High-Concentration System Peaks

Nicardipine is almost completely absorbed after po adrninistration. Administration of food decreases absorption. It undergoes extensive first-pass metaboHsm in the Hver. Systemic availabiHty is dose-dependent because of saturation of hepatic metaboHc pathways. A 30 mg dose is - 35% bioavailable. Nicardipine is highly protein bound (>95%). Peak plasma concentrations are achieved in 0.5—2.0 h. The principal path of elimination is by hepatic metaboHsm by hydrolysis and oxidation. The metaboHtes are relatively inactive and exert no pharmacological activity. The elimination half-life is 8.6 h. About 60% of the dose is excreted in the urine as metaboHtes (<1% as intact dmg) and 35% as metaboHtes in the feces (1,2,98,99). 

The use of TFA as a mobile-phase additive in LC-MS can be problematical when using electrospray ionization. In negative ion detection, the high concentration of TFA anion can suppress analyte ionization. In positive ion detection, TFA forms such strong ion pairs with peptides that ejection of peptide pseudo-molecular ions into the gas phase is suppressed. This problem can be alleviated by postcolumn addition of a weaker, less volatile acid such as propionic acid.14 This TFA fix allows TFA to be used with electrospray sources interfaced with quadrupole MS systems. A more convenient solution to the TFA problem in LC-MS is to simply replace TFA with acetic or formic acid. Several reversed-phase columns are commercially available that have sufficient phase coverage and reduced levels of active silanols such that they provide satisfactory peptide peak shapes using the weaker organic acid additives.15... 

Figure 13.7 Electropherograms showing the simultaneous measurement of low- and high-energy explosives as recorded with the (a) conductivity and (b) amperometric detectors. Analytes, ammonium (1), methylammonium (2), sodium (3), TNB (4), TNT (5), 2,4-DNB (6), and 2-Am-4,6-DNB (7), system peak (SP). Explosive concentration, 2 mM (1,2,3) and 15 ppm (4,5,6,7). Conditions MES/His buffer (20 mM, pH 6.1) containing 15 mM lithium dodecyl sulfate as the run buffer separation field strength, +250 V/cm injection field strength, +250 V/cm for 2 s detection at 200 kHz, (a) 5 Vp p and at (b) —0.5 V. (Reprinted in part with permission from [34]. Copyright 2002 American Chemical Society.)...

An exothermic peak similar to P was also observed in the case of highly concentrated aqueous solutions of NaCS and NaPSS (1,2). Therefore, it is supposed that P is observed when the molecules in the water-polyelectrolyte systems rearrange to a stable state having lower energy than the normal state of water. This suggests that the molecules in the water-NaLS system assume a loosely ordered state in the range between the temperatures where P and P/ appear. 

Four replicate columns were run, and all produced the same type of elution curve. Lyophilized samples of another aliquot of the same soil used in the columns yielded soil water with a tritium concentration the same as that in the first aliquot drawn from the column. The first aliquot was equivalent to 1 inch of rain on the column and represents about one-third of the free water in the original soil material in the column. Zimmerman (13) reported a similar phenomenon when he used tritiated water to trace pulses of rainwater in soil systems. He found a relatively discrete front associated with the tritium injection after rainfall had occurred and eluted the tritium pulse into the soil profile. The frontal movement of high specific activity tritiated water from the surface layers in ejecta strata Sedan accounts for the peak tritium concentrations observed at depths just below the maximum penetration of rainfall. 

The volume of the NMR detection cell is relatively large (30-240 xl) when compared with the peak volumes and other void volumes in the chromatographic system. This leads to a considerable broadening of the peaks when they pass the NMR detection cell. It takes a long time until a peak is completely washed out of the flow cell, i.e. a tailing is observed. This is especially critical when traces of a high-concentration first peak interferes with the spectrum of a minor compound. 

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