Revisiting Solvent Impurities with Log-Based Fluorescence Detection and GC–MS

Solvents are needed in all branches of chemistry, and they must be eliminated after use. Impurities present in these solvents could concentrate detrimentally during chemical and, especially, pharmaceutical processes. So it is critically important to know the purity of the solvents used. The label-claimed purity of 24 solvents, including polar protic alcohols, polar aprotic carbonate esters and acetonitrile, dipolar ethers, aromatic solvents, and apolar alkanes, was tested by liquid chromatography with fluorescence detection (HPLC-FLD), gas chromatography with flame ionization or mass spectrometry detection (GC-FID or GC–MS). For all 24 tested solvents, the label-indicated purities were confirmed. The study showed that minute amounts of fluorescent impurities could be better visualized on a logarithmic scale of the emitted light rather than on the classical linear scale. Comparing the results obtained for the same solvent by HPLC-FLD and GC-FID showed that the fluorescence detector is so sensitive that it could detect an extremely low concentration of a fluorescent compound that did not produce a signal in the FID detector. However, the FID detected several more impurities than the fluorescence detector since many do not fluoresce. GC-FID and GC–MS chromatograms of the same sample on the same column showed similarities. But the strong advantage of the MS detector is impurity identification using extended modern spectral databases.