Meeting Modern Regulatory Requirements
As regulators have redefined environmental methods for low-abundance analytes, and demanded the analysis of more analytes, column manufacturers have done well to provide greater sensitivity and selectivity. “Adding more compounds to the list causes co-elution issues that are difficult to resolve,” says Kory Kelly, technical manager for GC and environmental at Phenomenex (Torrance, CA).
One strategy for meeting the challenges of modern environmental analysis is to fine-tune selectivity to specific methods, e.g., pesticides in water or halogenated contaminants in soil, or to specific analyte categories. At the same time vendors have improved detectors, offered multiple detector modes, and introduced sample preparation methods that remove matrix while enriching analytes.
Overcoming undesirable activity
Reducing GC systems’ undesirable chemical activity is an approach focused on, but not limited to, columns. “Activity” refers to the chemical interactions between analytes and their surroundings, most of which occur within columns. Chemical activity in the stationary phase arises from the interaction between labile analytes and silicon-oxygen species.
“When you inject compounds at high temperatures, you vastly speed up chemical kinetics,” Kelly explains. “Compounds break down, fall apart, or otherwise change.” Analysts testing for a compound at 100 ppb will be surprised to find three peaks, at different retention times, each at 33 ppb. These degradation products may be below the instrument’s detection limit, even though the parent compound was well above it. Chemical activity becomes more of a problem with lower concentrations.
Phenomenex and other manufacturers have countered by inactivating columns toward certain chemical interactions for a particular compound type. That approach leaves columns suitable for one class of analytes, but not for others that may be found within the same sample.
“Now the trend is to deactivate for everything,” Kelly says, “so the column works for all compounds. Because if a column fails for one component of a method, it fails for the entire method.”
Capping those silanols
Chris English, who manages Restek’s (Belfonte, PA) Innovations Laboratory, explains that GC columns consist of fused borosilicate glass coated with a polyimide resin that provides a physical barrier, mechanical strength, and “bendability.”
Columns are basically just glass tubes containing innumerable free silanol groups onto which the desired chemistry (a liquid polymer) is attached. But if the silanols are not completely coated, if they don’t react properly with the stationary phase, they remain active and, in many cases, can react with analytes.
Manufacturers once used dimethyldichlorosilane exclusively to inactivate the silanols. Newer techniques provide more options for cross-linking to the column’s chemistry or stationary phase. Absent cross-linking, the liquid polymer would eventually bleed off the column.
Several tests exist for quantifying active hot spots on columns. The most common uses 2,4-dinitrophenol (DNP), which reacts readily with the -Si-OH group. “It can tell us a lot about the column surface, but it’s also a very common compound in environmental analysis,” English says. The test involves co-injecting DNP with a compound that is inert to silanol groups, such as polyaromatic hydrocarbons (PAHs). If active sites exist, the PAH peak will be sharp, while the DNP peak will tail and lose response.
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