What should I do if the molecular ion peak in the mass spectrum does not appear or the abundance is extremely low?
Molecular ion is the most valuable information in the mass spectrum. It is not only the basis for determining the molecular weight of the compound, but also the molecular formula of the compound can be inferred. High-resolution mass spectrometry can be used to directly determine the molecular formula of the compound.
Generally speaking, the electrons lost from the molecule should be the weakest bound electrons in the molecule, such as π electrons in double or triple bonds, and non-bonding electrons on heteroatoms.
The abundance of a molecular ion mainly depends on its stability and the energy required for molecule ionization. Compounds that easily lose electrons, such as cyclic compounds and double-bonded compounds, have stable molecular ions and strong molecular ion peaks; while long carbon chain alkanes, branched chain alkanes, etc. are just the opposite.
The order of molecular ion stability of various compounds is roughly as follows: aromatic ring (including aromatic heterocycle) > conjugated ene > alkene > alicyclic ring > thioether, thione > amide > ketone > aldehyde > linear alkane > ether > ester > amine >Carboxylic acids >Nitriles >Primary alcohols >Secondary alcohols >Tertiary alcohols >Highly branched hydrocarbons.
The molecular ion peaks of aromatic rings (including aromatic heterocycles), alicyclic compounds, thioethers, thiones, and conjugated alkenes are relatively obvious. Straight-chain ketones, esters, acids, aldehydes, amides, halides, etc. usually show molecular ion peaks, and fats Family alcohols, amines, nitrites, nitrates, nitro compounds, nitriles and multi-branched compounds are easy to cleave, and the molecular ion peak is usually very weak or does not appear. It is difficult to confirm if the molecular ion peak does not appear or the abundance is extremely low. The experimental conditions can be changed according to different situations for verification.
(1) Reduce the energy of bombarding electrons
The commonly used 70eV is changed by 15eV to reduce the probability of continued fragmentation of the formed molecular ions, reducing the abundance of fragment ions and increasing the relative abundance of the molecular ion peak, making it possible to identify the molecular ions.
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(2) Use soft ionization methods such as CI, FI, FD, etc.
Reducing the energy of bombarding electrons will reduce the sensitivity of the instrument. Although the abundance of the molecular ion peak has increased, the absolute intensity of the ion has decreased. Some compounds do not have molecular ion peaks due to thermal instability and low volatility. , using this method will not achieve the expected effect. In this case, various soft ionization methods can be adopted. Although the fragment ions are greatly reduced, the molecular ion peak can be highlighted.
(3) Reduce the vaporization temperature of the sample
The reduction of vaporization temperature can reduce the possibility of further fragmentation of molecular ions, and the relative abundance of molecular ion peaks increases. For example, when triacontane is vaporized at 340°C, there is no molecular ion peak. When it is vaporized at 70°C, the abundance of the molecular ion peak is close to the base peak.
(4) Preparation of derivatives
Some compounds are not volatile or have poor thermal stability and can be derivatized. For example, a certain organic acid can be prepared into the corresponding ester. The ester is easy to vaporize and the molecular ion peak is easy to be obtained, from which the molecular weight of the organic acid can be inferred.