Abstract
Desorption electrospray ionization (DESI), easy ambient sonic-spray ionization (EASI) and low-temperature plasma (LTP) ionization are powerful ambient ionization techniques for mass spectrometry. However, every single method has its limitation in terms of polarity and molecular weight of analyte molecules. After the miniaturization of every possible component of the different ion sources, we finally were able to embed two emitters and an ion transfer tubing into a small, hand-held device. The pen-like interface is connected to the mass spectrometer and a separate control unit via a bundle of flexible tubing and cables. The novel device allows the user to ionize an extended range of chemicals by simple switching between DESI, voltage-free EASI, or LTP ionization as well as to freely move the interface over a surface of interest. A mini camera, which is mounted on the tip of the pen, magnifies the desorption area and enables a simple positioning of the pen. The interface was successfully tested using different types of chemicals, pharmaceuticals, and real life samples. Moreover, the combination of optical data from the camera module and chemical data obtained by mass analysis facilitates a novel type of imaging mass spectrometry, which we name “interactive mass spectrometry imaging (IMSI)”.
Highlights
Desorption electrospray ionization (DESI), easy ambient sonic-spray ionization (EASI) and low-temperature plasma (LTP) ionization are powerful ambient ionization techniques for mass spectrometry
The combination of optical data from the camera module and chemical data obtained by mass analysis facilitates a novel type of imaging mass spectrometry, which we name “interactive mass spectrometry imaging (IMSI)”
A variety of both direct as well as indirect surface sampling methodologies are available to ionize chemicals on surfaces at ambient conditions. The former include desorption electrospray ionization (DESI),[3] direct analysis in real time (DART),[4] low-temperature plasma (LTP) ionization,[5] rapid evaporative ionization mass spectrometry (REIMS),[6] easy ambient sonic-spray ionization (EASI),[7] picosecond infrared laser desorption (PIRL),[8] SpiderMass,[9] or sources, which combine different ionization technologies.[10−12] the importance and impact of direct surface sampling techniques is emphasized by numerous important applications.[13−34] In contrast, a common feature of indirect surface sampling methods is the local separation of the extraction and the ionization process
Summary
We selected a set of 34 chemicals with a variety of functional groups and different combinations thereof to demonstrate advantages as well as capabilities of the 3-in-1 ionization source. The pure chemicals applied to glass or paper surfaces were directly analyzed using LTP, DESI, and EASI ionization in positive as well as negative ion-mode (for a full list of compounds as well as of obtained molecular ions, see Table S2). We are convinced that the EASI mode is an important feature of our interface It is a zero-voltage ionization technique, which allows harmless mass spectrometric analyses of chemicals on human skin, in particular when EASI-MS is performed with nonhazardous spray solvents, e.g. ethanol or isopropanol mixed with water. A user has to define a region of interest, which is analyzed spot by spot within a relatively long period of time (in microprobe mode).[47] microprobe mass spectrometry imaging does not offer a “preview” function, unless the whole sample is analyzed pointwise at a very low lateral resolution. As shown in Figure S-7, the lateral resolution of the first trial runs was observed to be
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