Monitoring organic compounds in aqueous solution by rotating ball inlet mass spectrometry with continuous wave infrared laser desorption

The rotating ball inlet (ROBIN) is a mechanical device for introduction of a small stream of sample solution into a mass spectrometer or other systems by adherence of sample to the surface of a rotating ball. Using a simple quadrupole mass spectrometer with an open ion source we have determined response times and detection limits of ROBIN-MS for a variety of organic compounds dissolved in water. The dependency of the detection limits on temperature and the effect of additional local heating of the vacuum exposed surface of the ball by continuous wave IR-laser were studied. The 0–90% response time for t-butanol and similar low molecular weight compounds with little affinity for binding to surfaces inside the vacuum system is about 0.5 s, in both rising and falling direction. With caffeine and similar compounds with significant affinity for binding to surfaces both the rising and the falling transient are biphasic with an initial rapid phase with a 0–90% response of about 0.5 s. The subsequent slow phase has a response time of several minutes. Because of the wide separation of time scales of the fast and the slow response, the fast response can be used to quantify the analyte in flow injection analysis with a cycle time of 25 s. Detection limits for highly volatile organic compounds are unaffected by the temperature whereas detection limits for less volatile compounds decrease with increasing temperature. Compounds of low volatility can only be detected with IR-laser desorption. The lowest detection limits found are around 1 μM. Considerable improvement of the detection limits is expected with an optimal mass spectrometer.