Mobile phase mobility

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Separation science focuses on room temperature ionic liquids (RTlLs), salts that are liquid at ambient temperature. They have been studied as extracting solvents, stationary and mobile phases, mobile phase additives, and other uses. Common RTILs consist of a bulky nitrogen- or phosphorus-containing organic cation (pyridinium or pyrrolidinium, alkyl-imidazolium, ammonium or phosphonium) and a variety of organic and inorganic anions (triflate, dicyanamide, trifluoroacetate, acetate trifluo-romethylsulfate, nitrate, perchlorate, bromide, chloride, chloroaluminate, tetrafluo-roborate, hexafluorophosphate). 

The concentration of the organic solvent is lower in the initial mobile phase (mobile phase A) than it is in the final mobile phase (mobile phase B). The gradient then, regardless of the absolute change in percent organic modifier, always proceeds from a condition of... 

One of the most important developments of the last 15 years in homogeneous catalysis is the introduction of the aqueous two-phase ( biphase ) technique. This method uses a homogeneous catalyst, dissolved in water, as a mobile phase ( mobile support ). By simple phase separation (decantation), catalyst and reac-tants/reaction products are separated just after reaction and at approximately the same temperature (cf. Figure 1). In relation to the reaction products the catalyst is thus immobilized as well as heterogenized on liquid supports , but not anchored . So the manifold advantages of homogeneous catalysis are supplemented by the argument that catalyst and reaction products may be separated immediately after reaction without any chemical stress [9]. Only those systems... 

Hi kino et al.4 developed two HPLC methods for the analysis of aconitine and related alkaloids in crude drugs. In the first one the alkaloids were separated on an octadecyl column using tetrahydrofuran - 0.05 M phosphate buffer (11 89) as mobile phase. Mobile phases containing methanol and acetonitrile gave broader peaks and less resolution of some of the alkaloids. In the pH range 2-5, little variation in k1 was found for the alkaloids, but above pH 5 some alkaloids showed increased k1 values. Best results were obtained at pH 2.7 (Fig. 12.1), A second method was developed to minimize the risk of interference of co-eluting compounds from the plant material. The same type of column as above was used in the reversed--phase ion-pair mode, and, as pairing-ion, 0.01 M hexanesulfonate was added to the mobile phase (tetrahydrofuran - 0.05 M phosphate buffer (pH 2.7)(15 85)). 

Column Lichrosorb Diol 10 pm (150x3.2 mm ID) loaded with 0.1 M naphtalene-2-sulfonate in 0.1 M aqueous phosphate buffer (pH 2.1) by subsequently pumping 30 ml of phosphate buffer (pH 2.1) and 50 ml of the stationary phase through the column, followed by the mobile phase until no more droplets could be observed in the eluate (ca. 20 ml). Finally the column was recycled with 500 ml of mobile phase. Mobile phase chloroform - n-propanol (9 1) and chloroform - n-propanol (9 1) saturated with the stationary phase mixecf in a ratio (1 9), flow rate 0.1> ml/min, detection UV 254 nm. Peaks 1, tetrabutylammonium 2, tributylmethylammonium 3, tetrapropylanmonium 4, tripropylmethylanmonium. (reproduced with permission from ref. 21, by the courtesy of Acta Pharmaceutica Suecica). 

Figure 10.12. An example of many ghost peaks in a gradient analysis of a blank sample. These peaks are impurities or contaminants in the initial weaker mobile phase (mobile phase A), which are concentrated and eluted during gradient analysis. A more normal baseline can usually be established by preparing a fresh batch of MPA with purified reagents. Figure courtesy of Academy Savant.

An area of analytical chemistry very well suited to optimization strategies is high-performance liquid chromatography (HPLC). Many papers and a recent book have focused on simplex optimization experiments in this area. Among the factors that influence a chromatographic experiment, many are controllable and thus are susceptible to optimization, but some are not. Examples of uncontrollable faaors include noise, drift, and column performance. Examples of controllable factors include flow rates of mobile phase, mobile phase composition, and temperature. 

The flow rate was set at 1 mL min. Two mobile phases (mobile phase A 0.1% formic acid in water, mobile phase B 0.1% formic acid in methanol) were employed to run in 10 min a linear gradient from 5% B to 95% B, which was maintained for 2 min this was followed by re-equilibration at 5% B for the next 3 min. The run time was 15 min injection volume 10 pL, autosampler temperature 25 °C, detection wavelength 220 nm. 

Fig. 20 Dependence of the elution volume of polystyrtme standards on the composition of the mobile phase. Mobile phase decalin-cyclohexanone (in vol.%). Column Lichrosorb 100,250 mm X 4.6 mm l.D. Temperature 140°C. Detector PL-ELS 1000. Flow rate 1 mL/min. (Reprinted from [149] with permission of Taylor Francis)...

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