Solvent/phase polarity mismatch is a phrase associated with troubleshooting of peak shape in splitless injection mode, but what does it actually mean, and why can it cause problems?
When a GC injection is made there is generally a large excess of injection solvent, compared to a relatively small amount of analyte molecules. In an ideal situation the solvent will coat the stationary phase evenly, distributing the analyte molecules on the column stationary phase uniformly. However when a solvent has a significantly different polarity to the stationary phase of the column the solvent will tend to form “beads” rather than coating the phase. These beads can then be blown through the column by the carrier gas, further exacerbating the situation.
Chromatographic problems associated with solvent phase polarity mismatch are:
- Tailing peaks, especially those close to the solvent peak
- Peak splitting
- Multiple peaks for the same analyte
Steps that can be employed to reduce or solve the problem include:
- Changing injection solvent, so that the polarity more closely matches that of the column phase
- Reducing injection volume
- Use of a retention gap to limit the effects of solvent beading, together with that of the beads blowing through the column
Low polarity phases (100% dimethylpolysiloxane, 5% phenyldimethylpolysiloxane) favour low polarity injection solvents like hexane. Mid polarity phases (50% phenyl, 6% cyanopropylphenyl etc) favour solvents like ethyl acetate, whilst high polarity phases (polyethylene glycol, high % cyanopropylphenyl) can work well with methanol. If sample solubility is an issue, then adding a portion of a different solvent (toluene to acetonitrile) so that the mixture more closely matches the polarity of the column phase can be advantageous.
Conclusion: Solvent polarity mismatch occurs when an injection solvent used in splitless injection mode has a polarity greatly different to the column stationary phase. Altering the injection solvent, reducing the volume injected and using a retention gap are all possible solutions to alleviate the issue.
