Centrifugation and ultracentrifugation

Example. How long would one need to centrifuge the 0.2 pm particles, from the previous example, in a centrifuge capable of 3000 rpm with an effective x = 10 cm Assume we need to settle the particles by 3 cm to strike the wall in an angled centrifuge tube. 

In this case, to sediment the 3 cm in the tubes takes (3 cm)/(0.132 cm/ min) = 23 min. This is better than the 16 days needed in a column  

Example. Bitumen is recovered in the form of a froth when a separation-flotation process is applied to surface mined oil sand. Once de aerated, this bituminous froth is a W/O emulsion from which the water must be removed prior to upgrading and refining. At process temperature (80 °C) the emulsion viscosity is similar to that of the bitumen, but the density, due to entrained solids, is higher. Taking t) = 500 mPa-s and f = 1.04 g/mL, the rate of creaming of 20 pm diameter water droplets under gravitational force will be very slow  

In the commercial process, a centrifuge process is used to speed up the separation. The continuous centrifuges can operate at 2500 gs, the droplets hav- 

This is 2500 times faster than with gravity alone, but the residence time in the centrifuge would have to be about 20 minutes, which is not practical. To speed up the separation, naphtha is added to the level of 25%. This lowers the viscosity to about 4.5 mPa-s and lowers the density of the continuous phase to 0.88 g/mL. Note that now the water drops would sediment rather than cream under gravitational force, and while the emulsion density is much reduced, the absolute value of the density difference changes very little Ap = -0.07 g/mL originally, and becomes Ap = +0.09 g/mL The overall effect is to lower the viscosity by about two orders of magnitude. The droplet velocity now becomes (dx/dt) = 1.1 cm/s, which yields a satisfactory residence time of about 8 seconds. 

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Particles that are smaller and less dense than the nuclei can be obtained by step-by-step acceleration of the gravity on the supernatant left over from the first centrifugation. However, this requires very powerful centrifuges (high-speed centrifuges and ultracentrifuges). The sequence in which the fractions are obtained is mitochondria, membrane vesicles, and ribosomes. Finally, the supernatant from the last centrifugation contains the cytosol with the cell s soluble components, in addition to the buffer. 

Fig. 6. Percentage of uranyl sorbed on quartz at a total carbonate concentration of 10-3 M (series Cl) or 10 2m (series CIO) obtained after sample centrifugation and ultracentrifugation (after Mignot 2001).

There are three major types of centrifuge commonly encountered in laboratory settings namely low-speed (clinical) centrifuge, high-speed centrifuge and ultracentrifuge. These have different characteristics and applications, which are briefly considered below. 

D-isomer. It has been possible to separate and identify the different variants of alkaline phosphatases in human placenta by Sephadex-gel filtration, sucrose density-gradient centrifugation, and ultracentrifugation, and in these studies starch-gel electrophoresis has proved to be unique in characterization of different isoenzyme fractions. Figure 17... 

Several methods can be used to separate specific size fractions of droplets or particles from emulsions, suspensions and aerosols. Some of these, such as filtration, sieving, centrifugation and ultracentrifugation have been introduced in Sections 2.2 and 2.4. 

Transfer the homogenate into precooled centrifuge bottles and centrifuge for 10 min at 2,400 x, 4°C. Collect the supernatant and discard the pellet. Transfer the supernatant to a precooled ultracentrifuge bottle and ultracentrifuge for 90 min at 34,000 x, 4°C. Discard the supernatant and resuspend the... 

It is our objective in this chapter to outline the basic concepts that are behind sedimentation and diffusion. As we see in this chapter, gravitational and centrifugal sedimentation are frequently used for particle-size analysis as well as for obtaining measures of solvation and shapes of particles. Diffusion plays a much more prevalent role in numerous aspects of colloid science and is also used in particle-size analysis, as we see in Chapter 5 when we discuss dynamic light scattering. The equilibrium between centrifugation and diffusion is particularly important in analytical and preparative ultracentrifuges. 

To obtain resin, the hot product of condensation is poured through the lower drain of condenser 18 to obtain varnish, the resin in condenser 18 is dissolved with ethyl alcohol, which self-flows into the apparatus from batch box 21. While the resin is dissolved, cooler 19 operates in the inverse mode. The obtained varnish is loaded by vacuum into settling box 22, where it is settled at ambient temperature for a long time (24-48 hours) to separate mechanical impurities. There is also a possibility for additional centrifuging in ultracentrifuge 23 for complete elimination of mechanical impurities, as well as clarification" of the varnish. The finished poly-methyl(phenyl)silsesquioxane varnish is sent from the centrifuge into container 24 and packaged. 

To analyze free amino acids in plasma or tissue homogenates, it is necessary to remove proteins and peptides present in solution. The most widely used deproteinization method is precipitation with 5-sulfosalicylic acid followed by centrifugation for separating the precipitate. In comparison to other precipitation agents such as trichloroacetic acid, perchloric acid, picrinic acid, or acetonitrile, the best results with respect to completeness of precipitation are obtained with 5-sulfosalicylic acid [39]. Other deproteinization methods comprise ultrafiltration and ultracentrifugation [40], which have only recently been considered as sample preparation methods for amino acid analysis. 

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