1 of 30© boardworks ltd 2009. 2 of 30© boardworks ltd 2007

1 of 30 © Boardworks Ltd 2009


Detecting elements and compounds

Mass spectrometry is a highly sensitive technique that can be used to help identify compounds. Samples of only a few milligrams are required.

This makes it useful in forensic science. Spectra of samples could be compared, for example, to link a suspect to a weapon that has been fired.

Mass spectrometry is useful in the identification of unknown materials, because the mass spectrum of an unknown sample can be compared with a database of known spectra.


Mass spectroscopy in space

Mass spectrometers have been included onboard several space probes.

During the descent towards the surface of Titan, the mass spectrometer analyzed the atmospheric composition at various heights. It was found that the atmosphere consists mainly of nitrogen and methane. The presence of argon-40 was also detected.

In 2005, the Huygens probe landed on the surface of Titan, the largest moon of Saturn.


Molecular ions

When a compound is analyzed in a mass spectrometer, gaseous molecules are bombarded with high-speed electrons from an electron gun.

M(g) + e- M+(g) + 2e-

The peak with the highest mass-to-charge ratio (m/z) is formed by the molecular ion, and the value of m/z is equal to the relative molecular mass of the compound.

These knock out an electron from some of the molecules, creating molecular ions, which travel to the detector plates:


Molecular ions and peaks

The peak at the highest m/z on the mass spectrum is formed by the heaviest ion that passes through the spectrometer. Unless all molecules of the original substance break up, this corresponds to the molecular ion of the sample substance.

molecular ion peakmass spectrum of paracetamol

40 80 120 160

20

40

60

80

100

00

abu

nd

ance

(%

)

m/z


High resolution mass spectroscopy

High resolution mass spectrometry can determine the m/z of a peak to several decimal places. This can distinguish between compounds that have very similar relative molecular masses (Mr) but different empirical formulae.

To integer values, hexane (C6H14) and pentanal (C5H10O) both have a Mr of 86. However, using atomic masses to four decimal places, the Mr of hexane is 86.1106, while the Mr of pentanal is 86.0739.

With a high enough resolution, it is therefore possible to distinguish between hexane and pentanal, which are both colourless liquids.


Determining molecular formulae


What is fragmentation?


Interpreting mass spectra


Mass spectroscopy


IR energy and molecular vibrations


IR energy and dipoles


The infrared spectrometer


Interpreting infrared spectra

An infrared spectrum is a plot of transmission of infrared radiation against wavenumber (1 / wavelength). Any wavelength that is absorbed by the sample will transmit less than the others, forming a dip or ‘peak’ in the graph.

0

20

40

60

80

100

3000 2000 1000

wavenumber (cm-1)

tran

smis

sio

n (

%)For instance, C–H

bonds absorb radiation at a wavenumber of 2950 cm-1, which produces a peak in an infrared spectrum as shown on the right.


IR radiation and greenhouse gases

Nitrogen and oxygen, the most abundant gases in the atmosphere, do not absorb infrared energy. This is because the vibrations caused would not change the dipole of the molecules.

These gases absorb infrared radiation in the atmosphere, stopping it escaping into space: they are greenhouse gases.

However, methane, water and carbon dioxide do absorb strongly in the infrared region.


Greenhouse gases


Using infrared spectroscopy

When breathalyzed by police, motorists blow into a handheld device, which gives an indication of the amount of alcohol in their breath. However, the result is not accurate enough to be used as evidence.

At the police station, the motorist blows into a more accurate breathalyzer, containing an IR spectrometer. The breathalyzer calculates the percentage of alcohol in the breath by looking at the size of the absorption due to the C–H bond stretch in the alcohol.


Infrared spectroscopy: summary


Identifying C–H and C–C bonds

Most organic compounds contain C–H and C–C bonds. These are therefore often visible in an infrared spectrum.

C–H

0

20

40

60

80

100

3000 2000 1000

wavenumber (cm-1)

tran

smis

sio

n (

%)

C–C

In the spectrum of an alkane, such as butane shown here, there is a peak at 2950 cm-1 (due to C–H) and a peak below 1500 cm-1 (due to C–C).

The peak due to C–H is the stronger of the two because there are many more C–H bonds than C–C bonds in the compound.


Using the fingerprint region

The region below 1500 cm-1, usually called the fingerprint region, has many peaks that are difficult to assign. The pattern of these peaks is unique to a particular compound.

A substance may be identified by comparing the IR spectrum to a database of reference spectra.

An exact match in the fingerprint region identifies a compound.

0

20

40

60

80

100

3000 2000 1000

wavenumber (cm-1)

tran

smis

sio

n (

%)

fingerprint region


Identifying chemical groups


Spotting characteristic bonds


Interpreting IR spectra


Glossary


What’s the keyword?


Multiple-choice quiz

1 of 30© boardworks ltd 2009. 2 of 30© boardworks ltd 2007

Documents

mass spectroscopy slide

mz slide

infrared spectrometer

summary slide

greenhouse gases slide

infrared spectrum

highest mass

infrared spectroscopy