Matrix-assisted laser desorption electrospray ionization

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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). [1] 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. [2]

Contents

The schematic of IR-MALDESI imaging source IR-MALDESI Imaging Schematic.png
The schematic of IR-MALDESI imaging source

The IR-MALDESI source can be used for mass spectrometry imaging (MSI), a technique using MS data analyzed from a sample area to detect hundreds to thousands of biomolecules and visualize their spatial distributions. The IR-MALDESI MSI source was designed and implemented in 2013, [3] and is coupled to a high resolving power hybrid Quadrupole-Orbitrap mass spectrometer. The computer controlled motorized stage and a charge-coupled device (CCD) camera are placed in a nitrogen purged enclosure where ambient ions and relative humidity can be regulated. A water-cooled Peltier thermoelectric plate is used to control the sample stage temperature (−10 °C to 80 °C). The source has single- or multi-shot capabilities with adjustable laser fluence, repetition rate, and delay between the laser trigger and MS ion accumulation.

Over the past ten years, transformative developments in the laser technology, [4] data acquisition, motor-control software (RastirX [5] ), and imaging processing software (MSiReader [6] [7] ) have promoted IR-MALDESI as a powerful tool for the direct analysis and MSI of diverse biological, forensic, and pharmaceutical samples.

Principles of operation

In a typical MALDESI MS experiment, a thin layer of ice is deposited on the sample as the energy-absorbing medium. A mid-IR laser [4] that excites the O-H stretching mode of water is used to desorb the neutral materials from the biological samples. The plume of desorbed compounds interacts with an orthogonal electrospray plume where they partition into charged electrospray droplets and are ionized via a process similar to ESI. The ions are subsequently introduced into a mass spectrometer. [2] The ESI-like ionization mechanism has been experimentally demonstrated and studied in depth, [8] [9] [10] showing equivalent softness as ESI and enabling the detection of intact protein complexes.

The formation of ice matrix starts with purging the enclosure to below 12% relative humidity with the dry nitrogen gas source prior to cooling the Peltier stage down to −10 °C. Afterwards, the cooled sample is exposed to ambient relative humidity, causing the rapid formation of an ice layer on the sample. The ice thickness is maintained throughout the entire experiment by keeping the relative humidity of the enclosure at 10%±2% via the nitrogen purged enclosure. [2] Previously, ice was used as a matrix in IR-MALDI experiments; [11] however, the ion yields for such experiments have been very low. The electrospray post-ionization employed in IR-MALDESI helps alleviate issues associated with low ionization yield. An exogenously deposited ice matrix has been shown to improve the ion abundance by a factor of approximately 15. [12]

MALDESI is a complex interactive system, making it challenging to generate the optimum geometry in just one single experiment. The experimental settings involved in the communication between the sampling stage, mass spectrometer and laser desorption, such as stage height, ESI-Spot distance and laser repetition rate, have undergone progressive optimization by several design of experiments (DOE). [13] [9] [2] In addition to the source geometry optimization, the electrospray solvent composition has an effect on the MALDESI signals (i.e. influencing molecular coverage and ion abundance). In a study to improve the detection of tissue-specific lipids, the electrospray parameters have been tailored for positive and negative ionization polarities. Under the optimal parameters, lipid abundances increased by 3-fold with 15% greater coverage in positive polarity, while in negative mode lipid abundance achieved 1.5-fold increase with 10% more coverage. [14]

While the first generation of MALDESI system was coupled to Fourier transform (FT) ion cyclotron resonance mass spectrometer, [3] the current source is interfaced with a Thermo Exploris 240 or a Q Exactive Plus mass spectrometer equipped with an Orbitrap mass analyzer. These mass analyzers offer equally accurate mass measurements but generally shorten the data acquisition duration. MALDESI source can also be coupled to drift tube ion mobility spectrometer-mass spectrometer (IMS-MS) for high-throughput screening. [15] These integrations provide more reliable raw data for MSI as well as direct analysis of various biological specimens because long analysis time could cause physiological changes to the samples under interrogation.

MSiReader

MSiReader is a Matlab application, providing visualization and analyses of high-resolution accurate mass data collected via MALDESI. MSiReader was developed at North Carolina State University and was first released in 2013. [6] It has now become one of the most important free, open-source software options for MSI data, [7] and is compatible with most common MSI data formats (e.g., mzXML, imzML, img, ASCII). There are many essential functions available in MSiReader, namely heat map generation with high mass measurement accuracy, [7] peak normalization, [16] absolute and relative quantification, [7] [17] and polarity switching. [18] Advanced features like principal component analysis (MSiPCA), MSiCorrelation, [19] and 3D visualization [20] [21] have also been added to MSiReader. To date, MSiReader is used by over 1250 researchers and cited in more than 325 publications since 2013. It is released under the BSD 3 open-source license and can be downloaded freely from the public MSiReader website www.msireader.com. A standalone version that does not require a Matlab license is also available.

RastirX

The Rastir software, also built on a Matlab platform, was developed to enable users to visually define a rectangular region-of-interest (ROI) surrounding a tissue section for MALDESI experiments. Motion control commands and laser and instrument synchronization parameters are automatically generated by Rastir to move the sample stage under the laser beam to acquire the MS data voxel-by-voxel. The latest software version, denoted RastirX, allows users to draw arbitrary ROI on a live video image of a sample with the computer mouse. The ROI can be further modified scan-by-scan, which leads to a significantly shorter acquisition time and less contamination from off-tissue compounds than using a rectangular ROI. This arbitrary ROI tool has been applied in several projects. [5] [22]

Applications

Since its debut in 2006, MALDESI has experienced great advancements in laser technology (from UV lasers to various mid-IR wavelength lasers), [1] [4] [23] ESI settings [9] [13] [14] and associated software. [5] [6] [7] [8] [9] All the progress has made IR-MALDESI a competitive imaging and direct analysis tool to investigate a wide variety of biological samples. By associating m/z with positions where they are acquired, unique ion maps of analytes can be generated, which provides valuable information for spatial distributions of lipids, peptides, metabolomics, and other small biomolecules from mammalian samples [1] [12] [21] [23] [14] [24] to plant tissues. [25] [26] [27]

Biological

Proteins and peptides

The ESI-like process enabled MALDESI to detect multiply charged peptides and proteins, which could not be realized by MALDI because MALDI generates primarily singly- and doubly-charged ions. [1]

Lipids and metabolomics

IR-MALDESI has been successfully applied for visualizing lipid distributions on the whole-body level, namely neonatal mouse [24] and zebrafish. [28] By employing oversampling method [29] where the stage moving distance is smaller than the laser beam diameter, IR-MALDESI MSI at the cellular level was achieved at a spot-to-spot distance of ~10 micrometer. [30] Moreover, the lipidomic detectability of IR-MALDESI at the single-cell level has been demonstrated with isolated HeLa cell. [31]

In addition to soft tissues (e.g., hen ovaries [32] and rat liver tissues [10] ), IR-MALDESI is capable to detect metabolites from unmodified healthy and stroke-affected mouse bones embedded in plaster of Paris, resulting in putatively annotated 826 and 669 tissue-specific species. [22]

IR-MALDESI is an invaluable tool to study metabolism of plant and vegetable samples, as these samples are naturally rich in water. An extensive metabolomic analysis of cherry tomatoes by IR-MALDESI was recently reported. [25] In the analysis of Arabidopsis seedlings, the amounts of auxin-related compounds, was relatively quantified using stable-isotope-labelled (SIL) indole-3-acetic acid (IAA). Moreover, in this study, agarose was used as an appropriate substrate for first time used in IR-MALDESI. [27]

Neurotransmitters

IR-MALDESI has shown the ability to measure selected neurotransmitters in rat brains exposed to flame retardant tetrabromobisphenol A without any chemical derivatization. [33] The follow-up work that focused on neurotransmitters and their pathway-related metabolites in rat placenta sections reported 49 putatively identified neurotransmitters and metabolites. [34]

Glycans

Glycans are structurally complex molecules that pose many analytical challenges. The direct analysis of N-linked glycans from bovine fetuin by IR-MALDESI in both positive and negative ionization modes was presented, which indicates that IR-MALDESI can be an alternative technique for N-linked glycans profiling. [35]

Three dimensional MSI

Although most traditional 3D MSI build on reconstruction of 2D images of analytes from serial sections, the serial-section-based 3D imaging is time-consuming and can lose significant biological information. IR-MALDESI is an ablation-based technique, which is able to measure and generate 3D heat maps of biomolecules from pharmaceutical tablets [20] and nude mouse skin [21] by sequentially imaging a sample with consecutive ablation events.

Pharmaceutical

In a proof-of-concept study, HIV antiretroviral drugs in incubated cervical tissues were imaged and quantified using IR-MALDESI, and the results were evaluated by a validated LC-MS/MS method. [12] So far, the pharmaceutical application of IR-MALDESI has extended from tissue sections [12] [24] [36] to single hair strands. [17]

Forensic

Textile fibers are an important form of trace evidence for criminal investigation. One of the characteristics of fibers that can be used to distinguish the victim from the suspect is color generated by a fiber dye along with its associated impurities. [37] A direct analysis of the dye from the fabric was successfully performed using IR-MALDESI without prior separation by chromatography. [38] [39]

There are other hybrid ionization methods which combine resonant or non-resonant laser desorption with post electrospray ionization. One example is electrospray laser desorption ionization (ELDI), which uses an ultraviolet laser to form ions by irradiating the sample directly, then interacting with the electrospray plume without using any matrices. [40] The infrared version of ELDI has been referred to as laser ablation electrospray ionization (LAESI). IR-MALDESI differs from these two methods as an exogenous ice matrix is used to enhance the desorption of the sample as well as the geometric parameters of the source. Another technique, desorption atmospheric pressure photoionization (DAPPI), uses a jet of heated solvent vapor to desorb analytes from the sample surface via a nebulizer microchip. The desorbed molecules are then ionized by photons emitted by a UV lamp and introduced to mass spectrometers. [41]

Related Research Articles

<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">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.

Mass spectrometry imaging (MSI) is a technique used in mass spectrometry to visualize the spatial distribution of molecules, as biomarkers, metabolites, peptides or proteins by their molecular masses. After collecting a mass spectrum at one spot, the sample is moved to reach another region, and so on, until the entire sample is scanned. By choosing a peak in the resulting spectra that corresponds to the compound of interest, the MS data is used to map its distribution across the sample. This results in pictures of the spatially resolved distribution of a compound pixel by pixel. Each data set contains a veritable gallery of pictures because any peak in each spectrum can be spatially mapped. Despite the fact that MSI has been generally considered a qualitative method, the signal generated by this technique is proportional to the relative abundance of the analyte. Therefore, quantification is possible, when its challenges are overcome. Although widely used traditional methodologies like radiochemistry and immunohistochemistry achieve the same goal as MSI, they are limited in their abilities to analyze multiple samples at once, and can prove to be lacking if researchers do not have prior knowledge of the samples being studied. Most common ionization technologies in the field of MSI are DESI imaging, MALDI imaging, secondary ion mass spectrometry imaging and Nanoscale SIMS (NanoSIMS).

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">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">Capillary electrophoresis–mass spectrometry</span>

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.

<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 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.

<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">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.

<span class="mw-page-title-main">Laser diode thermal desorption</span>

Laser diode thermal desorption (LDTD) is an ionization technique that is coupled to mass spectrometry to analyze samples with atmospheric pressure chemical ionization (APCI). It uses a laser to thermally desorb analytes that are deposited on a stainless steel sheet sample holder, called LazWell. The coupling of LDTD and APCI is considered to be a soft-ionization technique. With LDTD-APCI, it is possible to analyze samples in forensics, pharmaceuticals, environment, food and clinical studies. LDTD is suitable for small molecules between 0 and 1200 Da and some peptides such as cyclosporine.

<span class="mw-page-title-main">Ron Heeren</span> Dutch mass spectrometry researcher

Ron M.A. Heeren is a Dutch scientist in mass spectrometry imaging. He is currently a distinguished professor at Maastricht University and the scientific director of the Multimodal Molecular Imaging Institute (M4I), where he heads the division of Imaging Mass Spectrometry.

<span class="mw-page-title-main">Julia Laskin</span> Russian–American Chemist

Julia Laskin is the William F. and Patty J. Miller Professor of Analytical Chemistry at Purdue University. Her research is focused on the fundamental understanding of ion-surface collisions, understanding of phenomena underlying chemical analysis of large molecules in complex heterogeneous environments, and the development of new instrumentation and methods in preparative and imaging mass spectrometry.

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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