Improved Peak Symmetry as a Result of Derivatization of Phenols

Peak symmetry is important for determination and quantification of compounds. In order to create a symmetric peak, the compound must be able to elute all at more or less the same time. If a compound interacts with the column itself (ex: by temporarily adsorbing to the fused silica surface), it delays travel and effects peak morphology, causing “tailing”.

When comparing EPA method 525.2 to NF EN 12673 for the extraction of phenols, peak symmetry was notably improved using NF EN 12673.

The hydroxyl group on phenol compounds is highly active and able to hydrogen bond with the silanol groups present to some extent in all fused silica GC columns. This is one possible causes of phenol peak tailing. Phenyl acetates, however, have an acetic acid ester group rather than a hydroxyl group, which greatly reduces compound activity,  resulting in cleaner, symmetric peaks.

Relative response factors (RF) were calculated for each of 10 calibration points for pentachlorophenol and 12 calibration points for pentachlorophenyl acetate, to illustrate the effect of peak tailing. Ideally, relative response factors should be equal at each calibration point because they are normalized by concentration. Peak tailing, however, complicates the peak integration, which pushes the RF further from the “ideal”. The relative standard deviation (RSD) shows how close the RFs are to one another. The higher RSD in Figure 1 shows that the RFs deviated more from one another in the pentachlorophenol calibration, than they did in the pentachlorophenyl acetate calibration.

Figure 1: A graphical representation of RRF’s for pentachlorophenol (right) and pentachlorophenyl acetate (left) peaks used for calibration purposes. Concentrations used for calibration represent the concentration of phenols in the original water sample.

Accordingly, the peaks in the pentachlorophenol calibration exhibited more tailing than the peaks in the pentachlorophenyl acetate calibration.

Figure 2: A chromatogram showing peak shapes for pentachlorophenol (left) and pentachlorophenyl acetate (right).