Viscosity mobile phases

Figure 4.21 demonstrates the effect of temperature on the resolution of PEOs on a TSK-GEL G6000PWxi. and G3000PWxl in series. Increased temperature will decrease mobile phase viscosity and improve diffusion, which will improve resolution. 

In the pneumatic pumping system, the pressure (and not the flow rate) is maintained constant as variations in chromatographic conditions occur. Thus, a change in mobile phase viscosity (e.g. gradient elution) or column back pressure will result in a change in flow rate for these types of pumps. The gas displacement pump in which a solvent is delivered to the column by gas pressure is an example of such a pneumatic pump. The gas displacement system is among the least expensive pumps available and is found in several low cost instruments. While the pump is nonpulsating and hence, produces low noise levels with the detectors in current use, its flow stability and reproducibility are only adequate. In addition, its upper pressure limit is only 2000 psi which may be too low in certain applications. 

Selection of columns and mobile phases is determined after consideration of the chemistry of the analytes. In HPLC, the mobile phase is a liquid, while the stationary phase can be a solid or a liquid immobilised on a solid. A stationary phase may have chemical functional groups or compounds physically or chemically bonded to its surface. Resolution and efficiency of HPLC are closely associated with the active surface area of the materials used as stationary phase. Generally, the efficiency of a column increases with decreasing particle size, but back-pressure and mobile phase viscosity increase simultaneously. Selection of the stationary phase material is generally not difficult when the retention mechanism of the intended separation is understood. The fundamental behaviour of stationary phase materials is related to their solubility-interaction... 

Efficiency the organic modifier can be used to adjust solvent selectivity as normally practiced in reversed-phase chromatography. Lowers mobile-phase viscosity and improves solute mass-transfer kinetics. 

Mobile-phase viscosity Low value gives high value of N... 

Permeability is further influenced by the linear mobile phase velocity u (cm/s), mobile phase viscosity g (g/cms) and the length of the HPLC column (cm) ... 

McCalley [82,83] showed that the retention of some bases at neutral pH increased with temperature over the range ambient to 60" C, in contrast to the usual effect of decrease in retention for neutral compounds. This observation accounted for some of the marked selectivity differences that can be observed in the separation of mixtures containing different classes of compound with increasing temperature. Pronounced increases in efficiency with increasing temperature were demonstrated for basic compounds at a mobile phase pH of 7. These increases were over and above any expected due to decreased mobile phase viscosity and increased solute diffusivity, which were shown for the same compounds at pH 3. It was later demonstrated that the increase... 

Higher flow rates are possible because of the low mobile phase viscosity... 

The fastest HPLC separations are achieved using the maximum available pressure drop. Using reduced variables, Equation 9.6 illustrates a linear relationship between retention time and mobile phase viscosity for packed columns and fixed values of AP (pressure drop), Areq (required efficiency for a given separation) and (a constant that describes the permeability of the packed bed) [4]... 

Temperature is an important variable in all modes of chromatography since it affects the mobile phase viscosity, as well as solute partitioning, solute diffusivity, the degree of ionization of buffers, and the buffer pH. Increased temperature (T reduces the mobile phase... 

The importance of temperature control of the GPC column cannot be overstated. The use of temperatures above ambient results in lower mobile-phase viscosity, which in turn reduces the back pressure generated by the column. Column life is prolonged, and in some cases higher flow rates may be employed. The reduction in mobile-phase viscosity improves both the rate and efficiency of mass transfer processes, enhancing column performance. While... 

Available particle size Mobile phase viscosity... 

It is seen that the column length varies inversely as the product of the solute diffusivity in the mobile phase and the mobile phase viscosity in much the same way as the column efficiency does when operating at the optimum velocity. As would be expected the column length is directly proportional to the inlet pressure but, less obviously is also proportional to the cube of the particle diameter. 

It is seen from equation (18) that the analysis time is proportional to the fourth power of the particle diameter and inversely proportional to the square of the diffusivity in the mobile phase. In a similar manner to column length, the analysis time is also directly proportional to the applied inlet pressure and inversely proportional to the mobile phase viscosity. 

Capacity Ratio of the Last Eluted Peak "Diffusivity of Solute in Mobile Phase "Viscosity of Mobile Phase... 

Do not try to open or repack polymeric columns. They are usually under some pressure and come out of the tube like toothpaste. The column is of no use. Polymeric columns are usually packed in one solvent, then switched to a second solvent, which causes the packing to swell and squeeze out voids. They are then designed to be run in the second solvent. Polymeric ion exchangers are usually run at elevated temperature. This serves two purposes it decreases mobile phase viscosity, thereby reducing operating pressures, and it speeds... 

HTLC operates chromatographic separation at elevated temperature (60-120 °C) resulting in a significant reduction of mobile phase viscosity, leading to higher diffusion coefficients for the compounds and improved mass transfer [82], The authors also highlighted the practical limitations due to the instrumentation moreover, limited number of stable stationary phases compatible with elevated temperature makes HTLC to be rarely used in routine analysis and only investigated in academic laboratories [81]. 

Figure 1. Equation for calculating the number of theoretical plates (number may vary with mobile-phase viscosity-column temperature test solute type and molecular weight mobile-phase velocity)...

Measurement of column permeability is not clear-cut. The permeability of a column can be determined in several different ways. For example, it can be measured as the specific permeability, K° (EQ. 3), which corrects for mobile phase viscosity and column length (5) ... 

External porosity Mobile phase viscosity Column section... 

Another factor to consider with gradients is that, as the composition of the mobile phase changes, mobile-phase viscosity and refractive index change. 

FIGURE 7-27. Mobile phase viscosities and refractive indices, (a) Methanol/water mixtures at 20°C. (b) Acetonitrile/water mixtures at 20°C. 

The small heat capacity of silica CEC columns means that column temperature is easily changed. Temperature influences EOF velocity through its effect on zeta potential and mobile-phase viscosity [Eq. (4)]. Increased temperature reduces r via an exponential relation ... 

At low selectivity to achieve the same resolution, one has to use a longer column to increase efficiency and consequently operate under higher-pressure conditions. The relationship between the column length, mobile-phase viscosity, and the backpressure is given by equation (2-17), which is the variation of the Kozeny-Carman equation. Expression (2-17) predicts a linear increase of the backpressure with the increase of the flow rate, column length, and mobile phase viscosity. The decrease of the particle diameter, on the other hand, leads to the quadratic increase of the column backpressure. 

---