Process Gas Chromatographs. Tony Waters

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Figure 1.4 Typical Gas Chromatographic Columns.

      Source: Ohio Valley Specialty Company, Inc. Reproduced with permission.

      The carrier gas carries the injected sample molecules into the column, where they touch the selected stationary phase. It's the contact with the stationary phase that causes separation. The stationary phase delays the sample molecules − some more than others − so different components end up with different transit times through the column. Each component emerges from the column after its own characteristic retention time.

      It takes time

      A chromatographic separation takes time. In most process applications, the analysis time is from one to ten minutes, depending on the complexity of the analyzed mixture. Some complex separations take longer.

It's important to realize that separation is just a prelude to analysis. The enormous power of the gas chromatograph comes from its ability to physically separate almost any chosen component from all other components, and then to measure it. Other analytical techniques attempt to measure the concentration of one substance in the presence of all the other substances, a goal that few accomplish well. Gas chromatographs separate the analytes first, and then measure them individually. When properly designed, a process gas chromatograph may be the only process analyzer that doesn't suffer interference from other stuff in the sample.

      Of course, it's possible for two or more components to have about the same retention time in a column, so a column might not separate every component from every other component present in the sample. The task of a PGC column system is to separate the measured components from all the others. It's neither necessary nor desirable to separate everything.

      Multiple columns

      The choice of separating column is always the key to a successful analysis. In practice, it's difficult to achieve the desired separation using just one column, so process gas chromatographs usually employ multiple columns to achieve the necessary separation in the shortest possible time.

With multiple columns, certain partially separated components from one column must flow into another column to achieve the desired separation. To divert flows between columns, an online gas chromatograph typically uses one or more column valves that are usually similar in design to its sample injection valve. Figure 1.5 shows an example of a simple column system.

      For simplicity, the figure shows a rotary valve that rotates 90° when actuated, thereby flushing later peaks to vent. Other valves have a similar function.

Figure 1.5 A Simple Column Switching System.

      Intercolumn valves must not leak. They must also have very low internal volume and smooth flow paths, lest separated components start to remix. For the same reason, a PGC typically employs ‐inch o.d. tubing for all its internal plumbing.

      PGCs are individually configured for a particular application. During this procedure, known as application engineering, the application engineer chooses a column system to perform the desired separation and decides on the stationary phase needed for each column. Refer to Chapter 9 for a review of some standard column configurations and the function of each column.

      SCI-FILE: On Column Types

      Introduction to SCI‐FILEs

The more theoretical and mathematical content of the book resides in separate segments called SCI‐FILEs. These contain optional reading that may or may not be part of a course of study.

      Each SCI‐FILE is a supplement to the main text that you can safely omit if not of immediate interest. Treat them as reference sources to consult when needed.

      Two kinds of column

      The stationary phase must be secure inside the column so it doesn't move. The packed column and the open‐tubular column differ per the method they use to anchor the stationary phase in place.

      Packed columns

      A packed column most often uses several meters of ⅛‐inch o.d. stainless steel tubing, although early PGCs used larger diameters, and some PGCs now employ ‐inch o.d. “micropacked” columns.

      In the traditional packed column, the packing is a granular porous solid with particles about the same size as granulated sugar. These particles pack tightly together inside the tube so that any sample molecules moving with the carrier gas are in intimate contact with them.

      The type of column so produced depends on the role of the solid particles:

      Many columns now use proprietary stationary phases, often made from specialized polymer material. These columns don't easily fit into the old classifications СКАЧАТЬ