Stationary phase precautions

Other modes of LC operation include liquid-liquid partition chromatography (LLC) and bonded phase chromatography. In the former, a stationary liquid phase which is immiscible with the mobile phase is coated on a porous support, with separation based on partition equilibrium differences of components between the two liquid phases. This mode offers an alternative to ion exchange in the fractionation of polar, water soluble substances. While quite useful, the danger exists in LLC that the stationary phase can be stripped from the column, if proper precautions are not taken. Hence, it is typical to pre-equil-ibrate carefully the mobile and stationary phases and to use a forecolimn, heavily loaded with stationary phase 9). 

Many problems that arise in the application of adsorption chromatography can be related to the slow attainment of equilibrium distribution of the omnipresent water and other modulators tetween eluent and stationary phase. With suitable precautions, such as moisture control, reproducible work is possible with both silica and alumina as the stationary phase. 

Nevertheless, the same precautions should be taken as those advised for the FED. If used with silicone stationary phases, or for the separation of silyl derivatives, the electrodes and jet will also become covered with silica and must be cleaned. In general, problems with detector electronics are very rare indeed. Contemporary detector electronics are almost exclusively solid state and consequently, if the... 

Analogous to HPLC, CCC fractionation permits the use of either stepwise elution or gradient elution, provided that some precautions are taken. Eor example, in the isolation of extracts of Ginkgo biloba, one starts with water as a stationary phase, eluting with ethyl acetate with... 

CE techniques have great advantages over conventional chromatographic methods for characterization of polyelectrolytes, because in the pure aqueous system unwanted interactions with a stationary phase are excluded. However, strongly basic polyelectrolytes may adhere to the capillary surface, leading to peak distortion. In this case similar precautions have to be taken as in the separation of proteins and DNA, i.e. use of hydrophilically coated capillaries. 

The fact that particles of 5 jam and less, as used in HPLC, may pass into the lungs (they are not retained by the bronchial tubes but pass straight through) is less well known and the potential long-term risk to health has not yet been adequately researched. Amorphous silica, as used for stationary phases, is not hazardous but inhalation should be avoided anyway. As a safety precaution, any operation involving possible escape of stationary phase dust (opening phials, weighing etc.) must be carried out in a fume cupboard. 

The need for elaborate precautions for saturation of the paper with water (or other stationary phase), the mobile phase with stationary phase, and the chamber with the vapors of both phases should be evaluated in each case to be sure they lead to improve reproducibility. For example, Cassidy [38] has shown that even after taking great pains to achieve maximum equilibration, the mobile phase composition varies along the paper, sometimes leading to a double solvent front. Especially with low volatility solvents and ascending development, prior saturation or equilibration is usually not now used for normal analytical work. 

Metal contamination of the matrix of the silica, especially by aluminum and iron, also increases the acidity of surface silanols and the heterogeneity of the surface. This is a problem that largely plagues older stationary phases, which are based on silicas derived from inorganic raw materials. Many modem silicas are manufactured from organic silanes. In such processes, a high purity of the silica can be maintained with the appropriate precautions. In turn, this results in reversed-phase bonded phases with superior behavior toward basic compounds, that is, without excessive retention or taOing. 

The general problem in all chromatographic separations is the selection of conditions such that the distribution ratios of the solutes to be separated differ as widely as possible. With chromatographic columns of practical dimensions, having up to 500 equivalent theoretical plates, differences in distribution ratio of at least 10% are desirable for adequate separations (12). In addition, the maximum amount of solute retained by the resin stationary phase should not exceed about 2% of the maximum exchange capacity, in order to obtain linear exchange isotherms. This precaution also ensures the maximum rate of solute exchange for a particular solute-resin system. 

Finally, analytes can also be enriched on a stationary phase in the PTV insert after the evaporation of the water in the PTV [80-82], Also in this mode, Tenax appeared to be the best choice [83]. However, sampling took a rather long time due to the low evaporation rate of water (approximately 10 Xl/min) [81], and the insert had to be exchanged rather often, because deposition of matrix constituents such as salts in the liner caused the decomposition of chemically less stable compounds [82], If proper precautions were taken, analyte recoveries were, however, fully satisfactory (80 to 104%), and detection limits of 10 to 20 ng/L were obtained (0.5-ml samples NPD detection). Actually, the procedure seems to be interesting mainly for compounds with high water solubility. 

If the sample contains very polar constituents, it is advisaUe to remove them through sample preparation techniques like solid-phase extraction. Samples dissolved in water should not be injected directly onto a normal-phase column. When an aminopropyl column is used, additional precautions are necessary, which were outlined above when we discussed this stationary ffiase. 

---