Relative peak heights, quantitative

Both absolute quantitation and relative quantitation of species in mixtures is of interest in some circumstances. Quantitation in a 5-minute analysis can be achieved by addition of an internal standard, ideally the target microorganism grown in special media to incorporate heavy isotopes92-95 and determination of the relative peak heights of pairs of proteins from the analyte and the standard. Isotope-labeled proteins or peptides, selected to match proteins or peptides characteristic of target microorganisms, can also serve as internal standards for isotope ratio measurement. The addition of unmatched proteins or peptides is less reliable for either ESI or MALDI measurements because of unpredictable suppression in the variable mixture. 

If the analysis to be optimized involves a sample in which the relative peak areas are expected to be constant (for instance in a quality control situation), then a criterion may be used that is affected by the relative peak height (A), i.e. FO or Pv may be used. If this is not the case, then a criterion should be selected that does not vary with A ( Rs or S P or P"). This will avoid the very unattractive situation in which the location of the optimum is a function of the (quantitative) composition of the sample, so that in theory there may be different optimum conditions for every single sample ... 

Although less important than peak frequencies, peak intensities obviously play a role in quantitative analysis and, in many cases, qualitative identification. Multivariate calibration techniques and their transferability depend on reproducible relative peak heights. A possibly lengthy method development procedure may fail when a different spectrometer is used, if the observed intensities vary. Reproducibility of absolute signal is difficult to achieve between labs or even between instruments of the same design, but it is important for a particular instrument. Absolute intensities can at least be used to evaluate day-to-day instrument performance and to detect hardware or alignment problems. 

Capillary electrophoresis technology has become an indispensable tool for forensic scientists in the biology field since it is able to provide valuable information to aid in the process of law enforcement. The primary application of the technique is in the qualitative analysis of STRs. Isolation of STR mixtures is also possible using relative peak heights. Other applications of CE include quantitative analysis of PCR products, mtDNA sequencing, and mutation detection for the analysis of plant and bacterial DNA. Based on the performance of the methods illustrated above, it is reasonable to expect future researchers and practitioners to continue working to exploit the capabilities of this robust scientific technique and its application to criminal investigations. 

As mentioned in the results section, a thorough analysis of the XANES region is not possible. However a tentative comparison can be made with the results of Mountjoy et al. [22], who reported reference peak heights and positions shown in Table 4.10. The relative peak positions of the five samples (Table 4.7) imply 4 coordinated Ti is present. Although the XANES peak height is monochromator dependent, as Mountjoy et al. [22] also used an Si(lll) monochromator a semi-quantitative comparison will be made. From Table 4.7 it can be seen that the relative peak heights of the five samples lie between the 4/5 and 6 coordinated heights... 

The measurement of the current for a redox process as a fiinction of an applied potential yields a voltaimnogram characteristic of the analyte of interest. The particular features, such as peak potentials, halfwave potentials, relative peak/wave height of a voltaimnogram give qualitative infonnation about the analyte electrochemistry within the sample being studied, whilst quantitative data can also be detennined. There is a wealth of voltaimnetric teclmiques, which are linked to the fonn of potential program and mode of current measurement adopted. Potential-step and potential-sweep... 

Successful use of modern liquid chromatography in the clinical laboratory requires an appreciation of the method s analytical characteristics. The quantitative reproducibility with respect to peak height or peak area is quite good. With a sample loop injector relative standard deviations better than 1% are to be expected. The variability of syringe injection (3-4% relative standard deviation) requires the use of an internal standard to reach the 1% level (2,27). 

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... 

To run a patient sample, you will need to go through exactly the same deproteination, SFE cartridge extraction, IS addition, mobile phases dilution, and injection steps (Fig. 12.4f). From the peak heights relative to the IS height, we can now quantitate the amount of each drug in the patient s blood. To insure linearity, you may need to dilute our windowed plasma blank and spike it with different levels of each standard and plot calibration curves for each compound, but basically, our methods development is done. 

Calibration standards can be of two types external standards and internal standards. With external standards, multiple concentrations of the standards are injected, areas are measured, and a calibration curve is platted. Unknown samples are then injected, chromatograms run, and areas are calculated and compared with the calibration curves to determine amounts of each compound present. With internal standards, known amounts of an internal standard are added to each known concentration of standard compound and areas or peak height response factors relative to those of the internal standard are calculated. When unknowns are run, a known amount of internal standard is added to the unknown sample, response factors are calculated relative to the internal standards, and amounts of each unknown present are calculated from the standards calibration factors. Internal standards are usually used to correct for variations in injection size due to different operators and injection techniques. Internal standards can also be used to correct for extraction variation in GC/MS target compound quantitation, this standard is referred to as a surrogate standard. Generally, an internal standard is used for one purpose or the other, not both at the same time. 

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