David H. Freeman

Chemical Microstandards from Ion Exchange Resin

ION exchange beads which are predominantly spherical and chemically uniform may be loaded with one or a combination of electrically charged counterions. The mass of such a bead, and of its exchangeable counterion content, is related to the particle diameter1, which can be measured precisely. This material is ideally suited for the stable localization of extremely small quantities of matter, so that the individual particles may be used as chemical microstandards. This capability depends on a high degree of chemical uniformity the attainment of which in carefully controlled conditions has been established2–4. It also depends on effective stabilization of the counterion content; this is attained by drying to prevent the diffusive processes of ion exchange.

LASER DETECTION OF SMALL PARTICLES IN LIQUIDS

Particulate contamination is recognized as a source of uncertainty or possible trouble in many technical and scientific areas. The significance to trace chemical measurements motivated the present study. As illustrations : Sloan2j has suggested the transport of dust during the sublimation of anthracene; Ma22 has characterized the abrasive molecular fragmentation of polyethylene containers. Other mechanisms for particle generation, transport and contamination are known, and have prompted an interest in developing a simple, convenient, and sensitive means for the observation and control of particulate contamination. For over a century, light-scattering has been used for the measurement of liquid-borne particle concentration in the form of increasing refinements of turbidimeters and nephelometers.16 These instruments generally use a filament type3.23.24 or a mercury vapor”8.17 light source with the attendant collimating optics. Advances have been made in the use of lasers for the study of stimulated Brillouin scattering,5,6.l”,lx,*i measurements of Rayleigh scattered light,ll,12,1”~”,?1 fluctuations near the critical temperaturel,”l’,l,j,~K and the polarization,Q but the application to turbidimetry or nephelometry has received little impetus.i A continuous laser with an output of 0.3 mw is an inexpensive source of a narrow light beam. Typically, the spreading of a 1 mm beam is small for dilute suspensions of particulate matter in liquid media. Focusing or collimating lenses are not necessary, and sampling tubes of less than 10 mm in diameter may be used for the convenient measurement of scattered light along several centimeters of path.