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How Atmospheric Pressure Ionization Sources Work in Mass Spectrometry

Design for atmospheric pressure ionization sources came of age in the late 1980s to provide a powerful analytical tool—the mass spectrometer —the ability to characterize analytes amenable to liquid chromatography as gas-phase ions removed from the liquid. The motivating force behind such invention has always been the need to answer questions better and faster with tools that provide greater utility.

by Waters Corporation
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Problem: With all that is known currently, the transfer of the ions generated at atmosphere into the vacuum system still requires artful and scientific consideration. Whether electrospray (ESI) for polar analytes or atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) for less polar and neutral analytes, transfer inefficiencies can develop losses requiring from 100 to 5000 molecules in the original solution to yield a single ion that is actually detected. The efficiency is highly dependant on the flow rate of the original solution as well as the chemical characteristics of the molecules themselves.

Aerosol droplet dispersion remote from the ion entrance to the mass spectrometer and incomplete droplet desolvation play a major role in ion losses. Both behaviors are controlled by fundamental gas dynamic principles that can be modeled analytically and, using computational fluid dynamic models, reveal that the physical processes of gas entrainment and recirculation dominate the trajectory of the aerosols.

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Waters’ Xevo™ source

Solution: Control of heat and gas dynamics are obviously important when transitioning reproducibly between the conditions amenable to ESI and the drier vapor environs needed for APCI and APPI. Low adsorptivity surfaces for reduced sample memory or persistence and drafting exhaust vapors after the aerosol is formed, are critical.

Essentially all manufacturers attempt to understand and employ similar physics as added instrument capability. For instance, nitrogen gas is swept across the entering stream, which reduces contamination and declusters weakly held adducts. MDS/Sciex instruments’ Turbo V source allows ESI and APCI to be optimized in the same source using naturally occurring hydrophilic differences to allow time to heat or cool the source to provide optimum performance. Others, such as Thermo Electron’s Ion Max source, similarly rely on controlling heating and gas dynamics as well as positioning the exhaust or drain port directly across form the aerosol to minimize additional gas interaction.

The recent Xevo development from Waters, based on the industry standard Z-spray ion path design developed by Bajic found in many versions of current sources, has refined the housing through the use of specific materials, heating and gas dynamics to be optimized for specific ionization modes. The easily unlatched and replaced housing provided designers the chance to avoid the necessity of making compromises to focus on what features worked best for each. This new design, in addition to the standard atmospheric pressure ionization modes, supports novel techniques such as multimode ESCi® high speed millisecond switching between ESI and APCI and an atmospheric pressure gas chromatography (APGC) interface to perform high sensitivity GC in the same source used for LC.

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