Mobile phases range

A variety of sorbents have been used as the stationary phase in TLC, including silica gel, cellulose, alumina, polyamides, ion exchangers, chemically modified silica gel, and mixed layers of two or more materials, coated on a suitable support. Currently in the pharmaceutical industry, commercially precoated high-performance TLC (HPTLC) plates with fine particle layers are commonly used for fast, efficient, and reproducible separations. The choices of mobile phase range from single component solvent systems to multiple-component solvent systems with the latter being most common. The majority of TLC applications are normal phase, which is also a complementary feature to HPLC that uses mostly reverse-phase columns. 

The separation factor was determined at various flow rates of the mobile phase ranging from 0.3 to 3 mL/min, i.e., the residence times ranged from 4.6 to 46 s in the BS A-multilayer-adsorbed porous hollow-fiber membrane. As a result, the separation factor was constant... 

Armstrong et al. [110) studied 19 sets, with two to five compounds in each set, of structural isomers (e.g., cresols and xylenes to methylindoles and prostaglandins) on a ) -cyclodextrin colunm (X = 254 nm or 280 nm). Good peak shapes and resolution were obtained for all sets using mobile phases ranging from 30/70 methanol/water (for cresols) to 90/10 methanol/water (for vitamin D2). It was noted that retention for disubstituted benzenes (e.g., the hydroxy/methyl, dimethyl, nitto/hydroxy, nitro/amine and amine/carboxy substituent pairs) usually decreased in the order para > ortho > meta on the ) -cyclodextrin column. 

Smith and Burr [126] studied the retention of 73 disubstituted benzenes (functional groups included alkyl, nitro, bromo, chloro, carboxyl, nitrile, amide, amino, hydroxyl, methoxy, and phenyl) on a C g column (A = 254nm) usmg a series of methanol/water (1.37 g NaH2P04 with 1.58 g Na2HP04 to pH 7) mobile phases. The V values for these compounds were tabulated for mobile phases ranging in composition from 40/60 to 80/20 methanol/buffer. 

In a similar fashion, the retention behavior of 23 tripeptide epimers (with the basic structure Z-Ala-X-Val-OMe, where X is either the d or l form of the inserted amino acid) was studied on a C g column (A = 220 nm) using a series of isocratic methanol/water mobile phases [471]. The k values for all the lll and ldl forms and the resulting a values are tabulated for each epimer. Reasonable elution times were achieved, with mobile phases ranging fixim 60% to 80% methanol. The LDL-epimers were consistently more retained than the LLL-epimers. The authors attribute this to a larger surface area presented to the packing material in the ldl conformation. 

Methanol/DCM mobile phases (ranging fiom 25 to 75% DCM) were used to characterize polystyrene polymers ranging in molecular weight liom 3600 to 2.7 X 10 on Ci8 columns with base silicas of varying pore diameter. Injections of 0.5 sample were used. This study was conducted to deconvolve adsorption from solubilization effects from size exclusion contributions to retention [750]. Peak retention shifted from elution with 65 to 72% DCM as the sample load increased from 0.5 to 80 pg injected. These effects were attributed to sample solubility effects. 

Vanadium(V) extracted from clam tissue was determined as its A -phenylbenzo-hydroxamic acid (PBHA) complex on a silica column (A = 430 nm) using a 97/3 chloroform/methanol (0.9 mM PBHA) mobile phase [789]. Absorption spectra are presented for the PBHA-V(V) complexes in chloroform/methanol mobile phases ranging from 5% to 30% methanol. Choice of mobile phase is important not only because of its effects on the spectral characteristics of the complex, but also because of its effects on complex stability. Linear concentration curves were obtained up to 200 ig/L, with detection limits reported as 8 pg/L. 

The retention behavior of flie 2 -deoxyribonucleosides (adenosine, cytidine, uridine, thymidine, guanosine) were studied on a C g column (2 = 254 nm) with a series of mobile phases ranging from 100% water to 12.5/87.5 acetonitrile/water... 

An optimization procedure based on the Geiss (1987) structural approach uses a Vario KS chamber (Chapter 7) with three strong solvents, methyl-r-butyl ether, acetonitrile, and methanol, that are diluted with a weak solvent such as 1,2-dichloroethane to produce a series of mobile phases ranging in e values from 0.0 to 0.70 in 0.05 increments. Once the appropriate solvent strength is determined, the separation is fine-tuned by blending solvent mixtures of this strength but with different selectivity (Szepesi and Nyiredy, 1996). 

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. 

A UV/Vis absorbance detector can also be used if the solute ions absorb ultraviolet or visible radiation. Alternatively, solutions that do not absorb in the UV/Vis range can be detected indirectly if the mobile phase contains a UV/Vis-absorbing species. In this case, when a solute band passes through the detector, a decrease in absorbance is measured at the detector. 

In general, the synthetic polymeric phases seem to have polarities analogous to diol-type phases and a wide range of mobile phase conditions have been used including hexane, various alcohols, acetonitrile, tetrahydrofuran, dichioromethane and their mixtures, as well as aqueous buffers. 

The mixture of acetonitrile/water (1 1, v/v) was selected as most effective mobile phase. The optimum conditions for chromatography were the velocity of mobile phase utilization - 0,6 ml/min, the wave length in spectrophotometric detector - 254 nm. The linear dependence of the height of peack in chromathography from the TM concentration was observed in the range of 1-12.0 p.g/mL. 

Liquid chromatography was performed on symmetry 5 p.m (100 X 4.6 mm i.d) column at 40°C. The mobile phase consisted of acetronitrile 0.043 M H PO (36 63, v/v) adjusted to pH 6.7 with 5 M NaOH and pumped at a flow rate of 1.2 ml/min. Detection of clarithromycin and azithromycin as an internal standard (I.S) was monitored on an electrochemical detector operated at a potential of 0.85 Volt. Each analysis required no longer than 14 min. Quantitation over the range of 0.05 - 5.0 p.g/ml was made by correlating peak area ratio of the dmg to that of the I.S versus concentration. A linear relationship was verified as indicated by a correlation coefficient, r, better than 0.999. 

Consider a distribution system that consists of a gaseous mobile phase and a liquid stationary phase. As the temperature is raised the energy distribution curve in the gas moves to embrace a higher range of energies. Thus, if the column temperature is increased, an increasing number of the solute molecules in the stationary phase will randomly acquire sufficient energy (Ea) to leave the stationary phase and enter the... 

Alhedai et al also examined the exclusion properties of a reversed phase material The stationary phase chosen was a Cg hydrocarbon bonded to the silica, and the mobile phase chosen was 2-octane. As the solutes, solvent and stationary phase were all dispersive (hydrophobic in character) and both the stationary phase and the mobile phase contained Cg interacting moieties, the solute would experience the same interactions in both phases. Thus, any differential retention would be solely due to exclusion and not due to molecular interactions. This could be confirmed by carrying out the experiments at two different temperatures. If any interactive mechanism was present that caused retention, then different retention volumes would be obtained for the same solute at different temperatures. Solutes ranging from n-hexane to n hexatriacontane were chromatographed at 30°C and 50°C respectively. The results obtained are shown in Figure 8. 

---