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  Section: Practical Skills in Chemistry » Instrumental techniques
 
 
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Optimizing chromatographic separations

 
     
 
Content
Instrumental techniques
  Basic spectroscopy
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    UV Ivisible spectrophotometry
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    Fluorescence spectrophotometry
    Phosphorescence and luminescence
    Atomic spectroscopy
  Atomic spectroscopy
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  Infrared spectroscopy
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    1H-NMR spectra
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  Mass spectrometry
    Interfacing mass spectrometry
  Chromatography ~ introduction
    The chromatogram
    Resolution
    Detectors
  Gas and liquid chromatography
    Gas chromatography
    Liquid chromatography
    High-performance liquid chromatography
    Interpreting chromatograms
    Optimizing chromatographic separations
    Quantitative analysis
  Electrophoresis
    The supporting medium
    Capillary electrophoresis
    Capillary zone electrophoresis (CZE)
    Micellar electrokinetic chromatography (MEKC)
  Electroanalytical techniques
    Potentiometry and ion-selective electrodes
    Voltammetric methods
    Oxygen electrodes
    Coulometric methods
    Cyclic voltammetry
  Radioactive isotopes and their uses
    Radioactive decay
    Measuring radioactivity
    Chemical applications for radioactive isotopes
    Working practices when using radioactive isotopes
  Thermal analysis
    Thermogravimetry
    Applications

In an ideal chromatographic analysis the sample molecules will be completely separated, and detection of components will result in a series of discrete individual peaks corresponding to each type of molecule. However, to minimize the possibility of overlapping peaks, or of peaks composed of more than one substance, it is important to maximize the separation efficiency of the technique, which depends on:
  • the selectivity, as measured by the relative retention times of the two components, or by the volume of the mobile phase between the peak maxima of the two components after they have passed through the column; this depends on the ability of the chromatographic method to separate two components with similar properties;
  • the band-broadening properties of the chromatographic system, which influence the width of the peaks; these are mainly due to the effects of diffusion.
The resolution of two adjacent components can be defined in terms of k', α and N, using eqn [31.6]. In practical terms, good resolution is achieved when there is a large 'distance' (either time or volume) between peak maxima, and the peaks are as narrow as possible. The resolution of components is also affected by the relative amount of each substance: for systems showing low resolution, it can be difficult to resolve small amounts of a particular component in the presence of larger amounts of a second component. If you cannot obtain the desired results from a poorly resolved chromatogram, other chromatographic conditions, or even different methods, should be tried in an attempt to improve resolution. For liquid chromatography, changes in the following factors may improve resolution:
  • Stationary-phase particle size - the smaller the particle, the greater the area available for partitioning between the mobile phase and the stationary phase. This partly accounts for the high resolution observed with HPLC compared with low-pressure methods.
  • The slope of the salt gradient in eluting IEC columns, e.g. using computer-controlled adapted gradients.
  • In low-pressure liquid chromatography, the flow rate of the mobile phase must be optimized because this influences two band-broadening effects which are dependent on diffusion of sample molecules:
    1. the flow rate must be slow enough to allow effective partitioning between the mobile phase and the stationary phase: and
    2. it must be fast enough to ensure that there is minimal diffusion along the column once the molecules have been separated. To allow for these opposing influences, a compromise flow rate must be used.
  • If you prepare your own columns, they must be packed correctly, with no channels present that might result in uneven flow and eddy diffusion.
 
     
 
 
     



     
 
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