Basic Concepts of Mass Spectrometry
It is the analytical technique Which was discovered by J.J Thomson and It is used to identify the unknown compound within a sample and also helps to elucidate the structure by the effect of ionization energy on molecules.
This is an analytical tool useful for measuring the mass-to-charge ratio (m/z) of one or more molecules present in a sample which can often be used to calculate the exact molecular weight of the sample components. .
This branch of the technique have wide variety of applications like-
- It gives detail information of the composition of organic compound
- It gives the position of functional group
- It is used for the determination of molecular weight
- Analyze a purified protein
- Study the protein content of a sample of cells.
Working strategy of an Instrument-
An instrument generates the beam of positively charged produce ions of sample from ionization source then separate these ions according to mass charge ratio in the mass analyser after that fragment of selected ions and analyse or detect the ions emerge in the analyser which measures the abundance with the detector that converts the into electrical signals. This further process to give results.
- Inlet system
- Ionization mechanism
- Ionization source
- Mass Analyzer
- Detector
Inlet system
The selection of sample inlet depends upon the sample matrix. It is very important to note that inlet must transfer the analyte into the source as a gas phase molecule if sample is in gaseous state but for liquid sample It is heated to increase vapour pressure and handled by hypodermic needle.
Ionization mechanism
It refers to mechanisms of ionization followed by various methods like Protonation, deprotonation , cationization , electron ejection and electron capture.
Ionization source
It is the device which allows ionization to occur and the amount of minimum energy to ionize the sample or organic molecule.
It has a function of producing mass without mass discrimination of the sample and accelerating the ions into mass analyser.
Mass analyser
It is the component of the mass spectrometer that ionized masses and separates them based on charge to mass ratios and outputs them to the detector where they are detected.
Detector
Once the ions are separated by the analyser, it will reach a detector which generates a current signal from the incident ions.
The most commonly used detectors in mass spectrometer are-
- Faraday cup
- Electron multiplier
- Photon multiplier
- Charge or inductive detector
Metastable ion
Consider that M1 Is the parent ion and m1 is daughter ion, If the reaction
M1→m1 takes place in the source, then m1 may travel the whole analyser region.
m= m1^2/M1
Characteristics of Metastable ions
- They do not necessarily occur at the integral m/e value.
- They have low kinetic energies.
- These are of relatively low abundance.
- These are much broader than the normal peaks.
Fragment ion
The energy more than 70ev, produces a fragment which has small masses.
It is formed by both homolytic and heterolytic cleavage of bonds by simple cleavage and rearrangement process.
Simple alkyl group split as
Loss of methyl 15
Loss of ethyl 29
Loss of propyl 43
Loss of butyl 57
Branched alkanes tend to give more stable carbocation.
It has different fragments for different functional group
Alcohol
It results in very small or missing parent ion peak.
May lose hydroxyl or water at
M=17 or 18
For example – primary alcohol has peak of m/z at 31 corresponding to CH2=OH+
Questions and Answers on Mass Spectrometry
Q1 Which species of the following is used to bombard with the sample ?
a) alpha particles
b) Neutrons
c) Electrons
d) Protons
Answer – c) Electrons
Q2 Separation of ions in the mass spectrometer take place on the basis of which of the following?
a) Mass
b) Charge
c) Molecular weight
d) Mass to charge ratio
Ans – D) Mass to charge ratio
Understand Mass Spectrometry, structure determination, and chemical analysis of the element
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Frequently Asked Questions
What is the basic principle of mass spectrometry?
In mass spectrometry, multiple ions are generated from a given compound and those ions are separated on the basis of specific mass to charge ratios under the influence of magnetic field as they are deflected to different angles. The relative abundance of each ion type is recorded by the machine.
How does the mass spectrometer work?
The mass spectrometer ionizes the atom or molecule into multiple ions by removing one or more electrons and these ions produced are then accelerated and then deflected according to their masses by magnetic field and then detected by the machine.
What are the four stages of mass spectrometry?
a) Ionization- The compound is ionized to form multiple ions by ejecting one or more electrons from the molecule or ion.
b) Acceleration- The ions produced are accelerated with some kinetic energy.
c) Deflection- Under the influence of magnetic field, the ions are deflected to different angles according to their mass to charge ratio.
d) Detection- The relative abundance of each ion type is recorded.
What does a mass spectrum show?
A mass spectrum is the graphical representation of ion abundance versus the mass to charge ratio (m/z) of the different ions separated in a mass spectrometer. The mass spectrum is useful in determination of masses of molecules and particles and to elucidate the chemical structure of compounds.