Laser-Induced Acoustic Desorption/Atmospheric Pressure Chemical Ionization Mass Spectrometry

Abstract

Laser-induced acoustic desorption (LIAD) was successfully coupled to a conventional atmospheric pressure chemical ionization (APCI) source in a commercial linear quadrupole ion trap mass spectrometer (LQIT). Model compounds representing a wide variety of different types, including basic nitrogen and oxygen compounds, aromatic and aliphatic compounds, as well as unsaturated and saturated hydrocarbons, were tested separately and as a mixture. These model compounds were successfully evaporated into the gas phase by using LIAD and then ionized by using APCI with different reagents. From the four APCI reagent systems tested, neat carbon disulfide provided the best results. The mixture of methanol and water produced primarily protonated molecules, as expected. However, only the most basic compounds yielded ions under these conditions. In sharp contrast, using APCI with either neat benzene or neat carbon disulfide as the reagent resulted in the ionization of all the analytes studied to predominantly yield stable molecular ions. Benzene yielded a larger fraction of protonated molecules than carbon disulfide, which is a disadvantage. A similar but minor amount of fragmentation was observed for these two reagents. When the experiment was performed without a liquid reagent (nitrogen gas was the reagent), more fragmentation was observed. Analysis of a known mixture as well as a petroleum cut was also carried out. In summary, the new experiment presented here allows the evaporation of thermally labile compounds, both polar and nonpolar, without dissociation or aggregation, and their ionization to predominantly form stable molecular ions.

HPLC/APCI Mass Spectrometry of Saturated and Unsaturated Hydrocarbons by Using Hydrocarbon Solvents as the APCI Reagent and HPLC Mobile Phase

“…Unfortunately, fragment ions are commonly formed. The same is true when using CS 2 as an APCI reagent to form molecular ions for saturated hydrocarbons [27,28] due to the low fragmentation threshold energies of their molecular ions. Recently, helium ionization mass spectrometry was implemented on a modified atmospheric-pressure ion source designed for electrospray ionization for ionization of aliphatic hydrocarbons [29].…”

Surface analysis of lipids by mass spectrometry: More than just imaging

“…Desorbed analytes are then ionized by subsequent interaction with the ion source. A variety of ionization techniques have been coupled with LIAD including chemical ionization [270], ESI [271] and APCI [272]. This desorption mechanism is not dependant on laser absorption by the analyte and is suitable for analysis of thermally labile and non-volatile analytes.…”

High molecular weight non‐polar hydrocarbons as pure model substances and in motor oil samples can be ionized without fragmentation by atmospheric pressure chemical ionization mass spectrometry

“…However, as they are difficult to ionize, non‐polar hydrocarbons are not readily examined by MS. Although electrospray ionization (ESI) and matrix‐assisted laser desorption/ionization (MALDI) have broadened the application of MS, these techniques are unable to ionize analytes lacking basic or acidic functional groups that allow proton transfer chemistry in the ion source …”

“…Several recent significant studies have examined the formation of hydrocarbon ions in different MS technologies …”

“…A further major advance in methodology by Kenttamaa's group involved the coupling of LIAD to APCI. This approach enabled the evaporation and ionization of polar and non‐polar analytes yielding predominantly molecular ions with minor fragmentation, when using carbon disulfide (CS 2 ) as a reagent . A recent APCI/CS 2 application by the same group demonstrated the production of a stable molecular ion (M +• ) for non‐polar aromatic, polar aromatic and alkene compounds.…”