System peaks Sensitivity

With the multitude of transducer possibilities in terms of electrode material, electrode number, and cell design, it becomes important to be able to evaluate the performance of an LCEC system in some consistent and meaningful maimer. Two frequently confused and misused terms for evaluation of LCEC systems are sensitivity and detection limit . Sensitivity refers to the ratio of output signal to input analyte amount generally expressed for LCEC as peak current per injected equivalents (nA/neq or nA/nmol). It can also be useful to define the sensitivity in terms of peak area per injected equivalents (coulombs/neq) so that the detector conversion efficiency is obvious. Sensitivity thus refers to the slope of the calibration curve. 

Based on the high peak capacity of CE, the separation speed, and the availability of numerous chiral selectors and the simplicity of the systems, chiral CE is superior to chiral HPLC separations. This is as well reflected by the high number of publications on chiral CE in recent years. Chiral HPLC is suffering from low peak capacity (broad peaks), system stability (often normal phase systems), pressure sensitivity of columns (often cellulose-based column materials), and as a consequence long separation times. 

Certain SEC applications solicit specific experimental conditions. The most common reason is the limited sample solubility. In this case, special solvents or increased temperature are inavoid-able. A possibility to improve sample solubility and quality of eluent offer multicomponent solvents (Sections 16.2.2 and 16.8.2). The selectivity of polymer separation by SEC drops with the deteriorating eluent quality due to decreasing differences in the hydrodynamic volume of macromolecules with different molar masses. The system peaks appear on the chromatograms obtained with mixed eluents due to preferential solvation of sample molecules (Sections 16.3.2 and 16.3.3). The multicomponent eluents may create system peaks also as a result of the (preferential) sorption of their components within column packing [144,145]. The extent of preferential sorption is often sensitive toward pressure variations [69,70,146-149]. Even if the specific detectors are used, which do not see the eluent composition changes, it is necessary to discriminate the bulk sample solvent from the SEC separated macromolecules otherwise the determined molecular characteristics can be affected. This is especially important if the analyzed polymer contains a tail of fractions possessing lower molar masses (Sections 16.4.4 and 16.4.5). 

The utility of UPLC has been demonstrated for both qualitative and quantitative analyses. In 2005, Castro-Perez et al. (2005) compared the performance of a HPLC with that of a UPLC and showed that improved chromatographic resolution and peak capacity attained with UPLC lead to reduction in ion suppression and increased MS sensitivity. Comparison of the mass spectrum obtained using HPLC with that from UPLC (Fig. 1.15) revealed that the higher resolving power of the UPLC-MS system resulted in a much cleaner mass spectrum than that obtained using the HPLC-MS system. The sensitivity improvement directly resulted in a higher ion count in the UPLC mass spectrum (855 vs. 176). 

The application of indirect methods in biomedical analysis has been reviewed by Schill and Arvidsson.23 Reversed-phase HPLC is the main field of application for indirect detection, and both charged and uncharged species can be visualized, although sensitivity is better for ionic solutes. With indirect UV detection for reversed-phase HPLC, the eluent contains a chromophoric group (probe), and peaks are obtained for injected solutes as well as for the mobile phase additives (system peaks). For solutes that... 

The sensitivity of a detector is not the minimum mass that can be detected. This would be the system mass sensitivity, which would also depend on the characteristics of the apparatus as well as the detector and, in particular, the type of column employed. During the development of a separation the peaks become broader as the retention increases. Consequently, a given mass may be detected if eluted as a narrow peak early in the chromatogram, but if eluted later, its peak height may be reduced to such an extent that it is impossible to discern it from the noise. Thus detector sensitivity quoted as the minimum mass detectable must be carefully examined and related to the chromatographic system and particularly the column with which it was used. If the data to do this are not available, then the sensitivity must be calculated from the detector response and the noise level in the manner described above. 

The concentration sensitivity of a chromatographic system (XJ is defined as that which will provide a peak with a height equivalent to twice the noise level and can be obtained directly from the system mass sensitivity. If the minimum detectable mass is dissolved in the maximum permissible sample volume [9] (that sample volume that will limit the increase in sample variance to 10% of the column variance), then this solute concentration will constitute the minimum detectable sample concentration. 

The sensitivity or MDC of a detector is not the same as the minimum mass that can be detected. This would be the system mass sensitivity, which will depend on the characteristics of the column and the chromatographic properties of the solute, as well as the detector specifications. In all chromatographic systems, the peak becomes broader as the retention increases. Con-... 

These systems had already proved suitable for routine use. However, their disadvantage was, that the measuring procedure was relatively slow and that all spots on the track surpassing the peak sensitivity value were registered. The easily accessible and very clearly designed software worked out by Ebel, Hocke and Kaal, however, allowed the user to make improvements 34,35). 

A system peak is a peak that originates from the chromatographic system itself, i.e., mobile phase and column, and not from the sample. Its appearance and size are sensitive to the sample composition, but its origins are generally the mobile phase components. 

The best w s to avoid system peaks are (1) to dissolve the sample in the mobile phase vdienever possible, and (2) to work at a wavelength where the sample matrix and the mobile phase have little ot no absorbance. It ould be noted that even if the second condition is satisfied, the refractive index difference between the umnatched san le solution and the solvent system might generate lefiactive index-related peaks or other baseline shifts. These peaks may elute at any time during the elution process and could also be very disruptive, especially when working at sensitive detector settings. 

In addition, we have proposed some other atom loealization schemes in four-level atomic systems. Phase-sensitive atom localization has been realized in a loop A -system[25]. Due to the sensitivity of the loop system to the relative phases between the eoupled fields, the detection probability of atoms within the sub-wavelength domain of the standing wave can be improved by a factor of 2 while appropriate choosing the relative phase. In a four-level alkaline earth atomie system[26], not only the positions bnt also the widths of the localization peaks are investigated systemically. It is shown that the nnmbers and the widths of the localization peaks can be controlled by adjusting the additional control field. 

Applications of DTA for Polymers. Table 2 (Ref 5, Chapt. l) describes some of the many applications of DTA and DSC. Both DTA and DSC can be used to determine the temperature of the transitions, cited in Table 2. However, the DSC peak area, in addition, gives quantitative calorimetric information (heat of reaction, transition, or heat capacity). DTA can only do so when calibration with a standard material allows the quantitative conversion of AT to heat flow and, ultimately, heat of transition (AH) or heat capacity (Cp). Also, the response of DTA with increasing temperature may be affected by poor heat transfer in the system, detector sensitivity, etc (4). For these reasons, when there is a choice between DSC and DTA, DSC is the preferred method. The illustrations shown below for applications of DSC in characterization of polymers also generally apply for DTA, with the limitations mentioned above. Therefore, DTA applications will not be considered here. Illustrations of polymer applications for DTA can be found in the Thermal Analysis section by Bacon Ke (6) in the previous edition of this encyclopedia. 

Alternatively, ultraperformance—ultrahigh-performance LC (UPLC— UHPLC) systems have the benefit over conventional LC of increased peak resolution, which may be sufficient to separate some of the analytes of interest from matrix components. The validated method of Badawi et al. [90] is an example of the application of a matched UHPLC—UPLC— tandem MS system to the analysis of oral fluid for DUID purposes. The additional benefits of higher peak sensitivity and the ability to shorten the run time of the analysis offered by UPLC systems make them ideal for DUID testing. To take full advantage of UHPLC—UPLC, fast tandem—MS instruments are required to obtain sufficient data points across such narrow peaks. 

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