In the world of scientific research, Mass Spectrometry is a symbol of accuracy and discovery. Picture it as a molecular investigator, enabling us to measure and recognize substances with an exceptional level of precision. This analytical tool has become a cornerstone in various scientific fields, uncovering the secrets of molecules in areas ranging from drug development to environmental studies.
Mass Spectrometry is like a super microscope for scientists. It helps them look closely at what things are made of and how they’re put together. Imagine it as a musical concert where each molecule is a different instrument playing a special note. This ‘symphony’ of analysis gives scientists a colorful picture of what’s happening in the tiny world of molecules.
So, without any further ado, let’s get into this article and get to know more about this technique.
Mass spectrometry(MS) is a quantitative analytical technique used to determine the components present in a compound with their structure and chemical properties. It is an instrumental method that uses the mass-to-charge ratio of a substance for the analysis. The unique feature of the technique is that it can elucidate the molecular mass of a compound, hence the name “mass spectrometry”.
The modern mass spectrometers were devised by Arthur Jeffry Dempster(1918) and F.W. Aston(1919).
Principle of Mass Spectrometry
The principle of Mass Spectrometry says that “The ions with different mass-to-charge ratios are deflected by different angles in an electric or magnetic field.”
When a beam of electrons is bombarded in the analyte compound, it leads to the removal of one electron from the analyte. Due to the removal of electrons, the molecule becomes positively charged which is known as a molecular ion which then splits into fragmented ions. The fragmented ions are then detected by the spectrometer.
The fragmented ions are separated in space or time, based on their mass-to-charge ratio and are quantified. All the ions are accelerated across the same distance by the same force and, therefore have the same kinetic energy, determined by the acceleration voltage of the instrument and the charge of the ion. This is given by:
KE = zV = ½ mv² Where,
KE = kinetic energy of the ions
z = charge to mass ratio
V = voltage of the instrument
m = mass of ions
v = velocity of ions
As charged ions move through the magnetic field, they are bent in an arc to follow a circular path of a certain radius in the direction perpendicular to the applied magnetic field.
Hzv = mv²/r Where,
H = magnetic field
r = radius of path
mv²/r = centripetal force
The magnetic force on the ions is balanced by the centripetal force.
The ions of a certain mass-to-charge ratio value have a unique path radius which can be determined if both, H and V are held constant.
All ions having the same mass-to-charge ratio value are deflected to the same degree and follow the same trajectory.
This deflection produces a mass spectrum of the analyte in the form of a plot of ion abundance versus mass-to-charge ratio. Ions provide information concerning the nature and the structure of their precursor molecule.
Instrumentation of Mass Spectrometry
The instrumentation of MS is composed of:
- Sample inlet
- High vacuum system
- Ionization source
- Mass analyzer
- Ion detector
- Amplifier and detector
I. Sample Inlet
Since Mass spectrometry only detects the ions, the analyte should be in gaseous form. But not all the analytes are available in a gaseous state (solid and liquid samples) so they are first converted into gaseous form and then injected into the instrument. Sample inlet commences the introduction of the analyte into the instrument.
The most common method used is a direct vapor inlet, in which a high vapor-pressure gas phase sample is introduced directly into the source region through a needle valve. The sample can also be introduced to the ionization source through a chromatographic system such as coupled HPLC or GC.
II. High Vacuum System
The vacuum system is required to maintain the smooth flow of the ions throughout the instrument. The ionization source, mass analyzer, and ion detector are enclosed in the vacuum system.
The high vacuum also reduces the collision of ions with other molecules to prevent scattering and fragmentation.
III. Ionization Source
MS is significantly dependent upon the ionization method. The sample molecules are excited so that they eject an electron to form a radical cation(M⁺) or are forced to undergo ion-molecule reactions to produce adduct ions(MH⁺) mostly by the addition of a proton(H⁺) to the molecule(M⁺).
M + H⁺ = MH⁺
The amount of fragmentation is controlled by substantial ionization energy observed in the mass spectrum. Some ionization methods are easy and only produce molecular ions while some are hard ionization techniques that are very active and cause ions to undergo extensive fragmentation.
Variation in the spectrum is introduced in terms of the number and intensity of peaks.
The ionization can be categorized as soft ionization and hard ionization. The soft ionization exhibits low energy that decreases the fragmentation. On the other hand, hard ionization possesses high energy which leads to increased fragmentation.
Various kinds of ionization methods can be carried out and are classified as:
- Gas phase ionization
- Desorption ionization
- Evaporation ionization
IV. Mass Analyzer
Once the electrons are ionized, they are sorted by the mass analyzer according to their mass-to-charge ratio. They can be continuous or pulsed mass analyzers. Continuous analyzers consist of specific quadrupole filters and magnetic sectors which work on single ion monitoring that improves the signal-to-noise ratio.
The pulsed analyzers consist of time-of-flight, ion cyclotron resonance, and quadrupole ion trap mass spectrometers which collect an entire spectrum from a single pulse of ion. And this results in high transmission efficiency which increases the signal-to-noise ratio.
V. Ion Detector
The sorted ions from the analyzer strike the detector which amplifies the signal and transfers it to the recorder. The detector detects the mass-to-charge ratio of the ions and records the relative abundance of each of the resolved ionic species. The detectors used are
- Photomultiplier Tubes: In Mass Spectrometry, the photomultiplier tube (PMT) acts like a super-sensitive detective for light. When molecules release faint signals of light during analysis, the PMT captures and amplifies these signals. It’s essentially a high-tech amplifier, turning weak light into a strong signal for further study. This tiny but powerful device helps scientists unveil the composition of samples, turning subtle molecular notes into valuable insights in the world of Mass Spectrometry.
- Electron Multiplier Tube: The electron multiplier tube (EMT) is like a silent powerhouse in Mass Spectrometry. Think of it as an amplifier for electric signals during analysis. In this molecular world, charged particles are like musical notes, and the EMT boosts their signals by creating a chain reaction of multiplying electrons. This tiny device ensures even the faintest signals become strong enough for precise measurement. In the symphony of Mass Spectrometry, the electron multiplier tube takes center stage, turning subtle electric whispers into a powerful composition of molecular insights.
- Micro-Channel Plate Detectors: Micro-channel plate (MCP) detectors are like high-speed cameras for Mass Spectrometry. Charged particles trigger a cascade effect in microscopic channels, multiplying the signal and turning faint interactions into strong, measurable data. These compact detectors enhance sensitivity, capturing the dynamic molecular dance and enriching the symphony of insights in analytical chemistry.
- Faraday Cup: The Faraday cup is the silent collector in Mass Spectrometry, akin to a precise accountant tallying charged particles. In its simplicity, it’s a metal cup that intercepts charged ions generated during analysis. By measuring the electric current produced when ions hit the cup, it quantifies the abundance of ions, providing valuable data on the sample’s composition. The Faraday cup is a straightforward yet essential tool, quietly contributing to the accuracy of Mass Spectrometry by diligently counting the charged particles and revealing the elemental makeup of substances.
The recorder is a computer system that records the signal in the form of a graph called the mass spectrum which is plotted between the mass-to-charge ratio and ion signals which may represent the molecular mass, number of components, and their relative abundance in the compound.
Mass Spectrometry is a qualitative and quantitative analytical method used in various scientific disciplines. It elucidated the structures and chemical properties of a substance based on its mass-to-charge ratio which is a unique feature of every substance.
Therefore it is a highly recommended instrumental technique in forensics. It can be coupled with other analytical techniques such as HPLC or GC to give quantitative results.