Analytical Techniques

Question 1

What is high resolution mass spectrometry used for?

Answer 1

To determine the formula of a compound from the accurate mass of the molecular ion.

• To find the relative molecular mass of a compound you look at the molecular ion (formed when molecules have electrons knocked off) peak; the one with the second highest mass/charge ratio.
• The smaller peak to the right of the peak is called the M+1 peak; caused by the prescence of carbon isotope ¹³C.
• A parent ion of mass 200 (to nearest whole number) could have the following molecular formulae: C₁₀H₁₆O₄, C₁₁H₄O₄, C₁₁H₂₀O₃.
  Adding up accurate atomic masses gives the following:

C₁₀H₁₆O₄ = 200.1049
C₁₁H₄O₄ = 200.0110
C₁₁H₂₀O₃ = 200.1413

Remember to say that each species responsible for the peaks must have a positive charge.

Question 2

How does infrared spectroscopy help us identify molecules?

Answer 2

Certain groups in a molecule absorb infrared radiation (by covalent bonds in molecules increasing vibrational energy) at characteristic frequencies, hence we can identify them by comparing the absorption to a table of values.

Question 3

What does the fingerprint region tell us?

Answer 3

Allows identification of a molecule by comparison of spectra; the area below 1500cm-1 usually has many peaks caused by complex vibrations of the whole molecule, unqiue for any particular substance.

A computer is used to match the fingerprint region of a sample with those on a database of compounds. An exact mathc confirms the identication of a sample.

Question 4

How do you use spectra to identify particular functional groups and impurities?

(limited to data presented in wavenumber form)

Answer 4

Functional groups

Look at the spectrum.

Identify where a wavenumber has been absorbed.

Match this with the data sheet.

Say “The absorbance at ??? cm^-1 indicates the presence of a X-Y bond.”

Impurities

Impurities are revealed by peaks that should not be there in the pure compound. Compare spectrums and identify rogue group.

Question 5

What is the the link between absorption of infrared radiation by bonds in CO₂, methane and water vapour and global warming?

Answer 5

• Some EM radiation from the Sun is of infrared form.
• Molecules of greenhouse gases such as CO₂, methane and H₂O are profficient at absorbing infrared energy; thus more absorption leads to global warming.