Specifications detector temperature sensitivity

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). 

Improvement in sensitivity can be obtained by increasing the temperature of the sample or by the salting-out effect, which is particularly useful for compounds such as phenols and fatty acids which form strong hydrogen bounds in aqueous solutions. With some compounds, the use of a more sensitive detector such as an electron-capture detector or an element-specific detector will enhance sensitivity. 

The rate of temperature programming or ramp rate can influence the bleed profile from a column. As the rate of temperature programming increases, column bleed also increases. Finally, the more sensitive element-specific detectors (e.g., an BCD or NPD) will generate a more pronounced bleed profile if the stationary phase contains a heteroatom or functional group (-CN or -F) to which a detector responds in a sensitive fashion. 

In the study of atmospheric temperatures and composition one is often interested in the emission from a particular atmospheric constituent. For the analysis of the vertical temperature profile on Earth, Venus, and Mars, thermal emission from the CO2 molecule can be used. If the same gas is contained in the absorption cell, the radiation of interest is being filtered out. How does one measure the radiation that has just been removed from the beam This can be accomplished in several ways. For example, consider an absorption cell with two windows on opposing ends exposed to a beam of radiation. Wavenumbers outside the gas absorption band and in the transparent gaps between lines will penetrate the cell without a noticeable effect. Radiation within the width of strong lines will be absorbed and will cause a temperature rise in the gas. The corresponding pressure increase may be registered by a sensitive pressure transducer. The resulting infrared detector is sensitive only to radiation specifically tuned to the gas in the cell. Such detectors have been produced (the Patterson-Moos cell), but have, as far as we know, never been applied to planetary work. 

In the pharmaceutical industry, it is essential to produce pure drug substance, suitable for human consumption, in a cost-effective manner. The purity of a drug substance can be checked by separation techniques such as GC, TLC, and HPLC. Both techniques tend to be more sensitive and specific than spectroscopic methods. HPLC has an advantage over GC as an analytical technique, since analytes need be neither volatile nor extremely stable to elevated temperatures. Highly accurate, almost universal detectors, such as... 

RI detectors measure this deflection, and are sensitive to all analytes that have a different R1 than the mobile phase. There are two major limitations First, Rl detectors are very sensitive to changes in the temperature, pressure, and flow rate of the mobile phase, and so these measurement conditions must be kept stable in order to obtain low background levels. Second, Rl detectors are incompatible with chromatographic separations using gradient elution. Furthermore, because Rl detectors are nonselective, they must be used in conjunction with other detection methods if specificity is required. Nevertheless, they have found wide application in isocratic chromatographic analysis for analytes that do not have absorptive, fluorescent, or ionic properties, such as polymers and carbohydrates. 

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