Packed Columns

For many years most gas chromatography was carried out with packed columns. Packed columns for analytical applications have internal diameters usually between 2 and 5 mm and lengths from 0.5 to 5 m, and contain particles around 100-250 |im in diameter with a range of + 25 |im, carrying the liquid phase. Packed columns have been constructed of various materials but the preferred materials are glass and stainless steel. The packing can be a solid adsorbent such as silica, alumina or graph-itized carbon for gas-solid chromatography or a porous support coated with a high boiling liquid for gas-liquid chromatography. The usual support for the liquid phase is diatomaceous earth, a form of naturally occurring silica, with a surface area of about 0.5-4.0m2g_1 and a capacity to retain physically 5-30% (w/w) of liquid phase. Celite, a commercial diatomaceous earth, was used by Martin and James in their early experiments and is still a common support for packed columns. Polar compounds such as those found in the biomedical field give severe tailing, possible decomposition and

Table 1 Important advances in gas chromatography

1941 1952 1955

1955-1960

1958

1960-1970

1970-1980

1979

1980-1990

1990-Today

Fundamentalpaperon partition chromatography (Martin and Synge) Fundamentalpaperintroducesgas chromatography (James and Martin) First commercial GC instrument (thermal conductivity detector) Rapid period of technological growth Invention of ionization detectors (FID, ECD) Direct coupling to mass spectrometry Microsyringes Temperature programming Fundamental paper describes open-tubular columns (Golay) Period of technical advancement Stainless steel open-tubular columns Transistors replace vacuum tubes Stable rubidium sources for AFID Improved FPD (several designs) Pulsed ECD Period of consolidation and refinement Microprocessor-based instruments introduced

Preparation of glass open-tubular columns mastered by some research groups Fundamentalpaperdescribes fusedsilica open-tubularcolumns (Dandeneau and Zerenner) Period of technical advancement Gum and immobilized phases Thick-film open-tubular columns Wide-bore open-tubular columns

On-column and PTV injection (greater understanding of the injection process) Large volume injection (LC-GC) Computing integrators for data handling Autosamplers Period of consolidation and refinement Keyboard instrumentation (PC control of operation and data handling) Electronic pneumatic control Selectable elemental detection (AED) Sensitive and versatile spectroscopic detectors (MS, FTIR)

structural rearrangements or even complete adsorption on untreated diatomaceous earth. Acid and/or base washing to remove metallic impurities and silan-ization of surface silanol groups are widely used to minimize these effects. Fluorocarbon powders have been used occasionally in the separation of reactive compounds such as hydrogen chloride and organo-metallic compounds. Glass beads have also been used in theoretical studies, but have no practical application.

With packed columns the only way to make a radical alteration in the selectivity is by changing the liquid phase. This gave rise to a large number of stationary liquids, many with similar separation properties. Rohrschneider developed an empirical classification method based on the comparison of Retention Index (see below) differences for a number of standard compounds on the liquid phase to be characterized, relative to squalane as a reference phase. The scheme was extended by McReynolds and although it had no fundamental basis it did allow a list of preferred liquid phases to be drawn up. By employing the Rohrschneider-McReynolds classification the number of liquid phases can be drastically reduced and seven phases can be recognized as preferred choices for packed column gas-liquid chromatog-raphy (Table 2). If sample volatility is also considered then a poly(dimethylsiloxane) such as OV-1 would have to be substituted for squalane.

Another factor in the move towards rationalization of the choice of stationary phase was the introduction by Kovats in 1958 of the Retention Index system for expressing retention times (or volumes) relative to a series of standards. This was not a new idea, but Kovats proposed that the normal paraffins be taken as the standards for the scale of reference. The Retention Index system took rather a long time to be accepted since it was originally published in an overcomplicated form in German and it was not until it was publicized by Ettre in Analytical Chemistry some years later that the system received wider acceptance.

Much work has been carried out on the relationship between structure and Retention Index and the concept has proved so useful that it has been transferred in a modified form to liquid chromatography.

Current uses of packed columns include large scale separations, physicochemical measurement of compounds used as stationary phases (inverse gas chromatography), separations employing stationary phases not easily immobilized on fused silica surfaces (see below) and the routine analysis of simple mixtures in a dirty matrix not tolerated by open-tubular columns.

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