Understanding Basic Principle and Theory of Mass Spectrometry | ChemFam #89 |

Greetings everyone! Ever wondered how scientists peek into the hidden world of molecules or solve complex puzzles in crime scenes like detectives in a mystery novel? The technique called as mass spectrometry (MS) helps to solve such mysteries. But why spectrometry and not spectroscopy? We call it so because we actually do not use electromagnetic radiations in this technique and so we call it as mass spectrometry and not spectroscopy.

Mass spectrometry is useful for characterization of organic compounds in two ways:

• It can give the exact molecular masses and thus gives the exact molecular formulae.
• It can provide information on the structure of the molecule by indicating the existence of specific structural units and the places at which they adhere to it.

Basic Principle

The most precise technique for figuring out a compound's molecular mass and elemental composition is mass spectrometry (MS). This method involves launching a powerful electron beam at molecules. A large number of fragments, some of which are positive ions, are formed as the molecules get ionized. Every type of ion has a unique mass to charge ratio, or m/e ratio. Since the charge of the majority of ions is 1, the m/e ratio is just the ion's molecular mass.

The separated ions are then detected and recorded as mass spectra, which provide valuable information about the composition, structure, and abundance of molecules within the sample. By analyzing the patterns and peaks in the mass spectra, scientists can identify specific compounds, quantify their concentrations

Let's take the example of neopentane. The molecular ion, here
C₅H₁₂⁺ is called parent ion and is usually designated as M⁺. The molecule has an unpaired electron and is positively charged.

A signal is obtained for each represented value of m/e by analyzing the set of ions (also known as daughter ions or fragmented ions). Each signal's intensity indicates the relative abundance of the ion that is generating the signal. The base peak, which is the largest peak in the structure, has an intensity of 100. The additional peaks' intensities are displayed in relation to the base peak.

A compound's mass spectrum is a plot that shows the signal intensities at different m/e values. It is a very distinctive property of a compound. It is impossible for two substances to have identical mass spectra.

Theory

When a substance's parent molecule loses one electron, a parent ion is produced.

M(g) + e → M⁺(g) + 2e

The m/e value of the parent ion is equal to the molecular mass of the compound. In a fewcases, the parent ion peak may be the base peak and can be easily recognised. In most of the cases, parent ion peak is not the base peak and is often of very small abundance. Many elements occur naturally as isotopes, out of these the lightest one greatly predominates. Three primary tasks have been designed to be performed by the mass spectrometer. These are the following:

• To evaporate different volatile chemicals.
• To produce ions in the vapour phase by using the neutral chemicals.
• To record and segregate ions based on their mass over charge ratio. The mass spectrum is the plot of the m/e values along the abscissa and their respective intensities along the ordinate.

M⁺(g) → m₁⁺ + m₂
or, M⁺(g) → m₁+m₂

Neutral particles produced in the process of fragmentation can not be detected in the mass spectrometer.

Applications of Mass Spectrometry

The versatility of mass spectrometry lends itself to a vast array of applications across numerous disciplines:

• Environmental Analysis: The monitoring of pollutants, poisons, and contaminants in soil, water and air is a common application of mass spectrometry. In order to protect ecosystems and human health, mass spectrometry is essential for tracking the spread of environmental pollutants and identifying hazardous compounds in industrial effluents.

• Forensic Science: Mass spectrometry is an effective method in criminal investigations that can be used to analyze trace evidence, including biological fluids, fibers, narcotics and explosives. Forensic specialists can uncover distinctive molecular signatures that connect suspects to crime sites, clear innocent parties and offer vital evidence during legal proceedings.

• Pharmaceutical Development: Quality control, drug development, and discovery all heavily rely on mass spectrometry. Before putting drug candidates through clinical trials, pharmaceutical companies use mass spectrometers to assess the candidates' purity, stability and bioavailability to make sure they are safe and effective. Additionally, mass spectrometry helps characterize metabolites, clarifying drug metabolic routes and directing pharmacokinetic research.

Mass spectrometry stands as a cornerstone in scientific research and plays an important role in the vast field of organic spectroscopy. I hope I was able to give a basic insight to the topic.

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B I B L I O G R A P H Y

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Organic Spectroscopy | Springer, LDS Yadav

Elementary Organic Spectroscopy | S. Chand Publication, YR Sharma

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