Concentration sensitive detector

Thermal conductivity detector. The most important of the bulk physical property detectors is the thermal conductivity detector (TCD) which is a universal, non-destructive, concentration-sensitive detector. The TCD was one of the earliest routine detectors and thermal conductivity cells or katharometers are still widely used in gas chromatography. These detectors employ a heated metal filament or a thermistor (a semiconductor of fused metal oxides) to sense changes in the thermal conductivity of the carrier gas stream. Helium and hydrogen are the best carrier gases to use in conjunction with this type of detector since their thermal conductivities are much higher than any other gases on safety grounds helium is preferred because of its inertness. 

Under many experimental conditions, the mass spectrometer functions as a mass-sensitive detector, while in others, with LC-MS using electrospray ionization being a good example, it can behave as a concentration-sensitive detector. The reasons for this behaviour are beyond the scope of this present book (interested readers should consult the text by Cole [8]) but reinforce the need to ensure that adequate calibration and standardization procedures are incorporated into any quantitative methodology to ensure the validity of any results obtained. 

The spectrometer is behaving as a concentration-sensitive detector as the signal intensity remains constant as the flow rate increases. If it were mass-sensitive, the detector response would increase. 

Concentration-sensitive detector A detector for which the intensity of response is proportional to the concentration of analyte reaching it. 

Size exclusion chromatography (SEC) separates molecules of a polymer sample on the basis of hydrodynamic volume. When the chromatograph is equipped only with a concentration-sensitive detector, i.e. conventional SEC, a molecular weight distribution (MWD) can be obtained from the chromatogram only through use of a calibration function relating molecular weight and elution volume V (2). 

Where Q, is the minimum detectable amount, R the detector noise level and s the detector sensitivity [135,146,151,152]. For a concentration sensitive detector the minimum detectable concentration is the product of Q, and the volumetric gas flow rate through the detector. The minimum detectable amount or concentration is proportional to the retention time, and therefore, directly proportional to the column radius for large values of n. it follows, then, that very small quantities can be detected on narrow-bore columns. 

Detection is also frequently a key issue in polymer analysis, so much so that a section below is devoted to detectors. Only two detectors, the ultra-violet-visible spectrophotometer (UV-VIS) and the differential refractive index (DRI), are commonly in use as concentration-sensitive detectors in GPC. Many of the common polymer solvents absorb in the UV, so UV detection is the exception rather than the rule. Refractive index detectors have improved markedly in the last decade, but the limit of detection remains a common problem. Also, it is quite common that one component may have a positive RI response, while a second has a zero or negative response. This can be particularly problematic in co-polymer analysis. Although such problems can often be solved by changing or blending solvents, a third detector, the evaporative light-scattering detector, has found some favor. 

Concentration-sensitive detectors, such as the refractive index detector or UV-VIS spectrophotometer... 

The output signal of concentration-sensitive detectors is proportional to the concentration or weight of polymer in the column eluent. Examples of this type include the differential refractometer and the ultraviolet-visible spectrophotometer. Infrared and fluorescence detectors are used infrequently. None of the detectors described above is truly universal i.e., the response of these detectors varies with the chemical species, and, in case of the DRI, response is also a function of the chromatographic eluent.156 Recently, an... 

Dramatic increase in mass sensitivity of concentration-sensitive detectors... 

For a detector to be of use in quantitative analysis, the signal output should be linear with concentration for a concentration-sensitive detector and with mass for a mass-sensitive detector. Some detectors have an additional time constant purposely introduced to remove the high-frequency noise. This should always taken into consideration, since a slow detector response can significantly broaden and attenuate chromatographic peaks relative to those actually sensed. Moreover, a versatile detector should have a wide linear dynamic range so that major and trace components can be determined in a single analysis, over a wide concenua-tion range. 

GC detectors can be grouped into concentration-sensitive detectors and mass-sensitive detectors. The signal from a concentration-sensitive detector is related to the concentration of solute in the detector, which does not usually destroy the sample. Mass-sensitive detectors usually destroy the sample, and the signal is related to the rate at which solute molecules enter the detector. The response of a mass-sensitive detector is unaffected by make-up gas, while that of a concentration-sensitive detector will lower with make-up gas. A summary of some important characteristics of the GC detectors specifically used in drug residue analysis is presented in Table 23.1. 

FID is mass-sensitive rather than concentration-sensitive detector this gives the advantage that changes in mobile phase flow rate do not affect detector response. It offers high sensitivity, a large linear response range, and low noise. Its precision is high and is not susceptible to contamination from dirty samples... 

Altitude Response. Pressure response is an issue that needs to be addressed for every instrument deployed on an aircraft. First, it must be decided how chemical abundances are to be reported. If standard practice is followed and they are reported as mixing ratios, then it must be determined whether the instrument is fundamentally a mass- or a concentration-depen-dent sensor, because this definition determines the first-order means by which instrument response is converted to mixing ratios as a function of pressure. In this context, a mass-sensitive detector is a device with an output signal that is a function of the mass flow of analyte molecules a concentration-sensitive detector is one in which the response is proportional to the absolute concentration, that is, molecules per cubic centimeter. 

For sensors that are truly mass sensitive and for which the mass flow of sample through the sensing element is held constant as a function of pressure (for example, by use of electronic mass-flow controllers), instrument response is proportional to the mixing ratio independent of the pressure. For concentration-sensitive detectors, such as simple spectrophotometric instruments measuring absorbance or fluorescence, instrument response is a function of the absolute concentration, and the response will decrease for a constant mixing ratio as the pressure decreases. For example, the response of a pulsed fluorescence SO instrument sampling air containing a fixed... 

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