hide
Free keywords:
-
Abstract:
The demands for adequate ionisation in mass spectrometry are met by a pool of different ionisation techniques on the market. For a mass spectrometric measurement, the choice of an appropriate ionisation source is of high importance since the methods have different characteristics and are mostly analyte-selective. In this study, a lab-built source for nano scaled atmospheric pressure photoionisation (NAPPI) is introduced. The aim was to supplement the present-day demands and to overcome basic limitations of conventional APPI, such as a relatively high sample consumption and intense baseline noise. On this account, the general performance of the different setup modifications was evaluated and the impacts of various measurement parameters were investigated.
Different source setups and parameters were tested by experiments of source modifications. Interdependent factors, like device and tuning parameters, and analytes at different concentrations were varied. Particularly, the signal intensity of the analyte and the baseline noise were combined in the parameter signal-to-noise. The ion injection time was noted to qualitatively evaluate the measurements. The limit of detection was determined for the best setup and compared to conventional APPI.
The NAPPI source was divided into three main source parts which were individually optimised and resulted in the following setup: In the transfer unit an external syringe pump injects the analyte solution into the source. Flow rates of approximately 60 μl/h were sufficient for adequate ionisation. At an angle of 45° in regard to the pump the sheath gas is injected into the sprayer. This carrier gas meets the analyte solution at the end of the sprayer inner sample capillary which has an inner diameter of 116 μm. An aluminium-block through which the sprayer is slotted generates the desolvation temperature. After desolvation, gas-phase neutral analytes leave the sprayer. A krypton UV-lamp is excited by two high-purity copper-electrodes each (positioned lengthways along the axis of the lamp; 90° sector coverage) and produces 10 eV photons of 120 nm wavelength to initiate ionisation. At coaxial injection a distance of 2.5 mm between sprayer and MS orifice is optimal for the generated ions to enter the analyser unit of the mass spectrometer.
One of the remarkable features of NAPPI is its extremely low sample consumption. However, NAPPI is more than just a miniaturised version of conventional APPI: Among nano to micro flows, NAPPI is capable of enhancing the sensitivity by reducing the baseline noise. NAPPI reduces the LoD by a factor of 100 compared to conventional APPI for phenanthrene in toluene. A concentration of below 10 ppm is enough for PAH analysis with NAPPI, instead of 100 ppm for APPI. Even for highly complex samples, just a few microliters of analyte suffice for extended mass spectrometric analyses. The NAPPI source holds great potential as a newly developed ionisation source. It benefits from its high simplicity in setup and operability at low noise levels. The optimisation of tuning parameters is of crucial importance to examine the best settings for the analytes.
