Retention volume measurements apparatus

Figure 7. Experimental high pressure gas chromatographic apparatus for retention volume measurements.

Scott and Reese (3) also measured the change in retention volume with solvent composition using the same LC apparatus as that used for investigating the effect of temperature. The column was thermostatted at 24.7°C and the results that were obtained are shown in figure 6B. 

Note The expression in a is defined from the retention volumes and remains useful with a flow gradient but it is very rare that it is employed since the apparatus does not measure the instantaneous rate of the column. Therefore the calculation of the retention volumes (other than by substituting in the relation, VR = D tR) is not permissible. 

There is a fundamental relationship described in chromatographic theory between the retention volume of a elution peak and the mid-point of a breakthrough curve achieved by operating the column under frontal analysis conditions (41 ). In the Henry s Law region of the adsorption isotherm, the net retention volume and its measurement can be used to describe the variation of sorbate breakthrough volume as illustrated in Figure 8. Utilizing the experimental apparatus described in the last section, retention volumes were measured as a function of pressure at 40°C (T =... 

The solutes chosen were (S) and (R) 4-benzyl-2-oxazolidinone. The apparatus consisted of an LC pump, a 1 pi internal loop sample valve and the column was situated in a thermostatically controlled oven. The oven could be operated at temperatures between 5°C and 85°C with a precision of 0.2°C. The mobile phase was passed through a preheater, consisting of 1 m length of coiled stainless steel tube, 0.010 in. ID., situated in the thermostat, between the sample valve and the column. The column outlet was connected directly to a UV detector operating at 220 nm. The eluent from the detector was passed to a 50 ml Grade A burette and the retention volumes were measured directly in ml of mobile phase. The burette was read with an accuracy of 0.02 ml. The output of the detector was displayed on a digital meter and an electronic recorder/integrator. 

Equation (8-3) is used to determine k values for experiments A-C, as indicated in the table. The Vr values for the dyes in this experiment are the volumes corresponding to the centers of the red and blue bands. No value is measured for Em, however. The best method for obtaining a true value of Em is to inject on the column a compound that is very similar, in chemical retentivity to the mobile phase and that responds to the detector. Such a compound would be unretained by the stationary phase and would elute after passing through the volume Em which is occupied by the mobile phase within the column. Due to the limitations of the apparatus used in these experiments, we will approximate Em- For a column filled with porous packing, the value of Em represents about 50% of the total empty column volume and can be estimated by the equation... 

The gas-phase tram-alkylation reaction was performed in an automated micro-flow apparatus containing a quartz fixed-bed reactor (i d. 10 mm) at lO Pa [16 vol% benzene (1, p.a., dried on molsieve), 3.2 vol% diethylbenzene (2, consisting of 25% ortho, 73% meta, 2% para isomers, dried on molsieve), N2 balance (50 mL/min), WHSV =1.5 h ] with 2.0 mL of the tube reactor filled with catalyst particles (500-850 pm sieve fraction, typically 1.4 g). Two separate saturators were connected to the inlet of the reactor for the supply of 1 and 2. The partial vapor pressure of 1 and 2 was controlled by adjusting the temperature of the saturator-condensers and the N2 flow rate. After equilibration for 30 min at the applied reaction temperatures (473 K and 673 K, heating rate 10 K/min) within a dry N2 flow (50 mL/min), benzene (1) and diethylbenzene (2) were passed throu the reactor. To prevent condensation of both reactants and products prior to GC analysis [Hewlet Packard 5710 A, column CP-sil 5CB capillary liquid-phase siloxane polymer (100% methyl) 25 m x 0.25 mm, 323 K, carrier gas N2, FID, sample-loop volume 1.01 pL], tubes were heat-traced (398 K). FID sensitivity factors and retention times were determined using ethene (99.5 %, dried over molsieve) and standard solutions of 1, 2, and ethylbenzene (3, 99%) in methanol (p.a.). The conversion of 2 was measured as a function of time [8]. 

The simplest approach to qualitative or semiquantitative analysis of products that are gases at ambient temperature (e.g., C02, S02, some nitrogen oxides, perhaps H20, etc.) is through retention with a refrigerant in a vacuum apparatus. Measurements are then made of the pressure exerted on subsequent volatilization of the condensed gases present within a vessel of known volume. By the use of several cold traps, estimations of the compositions of a limited range of simple mixtures are sometimes possible. 

A strong positive feature of SEC is that instrumentation is readily available in the form of HPLC apparatus. No special experience is needed for those acquainted with this widely practiced method. Relatively unskilled operators can quickly learn to perform the analysis satisfactorily. Average particle sizes are quickly measured by the peak-position method. However, it is also feasible to determine particle-size distributions if appropriate computer software is available. Separation times are predetermined, because all species elute between the total exclusion and total permeation volumes (provided the desired SEC process is the only retention). No special method development is required, other than ensuring that the proper mobile phase-stationary phase combination is selected. Particle diameter is directly a function of retention or elution times. 

Countercurrent Chromatography Procedure. The entire column (pair of coiled multilayer columns connected in series) was filled with the stationary phase. The apparatus was then rotated counterclockwise at 600 rpm in planetary motion while the mobile phase was pumped into the inlet of the column at a flow-rate of 2.2 mL/min (head to tail elution mode). Maximum pressure at the outlet of the pump measured 80 psi. After a 1-hour equilibration period, the sample was loaded into the Rheodyne injector loop and injected. Effluent from the outlet of the column was continuously monitored with a Shimadzu UVD-114 detector at 312 nm and fractions collected with a Gilson FC-lOO fraction collector to obtain approximately 8.8 mL of eluant in each tube (during a 4-min interval). Retention of the stationary phase was estimated to be 930 mL (74%) by measuring the volume of stationary phase eluted from the column before the effluent changed to mobile phase (330 mL) and subtracting this volume from the total column capacity of 1260 mL. 

Step 4. Finally, the apparatus can be emptied (for instance, by pushing with nitrogen gas) and the liquids collected in another graduated cylinder the volume of the stationary phase is V3. For simplification purposes, the extracolumn volumes are neglected. Three measurements of the stationary phase retention are available ... 

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