Article pubs.acs.org/ac
Characteristics of Low-Temperature Plasma Ionization for Ambient Mass Spectrometry Compared to Electrospray Ionization and Atmospheric Pressure Chemical Ionization Anastasia Albert and Carsten Engelhard* Institute of Inorganic and Analytical Chemistry, University of Muenster, Correnstrasse 30, 48149 Muenster, Germany S Supporting Information *
ABSTRACT: Ambient desorption/ionization mass spectrometry (ADI-MS) is an attractive method for direct analysis with applications in homeland security, forensics, and human health. For example, low-temperature plasma probe (LTP) ionization was successfully used to detect, e.g., explosives, drugs, and pesticides directly on the target. Despite the fact that the field is gaining significant attention, few attempts have been made to classify ambient ionization techniques based on their ionization characteristics and performance compared to conventional ionization sources used in mass spectrometry. In the present study, relative ionization efficiencies (RIEs) for a large group of compound families were determined with LTP-Orbitrap-MS and compared to those obtained with electrospray ionization mass spectrometry (ESI-MS) and atmospheric pressure chemical ionization mass spectrometry (APCI-MS). RIEs were normalized against one reference compound used across all methods to ensure comparability of the results. Typically, LTP analyte ionization through protonation/deprotonation (e.g., 4-acetamidophenol) was observed; in some cases (e.g., acenaphthene) radicals were formed. Amines, amides, and aldehydes were ionized successfully with LTP. A benefit of LTP over conventional methods is the possibility to successfully ionize PAHs and imides. Here, the studied model compounds could be detected by neither APCI nor ESI. LTP is a relatively soft ionization method because little fragmentation of model compounds was observed. It is considered to be an attractive method for the ionization of low molecular weight compounds over a relatively wide polarity range.
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desorption and ionization of the sample. The plasma itself is a dielectric-barrier discharge that is sustained in helium by applying a high-frequency alternating current (ac) voltage between a specially designed electrode configuration. The afterglow of the LTP probe, i.e., the portion of the plasma outside the discharge tube, can be used in direct contact with the sample surface. Surface analysis and surface scanning are spatially independent of the electrode configuration.5 This enables the direct mass spectrometric analysis of samples in the most different sizes and shapes. When not used for direct surface sampling, a nonthermal plasma can also be used, e.g., to ionize compounds in the effluent from a liquid chromatography separation.6 The mechanisms of desorption and ionization in the LTP probe are still not fully understood and an area of active research. Potential desorption mechanisms presumably include thermal desorption, chemical sputtering, and surface reactions. Different plasma species including neutrals, ions, radicals, metastables, and electrons interact with the sample surface and can lead to a variety of effects. Chemical sputtering through collisions of high-velocity species (e.g., a fast stream of discharge gas directed to the sample surface)7 or high-energy
mbient desorption/ionization mass spectrometry (ADIMS) is a relatively new, but attractive method for direct analysis of complex samples. The first ambient desorption/ ionization sources described in the literature were desorption electrospray ionization (DESI)1 and direct analysis in real time (DART).2 DESI and DART rely on electrospray droplets and plasma species, respectively, two fundamentally different means of desorption/ionization, which have been used to classify new ion sources that emerged soon thereafter. Today, over 25 electrospray- and plasma-based ADI sources have been published.3 Plasma-based sources are especially attractive because they are relatively cheap to build and do not require extensive use of gases, power, and solvents. They can even be made into hand-held and portable systems.4 Among plasmabased sources, considerable amount of research has been devoted to the low-temperature plasma (LTP) probe, which was first described for its use in mass spectrometry by Harper et al.5 LTP is considered a nondestructive plasma-based ion source that can be used, e.g., for temperature-sensitive samples such as biological materials, living cells, and explosives. Furthermore, because LTP features low power consumption (
