Measurements of Peak Area

The option of efficiency calibration is usually not available on simple emulator programs and until recently was restricted to off-line analysis packages. More recent software, such as Genie 2000, GammaVision and Fitzpeaks, provide online efficiency calibration facilities within the overall acquisition/analysis package. Regardless of which of the many mathematical functions the analyst might choose to fit to the data, the spectrum analyst is constrained by the choice of functions provided within 

Gunnink (1990) described polynomial equations of this type for calculating intrinsic efficiency, from which absolute efficiency can be calculated. By examining the efficiency calibrations of a large number of detectors, he was able to relate some of the parameters of these equations to the dimensions of the detector and other details of the detector system. Different equations were used from 50-90 keV, 90-200 keV (second order polynomial) and above 200 keV (sixth order polynomial). At first sight. 

This function is available in Genie 2000 and is referred to as the Empirical function for, as the manual says (obscurely), historical reasons . 

The data below the knee point is clearly unsatisfactory and is shown here merely as an example of how a 

Deconvolution is the term given to the process of extracting peak area information from a composite (multi-plet) peak. The results of deconvolution should be treated with caution. Most other spectrum analysis calculations 

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The mixture is identical in each example. The peaks are shown separated by 2, 3, 4, 5 and 6 (a) and it is clear that a separation of 6a would appear to be ideal for accurate quantitative results. Such a resolution, however, will often require very high efficiencies which will be accompanied by very long analysis times. Furthermore, a separation of 6o is not necessary for accurate quantitative analysis. Even with manual measurements made directly on the chromatogram from a strip chart recorder, accurate quantitative results can be obtained with a separation of only 4a. That is to say that duplicate measurements of peak area or peak height should not differ by more than 2%. (A separation of 4a means that the distance between the maxima of the two peaks is equal to twice the peak widths). If the chromatographic data is acquired and processed by a computer, then with modem software, a separation of 4a is quite adequate. 

The area of a peak is the integration of the peak height (concentration) with respect to time (volume flow of mobile phase) and thus is proportional to the total mass of solute eluted. Measurement of peak area accommodates peak asymmetry and even peak tailing without compromising the simple relationship between peak area and mass. Consequently, peak area measurements give more accurate results under conditions where the chromatography is not perfect and the peak profiles not truly Gaussian or Poisson. 

Foley, J. R, Systematic Errors in the Measurement of Peak Area and Peak Height for Overlapping Peaks, Journal of Chromatography 384, 1987, 301-313. 

Ordinarily the task of the laboratory chromatographer is first to obtain a separation of various components in a mixture and then to use that separation to carry out quantitative analysis of a number of samples. That task may be routine or quite challenging depending on the number of components to be separated, their relative concentrations, and the accuracy demanded of the analysis. Adequate accuracy may not be available from simple peak height data yet more accurate measurements of peak areas from a... 

Peak resolution is usually easier if well chosen background parameters are input and if constrained optimization methods are utilised. Misleading results can be obtained if the constraints are too limited and tests with unconstrained optimization are desirable if at all possible. In particular, the possible presence of paracrystalline or intermediate phase peaks must be tested with extreme care in order to avoid ambiguity. It is not sufficient to have a good mathematical resolution alone, all peaks must be significant in crystallographic or structural terms. The incidental measurement of peak-area crystallinity is considered to be of secondary importance to the resolution of overlapping peaks. 

Measurement of peak height, although an approximation, can be done with good accuracy and is simpler to perform than measurement of peak area. It is often preferred for manual data reduction because it is simple and fast. In addition, less resolution of components is required for accurate calculations.12 However, peak heights are more susceptible to variations in separation conditions, such as column temperature and aging, than are... 

The most accurate measurements of peak areas are those obtained by means of electronic data reduction with integrators or computers. The vast majority of laboratories now have electronic or computer methods for determining peak areas. Two main features are required for electronic processing of chromatographic data accurate digitization of the analog signal and software. The software is required for the detection of peaks, correction for baseline drift, calculation of peak areas, retention times, and concentrations of components in the sample, and production of the final report.5... 

The peak area or height is converted into numerical data by either manual or electronic means. The measurement of peak height is simpler than the measurement of peak area and is often preferred for manual data... 

An additional problem is the measurement of peak areas itself. The integration will give rise to errors, especially if the peaks are not completely resolved and if the baseline varies during the analysis. 

Integrals can be determined by the spectrometer or by measurements of peak areas on a recorded spectrum (Adler et al. 1987). When a peak of interest overlaps other peaks, it may be desirable to determine an upper and lower value for the peak area (Adler et al. 1987). The following methods have been employed to express the results of integrations in structural terms calculations starting from the total integral of the lignin spectrum and the amount of hydrogen/... 

Data were obtained from measurements of peak height. Data were obtained from measurements of peak area. 

Most analytical determinations employing liquid chromatography are based on the measurement of peak area (A) or peak height (fit). The peak area is equal to the zeroeth moment of the Ciaussian distribution... 

The measurement of peak areas radier than peak heights would undoubtedly eliminate or reduce some of the matrix effects on sensitivity reported below 13), However, the integration mode of the Perkin Elmer 503 does not provide true peak area integration, but instead provides signal averages for a preset time subsequent to initiation of the atomiza-... 

The LAS in soils was quantified through the use of a standard sample of a LAS (C10-C14) as an external standard. Calibration plots were constracted from measurements of peak areas versus known weights of the reference LAS. Both fluorescence and UV detection resulted in linear plots for 0-0.75 ug and 0-2 ug LAS injected respectively and with correlation coefficients (R ) greater than 0.99. The individual LAS homologues were shown to have similar molar responses for fluorescence detection as was found previously for the UV system (Matthijs and De Henau 1987 Kikuchi et al 1986). 

A sample of the deuteriated polymer (0.531 g) was ozonized as a suspension in CH2CI2, and the product treated with formic acid-hydrogen peroxide as described previously in this section. Cyclization to d2-succinic anhydrides and derivatization to form the d2 N-(o-biphenyl) succinimides were also performed by previously described methods. %-NMR (acetone-d6, 200 MHz, ) resonances at 6 2.62 (s), 2.62 (d, J 4.4 Hz), 2.84 (s), 2.84 (d, J 4.4Hz), 7.48 (m). Measurement of peak areas by triangulation indicated that the d2-N-(o-biphenyl) succinimide produced in the above experiment was 55% meso-isomer and 45% d>l-isomer. 

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