Mass Spectroscopy - Interpretation
- Mass spectroscopy is an analytical technique used to identify unknown compounds
- The molecules in the small sample are bombarded with high energy electrons which can cause the molecule to lose an electron
- This results in the formation of a positively charged molecular ion with one unpaired electron
- One of the electrons in the pair has been removed by the beam of electrons
- The molecular ion can further fragment to form new ions, molecules, and radicals
Fragmentation of a molecule in mass spectroscopy
- These fragmented ions are accelerated by an electric field
- Based on their mass (m) to charge (z) ratio, the ion fragments are then separated by deflecting them into the detector
- Most ions will only gain a charge of 1+ and therefore a ion with mass 12 and charge 1+ will have an m/z value of 12
- It is, however, possible for a greater charge to occur. For example, an ion with mass 16 and charge 2+ will have a m/z value of 8
- The smaller and more positively charged fragment ions will be detected first as they will get deflected the most and are more attracted to the negative pole of the magnet
- Each fragment corresponds to a specific peak with a particular m/z value in the mass spectrum
- The base peak is the peak corresponding to the most abundant ion
- The m/z is sometimes referred to as the m/e ratio and it is almost always 1:1
Isotopes
- Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
- These are atoms of the same elements but with different mass number
- For example, Cl-35 and Cl-37 are isotopes as they are both atoms of the same element (chlorine, Cl) but have a different mass number (35 and 37 respectively)
- Mass spectroscopy can be used to find the relative abundance of the isotopes experimentally
- The relative abundance of an isotope is the proportion of one particular isotope in a mixture of isotopes found in nature
- For example, the relative abundance of Cl-35 and Cl-37 is 75% and 25% respectively
- This means that in nature, 75% of the chlorine atoms is the Cl-35 isotope and 25% is the Cl-37 isotope
- The heights of the peaks in mass spectroscopy show the proportion of each isotope present
The peak heights show the relative abundance of the boron isotopes: boron-10 has a relative abundance of 19.9% and boron-11 has a relative abundance of 80.1%
Worked example
Calculating m/z ratio
In a sample of iron, the ions 54Fe2+ and 56Fe3+ are detected. Calculate their m/z ratio and determine which ion is deflected more inside the spectrometer.
Answer
-
- 56Fe3+ has a smaller m/z ratio and will therefore be deflected more.
- It also has the largest positive charge and will be more attracted to the negative pole of the magnet within the mass spectrometer.
Examiner Tip
A small m/z value corresponds to fragments that are either small or have a high positive charge or a combination of both.
Deducing molecular formulae
- Each peak in the mass spectrum corresponds to a certain fragment with a particular m/z value
- The peak with the highest m/z value is the molecular ion (M+) peak which gives information about the molecular mass of the compound
- The molecular ion is the entire molecule that has lost one electron when bombarded with a beam of electrons
- The [M+1] peak is a smaller peak which is due to the natural abundance of the isotope carbon-13
- The amount of naturally occurring C-13 is a little over 1%, so the [M+1] peak is very small
- The height of the [M+1] peak for a particular ion depends on how many carbon atoms are present in that molecule; the more carbon atoms, the larger the [M+1] peak is
- For example, the height of the [M+1] peak for an hexane (containing six carbon atoms) ion will be greater than the height of the [M+1] peak of an ethane (containing two carbon atoms) ion
Worked example
Analysing mass spectra
Determine whether the following mass spectrum corresponds to but-1-ene or pent-1-ene:
Answer
-
- The mass spectrum corresponds to pent-1-ene as the molecular ion peak is at m/z = 70
- The small peak at m/z = 71 is a C-13 peak, which does not count as the molecular ion peak
- But-1-ene arises from the C4H8+ ion which has a molecular mass of 56
- Pent-1-ene arises from the C5H10+ ion which has a molecular mass of 70
- The mass spectrum corresponds to pent-1-ene as the molecular ion peak is at m/z = 70