Peak area, analytical significance

Monochromatic detection. A schematic of a monochromatic absorbance detector is given in Fig. 3.12. It is composed of a mercury or deuterium light source, a monochromator used to isolate a narrow bandwidth (10 nm) or spectral line (i.e. 254 nm for Hg), a flow cell with a volume of a few pi (optical path 0.1 to 1 cm) and a means of optical detection. This system is an example of a selective detector the intensity of absorption depends on the analyte molar absorption coefficient (see Fig. 3.13). It is thus possible to calculate the concentration of the analytes by measuring directly the peak areas without taking into account the specific absorption coefficients. For compounds that do not possess a significant absorption spectrum, it is possible to perform derivatisation of the analytes prior to detection. 

Raman spectroscopy can also directly benefit TE analysis by non-invasively monitoring the growth and development of ECM by different cells on a multitude of scaffold materials exposed to various stimuli (e.g. growth factors, mechanical forces and/or oxygen pressures). Indeed the non-invasive nature of Raman spectroscopy enables the determination of the rate of ECM formation and the biochemical constituents of the ECM formed. Univariate (peak area, peak ratios, etc.) and multivariate analytical techniques (e.g. principal component analysis (PCA)) can be used to determine if there are any significant differences between the ECM formed on various scaffolds and/or cultured with different environmental parameters, and what these biochemical differences are. Least square (LS) modelling, for example, could allow the quantification of the relative components of the ECM formed (Fig. 18.3) [4, 38],... 

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. 

Hence, the proportionality between peak area and concentration still exists. Suppressor systems cannot be used in this case because they would convert the anions mentioned above into the corresponding acids, which are not suitable for conductivity detection because of their weak dissociation. Although sodium hydroxide fulfills all requirements for indirect conductivity detection, its elution power is in many cases not sufficient to elute anions of practical relevance. Retention times are significantly shortened by adding small amounts of sodium benzoate to the sodium hydroxide solution, which barely affects the conductivity difference between eluent and analyte ions. However, shorter retention times improve the peak shape, thus increasing the sensitivity of the method. Figure 3.145 shows such separation. 

In the absence of a detectable blank, the relationship of the logarithm of peak area to the logarithm of the mass of injected DMS should be linear over at least two orders of magnitude. However, blanks may occur as a consequence of decreasing system performance after prolonged use (e.g., decreased sulphur scrubber efficiency, decreased surface silylation) or because of DMS traces in the seawater samples used for calibration. In this case, significant deviations from linearity are observed at low analyte levels. Therefore, the blank contribution, Wb, to the total amount of injected DMS must be taken into account ... 

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