Matrix-assisted ionization

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Scheme of inlet ionization Inlet ionization.jpeg
Scheme of inlet ionization

In mass spectrometry, matrix-assisted ionization (also inlet 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. [1] [2]

Contents

Initial ionization occurs as the pressure drops within the inlet tube.

Inlet ionization is similar to electrospray ionization in that a reverse phase solvent system is used and the ions produced are highly charged, however a voltage or a laser is not always needed. [3] It is a highly sensitive process for small and large molecules like peptides, proteins [3] and lipids [4] that can be coupled to a liquid chromatograph.

Inlet ionization techniques can be used with an Orbitrap mass analyzer, Orbitrap fourier transform mass spectrometer, linear trap quadrupole and MALDI-TOF.

Types of inlet ionization

Matrix-assisted inlet ionization

Schematic of matrix-assisted inlet ionization (MAII) MAII schematic.jpeg
Schematic of matrix-assisted inlet ionization (MAII)

In Matrix-assisted inlet ionization (MAII), a matrix which can be a solvent is used at ambient temperature with the analyte of interest as a mixture. The matrix/analyte mixture is inserted into the heated inlet tube through tapping the mixture at the opening end of the tube. For the highly charged ions of the analyte to be produced from ionization, desolvation of the matrix molecules needs to occur. Matrices that can be used include: [4] 2,5-dihydroxybenzoic acid, 2,5-dihydroxyacetophenone, 2-aminobenzyl alcohol, anthranilic acid, and 2-hydroxyacetophenone.

Laserspray inlet ionization

Schematic of LSII Schematic of LSII.jpeg
Schematic of LSII

Laserspray inlet ionization (LSII) is a subset of MAII and uses a matrix-assisted laser desorption/ionization (MALDI) method. It was originally called atmospheric pressure matrix-assisted laser desorption/ionization however was renamed as LSII to avoid confusion with MALDI and as it was found to be a type of inlet ionization. [3] [5] As all inlet ionization techniques, highly multiply charged ions are produced. A nitrogen laser is used to ablate the solid matrix/analyte into the heated inlet tube, the observed ions are generated at the surface of the matrix/analyte and so the laser is not directly involved in the ionization as was originally thought. [4] LSII can determine protein molecular weights and has been found to detect masses of proteins up to 20,000 Da. The sensitivity of LSII, for protein detection, is higher by an order of magnitude compared to ESI. [6]

Solvent assisted inlet ionization

Schematic of SAII SAII Inlet ionization.jpeg
Schematic of SAII

Solvent assisted inlet ionization (SAII) is similar to matrix-assisted inlet ionization however the matrix is a solvent such as water, acetonitrile and methanol. [3] This ionization technique is highly sensitive to small molecules, peptides and proteins. [3] The analyte is dissolved in the solvent and can either be introduced to the heated inlet tube by a capillary column or directly injected into the inlet tube with a syringe or by pipetting. The capillary column is made of fused silica particles with one end submerged in the sample solvent and the other in the end of the heated inlet tube. The solvent flows through the capillary column without the use of a pump due to the pressure difference between ambient pressure and the vacuum. [7]

The temperature can vary in the inlet tube from 50 °C to 450 °C, with the lower temperature being used if the results obtained from a higher temperature are of good resolution.[ clarification needed ] [4] Solvent assisted inlet ionization can be coupled not only to liquid chromatography (LC) but also to nano LC.

Advantages of inlet ionization

Ionization at atmospheric pressure often leads to a loss of ions during the transfer of the ions from the ambient pressure region to the vacuum of the mass analyzer. [8] Ions are lost due to dispersion of analyte spray and 'rim loss' causing fewer ions to reach the vacuum for m/z separation to occur. Initial ionization occurs in the sub-atmospheric pressure region of the heated inlet tube which is directly attached to the vacuum of the mass analyzer and so ion loss is reduced as transfer of the ions does not occur.

In LSII the use of the laser increases the image quality of the results by producing better spatial resolution. [4] This is where more pixels are created and so a clearer image is obtained.

Multiply charged ions are produced further extending mass range.

Multiple methods can be used to fragment molecules producing fragmentation for structural information: electron transfer dissociation (ETD), collision-induced dissociation (CID), and electron capture dissociation (ECD).[ citation needed ]

When using a laser, only small volumes are needed. [9]

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">Matrix-assisted laser desorption/ionization</span> Ionization technique

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.

<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.

Surface-enhanced laser desorption/ionization (SELDI) is a soft ionization method in mass spectrometry (MS) used for the analysis of protein mixtures. It is a variation of matrix-assisted laser desorption/ionization (MALDI). In MALDI, the sample is mixed with a matrix material and applied to a metal plate before irradiation by a laser, whereas in SELDI, proteins of interest in a sample become bound to a surface before MS analysis. The sample surface is a key component in the purification, desorption, and ionization of the sample. SELDI is typically used with time-of-flight (TOF) mass spectrometers and is used to detect proteins in tissue samples, blood, urine, or other clinical samples, however, SELDI technology can potentially be used in any application by simply modifying the sample surface.

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

MALDI mass spectrometry imaging (MALDI-MSI) is the use of matrix-assisted laser desorption ionization as a mass spectrometry imaging technique in which the sample, often a thin tissue section, is moved in two dimensions while the mass spectrum is recorded. Advantages, like measuring the distribution of a large amount of analytes at one time without destroying the sample, make it a useful method in tissue-based study.

<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">Desorption atmospheric pressure photoionization</span>

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.

<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">Surface-assisted laser desorption/ionization</span>

Surface-assisted laser desorption/ionization (SALDI) is a soft laser desorption technique used for mass spectrometry analysis of biomolecules, polymers, and small organic molecules. In its first embodiment Koichi Tanaka used a cobalt/glycerol liquid matrix and subsequent applications included a graphite/glycerol liquid matrix as well as a solid surface of porous silicon. The porous silicon represents the first matrix-free SALDI surface analysis allowing for facile detection of intact molecular ions, these porous silicon surfaces also facilitated the analysis of small molecules at the yoctomole level. At present laser desorption/ionization methods using other inorganic matrices such as nanomaterials are often regarded as SALDI variants. As an example, silicon nanowires as well as Titania nanotube arrays (NTA) have been used as substrates to detect small molecules. SALDI is used to detect proteins and protein-protein complexes. A related method named "ambient SALDI" - which is a combination of conventional SALDI with ambient mass spectrometry incorporating the direct analysis real time (DART) ion source has also been demonstrated. SALDI is considered one of the most important techniques in MS and has many applications.

<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.

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).

Barbara Seliger Larsen is a mass spectrometrist, with a career in instrumentations and applications of mass spectrometry in industry, and served on the board of the American Society for Mass Spectrometry for several terms.

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