Silica-based packings, mass transfer with

SO3H) for cations, is the same in HPLC as in classical ion exchange chromatography. A variety of substrates ranging from cross-linked polystyrene, cross-linked polydextrans, cellulose and silica have been utilised. However, due to the problems of swelling, compressibility and mass transfer encountered with macroreticular polymeric supports and the limited pH stability of silica-based packings, the efficiencies achieved in lE-HPLC applications have been moderate. 

A liquid mobile phase is far denser than a gas and, therefore, carries more momentum. Thus, in its progress through the interstices of the packing, violent eddies are formed in the inter-particular spaces which provides rapid solute transfer and, in effect, greatly increases the effective diffusivity. Thus, the resistance to mass transfer in that mobile phase which is situated in the interstices of the column is virtually zero. However, assuming the particles of packing are porous (i.e., silica based) the particles of packing will be filled with the mobile phase and so there will... 

Pellicular or controlled surface porosity particles were introduced in the late 1960s these have a solid inert impervious spherical core with a thin outer layer of interactive stationary phase, 1-2 pm thick [13]. Originally, the inner sphere was a glass bead, 35-50 pm i.d., with a thin active polymer film or a layer of sintered modified silica particles on its surface. Such particles were not very stable, had very low sample load capacities because of low surface areas and are not used any more. Nowadays, this type of material is available as micropellicular silica or polymer-based particles of size 1.5 to 2.5 pm [14]. Micropellicular particles are usually packed in short columns and because of fast mass-transfer kinetics have outstanding efficiency for the separation of macromolecules. Because the solutes are eluted as very sharp narrow peaks, such columns require a chromatograph designed to minimise the extra-column contributions to band broadening. 

Gas chromatography, coupled with flame-ionisation, electron capture (for halogenated species) and mass spectrometric detectors, is the most popular tool for determination of SVOCs in melted snow samples [44]. A prerequisite is the efficient separation of the analytes from the aqueous matrix, which can be accomplished using filtration onto quartz fibre filters and sohd phase extraction [88]. Solid phase micro-extraction, which utilises equihbrium-based adsorption of analytes onto a polymer fibre bundle, has also been proposed and tested in laboratory studies [13, 89]. Both methods allow for an efficient transfer into the injection port of a gas chromatograph without water contamination. Directly coupled inlet sampler with GC-EID instrumentation has also been used [90]. The air sample was pre-concentrated using adsorbents (Carbotrap B, Carbosieve), followed by heating and collection on a cryofocuser (a fused silica capillary tube packed with... 

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