[17.1] mass spectrometry

Question 1

what can mass spectra be used for?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

how do you find the Mr?

Answer 3

• 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

Question 4

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

Answer 4

• 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

Question 5

how do you calculate the number of carbon atoms?

Answer 5

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

Question 6

what is fragmentation?

Answer 6

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

Question 7

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

Answer 7

• 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

Question 8

what does the simplest fragmentation break a molecular ion into?

Answer 8

two species:

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

Question 9

how can the identity of the peaks be calculated?

Answer 9

• 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

Question 10

what can mass spectrometry be used for?

Answer 10

• 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

Question 11

what is the base peak?

Answer 11

• 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

Question 12

what is the meaning of relative intensity?

Answer 12

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

Question 13

what is the meaning of m/z?

Answer 13

• 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