Particle measurement progress

by Robert P. Donovan

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Often a breakthrough comes about as a result of recognizing the applicability of a solution developed in a different, traditionally separate field to one's own particular discipline. Particle measurement technology has benefited from such technology transfers. I will cite a few examples all based on the condensation particle counter (CPC).

The CPC itself represents an adoption or an extension of the Wilson cloud chamber technology developed as a charged particle detector in the 1930s. The principle of operation of the Wilson cloud chamber was that high-energy nuclear or atomic particles passing through a vapor saturated or super-saturated volume induce condensation of liquid droplets that makes the otherwise invisible particle readily detectable.

This technique was used to detect atmospheric cosmic rays and particles created by nuclear reactions.

The CPC used by contemporary aerosol technologists uses the same vapor condensation technique to detect and count particles smaller than what can be detected by even today's optical particle counters (OPCs). Particles passing through a region of saturated vapor serve as condensation centers for the vapor. The condensate that collects on each particle increases the particle size sufficiently so that an OPC can easily detect its presence.

Commercial CPCs detect particles smaller than 10 nm, while the most sensitive OPCs struggle to count 50-nm particles. The condensation step employed by a CPC boosts the size of these small, sub-10-nm particles into the 2 to 3 micron range, easily countable by almost any OPC. However, all size information is lost, because the size of the condensed mass is relatively independent of the size of the nucleating particle.

This CPC technique is itself the primary principle by which the residue after evaporation (RAE) of a liquid can now be rapidly measured. The established technique for measuring RAE consists of heating a liquid sample at some temperature, typically 120 degrees C, until all its volatile components have been vaporized. What remains then is the nonvolatile residue — the RAE — which is weighed and ratioed to the volume (usually 1 liter) or weight of the initial liquid sample to express the sample RAE as either a density or a concentration. This traditional, gravimetric RAE measurement is a straightforward but somewhat lengthy measurement method.

The measurement time and sample size required by the traditional RAE measurement method can now be significantly reduced by using a CPC as the sensor that generates the data from which RAE is determined by a commercial analyzer called the Nonvolatile Residue Monitor (NRM).

This approach requires changing the liquid sample into an aerosol sample consisting of small liquid droplets with large area-to-volume ratios. A nebulizer (atomizer) easily performs this conversion. Because of the large surface area of the droplets, evaporation of the volatile constituents from the droplets in a flow of dry air or nitrogen is very rapid — on the order of milliseconds rather than the tens of minutes or even hours required to evaporate the 1-liter sample of the traditional RAE measurement.

What remains after droplet evaporation in this new method is an aerosol sample in which the aerosol particles consist of the now solid, nonvolatile material that was part of the initial liquid sample. Each atomized droplet produces one aerosol particle. The size of the aerosol particles making up the aerosol sample depends on the concentration of nonvolatile residue in the liquid sample as well as the volume of the droplets formed by the nebulizer. Because the size distribution of the droplets formed by the nebulizer of the NRM is designed to be reproducible from one sample to another, the size distribution of the aerosol particles formed after evaporation changes only with the concentration of the nonvolatile residue in the liquid sample.

Happily, ASTM has recognized the merits of this adopted technology and incorporated it into a contemporary method for measuring RAE (ASTM 5544-99).

Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc.


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