510 likes | 1.59k Vues
Molecular Mass Spectrometry. Mass Spectroscopy (MS). The elemental composition of matter of samples The structure of inorganic, organic and biological molecules The quantitative and qualitative composition of complex mixture Isotopic ratios of atoms in samples. MS - Components.
E N D
Mass Spectroscopy (MS) • The elemental composition of matter of samples • The structure of inorganic, organic and biological molecules • The quantitative and qualitative composition of complex mixture • Isotopic ratios of atoms in samples
MS - Components Components of Mass Spectrometer
MS - Ionization • Gas phase ionization (103 D): • Electron Impact (EI) – Energetic electron beam • Chemical Ionization (CI) – Reagent gaseous ions • Field Ionization (FI) – high potential electrode
MS - Ionization • Desorption Ionization (105 D): • Field Desorption (FD) – High- potential Electrode • Electrospray Ionization (ESI) – High Electric Field (20 kV) • Matrix – Assisted Desorption/Ionization (MALDI) • Plasma desorption (PD) – Fission fragments from 252Cf • Thermospray ionization (TS) – High temperature
MS - Ionization • Hard source – enough energy to rupture bonds and producing fragments (EI) • Soft source – provides always molecular weight of the molecule (CI; ESI, MALDI)
MS - Spectra CH2Cl2 MW=84 1-Pentanol MW=88
MS – Isotopes Abundance Most Other Percentage (%) H1 H2 0.015 C12 C13 1.08 N14 N15 0.37 S32 S33 0.8 S34 4.4 Cl35 Cl37 32.5 Br79 Br81 98.0 Si28 Si29 5.1 Si30 3.4
MS – Effect of Ionization Mode on Spectra EI Field Desorption Field Ionization Glutamic Acid HOOC-CHNH2 -CH2 – CH2 - COOH
MS – Ionization Mode and Spectra Electron- Impact 1- Decanol MW = 158 Chemical Ionization
MS – Chemical Ionization • Most often use reagent is CH4: • Electrons will form several ions: CH4+; CH3+; CH2+ • Reactions (MH – sample molecule): CH4+ + CH4 CH5+ + CH3 CH5+ + MHMH2+ + CH4 (M + 1) CH3+ + CH4 C2H5+ + H2 C2H5+ + MH M+ + C2H6 (M – 1) (M + 29)
MS – Direct Sample Introduction External Sample Introduction System Direct Sample Probe
MS – Mass Analyzer • Resolution: R = m/Dm Where: m – mass of the first peak Dm – difference between two adjacent peaks Commercial MS have resolution 500 to 500,000 Dm = m/R
MS – Fourier Transform Analyzer Ion Cyclotron Resonance Magnetic Field
Mass Spectrum MW 240 5-Ethyl-5n-hexyl barbituric acid
MS - Quantitative • SIM – single ion monitoring • Spectra mode • Precision – 2% to 10%
MS – Spectra Interpretation • Base peaks and Relative Ion Intensities: • Determine molecular ion mass. CI if needed. • Elemental composition from isotopic abundance: • Look for A+2 pattern elements (Cl, Br, S, Si, O) • Check A+1 ratios for absence/presence of S and Si • Use the nitrogen rule to determine number of N’s (If MW is even N=0 or even number. If MW is odd = odd number of nitrogen atoms • Estimate number of H, F, I, and P from isotopic ratios and MW balance (P is multivalent; F=19; I=127 mass units) • Check allowance for rings and double bonds. Number of double bonds or rings = x – 1/2y +1/2 z +1 (x=C, Si; y=H, F, Br, Cl; z=N, P, O, S)
MS – Spectra Interpretation • Use molecular ion fragmentation mechanism: • Check fragment masses differences for expected losses (Cl = 35; Br=79; Me=15; Et=29 etc…) • Look for expected substructures • Look for stable neutral loss (CH2 == CHR) • Look for products of known rearrangements • Postulate structures: • Search library data base • Run hit compound on the same instrument to confirm • Use MS/MS if further confirmation is needed