Particle Characterization Using a Helium MIP System

A helium microwave cavity plasma or MIP serves as the heart of a unique instmment dedicated to the analysis of particles with diameters between 1 and 20 pim. The analysis of small particles is becoming increasingly important in fields as diverse as the semiconductor industry where extremely low levels of contaminants can destroy semiconductor chips, to indoor and outdoor air quality and the health impact of respirable particles. This instrument is basically an atomic emission spectrometer, but designed to count particles, calculate particle size, and determine the chemical composition of each particle analyzed, one by one. 

An atmospheric pressure helium MIP is generated using a 2.45 GHz microwave generator and an electromagnetic cavity resonator, called a Beenakker cavity. The hehum gas is passed through a discharge tube placed in the cavity, as seen in Fig. 7.48. The plasma is initiated by a spark from a Tesla coil. The electrons produced by the spark oscillate in the 

The emission intensity should be proportional to the mass of the material in the plasma. If the particles of the material are spherical, the emission intensity is proportional to the diameter of the particle raised to the third power or  

From the emission intensity, an equivalent spherical particle size can be calculated for sample particulates. One material which is available as pure, spherical particles of well-controlled particle size is sihca, used for chromatography column packing. A micrograph of 3 p.m diameter Si02 beads collected on a polycarbonate filter is shown in Fig. 7.51(a). While relatively uniform in diameter (monodisperse), you can see that not all the beads are perfect single spheres. The silica particles are aspirated into the He 

MIP and the emission intensity at a silicon emission line is measured. The correlation between emission intensity and true particle size measured from the electron micrograph is excellent [Fig. 7.51(b)]. Only 397 individual particles were measured to obtain the size distribution of the material the number of particles is determined from the number of Si photons registered by the PMT detector. 

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