[17.1] mass spectrometry Flashcards

1
Q

what can mass spectra be used for?

A

to identify the molecular mass of an organic compound and to gain further information about its structure

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2
Q

what happens when an organic compound is placed in a mass spectrometer?

A
  • it loses an electron and forms a positive ion
  • molecular ion formed: M —> [M]⁺ + e⁻
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3
Q

how do you find the Mr?

A
  • locate the molecular ion peak (M⁺ peak)
  • the M⁺ peak is the clear peak at the highest m/z value on the right hand side of the mass spectrum
  • m/z value for [M]⁺ gives the Mr
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4
Q

why might a very small peak be seen after the M⁺ peak?

A
  • an M + 1 peak exists because 1.1% of carbon is presented as the carbon-13 isotope
  • M + 1 peak is one unit after the M⁺ peak
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5
Q

how do you calculate the number of carbon atoms?

A

number of carbon atoms = (height of M + 1 peak / height of M peak) × 100

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6
Q

what is fragmentation?

A

in the mass spectrometer, some molecular ions break down into smaller pieces known as fragments

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7
Q

what are peaks other than the one furthest to the right caused by?

A
  • fragment ions, formed from the breakdown of the molecular ion
  • this is due to the covalent bond breaking
  • [M] —> X⁺ + Y●
  • the more stable the ion, the greater the peak intensity
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8
Q

what does the simplest fragmentation break a molecular ion into?

A

two species:

  • a positively charged fragment ion (detected)
  • a radical (not detected)
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9
Q

how can the identity of the peaks be calculated?

A
  • by doing basic maths
  • eg. if the m/z value is 15, it is caused by a CH₃⁺ fragment
  • calculate which ion causes each peak at m/z = XYZ
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10
Q

what can mass spectrometry be used for?

A
  • fragmentation can be used to identify the structure of organic compounds because the mass spectrum of each compound is unique, so molecules will fragment slightly differently depending on their structures
    > ie. useful for identifying isomers
  • drug testing
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11
Q

what is the base peak?

A
  • the tallest peak
  • it’s relative intensity is set at 100%
  • all other relative intensities are expressed relative to the base peak
  • the base peak is usually the most stable ion formed
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12
Q

what is the meaning of relative intensity?

A

the height of the peaks in the mass spectrum shows the abundance of the ions of each mass

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13
Q

what is the meaning of m/z?

A
  • m/z means mass ÷ charge
  • however, most ions have a +1 charge so in practice we assume that m/z is the same as mass
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