Spin fluid

As = surface area of cyclone exposed to the spinning fluid, m2. 

Trichlorodifluoroethane (HCFC-122) is a co-spin agent, which lowers the cloud-point pressure. The cloud-point pressure means the pressure at which a single phase liquid solution begins to phase separate. At temperatures above the critical point, there cannot be any liquid phase present and therefore a single phase, supercritical solution phase separates into a polymer-rich/spin fluid-rich, two-phase gaseous dispersion. 

Data System. An IBM-PC desktop computer is interfaced to the instrument control module to provide automated data collection and analysis. The data collection and analysis programs are menu driven. A data management facility is an integral part of the data system. A modeling utility is provided to aid the operator in chosing the operating conditions (rotational speed, spin fluid volume, fluid density, etc.) for a sample. Programming is done in compiled BASIC and the 8087 math coprocessor is used to improve computational speed. 

DISC SPEED PARTICLE DENSITY SPIN FLUID SPIN FL DENSITY... 

At a specific radial distance the moving particles interrupt and attenuate a light beam, the intensity of which is measured by a photodetector. The particle size is related to the appearance time at the photodetector by means of Stokes Law for centrifugation. The concentration of particles is obtained from the photodetector response and the application of Mie scattering theory.(1) A very Important step in this operation is the formation of a density gradient within the spin fluid to allow a hydrodynamically stable separation of the suspended particles. An accepted method for forming this spin fluid density gradient within the disc cavity is the widely used and patented (2) Joyce-Loebl Buffered Line Start... 

BLS). This paper describes a novel and alternative method for performing DCP analyses and in particular a new procedure for the formation of the spin fluid density gradient external to the DCP disc. This new patented method ( 3), known as the External Gradient Method (EGM), will be compared to the BLS method of analysis. The quantitative precision of the results obtained with EGM and BLS will be compared by statistical analysis.In addition, qualitative advantages of the EGM over the BLS method will be given. 

The baseline was adjusted by the operator, when necessary, and the revised data calculated. These results were computed on the basis of the initial spin fluid temperature of 25°C. 

The simplicity of operation of the EGM and Its accuracy are best seen by comparing particle size distribution curves by the EGM and the BLS method and comparing data obtained from both methods. Figure 1 shows the separation of three latex standards by the BLS method. Twenty ml of water was used as spin fluid and 1 ml of 50% (V/V) methanol as a density buffer. A 0.25 ml sample of dilute latex (10... 

Figure 4 shows the results obtained on another sample containing four different standard latexes with particle sizes of 1.091, 0.822, 0.600 and 0.497pm, using a spin fluid of 11 ml of water topped with an external gradient of 9 ml of water and 1 ml of methanol and a disc speed of 3586 rpm. Peak appearance times of 3.6 minutes, 7.1 minutes, 12.5 minutes and 19 minutes correspond to calculated particle size values of 1.142, 0.813, 0.613 and 0.497 pm respectively, which are in excellent agreement with the nominal values of the standards. 

Figure 6 shows the separation of the latex mixture shown in Figure 5 including an additional 0.176pm polystyrene standard. In this case, the externally prepared spin fluid/density gradient consisted of 15 ml of water and 1 ml of methanol and the disc speed was 10,450 rpm. Baseline separation and peak resolution are obtained over a decade of size range in less than 25 minutes. 

For example, disc speeds could be changed from 3,000 to 4,000, 4,000 to 5,000, and 5,000 to 6,000 rpm without upsetting the delicately balanced spin fluid density gradient needed for effective particle size analysis. This can not be done with the BLS. Note that for quantitative accuracy minor volume changes need to be accounted for after repeat injection. However, for qualitative analysis or "fingerprinting" this procedure is acceptable. 

Subsequent experiments using dodecane (6) to inhibit evaporative cooling at the spin fluid/air Interface have demonstrated even more improvement in baseline stability and run-to-run reproducibility when using the EGM. 

Temperature control is an important factor in determining particle size by sedimentation methods. During a typical run changes in spin fluid temperature of 2-4°C were common. This temperature change (DELTEMP) was used as the covariate in the analysis of covariance. 

The covariate, DELTEMP, is highly significant in the determination of the Particle Size. This indicates that temperature control, from the statistical point of view, is more important than has been previously considered. This is not surprising physically, in view of the temperature dependence of the spin fluid viscosity and the density terms contained in Stokes Law. The cooling capacity of the two DCPs were known to be different. Both instruments had been physically recalibrated prior to running these experiments. 

It has been recognized that stable sedimentation, as a rule, requires a density gradient in the spin fluid (1). The buffer-layer method (2) is most often used to generate this gradient, but at times it has been unreliable. 

Sucrose solutions were usually used as aqueous spin fluids. Nonaqueous media such as dodecane/hexadecane and diethylforma-mide/dimethylformamide mixtures have also been used. The quality of the gradients was confirmed refractometrically in separate experiments. 

Figure 2. Sedimentation curves of polystyrene latexes with nominal diameters of 0.45 and 0.765 urn disk centrifuged at 4000 rpm in 20 mL of spin fluid with gradients of (a) 0/4%, (b) 0/3%, (c) 0/2% sucrose. Reproduced with permission from Ref. 3. Copyright 1985, Academic Press.

The gradient-variation experiments described in the previous section were repeated with dodecane-protected gradients. Much shallower gradients gave stable sedimentation under these conditions. The latex mixture could be separated in a 0/1% gradient as well as in a 4/4.5% gradient. This confirmed that evaporative cooling of the spin fluid was the primary cause of convection and hence of unstable sedimentation. 

Particle radii (R) are calculated from the sedimentation times (t) by means of the Stokes equation. For a spin fluid with density and viscosity gradients... 

The concentration and hence the viscosity and density (of the aqueous sucrose, e.g.) at any point in the spin fluid are calculated from the concentrations and volumes in the gradient apparatus and the dimensions of the rotor. 

While examining time-dependent phenomena of silver bromide particles, we made many repeat measurements. With care, standard deviations could be kept below 1%. Figure 6, for instance, contains the diameters calculated from the peak maxima for 44 injections of a silver bromide dispersion. Four consecutive injections were made into the same dodecane-protected spin fluid, so that experiments in 10 different spin fluid gradients are represented here. Monitoring of the spin fluid temperature was required, since this variable affects both density and viscosity. Corrections were also made for the volume change of the spin fluid caused by successive injections Cl, 3). 

The layer thickness ranged from 0.025 to 0.15 pm, depending on the sample, the ionic strength, and the pH of the spin fluid. 

Figure 9 shows plots of layer thickness as a function of spin-fluid ionic strength obtained for four silver bromide sols at pH 6. 

Figure 9. Gelatin-layer thickness versus spin-fluid ionic strength for four silver halide dispersions. The spin fluids contained NaN03 and were adjusted to pH 6. Reproduced with...

This effect can be suppressed by covering the spin-fluid surface with a thin layer of a nonvolatile oil such as dodecane. A density gradient protected in this manner can be used for several hours after injection into the rotor. Since many sample types can be introduced into the spin fluid through such an oil layer, consecutive runs can be made with the same spin fluid. The standard deviation in particle sizes calculated from peak positions can be reduced to <1% by using these techniques along with careful monitoring of the spin-fluid temperature. 

General criteria for disc centrifuge operation includes (1) the mean density of the suspending medium and particles must be less than the spin fluid (2) particle density must be greater than the spin fluid (3) viscosity of the suspending medium must be adjusted slighdy higher than the spin fluid (4) spin fluid must be compatiUe with the dispersion (5) a refiractive index difference must exist between the spin fluid and the particle. 

In the external gradient method, the spin fluid density gradient is formed external to the disc and injected into the disc while the disc is spiiming. The externally formed spin fluid contains at least two miscible or partially miscible liquids of different densities in an incompletely mixed condition. 

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