HPLC instrumentation conductivity detector

Ion-exchange columns can be substituted into the general HPLC instrument shown in Eigure 12.26. The most common detector measures the conductivity of the mobile phase as it elutes from the column. The high concentration of electrolyte in the mobile phase is a problem, however, because the mobile-phase ions dominate the conductivity, for example, if a dilute solution of HCl is used as the mobile phase, the presence of large concentrations of H3O+ and Ck produces a background conductivity that may prevent the detection of analytes eluting from the column. 

For many years, the concept of the conductivity detector could not work, however. Ion chromatography experiments utilize solutions of high ion concentrations as the mobile phase. Thus, changes in conductivity due to eluting ions are not detectable above the already high conductivity of the mobile phase. This was true until the invention of so-called ion suppressors. Today, conductivity detectors are used extensively in HPLC ion chromatography instruments that also include suppressors. 

The methyl esters of herbicides are analyzed by GC using an electron capture detector. Microcoulometric detector or electrolytic conductivity detector may alternatively be used. GC/MS, if available, should be employed to confirm the presence of analytes. Other instrumental techniques include GC-FID and HPLC. The latter can measure the acids for which esterification is not required. 

Very often baseline problems are related to detector problems. Many detectors are available for HPLC systems. The most common are fixed and variable wavelength ultraviolet spectrophotometers, refractive index, and conductivity detectors. Electrochemical and fluorescence detectors are less frequently used, as they are more selective. Detector problems fall into two categories electrical and mechanical/optical. The instrument manufacturer should correct electrical problems. Mechanical or optical problems can usually be traced to the flow cell however, improvements in detector cell technology have made them more durable and easier to use. Detector-related problems include leaks, air bubbles, and cell contamination. These usually produce spikes or baseline noise on the chromatograms or decreased sensitivity. Some cells, especially those used in refractive index detectors, are sensitive to flow and pressure variations. Flow rates or backpressures that exceed the manufacturer s recommendation will break the cell window. Old or defective source lamps, as well as incorrect detector rise time, gain, or attenuation settings will reduce sensitivity and peak height. Faulty or reversed cable connections can also be the source of problems. 

The aldehyde functional groups can be tested by classical wet methods. Individual aldehydes can be analyzed by various instrumental techniques such as GC, HPLC, GC/MS, colorimetry, polarography, and FTIR. Of these, GC, GC/MS, and HPLC are referred to here because of their versatility and wide application. Although an FID is commonly employed in GC, a thermal conductivity detector can also be suitable, especially for lower aliphatic aldehydes. The advantage of FID is that aqueous samples can be injected straight into the column. 

After optimization of the correct capillary parameters (ID, OD, Lj), detection at the microscale level became the next major challenge for the survival of CE. Despite the challenges, many of the common HPLC detectors have a CE complement, e.g., absorbance, fluorescence, conductivity, photodiode array, and mass spectroscopy. Small dimensions mean universal detectors such as refractive index cannot be used. A sample of detectors will be discussed. The technical aspects of each detector will not be covered except in relation to the CE instrument. Readers are advised to consult an instrumentation textbook for more details on theory of operation. 

Shimadzu has produced the Prominence HPLC, Agilent has developed the 1200 Rapid Resolution HPLC (Figure 3.22), Thermo Scientific has the Accela LC system and Waters has brought out the Acuity UPLC. Jasco s X-LC system allows two systems to fit in the footprint previously occupied by one traditional system. It is also a modular system of detectors and autosamplers, which allows it to be customised. LC Packings has launched the UltiMate 3000, which is a nanoflow LC system for use with columns of 50 im and larger. Modular systems are common now, e.g. Cecil Instruments produces both HPLC and ion chromatography systems and various detectors can be accommodated including UV-Vis, refractive index, conductivity and fluorescence. 

UV detection is used in most chiral analysis by HPLC and other liquid chromatographic modalities. However, some other detectors, such as conductivity, fluorescent and refractive index types, are also used. The choice of detector depends on the properties of the racemic compound to be resolved [41, 144]. Chiroptical detectors, which are based on the principle of polarimetry [145] or circular dichroism [146, 147], are also available. The enantiomer (+)- or (—)-notation is determined by these detectors. Some organochlorine pesticides are not UV-sensitive, and hence they are difficult to detect in liquid chromatography. The detection of these types of pollutant can be achieved by using a mass spectrometry (MS) detector, and therefore LC-MS instruments are now being put on the market for routine use [148, 149]. 

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