Onsite - Checking and troubleshooting pressure and flow parameters of gas chromatographs with high accuracte MFCs
Gas chromatographs (GCs) detect materials at the molecular level by creating a sample of the material that is “carried” into the GC with a carrier gas stream. The carrier gas stream, which is usually around 20-30 ml/min, is fed into small capillary flow tubes in the analytical column. Due to the different interactions with the column material, the individual components split up and then move at different speeds.
When they finally pass through the detectors, the separated materials are recorded and quantified as individual peaks. The carrier and capillary flow rates are critical to the correct performance of the gas chromatograph. Flow masses, volumes and pressures through the different gas flow paths vary depending on the manufacturer’s specifications. However, it is important that they are precisely maintained according to the specifications.
Increasing pressure or flow rate can cause decreasing retention times. A drifting or unstable pressure regulator leads to non-reproducible retention times, while increasing retention times could indicate a carrier gas leak. Such faults should be detected as early as possible so that septims and column connections can be checked accordingly and the problem rectified. Reliable and accurate pressure and flow control is therefore extremely important.
Too high a column flow leads to rapid peak broadening and too high an injection volume leads to tailing (asymmetry) of the peaks. Too high or too low a flow of combustion gas can also cause signal noise.
Temperature is also an influencing factor, not only the column temperature, which directly affects the process, but also the room temperature.
This should be kept stable if the pressure regulator does not have suitable compensation, as otherwise temperature fluctuations can also affect the pressure control. This could result in sinusoidal long-term fluctuations in the signal.
In addition, non-uniform sampling can cause rectangular signal fluctuations in a thermal conductivity detector.
The so-called make-up gas from the column also has another important function. It cools the detector and is therefore also responsible for the service life of the FID.
In order to control all these parameters appropriately, versatile devices for pressure and flow control are required, which are installed in the system. Various types of mass flow and pressure controllers are available from Alicat for installation in the system, both in the full version with multi-parameter display (left) and as an OEM version for installation in the system (right). In addition, Alicat has already integrated rapid temperature compensation into the controllers.
But it’s not just high-quality equipment that is important. What should you do if one or more of the above-mentioned signal faults occur during operation?
Then a mobile troubleshooting aid is often required, which can be used to effectively and quickly search for the cause when the above-mentioned error signals occur. Testing pressures, temperatures and mass flow rates cannot, of course, reveal all problems, as there are a number of other influencing factors in the handling of a GC. But as described above, they are responsible for a large part of the causes and therefore a good place to start troubleshooting.
The highly accurate Alicat handheld mass flow tester is ready to go all day with 18 hours of operation on a single charge. It is versatile for all of the above tests (pressure, temperature, mass flow and volume flow) so you can find and fix your system faults quickly.
The portable Alicat mass flow meter fulfills the following tasks, for example:
- Checking the mass measurement at the inlet and deaerator of the GC
- Checking the volumetric measurement at the vent of the GC
- Checking the pressure at the inlet and outlet of the GC
With the Gas Select ™ field selection for 30 of the most common gases, you can quickly set it to the required gas.
Important: Alicat does not use adjustment factors for the conversion between gases, which can lead to major inaccuracies, especially with small quantities. The complete calibration curves according to NIST for the corresponding gases are stored in this device, so that your measurement accuracy does not suffer from a gas change.
With an accuracy of 0.8% of the measured value + 0.2% of the measuring range, as well as a repeatability of 0.2%, the required accuracy for these tests is given.
The adjustment to pressure and temperature changes takes place in a few milliseconds (for flow measurement the response is approx. 10 ms). Due to the large measuring range of 100:1, these units can cover both the capillary and carrier gas flow ranges.
Signal processing can be carried out via an RS-232 serial interface, which allows you to transfer all parameters and the selected gas to any PC, PDA or other device for data logging.
These multiple functions integrated into a compact, portable instrument provide you with an invaluable tool for checking and troubleshooting GC operations.
For technical advice and distribution in Germany, please contact the TrigasDM GmbH team.
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