How to Commission a Gas Chromatograph Stream
Bringing a new stream onto a process gas chromatograph is more involved than a single-sensor commissioning, because the GC has to separate a mixture, identify each component, and report a normalized composition that downstream custody and quality calculations depend on. This procedure is for the analyzer engineer or technician commissioning a GC stream. It walks proving the sample and carrier gas, running the calibration standard, checking that the peaks separate and the components add up, and confirming the composition reaches the control system. The emphasis is that a GC is only trustworthy when its separation, its calibration, and its totals all hang together.
Commission a GC Stream in one line: To commission a gas chromatograph stream, first prove the sample reaches the GC representatively and the carrier gas is the right purity at the right pressure, then run the certified calibration standard and confirm each component peak is identified and separated with acceptable response, check that the unnormalized total sits near one hundred percent as a health indicator, and confirm the reported composition reaches the control system. A GC is only trustworthy when the separation, the calibration, and the component totals all agree.
Prove the Sample and the Carrier Gas
A GC cannot analyze a sample it never gets cleanly, so start with the sample delivery and the carrier gas. Confirm the sample is drawn representatively and conditioned so the GC sees clean, dry, single-phase gas at the pressure and flow it expects, because liquids or particulates reaching a GC damage columns and corrupt analyses. Confirm the stream reaches the analyzer without excessive lag, and understand how the GC selects among streams if it handles more than one. The analyzer itself and its role are described in the note on a gas chromatograph in oil and gas.
The carrier gas is the mobile phase that pushes the sample through the column, so its purity and pressure directly affect the analysis. Confirm the carrier gas is the correct type at the specified purity, because impurities in the carrier raise the baseline and skew response, as covered in the note on carrier gas purity in a custody GC. Confirm the carrier pressure and flow are stable and set to specification, since retention times and separation depend on a steady carrier flow, and a drifting carrier makes peaks wander and identification unreliable.
Run the Calibration Standard and Check the Peaks
With sample and carrier proven, run the certified calibration standard through the stream. The GC identifies each component by when its peak elutes and quantifies it by the peak area against the response established from the standard, so the calibration standard is the known truth the whole analysis is built on. Confirm the standard is certified, in date, and representative of the composition range the stream will see. Run it and watch the chromatogram.
Check that each expected component produces a clean, well-separated peak eluting at its expected retention time, because peaks that overlap or are misidentified corrupt the composition. Confirm the response for each component is reasonable and that the calibration sets the response factors correctly, as described in the note on a GC response factor. A component peak that will not separate, that shows up at the wrong time, or that gives an implausible response is a commissioning finding, whether a column problem, a carrier-flow issue, or a bad standard, and it is chased now rather than accepted.
Confirm the Unnormalized Total and Normalization
A GC normalizes the measured components so they sum to one hundred percent, which can hide a problem, so the key health check at commissioning is the unnormalized total before that scaling is applied. If the raw, unnormalized sum of the measured components lands near one hundred percent, the analysis is internally consistent and the response factors are about right; if it lands well above or below, the GC is over- or under-measuring and the normalization is papering over an error. This check is described in the note on a GC unnormalized total check.
Confirm the normalization is configured correctly and that the components measured actually account for the stream, because a component present in the gas but not measured by the method gets distributed across the others by normalization and biases every reported value. Confirm the composition the GC reports is physically plausible for the stream. Getting the unnormalized total and the normalization right at commissioning is what makes the later analyses defensible, because a GC that quietly normalizes away an error reports a clean-looking composition that is wrong.
Verify the Composition Reaches SCADA and Baseline It
The commissioning is complete only when the reported composition reaches the control system correctly. Confirm the component values and any derived quantities the GC calculates appear on the operator screen or historian with the right tags, units, and scaling, so a value that is right at the analyzer is proven right at the panel. A composition that looks correct on the GC but arrives wrong at SCADA is a mapping or scaling error, and this check catches it before a downstream calculation inherits it.
Record the calibration standard used, the response factors, the unnormalized total, and the confirmed composition as the commissioning baseline. When the component values, the unnormalized total, and the analysis cycle behavior are trended in a monitoring platform such as Merobix, a drift in the unnormalized total or a shift in a component is visible against this baseline, so a degrading column, a carrier problem, or a calibration slipping shows up as a trend rather than a surprise in a custody number. The clean commissioning analysis is the reference every later run is compared against, and the cycle timing that governs how often it updates is covered in the note on a GC analysis cycle.
Frequently Asked Questions
Why does carrier gas purity matter when commissioning a GC stream?
The carrier gas is the mobile phase that pushes the sample through the column, so impurities in it raise the baseline, skew component response, and can shift retention times, corrupting both identification and quantification. Commissioning confirms the carrier is the correct type at the specified purity and that its pressure and flow are stable, because retention times and separation depend on a steady carrier flow. A drifting or impure carrier makes peaks wander and the analysis unreliable no matter how good the calibration.
What does the unnormalized total tell me on a GC?
It is the raw sum of the measured components before the GC scales them to one hundred percent, and it is a direct health check. A total near one hundred percent means the analysis is internally consistent and the response factors are about right. A total well above or below means the GC is over- or under-measuring and the normalization is hiding an error by scaling a wrong composition to look clean. Checking it at commissioning is what makes later analyses defensible.
Why check peak separation during GC commissioning?
Because a GC identifies each component by when its peak elutes and quantifies it by peak area, so peaks that overlap or elute at the wrong retention time get misidentified or miscounted, corrupting the reported composition. Confirming that each expected component produces a clean, well-separated peak at its expected time proves the column and carrier flow are working before you trust the calibration. A peak that will not separate is a commissioning finding to chase, not accept.
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