Organic Soluble Samples

Polymer samples are size separated by dissolving them in an organic solvent, such as THF, then passing them through a linked series of sizing columns. These cross-linked polymer columns vary in pore size, with each column 

Switching solvents is a very poor practice because of bed swelling and shrinking from solvent to solvent. Usually, a solvent is selected with a broad solubility range the system is turned on and allowed to equilibrate for 24 hours and then kept in a constant flow recycle mode until needed. When a sample is to be shot, flow is switched out of recycle, the chromatogram run, and then immediately returned to recycle. The pump is left on at all times. 

Very-high-density polymers are run in a special high-temperature HPLC. This device can automatically dissolve, filter, inject and run these samples at 200°C in a solvent such as chlorobenzene. 

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Figure 6.47 Simplified guide to column selection (a) for organic-soluble samples (b) for water-soluble samples.

Many samples are more soluble or less prone to decompose in organic than in aqueous mobile phases. Such samples do not cause direct-injection problems in NPLC, unlike to RPLC. (5) The recovery of organic-soluble sample components is easy in preparative NPLC. (6) Very large changes in separation selectivity are possible by changing either the mobile phase or the stationary phases in NPLC. 

Implementation Elemental analysis shows that the organic portion of the sample is about 50% carbon, 8% hydrogen, and has no detectable nitrogen or sulfur. About 40 wt% of the sample is soluble in an organic solvent (2 1 chloroform methanol). GC analysis of the organic-soluble portion is found to be fatty acids (Fig. 21.15). 

To insure that all the molybdenum in the oil whether present in an organically soluble form or in varying forms as particulate matter would be recorded by AAS, a sulfuric and nitric acid wet digestion process was selected for sample preparation. This process differs from the wet ashing procedure in that the former is carried out in a liquid acid medium from start to finish at a relatively low temperature (280 C), whereas in the wet ashing procedure after acid charring the oil, the char is subjected to a 600 C muffling step. 

Oel filtration and gel permeation methods are complementary in that gel filtration is applied to water-soluble samples and gel permeation is used for substances in less-polar organic solvents. One useful application of the size-exclusion procedure is to the separation of high-moIccular-mass, natural-product molecules from low-molccular-mass species and from salts. For example, a gel with an exclusion limit of several thousand can clearly separate proteins from amino acids and low-molecular-mass peptides. 

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