Measuring Gas Flow for Gas Chromatography

Mar 01, 2013

"GC Connections" editor John Hinshaw compares various gas flow measurement methods in the laboratory for gas chromatography (GC) users, namely bubble, mass flow and volumetric flowmeters as well as the built-in capillary column flow measurements found in GC systems.

Obtaining and reproducing accurate, repeatable flow measurements is of great importance to chromatographers. The quantitative accuracy of their instruments depends on being able to accurately set and measure flow rates for columns, splitters and detectors, while the robustness of gas chromatography (GC) methods relies on the repeatability and reproducibility of flow measurements over time and across diverse laboratories. Gas flow measurements present unique challenges because of gas compressibility and the different operating principles of the various flow measurement devices.

Gas flow measurements for GC can be divided into three regimes. First are high flows from about 30 cm3 /min up to 400 cm3 /min. Flows of this magnitude are typically found in detector fuel or makeup gases and in split–splitless inlets. These flows generally are easy to measure and are usually not subject to large errors. The second flow regime falls between 5 cm3 /min and 30 cm3 /min. These are the carrier-gas flow rates of typical packed and larger-bore capillary columns. The lower end of this range can be difficult to measure with conventional flowmeters. The third flow regime lies at 5 cm3 /min and lower. Open-tubular (capillary column) carriergas flows are in this range and can be difficult to measure with sufficient accuracy using external flow measuring devices. The average linear carrier-gas velocity is a superior metric for gauging column flow.

Instead of relying on external meters to set and measure flows, chromatographers with modern GC systems can use computer-controlled pneumatics to supply and control pressures and flows across the entire range from <1 to >400 cm3 /min. These GC instruments use various types of flow control and measurement technologies, all of which should be validated and calibrated on a regular schedule, if possible.

Computer-controlled pneumatics require correct gas identity settings. For open-tubular column systems, additional values must be accurately stated for the column length, inner diameter and, preferably, the stationary phase film thickness. Gross errors such as setting helium carrier when hydrogen is being used or setting 320 µm for a 250-µm i.d. column will result in obvious deviations. A quick check of the average carrier-gas linear velocity will easily reveal such errors. The effects of slight errors in these column dimension settings were discussed in a recent "GC Connections" instalment (1).

Validation and calibration of computer-controlled pneumatics is a topic for a future "GC Connections" instalment. The various types of conventional external flowmeters also requires careful attention to individual meter characteristics, and this month's instalment addresses their care, use and operation.