206 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS from the system is employed some smaller ones will also be removed. In practice, however, the error is insignificant usually being less than 2% in terms of number, and decreasing, which is negligible on a number or weight basis. The calculation then proceeds as normal. The time taken for a typical single tube analysis including the calcu- lation is 20 min. Two tubes require perhaps another 5-10 min. Times can normally be halved on a routine basis. Figs. 5 and 6 show the size analysis of various other cosmetic materials obtained with the Coulter Counter. IOO qo 90. 70. 6o. fro- 7' 6 cunuL•rtvoe [voetc•lr p•l•coe/vr ,•ovoe 5rArc• stzoe Figure 8 Particle size distributions of various cosmetic materials The Coulter Counter being akeady partly automatic is ideally suited to further automation. New Models B and C now lend themselves to this system. The Model B has two counting circuits so that a frequency number distribution can be obtained directly. Among the many other advantages is that the Model B response is independent of electrolyte resistance so that one calibration factor will hold for changes in electrolyte resistivity caused by concentration change or temperature increase. It also lends itself to easier and faster data reduction. Using a Model J plotter coupled

PARTICLE SIZE ANALYSIS USING COULTER COUNTERS 207 •oo t •o 7 CUPIUL•TI•E •/EIC#T ?oe•[EAIT AfiOVoe 3T•TEP SIZE Fiõure • P•rt•cle s•ze distributions ooe ¾•r•ous cosmetic to the B Model a frequency or cumulative number histogram can be auto- matically obtained in 110 sec over 25 points with no operator attention. Each plot is only over a relatively narrow range, about 3-1 on a diameter basis, a wide range distribution can be built up if necessary by the pro- duction of several such plots. A further development is to use the information obtained by the above system, and by coupling the numbers and particle volume, automatically and instantly compile the cumulative volume or weight distribution. This is achieved with the Model M. The Coulter Counter Model C is the most elaborate model yet com- mercially available. By utilizing advanced circuitry it is possible to obtain complete a six, nine or twelve point size distribution in as little as 15 sec. Recording of the data can be done in many ways, e.g. simple digital print-out and direct connection to a computor. The model has many other advantages over the original Model A. Correlation of the Coulter Counter to other methods of particle size analysis has been well studied. This presents no problems if one appre- ciates the limitations of the various techniques. For instance gravitational