analytical spectroscopy

• most IR and NMR spectroscopy is

  qualitative
• uv-vis spectroscopy is

  quantitative
• qualitative analysis

  analyses are identified on the basis of the wavelength(s) that they absorb or emit
• quantitative analysis

  concentration of analytes are measured
• selectivity

  degree to which a method responds to the analyte and not to other components of the sample matrix
• selectivity must be

  high so the method is free from interference
• sensitivity

  change in response per unit change in concentration of the analyte
• sensitivity must be

  high so we can distinguish small differences in concentration of the analyte
• limit of detection

  lowest analyte concentration we can detect with a particular statistical certainty
• what is uv-vis spectroscopy

  light in the wavelength of 200-800 nm can induce electronic transitions (movement of valence electrons between orbitals)

  so atoms or molecules can absorb or emit uv-vis light

  the wavelength of light absorbed allows compounds to be identified

  the amount of light absorbed relates to concentration
• describe qualitative uv-vis spectroscopy

  the wavelength at which a substance shows maximum absorption is called the (lambda max) corresponds to the delta E of the transition occurring

  can be used to id unknowns since each substance has a different lambda max
• LAMBDA MAX does not correspond to the colour of the solution

  we see the un-absorbed portion of the visible spectrum, the complementary colour
• how is quantitative analysis preformed

  light of correct wavelength (mathching delta E of the analyte) is shone through the solution

  the proportion of light absorbed is measured
• incident radiation (I0) is ...

  100
• I (transmitted radiation)

  is x
• Absorbance (A)

  log10(I00/I)
• the beer lambert law relates

  absorption to concentration
• the beer lambert law is 


  A = e c l
• e

  molar absorptivity dm3 mol-1 cm -1
• c

  concentration mol dm-3
• l

  path length (cm)
• molar absorptivity relates to the probability of a particular electronic transition occurring.
• large e values correspond to

  strong and intense colours
• small e values correspond to

  pale and weak colours
• large e values give good sensitivity in spectroscopic analysis and so are desirable
• a chromophore is the specific functional group in the molecule that absorbed a particular wavelength of light
• different chromophores have different, characteristic, absorption wavelengths and e values
• common chromophores in uv-vis spectrometry include

  double or triple bonds and conjugated systems

  lone pairs

  metal ion complexes
• double or triple bonds, and conjugated systems such as benzene or butadiene have e values in the range of

  1000 - 100000 dm3 mol-1 cm-1
• lone pairs e.g. on O, N or S atoms have e values around

  1000 dm3 mol-1 cm-1
• metal ion complexes have e values in the range of

  0.3 - 100000 dm3 mol-1 cm-1
• examples of organic chromophores include

  amines, alkenes, conjugated alkenes, ketones, aromatics
• direct spectrophotometry finds the concentration of the unknown with respect to a series of standard solutions made up in the same solvent and measured at the same temperature
• direct spectrophotometry overcomes the fact that molar absorbivity is not a universal constant but instead varies because of temperature and the composition of the solvent the analyte is dissolved in which can make using the beer-lambert law to accurate difficult
• in DS

  1 - the wavelength which gives maximum absorbance for the analyte of the interest is found

  2 - set the spectrometer and determine the analyte concentration in the sample by

  making up a series of standard solution of known concentrations of the analyte and one blank

  analyse the standards

  plot a calibration graph

  analyse the samples

  use the calibration graph to obtain the concentration of analyte in the samples
• limitation of DS include

  only linear part of the graph is used (usually up to A = around 0.7)

  the response for the sample must lie within the linear portion of the graph

  new graph for each analysis
• light source (DS) - a tungsten and/or a deuterium lamp to provide visible and uv wavelengths, respectively
• monochromator (DS) - selects a small wavelength region from the lamps output spectrum to pass through the sample (typical bandpass is 2nm)
• cell (DS) - contains the sample solution
• detector (DS) - a photomultiplier which gives out an electrical signal when light falls upon it