Particle challenge testing

Some preliminary work has commenced on Part 3, the estimation of pore size by particle challenge testing. 

Particle challenge testing can be conducted in three ways as... 

Figure 7.9 An illustration of the model of Williams and Meltzer [7] to explain the discrepancy between membrane pore diameter measurements based on the microbial challenge test and the bubble point test. Reprinted from R.E. Williams and T.H. Meltzer, Membrane Structure, the Bubble Point and Particle Retention, Pharm. Technol. 7 (5), 36 (1983) with permission from Pharmaceutical Technology, Eugene, OR...

Other particles may also be used to test the membrane. Monodisperse latex spheres (produced by Dow Chemical) in concentrations of 106-109 particles per ml are sometimes used. Normally, a second membrane is used downstream of the test membrane to collect particles which pass. The number of spheres collected are Counted with the use of a scanning electron microscope. This is much more tedious and less accurate than the bacteria challenge test where bacteria collected on the second membrane may be grown into colonies visible to the naked eye. Alternatively, automatic particle counters are used before and after the membrane, but the method is not as sensitive and is limited to particles over 0.5 t in size. 

Gas niters may be evaluated prior to use by the same methods used for liquid filters (see Section 111 below), allowing that they can be effectively dried out and sterilized without loss of the qualities being tested. Alternatively they may be evaluated by exposure to particles in a gas stream, for instance by the sodium name test, which is also used for HEPA filters. Microbiological tests have to be considered in a somewhat different light. Leahy and Gabler 71 describe an aerosol challenge test of 10 Ps. diminuta bacteria per mL over a four-day period as an appropriate manufacturer s validation test. It would not be practical for routine use. 

The above tests are non-destractive since the filter medium is unchanged afl er conqiletion of the test. Challenge tests, involving the filtration of particles of known size are destractive since after testing, it is virtually m5>ossible to recover the original, unused characteristics of the medium. These challenge tests are considered in the next section. 

Although this technique is widely used to characterise pore size, other techniques are also used, not least amongst which are challenge tests (see Rideal, 2005) in which the filter medium is challenged by a quantity of calibrated micro-spheres, which are fluidised, typically by sonic means, to assist in their passage through the filter. Comparison is then made of the particle size distribution of the micro-spheres before and after filtration. 

The classic method of determining filter efficiency is by the bead challenge test, with a rating expressed as a beta ratio, thus a beta ratio > 75 specifies a filter efficiency of 98.6% or better relative to a particle size of x pm. 

The tests described so far have all been challenge tests, in which a known suspension is fed to a test filter. The bubble point test, on the other hand, measures a characteristic of the filter medium without use of particles. 

All gas phase filter tests are of the single-pass format using a particle challenge, but the methods differ widely, as do the particle formulations used in the challenge and the means of analysis to demonstrate performance. The materials used include natural sand/quartz mixtures, alumina dusts, methylene blue aerosols and di-octyl phthalate. A sand-quartz mix is specified for the valuation of air filters for IC engines and compressors, originally in BS 1701, now BS ISO 5011. 

Conventional filtration theory has been challenged a two-phase theory has been appHed to filtration and used to explain the deviations from paraboHc behavior in the initial stages of the filtration process (10). This new theory incorporates the medium as an integral part of the process and shows that the interaction of the cake particles with the medium controls filterabiHty. It defines a cake-septum permeabiHty which then appears in the slope of the conventional plots instead of the cake resistance. This theory, which merely represents a new way of interpreting test data rather than a new method of siting or scaling filters, is not yet accepted by the engineering community. 

CPMV particles that contained a 17-mer neutralizing epitope, 3L17, from the VP2 capsid of Mink enteritis vims (MEV) fused to the S protein were generated. When mixed with adjuvant, these particles protected all the test animals from clinical disease when challenged with vimlent MEV. A modified constmct, which presented the... 

The basic challenges for parallel test reactor development for high-throughput experimentation are, apart from technological challenges, related to technical demands that arise with the special issues for parallel test reactors, which are identical with the demands for conventional test reactors for gas-phase reactions. The criteria that must be fulfilled to obtain intrinsic catalyst properties from experimental data relate mainly to mass and heat transfer. A sufficient contact between the reactants and the catalyst must be insured to avoid mass transfer limitations inside and outside of the catalyst particles. Isothermal operation under laboratory conditions and avoidance of heat transfer limitations are also crucial. As a general quality check prior to operation intra- and extra-particle limitations should be... 

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