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They are available in different sizes, diameters, pore sizes, or they can have special materials attached. Packings available range from those needed for specific applications to those for allpurpose applications.The packings are attached to the internal column hull by resins or supports, which include oxides, polymers, carbon, hydroxyapatite beads, and silica, the most common type.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 noncovalent 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 <a href="http://www.targetmol.com/compound/Halobetasol-Propionate">buy Halobetasol Propionate</a> 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.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 and it involves a lot of try and error approach based on knowledge of compound to be analysed.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.At the onset of sample introduction, the compounds are initially retained at the inlet of the column.As the solute capacity, or k, for the compound decreases, the compound begins to migrate through the stationary phase.Each of the other compounds in the sample subsequently migrate as their k values decrease.It is positioned immediately posterior to the stationary phase in order to detect the compounds as they elute from the column.The bandwidth and height of the peaks may usually be adjusted using the coarse and fine tuning controls, and the detection and sensitivity parameters may also be controlled. There are many types of detectors that can be used with HPLC.This property for each molecule or compound is called its refractive index.For most RI detectors, light proceeds through a bimodular flowcell to a photodetector.One channel of the flowcell directs the mobile phase passing through the column while the other directs only the mobile phase.Detection occurs when the light is bent due to samples eluting from the column, and   this is read as a disparity between the two channels.Refractive index detectors are used to detect nonUV absorbing compounds, but they are less sensitive than UV detectors.

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