Mobile phase solvent systems

To experimentally determine a PC mobile phase for an unknown, a number of solvents from the middle part of the elutropic series (see TLC Section 5.2.3.3) are tried, after equilibrating the paper with water vapor. If the Rp value is too high, the mobile phase is too polar relative to the solutes and a solvent closer to the top of the table should be used. If the Rp value is too low, the solvent is too hydrophobic compared to the solute, and a more polar solvent is needed. The R p region between 

When a solvent is found that gives Rp values for the solutes near the middle of the paper but separation is not adequate, other single solvents or solvent mixtures that maintain a similar elution strength (polarity) should be tried until one that provides better resolution is found. For example, if benzene provides Rp values that are slightly too low, the more-polar solvent chloroform can be tried, or a small amount of ethanol or methanol can be added to benzene to increase polarity. In either case, different interactions will be involved in the chromatographic system due to the new solvent, and resolutions may be improved. Solvents can conveniently be 

It is necessary to use very pure solvents for preparation of mobile phases, since the presence of impurities can drastically alter Rp values or interfere with color development. For example, the polarities of absolute chloroform and chloroform with 2% ethanol stabilizer are very different. Herein lies the cause of many of the problems in reproducibility of countless methods that are described in the literature. 

A brief description of the six most popular types of solvent systems for paper chromatography follows. 

Reversed-phase systems are for separation of lipophilic compounds such as fatty acids. The paper is rendered nonpolar by acetylation or impregnation with a 5-25% solution of paraffin oil or vaseline in benzene, ether, CHClj, etc. Development is with polar solvents such as various mixtures of acetic acid, methanol, acetone and water. Presumably, the paper acts as an inert support. 

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Opiates can be identified by thin-layer chromatography (TLC) with many different combinations of mobile-phase solvent systems and detection reagents. A solvent system prepared with ethyl ace-tate-methanol-ammonium hydroxide (85 10 5) is commonly used to resolve heroin, codeine, acetylco-deine, morphine, and acetylmorphine. After development, opiates can be visualized by spraying or dipping the chromatogram in Dragendorff, iodoplatinate, or Marquis reagent. 

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. 

Snyder and Soczewinski created and published, at the same time, another model called the S-S model describing the adsorption chromatographic process [19,61]. This model takes into account the role of the mobile phase in the chromatographic separation of the mixture. It assumes that in the chromatographic system the whole surface of the adsorbent is covered by a monolayer of adsorbed molecules of the mobile phase and of the solute and that the molecules of the mobile phase components occupy sites of identical size. It is supposed that under chromatographic process conditions the solute concentrations are very low, and the adsorption layer consists mainly of molecules of the mobile phase solvents. According to the S-S model, intermolecular interactions are reduced in the mobile phase but only for the... 

Coupling HPLC to a mass spectrometer is far more complicated than in a GC system because of the large amount of mobile phase solvent expanding into the system (see Table 1 for expansion volumes). Typical mobile phase flow rates for HPLC are 0.5-2 mL min which translates into gas flow rates of 100-3000 mL min . ... 

THF and methanol employed as organic modifiers of mobile phase provided a considerable difference in selectivity based on the polar interactions between solutes and the organic solvent molecules in the stationary phase. Acidic compounds, phenols and nitroaromatics, were preferentially retained in the THF-based mobile phase, whereas esters and ketones were preferentially retained in the methanol (a hydrogen-bond donor) containing mobile phase. The system presented here seems to be very practical because any laboratory possessing two sets of HPLC equipment and two C j g columns can attempt similar 2D HPLC by simply changing the mobile phase for the two dimensions. 

After a plate has been exposed to the mobile-phase solvent for the required time, the compounds present can be viewed by several methods. Polynuclear aromatic hydrocarbons, other compounds with conjugated systems, and compounds containing heteroatoms (nitrogen, oxygen, or sulfur) can be viewed with long-and short-wave ultraviolet light. The unaided eye can see other material, or the plates can be developed in iodine. Iodine has an affinity for most petroleum compounds, including the saturated hydrocarbons, and stains the compounds a reddish-brown color. 

Tertiary systems. With methanol/carbon dioxide mixtures the addition of even the most polar additives has only a small impact on the mobile phase solvent strength as measured with Nile Red. With TFA concentrations below 1 to 2 % in methanol, ternary mixtures of TFA/methanol/carbon dioxide produce the same apparent solvent strength as binary methanol/carbon dioxide mixtures. As much as 5 or 10 % TFA in methanol is required to noticeably increase the solvent strength of TFA/methanol/carbon dioxide mixtures above those for binary methanol/carbon dioxide mixtures, as shown in Figure 4. 

With the demonstration of supercritical fluid extraction, an obvious extension would be to extract or dissolve the compounds of interest into the supercritical fluid before analysis with SFC.(6) This would be analogous to the case in HPLC, where the mobile phase solvent is commonly used for dissolving the sample. The work described here will employ a system capable of extracting materials with a supercritical fluid and introducing a known volume of this extract onto the column for analysis via SFC. Detection of the separated materials will be by on-line UV spectroscopy and infrared spectrometry. The optimized SFE/SFC system has been used to study selected nonvolatile coal-derived products. The work reported here involved the aliphatic and aromatic hydrocarbon fractions from this residuum material. Residua at several times were taken from the reactor and examined which provided some insight into the effects of catalyst decay on the products produced in a pilot plant operation. 

Another feature of modern HPLC systems that makes them desirable for both analytical and preparative applications is the complex mobile-phase gradients that they are capable of producing. Many systems come equipped with a pump integrator or controller (computer) that allow a number of different mobile-phase solvents to be simultaneously mixed and delivered to the stationary phase. Since this process is automated, complex gradients used for a particular application are quite reproducible. 

The noise level of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g. the katherometer and the refractive index detector) are often measured under static conditions (i.e. no flow of mobile phase). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held responsible for the precise control of the mobile phase, beitmay a gas flow controller or a solvent pump. However, all mobile phase delivery systems show some variation in flow rates (and consequently pressure) and it is the responsibility of the detector manufacturer to design devices that are as insensitive to pressure and flow changes as possible. 

The constants a, y can be determined from three experimental values of retention factors, k, A/ and Two of these values can be. selected to represent the data in binary mobile phases with the concentrations

ternary mobile phase, A at X = 0 and Ai at = 1. Only one experimental value. A , should be determined experimentally in a single ternary mobile phase at a concentration ratio X,. From A], At and Aj the constants a, /3, y can be calculated using Eiqs. (1.27)-( 1.29) and introduced into Eq. (1.26) to make possible prediction of retention in temaiy normal-phase solvent systems ... 

CPCs for separation of proteins which are not soluble in the PEG-phosphate system. This two-phase solvent system consists of the PEG-rich upper phase and dex-tran-rich lower phase. The cross-axis CPC may be operated in four different elution modes PiHO, PuTO, PiTI, and PnHI. The parameters Pi and Pn indicate the direction of the planetary motion where Pj indicates counterclockwise and Pn clockwise when observed from the top of the centrifuge. H and T indicate the head-tail elution mode, and O and I the inward-outward elution mode along the holder axis. In mode I (inward), the mobile phase is eluted against the laterally acting centrifugal force, and in mode O (outward), this flow direction is reversed. These three parameters yield a total of four combinations for the left-handed coils. Among these elution modes, the inward-outward elution mode plays the most important role in the stationary-phase retention for the polymer-phase sys-... 

Early work involving high-temperature GPC of polyesters utilized solvents such as meta-cresol and or-tho-chlorophenol as mobile-phase solvents. These solvents are very viscous and require system temperatures of between 140°C and 150°C. Both solvents are also very dangerous and difficult to handle [8,9]. 

For thicker layers, the attainment of equilibrium in the gas-mobile phase-adsorbent system to avoid complicating effects (solvent demixing, preadsorption) is more difficult. The solutes migrate in a nonequili-brated layer with differentiated velocity — more rapidly in the surface layer (because of the evaporation of solvent) and less rapidly closer to the carrier plate. 

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