Pulsed Desorption Electrospray Ionization Mass Spectrometry

Desorption electrospray ionization mass spectrometry (DESI-MS) is a powerful method for generating ions at ambient conditions without the need for sample preparation or pretreatment. In DESI-MS, a continuous stream of charged microdroplets impacts a surface containing a sample (cDESI-MS). Upon impact, analyte molecules are extracted from the surface into secondary microdroplets, from which gas-phase ions are eventually formed. We have developed a pulsed DESI-MS source (pDESI-MS) that demonstrates higher sensitivity. In addition, the pDESI-MS source is designed so that desorption only occurs during ion accumulation time (IT) of an ion trap mass spectrometer, increasing the sampling efficiency (SE) closer to 100%. This capability is particularly advantageous for high-resolution instruments such as the LTQOrbitrap hybrid mass spectrometers that require significant transient acquisition times after ion accumulation (e.g. for IT = 0.5 s and m/z-resolution = 100,000, and scan time = ~2.3 s, the SE increases by a factor of ~5 from 22% to 100%). In addition, pulsing the primary microdroplet spray reduces the total volume of deposited solvent per m/z scan, which minimizes the ‘washing effect’ reported for cDESI-MS on surfaces such as glass or Teflon®. The ability to control the amount of deposited solvent by varying the pulse widths offer the potential for improved spatial resolution in imaging mode. These results demonstrate that pDESI-MS offers the potential to transform current approaches for implementing spray-based ambient ionization techniques in chemical analysis and imaging