Selective detector, examples

Not all of the above-described statistical quantities are chromatographically observable. For example, s, d, t, and m are not directly observable unless selective detectors as a mass spectrometer is employed (Campostrini et al., 2005) and thus they are hidden quantities. The point will be discussed in the third section of this chapter. 

Other types of detectors such as mass selective detectors would need to have their own assessment performed. It should be clearly understood that the outcome of the risk assessment example is a personal assessment and is not meant to be definitive in any way. 

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. 

An example of the use of GC with nitrogen selective detection is in the quantification of bupivacaine in plasma. Bupivacaine contains two nitrogen atoms in its structure which makes it a good candidate for this type of analysis. The limits of detection which can be achieved with a nitrogen selective detector for this compound are much better than methods based on flame ionisation detection, which are much less selective. 

Monochromatic detection. A schematic of a monochromatic absorbance detector is given in Fig. 3.12. It is composed of a mercury or deuterium light source, a monochromator used to isolate a narrow bandwidth (10 nm) or spectral line (i.e. 254 nm for Hg), a flow cell with a volume of a few pi (optical path 0.1 to 1 cm) and a means of optical detection. This system is an example of a selective detector the intensity of absorption depends on the analyte molar absorption coefficient (see Fig. 3.13). It is thus possible to calculate the concentration of the analytes by measuring directly the peak areas without taking into account the specific absorption coefficients. For compounds that do not possess a significant absorption spectrum, it is possible to perform derivatisation of the analytes prior to detection. 

The selectivity universal detectors exhibit a response to every sample (except the mobile phase), whatever the chemical species. Selective detectors respond to samples that exhibit a physical or chemical property (UV absorbance, for example). In this mode the mobile phase should not interfere. 

In this manner, a nearly universal and very nonselective detector is created that is a compromise between widespread response and high selectivity. For example, the photoionization detector (PID) can detect part-per-billion levels of benzene but cannot detect methane. Conversely, the flame ionization detector (FID) can detect part-per-billion levels of methane but does not detect chlorinated compounds like CCl very effectively. By combining the filament and electrochemical sensor, all of these chemicals can be detected but only at part-per-million levels and above. Because most chemical vapors have toxic exposure limits above 1 ppm (a few such as hydrazines have limits below 1 ppm), this sensitivity is adequate for the initial applications. Several cases of electrochemical sensors being used at the sub-part-per-million level have been reported (3, 16). The filament and electrochemical sensor form the basic gas sensor required for detecting a wide variety of chemicals in air, but with little or no selectivity. The next step is to use an array of such sensors in a variety of ways (modes) to obtain the information required to perform the qualitative analysis of an unknown airborne chemical. 

The confirmation of pesticides by GC/MS should be more reliable than that on the GC-ECD using an alternate column. Presence of stray interference peaks, even after sample cleanup, and the retention time shift and coelution problem, often necessitate the use of GC/MS in compounds identification If a quantitative estimation is to be performed, select the primary ion or one of the major characteristic ions of the compounds and compare the area response of this ion to that in the calibration standard. Quantitation, however, is generally done from the GC-ECD analysis, because ECD exhibits a much greater sensitivity than the mass selective detector (MSD). For example, while ECD is sensitive to 0.01 ng dieldrin, the lowest MSD detection for the same compound is in the range of 1 ng. The primary and secondary characteristic ions for qualitative identification and quantitation are presented in Table 2.20.3. The data presented are obtained under MS conditions utilizing 70 V (nominal) electron energy under electron impact ionization mode. 

Second column confirmation is an imperfect technique, prone to false positive detection, particularly if non-selective or low selectivity detectors are used. Even the most selective detectors may not be fully capable of correctly identifying target analytes in complex environmental matrices as illustrated in Example 4.12. 

Always give preference to chromatography methods that employ selective detectors, for example, for PAHs choose EPA Methods 8310 and 8270 over EPA Method 8100 for phenols choose EPA Method 8270 over EPA Method 8041/FID. 

As summarized in Table 3, there are a number of examples in which internal standards have been effectively used for the quantitative analysis of PGS by GLC-MS. The prime limitation of this approach has been the high cost of the instrumentation. As will be discussed in a later portion of this manuscript, other selective detectors used on either a GLC or an HPLC should offer other choices for PGS analysis, but they are not as specific as GLC-MS. 

The detector converts a change in the column effluent into an electrical signal that is recorded by the data system. Detectors are classified as selective or universal depending on the property measured. Selective (solute property) detectors, such as fluorescence detectors, measure a physical or chemical property that is characteristic of the solute(s) in the mixture only those components which possess that characteristic will be detected. Universal (bulk property) detectors measure a physical property of the eluent. Thus, with refractive index (RI) detectors, for example, all the solutes which possess a refractive index different from that of the eluent will be detected. Selective detectors tend to be more sensitive than universal detectors, and they are much more widely used. Universal detectors are more commonly used in preparative chromatography, where a universal response is desired and sample size is large. 

GC analysis with element selective detectors and GC/MS analysis is usually performed with the same sample aliquot and under similar chromatographic conditions, for example, with the same type of stationary phase. This is done to achieve accurately comparable retention times on both instruments. This comparison in particular may prove its value when background materials interfere in a GC/MS run and complicate interpretation of the total ion chromatogram (TIC). 

Single Selective Detectors. In both GC and LC there are detectors that show a selectivity for particular groups of compounds or functional groups. Conventional examples will be given later in the individual chapters, but one unusual, specific detector is the moth (alive) used with a GC to detect the presence of sex pheromones in a column effluent.11 The physical response of the moth clearly indicates which peak represents its sex hormone. Humans also sniff column effluents to identify particular odors in the flavor and fragrance industry. 

The burner heads used in such cool flame emission studies are often simply quartz tubes. Figure 12 shows the burner system used by Arowolo and Cresser27 for automated gas-phase sulfide determination, for example. Other species determined by cool flame emission techniques include chloride, bromide, and iodide, which give intense emission in the presence of indium.29 The main application of cool flame emission techniques in environmental analysis is in speciation studies, for example for the separate determination of sulfite and sulfide, or as element-selective detectors in gas chromatography. 

Spectroscopic detectors measure partial or complete energy absorption, energy emission, or mass spectra in real-time as analytes are separated on a chromatography column. Spectroscopic data provide the strongest evidence to support the identifications of analytes. However, depending on the spectroscopic technique, other method attributes such as sensitivity and peak area measurement accuracy may be reduced compared to some nonselective and selective detectors. The mass spectrometer and Fourier transform infrared spectrometer are examples of spectroscopic detectors used online with GC and HPLC. The diode array detector, which can measure the UV-VIS spectra of eluting analytes is a... 

By the use of different modes of liquid chromatography it is possible to separate polymers selectively with respect to hydrodynamic volume (molar mass), chemical composition or functionality. Using these techniques and combining them with each other or with a selective detector, two-dimensional information on different aspects of molecular heterogeneity can be obtained. If, for example, two different chromatographic techniques are combined in a cross-fractionation mode, information on CCD and MMD can be obtained. Literally, the term chromatographic cross-fractionation refers to any combination of chromato-... 

However, modern analytical techniques, for example capillary gas-chromatogra-phy coupled with mass-selective detectors (GC-MS) allow the characterization of... 

PCDE analyses can be performed by high resolution gas chromatography (HRGC) combined with low resolution mass spectrometry (LRMS) or using HRGC combined with high resolution mass spectrometry (HRMS). For example, a Hewlett Packard 5970 mass selective detector system, which is a qua-drupole mass analyzer, has been used as an LRMS instrument in Finnish studies [33, 57, 113, 114, 122-125, 139]. Compared to LRMS, HRMS is more sensitive, allows less sample cleanup, and eliminates interference more effec-... 

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