Gradient analysis

An ion chromatographic system that included column switching and gradient analysis was used for the deterrnination of cations such as Na", Ca ", Mg ", K", and NH" 4 and anions such as Cf, NO, NO , and in fog water samples (72). Ion-exchange chromatography compares very well with... 

Stationary phase. Supelcosil Cig ABZ (Supelco Scientific, Bellefonte, PA, USA) was the most often employed support and gave the best correlations. This stationary phase should be selected in a first instance with a geometry adapted to the application for conventional gradient experiments, supports of 150X4.6 mm, 5 pm represent a good choice while a shorter column (i.e. 50 mm or lower) with smaller particle size (i.e. 3-3.5 pm) must be preferentially selected for fast gradient analysis. 

T. Alfredson, Reverse Phase Gradient Analysis of Polyolefin Polymer Additives, Publication No. LC-123, Varian Instmment Group, Walnut Creek, CA (n.d.). 

Gradient analysis (Parczewski [1981], Singer and Danzer [1984], Parczewski et al [1986], Parczewski and Danzer [1993]) which is based on two-dimensional regression models and adds pictorial information to statistical decisions. Figure 3.12 shows such a graphical representation of an element distribution on a surface. 

Most modern HPLC instruments include a column oven that can thermostat the column to at least 100°C. A typical HPLC analysis can be done in half the time by elevating the column temperature from ambient to 50 or 60°C. At temperatures above 100°C, it is not uncommon to decrease analysis time by a factor of 5.26 Also, re-equilibration time for the column is much shorter, so it is possible to achieve ultra-fast gradient analysis with HTLC. 

An ELSD converts the HPLC eluent into a particle stream and measures the scattered radiation. It offers universal detection for nonvolatile or semivolatile compounds and has higher sensitivity than the RI detector (in the low ng range) in addition to being compatible with gradient analysis. ELSD is routinely used in combinatorial screening. Response factors are less variable than that of other detectors. An ELSD consists of a nebulizer equipped with a constant temperature drift tube where a counter-current of heated air or nitrogen reduces the HPLC eluent into a fine stream of analyte particles. A laser or a polychromatic beam intersects the particle stream, and the scattered radiation is amplified by a photomultiplier. Manufacturers include Alltech, Polymer Laboratories, Shimadzu, Waters, Sedere, and ESA. 

Higher colnmn temperatnres (T) lower the viscosity of the mobile phase (and thns the pressnre) and increase the diffusion coefficient of the analytes (increase N). The effects of these operating parameters are dis-cnssed farther in the next section on gradient analysis. 

Knowledge of the pKa value is crucial for analyzing both lipophilicity and solubility of ionizable compounds, as discussed above. Ionization equilibria also affect several toxicokinetic parameters, such as intestinal absorption, membrane permeability, protein binding, and metabolic transformations. Therefore, much research has been invested in developing both experimental and computational tools for pKa determination. Experimentally, two high-throughput methods exist spectral gradient analysis and capillary electrophoresis. However, the most definitive methods are still... 

Stull R (1989) An Introduction to Boundary Layer Meteorology. Kluwer Academic Publ. ter Braak CJF (1986) Canonical correspondence analysis A new eigenvector technique for multivariate direct gradient analysis. Ecology 67 1167-1179... 

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