Desorption atmospheric pressure photoionization

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Desorption atmospheric pressure photoionization schematic DAPPI.jpg
Desorption atmospheric pressure photoionization schematic

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 allow for direct analysis of samples without pretreatment. [1] The direct analysis technique, such as DAPPI, eliminates the extraction steps seen in most nontraditional samples. DAPPI can be used to analyze bulkier samples, such as, tablets, powders, resins, plants, and tissues. The first step of this technique utilizes a jet of hot solvent vapor. [2] The hot jet thermally desorbs the sample from a surface. [2] The vaporized sample is then ionized by the vacuum ultraviolet light and consequently sampled into a mass spectrometer. [1] DAPPI can detect a range of both polar and non-polar compounds, but is most sensitive when analyzing neutral or non-polar compounds. [3] This technique also offers a selective and soft ionization for highly conjugated compounds. [4]

Contents

History

The history of desorption atmospheric pressure photoionization is relatively new, but can be traced back through developments of ambient ionization techniques dating back to the 1970s. [5] DAPPI is a combination of popular techniques, such as, atmospheric pressure photoionziation (APPI) and surface desorption techniques. [1] The photoionization techniques were first developed in the late 1970s and began being used in atmospheric pressure experiments in the mid 1980s. [6] Early developments in the desorption of open surface and free matrix experiments were first reported in literature in 1999 in an experiment using desorption/ionization on silicon (DIOS). [7] DAPPI replaced techniques such as desorption electrospray ionization (DESI) and direct analysis in real time (DART). This generation of techniques are all recent developments seen in the 21st century. DESI was discovered in 2004 at Purdue University, [8] while DART was discovered in 2005 by Laramee and Cody. [9] DAPPI was developed soon after in 2007 at the University of Helsinki, Finland. [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. [1]

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. [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 250-350C 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]

This mechanism shows the solvent (S) and the analyte (M) in desorption atmospheric pressure photoionization going through both positive ion and negative ion reaction. Photoionization Mechanism.png
This mechanism shows the solvent (S) and the analyte (M) in desorption atmospheric pressure photoionization going through both positive ion and negative ion reaction.

Types of component geometries

Reflection geometry

Figure A is a conventional DAPPI setup with a reflection geometry. Figure B is a transmission DAPPI technique. The UV Lamp (not seen in figure) is in the same place in both techniques. The UV Lamp is located above the surface space. DAPPI.png
Figure A is a conventional DAPPI setup with a reflection geometry. Figure B is a transmission DAPPI technique. The UV Lamp (not seen in figure) is in the same place in both techniques. The UV Lamp is located above the surface space.

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 . [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 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]

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 a matrix or discharge. [27] 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 angle into the coupled MS. [28] Studies have shown the detection of organic compounds such as: farnesene, squalene, tetradecahydroanthracene, 5-alpha cholestane, perylene, benzoperylene, coronene, tetradecylprene, dodecyl sulfide, benzodiphenylene sulfide, dibenzosuberone, carbazole, and elipticine. [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]

Mass spectrometry

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. [29] DAPPI allows for a broader range of polarities to be detected, and a range of molecular weights. [30] Without separation or sample preparation, DAPPI is able to thermally desorb compounds such as oak biochars. The study did cite an issue with DAPPI. 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.

Applications

DAPPI can analyze both polar (e.g. verapamil) and nonpolar (e.g. anthracene) compounds. [10] This technique has an upper detection limit of 600 Da. [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). [32] Performance of DAPPI has also been demonstrated on direct analysis of illicit drugs. [24] Other applications include lipid detection and drug analysis sampling. [33] Lipids can be detected through a coupling procedure with orbitrap mass spectroscopy. [24] DAPPI has also been known to couple with liquid chromotography and gas chromotography mass spectroscopy for the analysis of drugs and aerosol compounds. [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. [34]

See also

Related Research Articles

<span class="mw-page-title-main">Mass spectrometry</span> Analytical technique based on determining mass to charge ratio of ions

Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.

<span class="mw-page-title-main">Ion source</span> Device that creates charged atoms and molecules (ions)

An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.

<span class="mw-page-title-main">Electrospray ionization</span> Technique used in mass spectroscopy

Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized. ESI is different from other ionization processes since it may produce multiple-charged ions, effectively extending the mass range of the analyser to accommodate the kDa-MDa orders of magnitude observed in proteins and their associated polypeptide fragments.

<span class="mw-page-title-main">Liquid chromatography–mass spectrometry</span> Analytical chemistry technique

Liquid chromatography–mass spectrometry (LC–MS) is an analytical chemistry technique that combines the physical separation capabilities of liquid chromatography with the mass analysis capabilities of mass spectrometry (MS). Coupled chromatography – MS systems are popular in chemical analysis because the individual capabilities of each technique are enhanced synergistically. While liquid chromatography separates mixtures with multiple components, mass spectrometry provides spectral information that may help to identify each separated component. MS is not only sensitive, but provides selective detection, relieving the need for complete chromatographic separation. LC–MS is also appropriate for metabolomics because of its good coverage of a wide range of chemicals. This tandem technique can be used to analyze biochemical, organic, and inorganic compounds commonly found in complex samples of environmental and biological origin. Therefore, LC–MS may be applied in a wide range of sectors including biotechnology, environment monitoring, food processing, and pharmaceutical, agrochemical, and cosmetic industries. Since the early 2000s, LC–MS has also begun to be used in clinical applications.

<span class="mw-page-title-main">Atmospheric-pressure chemical ionization</span> Ionization method

Atmospheric pressure chemical ionization (APCI) is an ionization method used in mass spectrometry which utilizes gas-phase ion-molecule reactions at atmospheric pressure (105 Pa), commonly coupled with high-performance liquid chromatography (HPLC). APCI is a soft ionization method similar to chemical ionization where primary ions are produced on a solvent spray. The main usage of APCI is for polar and relatively less polar thermally stable compounds with molecular weight less than 1500 Da. The application of APCI with HPLC has gained a large popularity in trace analysis detection such as steroids, pesticides and also in pharmacology for drug metabolites.

In mass spectrometry, direct analysis in real time (DART) is an ion source that produces electronically or vibronically excited-state species from gases such as helium, argon, or nitrogen that ionize atmospheric molecules or dopant molecules. The ions generated from atmospheric or dopant molecules undergo ion-molecule reactions with the sample molecules to produce analyte ions. Analytes with low ionization energy may be ionized directly. The DART ionization process can produce positive or negative ions depending on the potential applied to the exit electrode.

<span class="mw-page-title-main">Thermospray</span>

Thermospray is a soft ionization source by which a solvent flow of liquid sample passes through a very thin heated column to become a spray of fine liquid droplets. As a form of atmospheric pressure ionization in mass spectrometry these droplets are then ionized via a low-current discharge electrode to create a solvent ion plasma. A repeller then directs these charged particles through the skimmer and acceleration region to introduce the aerosolized sample to a mass spectrometer. It is particularly useful in liquid chromatography-mass spectrometry (LC-MS).

<span class="mw-page-title-main">Desorption electrospray ionization</span>

Desorption electrospray ionization (DESI) is an ambient ionization technique that can be coupled to mass spectrometry (MS) for chemical analysis of samples at atmospheric conditions. Coupled ionization sources-MS systems are popular in chemical analysis because the individual capabilities of various sources combined with different MS systems allow for chemical determinations of samples. DESI employs a fast-moving charged solvent stream, at an angle relative to the sample surface, to extract analytes from the surfaces and propel the secondary ions toward the mass analyzer. This tandem technique can be used to analyze forensics analyses, pharmaceuticals, plant tissues, fruits, intact biological tissues, enzyme-substrate complexes, metabolites and polymers. Therefore, DESI-MS may be applied in a wide variety of sectors including food and drug administration, pharmaceuticals, environmental monitoring, and biotechnology.

Sample preparation for mass spectrometry is used for the optimization of a sample for analysis in a mass spectrometer (MS). Each ionization method has certain factors that must be considered for that method to be successful, such as volume, concentration, sample phase, and composition of the analyte solution. Quite possibly the most important consideration in sample preparation is knowing what phase the sample must be in for analysis to be successful. In some cases the analyte itself must be purified before entering the ion source. In other situations, the matrix, or everything in the solution surrounding the analyte, is the most important factor to consider and adjust. Often, sample preparation itself for mass spectrometry can be avoided by coupling mass spectrometry to a chromatography method, or some other form of separation before entering the mass spectrometer. In some cases, the analyte itself must be adjusted so that analysis is possible, such as in protein mass spectrometry, where usually the protein of interest is cleaved into peptides before analysis, either by in-gel digestion or by proteolysis in solution.

<span class="mw-page-title-main">Laser spray ionization</span>

Laser spray ionization refers to one of several methods for creating ions using a laser interacting with a spray of neutral particles or ablating material to create a plume of charged particles. The ions thus formed can be separated by m/z with mass spectrometry. Laser spray is one of several ion sources that can be coupled with liquid chromatography-mass spectrometry for the detection of larger molecules.

<span class="mw-page-title-main">Matrix-assisted laser desorption electrospray ionization</span>

Matrix-assisted laser desorption electrospray ionization (MALDESI) was first introduced in 2006 as a novel ambient ionization technique which combines the benefits of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI). An infrared (IR) or ultraviolet (UV) laser can be utilized in MALDESI to resonantly excite an endogenous or exogenous matrix. The term 'matrix' refers to any molecule that is present in large excess and absorbs the energy of the laser, thus facilitating desorption of analyte molecules. The original MALDESI design was implemented using common organic matrices, similar to those used in MALDI, along with a UV laser. The current MALDESI source employs endogenous water or a thin layer of exogenously deposited ice as the energy-absorbing matrix where O-H symmetric and asymmetric stretching bonds are resonantly excited by a mid-IR laser.

<span class="mw-page-title-main">Ambient ionization</span>

Ambient ionization is a form of ionization in which ions are formed in an ion source outside the mass spectrometer without sample preparation or separation. Ions can be formed by extraction into charged electrospray droplets, thermally desorbed and ionized by chemical ionization, or laser desorbed or ablated and post-ionized before they enter the mass spectrometer.

<span class="mw-page-title-main">Laser ablation electrospray ionization</span>

Laser ablation electrospray ionization (LAESI) is an ambient ionization method for mass spectrometry that combines laser ablation from a mid-infrared (mid-IR) laser with a secondary electrospray ionization (ESI) process. The mid-IR laser is used to generate gas phase particles which are then ionized through interactions with charged droplets from the ESI source. LAESI was developed in Professor Akos Vertes lab by Peter Nemes in 2007 and it was marketed commercially by Protea Biosciences, Inc until 2017. Fiber-LAESI for single-cell analysis approach was developed by Bindesh Shrestha in Professor Vertes lab in 2009. LAESI is a novel ionization source for mass spectrometry (MS) that has been used to perform MS imaging of plants, tissues, cell pellets, and even single cells. In addition, LAESI has been used to analyze historic documents and untreated biofluids such as urine and blood. The technique of LAESI is performed at atmospheric pressure and therefore overcomes many of the obstacles of traditional MS techniques, including extensive and invasive sample preparation steps and the use of high vacuum. Because molecules and aerosols are ionized by interacting with an electrospray plume, LAESI's ionization mechanism is similar to SESI and EESI techniques.

<span class="mw-page-title-main">Extractive electrospray ionization</span>

Extractive electrospray ionization (EESI) is a spray-type, ambient ionization source in mass spectrometry that uses two colliding aerosols, one of which is generated by electrospray. In standard EESI, syringe pumps provide the liquids for both an electrospray and a sample spray. In neutral desorption EESI (ND-EESI), the liquid for the sample aerosol is provided by a flow of nitrogen.

<span class="mw-page-title-main">Nanospray desorption electrospray ionization</span>

Nanospray desorption electrospray ionization (nano-DESI) is an ambient pressure ionization technique used in mass spectrometry (MS) for chemical analysis of organic molecules. In this technique, analytes are desorbed into a liquid bridge formed between two capillaries and the sampling surface. Unlike desorption electrospray ionization (DESI), from which nano-DESI is derived, nano-DESI makes use of a secondary capillary, which improves the sampling efficiency.

<span class="mw-page-title-main">Atmospheric-pressure photoionization</span> Soft ionization method

Atmospheric pressure photoionization (APPI) is a soft ionization method used in mass spectrometry (MS) usually coupled to liquid chromatography (LC). Molecules are ionized using a vacuum ultraviolet (VUV) light source operating at atmospheric pressure, either by direct absorption followed by electron ejection or through ionization of a dopant molecule that leads to chemical ionization of target molecules. The sample is usually a solvent spray that is vaporized by nebulization and heat. The benefit of APPI is that it ionizes molecules across a broad range of polarity and is particularly useful for ionization of low polarity molecules for which other popular ionization methods such as electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) are less suitable. It is also less prone to ion suppression and matrix effects compared to ESI and APCI and typically has a wide linear dynamic range. The application of APPI with LC/MS is commonly used for analysis of petroleum compounds, pesticides, steroids, and drug metabolites lacking polar functional groups and is being extensively deployed for ambient ionization particularly for explosives detection in security applications.

<span class="mw-page-title-main">Desorption/ionization on silicon</span> Soft laser desorption method

Desorption/ionization on silicon (DIOS) is a soft laser desorption method used to generate gas-phase ions for mass spectrometry analysis. DIOS is considered the first surface-based surface-assisted laser desorption/ionization (SALDI-MS) approach. Prior approaches were accomplished using nanoparticles in a matrix of glycerol, while DIOS is a matrix-free technique in which a sample is deposited on a nanostructured surface and the sample desorbed directly from the nanostructured surface through the adsorption of laser light energy. DIOS has been used to analyze organic molecules, metabolites, biomolecules and peptides, and, ultimately, to image tissues and cells.

<span class="mw-page-title-main">Matrix-assisted ionization</span>

In mass spectrometry, matrix-assisted ionization is a low fragmentation (soft) ionization technique which involves the transfer of particles of the analyte and matrix sample from atmospheric pressure (AP) to the heated inlet tube connecting the AP region to the vacuum of the mass analyzer.

Probe electrospray ionization (PESI) is an electrospray-based ambient ionization technique which is coupled with mass spectrometry for sample analysis. Unlike traditional mass spectrometry ion sources which must be maintained in a vacuum, ambient ionization techniques permit sample ionization under ambient conditions, allowing for the high-throughput analysis of samples in their native state, often with minimal or no sample pre-treatment. The PESI ion source simply consists of a needle to which a high voltage is applied following sample pick-up, initiating electrospray directly from the solid needle.

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  29. 1 2 Cho, Yunju; Jin, Jang Mi; Witt, Matthias; Birdwell, Justin E.; Na, Jeong-Geol; Roh, Nam-Sun; Kim, Sunghwan (2013-04-18). "Comparing Laser Desorption Ionization and Atmospheric Pressure Photoionization Coupled to Fourier Transform Ion Cyclotron Resonance Mass Spectrometry To Characterize Shale Oils at the Molecular Level". Energy & Fuels. 27 (4): 1830–1837. doi:10.1021/ef3015662. ISSN   0887-0624.
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  31. ↑ Suni, Niina M.; Lindfors, Pia; Laine, Olli; Östman, Pekka; Ojanperä, Ilkka; Kotiaho, Tapio; Kauppila, Tiina J.; Kostiainen, Risto (2011-08-05). "Matrix effect in the analysis of drugs of abuse from urine with desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS) and desorption electrospray ionization-mass spectrometry (DESI-MS)". Analytica Chimica Acta. 699 (1): 73–80. doi:10.1016/j.aca.2011.05.004. PMID   21704760.
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  33. ↑ Kauppila, Tiina J.; Syage, Jack A.; Benter, Thorsten (2015-05-01). "Recent developments in atmospheric pressure photoionization-mass spectrometry". Mass Spectrometry Reviews. 36 (3): 423–449. Bibcode:2017MSRv...36..423K. doi:10.1002/mas.21477. ISSN   1098-2787. PMID   25988849.
  34. ↑ Luosujärvi, Laura; Kanerva, Sanna; Saarela, Ville; Franssila, Sami; Kostiainen, Risto; Kotiaho, Tapio; Kauppila, Tiina J. (2010-05-15). "Environmental and food analysis by desorption atmospheric pressure photoionization-mass spectrometry". Rapid Communications in Mass Spectrometry. 24 (9): 1343–1350. doi: 10.1002/rcm.4524 . ISSN   1097-0231. PMID   20391607.
  35. ↑ Ifa, Demian R.; Wu, Chunping; Ouyang, Zheng; Cooks, R. Graham (2010-03-22). "Desorption electrospray ionization and other ambient ionization methods: current progress and preview". The Analyst. 135 (4): 669–81. Bibcode:2010Ana...135..669I. doi:10.1039/b925257f. ISSN   1364-5528. PMID   20309441.
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