Particle counting techniques

Special attention must be paid to the interpretation of particle size data presented in terms of either weight or number of particles. Particle weight data may be more useful in sedimentation studies, whereas number data are of particular value in surface-related phenomena such as dissolution. Values on the basis of number can be collected by a counting technique such as microscopy, while values based on weight are usually obtained by sedimentation or sieving methods. Conversion of the estimates from a number distribution to a weight distribution, or vice versa, is also possible using adequate mathematical approaches, e.g., the Hatch-Choate equations. 

Since most of the radioisotopes used in biochemical research are j8 emitters, only methods that detect and measure /3 particles will be emphasized. Two counting techniques are in current use, scintillation counting and Geiger-Miiller counting. 

Many experiments (see Section I.B) require the energy analysis and detection of two or more particles with time correlation in other words, coincidence counting techniques must be used. Coincidence methods have long been used in nuclear physics because of the convenient fast detectors that have long been available. The more recent availability of fast, high-gain electron multipliers has created the possibility of coincidence measurements in electron spectroscopy. Various aspects of coincidence measurements have been discussed elsewhere.100 102... 

Laser diffraction is most suitable for analyzing dilute emulsions that are fluid, and therefore competes directly with electrical pulse counting methods, which are applicable to similar systems (see Alternate Protocol). Most laser diffraction instruments can cover a wider range of particle sizes (i.e., 0.01 to 1000 pm) than electrical pulse counting instruments (i.e., 0.4 to 1000 pm using a number of different aperture sizes), and do not require the presence of electrolyte in the aqueous phase, which could destabilize some electrostatically stabilized emulsions. Nevertheless, electrical pulse counting techniques are considered to have greater resolution. 

The major disadvantage of the laser diffraction and electrical pulse counting techniques is that they are only directly applicable to dilute emulsions or emulsions that can be diluted without disturbing the particle size distribution. However, many food emulsions are not dilute and cannot be diluted, either because dilution alters the particle size distribution or because the original sample is partially solid. For concentrated systems it is belter to use particle-sizing instruments based on alternative technologies, such as ultrasonic spectrometry or NMR (Dickinson and McClements, 1996). 

What are the assumptions in using the alpha-particle counting technique with regards to the different energies associated with different radionuclides ... 

With sizing techniques that count the particles, it is desirable to know the accuracy of the size distribution after counting a given number of particles. Figure 2.4 gives the number of particles to be counted to give a specific accuracy [3]. For a less than 1% error in a given size... 

Fig. 10. LogIV agwnst togC. corve for a polystyrene latex (a = 0.21

As a consequence of the difference in counting technique, a shift of measured algae size to smaller values than those determined by microscopy was observed. With each algae species corresponded a typical particle size distribution plot which could be considered as a fingerprint size distribution of that particular planktonic organism. Various algae species were identified with their... 

One technique for doing this is to count the particles microscopically. In addition to particle size limitation, this is an extraordinarily tedious procedure. Light scattering can be also used for the kinetic study of aggregation, but experimental turbidities must be interpreted in terms of the number and size of the scattering particles. 

Emission measurement from the excited states is also a powerful method to investigate the ion beam radiation chemistry because a very sensitive time resolved photon-counting technique can be applied. In 1970s, temporal behavior of the emission from benzene excited states in 40 mM benzene in cyclohexane irradiated with pulsed proton and He ion particles was measured and compared with UV pulse irradiation. It was found that immediately after the irradiation there is a short decay (< 10 ns) followed by a longer decay corresponding to the life-time of the benzene excited states (26-28 ns). The fraction of the shorter decay component increases with increasing LET of the particle. This was explained by a quenching mechanism that radical species formed in the track core attack and quench the benzene excited states, which would take place only shorter period less than 10 ns after irradiation [69]. 

During limnological investigations of Lake Zurich, Switzerland, monthly measurements were made of the total particle count and the particle size distribution as a function of depth over a 12-month period. A Kemmerer sampler was used to collect a 1-L sample which was conserved with formaldehyde, stored at 4°C in the dark, and counted within 48 hr. Sample handling, preparation, and details of the microscope counting technique using a Zeiss Videomat image analyzer and particle counter are described in detail elsewhere (19). 

Electrodeposition is currently used in a minority of laboratories to prepare a thin, uniform, and reproducible source. The alpha-particle emitting isotopes of plutonium are electrodeposited on polished stainless steel, or platinum disk. In the co-precipitation technique, a small amount of a carrier (e.g., LaF 3) is used to co-precipitate the separated and purified plutonium from solution. The precipitate is then prepared for counting by either filtration or by evaporation of a slurry of the precipitate onto a stainless steel disk or planchet (ASTM 1982 1987). Recent methods use a glass fiber filter which can be used as the source for alpha counting techniques. It has been suggested that low yields result from electrodeposition due to the presence of traces of interfering elements (e.g., iron) (Bernhardt 1976). 

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