Chromatography liquid, types

Analytical separations may be classified in three ways by the physical state of the mobile phase and stationary phase by the method of contact between the mobile phase and stationary phase or by the chemical or physical mechanism responsible for separating the sample s constituents. The mobile phase is usually a liquid or a gas, and the stationary phase, when present, is a solid or a liquid film coated on a solid surface. Chromatographic techniques are often named by listing the type of mobile phase, followed by the type of stationary phase. Thus, in gas-liquid chromatography the mobile phase is a gas and the stationary phase is a liquid. If only one phase is indicated, as in gas chromatography, it is assumed to be the mobile phase. 

Despite their importance, gas chromatography and liquid chromatography cannot be used to separate and analyze all types of samples. Gas chromatography, particularly when using capillary columns, provides for rapid separations with excellent resolution. Its application, however, is limited to volatile analytes or those analytes that can be made volatile by a suitable derivatization. Liquid chromatography can be used to separate a wider array of solutes however, the most commonly used detectors (UV, fluorescence, and electrochemical) do not respond as universally as the flame ionization detector commonly used in gas chromatography. 

The second set of experiments describes the application of high-performance liquid chromatography. These experiments encompass a variety of different types of samples and a variety of common detectors. 

In contrast to vapour phase chromatography, the mobile phase in liquid chromatography is a liquid. In general, there are four main types of liquid chromatography adsorption, partition, ion-chromatography, and gel filtration. 

Ion-exchange chromatography involves an electrostatic process which depends on the relative affinities of various types of ions for an immobilised assembly of ions of opposite charge. The stationary phase is an aqueous buffer with a fixed pH or an aqueous mixture of buffers in which the pH is continuously increased or decreased as the separation may require. This form of liquid chromatography can also be performed at high inlet pressures of liquid with increased column performances. 

Most high-performance liquid chromatography (HPLC) pumps can be used in HOPC. The back pressure rating should be at least several thousand pounds per square inch (a few hundred kg/cm ). A type of pump that does not allow bypassing the pressure transducer or a pulse damper, if it is installed, must not be used. The dead volume should be as small as possible. Pumps with a single plunger are better than those with two plungers. 

Coupled liquid chromatography-gas chromatography is an excellent on-line method for sample enrichment and sample clean-up. Recently, many authors have reviewed in some detail the various LC-GC transfer methods that are now available (1, 43-52). For the analysis of normal phase eluents, the main transfer technique used is, without doubt, concurrent eluent evaporation employing a loop-type interface. The main disadvantage of this technique is co-evaporation of the solute with the solvent. 

One of the first examples of the application of reverse-phase liquid chromatography-gas chromatography for this type of analysis was applied to atrazine (98). This method used a loop-type interface. The mobile phase was the most important parameter because retention in the LC column must be sufficient (there must be a high percentage of water), although a low percentage of water is only possible when the loop-type interface is used to transfer the LC fraction. The authors solved this problem by using methanol/water (60 40) with 5% 1-propanol and a precolumn. The experimental conditions employed are shown in Table 13.2. 

The principle was demonstrated using triazine herbicides as templates and by varying the type of functional monomer and the monomer composition. With a final batch size of ca. 40 mg of monomer, the consumption of monomers and template is significantly reduced and the synthesis and evaluation can take place in standard high-performance liquid chromatography (UPLC) autosample vials. After synthesis. 

New types of ion exchange resins have also been developed to meet the specific needs of high-performance liquid chromatography (HPLC) (Chapter 8). These include pellicular resins and microparticle packings (e.g. the Aminex-type resins produced by Bio-Rad). A review of the care, use and application of the various ion exchange packings available for HPLC is given in Ref. 19. 

The present chapter is largely concerned with HPLC, together with a summary of developments in quantitative thin-layer chromatography, but a brief account of the various types of liquid chromatography is given first together with a guide to the choice of appropriate separation mode. 

There are four main types of liquid chromatography which require discussion. 

The development of bonded phases (Section 8.2) for liquid-liquid chromatography on silica-gel columns is of major importance. For example, the widely used C-18 type permits the separation of moderately polar mixtures and is used for the analysis of pharmaceuticals, drugs and pesticides. 

Various types of derivatisation have now been developed for both gas and liquid chromatography. For more detailed information regarding the choice of a suitable derivative for a particular analytical problem, the appropriate works of reference should be consulted.62,63... 

High performance liquid chromatography is an effective technique for the separation of compounds of high molecular weight. There are, however, two major problems with the use of mass spectrometry for the study of this type of molecule... 

The use of high performance liquid chromatography (HPLC) for the study of paralytic shellfish poisoning (PSP) has facilitated a greater understanding of the biochemistry and chemistry of the toxins involved. HPLC enables the determination of the type and quantity of the PSP toxins present in biological samples. An overview of the HPLC method is presented that outlines the conditions for both separation and detection of the PSP toxins. Examples of the use of the HPLC method in toxin research are reviewed, including its use in the determination of the enzymatic conversion of the toxins and studies on the movement of the toxins up the marine food chain. 

---