Qualitative analysis sensitivity

The amount of information, which can be extracted from a spectrum, depends essentially on the attainable spectral or time resolution and on the detection sensitivity that can be achieved. Derivative spectra can be used to enhance differences among spectra, to resolve overlapping bands in qualitative analysis and, most importantly, to reduce the effects of interference from scattering, matrix, or other absorbing compounds in quantitative analysis. Chemometric techniques make powerful tools for processing the vast amounts of information produced by spectroscopic techniques, as a result of which the performance is significantly... 

As XRF is not an absolute but a comparative method, sensitivity factors are needed, which differ for each spectrometer geometry. For quantification, matrix-matched standards or matrix-correction calculations are necessary. Quantitative XRF makes ample use of calibration standards (now available with the calibrating power of some 200 international reference materials). Table 8.41 shows the quantitative procedures commonly employed in XRF analysis. Quantitation is more difficult for the determination of a single element in an unknown than in a known matrix, and is most complex for all elements in an unknown matrix. In the latter case, full qualitative analysis is required before any attempt is made to quantitate the matrix elements. 

As may be obvious from previous chapters, quantitative analysis requires more substantial advancements to be made than qualitative analysis (library-based fingerprinting, screening, identification, recognition). For many polymer/additive problems, the classical methods are usually sensitive enough, and sophisticated instrumental methods are available, allowing analytical chemists to probe samples for components at much lower concentration levels. 

The stoichiometry of an interaction between gas molecules and preadsorbed species may thus be deduced from the modifications of the Q-6 curves for a given reactant which are produced by the presence of preadsorbed species on the solid. The results are, of course, particularly conclusive when the differential heats of adsorption of small doses of reactant are measured in a sensitive calorimeter. But, such a qualitative analysis of the calorimetric data, though very useful, does not allow definite conclusions. In the preceding example, for instance, a fraction of carbon dioxide may remain adsorbed on the solid ... 

For a qualitative analysis it is sufficient to be able to apply a test which has a known sensitivity limit so that negative and positive results may be seen in the right perspective. Where a quantitative analysis is made, however, the relation between measurement and analyte must obey a strict and measurable proportionality only then can the amount of analyte in the sample be derived from the measurement. To maintain this proportionality it is generally essential that all reactions used in the preparation of a sample for measurement are controlled and reproducible and that the conditions of measurement remain constant for all similar measurements. A premium is also placed upon careful calibration of the methods used in a quantitative analysis. These aspects of chemical analysis are a major pre-occupation of the analyst. 

Direct quantitative and qualitative analysis of the tracer by using highly sensitive absorbance or fluorescence techniques. 

Ciosek, P., Augustyniak, E., and Wroblewski, W. (2004a). Polymeric membrane ion-selective and cross-sensitive electrode-based electronic tongue for qualitative analysis of beverages. Analyst 129(7), 639-644. 

In food analysis, sensitivity is not the only requirement for analytical method development. Besides confirmation of the identity of pesticides, the identification of nontarget analytes is also important. One powerful tool is LC/MS, especially when it is combined with appropiate sample-treatment procedures it allows one to obtain detection limits adequate for trace-level analysis. Liquid chromatography-MS has demonstrated that it is an effective way to obtain both qualitative and quantitative information. 

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. 

As mentioned in the previous section, triple-quadrupole instruments are very good at finding low levels and structurally related compounds in the presence of biological matrices as well as being the gold standard technique for quantitation. Ion trap mass spectrometers, on the other hand, have the capabilities to obtain high-sensitivity full-scan MS and MS/MS spectra therefore, they are widely used for qualitative analysis, such as structural elucidation and unknown identification. For complete metabolite identification, it is important to have both the sensitivity and selectivity of triple-quadrupole instruments and the full-scan data quality of ion traps. 

The quadrupole/time-of-flight analyzer (QToF) has become a key option in the qualitative and quantitative analytical arena. Instruments with resolving power of 20000 (50% valley definition) can provide <5ppm mass accuracy for parent and product ion identification and for 20mda mass selection windows quantitation. While the triple quadra-pole retains the lead in sensitivity for quantification, the QToF has a decided edge on specificity (Micromass, 1999) and qualitative analysis. 

A plot of this sort is useful for qualitative analysis, especially of outliers. A study of the test conditions (e.g., the activating systems used) the particular strain of test organisms (e.g., its sensitivity to frame shifts vs. base substitutions), or the chemical nature of the mutagen may reveal the basis for the discrepancy and ultimately yield deeper insights. For example, the discrepancy in the two tests for furylfuramide (the extreme outlier in Figure 9-2) could be due to different nitroreductase activities in the two systems, which would lead to differences in capacity to convert the chemical into an active form. 

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