LOG/LOG Line Chromatograms

Line chromatograms for an optimized comparison of data with wide ranging concentrations or strongly differing numbers of substances.

Some chromatograms are really not easy to handle.
Although it looks, that a simple gas analysis is easy to read, it in fact it combines a wide range of concentration with a wide or very wide range of substances of differing molecular weight. Thus isothermal analyses are either quite limited in the substance range from methane to hexane plus backflush or temperature programmed analyses with a wide range of mol weight substances are confronted with quantitative working range limitations. In order to get a correct signal for the first and largest peak = methane often in the range of 70-99 %, 100 further substances exist only in or below the 100 ppb range of concentration. Thus such chromatograms are often over sampled to make traces measurable. It causes a falsified methane signal. This also falsifies ALL other substance concentrations.

To get wide range GC chromatograms done fast enough, the separation capillary must work with a steep temperature program. This causes quickly trouble with the starting temperature because of a wide temperature range to master. The cooling down time might be too long.

How can a LOG/LOG line chromatogram comparison help ?

In the following we check line chromatogram modes to compare two chromatograms with starting temperature problems. The reason to compare was an urgent case of lost sample. The run of one sample could not be repeated. Only the two EXPORT data files existed.

First trial: see the two linear chromatograms with log time scale (this is used because of temperature programming) and a linear quantity scale. To better see traces the quantity scale sensitivity is set to
1000%, that is a factor 10 of enlargement. On the y-axis we see the quantity value of the largest peak. It is 96.5 %. With other words: the total of all other substances in the sample adds up to 3.5 %. Figure 1 shows, that the retention time values of sample “CC0000556.EXP” differ strongly from the values given with sample “CC0000843.EXP”. These are Standard EXPORT data files.


		Figure 1: No hope to see traces. The time scale shows: too differing retention time values. Probably was sample “CC0000843.EXP” chromatographed on a shorter capillary as the first peak (surely methane) elutes much earlier.The LIN / LIN comparison does not help to find out, if the two samples with main compounds differ. Both samples contain the main compound at equal concentration. May be only the chromatograms differ, but not the sample ?

				Figure 2: Looking to only one gas sample in LIN/ over retention index scale shows: The first peak is methane, the second is ethane C-number 2, index 200; there is C3, C4, C5. But as the representation of the concentration is made at 100% full scale only (fsd = 100%) it only shows, that the next compounds besides methane have together only a few %. The index scale was calculated automatically.

	Figure 3: A much higher sensitivity for the quantity scale is helpful: the two samples differ strongly. “C0000556.EXP” has a much wider retention time range for traces. Probably the chromatogram for “C0000843.EXP” was back flushed after 1200 seconds, therefore a larger peak. But the retention time differences in both chromatograms make a qualified comparison impossible. Let us try a comparison using the y-axis as LOG scale

			Figure 4: The LOG Y-axis as quantity scale is equal for file 1 (from middle to top) and for file 2 (from middle to bottom). The LOG scaling gives well readable quantitative details. The main compound (at 96.5 %) has only one next large substance of above 2...3 %. Most of the by products range around .01 and .001 %. All data are mole-% values as the analysis is calibrated. However sample file 1 differs drastically from file 2 - at least it looks like. The retention times differ too much for a final evaluation.

Figure 5: The first and up to now the best version using line chromatograms for qualitative comparison: Both chromatograms are now shown as LOG quantity over the retention index. Even with linearized scaling. The gas sample “C0000843.EXP” is a wet natural gas analysis with backflush after C6 (hexane+). The sample “C0000556.EXP” is canadian natural gas with the added substance “M” between C3 and n-C4. As between both only iso-butane exists it follows the substance “M” must be the added methanol - see “M”.

Methanol is completely missing in sample “C0000843.EXP”. This means: the first sample is gas coming from countries with very cold winter climate. Methanol is the necessary add-on to avoid gas line blockage by methane hydrates. The second sample is gas coming from areas where a medium low temperature at high pressure does not cause transportation trouble. However figure 5 clears also the reason why the two samples look so different. The first gas sample was chromatographed at least up to C₁₀ - see the index scale, whilst the second sample was analyzed only up to hexane as usual for many gas producers quality control. Using the index scale any retention time problem is solved. The index scale is the best possible quality scale for the data quality control in gas chromatography.
There is also a further detail understandable: in the line chromatogram for sample “C0000843.EXP” there is a block of peaks visible between C₅ and C₆, which in fact does not exist as real substance mixture. Therefore this must be a systematic data error in the EXPORT file of “C0000843.EXP”. Thus one should look into the digital EXPORT data and remove false signals.
According to the rules of mathematical statistics a systematic error MUST be removed prior final quantitative evaluation. It would be a good idea to have software which allows such corrections - see the table below - automatically with given alerts. Such data correction has nothing to do with illegal data massage. The analytical raw data of each chromatogram remain untouched. Based on a given and documented ALERT qualified rules of regulation can be observed. Just a next integration of the binary raw data file would do it: result in error free data.
No such data correction is mathematically “illegal”. Even ONE wrong result based on false data may cause a wrong decision, and this may be an expensive result. Therefore it is a good idea, to let elegant software check for systematic errors, which is possible based on the many perfect chromatography laws for which in this site some examples have been discussed. See also “here”.

EXPORT file correction commands available in some Line Chromatogram software:

Auto identify index position

Calculate the retention indices for all peaks

Compare chromatograms LIN/LIN, LOG/LOG in y-axis and tms, C number or index on the x-axis; select time windows for comparison based on seconds

Correct the retention time scale

Correct values, remove spike data etc.

Enter index values

Exchange file one in two and file two in one

Show EXPORT data after the retention index is inserted

Show loaded file data

Subtract chromatogram two from one

Use substance specific signal correction