Laser diffraction sizing technology

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). 

The particle size distributions of 15 International Atomic Energy Agency and 16 National Institute of Standards and Technology reference materials (RMs) were measured by laser diffraction to determine their potential as reference or quality control materials in that less than 100 mg are required. Most of the materials are commercially available as environmental and biological reference materials (RMs) [137]. 

Smart J, Berg E, Nerbrink O, Zuban R, Blakey D, New M. TouchSpray technology Comparison of the droplet size measured with cascade impaction and laser diffraction. In Respiratory Dmg Delivery V Dalby RN, Byron PR, Farr SJ, eds. Interpharm Press Phoenix, AZ, 2002 525-532. 

With laser diffraction technology, the particle size distribution is determined on the basis of the scattering monochromatic light (at 750 nm). The measurement of particle diffraction pattern is characteristic of particle size in the range of 0.4 to 2000 ixm of particle diameters [20]. 

Figure 9.11 shows the characteristic operating ranges of a selected number of instruments. Sieve analysis is used for larger particles from as low as about 10 ttm to 1000 ttm. Microscopy is adequate from below 1 p,m to as much as 1000 xm. For fine particles, laser diffraction is the most broadly accepted technology and coincidentally can measure the widest range of particle sizes—it can measure particles as small as low as 40 nm. 

McGarvey M, McGregor D, McKay RB (1997) Particle size analysis by laser diffraction in organic pigment technology. Prog Org Coat 31 223-228... 

Droplet size analysis was used for characterization of the emulsions just prepared and to detect alterations of emulsions during storage. The analysis was conducted using laser diffraction with polarization intensity differential scattering (PIDS) technology (Coulter LS 230, Beckman-Coulter, Krefeld, Germany). 

Nathier-Dufor et al. Comparison of Sieving and Laser Diffraction for the Particle Size Measurements of Raw Materials used in Food Stuffs, Powder Technology, 76 (1993) 191-200. 

Having undergone rapid evolution during the past two decades laser diffraction technology has become one of the most important industrially applied methods for particle size analysis worldwide. 

In the common understanding of calibration, the calibration procedure may involve the measurement of some reference material whose size has been carefully determined and verified by other reference techniques, and the instrument s calibration constant will be adjusted to match the assayed value. However, since laser diffraction is an absolute measurement technology it is not required to use reference materials to calibrate the instrument, although one may use these materials to verify the validity and status of an instrument. For a laser diffraction analyzer, calibration is mainly related to the alignment of the optics (i.e., the laser and detectors). Since in most cases the detectors are etched onto a circuit board they are not individually mobile components. The task then is simply to align the laser, the focusing lens, and the detector array. 

Table 3.6 summarizes many other recent studies in the comparison of laser diffraction results to those of other technologies. Although the comparative ratios of the mean size for particles with different shapes cluster around some central value (the fourth column), laser diffraction always yields a larger apparent diameter than that obtained from other methods except the methods in [75] and [82]. The corresponding biases in the size distributions can be found in the original references. However, it seems there is a lack of a clear trend or... 

Table 3.6. Comparison of results of laser diffraction with that of other sizing technologies...

Since any bias is different for each technology if a spherical approximation is used for non-spherical particles, the variations in the size results of laser diffraction compared with that of sedimentation, for example, will be different than those obtained in a comparison with the ESZ method. Practically, when comparing results from laser diffraction to results from another technology, some scaling or weighting factors for size and/or density distributions are often employed to artificially shift or reweigh one of the instrument s results. In some cases, a correlation study may be needed to find the factors and their variations as a function of particle sizes. 

Khalili, M., W. L. Roricht and S. Y. L. Lee (2002). An Investigation to Determine the Precision for Measuring Particle Size Distribution by Laser Diffraction . In World Congress on Powder Technology, Paper 111. Sydney. 

Miniaturization Application of lasers as light sources resulted in a reduction in the size of refractive index detectors and made them compatible with in-line monitoring systems that are needed in capillary separation technology. The refractive index of minute amounts of solution may be determined by the measurement of the diffraction phenomenon produced in the interaction between the laser beam and a fluid-filled capillary. The laser beam passing through an off-center capillary produces a fan of scattered light in the plane perpendicular to the tube axis. The position of a single line corresponds to the refractive index of the fluid within the tube... 

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