Void volume, detection

The retention time of the non-adsorbing methane (ti) is the measure of the column void volume or holdup. Ethylene is adsorbed by the catalyst, hence it does not reach the detector until the available surface is saturated, at which point ethylene breaks through and is detected by the sensor (t2). The adsorbed volume of ethylene is given simply by ... 

Figure 3.9 Elution volumes (ml) of alternative void volume markers. Column, 5 im octadecyl-bonded silica gel, 15 cm x 4.5 mm i.d. eluents A and B, 10-90% aqueous acetonitrile, eluents C and D, 10-90% aqueous acetonitrile containing 50 mMphosphoric acid flow rate, 1 ml min temperature, 30 °C detection, UV 210 nm and refractometer. Sample a, acetonitrile b, methanol c, fructose d, 2,4-dinitronaphthol e, sodium nitrate, f, tetrahydrofuran g, deuterium oxide, and h, uric acid.

Note that Vg is proportional to the square of the inner radius of the column. It is important to have a rough idea of the void volume of the column since it often dictates the operating flow-rate range, sampleloading capacity, and mass sensitivity (the minimum detectable amount) of the assay. For instance, a typical analytical column (150mm x 4.6mm i.d.) has a Vg of about 1.5 mL and is operated at 1.0mL/min. In contrast, by reducing the inner diameter to 2.0 mm, a typical LC/MS column (150mm X 2.0mm i.d.) has a Vg of about 0.3mL and is operated at... 

Attempts to increase the size of nitrogen adsorption or desorption signals, by using larger sample cells, results in enhanced thermal diffusion signals due to the increased void volume into which the helium can settle. However, when krypton is used, no thermal diffusion effect is detectable in any of the sample cells shown in Fig. 15.10. 

Fig. 18 RP-HPLC of methylesters derived from three different fish oil sources sardine (A), menhaden (B), and cod liver (C). Identified fatty acid methyl esters in order of their elution 1. C20 5o 3 2. C14 0 3. C16 lw9 4. C22 6o>3 5. C18 2a>6 6. C16 0 7. C18 1 >9. BHT is eluted right after the void volume remaining peaks have not been positively identified. Mobile phase acetonitrile/THF/water (9 5 11) at 2.0 ml/min stationary phase WHATMAN ODS-3 RAC II (100 X 4.6-mm ID) detection, refractive index at ambient temperature.

Sample Extraction for the Two-Vessel Setup. The extraction of the 36 nitroaromatic compounds from sand spiked at 600 ng/g (per analyte) was begun at 150 atm/50°C/10 min (dynamic), continued at 200 atm/60°C/10 min (dynamic), and completed at 250 atm/70°C/10 min (dynamic) using carbon dioxide only. Sample size was 2.5 g. For all the experiments reported in this paper, the sample was sandwiched between two plugs of silanized glass wool to fill out the void volume. Two spiked samples identified as Experiments 1 and 2 in Figures 3a and 3b were extracted in parallel using the same conditions. Two additional sand samples spiked at the same concentration were extracted in parallel at 300 atm/70°C/30 min (dynamic) and are identified in Figures 3a and 3b as Experiments 3 and 4. To verify the completeness of the SFE technique, we collected an additional fraction for Experiments 1 and 2 at 300 atm/70°C/30 min (dynamic). No compounds were detected in the second fraction. 

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. 

Direct measurement [44] In spite of the disappointing results above, we were able to use this system for the detection of j -estradiol by virtue of its inherent fluorescence. The use of a MIP as the chromatographic stationary phase greatly facilitates the separation of the analyte from other species. It is noteworthy that estrone, which fluoresces at approximately the same wavelength as jS-estradiol due to its phenolic A-ring, was eluted in the void volume, while the fluorescence of /1-estradiol reached a maximum after approximately 9 min in pure MeCN (flow rate 1 mL min ). Thus, the only potential interfering species could be effectively removed from a sample containing a mixture of steroids. 

A variant on the dissolution methods discussed uses neither basket nor paddle. Convection is achieved by solvent flow through a chamber such as that drawn in Fig. 12.4. Dissolution data obtained from such a system where continuous monitoring of dmg concentration is achieved must be interpreted with care as the concentration-time profile will be dependent on the volume of solvent, its flow rate and the distance of the detection device from the flow cell, or rather the void volume of solvent. 

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