Ambient Mass Spectrometry

Abstract

Learning Objectives • Ion formation under ambient conditions • Interfaces for ambient mass spectrometry • Screening techniques for rapid quality control and safety applications • Real time examination of samples during physical manipulation All methods for the generation of ions for mass spectrometry described up to this point require the analyte for ionization to be presented either directly under high vacuum (EI, CI, FI, FD) or contained in a sort of solution from which ions are to be extracted into or generated in the gas phase (FAB, LDI, MALDI). Even the at-mospheric pressure ionization techniques employ processes that create ions from dilute (solid) solutions of the sample (ESI, APCI, APPI, AP-MALDI). This chap-ter deals with the manifold methods and interfaces which are allowing to over-come these limitations, and which have developed at a breathtaking pace within the short time since the publication of the first edition of this book. Desorption electrospray ionization (DESI) [1] was introduced at the end of 2004, and direct analysis in real time (DART) [2] soon after in 2005. The apparent potential of both DESI and DART in high-throughput applications soon led to the development of some " derivatives " with the intention to broaden the field of applications or to adapt the underlying methodology to specific analytical needs. Now, the repertoire of methods includes variations of the DESI theme such as desorption sonic spray ionization (DeSSI) [3], later renamed easy sonic spray ionization (EASI) [4] or extractive electrospray ionization (EESI) [5,6]. Then, there are the DESI analogs of APCI and APPI, i.e., desorption atmospheric-pressure chemical ionization (DAPCI) [7,8] and desorption atmospheric pressure photoionization (DAPPI) [9]. All these methods have one important characteristic in common: they direct a stream of ionizing or at least ion-desorbing fluid medium onto a sample surface from which analyte ions are withdrawn and transported through air into the mass analyzer via a standard API interface. The beauty of this approach lies in the fact that a sample needs just to be exposed to the ionizing medium under ambient con-ditions. In other words, DESI, DART and those numerous related methods enable the detection of surface materials like waxes, alkaloids, flavors, or pesticides from plants as well as explosives, pharmaceuticals, or drugs of abuse from luggage or banknotes. These and many more analytical applications are readily accessible by J. Gross, Mass Spectrometry © Springer-Verlag Berlin Heidelberg 2011 , 2nd ed., DOI 10.1007/978-3-642-10711-5_13, 622 13 Ambient Mass Spectrometry plainly exposing the corresponding items to the ionization region of the interface – even without harm to living organisms [10]. This reduced need for sample pre-treatment is key to the success of ambient MS. In ambient MS, samples are acces-sible to observation and may even be subjected to some kind of processing, either mechanical manipulations or chemical treatments, while mass spectra are continu-ously being measured. Recently, a fieldable ion trap mass spectrometer for use with commercial DESI and DART ion sources has been constructed [11]. Note: Although the features of DESI and DART are in many ways superior and " revolutionary " , one should be aware of intrinsic limitations. The detection of a compound largely depends on the matrix, e.g., whether it is on or eventually in skin, fruit, bark, stone etc. This also results in a lack of quantification abilities. However, also no other single ionization method, especially when used under just one set of conditions, can deliver ions of all constituents of a complex sam-ple. Nonetheless, DESI, DART and related methods can deliver a wealth of chemical information with unprecedented ease.