Interfering components

The methanol gradient from 50 to 60% releases quite a lot of interfering components. Omitting the step gradient does not provide enough selectivity and so the best conditions were obtained with a pH gradient. The experimental conditions are shown in Table 13.1. 

As was the case for PCR, we see that the PLS spectral residuals for a sample will be higher whenever there is something in the data that introduces a mode of variation into the spectrum that was not present in any of the training samples used to develop the basis space. The anomolous variation could be caused by instrument drift, an unexpected interfering component, a misaligned sample cell, or whatever. We can use this property of residuals as an indicator that can signal... 

Numerous stationary phases were investigated by Callahan et al. [135] but only OV-101 separated C14 5-sultone from the C14 unsaturated-y/2-chloro-y-sultone combination and also separated these from other components present in the sample. Apiezon L separated all three sultones of interest but other components in the neutral oil interfered with the C14 unsaturated y-sultone. Additional stationary phases were found which will separate these sultones, but again, interfering components preclude their use for neutral oil analysis. 

Scheme 7.3 Generalised supercritical fluid methods for extraction and analysis with and without separation of interfering components. After King [5], Reproduced from the Journal of Chromatographic Science, by permission of Preston Publications, a Division of Preston Industries, Inc.

Achiral-chiral LC-LC is most often used to separate the desired analyte from interfering components, such as matrix components, metabolites, excess derivatiza-tion reagent, or other impurities. Separating such interferents from the analyte allows for better analyte quantification or enantiomeric ratio determination. Also, achiral columns are seen as a way to protect the more expensive chiral columns from matrix components that might become irreversibly retained and deteriorate column performance. Short achiral columns (trap columns) are sometimes used to reconcentrate the chiral analyte after a previous separation (either chiral or achiral) as a type of online enrichment. Configurations that combine an achiral column for increased selectivity and trap column(s) for online enrichment are relatively common, though this type of configuration requires more columns and increases complexity. 

Each of the described filters only selects a group of ions with the right combination of mass, charge and energy. Thus, if we want to suppress the presence of interfering components, we can use two different fdters at least in order to transmit only those particles with the characteristics represented by the intersection point of the curves drawn in Figure 16.2. 

A large proportion of analytical measurements is subject to interference from other constituents of the sample. Newer methods increasingly employ instrumental techniques to distinguish between analyte and interference signals. However, such distinction is not always possible and sometimes a selective chemical reaction can be used to mask the interference. If this approach fails, the separation of the analyte from the interfering component will become necessary. Where quantitative measurements are to be... 

However, many analyses will require extraction of the component of interest before analysis is carried out. It may be essential to remove it from the soil matrix and/or interfering components before reliable analyses can be made. Extraction may be designed to extract only a portion of the analyte of interest. For example, extraction of a particular oxidation state may be important because oxidation states can affect the mobility and biological availability of chemicals. It may also be necessary to concentrate the extract after isolation. 

An interesting application of this region of the spectrum is the determination of nitrate and nitrite in soil extracts. Nitrate is very soluble in water and can be extracted using a simple water extraction procedure. Nitrate absorbs at 210 nm and nitrite at 355 nm, and both can be quantified using these absorption maxima ( max). Other materials extracted from soil, as noted previously, however, can obscure this region, and thus caution must be exercised to determine if there are any interfering components in such extracts [6-10],... 

The residuals for unknown 4 are predominantly random but have greater magnitude than expected (see Figure 5.23). This indicates that there are no interfering components in the sample but the noise level has increased. If this is the root cause, the predictions will in general be accurate, but will have poorer precision. 

How can the inverse method correct for the interferent when it was not explicitly included in the model For this example, it is easy to see. Recall that the spectrum of the interferent is ij - [3 0 0]. The estimated regression vectors (b) in Figure 5.63c have zeros for the variable on which the interferent responds (variable 1). In this case, the inverse approach has implicitly modeled the presence of the interferent by ignoring the response variable that is assooiated With the interfering component. This example demonstrates that, for this weU-... 

K15, and K25 the predictions for the validation set, A5, whose samples contain varying amounts of a 5th component that was never present in the training sets, are surprisingly good when compared to K13 , and K23 ,. But this is more an indication of how bad K13 , and K23 are rather than how good K15 , and K25 , are. In any case, these results are not to be trusted. Whenever a new interfering component turns up in an unknown sample, the calibration must be considered invalid. Unfortunatley, neither CLS nor ILS can provide any direct indication that this condition might exist. 

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