Peak Separation unexpected

Aliphatic and Olefinic Hydrocarbons. Of the first 30 fractions, only fractions 4-7 contained significant quantities of material. The exploratory gas chromatographic analysis of these four fractions showed them to be very similar they were pooled and analyzed by GC-MS. Figure 2 shows the total ionization plot (trace B) and the mass chromatogram of m/e 57 (trace C) for these combined fractions. The peak at scan 165 is elemental sulfur. Its mass spectrum shows peaks separated by 32 mass units and a molecular ion at m/e 256 corresponding to eight sulfur atoms. The presence of sulfur is not unexpected in such highly anoxic sediment... 

On a TLC plate, the entire sample is available for separation and visual detection. There are no problems with unrecognized loss of peaks or unexpected appearance of peaks from previous samples as in HPLC. Zone identification is facilitated in TLC by the visual nature of the detection using colors and shades and many different reagents and temperatures, and inspection in daylight and under short- and long-wavelength lamps. Separated substances can be subjected to subsequent analytical procedures (e.g., coupled TLC-UV-Vis, TLC-MS, TLC-FTIR) at a later time. 

Detection of impurities by separation, which almost always involves the finding of unexpected peaks in some kind of chromatogram, and which may lead to identification. 

Alias reduction for hybrid filter banks. One possible problem of all cascaded filter banks specific to hybrid filter banks needs to be mentioned. Since the frequency selectivity of the complete filter bank can be derived as the product of a single filter with the alias components folded in for each filter, there are spurious responses (alias components) possible at unexpected frequencies. Crosstalk between subbands over a distance of several times the bandwidth of the final channel separation can occur. The overall frequency response shows peaks within the stopbands. 

Reality, therefore, forces separation scientists to deal with various aspects of peak overlap. The subject is a complicated one. We present in this section a brief introduction to the nature, implications, and unexpectedly high frequency of overlap. Some theoretical methods for dealing with overlap are outlined in the following section. While this treatment is based on studies of peak overlap in chromatography [33, 34], the concepts are equally valid for electrophoresis and other zonal separation methods. 

Among the compounds identified besides isoprene and its oligomers are several aromatic hydrocarbons. Also, a few fatty acids were identified. Low levels of aldehydes were detected in the fresh rubber latex, and the presence of the acids in the pyrolysate is not unexpected [9]. However, these acids may also come from contaminants in the pyrolysis experiment. The peaks corresponding to pentamers and hexamers were not obvious in Figure 6.1.3, possibly due to the separation conditions or due to a higher pyrolysis temperature. Some compounds other than those indicated in Figure 6.1.3 were reported to be present in natural rubber pyrolysate [4,10], but their detection may depend on specific pyroiysis conditions and on the sensitivity of the analytical procedure. 

Peak purity analysis is very useful in chromatographic method development, to confirm that all components have been chromatographically separated, and in quality control, to warn the analyst that an unexpected coeluting impurity has appeared. 

A common speciation scheme after sample preparation involves a fractionation step followed by the element quantification in the fractions obtained. A clear trend exists toward using the techniques that combine separation and detection steps into one operating on-line system. In these coupled techniques, the selectivity is achieved by application of powerful separation modes (different chromatographic or electrophoretic methods), while the use of atomic spectrometric techniques assures high sensitivity of detection. It should be stressed, however, that coupled techniques with element-specific detection do not provide structural information for the species. If the appropriate standards are available, the assignment of chromatographic peaks can be accomplished by spiking experiments. On the other hand, the identification of unknown forms and/or ultimate confirmation of unexpected compounds observed in the sample require the use of complementary techniques (molecular mass spectrometry or NMR). ... 

Unlike other forms of liquid chromatography, the separation is not primarily dependent on the nature of the eluent, but rather on the pore size distribution of the column packing, provided that the solvent is of reasonably high polarity. The use of lower polarity solvents may lead to loss of sample components by adsorption on the column, and care should be taken to guard against this when sample peak areas appear unexpectedly low. Tetrahydro-furan is a nearly ideal solvent since it has low viscosity, high solvent power, low refractive index and is water-miscible, and it is therefore recommended as first choice for all separations. 

Also interesting are the results obtained on mixtures of solutions of two SIS copolymers with different polystyrene masses and equal polyisoprene masses. One would expect that the loss peak would broaden due to the strong dependence of the relaxation times on the polystyrene mass (see Fig. 124). However, the resulting loss peak is of the same form as those of the umnixed polymer solutions the G" values of the mixture are an intermediate between the G" values of either polymer solution separately [357], This unexpected result was attributed to the decrease in the time constants of polymers with relatively large S-blocks in the mixed domains and an increase in the time constants of the relatively small S-blocks. The resulting relaxation times seem to depend on the domain size of the S-blocks (rather than the individual S-sizes), which in turn will depend on the (average) S-block mass. 

Another effect of the mobile phase/sample solvent mismatch is a change in the sensitiviy (AR/AC) of the method for an analyte. As an example, Perlman and Kirschbaum [889] prepared a series of solutes (e.g., captopril, nadolol, o-nitroani-line, triamcinolone acetate, methylparaben) in neat solvents acetonitrile, methanol, DMSO, and dichloromethane. These solutions were then injected onto a C,g or I enyl column (2 = 214 run or 270 run) and eluted with 50/50 methanol/water or 38.8/1.1/960 ethanol/water/dichloromethane mobile phases. Significant differences in the peak areas resulted for some but not all analytes. Deterioration of peak shapes was also common. Prediction of these changes was nearly impossible. For example, o-nitoaniline (in methanol) exhibited an increased peak area in methanol/water, whereas p-nitroaniline was unaffected. An awareness of the unexpected and unpredictable effects the sample solvent has on both the quantitative results and the overall separation is critical when developing a method. 

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