Monday, May 25, 2009

HPLC - Mobile Phase

The mobile phase in HPLC refers to the solvent being continuously applied to the column, or stationary phase. The mobile phase acts as a carrier for the sample solution. A sample solution is injected into the mobile phase of an assay through the injector port. As a sample solution flows through a column with the mobile phase, the components of that solution migrate according to the non-covalent interactions of the compound with the column. The chemical interactions of the mobile phase and sample, with the column, determine the degree of migration and separation of components contained in the sample. For example, those samples which have stronger interactions with the mobile phase than with the stationary phase will elute from the column faster, and thus have a shorter retention time, while the reverse is also true. The mobile phase can be altered in order to manipulate the interactions of the sample and the stationary phase. There are several types of mobile phases, these include: Isocratic, gradient, and polytyptic.
In isocratic elution compounds are eluted using constant mobile phase composition. All compounds begin migration through the column at onset. However, each migrates at a different rate, resulting in faster or slower elution rate. This type of elution is both simple and inexpensive, but resolution of some compounds is questionable and elution may not be obtained in a reasonable amount of time.

In gradient elution different compounds are eluted by increasing the strength of the organic solvent. The sample is injected while a weaker mobile phase is being applied to the system. The strength of the mobile phase is later increased in increments by raising the organic solvent fraction, which subsequently results in elution of retained components. This is usually done in a stepwise or linear fashion.

Isocratic Vs. Gradient Elution
The Knox equation describes column efficiency or plate number N in relation to certain experimental conditions, such as column length, column diameter, temperature, flow-rate, molecular weight, etc.

Plate number N is equal to plate height value H divided by particle diameter (dp). Plate height value H is in turn equal to column length L divided by N. Two of the Knox coefficients, B and C, depend on k' and size of the compound. In the equations above, k' in the isocratic equations is replaced with average k' in the gradient equations. In fact, this is the only difference in the bandwidth and resolution equations between the two. Thus, separation and height of the peak are dictated by the exact same conditions for both isocratic and gradient elution (snyder -1983).


From the equation for capacity factor in gradient elution, it can be seen that average k' value depends on flow-rate, gradient time, and column dead volume. This differs in isocratic elution where k' is not dependent on time of separation, flow- rate, or column dimensions.
A special feature in gradient elution is linear-solvent strength (LSS) gradients. These give approximately equal values of average k' for samples eluting at different times during separation. This is the reason why gradient elution can yield constant bandwidths for different compounds and equal resolution for pairs of compounds which have similar alpha or separation factor values.
Polytyptic Mobile Phase, sometimes referred to as mixed-mode chromatography, is a versatile method in which several types of chromatographic techniques, or modes, can be employed using the same column. These columns contain rigid macroporous hydrophobic resins covalently bonded to a hydrophilic organic layer. SEC, IEC, hydrophobic or affinity chromatography are some of the methods that may be utilized. By changing the the mobile phase, the mode of separation is thereby changed which allows the chromatographer to achieve the desired selectivity in the separations.

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