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Our work with aerosol mass spectrometry has focused on generating ions from large biological molecules contained in small particles. The Murray Group is working to develop methods for bioaerosol analysis using both on-line and off-line approaches. We are also collaborating with the small business Ionwerks in Houston, Texas, to develop an ion mobility MALDI mass spectrometer for biological agent detection. This research will pave the way for portable instruments that can be used in homeland security and public health applications.

Research

MALDI TOF-MS of Bioaerosols

The goal of the bioaerosol project is to develop a instrumentation for rapid analysis of bioaerosols using MALDI TOF mass spectrometry. Our current focus is on near real-time analysis of collected particles by MALDI and matrix-free IR-LDI. We have also explored on-line single aerosol particle MALDI in which the matrix is added to the particles by condensation (Jackson et al., 2004).

MALDI mass spectra of test bioaerosols deposited on a target pre-coated with a) sinapic acid matrix and no additional treatment, b) CHCA plus 200 Ã‚ÂµL of solvent, and c) SA plus 200 Ã‚ÂµL of 1:1 (v/v) acetonitrile/water solvent. Ion suppression was used below m/z 3000, resulting in some baseline discontinuity.

Mass spectra obtained by depositing bioaerosols collected on matrix-coated targets in an Andersen aerosol impactor are shown in the figure above. The matrix-coated targets were prepared by depositing 400 Ã‚ÂµL of matrix solution on the target and allowing it to dry before the target was placed in the impactor. A target was removed from the impactor after bioaerosol deposition and inserted directly into the mass spectrometer for analysis with further treatment; the mass spectrum (a) above was obtained as a result. The jump in signal at m/z 3000 reflects the ion suppression pulse and resulting shift in baseline. No signal was observed above 3000, and only unresolved low m/z signal was recorded. However, signal could be recovered if solvent was added to the target after the bioaerosol was collected on the matrix pre-coated target; (b) and (c) show the result of adding 200 Ã‚ÂµL of a 1:1 (v/v) mixture of acetonitrile and water on the spot of collected particulate deposited on ÃŽÂ±-cyano-4-hydroxy-cinnamic acid (CHCA) and sinapic acid (SA) matrix-coated targets, respectively. The SA target mass spectrum in (c) is similar in terms of number of peaks, S/N and mass resolution (greater than 800) compared with conventional dried droplet mass spectrum. The CHCA target mass spectrum in (b) is somewhat lower in quality with fewer peaks with worse mass resolution (approximately 200) at one-third higher laser fluence compared to a dried droplet mass spectrum.

We are also using a QSTAR hybrid quadrupole TOF mass spectrometer for tandem mass spectrometry (MS/MS) analysis of bacteria and collected bioaerosols. The MS/MS approach will enable the identification of biomolecules in the bioaerosols that can potentially be used to identify the particles and their constituents.

Ion Mobility MALDI Mass Spectrometry

The principal challenge to MS development for portable and transportable instrumentation is in making the instrument small while maintaining requisite speed, sensitivity, selectivity and specificity. One approach to the problem is coupling ion mobility (IM) separation to a miniature time-of-flight mass spectrometer (TOF-MS). IM-TOF-MS can be used to separate and identify peptides, proteins, lipids and small organic and inorganic molecules that are found in bioaerosols. IM-MS allows fast two-dimensional separation of ions according to their shape and mass and thus has a great potential for the complex mixture analysis. I have recently started a collaboration with the Ionwerks in Houston, Texas, that is aimed at developing such an instrument.

In an IM-TOF-MS, ions are created by a UV laser and ejected into an ion mobility drift tube held at a pressure of a few Torr helium gas. Both a 337 nm nitrogen laser and a 1 kHz repetition rate Nd:YLF laser at 350 nm have been employed. Ion mobility resolution up to 20 has been achieved with applied voltage on the drift tube of 700 V. After IM separation, the ions are orthogonally accelerated into a 20 cm reflectron TOF mass spectrometer. The mobility drift times are typically several milliseconds while the flight times within the mass spectrometer are typically less than one hundred microseconds. The mass spectrometer has a mass resolution of up to 2000 and a sensitivity of 5 fmoles for peptide standards.

Ion mobility MALDI mass spectrum of B. Subtilis. The UV MALDI mass spectrum is shown at bottom.

Ion mobility spectra and MALDI mass spectra for B. Subtilis obtained from a dried droplet samples is shown in the figure above. For comparison with IM-MALDI, the bacteria samples were analyzed with a Bruker Omniflex reflectron time-of-flight mass spectrometer and a 337 nm laser. The IM-MALDI data for both bacteria fall onto three distinct trend lines with a roughly constant ratio of mass to drift time. This behavior suggests that three different classes of compounds are desorbed and ionized from the bacteria. The ions in the region above 5000 m/z have a faster drift time and fall on the trend line with the lowest slope. The ions giving rise to the fast drift trend line are most likely peptides and proteins. The trend line of intermediate slope corresponds to ions with approximately 60% lower drift velocity than the peptide/protein trend line and with masses below 5000 Da. The ions giving rise to the intermediate slope line are most likely lipo-polysaccharides or glycans, which are known to be components of bacterial cell wall surfaces. A third trend line with weak signal can be seen above the intermediate slope trend line. Ions contributing to this trend line have m/z below 5000 and nearly double the drift time of peptides of the same mass, indicating an anomalously large collisional cross section for their mass. The exact nature of these low mobility ions is not clear at this time.

References

S. N. Jackson, S. Mishra, and K. K. Murray, Ã¢â‚¬Å“On-line laser desorption/ionization mass spectrometry of matrix-coated aerosols.Ã¯Â¿Â½? Rapid Commun Mass Spectrom 18, 2041 (2004).

Publications

Kim, J. K.; Jackson, S. N.; Murray, K. K., Matrix-assisted laser desorption/ionization mass spectrometry of collected bioaerosol particles. Rapid Commun Mass Spectrom 2005, 19, (12), 1725-9.

Jackson, S. N.; Mishra, S.; Murray, K. K., On-line laser desorption/ionization mass spectrometry of matrix-coated aerosols Rapid Commun Mass Spectrom 2004, 18, (18), 2041.

Jackson, S. N.; Murray, K. K., Matrix addition by condensation for matrix-assisted laser desorption/ionization of collected aerosol particles. Anal Chem 2002, 74, 4841.

He, L.; Murray, K. K., :337 nm matrix-assisted laser desorption/ionization of single aerosol particles. J. Mass Spectrom. 1999, 34, 909.

Murray, K. K., Coupling matrix-assisted laser desorption/ionization to liquid separations. Mass Spectrom. Rev. 1997, 16, 283.

He, L.; Wei, G.; Murray, K. K., Matrix Dependent Fragmentation of Vitamin B₁₂ by Aerosol Matrix-Assisted Laser Desorption Ionization. J Am Soc Mass Spectrom 1997, 8, 140.

He, L.; Murray, K. K., A Laminar Flow Nebulizer for Aerosol MALDI. Anal Chem 1997, 69, 3613.

Fei, X.; Wei, G.; Murray, K. K., Aerosol MALDI with a Reflectron Time-of-Flight Mass Spectrometer. Anal Chem 1996, 68, 1143.

Fei, X.; Murray, K. K., On-Line Coupling of Gel Permeation Chromatography with MALDI Mass Spectrometry. Analytical Chemistry 1996, 68, 3555.

Beeson, M. D.; Murray, K. K.; Russell, D. H., Aerosol Matrix-Assisted Laser Desorption Ionization: The Effects of Analyte Concentration and Matrix to Analyte Ratio. Analytical Chemistry 1995, 67, 1981-1986.

Murray, K. K.; Russell, D. H., Matrix-Assisted Laser Desorption Ionization of Aerosols: The Ionization Mechanism. In Laser Ablation: Mechanisms and Applications - II, Miller, J. C.; Geohegan, D. B., Eds. American Institute of Physics: New York, NY, 1994; pp 459-464.

Murray, K. K.; Russell, D. H., Laser spray ionization for biological mass spectrometry. American Laboratory 1994, 26, 38-44.

Murray, K. K.; Russell, D. H., Aerosol Matrix-Assisted Laser Desorption Ionization Mass Spectrometry. Journal of the American Society for Mass Spectrometry 1994, 5, 1.

Murray, K. K.; Lewis, T. M.; Beeson, M. D.; Russell, D. H., Aerosol Matrix-Assisted Laser Desorption Ionization for Liquid Chromatography/Time-of-Flight Mass Spectrometry. Analytical Chemistry 1994, 66, 1601.

Murray, K. K.; Russell, D. H., Liquid Sample Introduction for Matrix-Assisted Laser Desorption Ionization. Analytical Chemistry 1993, 65, 2534.