Confident Your Gas Chromatography is Operating Correctly and Accurately?

by Bonnie Crossland, product marketing manager – gas chromatographs, Rosemount Analytical

Whether your gas chromatograph (GC) is used for custody transfer measurement or process control, it is critical to know your GC is providing accurate data and operating as it should. Validation is the testing of the correct calibration and operation of the GC. Many times we have heard customers say they run the calibration gas as an unknown to validate the GC – however, this is not an effective method. This only shows that the GC is doing what it is told. The GC will always read what it was forced to read in the morning calibration run. If the GC is set up incorrectly, has an issue, or the calibration gas blend is bad, the daily calibration may hide the issue and result in inaccurate analysis for the sample stream.

The validation of the GC can be completed in three steps:

  1. Validate the operation of the GC for the previous period
  2. Validate the current accuracy of the GC
  3. Check for changes in operation that may affect future reliability

Validating the operation of the GC for the previous period is done by checking alarm logs, event logs, and un-normalized total trend for the past 30 days. Reviewing the logs will yield clues as to whether the GC was running correctly. All alarms during the period should be investigated and the cause determined before moving to step 2.

Validating the current accuracy of the GC is done by confirming that the As Found calibration is correct, and then observing the correct operation and repeatability of the GC. Check the latest calibration report to be sure that the calibration gas concentrations entered into the GC match the certificate of the calibration gas cylinder. For natural gas applications, you will check the calibration report for Response Factor Order, Response Factor Deviation, and Retention Time Deviation. Once the existing calibration of the GC is confirmed to be accurate, run the GC through a calibration cycle. Check the results of the analysis of the calibration gas before and after the calibration cycle for repeatability. If everything looks good, move onto step 3.

The third and last step is to check for changes in operation that may affect future reliability of the GC. This is best accomplished by checking the retention times of the individual components peaks over the last 30 days. Overtime, retention times gradually increase from contamination of the analytical flow path. This is called Retention Time Drift and it can cause two issues – incorrect peak detection and incorrect component separation. The current calibration chromatogram is compared to the chromatogram from 30 days ago to assess the amount of drift that has occurred. This information is used to make a judgment on the likely amount of drift to occur over the next 30 days. If the drift will not impact peak detection or component separation, validation is completed. If the drift will impact peak detection or component separation, the GC should undergo planned maintenance during the next 30 days.

For more information on validating the operation of your gas chromatograph, click HERE to view the recorded webinar, “Validating the Operation of Your Gas Chromatograph,” the first in a new series of FREE webinars from Rosemount that we’ve prepared to help you get optimum ROI from your gas chromatographs. This new educational webinar series is called “Maximizing Your Gas Chromatograph’s Capabilities” and is hosted by Emerson’s top analyzer experts who will be covering the most critical aspects of the GC, sharing best practices, and addressing users’ frequently asked questions and challenges faced in the field.

Click HERE to register for this FREE new webinar series today! Next ones up include:

  • Webinar #2 – April 21: “Communicating with Your Gas Chromatograph Through Modbus: RS-2332, RS-485, or Ethernet”
  • Webinar #3 – June 16: “Installation Considerations for the Rosemount™ 370XA Gas Chromatograph”
  • Webinar #4 – August 11: “Maintaining Your Gas Chromatograph at Optimal Performance Levels”
  • Webinar #5 – October 20: “Sample Handling System Considerations for Your Gas Chromatograph”

And if you end up missing any, recorded versions will be available for all.