Novel Ambient Ionization Sampling Methodologies With Mass Spectrometry
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Expanding on the current paradigm of ambient ionization, herein is reported the use of a metal surface for metal spray ionization and the use of online derivatization to separate isomass compounds. A metal surface has several advantages over some of the newer ambient ionization sampling methodologies in the fact that it is reusable, relatively inexpensive, has a fixed geometry, does not require wicking to initiate a spray, and can be functionalized with many coatings depending on the metal used.
A bare copper surface was used as a spray initiator and the sensitivity, limits of detection and quantitation were compared to a copper surface that was functionalized with a superhydrophobic coating on singly and doubly charged peptides. These surfaces were tested with a 30°, 60°, and 90° tip with a channel extending from the proximal tip of the surface, where 0.5 μL of analyte was loaded prior to each spray event. Overall the superhydrophobic surface displayedgreater sensitivity, lower limits of detection and quantitation compared to the bare copper surface, and the 30° tip resulted in the highest sensitivity. There were several limitations to using a copper surface as a spray initiator, in that over time, it seems that the copper underneath the superhydrophobic coating oxidized due to the applied external voltage. Reliable calibration curves could not be obtained with nonpolar analytes, such as lipids. This is likely due to the attractive forces between the superhydrophobic coating and the analyte, which caused the analyte to adhere to the surface instead of being ejected.
To combat many of the limitations of the superhydrophobic surface, gold was electroplated onto a copper surface. Gold is inert and will not oxidize over time, and also has some very unique properties in creating a very strong bond with thiols, which can only be destroyed under very harsh oxidizing conditions. This surface can be reused, and re-functionalized with different thiols to give the desired surface properties. Depending on the functional groups the thiol contains, (hydrophobic/philic) properties can be further enhanced on the surface with other coatings to give greater sensitivity depending on the polarity of the analyte. A stepwise procedure was employed to create an omniphobic surface. Omniphobic surfaces maintain a static contact angle and have repellency to all oils and liquids. This provides an opportunity to make reliable and reproducible calibration curves with nonpolar analytes that could not be established with superhydrophobic surfaces. Sensitivity, limits of detection (LOD) and limits of quantitation (LOQ) of the omniphobic surface were compared to that of a heated electrospray ionization source, and overall the results obtained using an omniphobic metallic surface were similar, and sometimes better than those using electrospray.
An atmospheric solids analysis probe (ASAP) was used to rapidly differentiate isomass drugs such as Δ9-tetrahydrocannabinol (THC) from cannabidiol (CBD), and hydromorphone from morphine. These pairs cannot be distinguished on the basis of tandem mass spectra via collision induced dissociation (CID) because their fragmentation patters are so similar. Thispresents a unique problem to law enforcement and forensic analysts. These compounds however have different number of exchangeable hydrogens, so known mixtures of these compounds were derivatized with N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA) through direct infusion in order to shift the m/zenvelope.