Matrix-assisted laser desorption/ionization

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File:MALDITOF.jpg
MALDI TOF mass spectrometer

Matrix-assisted laser desorption/ionization (MALDI) is a soft ionization technique used in mass spectrometry, allowing the analysis of biomolecules (biopolymers such as proteins, peptides and sugars) and large organic molecules (such as polymers, dendrimers and other macromolecules), which tend to be fragile and fragment when ionized by more conventional ionization methods. It is most similar in character to electrospray ionization both in relative softness and the ions produced (although it causes much fewer multiply charged ions).

The ionization is triggered by a laser beam (normally a nitrogen laser). A matrix is used to protect the biomolecule from being destroyed by direct laser beam and to facilitate vaporization and ionization.

Matrix

UV MALDI Matrix List
Compound Other Names Solvent Wavelength (nm) Applications
2,5-dihydroxy benzoic acid[1] DHB, Gentisic acid acetonitrile, water, methanol, acetone, chloroform 337, 355, 266 peptides, nucleotides, oligonucleotides, oligosaccharides
3,5-dimethoxy-4-hydroxycinnamic acid[2][3] sinapic acid; sinapinic acid; SA acetonitrile, water, acetone, chloroform 337, 355, 266 peptides, proteins, lipids
4-hydroxy-3-methoxycinnamic acid[2][3] ferulic acid acetonitrile, water, propanol 337, 355, 266 proteins
α-cyano-4-hydroxycinnamic acid[4] CHCA acetonitrile, water, ethanol, acetone 337, 355 peptides, lipids, nucleotides
Picolinic acid[5] PA Ethanol 266 oligonucleotides
3-hydroxy picolinic acid[6] HPA Ethanol 337, 355 oligonucleotides


The matrix consists of crystallized molecules, of which the three most commonly used are 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid), α-cyano-4-hydroxycinnamic acid (alpha-cyano or alpha-matrix) and 2,5-dihydroxybenzoic acid (DHB). A solution of one of these molecules is made, often in a mixture of highly purified water and an organic solvent (normally acetonitrile (ACN) or ethanol). Trifluoroacetic acid (TFA) may also be added. A good example of a matrix-solution would be 20 mg/mL sinapinic acid in ACN:water:TFA (50:50:0.1).

File:Maldi cartoon.gif
Matrix-assisted laser desorption ionization depicted with matrix in blue and analyte in red

The identity of suitable matrix compounds is determined to some extent by trial and error, but they are based on some specific molecular design considerations:

  • They are of a fairly low molecular weight (to allow facile vaporization), but are large enough (with a high enough vapor pressure) not to evaporate during sample preparation or while standing in the spectrometer.
  • They are acidic, therefore act as a proton source to encourage ionization of the analyte.
  • They have a strong optical absorption in the UV, so that they rapidly and efficiently absorb the laser irradiation.
  • They are functionalized with polar groups, allowing their use in aqueous solutions.

The matrix solution is mixed with the analyte (e.g. protein-sample). The organic solvent allows hydrophobic molecules to dissolve into the solution, while the water allows for water-soluble (hydrophilic) molecules to do the same. This solution is spotted onto a MALDI plate (usually a metal plate designed for this purpose). The solvents vaporize, leaving only the recrystallized matrix, but now with analyte molecules spread throughout the crystals. The matrix and the analyte are said to be co-crystallized in a MALDI spot.

Laser

Lasers Used for MALDI
Laser Wavelength (nm) Reference
Nitrogen laser 337 (Tanaka 1988)[7]
Nd:YAG 355, 266 (Karas 1985)[8]
Er:YAG 2940 (Overberg 1990)[9]
CO2 10,600 (Overberg 1991)[10]

The laser is fired at the crystals in the MALDI spot. The spot absorbs the laser energy and it is thought that primarily the matrix is ionized by this event. The matrix is then thought to transfer part of its charge to the analyte molecules (e.g. protein), thus ionizing them while still protecting them from the disruptive energy of the laser. Ions observed after this process are quasimolecular ions that are ionized by the addition of a proton to [M+H]+, or other cation such as sodium ion [M+Na]+, or the removal of a proton [M-H]- for example. MALDI generally produces singly-charged ions, but multiply charged ions ([M+nH]n+) can also be observed, usually as a function of the matrix, the laser intensity and/or the voltage used. Note that these are all even-electron species. Ion signals of radical cations can be observed eg. in case of matrix molecules and other stable molecules.

AP-MALDI

Atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) is an ionization technique (ion source) that in contrast to vacuum MALDI operates at normal atmospheric environment.[11] The main difference between vacuum MALDI and AP-MALDI is the pressure in which the ions are created. In vacuum MALDI, ions are typically produced at 10 mTorr or less while in AP-MALDI ions are formed in atmospheric pressure. Disadvantage of the AP MALDI source is the limited sensitivity observed and the limited mass range.

AP-MALDI is used in mass spectrometry (MS) in a variety of applications ranging from proteomics to drug discovery fields. Popular topics that are addressed by AP-MALDI mass spectrometry include: proteomics, DNA/RNA/PNA, lipids, oligosaccharides, phosphopeptides, bacteria, small molecules and synthetic polymers, similar applications as available also for vacuum MALDI instruments.

The AP-MALDI ion source is easily coupled to an ion trap mass spectrometer[12] or any other MS system equipped with ESI (electrospray ionization) or nanoESI source.

Mass spectrometer

File:MALDI Target.jpg
Sample target for a MALDI mass spectrometer

The type of a mass spectrometer most widely used with MALDI is the TOF (time-of-flight mass spectrometer), mainly due to its large mass range. The TOF measurement procedure is also ideally suited to the MALDI ionization process since the pulsed laser takes individual 'shots' rather than working in continuous operation. MALDI-TOF instruments are typically equipped with an "ion mirror", deflecting ions with an electric field, thereby doubling the ion flight path and increasing the resolution. Commercial reflectron TOF instruments reach today a resolving power m/Δm of well above 20'000 FWHM (full-width half-maximum, Δm defined as the peak width at 50% of peak height).

History

The term matrix-assisted laser desorption ionization (MALDI) was coined in 1985 by Franz Hillenkamp, Michael Karas and their colleagues.[13] These researchers found that the amino acid alanine could be ionized more easily if it was mixed with the amino acid tryptophan and irradiated with a pulsed 266 nm laser. The tryptophan was absorbing the laser energy and helping to ionize the non-absorbing alanine. Peptides up to the 2843 Da peptide melittin could be ionized when mixed with this kind of “matrix”.[14] The breakthrough for large molecule laser desorption ionization came in 1987 when Koichi Tanaka of Shimadzu Corp. and his co-workers used what they called the “ultra fine metal plus liquid matrix method” that combined 30 nm cobalt particles in glycerol with a 337 nm nitrogen laser for ionization.[15] Using this laser and matrix combination, Tanaka was able to ionize biomolecules as large as the 34,472 Da protein carboxypeptidase-A. Tanaka received one-quarter of the 2002 Nobel Prize in Chemistry for demonstrating that, with the proper combination of laser wavelength and matrix, a protein can be ionized.[16] Karas and Hillenkamp were subsequently able to ionize the 67 kDa protein albumin using a nicotinic acid matrix and a 266 nm laser.[17] Further improvements were realized through the use of a 355 nm laser and the cinnamic acid derivatives ferulic acid, caffeic acid and sinapinic acid as the matrix.[18] The availability of small and relatively inexpensive nitrogen lasers operating at 337 nm wavelength and the first commercial instruments introduced in the early 1990s brought MALDI to an increasing number of researchers.[19] Today, mostly organic matrices are used for MALDI mass spectrometry.

Use

In Biochemistry

In proteomics, MALDI is used for the identification of proteins isolated through gel electrophoresis: SDS-PAGE and two-dimensional gel electrophoresis. One method used is peptide mass fingerprinting by MALDI-MS, or with post ionisation decay or collision-induced dissociation (further use see mass spectrometry).

In Organic Chemistry

Some synthetic macromolecules, such as catenanes and rotaxanes, dendrimers and hyperbranched polymers, and other assemblies, have molecular weights extending into the thousands or tens of thousands, where most ionization techniques have difficulty producing molecular ions. MALDI is a simple and rapid analytical method that can allow chemists to analyze the results of such syntheses and verify their results.

Reproducibility and performance

The sample preparation for MALDI is important for the result. Inorganic salts which are also part of protein extracts interfere with the ionization process. The salts are removed by solid phase extraction or washing the final target spots with water. Both methods can also remove other substances from the sample. The matrix protein mixture is not homogenous because the polarity difference leads to a separation of the two substances during crystallization. The spot diameter of the target is much larger than that of the laser, which makes it necessary to do several laser shots at different places of the target, to get the statistical average of the substance concentration within the target spot. The matrix composition, the addition of trifluoroacetic acid and formic acid, delay between laser pulses, delay time of the acceleration power, laser wavelength, energy density of the laser and the impact angle of the laser on the target are among others the critical values for the quality and reproducibility of the method.

See also

References

  1. Strupat K, Karas M, Hillenkamp F (1991). "2,5-Dihidroxybenzoic acid: a new matrix for laser desorption-ionization mass spectrometry". Int. J. Mass Spectrom. Ion Processes. 72 (111): 89–102.
  2. 2.0 2.1 Beavis RC, Chait BT (1989). "Matrix-assisted laser-desorption mass spectrometry using 355 nm radiation". Rapid Commun. Mass Spectrom. 3 (12): 436–9. PMID 2520224.
  3. 3.0 3.1 Beavis RC, Chait BT (1989). "Cinnamic acid derivatives as matrices for ultraviolet laser desorption mass spectrometry of proteins". Rapid Commun. Mass Spectrom. 3 (12): 432–5. PMID 2520223.
  4. "-α-Cyano-4-hydroxycinnamic acid as a matrix for matrix-assisted laser desorption mass spectrometry". Org. Mass Spectrom. 27: 156–8. 1992. Text "Beavis RC, Chaudhary T, Chait BT " ignored (help)
  5. Tang K, Taranenko NI, Allman SL, Cháng LY, Chen CH (1994). "Detection of 500-nucleotide DNA by laser desorption mass spectrometry". Rapid Commun. Mass Spectrom. 8 (9): 727–30. PMID 7949335.
  6. Wu KJ, Steding A, Becker CH (1993). "Matrix-assisted laser desorption time-of-flight mass spectrometry of oligonucleotides using 3-hydroxypicolinic acid as an ultraviolet-sensitive matrix". Rapid Commun. Mass Spectrom. 7 (2): 142–6. PMID 8457722.
  7. Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T., Protein and Polymer Analyses up to m/z 100 000 by Laser Ionization Time-of flight Mass Spectrometry. Rapid Commun Mass Spectrom 1988, 2, 151-153.
  8. Karas, M.; Bachmann, D.; Hillenkamp, F., Influence of the Wavelength in High-Irradiance Ultraviolet Laser Desorption Mass Spectrometry of Organic Molecules. Anal. Chem. 1985, 57, 2935-2939.
  9. Overberg, A.; Karas, M.; Bahr, U.; Kaufmann, R.; Hillenkamp, F., Matrix-assisted Infrared-laser (2.94 μm) Desorption/Ionization Mass Spectrometry of Large Biomolecules. Rapid Commun. Mass Spectrom. 1990, 4, 293-296.
  10. Overberg, A.; Karas, M.; Hillenkamp, F., Matrix-assisted Laser Desorption of Large Biomolecules with a TEA-CO2-Laser. Rapid Commun. Mass Spectrom. 1991, 5, 128-131.
  11. Laiko VV, Baldwin MA, Burlingame AL (2000). "Atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry". Anal. Chem. 72 (4): 652–7. PMID 10701247.
  12. Laiko VV, Moyer SC, Cotter RJ (2000). "Atmospheric pressure MALDI/ion trap mass spectrometry". Anal. Chem. 72 (21): 5239–43. PMID 11080870.
  13. Karas, M.; Bachmann, D.; Hillenkamp, F. (1985). "Influence of the Wavelength in High-Irradiance Ultraviolet Laser Desorption Mass Spectrometry of Organic Molecules". Anal. Chem. 57: 2935–9.
  14. Karas, M.; Bachman, D.; Bahr, U.; Hillenkamp, F. (1987). "Matrix-Assisted Ultraviolet Laser Desorption of Non-Volatile Compounds". Int J Mass Spectrom Ion Proc. 78: 53–68.
  15. Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshida, T. (1988). "Protein and Polymer Analyses up to m/z 100 000 by Laser Ionization Time-of flight Mass Spectrometry". Rapid Commun Mass Spectrom. 2 (20): 151–3.
  16. Markides, K. "Advanced information on the Nobel Prize in Chemistry 2002" (PDF). Unknown parameter |coauthors= ignored (help)
  17. Karas M, Hillenkamp F (1988). "Laser desorption ionization of proteins with molecular masses exceeding 10,000 daltons". Anal. Chem. 60 (20): 2299–301. PMID 3239801.
  18. Beavis RC, Chait BT (1989). "Matrix-assisted laser-desorption mass spectrometry using 355 nm radiation". Rapid Commun. Mass Spectrom. 3 (12): 436–9. PMID 2520224.
  19. Karas, M.; Bahr, U. (1990). "Laser Desorption Ionization Mass Spectrometry of Large Biomolecules". Trends Anal. Chem. 9: 321–5.

Bibliography

  • Hillenkamp F, Karas M, Beavis RC, Chait BT (1991). "Matrix-assisted laser desorption/ionization mass spectrometry of biopolymers". Anal. Chem. 63 (24): 1193A–1203A. PMID 1789447.
  • Ragoussis J, Elvidge GP, Kaur K, Colella S (2006). "Matrix-assisted laser desorption/ionisation, time-of-flight mass spectrometry in genomics research". PLoS Genet. 2 (7): e100. doi:10.1371/journal.pgen.0020100. PMID 16895448.
  • Hardouin J (2007). "Protein sequence information by matrix-assisted laser desorption/ionization in-source decay mass spectrometry". Mass spectrometry reviews. 26 (5): 672–82. doi:10.1002/mas.20142. PMID 17492750.
  • Jasna Peter-Katalinic; Franz Hillenkamp. MALDI MS: A Practical Guide to Instrumentation, Methods and Applications. Weinheim: Wiley-VCH. ISBN 3-527-31440-7.
  • W. Schrepp; Harald Pasch. Maldi-Tof Mass Spectrometry of Synthetic Polymers (Springer Laboratory). Berlin: Springer-Verlag. ISBN 3-540-44259-6.

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