Five batches of samples were sent out to one lab. 6th batch which was not sent out failed refractive index (RI) specs. The lab I sent shut down, so I send one of the batches of the five and the 6th batch to another lab. I am interested in the noise level peaks to see if there is any contamination which cause RI to fail. Both labs used same columns, GC/MS but new lab does not show any noise level peaks which other lab showed. The batch analyzed at first lab showed several low intensity including noise level peaks which are not see at this lab at all. Analyst try to change liner (different), but it flipped the intensity of two main peaks of the compound (two isomers). What could be the reason? Test parameters are same at both labs for GC and MS.
How to Read GC/MS Chromatograms
The X-Axis: Retention Time
Usually, the x-axis of the gas chromatogram shows the amount of time taken for the analytes to pass through the column and reach the mass spectrometer detector. The peaks that are shown correspond to the time at which each of the components reached the detector.
The type of column used during the analysis, as well as the GC parameters (e.g. flow rate, injection temperature, oven temperature, etc.), have a large impact on the retention time. As a result, when comparing retention times from different analyses or different labs, it’s critical that the same parameters are used to ensure accuracy.
The Y-Axis: Concentration or Intensity Counts
Typically, the y-axis, or the area of the peak, is a reflection of the amount of a specific analyte that’s present. When looking at a GC/MS chromatogram, the area will be based on the number of counts taken by the mass spectrometer detector at the point of retention.
However, it’s important to note that some compounds will have a better affinity with the detector and the peaks will appear larger than the actual concentration would be in relation to the other peaks on the chromatogram, which we often see in compounds that ionize readily. To overcome this challenge, our experts run standards with known concentrations of compounds to ensure accurate counts. In addition, unknown compounds are identified based on their retention times of known standards with other detectors. The mass spectrometer detector then allows identification of a compound by mass spectrum obtained at the time of testing.
Differences in Gas Chromatogram Models
At a high level, depending on the type of sample and the desired outcome, gas chromatography analysis can use several different means of sample introduction, such as static headspace analysis, thermal desorption, and direct injection, as well as different types of detectors, such as flame ionization (FID), electron capture (ECD), and—of course—mass spectrometry.
As a result, chromatograms can often differ in how they appear. However, even with variations, the basics of understanding chromatograms, as outlined above, remain the same. But one key point we’d like to reiterate is that it’s important to remember that the systems and parameters used for an analysis need to be similar when comparing results from two or more different analyses. This ensures you’re getting the most accurate comparison and drawing meaningful insights.
GC/MS Chromatogram Examples
Now that you understand the basics of how to read a gas chromatogram, below we share some examples of various GC/MS testing results
Epoxy Outgassing
A manufacturing client of ours discovered an epoxy used in their device was failing to function as intended. After performing epoxy outgassing analysis, you can see that we tested and compared two different samples: a reference epoxy (top) and the failed epoxy (bottom). Results showed that the failed epoxy has a large cluster of Benzene compounds (y-axis) at approximately the 9-minute mark on the retention timeline (x-axis)—which is something that wasn’t seen in the reference epoxy.
It is about difference of sensitivity of instrumwnts and maybe (also) to high cutoff level of instrument whith no noise indication.
Regards
GB
Sorry, I am not in replying. I did reply yesterday to both Grzegorz and Safwan, but looks line messages did not go though.
Dear Gezegorz,
How can we change cut off level or increase sensitivity? Since I don't have instrument, I have to recommend that to test lab.
Can different types of liner in GC will change peak intensity dramatically?
Test lab changed to different type of liner and two major peaks' intensity reversed (almost like doublet), Also base line showed wave and few small noise level/low intensity peaks were observed but not where I am expecting, also peak width and shape changed? Since one of these samples is already ran by other lab and that lab has analyzed at least 20 or so samples even before my time at CTC Global.
Instrument is similar, MSD may be different and liner used could be different. Same parameters were used for analysis.
Dear Safwan,
Thanks for describing a detailed message on how to read GC/MS chromatogram. Actually, I am well versed in using GC/MS, have used it for 7+ years . However, right now I do not have a instrument, I am sending samples out and now to a new lab, where I am not able to get the same chromatograms (with low level peaks) as we got earlier from other lab which is no longer open for analysis.
Please see attached.
Thank you both for quick response and suggestions.
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