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Parameters influencing retention time

  • Page ID
    61073
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    Nico Vonk, Avans+, Breda, The Netherlands

    Abstract

    In the previous sections the effects of column and system parameters on the separation have been discussed. The separation result is not the only important parameter in practice. This chapter touches on a number of other parameters e.g. the analysis time and optimisation.

    Level: Basic

    The time in which an analysis is carried out is of major economic importance. The ideal situation would be if all components from a given sample have baseline separation in a very short time.

    Analysis time vs column ID

    ZorpwvkHO.jpg

    Column and system parameters affect the resolution, analysis time, and sample capacity. They do not act independently of each other. Column and system parameters have a complex interactive effect on the separation result. Usually, any improvement in one of these parameters results in some decrease in one or both of the others. Therefore, a column giving the best possible resolution is not necessarily the fastest, or with the highest capacity. This should be taken into consideration during optimization.

    Remember also that parameters do not always give linear effects on the particular objective to be attained. These effects can always be predicted from the resolution equation. For instance, shortening the column length say to one half will half the retention times. The resolution, however, only decreases by the square root of the length, to about 70%.

    Overview of parameters in a chromatographic process which determine the analysis time:

    • Volatility of the component
      Volatile components travel through a column faster than non-volatile components. The volatility is related to the boiling point and to the size of the molecules. This means that the smaller molecules within a homologous series have shorter retention times than the larger molecules
    • Stationary phase
      Not the stationary phase itself, but the affinity of the sample component for the stationary phase is important. The stronger this affinity, the stronger is the interaction, and therefore the longer the retention time.
    • Amount of stationary phase
      The k-values of sample components are related to the amount of stationary phase. The more stationary phase is present, the larger the k-values and thus the longer the retention time.
    • Column diameter
      In a narrow bore column the optimum gas velocity is much higher than in a wide bore column. This has immediate effect on the analysis time. A narrow bore column is a 'fast' column; a wide bore column is a 'slower' column.
    • Column length
      The length of a column affects the retention time proportionally. Thus halving the column length yields a considerable profit: the retention time is cut in half. Evidently, the quality of the separation is reduced as well.
    • Type of carrier gas and linear gas velocity
      In the previous section we have discussed the effect of the type of carrier gas. Hydrogen as a carrier gas can provide very short analysis times. The linear gas velocity (related to the flow) is a powerful factor for shortening the retention time.
    • Column temperature
      The volatility of a sample component depends on its boiling point, its vapor pressure and temperature. A high oven temperature results in small k-values and therefore in short analysis times. The temperature and the carrier gas flow, are the most commonly used and most powerful parameters to shorten the retention time.


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