Developing and characterizing improved low-temperature plasma probes for ambient ionization mass spectrometry
Abstract
The emergence of ambient ionization mass spectrometry within the last decade with its minimal or no sample pretreatment has dramatically improved direct sampling measurements. With the introduction of the first ambient ionization techniques of desorption electrospray ionization (DESI) in 2004 and direct analysis in real time (DART) in 2005, there have been numerous sources described in the literature including low-temperature plasma (LTP) ionization sources. LTP ionization is notable for its simple design, low thermal temperatures, and low power and gas requirements that allow nondestructive sampling of compounds from sensitive surfaces such as human skin. This thesis details the development of improved LTP sources, the integration of these probes with miniature mass spectrometers, and the application of these sources to the quantitation of compounds. The first part of this thesis increases the utility of LTP sources by increasing the analyzable area that can be interrogated through the use of bundled source arrays. The sources have a larger sampling area that allows faster detection of each analyte, and selectivity towards the selected compounds is enhanced by adding reagents directly into the plasma stream. The second part of this thesis integrates LTP sources with miniature mass spectrometers. This work was inspired by the idea of having a completely field portable mass spectrometer that is fully self contained without the need for external gas cylinders or solvents. The miniaturization of these probes allows for gases other than helium to be used with lower power requirements which simplifies the instrumentation. It is shown that a miniature mass spectrometer equipped with LTP probes can be used to detect explosives in situ without the need of solvents or gas cylinders. Unlike DESI or spray techniques, LTP-MS systems benefit most from probe-surface sampling angles ranging from perpendicular to geometry independent (GI) configurations. The optimal sampling angles and heating of the surface to improve detection of compounds is demonstrated. The final chapter details the application of LTP-MS to the semi-quantitation of volatile and semi-volatile compounds. A fast and direct method is presented for the analysis of semi-volatile fragrance compounds without sample pretreatment. LTP is used to perform high-throughput analysis, determine limits of detection (LODs), and obtain semi-quantitation information on various surfaces. A peltier cooling device is used to enhance the retention time of compounds on a surface. The method allows the detection and quantitation of semi-volatile compounds in the low pg ranges with a commercially available cleaning product after 1 min of being applied to a vinyl tile surface.
Degree
Ph.D.
Advisors
Cooks, Purdue University.
Subject Area
Chemistry|Analytical chemistry
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