Specifications detector noise level

Detector specifications have been discussed in chapter 5. They reveal the accuracy and precision attainable in quantitative analysis and also the lower concentration levels that are possible in trace analysis. As in GC, the five specifications of prime importance are detector response, detector noise level, detector sensitivity, or minimum detectable concentration, detector linearity and linear dynamic range [1], The detector response, detector woAe level and the detector are relevant to trace... 

Essential features of an automated method are the specificity, ie, the assay should be free from interference by other semm or urine constituents, and the sensitivity, ie, the detector response for typical sample concentration of the species measured should be large enough compared to the noise level to ensure assay precision. Also important are the speed, ie, the reaction should occur within a convenient time interval (for fast analysis rates), and adequate range, the result for most samples should fall within the allowable range of the assay. 

The noise level of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g. the katherometer and the refractive index detector) are often measured under static conditions (i.e. no flow of mobile phase). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held responsible for the precise control of the mobile phase, beitmay a gas flow controller or a solvent pump. However, all mobile phase delivery systems show some variation in flow rates (and consequently pressure) and it is the responsibility of the detector manufacturer to design devices that are as insensitive to pressure and flow changes as possible. 

The dissimilarity score for two identical spectra will never reach zero because of the presence of noise in the detector that cannot be compensated. There will always be some residual dissimilarity, which imposes a practical limit on the definition of a match threshold. The noise level of a given spectrum at a specific analyte concentration depends on the spectrometer, the characteristics of the diode array, and the scan speed of the detector, the lamp, and detector electronics. 

The noise levels of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g., the katharometer) are sometimes measured under static conditions (i.e., no flow of carrier gas). Such specifications... 

Once the depletion voltage (noted on the specification sheet) is reached, the noise level will stay constant The 45 % n-type detector used to derive the data in Figure 11.2 had a depletion voltage of 2000 V and an operating voltage of 3500 V. 

Noise Level - The noise level of a detector is measured in mV and is taken as the maximum amplitude of the combined short and long term noise taken over a period of about 10 min. It has been given the symbol Np and is used in determining the detector sensitivity. It can not be too strongly emphasized that detectors must not be compared on the basis of the magnitude of their noise or response. They can only be compared on the basis of their relative signal-to-noise ratio at a specific solute concentration. 

The pressure sensitivity defined as that pressure equivalent to the noise level is very important as it determines both the limits of pressure and flow variation that can be tolerated from the pump. The specification of the maximum working pressure is also important where multidimensional column systems are required to be employed since there is a significant flow impedance subsequent to the detector cell,... 

A similar specification, the flow sensitivity should also be defined in terms of that flow change which will provide a signal equivalent to twice the noise level. The value of the flow sensitivity is also important in the design of the pump to be used with the detector. 

An FT-IR spectrometer is used optimally when detector noise exceeds all other noise sources and is independent of the signal level. This is the usual case for mid-infrared spectrometry but may not be so for shorter wavelengths. The sensitivity of mid-infrared detectors is commonly expressed in terms of the noise equivalent power (NEP) of the detector, which is the ratio of the root mean square (rms) noise voltage, P , in V Hz to the voltage responsivity, R, of the detector, in V W . It is effectively a measure of the optical power that gives a signal equal to the noise level thus, the smaller the NEP, the more sensitive is the detector. The NEP is proportional to the square of the detector area, Ao, with the constant of proportionality being known as the specific detectivity, D that is. 

In addition to controlling the bias, there are several constraints on the design of the front-end electronics the noise levels, frequency response, dynamic range, power dissipation, and size must be consistent with system-specific requirements. Even though most modern detectors include ESD protection circuitry, our front-end circuitry should not generate large transient voltages. 

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