294 zyxwvutsrqpon Part. Part. Syst. Charact. zyx 8 (1991) 294-296 z Mass Calibration of the Coulter Counter Model ZM M. Margarida Figueiredo, M. Graga Rasteiro, Cristina Santos, Cristina Monteiro * (Received: 17 December 1990; resubmitted: 15 June 1991) Abstract The calibration of electrical sensing zone instruments is nor- mally achieved by using spherical particles with a certified size. An alternative and more fundamental procedure, known as mass or self-calibration, is to use particles of the material under test. This work concerns the mass calibration technique, in par- ticular the equations used to calculate the mass calibration con- stant. It is demonstrated that some of the published expressions are inconsistent. An expression particularly suitable for the Coulter Counter Model ZM has been derived and validated. Some experiments were also performed using irregular particles, in order to compare both calibration methods. 1 Introduction The electrical sensing zone technique, sometimes called the Coulter Counter method, is widely used in particle size analysis, and often as a reference method. The Coulter Counter deter- mines both the number and size of particles suspended in an electrically conducting liquid. The operating principles of this instrument have been extensively described in instrument in- struction manuals, textbooks [l] and also in British Standard BS 3406 [2]. The calibration of these instruments is usually achieved by using spherical particles which have a known and certified size. The most popular standards are polymer latex particles which have a narrow particle size distribution. However, the instru- ment can also be self-calibrated, that is, calibrated with the material under analysis. This method, often known as the mass integration method, is recommended in BS 3406 as a primary calibration procedure. It is superior to latex calibration because the particle response may be affected by the shape, porosity and conductivity of the material. The procedure is therefore, more direct and traceable. This method was formerly employed for sizing BCR reference materials and polymer latex particles [3]. It is not a routine procedure, however, since it is time con- suming. The purpose of this paper is twofold : first, to compare the two calibration techniques for the Coulter Counter Model ZM, i. e. latex calibration and mass calibration, and second, to correct some misleading expressions which have been published for the mass calibration method, especially for the Coulter ZM. A cor- rected expression is given. ~~ * Prof. zyxwvutsrqponm M. M. zyxwvutsrqpon Figueiredo, M. G. Rasteiro, C. Santos, C. Monteiro, Departamento de Engenharia Quimica, Universidade de Coimbra, Largo MarquSs de Pombal, 3000 Coimbra (Portugal). 2 Determination of the Mass Calibration Constant Electrical sensing zone instruments require calibration in order to assign a particle size to each of the threshold levels. The par- ticle diameter is then calculated from the equation d=Kd31/T. Typically, a preliminary calibration of the instrument, fitted with the appropriate orifice tube, is achieved by applying the certified latex particles and using the "half-count'' method to identify the median value zyxw [4]. The calibration constant obtained in this way is automatically computed by the instrument. The alternative calibration technique is to use the powder under analysis. In this case it is necessary to prepare a suspension of a narrow size fraction of the material and of known concentra- tion [Z]. The volume of the particles in a metered volume of suspension will be given by This volume can also be obtained by measuring the particle size distribution, resulting in 71 Vp = - ZhnJ3 6 or, substituting Eq. (l), Comparison of Eqs. (2) and (4) gives (3) z 0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1991 0934-0866/91/0412-0294 $3.50 + .25/0