Desorption Electrospray Ionization

The LTQ XL is a high resolution linear ion trap mass spectrometer. The ionization can be done here by three different methods; electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and desorption electrospray ionization (DESI). This instrument is capable of scanning a mass range of up to 4000 m/z and MSⁿ is possible where n ranges from 2 to 10.

This instrument, other than the molecular mass, provides a lot of information about the structure of the molecule due to the capability of performing MSⁿ scans. Quantitative analysis can be performed using ESI probe for samples in solution. The DESI probe allows to analyze samples in solid state and give information about the surface of the samples. The sample stage in this probe can be moved in xy plane to acquire data from different points and from these data it is possible to construct mass spectrometric images of the surface for various species present on it.

In ESI, samples are introduced through syringe pump. At the tip of the probe it ionizes due to the applied voltages and enters into the analyzer portion of the instrument. But in case of DESI, a pneumatically assisted charged solvent droplets are sprayed on the sample surface which dissolves and ionizes the samples. Further droplets splashes the solvents containing dissolved ions towards the mass spectrometer inlet. The ions then pass through ion transfer tube, quadrupole, octopole and finally get trapped in the linear ion trap. When appropriate voltages are applied the ions come out of the trap and reaches the detector which gives the ion intensity for different m/z. For performing MSⁿ scans ions in the trap are allowed to collide with damping gas (helium) which fragments the ions into smaller parts and again it reaches to the detector to give the signals.