**2.3 Atmospheric pressure photo ionization (APPI)**

APPI by design is similar to APCI, with only difference being the replacement of corona discharge needle with gas discharge krypton lamp (10.0 eV) that produces ultraviolet photons [28–30]. Evaporation of liquid phase happens in pneumatic nebulizer. While ionization potential for most of the analytes is less than 10 eV; mobile phase constituents such as water, acetonitrile and methanol has higher potential requirements. Presence of dopants such as toluene or acetone will help in enhancing the sensitivity of analyte ions. Dopant molecules absorb photon energy and eject an electron, resulting in the formation of radical cation. Ionization of analytes can happen by two processes: 1) Charge transfer between analyte and radical cations generated from dopant molecules. 2) Charge transfer between dopant molecules and mobile phase components and finally from mobile phase components to analytes (**Figure 3**). Similar to other atmospheric pressure ionization techniques, APPI is also suitable for negative mode of ionization. Sensitivity of APPI is flow rate dependent and better sensitivities have been reported at low flow rates. When compared to APCI, APPI offers lesser matrix effects and minimal source contamination. Success of APPI as an ionization technique was reported in the analysis of steroids and quinones [31–33].

**43**

**Figure 4.**

**Figure 3.**

*Mass Spectrometry as a Workhorse for Preclinical Drug Discovery: Special Emphasis on Drug…*

In a nut shell, ESI is useful for the analysis of moderately polar to highly polar analytes; APCI covers moderately polar to non-polar analytes, whereas APPI suits

MALDI operates on principle of ionization of analytes dissolved in a matrix consisting of organic compound (sinapinic acid, α-cyano-4-hydroxycinnamic acid, 2,5-dihydroxybenzoic acid) and evaporated to dryness on a target plate [34]. Matrix crystallizes up on drying and the analyte dissolved with in it also gets co-crystallized. Firstly, laser beam hits the dried sample and ionize organic compound, which later ionize the analyte molecules. Laser beam causes both desorption and ionization of analytes. Nitrogen laser emitting at 337 nm and Nd: YAG laser emitting at 355 nm are the most widely used ones (**Figure 4**). MALDI is considered as a very

*DOI: http://dx.doi.org/10.5772/intechopen.88385*

for non-polar to moderately polar analytes.

**2.4 Matrix assisted laser desorption ionization (MALDI)**

*Schematic representation of atmospheric pressure photo ionization process.*

*Mechanism of matrix assisted laser desorption ionization process.*

**Figure 2.** *Mechanism of atmospheric pressure chemical ionization process.*

*Mass Spectrometry as a Workhorse for Preclinical Drug Discovery: Special Emphasis on Drug… DOI: http://dx.doi.org/10.5772/intechopen.88385*

In a nut shell, ESI is useful for the analysis of moderately polar to highly polar analytes; APCI covers moderately polar to non-polar analytes, whereas APPI suits for non-polar to moderately polar analytes.
