Users value application specificity
Phenomenex, for example, recently introduced a column specifically for chlorinated pesticides, which joins similar products from Agilent, Restek, and Supelco (Sigma Aldrich). Polyaromatic hydrocarbons (PAHs) are another applicationspecific favorite. PAHs are produced during manufacturing processes involving high heat. European regulators specify allowable PAH levels in food and consumer products. European and U.S. standards differ substantially enough that those markets are best served by different method-specific columns.
Column manufacturers are also addressing columns’ chemical activity, which arises from incomplete coverage of base silica with the stationary phase. Active sites promote undesirable chemical interactions or analyte breakdown. Capping free Si-OH bonds produces lighter coatings, shorter analyte residency times, and sharper peaks.
Application specificity extends to tests as well as to columns. Chris English, lab manager at Restek (Bellefonte, PA), describes tests as “the specific things customers are analyzing.” Column companies create application-specific tests or methods so that when customers receive a column they can count on it.
One example is simulated distillation, a high-temperature analysis common in the petroleum industry. The test allows labs to recreate conditions and results of a lengthy distillation through a much shorter, high-temperature (up to 430°C) column run.
“Historically, many manufacturers supplied the column but did not actually test it to 430°C, English says. “Without running up the temperature, there’s no way to know if the column can handle those conditions or what the bleed will look like.”
Other top column companies have jumped on the test/ column bandwagon. Agilent and Phenomenex, for example, provide specific tests for EPA method 8270 for semivolatile organic compounds and even offer an 8270-specific test mix as a reference standard.
A robust test includes components that do not easily separate, including contaminants or matrix. “We include the most difficult probes in our test mix,” English says. “If a column can separate those, then the customer is assured it will work on all the others.”
A test mix extends to column conditions as well as analytes and provides a rough estimation of column life. “End users need GC columns that can cope with actual separation conditions,” says Cristian Cojocariu, senior applications specialist at Thermo Fisher Scientific (Runcorn, UK). Columns that last longer ultimately lead to improved productivity and provide superior return on investment.
“We use standards to test columns for critical isomers in the 8270 semivolatile mix, for example,” says Cojocariu. “We also test for ruggedness using difficult matrix samples spiked with target analytes like pesticides, and we test columns as part of a workflow such as extraction, detection, and quantification of pesticides in difficult matrices.”
GC column performance depends on how the analyst cares for it, beginning from when it is taken out of the box. Cojocariu provides the following best practices:
- Never exceed the recommended maximum column temperature (both isothermal and programmed).
- Do not inject highly acidic or very alkaline samples.
- Store columns under oxygen- and water-free conditions, as even tiny amounts of oxygen and moisture can damage the stationary phase. To achieve this, install one end in the GC inlet, leaving the other end free and uncapped inside the GC. Start the carrier gas flow, set the GC oven temperature low, and run for at least 30 minutes.
- Appropriate column conditioning ensures good performance and enhances column shelf life. Condition columns at 10-20°C above maximum operating temperature to allow column bleed to stabilize. This takes up to two hours, sometimes longer, depending on column dimensions, stationary phase properties, and film thickness.
Many GC problems arise from non-eluting components. Anderson advises planning a periodic bake-out to expel “leftovers.” Phenomenex has developed high-temperature versions of several of its phases for this very purpose to allow regeneration at 430°C. Anderson also suggests using a guard column to trap matrix and non-volatiles, covering the ends of stored columns with rubber septa, and periodically checking injector linings for non-volatiles.
For additional resources on GC columns, including useful articles and a list of manufacturers, visit www.labmanager.com/GC