Detector specialised

In this case, no profound specialised knowledge is required for determination of the type of discontinuities. On the other hand, determination of the nature of damage using a regular flaw detector for manual UT, can be a complicated task for any flaw detection experts, in any case, its solution will require time and qualification of the operator, but. the presence of these conditions will not eliminate the subjective nature of the assessment. 

The detector. The function of the detector, which is situated at the exit of the separation column, is to sense and measure the small amounts of the separated components present in the carrier gas stream leaving the column. The output from the detector is fed to a recorder which produces a pen-trace called a chromatogram (Fig. 9.1fr). The choice of detector will depend on factors such as the concentration level to be measured and the nature of the separated components. The detectors most widely used in gas chromatography are the thermal conductivity, flame-ionisation and electron-capture detectors, and a brief description of these will be given. For more detailed descriptions of these and other detectors more specialised texts should be consulted.67 69... 

Compared with the flame ionisation detector, however, the ECD is more specialised and tends to be chosen for its selectivity which can simplify chromatograms. The ECD requires careful attention to obtain reliable results. Cleanliness is essential and the carrier gases must be very pure and dry. The two most likely impurities in these gases are water and oxygen which are sufficiently electronegative to produce a detector response and so give a noisy baseline. 

In order to measure peak areas, recording integrators are used. Alternatively specialised software for data treatment can be loaded onto a microcomputer linked to the detector by an appropriate interface card. 

Infrared analyses are conducted on dispersive (scanning) and Fourier transform spectrometers. Non-dispersive industrial infrared analysers are also available. These are used to conduct specialised analyses on predetermined compounds (e.g. gases) and also for process control allowing continuous analysis on production lines. The use of Fourier transform has significantly enhanced the possibilities of conventional infrared by allowing spectral treatment and analysis of microsamples (infrared microanalysis). Although the near infrared does not contain any specific absorption that yields structural information on the compound studied, it is an important method for quantitative applications. One of the key factors in its present use is the sensitivity of the detectors. Use of the far infrared is still confined to the research laboratory. 

In order to have a sensible model, the number of compounds must be less than or equal to the smaller of the number of experiments or number of detectors. In certain specialised cases this limitation can be infringed if it is known that there are correlations between the concentrations of different compounds. This may happen, for example, in environmental chemistry where there could be tens or hundreds of compounds in a sample, but the presence of one (e.g. a homologous series) suggests the presence of another, so, in practice there are only a few independent factors or groups of compounds. Also,... 

There is another important benefit in using smaller bore columns. Because of the smaller geometries involved, the same amount of solute injected onto a small-bore column will elute in a smaller volume than that of a larger column. This leads to a higher response at the detector for a particular mass, and therefore to a significant improvement in sensitivity. The major disadvantage associated with the use of miniaturised HPLC systems is the need for certain specialised components. Because the... 

An alternative is to determine the polymers molecular weight/mass in the SEC eluent in situ, by use of on-line molecular mass sensitive detectors. Two such detectors are commercially available, the light scattering detector and the viscosity detector. These detectors are usually used in series with a mass concentration detector and require specialised data handling/software to compute the outputs from the twin detectors and to produce molecular weight/masses and distributions. 

Some new specialised detectors are mass spectrometry detectors (MS), evaporating light scattering detectors (ELSC) and atom specific detectors. They all have in common that the mobile phase stream has to be split to secure a safe operation of the fragile detection system. As pointed out earlier, the eluent stream can be split by active and passive principles. For certain applications, mainly in the isolation of impurities with unknown structures in the mg range, the combination of these detectors to preparative equipment is of high value. As soon as the diameter of the... 

The basic components of a UV-Visible detector are a radiation source, normally consisting of a deuterium lamp for the UV region and a tungsten lamp for the visible region (mainly used for specialised applications such as chlorophylls), a lens, a filter or a monochromator to isolate the narrow wavelength band desired, a sht, a flow cell and a hght detector. 

The use of spectroscopic techniques as detectors for GC is discussed in Chapter 7. The detectors already mentioned are those commonly used in present day instruments, however, there are a large number that have been developed over the years either for specialised applications or in the quest for the ideal universal detector. Some of these are outlined below. The reader is referred to specialised texts for further details of these and other detectors [61, 62, 68). 

The most common method of detection in HPLC exploits the ultraviolet and visible regions of the electromagnetic spectrum (EM see Figure 5.1) in order to detect the analytes of interest. The detectors employed that utilise these regions of the EM spectrum are the ultraviolet (UV) and diode array detectors. However, other, more specific detectors can be used for specialised applications, such as conductivity and refractive index (not discussed here), and with the introduction of hyphenated techniques, mass spectrometry has become a widely used detector method. 

Ultra-violet detectors are used in specialised areas such as aircraft engine compartments. 

Knoery and Cutter [817] have described a method for the determination of sulphide and carbonyl sulphide in nm saline waters using specialised collection procediues followed by gas chromatography using flame photometric detector. The following species were determined HS, ... 

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