Desorption atmospheric pressure photoionization
[1] The photoionization techniques were first developed in the late 1970s and began being used in atmospheric pressure experiments in the mid 1980s.
[7] DAPPI replaced techniques such as desorption electrospray ionization (DESI) and direct analysis in real time (DART).
[1] The development of DAPPI widened the range of detection for nonpolar compounds and added a new dimension of thermal desorption of direct analysis samples.
[10] The nebulizer microchip is a glass device bonded together by pyrex wafers with flow channels embedded from a nozzle at the edge of the chip.
[10] Momentum transfer or liquid spray desorption is based on the solvent interaction with the sample, causing the release of specific ions.
The gaseous solvent can also undergo photoionization and act as an intermediate for ionization of the sample molecules.
[21] Considered the normal or conventional geometry of DAPPI, this mode is ideal for solid samples that do not need any former preparation.
[23] The UV lamp is directly above the sample and it releases photons to interact with the desorbed molecules that are formed.
from the nebulizer microchip and the mass spec inlet, with the lamp directing photons to the area where the mesh releases newly desorbed molecules.
Transmission mode uses a lower microchip heating power which eliminates some of the issues seen with the reflection geometry above, including low signal noise.
Thin layer chromatography (TLC) is a simple separation technique that can be coupled with DAPPI-MS to identify lipids.
[25][26] DAPPI was used for its ability to ionize neutral and non-polar compounds, and was seen to be a fast and efficient method for lipid detection as it was coupled with both NP-TLC and HPTLC plates.
[27] This method was also seen to be coupled with the mass spectroscopy technique, FTICR, to detect shale oils and some smaller nitrogen containing aromatics.
[28][29] Fourier transform ion cyclotron resonance (FTICR) is a technique that is normally coupled with electrospray ionization (ESI), DESI, or DART, which allows for the detection of polar compounds.
If the sample is not homogeneous, then the neutral ions will ionize only the surface, which does not provide an accurate detection for the substance.
The scanning of the FTICR allows for the detection of complex compounds with high resolution, which leads to the ability to analyze elemental composition.
[2] Compared to desorption electrostray ionization (DESI), DAPPI is less likely to be contaminated by biological matrices.
[31] DAPPI was also seen to be more sensitive and contain less background noise than popular techniques such as direct analysis in real time (DART).
[14] Studies have also shown where DAPPI has been used to find harmful organic compounds in the environment and in food, such as polycyclic aromatic hydrocarbons (PAH) and pesticides.
