Supercritical fluid jet expansions of polar aromatic carboxylic acids using simple derivatization with detection by resonant two-photon ionization

Abstract

In this work simple derivatization is used as a means of enhancing the solubility of polar aromatic carboxylic acids in supercritical CO2 and N2O. The supercritical fluid is then expanded from 200 atm and 40[deg]C through a pulsed injection orifice into vacuum as a supersonic jet for analysis by laser resonant two-photon ionization spectroscopy in a time-of-flight mass spectrometer. The use of derivatization reduces the polarity of these carboxylic acids and thus greatly enhances their solubility in these fluids. The result is that strong ionization signals from the jet are detected in the mass spectrometer at relatively low temperatures where these molecules, many of which are thermally labile, will not decompose. The laser ionization method allows soft ionization of these compounds where the molecular ion is generally the base peak and a characteristic fragment due to simple C[alpha]-C[beta] cleavage is often observed. By monitoring the molecular ion as a function of wavelength a cold laser ionization spectrum with sharp spectral features can be obtained in these supersonic expansions from high pressure supercritical fluids thus demonstrating that optical selectivity is still retained even after derivatization.