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Hey guys and gals ,watchout this new article on MASS SPECTROMETER!!!!!!

MASS SPECTROMETER

The mass spectrometer is an instrument by which you can separate ionised/charged (+) particles of different mass and determine the amounts of each particle in a mixture.
The technique is called mass spectroscopy or mass spectrometry ('mass-spec' and 'MS' in shorthand!).
The substance to be analysed is introduced into a high vacuum (extremely low pressure) system where the particles are ionised by colliding with beam of high speed electrons.
The resulting (+) ions are accelerated down a tube (from + to - plates) and then through a powerful magnetic field.
The charged or ionised particles are deflected by this powerful magnetic field.
How much they are deflected depends on the particle mass i.e. lighter particles of lower mass (and momentum) are deflected more than heavier particles of bigger mass (see right diagram below).
By varying the magnetic field, it is possible to bring into focus onto an ion detector at the end of the tube, every possible mass in turn and measure the strength of the ion current, a measure of how much of that ion has been formed from the sample under analysis.
A simplified diagram of a mass spectrometer tube system is shown below (left) with further explanation as to what is going on and further diagram to show the relative paths of light to heavy ions for a given strength of magnetic field.

The relative paths of light to heavy ions in a mass spectrometer tube

Key to diagram and function explanation.
- K = sample injection point, it must be a gas, so a liquid/solid must be vaporised at the injection point.
- Q = high voltage (high +/- p.d.) electron gun which fires a beam of high speed/energy electrons from a heated 'metal element' into the vaporised sample under analysis and causes ionization of the atoms (or molecules) forming positive ions (mainly monopositive in charge).
  - The collision of high KE electrons with atoms or molecules causes another electron to be knocked off the particle leaving a negative deficit i.e. a positively charged particle is formed e.g.
    
    M_(g) + e^-==> M⁺_(g) + 2e^-, usually written as just
    
    M_(g) ==> M⁺_(g) + e^- (M might represent e.g. a metal atom or a molecule)
    
    The ions formed should be written as [M]⁺, a notation that is handy if you are dealing with ionised molecule fragments with an overall single positive charge e.g. [CH₃]⁺ is seen in the mass spectrum of methane gas, CH₄.
    
    The low pressure (~vacuum) is needed to prevent the ions from colliding with air particles which would stop them reaching the ion detector system.
- P = are negative plates which accelerate the positive ions down the tube (there are positive plates at the start of the tube). A moving beam of charged particles creates a magnetic field around itself, and this 'ion beam' magnetic field interacts with the magnetic field at R.
- R = the magnetic field that causes deflection of ions, this is can be varied to change the extent of deflection for a given mass and to focus a beam of it down onto the detector. Hence, by programming the mass spectrometer to 'sweep' through all likely particle masses, in terms of the right hand diagram, you can increase the strength of the magnetic field to bring into focus onto the ion detector monopositive ions of increasing mass.
- N = an ion detection system which essentially generates a tiny electrical current when the ions hit it. The strengths of the 'electronic' signals from the various ion peaks are sent to a computer for analysis, computation and display. They tell you the particle masses present and their relative abundance (see the mass spectrum diagram for the element strontium below).
- The resulting record of the ion peaks is called the mass spectrum or mass spectra. The highest peak is called the base peak and is often given the relative and arbitrary value of 100, particularly in the mass spectra of organic compounds).
- For elements you get a series of signals or ion peaks for each isotope present and the ratio of peak heights gives you the relative proportion of each isotope in the element so that you can calculate the relative atomic mass of an element. This 'simple' spectra is only true for non-molecular elements like metals (see strontium below) or noble gases, but for molecular elements like nitrogen or the halogens things are not so simple (see chlorine example below).
- For larger e.g. organic molecules, things can be very complex indeed, as molecules fragment and many different ions can be formed.
Chlorine is a good example of a molecular element whose mass spectra can be a bit tricky when first encountered ...
- Chlorine consists of two principal isotopes, chlorine-37 (25% is ³⁷Cl) and chlorine-35 (75% is ³⁵Cl).
- BUT, chlorine consists of Cl₂ diatomic molecules, which, on ionisation, can split into chlorine atoms.
- The result is a series of 5 different mass peaks from the various isotopic ion possibilities...
- [³⁷Cl³⁷Cl]⁺ m/z = 74, [³⁷Cl³⁵Cl]⁺ m/z = 72, [³⁵Cl³⁵Cl]⁺ m/z=70, [³⁷Cl]⁺ m/z=37, [³⁵Cl]⁺ m/z=35
- m/z means the relative mass of the ion over its charge, which for our purposes the charge is taken +1 and the mass is the relative atomic/formula mass of the particle.
The mass spectra of organic compounds can be very complex as the molecules fragment, but mass spectra can used to identify compounds from their 'finger-print' pattern of ion peaks of different mass and in particular proportions.
- The largest m/z value gives the molecular mass of a molecule, i.e. the ion of largest mass, prior to fragmentation, is formed when the original whole and neutral molecule, loses one electron e.g. for ethane it would be due to the formation of [C₂H₆]⁺, m/z = 30 and is called the molecular ion peak.
Example of a relative atomic mass calculation based on the mass spectrum of the element

The mass spectrum of the element strontium

The relative atomic mass of an element, A_r, is the weighted average mass of the isotopes present, compared to 1/12th of the relative mass of the carbon-12 isotope. [ ¹²C is given the relative mass value of 12.0000 ]
Quite often the highest peak is arbitrarily given the value of 100, as in this case, but the peak lines might well indicate % abundance of isotopes. relative peak height = relative abundance
The mass spectrum shows strontium consists of four isotopes, ⁸⁴Sr (peak height = 0.68), ⁸⁶Sr (peak height = 12.0),⁸⁷Sr (peak height = 8.47) and⁸⁸Sr (peak height = 100)
The sum of the heights = 0.68 + 12.0 + 8.47 + 100.0 = 121.15, so we can now calculate the weighted average mass.
Therefore A_r = {(0.68 x 84) + (12 x 86) + (8.47 x 87) + (100 x 88)}/121.15 = 87.7
The book value is 87.62, BUT this calculation does NOT take into account the very accurate relative atomic masses based on the carbon-12 scale, it merely uses the mass numbers, which are always integer. The very accurate isotopic masses are usually a tiny fraction different from a whole number.
Modern mass spectrometers are exceedingly accurate and very sophisticated instruments and can measure mass to at least 4 decimal places. They can distinguish between N₂ and CO molecules, both with an M_r of 28.

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