Laser spray ionization refers to one of several methods for creating ions using a laser interacting with a spray of neutral particles [1] [2] [3] or ablating material to create a plume of charged particles. [4] 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. [5]
In one version of the laser spray interface, explosive vaporization and mist formation occur when an aqueous solution effusing from the tip of the stainless steel capillary is irradiated from the opposite side of the capillary by a 10.6 μm infrared laser. [1] Weak ion signals could be detected when the plume was sampled through the ion sampling orifice. When a high voltage (3–4 kV) was applied to the stainless-steel capillary, strong ion signals appeared. The ion abundances were found to be orders of magnitude greater than those obtained by conventional electrospray ionization in the case of aqueous solutions. This approach to laser spray ionization is a hybrid of three basic techniques for the generation of gaseous ions from the condensed phase, i.e., energy-sudden activation, nebulization and the action of an electric field. [1]
Laser spray mass spectrometry can faithfully reflect the solution-phase characteristics of biomolecules. It has been successfully applied to evaluate the binding affinities of protein-DNA.
Laser spray has better ionization efficiency than conventional electrospray ionization (ESI). [1] In particular, the sensitivity became more than one order of magnitude higher in negative ion modes. It was also found that this technique has a potential benefit for the low concentration samples due to condensation effect of the formed droplet by the irradiation of laser. Higher the solvation energies of triply charged metal ions, stronger are the signals for ions. [6]
Laserspray Ionization (LSI) is a newer mass spectrometric technique commonly used with biomolecules, such as proteins. This method is similar to matrix-assisted laser desorption/ionization (MALDI) at atmospheric pressure in that it involves an analyte and matrix mixture. It also contains features from electrospray ionization, in which it produces a similar mass spectra. The mechanism was initially thought to involve laser induced production of highly charge matrix/analyte clusters that upon evaporation of the matrix produces ions by the same mechanism as ESI. LSI's ability to ablate proteins at atmospheric pressure in order to form a multiple of charged ions with a mass resolution of 100,000 when coupled with a quadrupole orbitrap mass spectrometer. [7] The advantages of using LSI includes a solvent-free ionization technique, fast data acquisition, simply to use, and the improved fragmentation through multiple charging. [8]
Due to recent innovations to the laser spray technique, a new method of laser ablation using the spray method has surfaced. Laserspray inlet ionization (LSII) involves a matrix/analyte sample at atmospheric pressure being ablated, and the ionization process will take place in an ion transfer capillary tube located in the mass spectrometer inlet. [9] The LSII method is also known as laserspray ionization vacuum (LSIV). [10]
Matrix-assisted inlet ionization (MAII) has shown that the laser is not necessary for the ionization process. Ions are formed when matrix-analyte is introduced to the vacuum of a mass spectrometer through an inlet aperture. LSI is a subset of MAII and is now called laserspray inlet ionization (LSII). [11] Laser spray inlet ionization and matrix-assisted inlet ionization can be coupled to a fourier transform ion cyclotron resonance (FT-ICR) mass analyzer to improve detection of peptides and proteins. [12]
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.
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.
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.
In mass spectrometry, matrix-assisted laser desorption/ionization (MALDI) is an ionization technique that uses a laser energy-absorbing matrix to create ions from large molecules with minimal fragmentation. It has been applied to the analysis of biomolecules and various organic molecules, which tend to be fragile and fragment when ionized by more conventional ionization methods. It is similar in character to electrospray ionization (ESI) in that both techniques are relatively soft ways of obtaining ions of large molecules in the gas phase, though MALDI typically produces far fewer multi-charged ions.
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.
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).
The compound 3-nitrobenzyl alcohol is an organic compound with the formula C7H7NO3.
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.
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.
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. 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. The hot jet thermally desorbs the sample from a surface. The vaporized sample is 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. This technique also offers a selective and soft ionization for highly conjugated compounds.
Capillary electrophoresis–mass spectrometry (CE–MS) is an analytical chemistry technique formed by the combination of the liquid separation process of capillary electrophoresis with mass spectrometry. CE–MS combines advantages of both CE and MS to provide high separation efficiency and molecular mass information in a single analysis. It has high resolving power and sensitivity, requires minimal volume and can analyze at high speed. Ions are typically formed by electrospray ionization, but they can also be formed by matrix-assisted laser desorption/ionization or other ionization techniques. It has applications in basic research in proteomics and quantitative analysis of biomolecules as well as in clinical medicine. Since its introduction in 1987, new developments and applications have made CE-MS a powerful separation and identification technique. Use of CE–MS has increased for protein and peptides analysis and other biomolecules. However, the development of online CE–MS is not without challenges. Understanding of CE, the interface setup, ionization technique and mass detection system is important to tackle problems while coupling capillary electrophoresis to mass spectrometry.
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.
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 Dr. 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 Dr. 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.
Electrostatic spray ionization (ESTASI) is an ambient ionization method for mass spectrometry (MS) analysis of samples located on a flat or porous surface, or inside a microchannel. It was developed in 2011 by Professor Hubert H. Girault’s group at the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland. In a typical ESTASI process, a droplet of a protic solvent containing analytes is deposited on a sample area of interest which itself is mounted to an insulating substrate. Under this substrate and right below the droplet, an electrode is placed and connected with a pulsed high voltage (HV) to electrostatically charge the droplet during pulsing. When the electrostatic pressure is larger than the surface tension, droplets and ions are sprayed. ESTASI is a contactless process based on capacitive coupling. One advantage of ESTASI is, that the electrode and sample droplet act contact-less avoiding thereby any oxidation or reduction of the sample compounds at the electrode surface, which often happens during standard electrospray ionization (ESI). ESTASI is a powerful new ambient ionization technique that has already found many applications in the detection of different analytes, such as organic molecules, peptides and proteins with molecule weight up to 70 kDa. Furthermore, it was used to couple MS with various separation techniques including capillary electrophoresis and gel isoelectric focusing, and it was successfully applied under atmospheric pressure to the direct analysis of samples with only few preparation steps.
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.
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.
in mass spectrometry, an ion funnel is a device used to focus a beam of ions using a series of stacked ring electrodes with decreasing inner diameter. A combined radio frequency and fixed electrical potential is applied to the grids. In electrospray ionization-mass spectrometry (ESI-MS), ions are created at atmospheric pressure, but are analyzed at subsequently lower pressures. Ions can be lost while they are shuttled from areas of higher to lower pressure due to the transmission process caused by a phenomenon called joule expansion or “free-jet expansion.” These ion clouds expand outward, which limits the amount of ions that reach the detector, so fewer ions are analyzed. The ion funnel refocuses and transmits ions efficiently from those areas of high to low pressure.
In mass spectrometry, a matrix is a compound that promotes the formation of ions. Matrix compounds are used in matrix-assisted laser desorption/ionization (MALDI), matrix-assisted ionization (MAI), and fast atom bombardment (FAB).
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