What impurities are hiding in your monocyclic aromatic solvents?

As we approach the end of the year we are indulging in a bit of nostalgia and most experts agree that reminiscing about the past is a good thing1. It makes us more optimistic and generous. Researchers also noticed that in cold rooms we are more inclined to nostalgize than if we are in a warm environment. And, since my boss, Chris, is a fan of cold office spaces, I will spend a few minutes on what we did last year.

Past year, we devoted some time to validate the products for the ASTM Petrochemical Method Chromatography Product Guide. The ASTM guide contained mostly D02 GC methods – methods for the analysis of petroleum products. Now, we have expanded it to include D16 ASTM chromatographic methods. D16 Committee is responsible to standardize testing of the aromatic, industrial, specialty, and related Chemicals.

One of the more popular methods in the D16 section is ASTM D75042, a method for the analysis of trace impurities in monocyclic aromatic solvents by GC and the effective carbon number. This method uses one analytical procedure and column for an analysis of a variety of industrial solvents from different sources. This includes synthesized or refined solvents such as benzene, toluene, p,m,o-xylene, ethylbenzene, and styrene. Check out our ASTM guide for chromatograms of a few commercially available solvents.

The analysis is quite long, and if we simplify, the only resolution requirement in the method is to separate any impurity from the solvent being analyzed. The impurities in the solvent are reported as a sum of total non-polar hydrocarbons, a few individual mono-aromatic compounds, and later eluting aromatic compounds are summarized and reported as a group. Reported analytes can be easily separated on a long column which led to the use of shorter columns cited in the literature.

I evaluated the 20mm x 0.18mm x 0.36µm Stabilwax column and found it to have sufficiently separated the method resolution mixture in p-xylene.

Figure 1: Chromatogram of a method resolution mixture in p-xylene overlaid with a synthetic blend of compounds most commonly found as an impurity in monocyclic aromatic solvents on a 20m column. The analysis conditions were optimized using the Restek EZGC method development suite (method translator and chromatogram modeler)

Columns with less film thickness didn’t have enough loading capacity/efficiency to separate all the compounds with the required method resolution. The main problem with the analysis of solvents with high boiling temperatures, like p-xylene, is the so-called “solvent trapping effect”. In split injection solvent usually does not affect the chromatography because most of it vaporizes and is vented. However, in cases where the solvent elutes after the peaks, and the solvent temperature is more than ~80°C higher than the starting oven temperature, the solvent will have a direct effect on the retention and peak shape of the compounds eluting in the proximity of the solvent. The compounds eluting before the solvent will be distorted and retention time of the analytes eluting after the solvent will increase3,4. Since our sample solvent is the analyte, there is not much we can do to change this. Therefore, when analyzing p-xylene, the ethylbenzene peak will be slightly distorted. (Compare the ethylbenzene peak in the p-xylene solvent and ethylbenzene peak in a synthetic blend.)

To compare both methods I analyzed 9 injections of commercially available p-xylene using both columns and found very little difference between the two of them. Both techniques resulted in a similar solvent purity: 99.737 ± 0.002 for the 60m column and 99.739 ± 0.002 for the 20m column (Figure2).

Analysis conditions:

Figure 2: Comparison of the purity analysis of commercially available p-xylene using 60m and 20m column.

And now, excuse me, while I go grab some donuts and coffee for the department, hug my space-heater, and dream about the spring:).

Sources:
1. http://archive.nytimes.com/www.nytimes.com/2013/07/09/science/what-is-nostalgia-good-for-quite-a-bit-research-shows.html
2. https://www.astm.org/Standards/D7504.htm
3. K. Grob, Jr., Solvent Effects in Capillary Gas Chromatography, Journal of Chromatography, 279 (1983) 225-232
4. L. Ghaoui, Solvent effect in split injection in capillary gas chromatography, Journal of High Resolution Chromatography, May 1988

 

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