Open Tubular Columns

In 1958 Golay, wishing to simplify the mathematics of the flow of gas in a packed column with many tortuous paths, used a model consisting of a tube of capillary dimensions. He was able to demonstrate theoretically that such a capillary coated with a thin film of liquid would give columns with very high numbers of theoretical plates. The fundamental difference between packed and open-tubular columns is the much lower resistance to gas flow of the latter, which means that in practice very much longer columns can be used and very high efficiencies obtained. The reason for the need for a column of capillary dimensions can be understood from the equation derived by Golay stated in its modern form:

Here H is the height equivalent to a theoretical plate, f1 and f2 are pressure correction factors, k is the retention factor (frequently called the capacity factor), Dmo is the solute diffusion coefficient in the mobile phase at the column outlet pressure, Ds is the solute diffusion coefficient in the stationary phase, uo is the velocity of the mobile phase at the column

Table 2 Selection of preferred stationary phases for method development at intermediate column temperatures

Representative phase

Solvent characteristics

Table 2 Selection of preferred stationary phases for method development at intermediate column temperatures

Representative phase

Solvent characteristics

Squalane

Low cohesion and minimal capacity for polar interactions

OV-17

Low cohesion with a weak capacity for dipole-type interactions and weak hydrogen-bond basicity

QF-1

Low cohesion and intermediate capacity for dipole-type interactions combined with low hydrogen-bond

basicity

U50HB

Low cohesion and weak dipole-type interactions with interactions with intermediate hydrogen-bond

basicity

CW 20M

Low cohesion and intermediate capacity for dipole-type interactions and intermediate hydrogen-bond

basicity

QTS

Intermediate cohesion with a large capacity for dipole-type interactions and strong hydrogen-bond

basicity

OV-275

Very cohesive solvent with a large capacity for dipole-type interactions and intermediate hydrogen-bond

basicity

outlet, dc is the column internal diameter and df is the thickness of the film of stationary phase.

The three additive components of the plate height represent the contributions of longitudinal diffusion, resistance to mass transfer in the mobile phase and resistance to mass transfer in the stationary phase, respectively. Open-tubular columns minimize the contribution from resistance to mass transfer in the mobile phase. The stationary phase mass transfer term becomes increasingly important as the liquid film thickness increases beyond about 1 |im.

Soon after his theoretical investigation, Golay was able to demonstrate successfully the practical application of the theory. An important factor in the development of open-tubular (capillary) columns was the discovery of the flame ionization detector capable of giving adequate signals for the small sample sizes required by these columns. These early advances gave some spectacular separations, especially for complex hydrocarbon mixtures, but the general use of open-tubular columns evolved only slowly because many difficult problems remained that required a further 20 years of development. Stainless steel capillaries gave satisfactory performance for hydrocarbon mixtures but for more polar compounds they gave tailing peaks and poor efficiencies, owing to adsorption. Desty and co-workers described a glass-drawing machine capable of producing long lengths of coiled glass capillary tubing as early as 1960 but it was soon found that it was difficult to obtain uniform and stable thin films on glass columns. The surface was too smooth to allow good adhesion by physical adsorption of many of the phases commonly employed at the time, and the many metals included in glass, although at relatively low concentrations, gave rise to adsorption of polar analytes. Considerable effort was expended to overcome these problems, particularly notable being the work of G. and K. Grob. Leaching with aqueous hydrochloric acid to remove some of the surface metal ions followed by treatment with silanes was one approach. Deposition of a layer of barium carbonate to retain the stationary liquid was another practical solution. Such treatments, although successful in skilled hands, were regarded by many as a black art and the resulting columns were fragile and easily destroyed. Figure 1 shows the separation of peppermint oil on three generations of Carbowax 20M columns up to 1980.

In addition to the work on true open-tubular columns, intermediate variants between these and packed columns were developed. These include micro-packed columns and support-coated open-tubular (SCOT) columns where a thin coating of a very fine diatomaceous support was deposited on the inner wall of a stainless steel capillary. Another

Figure 1 Three generations in gas chromatography. Peppermint oil separated on (A) 6ftxJ in. i.d. packed column, (B) 500 ft x 0.03 in. i.d. stainless steel open-tubular column, and (C) 50 m x 0.25 mm i.d. glass open-tubular column. All columns contained Carbowax 20 M stationary phase and were operated under optimized conditions. (Reproduced with permission from Jennings W (1979) The use of glass capillary columns for food and essential oil analysis. Journal of Chromatographic Science 17: 636-639, Copyright Preston Publication, Inc.)

Figure 1 Three generations in gas chromatography. Peppermint oil separated on (A) 6ftxJ in. i.d. packed column, (B) 500 ft x 0.03 in. i.d. stainless steel open-tubular column, and (C) 50 m x 0.25 mm i.d. glass open-tubular column. All columns contained Carbowax 20 M stationary phase and were operated under optimized conditions. (Reproduced with permission from Jennings W (1979) The use of glass capillary columns for food and essential oil analysis. Journal of Chromatographic Science 17: 636-639, Copyright Preston Publication, Inc.)

variant was the porous-layer open-tubular (PLOT) column with a thin coating of an adsorbent on the inner wall of the capillary. PLOT columns still find application for the analysis of low boiling mixtures such as light hydrocarbon gases, but the other types are now no longer used.

The introduction of open-tubular columns made from fused silica in 1979 made glass capillaries obsolete almost overnight and also caused packed columns to be displaced as the dominant type. Currently the dimensions of commercially available open tubular columns range from 100 |im to 530 |im in inner diameter and from 5 m to 100 m in length.

Today nearly all open-tubular columns are prepared from either fused silica or metal capillaries lined with fused silica. Fused silica is essentially pure silicon dioxide containing less than 1 ppm of metallic impurities. When drawn, these capillaries are very fragile and must be protected from moisture and surface imperfections by the application of an outer coating of a polymer or aluminium layer immediately on production. After such treatment the tubing is rugged and sufficiently flexible to be coiled into a circle of 10 cm diameter or less. Typical performance characteristics of modern wall-coated open-tubular (WCOT) columns are summarized in Table 3 and compared those of classical packed columns.

To achieve a high separation efficiency in any type of open-tubular column it is essential that the stationary phase be deposited as a smooth, thin and homogeneous film that maintains its integrity without forming droplets when the column temperature is varied. Phases showing little variation in viscosity with temperature are preferred for this purpose. Many of the stationary phases developed for packed columns are of limited use for WCOT columns and have been replaced by specially synthesized poly(siloxane)s or poly(ethylene glycol)s.

A great advance in column technology that took place about the same time as the introduction of fused silica columns was the immobilization of phases by reaction with the column wall and crosslinking to form a three-dimensional polymer to further stabilize the poly(siloxane) films without destroying their favourable solute diffusion properties. Thermal condensation of nonpolar and medium polar poly(siloxanes) at high temperature with silanol groups on the fused silica surface results in chemical bonding of the phase to the surface to give columns suitable for use up to 400-425°C. High polarity phases are more difficult to bond; they require careful surface preparation to avoid film disruption during the bonding process and generally yield columns of lower thermal stability. The general approach for immobilization of stationary phases is by free-radical cross-linking of the polymer chains initiated with peroxides, azo compounds, or y-radiation. With increasing substitution of methyl by bulky or polar functional groups the difficulty of obtaining complete immobilization increases, and moderately polar poly(siloxane) phases are prepared with various amounts of vinyl, tolyl, or octyl groups that increase the success of the cross-linking reaction. Cross-linking is also important in enabling columns to be prepared with thicker films ( > 0.5 |im) than was possible with physically adsorbed stationary phases. Immobilization of polar phases remains a problem and so far cross-linking reactions are limited, in the main, to poly(siloxane)s and poly(ethyleneglycol)s, which limits the range of selective phases available for open tubular columns. The development of bonding and immobilization techniques facilitated large volume injection and online interfacing with liquid chrom-atography and supercritical fluid chromatography (or extraction), which require columns with a stationary phase resistant to solvent stripping.

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