Matrix-Assisted Laser Desorption/Ionization (MALDI) is a soft ionization technique used in Mass Spectrometry to analyze larger molecules, which are either non-volatile or thermally unstable. This technique allows for identification and spatial distribution studies of biomolecules (e.g. DNA, proteins, peptides and sugars) and large synthetic organic molecules (e.g. polymers, dendrimers and other macromolecules).
MALDI is a unique technique allowing for desorption and ionization of entire molecules, unlike more conventional ionization techniques, which often lead to substantial fragmentation of the species of interest. Initially, the sample is mixed with a selected matrix, such α-cyano-4-hydroxycinnamic acid, and deposited on a target plate. This plate is bombarded by photons from a pulsed laser, resulting in the desorption and ionization of the matrix. Subsequently, the energy is transferred from the matrix to the sample molecules. This gentle energy transfer process leaves the sample molecules intact, but now in the gas phase, yielding protonated/ cationized or deprotonated/anionized molecular ions.
The ions are analyzed with a Time-of-Flight Mass Spectrometer (ToF MS). Using this technique, the mass of the ions is determined by separation of the ions in time according to their mass/charge (m/z) ratio. Scans of the whole mass range are taken in ToF MS to obtain a full mass spectrum. ToF MS enables one to determine the molecular masses of the ions with high accuracy. In most cases, obtaining the mass of a specific molecule is not sufficient for unique identification. A route to acquire this information is tandem mass spectrometry (MS/MS or ToF/ToF). First, a selected ion is isolated and subsequently fragmented. The parent ion and fragment ions are then separated in the second ToF mass spectrometer, yielding a pattern of fragments, forming a characteristic fingerprint of the molecule of interest.
Ideal Uses of MALDI
Accurate molecular weight measurements
Sample identification
Determination of the purity of a sample
Verification of amino acid substitutions
Verification of post-translational modifications
Reaction monitoring
Enzyme reactions
Chemical modification
Protein digestion
Amino acid and oligonucleotide sequencing
Confirmation of amino acid sequence
De novo characterization of peptides
Identification of proteins by database searching with a sequence “tag” from a proteolytic fragment
Characterization or quality control of oligonucleotides
Spatial distribution of compounds withinbiological tissues or non biological systems
Strengths
Rapid, sensitive, high mass accuracy, high throughput
Limitations
Soft ionization not suitable for all molecules. Mass discrimination for wide polydispersity polymers
MALDI Technical Specifications
Information obtained:
Composition, molecular weight and structural information on individual components
Qualitative and (semi-)quantitative
Sample type: Solid or liquid (dissolved in aqueous buffers or solvents such as DMF or DCM)
Mass range: 50 – 100000 g/mol
Mass accuracy:
0.1% (1000 ppm) for masses
> 4000 g/mol
0.005% (50 ppm) or better for masses
< 4000 g/mol
50×10-3g/mol in MS/MS mode
Mass sensitivity: Compound and matrix dependent
Lateral resolution (imaging): 20 x 20 μm
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