Stationary phase choice

This derivation shows that retention time is dependant on three factors temperature, energies of intermolecular interactions and flow rate. Temperature and flow rate are controlled by the user. Energies of intermolecular interactions are controlled by stationary phase choice. This theory is also the basis for the popular software programs that are available for computer-assisted method development and optimization [4,5,6,7]. More detailed descriptions of the theory behind retention times can be found in the appropriate chapters in the texts listed in the bibliography. 

Stationary phase Choice of C-18, C-8, phenyl, cyano or trimethyl has varying impact, depending on type of sample ... 

In MPLC, the columns are generally filled by the user. Particle sizes of 25 to 200 pm are usually advocated (15 to 25, 25 to 40, or 43 to 60 pm are the most common ranges) and either slurry packing or dry packing is possible. Resolution is increased for a long column of small internal diameter when compared with a shorter column of larger internal diameter (with the same amount of stationary phase).Choice of solvent systems can be efficiently performed by TLC or by analytical HPLC. Transposition to MPLC is straightforward and direct. 

Modification of the reversed-phase stationary phase choice of the ion-interaction reagent detection. 190... 

Solvent and stationary phase choice are flexible i.e., the solvent system can be changed quickly during a run. 

The most ubiquitous type of stationary phase used within liquid chromatography is of a reversed-phase (RP) nature, with almost 700 differing types being commercially available worldwide. Hence, experienced and novice chromato-graphers alike are regularly faced with a bewildering choice of stationary phases for any particular application. The situation is further complicated by the many claims and counter claims made by manufacturers. To date, very little work has been done to compare all these stationary phases and to standardize the approach to stationary phase choice. Unfortunately, stationary phase selection appears to be the weak link in the method development cycle for many chromatographers. 

The most common mobile phases for GC are He, Ar, and N2, which have the advantage of being chemically inert toward both the sample and the stationary phase. The choice of which carrier gas to use is often determined by the instrument s detector. With packed columns the mobile-phase velocity is usually within the range of 25-150 mF/min, whereas flow rates for capillary columns are 1-25 mF/min. Actual flow rates are determined with a flow meter placed at the column outlet. 

The Diacel columns can be used for the separation of a wide variety of compounds, including aromatic hydrocarbons having hydroxyl groups, carbonyls and sulfoxides, barbiturates, and P-blockers (35,36). There are presendy nine different cellulose derivative-based columns produced by Diacel Chemical Industries. The different columns each demonstrate unique selectivities so that a choice of stationary phases is available to accomplish a separation. 

Giddings made a stalwart effort to provide values for the different constants that would apply to diverse stationary phase and support conditions [2]. However, at best, his values are the closest estimates from an assumed set of conditions that may fit, to a greater or lesser extent, the properties of the actual stationary phase or support in use. In some cases, his constants may be used in column design and to help in the choice of those operating conditions that will provide the required... 

The analyst must remember that solubility of a polymer in the chosen eluant is a necessary, but not sufficient, requirement for ideal GPC separations. Once injected on the column, the polymer has a choice of partitioning onto the stationary phase or remaining in the solvent. It is imperative that the analyst choose solvent and column conditions such that the ideal, nonadsorptive, GPC mechanism can occur. 

YHiile THF is the solvent of choice for ordinary acrylate ester polymers, there are numerous monomers that can be incorporated into those acrylic polymers that cause problems either with solubility or with adsorption onto the stationary phase. In some cases, these problems can be overcome by switching to a solvent other than THF. 

HPLC separations are one of the most important fields in the preparative resolution of enantiomers. The instrumentation improvements and the increasing choice of commercially available chiral stationary phases (CSPs) are some of the main reasons for the present significance of chromatographic resolutions at large-scale by HPLC. Proof of this interest can be seen in several reviews, and many chapters have in the past few years dealt with preparative applications of HPLC in the resolution of chiral compounds [19-23]. However, liquid chromatography has the attribute of being a batch technique and therefore is not totally convenient for production-scale, where continuous techniques are preferred by far. 

Despite the difficulties caused by the rapidly expanding literature, the use of chiral stationary phases (CSPs) as the method of choice for analysis or preparation of enantiomers is today well established and has become almost routine. It results from the development of chiral chromatographic methods that more than 1000 chiral stationary phases exemplified by several thousands of enantiomer separations have been described for high-performance liquid chromatography (HPLC). 

At the current time, there is considerable interest in the preparative applications of liquid chromatography. In order to enhance the chromatographic process, attention is now focused on the choice of the operating mode [22]. SMB offers an alternative to classical processes (batch elution chromatography) in order to minimize solvent consumption and to maximize productivity where expensive stationary phases are used. 

The versatility of chiral stationary phases and its effecitve application in both analytical and large-scale enantioseparation has been discussed in the earlier book A Practical Approach to Chiral Separation by Liquid Chromatography" (Ed. G. Sub-ramanian, VCH 1994). This book aims to bring to the forefront the current development and sucessful application chiral separation techniques, thereby providing an insight to researchers, analytical and industrial chemists, allowing a choice of methodology from the entire spectrum of available techniques. 

High-performance liquid chromatography is in some respects more versatile than gas chromatography since (a) it is not limited to volatile and thermally stable samples, and (b) the choice of mobile and stationary phases is wider. 

Unfortunately, exclusion chromatography has some inherent disadvantages that make its selection as the separation method of choice a little difficult. Although the separation is based on molecular size, which might be considered an ideal rationale, the total separation must be contained in the pore volume of the stationary phase. That is to say all the solutes must be eluted between the excluded volume and the dead volume, which is approximately half the column dead volume. In a 25 cm long, 4.6 mm i.d. column packed with silica gel, this means that all the solutes must be eluted in about 2 ml of mobile phase. It follows, that to achieve a reasonable separation of a multi-component mixture, the peaks must be very narrow and each occupy only a few microliters of mobile phase. Scott and Kucera (9) constructed a column 14 meters long and 1 mm i.d. packed with 5ja... 

It is clear that the first challenge facing the analyst is the choice of the phase system that is appropriate for the particular sample to be analyzed. Only after the phase system has been chosen, can the correct column be selected. It is therefore necessary to know the types and properties of the different stationary phases that are available and how to formulate the pertinent mobile phases that must be used with them. 

Returning now to the subject of the chapter, in addition to appropriate retentive characteristics, a potential stationary phase must have other key physical characteristics before it can be considered suitable for use in LC. It is extremely important that the stationary phase is completely insoluble (or virtually so) in all solvents that are likely to be used as a mobile phase. Furthermore, it must be insensitive to changes in pH and be capable of assuming the range of interactive characteristics that are necessary for the retention of all types of solutes. In addition, the material must be available as solid particles a few microns in diameter, so that it can be packed into a column and at the same time be mechanically strong enough to sustain bed pressures of 6,000 p.s.i. or more. It is clear that the need for versatile interactive characteristics, virtually universal solvent insolubility together with other critical physical characteristics severely restricts the choice of materials suitable for LC stationary phases. 

Nevertheless, silica gel is the material of choice for the production of the vast majority of LC stationary phases. Due to the reactive character of the hydroxyl groups on the surface of silica gel, various organic groups can be bonded to the surface using standard silicon chemistry. Consequently, the silica gel surface can be modified to encompass the complete range of interactive properties necessary for LC ranging from the highly polar to almost completely dispersive. 

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