Desorption atmospheric pressure photoionization

Desorption atmospheric pressure photoionization (DAPPI) is an

The first operation to occur during desorption atmospheric pressure photoionization is desorption. Desorption of the sample is initiated by a hot jet of solvent vapor that is targeted onto the sample by a nebulizer microchip.^[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.^[11] The microchip is heated to 250-350${\displaystyle ^{\circ }}$C in order to vaporize the entering solvent and create dopant molecules.^[12] Dopant molecules are added to help facilitate the ionization of the sample.^[13] Some of the common solvents include: nitrogen, toluene, acetone, and anisole.^[14] The desorption process can occur by two mechanisms: thermal desorption or momentum transfer/liquid spray.^[10] Thermal desorption uses heat to volatilize the sample and increase the surface temperature of the substrate.^[15] As the substrate's surface temperature is increased, the higher the sensitivity of the instrument.^[10] While studying the substrate temperature, it was seen that the solvent did not have a noticeable effect on the final temperature or heat rate of the substrate.^[10] Momentum transfer or liquid spray desoprtion is based on the solvent interaction with the sample, causing the release of specific ions.^[16] The momentum transfer is propagated by the collision of the solvent with the sample along with the transfer of ions with the sample.^[17] The transfer of positive ions, such as protons and charge transfers, are seen with the solvents: toluene and anisole.^[10] Toluene goes through a charge exchange mechanism with the sample, while acetone promotes a proton transfer mechanism with the sample.^[13] A beam of 10 eV photons that are given off by a UV lamp is directed at the newly desorbed molecules, as well as the dopant molecules.^[18] Photoionization then occurs, which knocks out the molecule's electron and produces an ion.^[18] This technique alone is not highly efficient for different varieties of molecules, particularly those that are not easily protonated or deprotonated.^[19] In order to completely ionize samples, dopant molecules must help. The gaseous solvent can also undergo photoionization and act as an intermediate for ionization of the sample molecules. Once dopant ions are formed, proton transfer can occur with the sample, creating more sample ions.^[1] The ions are then sent to the mass analyzer for analysis.^[1]

Ionization mechanisms

The main desorption mechanism in DAPPI is thermal desorption due to rapid heating of the surface.^[20] Therefore, DAPPI only works well for surfaces of low thermal conductivity.^[21] The ionization mechanism depends on the analyte and solvent used. For example, the following analyte (M) ions may be formed: [M + H]⁺, [M - H]⁻, M^+•, M^−•.^[21]

Types of component geometries

Reflection geometry

Considered the normal or conventional geometry of DAPPI, this mode is ideal for solid samples that do not need any former preparation.^[22] The microchip is parallel to the MS inlet.^[23] The microchip heater is aimed to hit the samples at ${\displaystyle 45^{\circ }}$.^[23] The UV lamp is directly above the sample and it releases photons to interact with the desorbed molecules that are formed.^[21] The conventional method generally uses a higher heating power and gas flow rate for the nebulizer gas, while also increasing the amount of dopant used during the technique.^[23] These increases can cause higher background noise, analyte interference, substrate impurities, and more ion reactions from excess dopant ions.^[23]

Transmission geometry

This mode is specialized for analyzing liquid samples, with a metal or polymer mesh replacing the sample plate in reflection geometry.^[23] The mesh is oriented ${\displaystyle 180^{\circ }}$ from the nebulizer microchip and the mass spec inlet, with the lamp directing photons to the area where the mesh releases newly desorbed molecules.^[21] The analyte is thermally desorbed as both the dopant vapor and nebulizer gas are directed through the mesh.^[23] It has been seen that steel mesh with low density and narrow strands produces better signal intensities. This type of mesh allows for larger openings in the surface and quicker heating of strands. Transmission mode uses a lower microchip heating power which eliminates some of the issues seen with the reflection geometry above, including low signal noise. This method can also improve the S/N ratio of smaller non-polar compounds.

Instrument coupling

Separation techniques

Thin layer chromatography (TLC) is a simple separation technique that can be coupled with DAPPI-MS to identify lipids.^[24] Some of the lipids that were seen to be separated and ionized include: cholesterol, triacylglycerols, 1,2-diol diesters, wax esters, hydrocarbons, and cholesterol esters. TLC is normally coupled with instruments in vacuum or atmospheric pressure, but vacuum pressure gives poor sensitivity for more volatile compounds and has minimal area in the vacuum chambers.^[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.^[25]

This method is able to help with smaller compounds, and generates both positive and negative ions for detection. A transmission geometry is taken as the beam and spray are guided at a ${\displaystyle 180^{\circ }}$angle into the coupled MS.

• Orbitrap
• Atmospheric pressure chemical ionization
• Desorption atmospheric pressure chemical ionization^[35]

References