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Unformatted text preview: Mass Spectrometry Notes: Mass Spectrometry determines the mass/charge ratio of molecular or atomic IONS in amu (atomic
mass units), or Daltons (1.6 x 10'24 g). A typical Electron Impact (E.I.) ionization Quadrupole Mass Filter. lon Focus and Accelerate V " lon Detector ,l'
""" " 'il‘ Pulse Counting
Computer Electron Impact 3
Ionization (70 ev.) QuadrUP0'9 Mass Filter < ------ -- Vaccum Pumps (10'5 — 10'3 torr) Important Principles necessary to understand and interpret mass spectra: 1. Electron Impact Ionization uses a 70 ev electron beam operating at the equivalent of over
1600 kcal/mole. Usually 106 - 1010 ions are formed, most with an excess of vibrational energy. 2. Ionization Chemistry: M EJ. M ‘l‘ + 89 Electron Impact usually causes the ionization
of a single electron. Since almost all organic
Elation-Radical) molecules have an EVEN # of electrons, this E I results in the formation of a cation-radical. M M+2 + 2 ea Occasionally E.I. may result in double ionization,
but this is rare. 3. At the operating pressure of about 10'6 torr (one billionth of an atmosphere), the distance a
particle would travel before colliding is about 1 meter. Therefore there are absolutely no
collisions and no inter-molecular chemistry. All mass spec. chemistry is unimolecular and or
intra-molecular. 4. Electrons have a minus one (—1) charge but nearly zero mass (1/1830 amu, or 8.7 x 10‘28 g).
5. All observed (recorded) mass spec “peaks” are for CATIONS (+). 6. Fragmentation will occur for some or all of the original ions during the approx 10’6 sec
between ionization and mass ﬁltering This is caused by the large amount of energy the ion
contains as the result of the collision with the 70 ev electron beam. 7. Ions are formed, focused, accelerated, mass analyzed, and detected as individuals, try to
remember that Avogadro’s Number has very little bearing on ms experiments. 8. Isotopes are important! 9. The mass spec measures “mass/charge" ratios, to the nearest unit of mass. The “nominal” CH4._EL.___, 120mg: [13C1HJI E2C1H32D]: mass/charge = 16 mass/charge = 17
with the common both detected at the same m/z
isotopes of C and H with heavy isotopes of C or H mass of an atom is the nearest whole number in amu.
10. isobaric Masses. There are often several, or even many, combinations of elements that add up to the same unit mass. ? E4sz @2120”le EH2120=1201HJ m/z = 28 m/z = 28 m/z = 28 m/z = 28 11. Fragments of the original “molecular ion" are formed and are detected as part of the mass
spectrum. There are several fragmentation modes and the fragments may undergo further
reactions. The ionic fragment is detected and mass analyzed, the neutral’s structure is
deduced from the mass difference between the molecular ion (if detected) and the fragment. [F113 [Ni Cation—Radical Fragment Ion Neutral
"Parent lon" Notice that in this example, the molecular ion is an odd-electron cation-radical, while fragment ion
is an even—electron cation and the neutral is an odd-electron radical. 12. Fragmentation of the initially formed molecular ions may be extensive and multi—staged.
Allow for the extra energy these ions may have. For example, the total bond energy in ethanol is
only 760 kcal/mole, therefore it is possible that some of the ethanol molecular ions are literally
“blown into atoms” as a result of El. at > 1600 kcal/mole. [CH3CH20H] T 2c + 6 H- + :o: A small molecule may absorb enough energy on ionization to totally dissociate
into atoms. Any one of the atoms might carry the charge, so the ms may show
ions at m/z 12, m/z 1, and m/z 16. TABLE OF THE MASS AND ABUNDANCES OF THE
NATURALLY OCCURRING ISOTOPES Element Relative Abundance l°ol Exact Mass
1H 99.98 1.0078
12C 98.93 12.0000
“‘N 99.62 14.0031
16O 99.76 15.9949
19F 100.00 18.9984
31P 100.00 30.9738
328 95.02 31.9721
35CI 75.53 34.9689
37CI 24.47 36.9659
79Br 50.52 78.9183
1“Br 49.48 80.9163 127l 100.00 126.9044 MASS SPECTROMETRY Fundamental Equation: m/e = H2 r2
V m = mass e = charge H = magentic field strength r : radius of curvature
V = accel. voltage 8 ...
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This note was uploaded on 02/02/2011 for the course PHYS 206 taught by Professor Pickett during the Spring '11 term at University of Evansville.
- Spring '11