as nearly all dissolved solutes alter the refractive index of the mobile phase. They are differential detectors, generating a signal that depends on the difference between the RI of the pure mobile phase and the modified value caused by the dissolved solute, which can, therefore, be positive or negative. 

The vast majority of refractive index detectors are differential detectors where the refractive index of the sample is measured relative to a reference liquid. This enables them to be used in a wide range of applications but requires a fairly delicate flow cell. Absolute refractive index detectors are available which although only covering a limited range and less sensitive, are more robust and have sensor probes which can be easily inserted in the process stream. These are especially suited to explosion proof applications. 

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