Overview of AMS Mass Spectrometry Analysis: Low vs. High Mass Resolution

Transcription

1 Overview of AMS Mass Spectrometry Analysis: Low vs. High Mass Resolution Qi Zhang Department of Environmental Toxicology University of California at Davis Aerodyne/Nanjing University Chinese AMS/ACSM Clinic Nanjing, China April -22, 18 1 Raw MS (Low Mass Resolution) 1

2 Ensemble MS Analysis Mass Conc. of Species Ensemble spectrum of gas and NR-PM materials. MS analysis Partial spectra for chemical species based on known relationships between peaks: Fragmentation pattern of inorganic & gas species Isotopic ratios of elements Ensemble ms ensemble = c air ms air + c SO4 ms SO4 + c NO3 ms NO3 + + c org ms org Summing signals in the partial mass spectrum of a species its mass concentration e.g., C org ( g m -3 ) = m/z i org Ion rate (Hz) x Ion rate (Hz) m/z NO 3 80 NH Use fragmentation table Qi Chen s talk 0x Ion rate (Hz) m/z 60 m/z 80 SO Allan et al. J. Aerosol Sci., 04 3 AMS Organic Mass Spectrum m/z 43: tracer ion for less oxidized species (e.g., carbonyls) f43: indicator for average oxidation degree m/z 44: tracer ion for highly oxygenated species (e.g., acids) f44: indicator for average oxidation degree m/z = 7: tracer ion for long chain HCs (HOA) Obtain information on individual organic species / classes MS deconvolution based on fragmentation table (works with 1 mass spectrum) This approach is not suitable for identifying organic components b/c thousands or more organic compounds in atmospheric aerosols Not able to definitively describe the relationships between peaks Can use multivariate analysis (e.g., PMF, ME2) b/c the quantitative and additive nature of AMS/ACSM data Zhang et al. ES&T, 0 2

3 A Simplified View of OA Oxidation f OOA LV-OOA SV-OOA Mexico City (flight) LV-OOA Mexico City (flight) SV-OOA Mexico City (flight) T0 Mexico City (flight) T1 Mexico City (flight) T2 Mexico City (ground T0) OOA HULIS Fulvic acid Component wtih biogenic influence O/C Ratio Ng et al., ACP, f Raw MS (High Mass Resolution) 3

4 Exact Mass and Mass Defect/Excess Mass Excess or Defect 0.1 Mass number of elements: 0.08 H = C = O = N = S = Atomic Number Due to setting 12 C = and the mass lost to binding energy, the mass of every other element is NOT a whole number. The most common isotopes of H and N both weigh more than their assigned whole number atomic mass. The most common isotopes of O and S weighs less than their assigned whole number atomic mass. 7 What Can High Mass Resolution Do for Us? Signal (Arbitrary units) Increase in resolution typically means a decrease in signal CHNO DeCarlo, Kimmel et al., Analytical Chemistry, C 2 H 3 O C CH 3 N 2 H N C 3 H

5 Mass Analyzer: Resolving Power Mass accuracy 4 Mass peak width ( Δm 0% ) Full width of mass spectral peak at half-maximum peak height Mass resolution / Resolving Power (m / Δm 0% ) Quantifies ability to isolated single mass spectral peak Mass accuracy is the ability to measure or calibrate the instrument response against a known entity. Difference between measured and actual mass 9 Exercise: Calculate R & A Argon Atomic Weight (Da): Atomic mass (m a /u) Abundance % % % R A ppm

6 Multiple Peaks at Integer m/z (isobaric ions) Are Commonly Seen in the HR-ToF-AMS Data Distinguish different classes of organics and inorganic species Resolution of 000 allows for unambiguous identification of isobaric ions at m/z < 100. Provides a powerful check for values in the frag list 6

7 HR-ToF-AMS Data Analysis Goal: apportion signal to all contributing ions, be as quantitative as possible, for as many m/z s as possible in HR-ToF-AMS mass spectra. m/z cal, peak width, peak shape are all critical for HR-MS analysis to work properly. (wiki page and Donna s tutorials) Mass Calibration Needs to be as accurate as possible Peak Width Sigma parameter from Gaussian Peak Shape Needs to be consistent Modified Gaussian HR-ToF-AMS Data Analysis Software: SQUIRREL and PIKA Donna Sueper s HR-ToF- AMS Data Analysis Tutorial Videos group/wiki/index.php/tof- AMS_Analysis_Software#Are_tutoria ls_available.3f 7

8 m/z cal, peak width, peak shape are all critical for HR- MS analysis to work properly. Extra efforts are needed to do these correctly. Peak width and shape need high signal, C x H y+ and C x H y O z+ ions are usually easier to determine well, but be extra cautious if you want N-containing ions. Don t over-fit! Sometimes you have to just accept that your data is not good enough for HR analysis in PIKA Separation of Organic Ion Classes thousands of compounds C 2 H 3 O + (m/z = 43): tracer ion for less oxygenated species (e.g., carbonyls) % of total O/C = 0.7, H/C = 1.38, N/C = 0.002, OM/OC = 1.88 CO 2 + (m/z = 44): tracer ion for highly oxygenated species (e.g., acids) f44: indicator for average oxidation degree C 2 H 4 O 2 + (m/z = 60): tracer ion for levoglucosan f60: indicator for BB influence 2 C x H y C x H y O 1 C x H y O 2 H y O 1 C x H y N p C x H y O z N p m/z (amu) UMR instrument would only resolve peak height 8

9 References Allan, J.D., Delia, A.E., Coe, H., Bower, K.N., Alfarra, M.R., Jimenez, J.L., Middlebrook, A.M., Drewnick, F., Onasch, T.B., Canagaratna, M.R., Jayne, J.T., Worsnop, D.R., 04. A generalised method for the extraction of chemically resolved mass spectra from Aerodyne aerosol mass spectrometer data. Journal of Aerosol Science 3, Ng, N.L., Canagaratna, M.R., Zhang, Q., Jimenez, J.L., Tian, J., Ulbrich, I.M., Kroll, J.H., Docherty, K.S., Chhabra, P.S., Bahreini, R., Murphy, S.M., Seinfeld, J.H., Hildebrandt, L., DeCarlo, P.F., Lanz, V.A., Prevot, A.S.H., Dinar, E., Rudich, Y., Worsnop, D.R., 10. Organic aerosol components observed in Northern Hemispheric datasets from Aerosol Mass Spectrometry. Atmos. Chem. Phys. 10, Zhang, Q., Alfarra, M.R., Worsnop, D.R., Allan, J.D., Coe, H., Canagaratna, M.R., Jimenez, J.L., 0. Deconvolution and quantification of hydrocarbon-like and oxygenated organic aerosols based on aerosol mass spectrometry. Environmental Science & Technology 39, DeCarlo, P.F., Kimmel, J.R., Trimborn, A., Jayne, J., Aiken, A.C., Gonin, M., Fuhrer, K., Horvath, T., Worsnop, D.R., Jimenez, J.L., 06. A Field-Deployable High-Resolution Time-of- Flight Aerosol Mass Spectrometer. Analytical Chemistry. 9