Optical and Mass Spectrometric Studies of a Helium Dielectric-Barrier Atmospheric-Pressure Plasma Jet Used as an Ambient Desorption Ionization Source.

摘要

Recently there has been a surge in the field of mass spectrometry centered around the concept of rapid analysis of target analytes with minimal or no sample preparation. The target analyte undergoes desorption from its surface of origin and is subsequently ionized under ambient conditions. The technique is termed ambient desorption/ionization mass spectrometry (ADI-MS). Since the introduction of ADI-MS in 2004, there has been an explosion of research based around the development of novel ambient desorption/ionization (ADI) sources with the capability of desorbing and ionizing a variety of target analytes from various sampling surfaces. One type of ADI source uses the properties of an electrical discharge, typically a helium gas plasma, for desorption and ionization. For electrical-discharge-based sources, ionization is the result of an atmospheric pressure chemical ionization (APCI) process. The initiation of the APCI process it generally attributed to the Penning ionization of atmospheric nitrogen (N2) by highly energetic helium metastable species (Hem). In this work, I describe the direct imaging of the densities of helium metastable atoms in atmospheric pressure plasma jet (APPJ) of a helium-based dielectric-barrier discharge (DBD) using collisionally-assisted laser-induced fluorescence. Axial Hem distributions are compared to the emission of excited helium (He*) and nitrogen ion (N2+*) species in the plasma. A correlation is found between Hem densities and the performance of the ionization source in ADI-MS. Fluorescence images also show that Hem densities increase substantially when a glass slide is placed 10 mm from the discharge capillary in a geometry typical for desorption/ionization experiments. Advantage is taken of the time-varying nature of the plasma to produce axial profiles of temporally and spectrally resolved fluorescence images of Hem atoms and ground state nitrogen ions in the plasma jet. The axial distribution and similarities in the temporal behavior of the helium metastable and ground state nitrogen ion species give strong evidence that nitrogen ion species are created via Penning ionization by helium metastable atoms. Although axial distributions of He*,N2+*, and N 2* emission support the fluorescence data, temporally-resolved emission measurements show that emission from key plasma species is almost entirely the result of excitation by a temporal energy wave. The effect that hydrogen (H2) has on the helium metastable atom densities is also presented. The addition of hydrogen to the discharge gas severely quenches the metastable state, leaving it virtually undetectable. The addition of 0.9% H2 to the helium in the source provides an order of magnitude increase in ADI-MS signal for target analytes despite the quenching of the He m population.