Question

Why is the retention time for solvents like CH2Cl2 and pentane much shorter than the compounds we dissolve in these solvents to analyze using gas chromatography?

Answer 1

In GC, it is desirable that the analyte mixture interacts with the stationary phase and be separated into bands. This will allow the separation and quantitation of the different components in the analyte mixture. The solvents chosen to dissolve the analyte mixture, like CH₂Cl₂ and pentane have short retention times and are easily removed from the analyte mixture. This is due to the fact that these solvents have lower boiling points as compared to the analyte of interest. Since in GC, a temperature gradient is usually applied, the compounds (or solvents) having lower boiling points will be easily removed. This is essential to rule out unwanted contamination of the components in the mixture. GC is a separation technique and the whole purpose of GC is to purify and separate the different components present in an analyte mixture. Hence, low boiling solvents are chosen to dissolve the organic analytes.