LC and IC analyses enable us to analyze for compounds that are not readily determined by GC such as some pesticides, explosives, natural products, pharmaceuticals, and many others. Introduction Chromatography is the science of separating mixtures of compounds, elements, or ions. The separation of these mixtures into components occurs as the result of the components having different partition ratios between the mobile phase (gas or liquid) and the stationary phase (column), and therefore different rates of travel through the column. Chromatography comes in a variety of forms and applications, but all forms of liquid chromatography (LC) use a liquid as the mobile phase to carry the components of the mixture through the column. In gas chromatography (GC) a gas is used as the mobile phase, but because of the heated injectors and ovens in GC, components must be thermally stable and have reasonable volatility (boiling points below 400-500oC). LC is usually carried out at or near room temperature and can be used for thermally labile and nonvolatile components. LC only requires that the component be slightly soluble in the liquid phase. GC has the advantages of higher resolving power with capillary columns and the availability of a large variety of sensitive and specific detectors. For sensitive detectors, LC and IC are limited to UV/VIS, fluorescence, evaporative light scattering, electrochemical, and conductivity. Generally GC is the method of choice if it can be successfully applied, but there are some separations that can only be done by LC. HPLC / IC Lab
Forms of LC and IC Reverse Phase (RP) LC is the most common form of LC and is used for lipophilic organic compounds. Normal Phase LC is used where RP does not adequately separate components. RP LC is used at WCAS for the types of analytes listed on the front. The use of ion exchange resins as the stationary phase in columns is the basis of Ion Chromatography (IC) for separating ionic components. WCAS uses IC for the analytes listed on the front. Ion Pair Chromatography (IPC) is a cross between IC and RP LC, and is used for ions which can be complexed with a compound having a lipophilic functional group. Usually the complexing agent has been added to the mobile phase, and the mixture is chromatographed on a RP column. We have used IPC to chromatograph various arsenic species, aminoethanol compounds, and other analytes. Gel Permeation (or Size Exclusion) chromatography (GPC) is used to separate components based on their molecular size and is usually applied to large molecules such as polymers and biomolecules. At WCAS we use GPC for separating smaller analyte components from larger biomolecules (fats, etc.). Detectors Because of the limited number of LC detectors, post column reactions have been developed to extend the sensitivity and selectivity of some applications. Reagents added to the eluant after the column can form highly UV/VIS absorbent or fluorescent products from certain analytes which would otherwise not be detectable. Post column reactors are used in the determination of N-methylcarbamates, hexavalent chromium, glyphosate, etc. Just as Gas Chromatography-Mass Spectrometry (GCMS) is such a powerful tool, the coupling of mass spectrometry (MS) to LC has also proven to be very useful. A mass spectrometer can be both a universal as well as a very selective detector in addition to providing information which will help determine the molecular structure of an unknown component.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||
