IR spectroscopy and mass spectrometry

How do you calculate the mass-to-charge ratio?

(relative mass of ion) / (relative charge of ion)

Where is the molecular ion (M⁺) peak on a mass spectrum?

The clear peak at the highest m/z value on the right hand side of the mass spectrum

Where is the M+1 peak on the mass spectrum?

It is the small peak to the right of the M⁺ peak

Why does the M+1 peak exist?

• Exists due to the small amount of the carbon-13 isotope

  • If the compound contains chlorine, there will be a peak 2 and 4 m/z values after the M⁺ peak, due to the existence of the chlorine-37 isotope

Fragmentation

The process in mass spectrometry that causes a positive ion to split into smaller pieces, one of which is a positive fragment ion.

Fragment ion

Ions formed from the breakdown of the molecular ion in the mass spectrometer

During fragmentation, what does the positive ion split into?

• A positively charged fragment ion (detected by mass spectrometer, so causes other peaks on a mass spectrum.

  • A radical

How do you calculate the number of carbon atoms, if you know the heights of the M⁺ and M+1 peaks?

[(height of M+1 peak) / (height of the M⁺ peak)] x 100

What are the 2 types of vibration?

• Stretch - movement along the line between the atoms, In which the distance between the 2 atomic centres increases or decreases

• Bend - results in a change in bond angle

What does the amount that a bond vibrates depend on?

• mass of atoms in the bond (heavier atoms vibrate more slowly)

• Strength of bond (stronger bonds vibrate faster)

What frequency of radiation can bonds absorb?

Bonds can only absorb radiation that has the same frequency as the natural frequency of the bond

What type of radiation do greenhouse gasses absorb and how do they lead to global warming?

Greenhouse gasses absorb longer-wavelength IR radiation (as it has the same frequency as the natural frequency of their bonds). Eventually, the bonds will re-emit this energy as radiation, this increases the temperature of the atmosphere, leading to global warming.

What are the steps involved in Infrared spectroscopy?

1. A sample is placed inside the IR spectrometer

2. A beam of IR radiation with the range 200 - 4000 cm^-1 is passed through the sample

3. Molecule absorbs some IR frequencies, and the emerging beam of radiation is analysed to identify the frequencies that have been absorbed (by the sample)

4. IR spectrometer is usually connected to a computer which plots a graph of transmittance against wavelength

What are some applications of infrared spectroscopy?

• IR bases breathalysers, pass a beam of IR radiation through captured breath, to measure and detect the amount of ethanol

• Sensors can analyse IR spectra of vehicle emissions to detect and measure levels of carbon monoxide, carbon dioxide and hydrocarbons, in town centres or motorways

Fingerprint region

Below 1500cm^-1 on IR spectrum

Fragment ion with m/z value of 15

CH₃⁺

Fragment ion with m/z value of 29

C₂H₅⁺

Fragment ion with m/z value of 43

C₃H₇⁺

Fragment ion with m/z value of 57

C₄H₉⁺