Mass spectrometry prosess

Transcription

1 1 Mass spectrometry prosess Ionization = ion source Ion acceleration and separation = Analyzer Data collection = Detector 2 Data analysis? 1

2 3 Analytical information CONCEPTS : nominal mass, atomic weight, isotopic mass.etc HIGH RESOLUTION-MASS DEFECT PRECISION AND ACCURACY MASS ACCURACY MASS RESOLUTION RESOLVING POWER = 1%valley and FWHM ION CHROMATOGRAM = TIC vs RIC 4 CONCEPTS Nominal mass is defined as the integer mass of the most abundant naturally occurring stable isotope of an element. The nominal mass of an element is often equal to the integer mass of the lowest mass isotope of that element, e.g., for H, C, N, O, S, Si, P, F, Cl, Br, I. The nominal mass of an ion is the sum of the nominal masses of the elements in its empirical formula. The isotopic mass is the exact mass of an isotope. It is very close to but not equal to the nominal mass of the isotope. The relative atomic mass or the atomic weight as it is also often imprecisely termed is calculated as the weighted average of the naturally occurring isotopes of an element. 2

3 5 Analitical information : High resolution-mass DEFECT Hydrogen atom (proton plus electron) Neutron predicted mass of deuterium actual mass of deuterium The missing mass is the mass defect, and represents the energy required to bind the atomic nucleus together in other words the energy that has been realeased in the process of combination. Mass defect = difference between exact mass and integer mass of a nuclide* (*characterised by the no. of protons and neutrons in nucleus). 6 Analytical information : mass accuracy ( requires High resolution) Since the mass defect of an ion, molecule or radical is the sum of the mass defects of all atoms of its constituent nuclides, accurate mass measurement allows distinction between molecules of similar nominal mass but different elemental composition. E.g.: chemical compound formula nominal exact mass mass carbon monoxide CO molecular nitrogen N ethylene CH 2 =CH Gln C 5 H 1 N 2 O Lys C 6 H 4 N 2 O MASS ACCURACY MEASUREMENTS 3

4 7 Precision and accuracy Mass Accuracy : The difference between measure and actual mass. Either defined in mmu or ppm: ppm = 1 x 16 (measured mass theoretical mass) theoretical mass Note how ppm definition varies with m/z (z=1): At m/z 1, an error of 5ppm 5mmu At m/z 5, an error of 5ppm 2.5mmu At m/z 1, an error of 5ppm.5mmu 8 Synthetic organic chemist: Mass accurate measurements are used for confirmation of the formula or for elemental composition analysis. You can NOT PUBLISH,with a mass accuracy higher than 2ppm * (M+NH 4 ) + 4

5 9 Mass Resolution and mass resolving power Mass Resolution: If m is the smallest mass difference for which two peaks with masses m1 and m2 are resolved, : m1 m1 R m m m % Unresolved Peaks (Mass) Resolving power: The ability of a device (mass spectrometer or chromatograph) to separate closely spaced components/peaks 1 % Resolved Peaks FWHM vs 1%valley mass 25 Mass (m/z) 1 Definitions of mass resolution 1% valley Analysers with constant peak width Quadrupole Ion Trap (QIT). e.g. unit resolution : pw ( m)=1: then at m/z 1, R =1 (= 1/1) and at m/z 1, R =1 (= 1/1); FWHM =.5 if peak is triangular. 5

6 11 Definitions of mass resolving power Analysers with constant mass resolution e.g. Magnetic sector, FTICR, TOF. e.g. If R = 1: m = 1 at m/z 1 and m =.1 at m/z 1 (peak width varies) Relationship between definitions R by FWHM = half R by 1% Valley e.g. R= 2, (FWHM) is equivalent to R = 1, (1% Valley) 12 Expansion of 92 Mo (lowest m/z) isotope species at: (A) R=5, (B) R=1, (C) R=2,5 gap31 MW="1563"? EPSRC National Centre, Swansea 6/7/26 9:56:18 G.PIERCE MAT95 EI NL: 1.4E (A) C 33 H 31 IMo P 2: C 33 H 31 I1 Mo 1 P 2 p (gss, s /p:8) Chrg 1 R: NL: 1.4E5 Relative Abundance (B) C 33 H 31 IMo P 2: C 33 H 31 I1 Mo 1 P 2 p (gss, s /p:8) Chrg 1 R: NL: 1.4E (C) C 33 H 31 IMo P 2: C 33 H 31 I1 Mo 1 P 2 p (gss, s /p:8) Chrg 1 R: m/z 6

7 13 Average Mass and Resolution Mass spectrometrically unresolved peaks will show maxima at the average mass! 1 C 11 H 19 O 5 N Average mass Mono-Iso mass Unresolved Peaks % Resolved Peaks % Mass (m/z) mass 14 Mass Chromatogram: intensity vs time The total ion current (TIC) chromatogram represents the summed intensity across the entire range of masses being detected at every point in the analysis In an extracted ion chromatogram (XIC or EIC), also called a reconstructed ion chromatogram (RIC), What is BPI Chromatogram? 7

8 15 Is a resolution of R = 5 sufficient to separate C5H9.+ from CF3.+ ions? C=12.; H= ; electron mass =.548; F= First, one has to calculate the accurate masses of both ions. Then the relationship R = m / m can be used to check the C5H9+ : 5 * 12 u + 9 * u u = u CF3+: 12u + 3 * u u = u Thus we obtain R = 69 u / ( ) u = 917 i.e., R = 1 already separates these isobaric ions. 8