PerspectivesAre you interested in submitting a Perspective Article? Be sure to read The Science Advisory Board's Editorial Guides for Perspective Articles. Click here. How two new ambient ionization methods have changed the way, with mass spectrometric analysis now being carried out, outside in the open. by Marek A. Domin  Since 2004 a number of new ‘open air’ sources where introduced that have revolutionized the way that samples are introduced into an ion source for mass spectrometric analysis. This has resulted in the rapid analysis, with no special sample preparation or treatment for a wide variety of samples not suitable or difficult to analyze prior to their introduction 1-2. The first of these new ambient ion sources were direct analysis in real time (DART) 3-4 and desorption electrospray ionization (DESI) 5. The introduction of these two ion sources has enabled one to be able to analyze spots directly from TLC plates 6-8, follow and monitor reactions in real time 9, analysis of highly insoluble polycyclic aromatic compounds 10, counterfeit drug identification 11-12, metabolic profiling of blood sera of ovarian cancer patients 13, forensic science 14-15, homeland security 16-18, to molecular imaging and depth profiling 19. Unlike DESI, analysis via DART does not involve exposure to any high voltages or to electrical discharges. Ionization during DART analysis can come about by either one of two processes, the first is direct Penning ionization, the second being proton transfer which is the most common on the two. During Penning ionization, the analyte molecules (S) are ionized by interacting with excited metastable species (M*). These excited gas species collide with the sample surface, resulting in an energy transfer to the neutral analyte molecule, which in turn results in an electron being released forming a radical cation of the analyte molecule, which is carried along in the gas stream (Nitrogen or Helium) into the mass analyzer.  Proton transfer occurs when Helium generated metastables interact with atmospheric water molecules resulting in the formation of protonated water clusters, and it’s these clusters that interact with the analyte generating protonated molecular ions.    [B]Figure 2[/B] DART Spectrum on an insoluble polycyclic aromatic compound, analyzed directly from the tip of a capillary tube.  Unlike DART, DESI uses charged electrosprayed solvent droplets and ions, which are directed on to an analyte surface to be analyzed. As these electrosprayed charged microdroplets impact the analyte surface, gaseous ions are produced results in electrospray type spectra exhibiting peaks mainly corresponding to single or multiple protonated species, but unlike DART we do see alkali-ion adducts of the analytes.  DESI has an advantage over DART in that due to the multiply charged droplets that are formed during the analysis, this technique has been applied to the analysis of both peptides and proteins something that DART is unable to do, it has also had great success in the field of biological imaging. But never the less these two pioneering ionization techniques have led to an explosions in the field of ambient ionization mass spectrometry were gases, liquids and solids are now being analyzed out the vacuum chamber, something pervious older methods have been unable to do 16-18. [B]References[/B] 1. A. Venter, M. Nefliu and R. G. Cooks, TrAC, Trends Anal. Chem., 2008, 27, 284-290 2. R. T. Covey, A. B. Thompson and B. B. Schneider, Mass Spectrom. Rev., 2009, 28, 870-897 3. R. B. Cody, J. A. Laramee and H. D. Durst, Anal. Chem., 2005, 77(8), 2297-2303 4. R. B. Cody, J. A. Laramee and H. D. Durst, JEOL News, 2005, 40(1), 8-12 5. Z. Takats, J. M. Wiseman, B. Gologan and R. G. Cooks, Science, 2004, 306, 471-473 6. G. Morlock and W. Schwack, Anal. Bioanal. Chem., 2006, 385(3), 586-595 7. G. Morlock and Y. J. Ueda, J. Chromatogr., A, 2007, 1143(1-2), 243-251 8. N. J. Smith, M. A. Domin and L. T. Scott, Org. Lett., 2008, 10(16), 3493-3496 9. C. Petucci, J. Diffendal, D. Kaufman, B. Mekonnen, G. Terefenko and B. Musselman, Anal. Chem., 2007, 79(13), 5064-5070 10. M. A. Domin, B. D. Steinberg, J. M. Quimby, N. J. Smith, A. K. Greene and L. T. Scott, Analyst., 2010, 135, 700-704 11. F. M Fernandez, R. B. Cody, M. D. Green, C. Y. Hampton, R. McGready, S. Sengaloundeth, N. J. White and P. N. Newton, ChemMedChem, 2006, 1(7), 702-705 12. F. M Fernandez, M. D. Green and P. N. Newton, Ind. Eng. Chem. Res., 2008, 47(3), 585-590 13. M. Zhou, W. Guan, L. D. Walker, R. Mezencev, B. B. Benigno, A. Gray, F. M. Fernandez and J. F. McDonald, Cancer Epidemiol. Biomark. Prev., 2010, 19, 2262-2270 14. R. W. Lones, R. B. Cody, M. D. Green, C. Y. Hampton, R. McGready, S. Sengaloundeth, N. J. White and P. N. Newton, J. Forensic Sci., 2006, 1(7), 702-705 15. C. M. Coates, S. Cotocone, P. D. Barreto, A. E. Cobb, R. B. Cody and J. C. Barreto, J. Forensic Ident., 2008, 58(6), 624-631 16. J. A. Laramee, H. D. Durst, T. R. Connell and J. M. Nilles, Am. Lab., 2008, 40(16), 16-20 17. J. A. Laramee, H. D. Durst, J. M. Nilles and T. R. Connell, Am. Lab., 2009, 41, 24-27 18. J. M. Nilles, T. R. Connell and H. D. Durst, Anal. Chem., 2009, 81, 6744-6749 19. J. C. Vickerman, Analyst, 2011, 136, 2199-2217 ### << Previous Next >> [ View All Perspectives ] |
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