Gas chromatography applications come with a multitude of inherent complications, one of which is the issue of column bleed. Colum bleed refers to the degradation of the polymer that makes up the column’s stationary phase and its subsequent elution alongside sample analytes. Detection of the stationary phase causes background signals which can hamper the quantitation process.
We delve deeper into the concept of column bleed and how to avoid it impacting your analysis.
The Impact of Column Bleed
Column bleed happens to some extent in every column. When it is significant, it is referred to as ‘high bleed’ as opposed to the less prominent ‘low bleed.’ Column bleed is usually fairly simple to spot as it will show up on detector readings as a gradual increase in the baseline.
Several factors can contribute to column bleed, but generally, you can expect to see a higher bleed level in columns with thicker films (as there is a larger amount of stationary phase in the column).
It should be noted that column bleed should not be confused with other common column issues such as problems with the injector, septum, carrier gas, or detector. These issues are often indicated by a high baseline at low temperatures, which is different from the gradual baseline increase that indicates column bleed.
Causes and Prevention of Column Bleed
So what causes column bleed, or more specifically, what causes the degradation of the column stationary phase? As GC/MS expert Don Hilton highlights in this Chromatography Forum discussion, there are several key factors that can cause column bleed, including excessive heat, exposure to caustic chemicals, and the presence of oxygen. Hilton notes that the bleed can be particularly exacerbated when these factors are combined.
And what can be done to mitigate these factors? Hilton’s primary recommendation is to practice diligent column care based on the manufacturer’s instructions. He also offers general advice in several key areas:
- Avoid oxygen exposure: Hilton instructs chromatographers to keep oxygen out of columns. “Use high-grade gas,” he advises. “Put a scrubber on it and keep the scrubber changed per schedule or use an indicating scrubber.” He adds that users should keep the septum changed on a regular basis and the ferrules snug to ensure the instrument is leak-tight. One more piece of advice is to keep carrier flow through the column when the instrument is not in use.
- Avoid high temperatures: “Do not apply excessive heat to the column,” advises Hilton. “Use a GC ramp that gets the job done and clears high boiling residues from the column.”
- Run clean samples: It’s vital to practice caution with the types of samples being applied to the column. Hilton warns that dirty samples will have residues that work their way onto the column. He notes that these residues may be reactive, particularly after being baked into the column, and may accelerate polymer degradation.
- Dispose of columns: “Remember that a GC column is a consumable,” guides Hilton. “You will have to heat the column, and unless you are running one of a very limited number of sample types, you will leave residues on the column. Keep in mind that an extra $5 per sample in cleanup will exceed the cost of a new column in just about 100 samples, which for some of us is less than a day's work.” He adds that in some cases, you may be able to skip the $5 cleanup and obtain good results for 200 injections before disposing of the column.
Conclusions
Column bleed is an inevitable aspect of chromatography, but understanding its causes and implementing preventive measures can significantly reduce its impact on your analyses. By maintaining your system, using columns optimally, and recognizing early signs of degradation, you can ensure accurate and reliable chromatographic results.
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