Chromatographic analysis mobile phases

Gas chromatography is useful in the pharmaceutical, food, environmental, and petrochemical fields as well as others. It is used for the separation of volatile and semi-volatile compounds. In gas chromatography, the compounds of interest are in a gaseous state or can be vaporized upon their introduction in the gas chromatograph. The mobile phase is a gas the most commonly used are helium (He), hydrogen (H2), argon (Ar) and nitrogen (N2). We call the mobile phase the carrier gas. The choice of carrier gas depends on the detector (detectors will be discussed later), the separation efficiency, and the speed of analysis. 

In the analytical chromatographic process, mixtures are separated either as individual components or as classes of similar materials. The mixture to be separated is first placed in solution, then transferred to the mobile phase to move through the chromatographic system. In some cases, irreversible interaction with the column leaves material permanently attached to the stationary phase. This process has two effects because the material is permanently attached to the stationary phase, it is never detected as leaving the column and the analysis of the mixture is incomplete additionally, the adsorption of material on the stationary phase alters the abiHty of that phase to be used in future experiments. Thus it is extremely important to determine the ultimate fate of known materials when used in a chromatographic system and to develop a feeling for the kinds of materials in an unknown mixture before use of a chromatograph. 

This type of chromatographic development will only be briefly described as it is rarely used and probably is of academic interest only. This method of development can only be effectively employed in a column distribution system. The sample is fed continuously onto the column, usually as a dilute solution in the mobile phase. This is in contrast to displacement development and elution development, where discrete samples are placed on the system and the separation is subsequently processed. Frontal analysis only separates part of the first compound in a relatively pure state, each subsequent component being mixed with those previously eluted. Consider a three component mixture, containing solutes (A), (B) and (C) as a dilute solution in the mobile phase that is fed continuously onto a column. The first component to elute, (A), will be that solute held least strongly in the stationary phase. Then the... 

The great leap forward for chromatography was the seminal work of Martin and Synge (7) who in 1941 replaced countercurrent liquid-liquid extraction by partition chromatography for the analysis of amino acids from wool. Martin also realized that the mobile phase could be a gas rather than a liquid, and with James first developed (8) gas chromatography (GC) in 1951, following the gas-phase adsorption-chromatographic separations of Phillips (9). 

Figure 15.4 Separation of mixtures of beta-blockers by using micellar HPLC, employing the following mobile phases (a) 0.12M SDS, 5% propanol, 0.5% tiiethylamine (b) 0.06 M SDS, 15% propanol (c) 0.1 IM SDS, 8% propanol. Adapted from Journal of Chromatographic Science, 37, S. Carda-Broch et al., Analysis of urine samples containing cardiovascular drugs by micellor liquid chromatography with fluorimetric detection , pp. 93-102, 1999, with permission from Preston Publications, a division of Preston Industries, Inc.

The use of both sub- and supercritical fluids as eluents yields mobile phases with increased diffusivity and decreased viscosity relative to liquid eluents [23]. These properties enhance chromatographic efficiency and improve resolution. Higher efficiency in SFC shifts the optimum flowrate to higher values so that analysis time can be reduced without compromising resolution [12]. The low viscosity of the eluent also reduces the pressure-drop across the chromatographic column and facilitates the... 

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

For the same adsorbent, different mobile phases can be used according to the aim of chromatographic analysis. Rather than preparing an endless line of chromatographic plates of different thickness, it is easier to change the mobile phase up to the most convenient composition, keeping the same characteristics of the stationary phase. In the case of a hygroscopic adsorbent, the adsorbed water influences its activity. 

In situ densitometry has been the most preferred method for quantitative analysis of substances. The important applications of densitometry in inorganic PLC include the determination of boron in water and soil samples [38], N03 and FefCNfg in molasses [56], Se in food and biological samples [28,30], rare earths in lanthanum, glass, and monazite sand [22], Mg in aluminum alloys [57], metallic complexes in ground water and electroplating waste water [58], and the bromate ion in bread [59]. TLC in combination with in situ fluorometry has been used for the isolation and determination of zirconium in bauxite and almnimun alloys [34]. The chromatographic system was silica gel as the stationary phase and butanol + methanol + HCl -H water -n HF (30 15 30 10 7) as the mobile phase. 

A major disadvantage of gradient elution in terms of fast analysis remains the time to adequately equilibrate the chromatographic column between two experiments. However, Carr et al. recently demonstrated an excellent repeatability ( 0.002 min in retention time) obtained with two column volumes of re-equilibration instead of the usual 10 column volumes when a small amount of ancillary solvent (1-3% of 1-butanol or 1-propanol) is added to the mobile phase [45, 58]. 

Nova-Pak C18 column in a methanol water chloroform gradient.92 Choline chloride was added to the mobile phase. One review of techniques used in the analysis of triacylglycerols lists over 300 references on separations of the triglyceride fraction of fats using nonaqueous RPLC, aqueous RPLC, argen-tation chromatography, and other chromatographic methods.93... 

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