Explanation about mass spectroscopy introduction and instrumentation

1. Mass Spectroscopy
INTRODUCTION & INSTRUMENTATION
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2. Principle:
• Mass spectra is also called positive ion spectra or line spectra. Unlike other kinds of
spectroscopy, we do not use any electromagnetic radiation (EMR) for excitation. We use
electron bombardment to convert a neutral molecule to a positive charged one. Also there is
no ground or excited state like other types of spectroscopy.
Conditions to obtained mass spectra:
• Conversion of neutral molecule into a charged molecule (positive molecule).
• Separation of the positively charged fragments formed, based on their masses by using
electrical or magnetic field or both.

3. The sample is bombarded with high energy electron beam (70eV), where an electron is
knocked off from every molecule. Hence the molecule becomes positively charged. When a
positive potential is applied as the molecules are positively charged, they get repelled and travel
with great speed, in a straight path.
Potential energy = kinetic energy of molecule
eV = ½ mv2
where e = charge of ion
V = acceleration voltage
M = mass
V = velocity after acceleration

4. When a magnetic field or an electric field is applied, the positive charged fragments
which were travelling in straight path, now travels in a curved path. When they travel in a
curved path under the influence of mag.field, the fragment are separated into different
masses because the radius of curvature depends upon their respective masses.
Under magnetic field: Hev = 𝑚 𝑣 2
𝑟 v = r e H/ m

5. INSTRUMENTATION
Mass Spectrometry instrumentation:
• Mass Spectrometry is widely used to determine and identify the elements present in samples and to
determine their concentrations.
• Mass Spectrometry is also used to measure the relative atomic mass of an element and to measure the
relative molecular mass of a substance.

6. Stage 1 Ionization:
 Electro Ionisation is the most common type of ionisation.
 The sample is bombarded by electrons which come from a heated filament.
 The electrons run in a stream between the cathode and anode.
 When the sample passes through the electron stream, the high energy electrons in the stream knock electrons out of the
sample to form ions.

7. Stage 2 Acceleration:
 Acceleration is a simple step where the ions are placed between a set of charges parallel plates.
 The ions will then be repelled by one plate and attracted to the other.
 There is a slit cut in the plate which the ions are attracted to. the force of attraction and repulsion forces the ions
through the slit at an accelerated rate.
 The speed of acceleration can be adjusted by changing the charge on the plates.

8. Stage 3 Deflection:
 Ions are deflected by the magnetic field surrounding the instrument.
 The amount of deflection depends on the mass and charge of the ions.
 The heavier ions and ions with a positive charge of 2 or more, are deflected the least (Ion stream C)
 The lightest ions and ions with 1 positive charge are deflected the most (Ion Stream A)
 The ions at the correct mass and charge travel to the detector. (Ion Stream B)
 The mass to charge ratio (m/z) is determined from the ion that hits the detector.

9. Step 4 Detection:
 When the ion stream reached the detector the hit a wire. On hitting the wire they become
neutralized by an electron jumping from the metal wire to the ion.
 The amplifier picks up on this current being created between the wire and the ion and
amplifies the signal being detected.
 The computer picks up on this and converts it to mass/charge ratio and a spectrum is
produced.

10. Applications of Mass Spectrometry:
 Mass Spectrometry as a technique can be coupled with other techniques such as HPLC and GC.
 As it is used in the identification of compounds it is used in all areas of science.
Some of its uses are:
 Trace Gas Analysis
 Pharmaceutical Industry
 Space Exploration
 Forensic Toxicology
 Archaeological Dating.