Process Gas Chromatographs. Tony Waters

use several types of valve. As an example, this sketch shows a rotary valve. The rotor turns 60° to inject a sample.

Figure 1.3 Typical Gas Sample Injector Valve.

      Plug injection

      The injector valve must inject the measured sample volume all at once, in the form of a compact plug. If the injection is slow and the sample starts to mix with the carrier gas, the sample molecules will start to spread out in time even before they reach the column. This would not be good because it's more difficult to separate a wide band of molecules than it is to separate a narrow band. Separation is easier when the injected molecules tightly pack together.

The sample volume is determined by the application. It's crucial to inject the same volume of sample each time because the detector output signal is proportional to the number of molecules it sees. Should the injected volume change, so would the output signal, even if the concentration of the analyte remained the same.

      Gas sample injection

      The injected volume of a gas sample is typically less than 0.25 mL.

      The number of molecules in a fixed gas volume increases with pressure, so it's necessary to maintain constant sample pressure for each injection. Therefore, most PGCs block and bleed the sample line to allow the sample gas to come to atmospheric pressure, a technique known as atmospheric referencing. Chapter 7 discusses some valve systems to achieve this.

      But that still leaves the normal variation of atmospheric pressure, which is quite small, as you can see by inspecting any barometer. Discounting stormy weather, the jobsite pressure variation should not be more than about ±2 %.

      In practice, atmospheric referencing works well enough for most applications. If greater precision is desired, it's best to measure local barometric pressure and adjust the measurement values to compensate for any variation found.

      Some PGCs have a sensor to measure the absolute pressure of the gas sample and use an algorithm to correct for detected changes.

      Liquid sample injection

      With liquid samples, the main challenge is to avoid gas bubbles in the injected sample as these will cause erratic measurements. To guard against bubbles, keep the pressure of a liquid sample as high as possible, consistent with the pressure rating of the sample injector valve.

      A volume of liquid contains about 300 times as many molecules as an equal volume of vapor. Therefore, to inject the same number of molecules, a liquid sample volume needs to be very small, usually less than one microliter (1 μL). In such a small volume, even the smallest bubble will displace a significant amount of the sample volume and cause low measurement values.

      It's easy to visualize a microliter since it's the same size as a one‐millimeter cube (1 mm³). A volume of one thousand microliters is equal to one milliliter (1 mL) and to one cubic centimeter (1 cm³), commonly called a cc.

      Another challenge with liquid samples is getting a complete and instant vaporization without making the sample too hot, lest it start to react or decompose. The small volume is helpful, and most process liquids quickly vaporize without significant decay.

The column

      The separating device

The chromatographic column is the heart of any gas chromatograph. It separates the analytes from the other sample components, and from each other, so the detector can measure them individually.

Figure 1.4 pictures some typical chromatographic columns.

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