Embedded peaks

The second case study is of two-way diode array detector (DAD) HPLC data of a small embedded peak, that of 3-hydroxypyridine, buried within a major peak (2-hydroxypyr-idine). The concentration of the embedded peak varies between 1 and 5% of the 2-hydroxypyridine, and a series of 14 chromatograms (including replicates) are recorded whose concentrations are given in Table 3. 

Window factor analysis (WFA) was described by Malinowski and is likely the most representative and widely used noniterative resolution method [34, 35], WFA recovers the concentration profiles of all components in the data set one at a time. To do so, WFA uses the information in the complete original data set and in the subspace where the component to be resolved is absent, i.e., all rows outside of the concentration window. The original data set is projected into the subspace spanned by where the component of interest is absent, thus producing a vector that represents the spectral variation of the component of interest that is uncorrelated to all other components. This specific spectral information, combined appropriately with the original data set, yields the concentration profile of the related component. To ensure the specificity of this spectral information, all other components in the data set should be present outside of the concentration window of the component to be resolved. This means, in practice, that component peaks with embedded peak profiles under them cannot be adequately resolved. 

How many peaks in a cluster Can we detect small impurities Can we detect metabolites against a background Can we determine whedier there are embedded peaks ... 

These methods can be extended to cases of embedded peaks, in which the purest point for the embedded peak does not correspond to a selective region a weakness of using this method of ratios is that it is not always possible to determine whether a maximum (or minimum) in the purity curve is genuinely a consequence of a composition 1 region or simply the portion of the chromatogram where the concentration of one analyte is highest. 

Obtaining the spectra of each pure compound. This allows identification or library searching. In some cases, diis procedure merely uses the multivariate signals to improve on die quality of die individual spectra, which may be noisy, but in odier cases, such as an embedded peak, genuinely difficult information can be gleaned. This is particularly useful in impurity monitoring. 

If pure variables such as spectral frequencies or mlz values can be determined, even if there are embedded peaks, it is also possible to use these to obtain first estimates of... 

More difficult situations occur when only some components exhibit selectivity. A common example is a completely embedded peak in HPLC-DAD. In the case of LC-MS or LC-NMR, this problem is often solved by finding pure variables, but because UV/vis spectra are often completely overlapping it is not always possible to treat data in this manner. 

Table 6.12 Determing spectrum and elution profiles of an embedded peak.

Finally, it is important to mention another class of methods. In many cases it is not possible to obtain a unique mathematical solution to the multivariate resolution of complex mixtures, and the problem of embedded peaks without selectivity, which may occur, for example, in impurity monitoring, causes difficulties when using many conventional approaches. 

Problem 6.2 Evolutionary and Window Factor Analysis in the Detection of an Embedded Peak... 

There are clearly two components in this mixture. Show how you could distinguish the situation of an embedded peak from that of two peaks with a central region of coelution, and demonstrate that we are dealing with an embedded peak in this situation. 

The result is used to determine regions of maximum purity. These approximate to the elution profiles or spectra (in coupled chromatography) of each component in a mixture that can then be employed in factor analysis as some information on each pure component is known. Sometimes there are embedded peaks, for which there is no pure (or selective or composition 1) region. The eigenvalue plots can still provide valuable information as to where each component elutes but sometimes it is hard to obtain unique mathematical solutions to the determination of information on each compound. 

In the single fiber pull out test (SFPO), a small portion of the fiber is embedded in the bulky matrix and the interfacial strength is calculated from the peak load when the fiber is pulled out of the composite. 

We analyzed the embedded particles with differential scanning calorimetry to identify the property of polystyrene. As shown in Fig, 1, fhe embedded particles show a small peak around 100 °C, which is typical in atactic polystyrene [7]. It is desirable that embedding polymer has a similar melting tempeiature as the final polymer (polyethylene) because a big difference in the melting temperatures between the two polymers may cause a gel problem and poor mechanical properties. 

Figure 3.3 Molecular structure of G-protein-coupled receptors. In (a) the electron density map of bovine rhodopsin is shown as obtained by cryoelectron microscopy of two-dimensional arrays of receptors embedded in lipid membrane. The electron densities show seven peaks reflecting the seven a-helices which are predicted to cross the cell membrane. In (b) is shown a helical-wheel diagram of the receptor orientated according to the electron density map shown in (a). The diagram is seen as the receptor would be viewed from outside the cell membrane. The agonist binding pocket is illustrated by the hatched region between TM3, TM5 and TM6. (From Schertler et al. 1993 and Baldwin 1993, reproduced from Schwartz 1996). Reprinted with permission from Textbook of Receptor Pharmacology. Eds Foreman, JC and Johansen, T. Copyright CRC Press, Boca Raton, Florida...

Polyatomic molecules provide a still richer environment for studying phase control, where coupling between different dissociation channels can occur. Indeed, one of the original motivations for studying coherent control was to develop a means for bond-selective chemistry [25]. The first example of bond-selective two-pathway interference is the dissociation of dimethyl-sulfide to yield either H or CH3 fragments [74]. The peak in Fig. 11 is indicative of a resonance embedded in an elastic continuum (case 4). 

Figure 21.1 Decrease in detection sensitivity of the ions originating from proteins by contamination of OTC. Adhering OTC to the tissue section diminishes the detectable peaks, (a) A case in which OTC was used only to support the tissue block, (b) A case in which the tissue block was completely embedded with OTC. Reprinted with permission from Schwartz et al.9...

Zsolnay and Kiel [26] have used flow calorimetry to determine total hydrocarbons in seawater. In this method the seawater (1 litre) was extracted with trichlorotrifluoroethane (10 ml) and the extract was concentrated, first in a vacuum desiccator, then with a stream of nitrogen to 10 pi A 50 pi portion of this solution was injected into a stainless steel column (5 cm x 1.8 mm) packed with silica gel (0.063-0.2 mm) deactivated with 10% of water. Elution was effected, under pressure of helium, with trichlorotrifluoroethane at 5.2 ml per hour and the eluate passed through the calorimeter. In this the solution flowed over a reference thermistor and thence over a detector thermistor. The latter was embedded in porous glass beads on which the solutes were adsorbed with evolution of heat. The difference in temperature between the two thermistors was recorded. The area of the desorption peak was proportional to the amount of solute present. 

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