1 Ion Information and Elemental Patterns Chapter 6 (Hoffmann & Stroobant) Chapter 1 (Johnstone and Rose, 1996)

2 Two Main Facets of Mass Spectrometry Compared with most other common methods of physicochemical spectroscopic analysis, some or all of the samples is consumed. MS does not deal with a well-defined property of a molecule. The appearance of a mass spectrum depends on the compounds itself but also upon the interval of time between ionization and detection, initial energy distribution in the molecule, ionization method, etc…

3 MS Spectral Collections and Database NIST/EPA/NIH: >190,000 spectra (over 163,000 compounds) Wiley Registry of Mass Spectral Data: 380,000 spectra (over 200,000 compounds) Eight-Peak Index of Mass Spectra: 81,000 spectra (over 65,000 compounds)

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5 %RIC? Used when background and unwanted impurities are removed %RA is affected by the error in measuring base peak

6 Multiply Charged Ions

7 The ions in a mass spectrometer that have sufficient energy to fragment sometime after leaving the ion source but before arriving at the detector. M + A + + N (m 1 /z) (m 2 /z) (m 1 -m 2 ) 1.M + with large amount of internal energy will fragment in the ionization source, producing “normal” A + ions. These A + ions will be seen as narrow peaks at m/z values correct for the mass and charge on the ion A M +, having only a small excess of internal energy, reach detector before decomposition can occur. Narrow peaks for “normal” M + appear. 3. M + which posses excesses of internal energy that are in between the those in above two cases, may fragment after leaving the ion source and before reaching the detector. The product ions, A+, are seen in the mass spectrum as broad peaks, centered at m/z values that are nor correct for the mass and charge on the ion A +. Metastable ions

8 These broad peaks are called “metastable ion peaks” These ““metastable ion peaks” do not represent metastable M + ions, but represent products of decomposition of metastable ions. The cause of A + ions from metastable ion decomposition being detected differently form “normal” A+ ions is due to their different momenta. Metastable ions

9 Metastable Ions

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12 Just for clarification Atomic mass amu, atomic mass units (uma??) “Da” or Dalton. kD (kiloDalton) 1 amu = * kg. proton, mp = * kg, neutron, mn = * kg.

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14 Exact Masses of Some Common Elements and Their Isotopes: ElementSymbolExact Mass (u)Rel. Abundance % Hydrogen1H Deuterium 2H or D Carbon 1212C Carbon 1313C Nitrogen 1414N Nitrogen 1515N Oxygen 1616O Oxygen 1717O Oxygen 1818O Fluorine19F Sodium23Na Silicon 2828Si Silicon 2929Si Silicon 3030Si Phosphorus31P Sulfur 3232S Sulfur 3333S Sulfur 3434S Sulfur 3636S Chlorine 3535Cl Chlorine 3737Cl See Table 4A for more elements

15 Isotopic peaks Mass spectrum of methane CH 4 m/z =16 ( 12 CH 4 ) and m/z = 17 ( 13 CH 4 ) relative abundance = 99:1 For a compound with 10 carbons (C 10 H 18 ) The chance of incorporating one 13 C into the molecule will be 10 times more. Therefore [M+1] + · would be 10 x1.08 = 10.8 percent of the abundance of [M] + · (see slide 5) For a compound with 10 carbons, the approximate relative heights of the [M] + ·, [M+1] + ·, [M+2] + ·, and [M+3] + · peaks are 100:10:0.45:0.01

16 Isotopic Peaks

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19 (a) only one chlorine atom (b) only one bromine atom c) one chlorine and one bromine atom 3:1 1:1 3:4:1

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26 Molecular ions

27 Normalized Mass Spectra

28 Normalized Mass Spectra

29 Directly Recorded Mass Spectra

30 Computerized Mass Spectra

31 Accurate mass determination to several decimal places, using a double–focusing mass spectrometer, enables the determination of molecular formulae and elemental compositions High Resolution Accurate Mass Measurement: Elemental Compositions For an ion of NH 3, the measured integer mass would be 17, viz., m/z = 17/1 = 17, for z = 1. In theory, this process can be reversed in that any measured mass leads to an elemental composition. For example, a measured value of 17 would imply the composition, NH 3. In practice, there are other elemental compositions which could add up to 17. For example, OH (oxygen = 16, hydrogen = 1), CD 2 H (carbon = 12, deuterium = 2*2, hydrogen = 1).

32 High Resolution Accurate Mass Measurement: Elemental Compositions For larger masses, the possibilities increase enormously. –At mass 100, there would be literally thousands of possible elemental compositions  unit mass is not enough! In fact atomic masses are not integers! –C = –O = , –N = , –H = and so on… The accurate mass for ammonia (NH 3 ) is: x = The accurate mass of OH is: = , giving a mass difference of units between NH 3 and OH, which are potentially separable. A mass spectrometer which can measure mass correct to several decimal places rather than just integer mass can be used to measure such differences  RESOLUTION IS IMPORTANT  ELEMENTAL ANALYSIS

33 m/z measured C3H6OC3H6O Accurate Mass Determination

34 Exact masses and corresponding formulae for various possible ions of m/z 180 containing only carbon, hydrogen, nitrogen and oxygen atoms in limited number (C 6-15, H 0-24, N 0-4, and O 0-4 )

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36 What is mass resolution?

37 The accurate mass for ammonia (NH 3 ) is: x = The accurate mass of OH is: = , giving a mass difference of units between NH 3 and OH, which are potentially separable.   m = amu  m= amu for NH 3 Minimum resolution required: R = / = Lets go back to resolution: what do I need to separate them?

38 How good is good? 5000 resolution  separate m/z from m/z , m/z from m/z , m/z from m/z m/z from m/z Unit resolution  m/z 50 from m/z 51, m/z 100 from m/z 101, m/z 1000 from m/z H h (h/H)x100 ≤ 10

Clenbuterol m/z 355

40 Sensitivity vs. selectivity When resolution is not a good thing?

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