pH mediated electrospray ionisation mass spectrometry

The direct analysis by Negative ion Electrospray Ionisation Mass Spectrometry (ESI-MS) of mixtures containing acidic compounds of widely varying pKa is inherently difficult as ionisation of the higher pKa acids is suppressed by those with a lower pKa. This behaviour is particularly prevalent in the ESI-MS analysis of fulvic acids, which, being complex mixtures of acidic compounds, produce non-representative spectra where only the most acidic components are observed, less acidic components being suppressed. Thus, a flow injection manifold was developed to introduce a pH mediated sample zone into the mass spectrometer. The pH mediation involves introducing the sample into a defined pH gradient before introduction to the mass spectrometer producing a spectra of mass distribution as a function of pH. The system has been successfully demonstrated with model compounds and standard fulvic acids to show different ion abundances relative to the pH. Application of this system to further fulvic acid studies could yield much information about acidity, ionisation efficiency and origins. Furthermore, the system shows great potential for application to many diverse systems that could be better understood if measured relative to pH change.