Selection of a detector

What is meant by the selectivity of a detector Define the limit of detection of a detector. 

The selectivity of a detector is its ability to determine an analyte of interest without interference from other materials present in the analytical system, i.e. the sample matrix, solvents used, etc. 

The selectivity of a detector is often related to its limit of detection, i.e. the more selective it is, then the lower the background noise is likely to be, and consequently the lower the limit of detection. 

Other important parameters in providing successful GC are the column packing, temperature conditions, and selection of a detector as specific to the analyte as possible. Maximum resolution of the halocar-bons is achieved with an 8-ft X 0.1-in. i.d. column of Carbopack-B coated with 1% SP-1000. The initial temperature of 45 °C is held for 3 min and then programmed at 8 °C/min to 220 °C. An organohalogen detector (OHD) is used. The aromatics are best resolved with a 6-ft X 0.085-in. i.d. column of Supelcoport coated with 5% SP-1200 plus 1.75 Bentone-34. They are measured with a photoionization detector. The temperature conditions are as follows 50 °C for 2 min then programmed at 6 °C/min to 90 °C. A 10-ft X 2-mm i.d. Porapak-QS (80-100 mesh) column at a temperature of 110 °C for 1.5 min and rapidly heated to 150 °C is now used for acrolein and acrylonitrile. This method employs a flame ionization detector (FID). 

GC detection systems for trichothecenes primarily featured classical detectors such as ECD, especially for fiuorinated derivatives however, mass spectrometers are nowadays routinely used for this purpose. The selectivity of a detector is an important issue as trichothecenes are present in relatively complicated matrices and create high background during the analysis. Three types of mass spectrometers have been used for trichothecene analysis by GC/MS, and quadrupole instruments are used most frequently. 

The choice of a detector for a specific appHcation should be made in order to minimize the cooling requirements and the magnitude of the background radiation noise therefore, in detector selection the cutoff wavelength should be only slightly greater than that required by the appHcation. If the... 

The high degree of sensitivity, selectivity, and efficiency of gas chromatography allows the elucidation of a complete profile of the volatile components of distilled spirits. The wide selection of chromatographic columns and techniques, such as gc-ms, gc-ftir, and gc-ms-ftir, has allowed the chemist to routinely identify and quantify individual constituents on a parts-per-biUion level. The two most critical variables in the analysis of volatile components of distilled spirits by gas chromatography are the selection of a suitable chromatographic column and of the most appropriate detector. 

With dicamba, a more polar chlorobenzoic acid herbicide, a gradient step is needed to elute all of the compounds in one chromatographic run. Depending on the buffer and selectivity of the detector, the baseline can be severely disturbed. If this happens, a step-gradient elution is recommended (52), and in this way the method can detect all of the compounds at very low levels. 

Quantitative accuracy and precision (see Section 2.5 below) often depend upon the selectivity of the detector because of the presence of background and/or co-eluted materials. The most widely used detector for HPLC, the UV detector, does not have such selectivity as it normally gives rise to relatively broad signals, and if more than one component is present, these overlap and deconvolution is difficult. The related technique of fluorescence has more selectivity, since both absorption and emission wavelengths are utilized, but is only applicable to a limited number of analytes, even when derivatization procedures are used. 

In order to answer this question, we should not consider the chromatographic resolution in isolation but in conjunction with the selectivity of the detector. If the detector is not selective, i.e. we cannot isolate the signal resulting from the analyte from those representing the other compounds present, we must rely on the chromatographic resolution to provide a signal which is measurable with sufficient precision and accuracy. If, however, the detector has sufficient selectivity... 

The principle of ec detection is that we measure the current in a flow cell at the column outlet. We can change the selectivity of the detector by changing the potential of the electrode. For instance, in the figure, if the potential of the electrode was X then the electrode would detect none of the compounds. At point Y the electrode would detect B but not A. 

Many frequency lines of infared radiation are absorbed by hydrocarbon gases. By selection of a particular frequency, a detector can be made which is either specific to a particular gas or if the frequency is common to several gases, a particular group of gases may be detected. 

The current individual methods differ in procedure, compounds detected, extraction techniques, and extraction solvents used. Some methods may include a cleanup step to remove biogenic (bacterial or vegetation-derived) material, while others do not. The methods have in common a boiling point type of column and a flame ionization detector. Selection of a method depends on the type of hydrocarbon suspected to be in the sample. 

Now the specimen is perfectly aligned and it remains to measure the rocking curve. This is normally done by selecting a step size on the axis and a counting time per step for the detector. Some systems will allow instead the selection of a total number of counts at each step, so that constant statistical accuracy is... 

Where the target analyte contains heteroatoms such as nitrogen, phosphorus and sulfur, atom-selective detectors can provide an ideal detection method. A number of examples appear in the literature of the use of a detector called a thermal energy analyser (TEA) for the measurement of A-nitroso compounds [14-17] and aromatic nitro compounds [18]. This has also been used as an HPLC detector [19, 20], and a modified TEA has been reported to be useful for analysis of amines and other nitrogen-containing compounds [17]. Unfortunately, this technique appears not to have gained in popularity, since no reports have appeared in the literature for over two decades. 

Sometimes it is not necessary to use the selectivity of a chromatographic technique. Sensitive analysis can sometimes be achieved with selective detection in flow injection analysis (FIA). Whilst some of the detectors described below may be appropriate in themselves in favourable cases, in most cases more sophisticated detection regimes are necessary, such as post-injection derivatisation of the analyte. Strategies involving some of the derivatisation methods outlined in Section 4.9.2 may be considered. 

Selectivity in a detector is most often required for sensitive bioanalytical methods where trace amounts of compounds are being analysed in the presence of interferants which are also present in the sample matrix. The properties of some commonly used detectors are summarised in Table 11.3. 

Selectivity is another highly desirable property of a detector. As a rule of thumb, the more selective the detection, tlie lower signal noise and the higher... 

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