User:Michael.p.hayes/sandbox

Desorption atmospheric pressure photoionization (DAPPI) is an ambient ionization technique for mass spectrometry that uses hot

solvent vapor for desorption in conjunction with photoionization. Ambient Ionization techniques allows for direct analysis of samples without pretreatment. The direct analysis technique, such as DAPPI, eliminates the extraction steps seen in most nontraditional samples. DAPPI can be used to analyze more bulky samples, such as, tablets, powders, resins, plants, and tissues. The first step of this technique utilizes a jet of hot solvent vapor. The hot jet thermally desorbs the sample from a surface. The vaporized sample are then ionized by the vacuum ultraviolet light and consequently sampled into a mass spectrometer. DAPPI can detect a range of both polar and non-polar compounds, but is most sensitive when analyzing neutral or non-polar compounds.

Principle of operation
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. The nebulizer microchip is a glass device bonded together by pyrex wafers with flow channel embedded from a nozzle at the edge of the chip. The microchip is heated in order to vaporize the entering solvent and create dopant molecules. Some of the common solvents include: nitrogen, toluene, and anisole. The desorption process can occur by two mechanism: thermal desportion or momentum transfer/ liquid spray. Thermal desorption use heat to volatilize the sample and increase the surface temperature of the substrate. As the substrates's surface temperature is increased, the higher the sensitivity of the instrument. . While studying the substrate temperature, it was seen that the solvent did not have an noticeable affect on the final temperature or heat rate of the substrate. Momentum transfer or liquid spray desoprtion is based on the solvent interaction with the sample, causing the release of specific ions. The momentum transfer is propagated by the collision of the solvent with the sample along with the transfer of ions with the sample. The transfer of positive ions, such as, proton and charge transfers are seen with the solvents: toluene and anisole. 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. Photoionization then occurs, which knocks out the molecule's electron and produces an ion. This technique alone is not highly efficient for different varieties of molecules, particularly those that are not easily protonated or deprotonated. 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. The ions are then sent to the mass analyzer for analysis.

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

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

Transmission geometry
This mode is specialized for analyzing liquid samples, with a metal or polymer mesh replacing the sample plate in reflection geometry. The mesh is oriented $$108^\circ$$ from the nebulizer microchip and the mass spec inlet, with the lamp directing photons to the area where the mesh releases newly desorbed molecules. The analyte is thermally desorbed as both the dopant vapor and nebulizer gas are directed through the mesh. It has been seen that steel mesh with low density and narrow strands produce better signal intensities. This type of mesh allows for larger opening 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 relfection geometry above, including, low signal noise. This method can also imporve the S/N ratio of smaller non-polar compounds.

Separation techniques
Thin layer chromatography (TLC) is a simple separation technique that can be coupled with DAPPI-MS to identify lipids. 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. 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.

Laser Desorption is normally used in the presence of a matrix such as matrix assisted laser desorption ionization (MALDI), but research has combined techniques of laser desoprtion in atmospheric pressure conditions to produce a method that does not use matrix or discharges. This method is able to help for 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 $$108^\circ$$ angle into the coupled MS. Studies have shown the detection of organic compounds, such as: farnesene, squalene, tetradecahydro anthracene, 5-alpha cholestane, perylene, benzo perylene, coronene, tetra decylprene, dodecyl sulfide, benzodiphenylene sulfide, dibenzosuberone, carbazole, and elipticine. This method was also seen to be coupled with the mass spectroscopy technique FTICR to detect shale oils and some smaller nitrogen containing aromatics.

Mass spectroscopy
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. DAPPI allows for a broader range of polarities to be detected and a range of molecular weights. Without separation or sample preparation, DAPPI is able to thermally desorb compounds such as oak biochars. The study did site an issue with DAPPI could be 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 compsition.

Applications
DAPPI can analyze both polar (e.g. verapamil) and nonpolar (e.g. anthracene) compounds. Compared to desorption electrostray ionization (DESI), DAPPI is less likely contaminated by biological matrices. DAPPI was also seen to be more sensentive and containing less background than popular techniques such as direct analysis in real time (DART). Performance of DAPPI has also been demonstrated on direct analysis of illicit drugs. Other application include lipid detection and drug analysis sampling. Lipids are can be detected through a coupling procedure with orbitrap mass spectroscopy. DAPPI has also been known to couple with liquid chromotography and gas chromotography mass spectroscopy for the analysis of drugs and aerosol compounds. Studies have also shown where DAPPI has been used to find harmful organics, such as PAHs and pesticides, in the environment and food.