Concreto, Albañilería e Impermeabilizantes 1.-Muros

The coiled, tubular gas chromatographic column is located within a temperature-controlled oven. Generally, one end of the column is attached to the inlet, while the other end is attached to the detector. Columns vary in length, diameter, and internal coating.

The column may arguably be considered the key component of a gas chromatograph, as it is here that the separation takes place. However, it is important to note that the total variance of a separation (σT) will conform to principles of error propagation and be a sum of variances from the injector (σi), column (σc), detector (σd), and data system (σds), i.e.

Equation 50

𝜎

= ��(𝜎

+ 𝜎

+ 𝜎

+ 𝜎

)

Thus, each of these components contributes to the overall efficiency of a GC separation and merits individual attention[23]_.

Jaydene Halliday 107 | P a g e Two general types of columns are encountered in gas chromatography, packed and open tubular, or capillary. Capillary columns are the more efficient of the two types. Their high separating power is primarily due to their longer length and thin stationary phase films. There is a trade-off for attaining this high separating power. Column capacity, i.e. the mass of individual solutes that can be injected into the columns in ng, for every narrow bore 0.18 mm capillary column is limited to about 100 ng per component and increases to 1,000-2,000 ng for megabore (0.53 mm) columns. By comparison, the capacity for packed columns is considerably higher, on the order of 10,000 ng.

2.4.6.1 Packed Columns

Packed GC columns can be either glass or stainless steel. They represent one of the original types of GC columns and are typically assembled by the end- user. Glass columns are favoured when thermally labile materials are being separated, such as essential oils and flavour components. Stainless steel columns, however, are used as they can easily tolerate the elevated pressures necessary for long packed columns, which glass cannot. Before packing, glass columns are generally treated with an appropriate silanizing reagent to eliminate the surface hydroxyl groups, which can be catalytically active or produce asymmetric peaks. Stainless steel columns are usually washed with dilute hydrochloric acid, then extensively with water followed by methanol, acetone, methylene dichloride and n-hexane. This washing procedure removes any corrosion products and traces of lubricating agents used in the tube drawing process. Columns are then packed by placing a piece of deactivated glass wool at one end of the tube to keep the silica based stationary phase within the tubing as it is slowly poured into the opposite end. The pellicular solid particles are typically carbon or diatomaceous earth, and are typically between 30/40 mesh and 100/120 mesh. These will previously have been coated with the chosen stationary phase. A vacuum is then applied at the end of the tubing to remove the solvent that was used to prepare the stationary phase gel. Subsequently, the tubing is capped off with a piece of deactivated glass wool. The smaller the particle size the higher the column efficiency. A typical packed GC column will be between 2 and 4 m in length with an internal diameter of between 2 and 4 mm[32]_.

Jaydene Halliday 108 | P a g e To date, the vast majority of gas chromatography has been carried out on packed columns. This situation is, however, changing rapidly, with packed columns being replaced by the more efficient and faster capillary columns. Packed columns do, however, still have some features that can be advantageous in that they can accept high sample loading and can give a greater analytical dynamic range.

2.4.6.2 Capillary Columns

Capillary columns differ from packed columns in that the stationary phase is coated on the inner wall, either as a thin film, i.e. wall-coated open tubular (WCOT), or impregnated into a porous layer which is then coated on the inner wall, i.e. porous layer open tubular (PLOT). PLOT columns act as molecular sieves for the retention of room temperature or permanent gases, such as O2, CO, CO2, SO2, NO2, and NO.

As previously mentioned, Dandenau and Zerenner introduced flexible fused silica capillary columns using the quartz fibre drawing technique. They found that coating the capillary tube with a polyimide polymer immediately after drawing prevented moisture coming in contact with the surface and thus stabilized the tube and prevented stress corrosion. As with packed columns, surface treatment is necessary to reduce adsorption and catalytic activity, as well as to make the surface sufficiently wettable to coat with the selected stationary phase. The treatment may involve washing with acid, silanization and other types of chemical treatment, including the use of surfactants.

Stationary Phase

Fused Silica

Polyimide Coating

Figure 33: Typical cross-section of a wall coated open tubular (WCOT) capillary

Jaydene Halliday 109 | P a g e Deactivation procedures used for commercial columns are kept highly proprietary.

Open tubular columns are coated internally with a liquid stationary phase or with polymeric materials that can be polymerized to form a relatively rigid, internal polymer coating. Column stability depends on the stability of the stationary phase film which, in turn depends on the constant nature of the surface tension forces that hold it to the column wall. These surface tension forces can be reduced by exposure to oxygen, extreme heat or reactive chemicals, resulting in the film suddenly breaking up, i.e. stationary phase bleed. This shows as a consistent disturbance in chromatogram baseline signal resulting from compound elution at elevated temperatures. Stationary phase bleed can be overcome by in-situ cross-linking of unsaturated groups in the stationary phase molecule by means of free-radical initiators, such as peroxides or azo compounds, to yield elastomers[10]_.

The majority of columns commercially available have undergone extensive crosslinking between the stationary phase and the polymer backbone, i.e. the column wall itself. This crosslinking gives extended temperature stability, extremely low bleed levels and longer column lifetimes. Whenever a column is first installed it will show some bleed due to oxygen exposure. Therefore, it is important to condition any column following installation at the maximum isothermal temperature to be used until bleed levels stabilize.

Table 12: Comparison of wall-coated capillary, support-coated capillary and packed columns

Wall-Coated Capillary Support-Coated Capillary Packed Length (m) 10 – 100 10 – 50 1 – 5 Internal diameter (mm) 0.1 – 0.8 0.5 – 0.8 2 – 4 Liquid film thickness

(µm) 0.1 – 1 0.8 – 2 10

Capacity per peak (ng) <100 50 – 300 10,000

Jaydene Halliday 110 | P a g e As well as displaying increased separation efficiency in comparison to packed columns, capillary columns also work at lower temperatures and achieve much better separation in equal times.

Stainless steel clad silica columns can be used for separations that require temperatures of over 400 °C. High temperature fused silica columns are also available from a number of suppliers. These have special, and proprietary, polyimide coatings which give extended high temperature stability without the need for using the specialist metal columns mentioned above.