Laser ablation electrospray ionization

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 is 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,[1][2][3] tissues,[4][5][6][7] cell pellets,[8] and even single cells.[9][10][11][12] In addition, LAESI has been used to analyze historic documents[13] and untreated biofluids such as urine and blood.[1] 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.

Schematic representation of laser ablation electrospray ionization (LAESI)

LAESI can be used to perform MS analysis of many different classes of compounds ranging from small molecules, such as pharmaceuticals, saccharides,[1][2][3][9][10] lipids,[5][7] and metabolites[1][2][3][4][5][6][7][8][9][10] to larger biomolecules like peptides[1] and proteins.[1] LAESI has also been shown to have a quantitative dynamic range of 4 decades and a limit of detection (LOD) of 8 fmol with verapamil, a small pharmaceutical molecule.[1] The technique has a lateral resolution of <200 μm for imaging applications[7][14] and has been used for 3D imaging of plant tissues.[3] Additionally, in cell-by-cell LAESI imaging experiments single cells can be used as the pixels of the molecular image.[12] This LAESI application uses etched optical fibers to produce laser spot sizes of <50 µm to deliver the laser energy and has also been utilized in single cell analysis experiments.[9][10][11][12]

Principle of operation

LAESI produces ions for MS analysis under normal atmospheric conditions for samples containing water.[15] The entire process can be divided into two steps.

Generation of analyte species

When a mid-IR laser beam is applied to a target which contains a hydroxide group, the target will absorb energy from this laser beam leading to evaporation of moisture from the targeted area.[16] A small-scale explosion occurs in the target and a small portion of the sample is ablated into the gas phase by a short (5 ns), mid-IR (2,940 nm) laser pulse.[17][18] The plume expands until it collapses into the sample due to the pressure exerted by the atmosphere. At this point a jet of material is ejected from the sample surface.[17][19] As mid-IR has low energy most of the ejected particles from sample remain neutral.[16][20]

Reacting analyte species with charged solvent species

An Electrospray ionization (ESI) source is located above the sample for post-ablation ionization.[21] The jet of ablated material is intersected and ionized by a spray plume from the ESI source located above the sample. The ionized molecules are then swept into the mass spectrometer for analysis. Because an ESI source is used for ionization, the LAESI mass spectra are similar to traditional ESI spectra, which can exhibit multiply charged analyte peaks, and extend the effective mass range of detection to biomolecules >100,000 Da in size.[19][20]

Applications

LAESI can be used to perform MS imaging experiments of diverse tissue samples, not only in three dimensions but also with respect to time. Similarly, LAESI can also be used for process monitoring applications because each individual analysis requires less than 2 seconds to perform. Because of the speed of a LAESI analysis, the technique is amenable to rapid, sensitive, and direct analysis of aqueous samples in 96- and 384-well microplates. These analyses can also be performed on liquid samples, such as biofluids, containing peptides, proteins, metabolites, and other biomarkers for clinical, diagnostic, and discovery workflows.[22] LAESI technology allows high throughput analysis of these sample types and the use of internal standards and calibration curves permit the absolute quantitation of targeted biomolecules.[23][22][20]

Advantages and limitations

Advantages

This technique needs very little or no sample preparation and it has high sensitivity.[22][15] This ionization technique does not need any external matrix. Therefore, the spatial resolution is not compromised by the presence of matrix crystal resulting in high spatial resolution. [20] This ionization technique can be carried out in natural and uneven biological surface.[24] Finally, as laser ablation and electronspray ionization work independently, they can be independently manipulated to achieve greater resolution.[20]

Limitations

LAESI is a relatively new technique for those samples which contain water and are relatively stable. However, it has limitations for those samples which have a lower water content. For example, this technique does not ionize dry skin, nails, tooth and bone well; this is due to low water content in these samples.[16] [22] Also, it needs a relatively large sampling area, compared to some other common ionization techniques.[20]

See also

References
  1. Nemes, P.; Vertes, A., Laser ablation electrospray ionization for atmospheric pressure, in vivo, and imaging mass spectrometry. Analytical Chemistry 2007, 79, (21), 8098-8106
  2. Nemes, P.; Barton, A. A.; Li, Y.; Vertes, A., Ambient Molecular Imaging and Depth Profiling of Live Tissue by Infrared Laser Ablation Electrospray Ionization Mass Spectrometry. Analytical Chemistry 2008, 80, (12), 4575-4582
  3. Nemes, P.; Barton, A. A.; Vertes, A., Three-Dimensional Imaging of Metabolites in Tissues under Ambient Conditions by Laser Ablation Electrospray Ionization Mass Spectrometry. Analytical Chemistry 2009, 81, (16), 6668-6675
  4. Nemes, P.; Vertes, A., Atmospheric-pressure Molecular Imaging of Biological Tissues and Biofilms by LAESI Mass Spectrometry. Journal of Visualized Experiments 2010, 43, 1-4
  5. Shrestha, B.; Nemes, P.; Nazarian, J.; Hathoutn, Y.; Hoffman, E. P.; Vertes, A., Direct analysis of lipids and small metabolites in mouse brain tissue by AP IR-MALDI and reactive LAESI mass spectrometry. Analyst 2010, 135, 751-758
  6. Sripadi, P.; Nazarian, J.; Hathout, Y.; Hoffman, E. P.; Vertes, A., In vitro analysis of metabolites from the untreated tissue of Torpedo californica electric organ by mid-infrared laser ablation electrospray ionization mass spectrometry. Metabolomics 2009, 5, (2), 263-276
  7. Nemes, P.; Woods, A. S.; Vertes, A., Simultaneous Imaging of Small Metabolites and Lipids in Rat Brain Tissues at Atmospheric Pressure by Laser Ablation Electrospray Ionization Mass Spectrometry. Analytical Chemistry 2010, 82, (3), 982-988
  8. Sripadi, P.; Shrestha, B.; Easley, R. L.; Carpio, L.; Kehn-Hall, K.; Chevalier, S.; Mahieux, R.; Kashanchi, F.; Vertes, A., Direct Detection of Diverse Metabolic Changes in Virally Transformed and Tax-Expressing Cells by Mass Spectrometry. PLoS ONE 2010, 5, (9), e12590
  9. Shrestha, B.; Vertes, A., Direct Analysis of Single Cells by Mass Spectrometry at Atmospheric Pressure. Journal of Visualized Experiments 2010, 43, 1-4
  10. Shrestha, B.; Vertes, A., In Situ Metabolic Profiling of Single Cells by Laser Ablation Electrospray Ionization Mass Spectrometry. Analytical Chemistry 2009, 81, (20), 8265-8271
  11. Shrestha, B.; Nemes, P.; Vertes, A., Ablation and analysis of small cell populations and single cells by consecutive laser pulses. Applied Physics A: Materials Science & Processing 2010, 101, 121-126
  12. Shrestha, B.; Patt, J.; Vertes, A., In Situ Cell-by-Cell Imaging and Analysis of Small Cell Populations by Mass Spectrometry. Analytical Chemistry 2011
  13. Stephens, C. H.; Shrestha, B.; Morris, H. R.; Bier, M. E.; Whitmore, P. M.; Vertes, A., Minimally invasive monitoring of cellulose degradation by desorption electrospray ionization and laser ablation electrospray ionization mass spectrometry. Analyst 2010, 135, 2434-2444
  14. Nemes, P.; Vertes, A., Laser Ablation Electrospray Ionization for Atmospheric Pressure Molecular Imaging Mass Spectrometry. In Mass Spectrometry Imaging, Rubakhin, S. S.; Sweedler, J. V., Eds. Springer Science+Business Media: 2010; pp 159-171
  15. Bartels, Benjamin; Svatoš, Aleš (2015). "Spatially resolved in vivo plant metabolomics by laser ablation-based mass spectrometry imaging (MSI) techniques: LDI-MSI and LAESI". Frontiers in Plant Science. 6: 471. doi:10.3389/fpls.2015.00471. ISSN 1664-462X. PMC 4498035. PMID 26217345.
  16. Nemes, Peter; Vertes, Akos (2007-11-01). "Laser Ablation Electrospray Ionization for Atmospheric Pressure, in Vivo, and Imaging Mass Spectrometry". Analytical Chemistry. 79 (21): 8098–8106. doi:10.1021/ac071181r. ISSN 0003-2700. PMID 17900146.
  17. Chen, Z.; Vertes, A., Early plume expansion in atmospheric pressure midinfrared laser ablation of water-rich targets. Physical Review E 2008, 77, 036316 1-9
  18. Chen, Z.; Bogaerts, A.; Vertes, A., Phase explosion in atmospheric pressure infrared laser ablation from water-rich targets. Applied Physics Letters 2006, 89, 041503 1-3
  19. Apitz, I.; Vogel, A., Material ejection in nanosecond Er:YAG laser ablation of water, liver, and skin. Applied Physics A: Materials Science & Processing 2005, 81, 329–338
  20. Huang, Min-Zong; Cheng, Sy-Chi; Cho, Yi-Tzu; Shiea, Jentaie (2011). "Ambient ionization mass spectrometry: A tutorial". Analytica Chimica Acta. 702 (1): 1–15. doi:10.1016/j.aca.2011.06.017. PMID 21819855.
  21. Vertes, A.; Nemes, P.; Shrestha, B.; Barton, A. A.; Chen, Z.; Li, Y., Molecular imaging by Mid-IR laser ablation mass spectrometry. Applied Physics A: Materials Science & Processing 2008, 93, (4), 885-891
  22. Kiss, András; Hopfgartner, Gérard (2016). "Laser-based methods for the analysis of low molecular weight compounds in biological matrices". Methods. 104: 142–153. doi:10.1016/j.ymeth.2016.04.017. PMID 27107904.
  23. Román, Jessica K.; Walsh, Callee M.; Oh, Junho; Dana, Catherine E.; Hong, Sungmin; Jo, Kyoo D.; Alleyne, Marianne; Miljkovic, Nenad; Cropek, Donald M. (2018-03-01). "Spatially resolved chemical analysis of cicada wings using laser-ablation electrospray ionization (LAESI) imaging mass spectrometry (IMS)". Analytical and Bioanalytical Chemistry. 410 (7): 1911–1921. doi:10.1007/s00216-018-0855-7. ISSN 1618-2642. PMID 29380018.
  24. Román, Jessica K.; Walsh, Callee M.; Oh, Junho; Dana, Catherine E.; Hong, Sungmin; Jo, Kyoo D.; Alleyne, Marianne; Miljkovic, Nenad; Cropek, Donald M. (2018-03-01). "Spatially resolved chemical analysis of cicada wings using laser-ablation electrospray ionization (LAESI) imaging mass spectrometry (IMS)". Analytical and Bioanalytical Chemistry. 410 (7): 1911–1921. doi:10.1007/s00216-018-0855-7. ISSN 1618-2642. PMID 29380018.
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