Peak Shapes in Gas Chromatography

The peak shape in Chromatography is very important. Ideally, one should get a symmetrical peak. This will ensure better resolution of different compounds in the sample and also better peak integration and quantitative results. But many times, chromatographers encounter problems with peak shapes due to various reasons. The three most common problems are Peak Tailing, Peak Fronting and Peak Splitting. All the three are briefly discussed below.

PeakTailing

As stated before, the ideal peak shape is a symmetrical one. In a symmetrical peak, if a perpendicular is drawn from the peak maximum to the baseline, it will be divided into two equal parts. If the width of the second half is more than the first half, then it becomes a tailing peak. Tailing of the peak ?is measured using Tailing Factor?or Asymmetry Factor. An asymmetry or tailing factor above 1.5 would indicate sufficient tailing?and should be investigated for the possible reasons and remedial actions. Tailing can cause a reduction in resolution between peaks and can make integration less accurate and reproducible. Hence it is something to be avoided.

Peak tailing can be caused by many factors. Some of the prominent reasons for tailing include: 1. Poor cutting of the column. 2. Improper column installation in the injector port 3. Presence of active sites in the injector liner or column.

If all the peaks in the chromatogram are tailing, then the reason is more likely to be poor column cut or incorrect installation of the column in the injector port.

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Tailing Peak

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Peak Fronting

As against the tailing peak, if the width of the first half of the peak is wider than the second half, then the peak is fronting or leading. It can also affect the resolution, reproducibility or accuracy of peak integration. In many cases, the reason for peak fronting is overloading of the GC column. ?One must realize that a GC capillary column contains a very small amount of stationary phase depending upon film thickness, and so the phase can be quickly overloaded by the sample quantity. This produces a concentration gradient that manifests as a peak “front” at the detector end of the column.

A remedial action will be to reduce the injection quantity with the help of split ratio or further dilution of the sample. If at all a higher quantity of sample is to be injected, then a column with a higher film thickness should be used.

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?????????????Fronting Peak

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Peak Splitting

This is typically exemplified by a peak with two apices or a “ragged” peak apex. This can affect the reproducibility of peak integration and therefore peak areas.

Likely causes for peak splitting include: Improper column cutting. 2. Improper column installation in the injection port. 3. Occluded stationary phase (that is, the phase has adsorbed involatile matrix material). 4. Issues with sample diluent or initial oven temperature during splitless injection.

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?????????????????????????Split Peak

All the three peak shapes, Tailing, Fronting and Splitting are totally undesirable and can affect the resolution, reproducibility and accuracy of the analysis. Care should be taken while selecting the column, installing the column, injection technique and optimizing the parameters. ???????????????

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