Mass Spectrometry

From drug testing and monitoring climate change to understanding viral dynamics inside a host cell, mass spectrometry has a host of different research applications in the real world. You might be wondering: what is Mass Spectrometry? As per Mass spectrometry definition, Mass spectrometry is an analytical technique of determining the molecular mass of compounds by measuring the mass-to-charge ratio of ions in the gaseous phase.  To understand it better, you need to know how mass spectrometry works and also the mass spectrometry principles and applications. 


Principle of Mass Spectrometry

In the process of mass spectrometry, the sample to be analyzed is ionized by the ionization source by using various methods like protonation* or deprotonation**. Subsequently, the ions formed in the gas phase are electrostatically channelled into a mass analyzer where the ions are separated according to their mass and detected through signals recorded on mass spectra. Thus, the mass spectrometry principle encompasses the three essential components of a mass spectrometer – the ionization source, the mass analyzer, and a detector.


What Are The Mass-to-charge Ratio And Mass Spectrum

The mass-to-charge ratio is the mass of an ion divided by its charge. A mass spectrum is a graphical plot of the relative abundance of ions versus the mass-to-charge ratio.


Mass Spectrometry Diagram

The instrumentation used in a typical mass spectrometer is shown in the following representative mass spectrometry diagram:

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Mass Spectrometry Instrumentation

Given below is a brief description of the primary components of the mass spectrometry instrumentation:

  1. Sample Inlet: Samples are steadily streamed at low pressure into the ionization chamber through a pinhole called "molecular leak."

  2. Ionizer: Samples are bombarded with a beam of electrons to generate positively charged ions.

  3. Accelerator: Positively charged sample ions pass through three slits, which have voltages in decreasing order. Acceleration ensures that all the ions have the same kinetic energy.

  4. Deflector: Due to differences in masses, the ions are deflected by an applied magnetic field. The lighter ions, as well as the ions carrying a more positive charge, are deflected more.

  5. Detector: The detector detects the ions reaching it through the mass analyzer. Detection is achieved based on the mass-to-charge ratio of ions.


How Does Mass Spectrometry Work

The working of the mass spectrometer involves the following steps:

Step 1: Ionization of the sample in the gas phase.


Step 2: Acceleration of the sample ions through an electric field. After acceleration, each ion emerges with a velocity that is proportional to its mass-to-charge ratio.


Step 3: Passage of the ions into a field-free region.


Step 4: Deflection of ions by a magnetic field.


Step 5: Passage of ions through the mass analyzer which detects the arrival times of the sample ions and records the mass spectrum. The following representative diagram illustrates the Mass spectrometry working steps:


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Advantages And Limitations Of Mass Spectrometry

Advantages:

  1. Works with a small sample size

  2. Fast

  3. Can differentiate isotopes

Limitations:

  1. Does not give direct structural information

  2. The requirement of pure samples

  3. Not ideal for non-volatile compounds


Applications of Mass Spectrometry

The uses of mass spectrometry are many. Be it pure or applied research, almost every discipline of science utilizes mass spectrometry for qualitative and quantitative analysis of macromolecules and low molecular weight compounds. Some of the most relevant applications of mass spectrometry are:

  1. Measurement of the molecular mass of biomolecules such as carbohydrates, proteins, and nucleic acids.

  2. Determination of the sequence of biopolymers like oligosaccharides, nucleic acids, and polypeptides.

  3. Determination of protein structure.

  4. Determination of elements and their isotopes.

  5. Determination of pesticide residues and toxins in food.

  6. Analysis of air, water, and soil quality for monitoring the environment and climate change.

  7. Monitoring the metabolic gas exchange of patients during surgery.

  8. Surveying gas deposits and locating oil deposits by measuring the petroleum precursors in rocks.

  9. Carbon dating of samples and determination of rock and soil composition.

  10. Quality control analysis in the chemical and petrochemical industries.

  11. Studies of particles in aerosols like perfumes.

  12. Identification of drug abuse cases through analysis of drug abuse metabolites in saliva, blood, and urine.

FAQ (Frequently Asked Questions)

1. How are Samples Converted to Ions in Mass Spectrometry?

  • Electron ionization: A high energy beam of electrons strikes the sample. The collision between an electron and the sample molecule removes an electron from the molecule, creating a cation.

  • Chemical ionization: The sample molecules are mixed with an ionized reagent gas. The collision between the ionized reagent gas and the sample molecules ionizes the latter by electron transfer, proton transfer, and adduct formation.

  • Electrospray ionization: The sample is dissolved in a volatile, polar solvent and electrostatically dispersed through a narrow capillary, which generates an aerosol of highly positively charged droplets. Due to solvent evaporation, the airborne droplets shrink, and their charge density increases, eventually releasing the charged sample ions.

  • Desorption ionization: The sample is dissolved in a suitable matrix. A short pulse of laser light is used to ionize the sample, which is desorbed from the matrix into a vacuum system for further analysis.

2. What is Time - of - Flight, and How is it Calculated?

Time-of-flight is a type of mass analyzer that measures the flight time of ions in a mass spectrometer. The principle is that two ions with the same kinetic energy will have different velocities based on their masses.


When accelerated through an electric potential V, the kinetic energy of an ion is given as:

E = zV = mv2/2........ (1) 


Where, E = kinetic energy

V = electric potential

v = velocity of an ion

z = charge on an ion

m = mass of an ion


The velocity of an ion is given by the length of the flight (L) divided by the time (t) for the ion's flight.


v = L/t....... (2)


Replacing the value of 'v' in (1) with that in (2):


zV = mL2/2t2


or, m/z = 2Vt2/L2 


m/z is the mass-to-charge ratio, which determines the separation of ions in the mass spectrometer. 


* Protonation is defined as the addition of proton to molecules or atoms to form a conjugate acid.


** Deprotonation is defined as the removal of a proton from a Bronsted-Lowry acid in an acid-base reaction.