There is a well-known problem in mass spectrometry of complex mixtures - variable ionisation efficiency. Briefly, every analyte has a different response to the ionisation technique (typically electrospray ionisation (ESI)), leading to a different peak height for different analytes that have the same concentration. Further to this, the spray conditions can easily change the peak height due to differences in pH, salt concentration, or even interactions between analytes. The result is that peak height can be a poor predictor of concentration.
The effect of ionisation efficiency in DOM mass spectrometry studies is largely unstudied, so we prepared a study to investigate it.
First we coupled our HPLC-ESI-MS system to a charged aerosol detector (CAD), which measures material abundance. Analysing different standards, we could prove that ESI-MS response varies while CAD response is quite uniform:
Then we applied the technique to complex DOM mixtures, and found that higher size fractions have less ionisable material, and that average sample ionisation efficiency (MS/µg) increases in samples taken from marine waters compared to terrestrial waters:
The paper was published in Analytical Chemistry, and was the last paper in Claudia Patriarca's excellent Ph.D. thesis: