Composition peaks

Figure 3. A Composite Peak Formed by Two Closely Eluting Peaks of Different Size...

Furthermore, in the example given, the peaks were considered to be truly Gaussian in shape. Asymmetric peaks can distort the position or the peak maximum of the envelope to an even greater extent. In general, the retention time of a composite peak should never be assumed to have a specific relationship with those of the unresolved pair. 

There is an interesting consequence to the above discussion on composite peak envelopes. If the actual retention times of a pair of solutes are accurately known, then the measured retention time of the composite peak will be related to the relative quantities of each solute present. Consequently, an assay of the two components could be obtained from accurate retention measurements only. This method of analysis was shown to be feasible and practical by Scott and Reese [1]. Consider two solutes that are eluted so close together that a single composite peak is produced. From the Plate Theory, using the Gaussian form of the elution curve, the concentration profile of such a peak can be described by the following equation ... 

A range of concentrations of the two substances were inserted in equation (6) and a curve constructed relating retention time of the composite peak (calculated by means of a computer) to mixture composition. The results are shown in Figure 7. 

Figure 7. Graph of Retention Time of a Composite Peak against Composition of Mixture...

In Figure 16 the elution profiles for samples from each group of seven plates are included together with the overall composite peak from the total charge. The calculations assumed a column efficiency of 5000 theoretical plates. The elution... 

Sometimes it is not possible to improve the resolution of a complex mixture beyond a certain level and, under these circumstances, the use of some de-convolution technique may be the only solution. The algorithms in the software must contain certain tentative assumptions in order to analyze the peak envelope. Firstly, a particular mathematical function must be assumed that describes the peaks. The function used is usually Gaussian and, in most cases, no account is taken of the possibility of asymmetric peaks. Furthermore it is also assumed that all the peaks can be described by the same function (i.e. the efficiency of all the peaks are the same) which, as has already been discussed, is also not generally true. Nevertheless, providing the composite peak is not too complex, de-convolution can be reasonably successful. 

The software will also function well if the same partial resolution is obtained but for peaks of unequal size. An example of the analysis of such a composite peak is shown in figure 9. 

It seen that the de-convolution is likely to be successful as the position of the peak maximum, and the peak width, of the major component is easily identifiable. This would mean that the software could accurately determine the constants in the Gaussian equation that would describe the profile of the major component. The profile of the major component would then be subtracted from the total composite peak leaving the small peak as difference value. This description oversimplifies the calculation processes which will include a number of iteration steps to arrive at the closest fit for the two peaks. 

Jin and Atrens (1987) have elucidated the structure of the passive film formed on stainless steels during immersion in 0.1 M NaCl solution for various immersion times, employing XPS and ion etching techniques. The measured spectra consist of composite peaks produced by electrons of slightly different energy if the element is in several different chemical states. Peak deconvolution (which is a non-trivial problem) has to be conducted, and these authors used a manual procedure based on the actual individual peaks shapes and peak positions as recorded by Wagner et al. (1978). The procedure is illustrated in Figure 2.8 for iron. 

GC-Computer System Nowadays, a large number of data-processing-computer-aided instruments for the automatic calculation of various peak parameters, for instance relative retention, composition, peak areas etc., can be conveniently coupled with GC-systems. A commercially available fairly sophisticated computer system of such type are available abundantly that may be capable of undertaking load upto 100 gas-chromatographs with ample data-storage facilities. In fact, the installation such as multi GC-systems in the routine analysis in oil-refineries and bulk pharmaceutical industries, and chemical based industries have tremendously cut-down their operating cost of analysis to a bare minimum. 

Resolution of a mixture of oligonucleotides of 15, 16, 17 and 18 nucleotides long, (b) Resolution of two oligonucleotides each of 18 nucleotides but differing in base composition. Peak 1 is TCACAGTCTGATCTCACC. Peak 2 is TCACAGTCTGATCTC-GAT. 

FIGURE 11.1. XRD patterns of Ni0-Ce02-Zr02 with varying Ce/Zr composition. Peaks marked as SS correspond to Ce02-Zr02 crystallites and those denoted as Ni correspond to NiO crystallites. ... 

Consider two solutes eluted close together such that a single composite peak is produced. From the Plate Theory the concentration profile of such a peak can be described by the following equation -... 

Some minor corrections of the values shown in Table I are required. The latest work (89a) shows that peaks A and B have identical amino acid composition. Peak A contains two residues of A-acetylglucosamine... 

Microparticulate alumina specially prepared for HPLC is available commercially and facilitates the isocratic separation of all-tram-/3-carotcnc from its lower potency cis isomers. Using an alumina column and a mobile phase of isooctane containing 0.5% stabilized tetrahydrofu-ran (161), the cis isomers of /3-carotene are eluted before the all-tram- isomer to form a single composite peak in the chromatogram. a-Carotcnc coelutes with the cis isomers of /3-carotene and therefore cannot be accurately quantified. y-Carotcnc, /3-cryptoxanthin, and canthaxanthin are not eluted. 

Nominal composition Peak position (20) Calc, composition (%) EPMA composition (%)... 

Using the test sample described below, periodically test the performance of and resolution provided by the gas chromatograph employed. The test sample must display results comparable in quantitative composition, peak shape, and elution order to those specified herein. The quantitative composition should not deviate from the results listed below by more than 10%. Analyze the GC test sample using the GC Conditions for Analysis given above. 

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