A number of ASTM gas-chromatographic methods are used to measure the purity of aromatic hydrocarbons. Ten of these have been unified into a single method. Using this method with the new HP 6850 Series gas chromatograph can simplify routine analyses, improve lab efficiency, and open the door to at-line analysis.

Aromatic hydrocarbons are among the most important materials produced by the chemical industry. They are used as industrial solvents, fuel additives, chemical intermediates, and polymer feedstocks, and their producers and users need extensive physical and chemical data to ensure production efficiency and quality. The American Society of Testing and Materials (ASTM) has developed analytical methods for many aromatic hydrocarbons. Gas chromatography is the most-referenced analytical technique for establishing both the purity of specific aromatic hydrocarbons and the amount of individual impurities typically found.

Table I. Ten separate ASTM methods for aromatic solvent analyses are combined into one unified, easy-to-use method.

Eliminating Methodological Diversity
Many production and QA/QC laboratories in the chemical industry dedicate a single GC instrument to run one specific ASTM method. In a lab supporting several processes, a number of GCs may each run a different method, with the inevitable disadvantages to this approach:

• Because each GC operates with a specific column and set of conditions, the results show different retention times for the same compounds found in aromatic samples. For instance, eight separate methods measure p-xylene, but the retention times obtained vary between 7 and 15 minutes. Such variation can be confusing in a production lab and may require extensive training for routine operators.
• A lab with dedicated GCs must keep several different columns on hand to be prepared in case of failure.
• If one GC is under routine maintenance or repair, its particular analysis cannot be performed, because the other GCs may not be configured as suitable backups.

One solution would be to unify these GC methods into a single one. Many ASTM aromatic-hydrocarbon GC methods are remarkably similar. Table I lists ten different methods that are combined into one GC method. The ten individual methods use six different columns to analyze sixteen different types of aromatic samples. A unified method with one column and one set of operating conditions, on the other hand, is more versatile and easier to use and troubleshoot. Running the unified method, any GC can be used as a backup for another in the lab during routine maintenance. Figure 1 shows the chromatogram of a standard containing all 27 compounds typically analyzed with the ten ASTM methods, only this separation was achieved with the unified aromatics method.

Figure 1. The unified method uses an HP-INNOWax column to separate 27 compounds typically found as contaminants in aromatic solvents.

Adding EPC and RTL...
Unprecedented retention time reproducibility can be achieved with the unified aromatics method by using HP 6850 or HP 6890 GCs. These instruments feature the same fourth-generation electronic pneumatics control (EPC), split/splitless inlet, HP 7683 auto-injector, and flame ionization detector. By also using the technique of retention time locking (RTL), virtually the same retention times can be obtained between any HP 6890 or HP 6850 running the unified method. Table II shows nearly identical retention times for several peaks from five different GCs, a reproducibility that greatly simplifies the comparison of results between instruments, labs, and locations. It also makes operator training easier and can give production users greater confidence in their results.

Table II. The unified aromatics method employs retention time locking (RTL) to achieve a high degree of retention time precision between HP 6890 and HP 6850 gas chromatographs.

...and the Right Column
The unified aromatics method uses the HP-INNOWax capillary column, which has been found to provide greater separation reproducibility than conventional wax columns. HP-INNOWax also provides excellent separation of meta- and para-xylene isomers. ASTM specifies that the valley point between para- and meta-xylene must not exceed 50% of the m-xylene peak height; the HP-INNOWax column has a valley point less than ten percent.

Figure 2. The analysis of trace impurities in benzene with the unified method yields the same quantitative results on both the HP 6890 and HP 6850 gas chromatographs.

The analysis of trace impurities in benzene (see Figure 2) demonstrates the quantitative reproducibility with the unified method. Both the HP 6890 and HP 6850 GCs achieved identical quantitative results. The autoranging flame ionization detectors of these two instruments make it unnecessary to select a sensitivity range, which can compromise results when trace and major components are measured in one run. It is therefore possible to identify and quantitate very small peaks, such as 1,4-dioxane, and the major peak, benzene, with just one injection. Quantitative analysis was never easier or saved more time!

Ideal for Process Control
The unified aromatics method is ideal for routine production and at-line analysis needed for process-control applications (especially true when used with the new HP 6850 Series gas chromatograph. This GC shares much of the technology of the industry-standard HP 6890, but it takes up only half the bench space and offers a simplified user interface. Using the unified aromatics method with the HP 6850 GC offers production QA laboratory personnel or at-line operators a small, easy-to-use system that delivers results identical to those obtained with an HP 6890 GC in R&D.