Quality control of GC makeup gas flow

1 . A method for quality control checking a gas chromatography (GC) apparatus, the method comprising:

providing a GC apparatus and a gas sample, the GC apparatus including at least a main column and a makeup gas flow line in fluid communication with a detector manifold, the main column configured to separate light hydrocarbon compounds in a gas stream;

using the GC apparatus to measure a first chromatogram of the gas sample with a first makeup gas flow rate;

using the GC apparatus to measure a second chromatogram of the gas sample with a second makeup gas flow rate, wherein the second makeup gas flow rate is greater than the first makeup gas flow rate; and

comparing elution times of at least one common peak in the first chromatogram and the second chromatogram to indicate a leak or an obstruction in the makeup gas flow line or the detector manifold.

2 . The method of claim 1 , wherein the first makeup gas flow rate is zero.

3 . The method of claim 1 , wherein the quality control check indicates that there are no leaks or obstructions in the makeup gas flow line or the detector manifold when the elution times of the at least one common peak in the first chromatogram and the second chromatogram are within a predetermined threshold of one another.

4 . The method of claim 1 , wherein the quality control check indicates a leak in at least one of the makeup gas flow line or the detector manifold when the elution times of the at least one common peak in the second chromatogram are less than the elution times of the at least one common peak in the first chromatogram.

5 . The method of claim 1 , wherein the quality control check indicates an obstruction in at least one of the makeup gas flow line or the detector manifold when the elution times of the at least one common peak in the second chromatogram are greater than the elution times of the at least one common peak in the first chromatogram.

6 . The method of claim 1 , wherein the using the GC apparatus to measure a first chromatogram, the using the GC apparatus to measure a second chromatogram, and the comparing the elution times are conducted automatically via a controller located in the GC apparatus.

7 . The method of claim 1 , wherein:

the gas sample comprises a plurality of methane, ethane, propane, butane, isobutane, pentane, and isopentane gas components; and

the comparing the elution times comprises comparing a plurality of common peaks in the first chromatogram and the second chromatogram to indicate a leak or an obstruction in the makeup gas flow line or the detector manifold.

8 . The method of claim 1 , further comprising computing a retention coefficient for at least one gas component in the gas sample.

9 . The method of claim 8 , further comprising computing a selectivity of a first gas component in the gas sample to a second gas component in the gas sample, the selectivity being a ratio of a first retention coefficient of the first gas component to a second retention coefficient of the second gas component.

10 . The method of claim 8 , further comprising estimating a column efficiency of the main column by computing a number of theoretical plates.

11 . A gas chromatography (GC) apparatus comprising:

a sample collection loop, a main column, and a carrier gas inlet in fluid communication with a multiport valve, the main column configured to separate light hydrocarbon gases in a gas stream;

a detector manifold in fluid communication with the main column and a makeup gas flow line;

a GC detector in fluid communication with the detector manifold; and

an electronic controller configured to

cause the GC apparatus to measure a first chromatogram of a gas sample with a first makeup gas flow rate;

cause the GC apparatus to measure a second chromatogram of the gas sample with a second makeup gas flow rate, wherein the second makeup gas flow rate is greater than the first makeup gas flow rate; and

compare elution times of at least one common peak in the first chromatogram and the second chromatogram to indicate a leak or an obstruction in the makeup gas flow line or the detector manifold.

12 . The GC apparatus of claim 11 , wherein the controller is configured to indicate that there are no leaks or obstructions in the makeup gas flow line or the detector manifold when the elution times of the at least one common peak in the first chromatogram and the second chromatogram are within a predetermined threshold of one another.

13 . The GC apparatus of claim 11 , wherein the controller is configured to indicate a leak in at least one of the makeup gas flow line or the detector manifold when the elution times of the at least one common peak in the second chromatogram are less than the elution times of the at least one common peak in the first chromatogram.

14 . The GC apparatus of claim 11 , wherein the controller is configured to indicate an obstruction in at least one of the makeup gas flow line or the detector manifold when the elution times of the at least one common peak in the second chromatogram are greater than the elution times of the at least one common peak in the first chromatogram.

15 . The GC apparatus of claim 11 , wherein:

the gas sample comprises a plurality of methane, ethane, propane, butane, isobutane, pentane, and isopentane gas components; and

the controller is configured to compare the elution times of a plurality of common peaks in the first chromatogram and the second chromatogram to indicate the leak or the obstruction in the makeup gas flow line or the detector manifold.

16 . A method for quality control checking a gas chromatography (GC) apparatus, the method comprising:

providing a GC apparatus and a gas sample including a plurality of light hydrocarbon gases, the GC apparatus including at least a main column and a makeup gas flow line in fluid communication with a detector manifold, the main column configured to separate light hydrocarbon compounds in a gas stream;

using the GC apparatus to measure a first chromatogram of the gas sample with no makeup gas;

using the GC apparatus to measure a second chromatogram of the gas sample with a nonzero makeup gas flow rate; and

comparing elution times of a plurality of common peaks in the first chromatogram and the second chromatogram to indicate a leak or an obstruction in the makeup gas flow line or the detector manifold.

17 . The method of claim 16 , wherein the quality control check indicates that there are no leaks or obstructions in the makeup gas flow line or the detector manifold when the elution times of the plurality of common peaks in the first chromatogram and the second chromatogram are within a predetermined threshold of one another.

18 . The method of claim 16 , wherein the quality control check indicates a leak in at least one of the makeup gas flow line or the detector manifold when the elution times of the plurality of common peaks in the second chromatogram are less than the elution times of the plurality of common peaks in the first chromatogram.

19 . The method of claim 16 , wherein the quality control check indicates an obstruction in at least one of the makeup gas flow line or the detector manifold when the elution times of the plurality of common peaks in the second chromatogram are greater than the elution times of the plurality of common peaks in the first chromatogram.

20 . The method of claim 16 , wherein the using the GC apparatus to measure a first chromatogram, the using the GC apparatus to measure a second chromatogram, and the comparing the elution times are conducted automatically via a controller located in the GC apparatus.