Molecular Spectrometry

mass spctrometry

Mass spectrometry

                                                   Spectrometry                                                                                                                                                                                    Spectrometry can be defined as:                                                                        

The study of molecular or atomic structure of a substance by observation of its interaction with electromagnetic radiation.

Two ways of studying this:                                                                                              Quantitatively:  For determining the amount  of material in a sample.                        Qualitatively  :   For identifying the chemical structure of a sample.

The Electromagnetic Spectrum                                                                                  

 The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object.

 The difference between these sources of radiation is the amount of energy  they radiate.

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radiation is transmitted in waveform

Long wavelength:  low radiation energy. Short wavelength:  high radiation energy


Wavelength: time or distance to complete one cycle of wave form 

Frequency: number of cycles in a given time

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                                         Laws of Specphotometry                                            

There are two very important basic laws and a third one which is a 

combination of the two. 

  • LAMBERTS LAW          - ABSORBANCE (A) proportional to the PATHLENGTH (l)  of the absorbing medium.
  • BEERS LAW -                -ABSORBANCE (A) proportional to the CONCENTRATION (c) of the sample. 
  • BEER- LAMBERT LAW - ABSORBANCE (A) proportional to c x l
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Molar extinction coefficient                                              

 should be for pure samples and with known molecular weights. If an E value has been calculated for a sample then the % purity of the sample can be calculated as follows:

                                  E sample / E standard x100                                                             

There are instances when either the MW or a pure sample is not  available and there are no literature E values.                            

 In these cases solutions are prepared at a concentration of 1% and absorbance measured in a 1cm pathlength cuvette to determine.

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  • Use of glass or plastic cuvettes for Visible                                                   Use of quartz or special plastic cuvettes for UV measurements
  • Glass cuvettes absorb UV radiation but not visible radiation whilst quartz cuvettes do not absorb any radiation at all


UV or Visible spectra can both be obtained form the same instrument                       There are two basic designs the DISPERSION system and the more modern DIODE ARRAY system. 

This design permits ALL wavelengths of light from source to pass sample in cuvette before reaching the diffraction grating and more sensitive.

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.Visible Spectrometry                                                                                            

  • WHAT IS COLOUR? Colour is a sensation which occurs when light enters the eye and focuses on the retina at the back of the eye. 
  • CONES - Give colour and three types which pick up red, blue and green    RODS  - Give grey/black and also used for night vision.

     Sunlight is white light and covers a wavelength range of 380-750nm

  • If the material completely REFLECTS all light it appears WHITE
  • If the material completely ABSORBS all light it appears  BLACK
  • If the material absorbs a constant fraction of the light across the spectrum it appears GREY
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Why Are Dye Molecules Coloured?                                                                       

Why Are Dye Molecules Coloured?                                                                       

Starting with ethanol, this is colourless because it only absorbs UV radiation.

The parts of a molecule which contain absorption features are called Chromophores and if a sufficient number are present then the molecule will be coloured. 

Increasing the number of chromphores means that the compound will absorb light at a longer wavelength and will therefore increase its v max value.

Auxochromes play major role in dye  chemistry as apart from changing the intensity depth of a colour they can have a profound influence on solubility and hence its ability to dye an object. 

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forensic application :

fluorescence-Fingerprints with use of dfo for maximum contrast on paper types

uv-visible -One of the current techniques for the analysis of ink samples is microspectrophotometry 

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calculations for this topic:

  • Radiation
  • transmittance
  • beer lambert law
  • Frequency:


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