Poor Peak Resolution

Poor Peak Resolution

Improving peak resolution to avoid overlap is a common question that arises during method development, as it directly influences the quantitation results.

If you’re using MS (mass spectrometry) detection, then there is always the option of extracting the ion ratios to deconvolute the peaks. But if you’re using FID (flame ionisation detector) or TCD (thermal conductivity detector), there’s no choice but to tweak the method to try and achieve baseline separation for your compounds of interest.

Two common causes of poor resolution are an insufficiently selective stationary phase and unoptimised column dimensions. Doing some literature research into the best columns can be useful here.

Assuming you’re working with an optimal column, then the GC method parameters might not be suitable for the application. You should try optimising the carrier gas linear velocity and the GC oven program.

Something else to consider is that sample overload of the column could be hampering your ability to achieve good resolution. Baseline separation could be achieved by simply adjusting the sample concentration or increasing the split ratio. However, this solution is less likely to be useful if you’re interested in a small peak that is being partly obscured by a larger one.

Poor vs. better resolution achieved by reducing sample loading:

Finally, there are some compounds that are always difficult to resolve. Sulphur compounds are notably ‘sticky’ and prone to poor peak shape, while ethylbenzene/xylenes, certain Polynuclear aromatic hydrocarbon (PAHs), and other structurally similar isomers are often difficult to separate chromatographically.

The chromatogram shows where the use of a column specifically designed for the analysis of PAHs can resolve Benzo[b]fluoranthene, Benzo[k]fluoranthene and Benzo[j]fluoranthene. These compounds are not separated on more common column phases.

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