Modern peak integration

Compared to flame excitation, random fluctuations in the intensity of emitted radiation from samples excited by arc and spark discharges are considerable. For this reason instantaneous measurements are not sufficiently reliable for analytical purposes and it is necessary to measure integrated intensities over periods of up to several minutes. Modern instruments will be computer controlled and fitted with VDUs. Computer-based data handling will enable qualitative analysis by sequential examination of the spectrum for elemental lines. Peak integration may be used for quantitative analysis and peak overlay routines for comparisons with standard spectra, detection of interferences and their correction (Figure 8.4). Alternatively an instrument fitted with a poly-chromator and which has a number of fixed channels (ca. 30) enables simultaneous measurements to be made. Such instruments are called direct reading spectrometers. 

Integration is a process of determining the area under a chromatographic peak for the quantitation of the analyte concentration. There are several scientifically valid techniques that may be used to integrate peaks. Modern chromatography computer software performs peak integration with a minimum of human intervention. Once an... 

Computer Techniques McLafferty (Ref 63) has pointed out that the usefulness of elemental composition information increases exponentially with increasing mass, since the number of elemental combinations with the same integral mass becomes larger. There are compilations of exact masses and elemental compositions available (Refs 12a, 13 18a). Spectral interpretation will be simplified in important ways if elemental compositions of all but, the smallest peaks are determined. Deriving the elemental compositions of several peaks in a spectrum is extremely laborious and time-consuming. However, with the availability of digital computers such tasks are readily performed. A modern data acquisition and reduction system with a dedicated online computer can determine peak centroids and areas for all peaks, locate reference peaks, interpolate between them to determine the exact masses of the unknown peaks, and find within minutes elemental compositions of all ions in a spectrum (Refs 28b 28c)... 

The cross-section of the primary X-ray beam is extended and not an ideal point. This fact results in a blurring of the recorded scattering pattern. By keeping the cross-section tiny, modern equipment is close to the point-focus collimation approximation - because, in general, the features of the scattering patterns are relatively broad. Care must be taken, if narrow peaks like equatorial streaks (cf. p. 166) are observed and discussed. The solution is either to desmear the scattering pattern or to correct the determined structure parameters for the integral breadth of the beam profile (Sect. 9.7). 

The relationship between the concentration of the solute and the peak produced in the chromatogram is, strictly speaking, only valid for peak area measurements, but in most instances it is more convenient to measure peak height. Such peak height measurements should only be used when all the peaks are very narrow or have similar widths. The tedium and lack of precision associated with non-automated methods of peak area measurements may be overcome using electronic integrators, which are features of most modern instruments. 

Although recorders and integrators are used in some older systems, generally in modern ion exchange chromatography results are stored in computer. Retention time and peak areas are the most useful information. Retention times are used to confirm the identity of the unknown peak by comparison with a standard. In order to calculate analyte concentration peak areas are compared with the standards which is in known concentration [10]. 

Modern HPLC systems commonly include computer software that automatically integrates peak areas. For older systems, manual integration may be necessary. 

Modern HPLC systems equipped with analog-to-digital converters (ADCs) and initial data acquisition and signal analysis are performed by either built-in microprocessor or attached computer [38,39]. Depending on the data acquisition rate and initial analysis criterion (peak height, peak area, or sensitivity thresholds, etc.), the integration limits or area calculation could be erroneous, and in some cases the whole peaks could be missed. 

Though most modern data acquisition systems can either sense or adjust integration parameters to accommodate baseline drift or permit manual setting of parameters to overcome drift-induced problems, it is still easy to mistake a system peak arising from the mobile phase as an impurity if the... 

The densitometer trace is used to measure peak positions (migration distance) and purity or relative concentrations of components by integration of the areas under the peaks. Modern densitometers perform this operation automatically. For quantitative work, overloading of the gels should be avoided, since this leads to very concentrated sample zones that may inhibit proportional staining and leads to underestimates of sample concentrations. 

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