Chromatography injection devices

The injection device is also an important component in the LC system and has been discussed elsewhere (2,18). One type of injector is analogous to sample delivery in gas chromatography, namely syringe injection through a self-sealing septum. While this injection procedure can lead to good column efficiency, it generally is pressure limited, and the septum material can be attacked by the mobile phase solvent. 

In liquid chromatography, liquid samples may be injected directly and solid samples need only to be dissolved in an appropriate solvent. The solvent need not to be the mobile phase, but frequently it is judiciously chosen to avoid detector interference, column/component interference or loss in efficiency. It is always best to remove particles from the sample by filtering, or centrifuging since continuous injection of particulate materials will eventually cause blockage of injection devices or columns. 

A saturation column situated between the pump and the injection device may be installed for two reasons. In a liquid-liquid partition system a saturation column containing a large amount of stationary phase on a suitable solid support may be used in order to ensure a proper equilibrium between the two phases. In other systems a saturation column containing bare silica may be installed in order to prevent dissolution of silica from the analytical column. This is an advantage even if the analytical colunm contains chemically modified silica for reversed phase chromatography. 

Cryogenic traps are convenient accumulation and injection devices for fast gas chromatography and interfaces for coupled-column gas chromatography, where a heartcut sample is collected and focused from the first column, and reinjected into the second column. The main requirement for a cryogenic trap used in these applications is efficient accumulation over time with rapid injection of the collected analytes as a narrow pulse in both time and space. Commercially available systems using a capacitance discharge for heating provide injection bandwidths of 5-20 ms. 

They were initially developed over 15 years ago and found some success in certain niche applications that could not be adequately addressed by other nebulization systans, such as introducing samples from a chromatography separation device into an ICP-MS or the determination of mercury by ICP-MS, which is prone to severe memory effects. Unfortunately, they were not considered particularly user-friendly, and as a result became less popular when other sample introduction devices were developed to handle microUter sample volumes. More recently, a refinement of the direct injection nebulizer has been developed, called the direct inject high-efficiency nebulizer (DIHEN), which appears to have overcome many of the limitations of the original design. The advantage of the DIHEN is its ability to introduce microliter volumes into the plasma at extremely low sample flow rates (1-100 pL/min), with an aerosol droplet size similar to a concentric nebnlizer fitted with a spray chamber. 

Another recent development is the advent of pulse amperometry in which the potential is repeatedly pulsed between two (or more) values. The current at each potential or the difference between these two currents ( differential pulse amperometry ) can be used to advantage for a number of applications. Similar advantages can result from the simultaneous monitoring of two (or more) electrodes poised at different potentials. In the remainder of this chapter it will be shown how the basic concepts of amperometry can be applied to various liquid chromatography detectors. There is not one universal electrochemical detector for liquid chromatography, but, rather, a family of different devices that have advantages for particular applications. Electrochemical detection has also been employed with flow injection analysis (where there is no chromatographic separation), in capillary electrophoresis, and in continuous-flow sensors. 

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