Detectors compound selectivity

These are by far the most popular detectors in hplc. The principle is that the mobile phase from the column is passed through a small flow cell held in the radiation beam of a uv/visible photometer or spectrophotometer. These detectors are selective in the sense that they will detect only those solutes that absorb uv (or visible) radiation. Such solutes include alkenes, aromatics and compounds having multiple bonds between C and O, N or S. The mobile phase we use, on the other hand, should absorb little or no radiation. 

The detectors described thus far do not give any information as to the nature of the compounds that are eluting from the column. They are, at best, selective. With these detectors, compound identification has to proceed with the use of internal calibration based on retention times. When the chromatogram is very complex, some confusion can occur. Because of these limitations, other detectors have been developed that can provide structural information based on spectroscopic data. In this case, one can use retention times and a specific characteristic for each compound to identify the components of a sample. These detectors lead to stand-alone analysis techniques for which the results depend only on the proper separation of the compounds eluting from the column. 

Flame ionization detector non-selective, responds to virtually all organic compounds with the carbon-hydrogen bonds... 

Electrolytic conductivity detector f KLCD) and electron capture detector (ECD) selective to chlorinated and oxygenated compounds... 

Nitrogen-phosphorus detector (NPD) selective to compounds containing nitrogen and phosphorus... 

Second detector confirmation is another compound confirmation technique. Two detectors with selectivity to different functional groups are connected in series to one column or in parallel to two columns. For example, two detectors, a UV/VIS detector and a fluorometer, connected in series to the HPLC column are used in the EPA Method 8310 for analysis of PAH compounds. If the second detector is connected to a second column of a dissimilar polarity, then the confirmation becomes even more reliable. An example of such a configuration is organophosphorus pesticides analysis the samples may be initially analyzed with an NPD, and then confirmed on a different column with an ECD or a FPD. 

An example of the use of the variable wavelength UV detector to select a specific wavelength to give a high sensitivity for a certain group of compounds is afforded by the separation of some carboxylic acids that is monitored by UV absorption at 210 nm. The separation is shown in figure 6. The separation of a series of common fatty acids was carried out on a reversed phase column using water buffered with phosphoric acid as the mobile phase. 

When large samples are used in isothermal analysis using PEG, water forms a chromatographic zone in the form of a step, which simplifies calibration and improves the accuracy [66]. This method permits the determination of water at concentrations of 0.2—1.2% (w/w) using a thermal conductivity cell. The application of reaction methods for the conversion of water in organic compounds using a flame-ionization detector or selective electrochemical detectors that are highly sensitive to water will undoubtedly enable the detection limit to be lowered. 

Using the information in Sections 13.1.4 and 13.1.6, construct a table contrasting the properties of various LC and CE detectors, indicating relative sensitivity, compound selectivity, gradient compatibility, buffer salt and pH compatibility, ability to extract analyte structural information, and mass or concentration mode of detection. 

The most frequently used detection system for the analysis of PCBs in various matrices is the ECD. This detector is selective for halogenated compounds. The very high sensitivity of ECD for PCBs theoretically allows detection limits below Ingkg. Its extreme sensitivity, on the other hand, makes ECD vulnerable to dirt and overloading. The ECD response varies from one detector to another. The limited linearity is a particular problem. Although Hnear ranges of several decades are often claimed by manufacturers, in practice the linear range of all ECDs is substantially lower than one decade. Consequently, the use of multilevel calibrations is essential. 

---