Anal. Chem. 1995,67, 257R-272R
Particle Size Analysis Howard 0. Barth* and Richard B. Flippen Central Research and Development Experimental Station, DuPont Company, P.0. Box 80228, Wilmington, Delaware 19880-0228 Review Contents General Books and Reviews Scattering Techniques Books and Reviews Photon Correlation Spectroscopy Classical Light Scattering Turbidimetry
Diffraction Optical Particle Counters Velocimetry
Neutron and X-ray Scattering Size Exclusion and Hydrodynamic Chromatography Field Flow Fractionation Electrozone Sensing Sedimentation and Centrifugation Sieving and Filtration Ultrasonic Measurements Other Measurement Techniques Intercomparison of Techniques Particle Shape and Image Analysis Data Interpretation Particle Size Standards Miscellaneous
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This is the tenth-year anniversary review issue of particle size analysis (A1-A5), an active field that still remains a fascinating area of research with unlimited challenges. The need for reliable particle size analyzes is driven by environmentalconcerns, process control and monitoring demands, and the development of new materials that require characterization. Particle size measurements are critical for understanding supramolecular,micellar, and colloidal systems in both synthetic and biological processes. As in previous review periods, methodologies based on light scattering measurements continue to dominate particle size instrumentation. We see extensive use of velocimetry techniques in industrial applications, which has expanded since the last review. The use of field flow fractionation techniques, which had been a growing area, appears to have leveled off, at least from the number of publications. Although microscopy is not emphasized in our literature coverage, we do see increased studies in the use of image analysis for particle size and shape analysis, ostensibly as a result of new imaging systems and improved software. In terms of new emerging instrumentation, the use of ultrasonic measurements appears to offer much promise. As in the past, application areas are dominated by environmental analyzes. Literature searches were based on Chemical Abstracts from 1993, volume 118, to 1994, volume 121, inclusive. All relevant papers dealing with new developments and interesting and useful applications were covered. With some exceptions, we have omitted aerosol and atmospheric measurements, as well as industrial-scaleparticle fractionation studies. Except for selected papers on image analysis, we have not included microscopy in 0003-2700/95/0367-0257$15.50/0 0 1995 American Chemical Society
the review. Although our focus is on particles, pertinent references dealing with macromolecules, micelles, and associated structures are given. Suggestionsfor expanding coverage in specific areas of particle size analysis are welcome. Also, please contact the authors if we have missed any significant study, which we will include in the next review. We gratefully acknowledge the assistance of Carol Perrotto in improving and implementing our literature search strategies. We are also grateful to Rebecca Pennington for her excellent typing skills and patience. Finally, this paper is dedicated to Dr. Shao-Tang Sun, a past coauthor who, in his infinite wisdom, had decided to enjoy his winter vacation rather than pore over hundreds of abstracts. GENERAL BOOKS AND REVIEWS
The Royal Society of Chemistry published a conference proceedings on particle size analysis (A@. Books have been issued on particle analysis in pharmaceuticals (Anand gases and liquids (AS). Also published were volumes on characterization of submicrometer aggregates (AS) and particle classification
(Am.
Groves (All)discussed special needs of size characterization for biological and pharmaceutical systems with emphasis on submicrometer particles. Poke et al. (A12, A13) reviewed the present state and trends in particle measurements and the importance of these measurements to process engineering. Henley (A14 reviewed particle analysis in ultrapure water and stressed the need for detecting less than 0.03-pm particles. Future applications of sensors to determine particle size distributionswere covered by Robert et al. 0 . General reviews have been published on particle size analysis for atmospheric and environmental particles (A16-A20),colloids and suspensions (A21-A23), catalysts (A24, A25), ceramics (A26-A28),clays (A29,A30), pharmaceuticalaerosols (431,A32) and emulsions (A33), and soil minerals (A34). SCATTERING TECHNIQUES
Light, neutron, and X-ray scattering techniques have been used extensively to study the structure and dynamics of particles and macromolecules in multicomponent systems. A wide range of sizes (angstrom to micrometers) can be determined under proper conditions. In a typical experiment, an incident electromagnetic beam of a given wavelength probes the material of interest, and the scattered radiation is analyzed with a suitable spectrometer. The momentum and energy differences between the scattered and incident radiation are used to characterize the structure and dynamics of the material. Scattering techniques are often nondestructive and nonperturbative to the medium investigated and thus useful for kinetics studies and the monitoring of events in situ. Analytical Chemistty, Vol. 67, No. 12,June 15, 1995 257R
Books and Reviews. Gouesbet et al. (B1) reviewed laserbased optical techniques used for the optical characterization of discrete particles contained in flows at rest or transported by moving flows. A review and discussion of the proper use of water liquid particle counters was given by Pohl (B2). Wiese (B3) discussed particle size determination by turbidimetry. A review of the use of optical methods for the determination of particle size and size distributions in solid propellant rocket motors and rocket exhaust plumes was prepared by Laredo and Netzer (B4). Bachalo (B5) reviewed, with many references, experimental scattering methods in multiphase studies, particularly velocimetry. Other reviews concerning velocimetry were published by Gautam (B6),Bakker et al. ( B q ,De Ramefort (B8),Heitor and co-workers (B9),and F'feifer (B10). Weiner (B11) reviewed advantages and disadvantages of particle sizing with photon correlation spectroscopy. Gouesbet et al. (BIZ) discussed work carried out in Rouen, France, during the last 10 years on light multiple scattering in particulate media. A review of fundamentals and instrument design of light scattering for measurement of particle size in combustion systems was given by Jones (B13). A review of light scattering and other techniques for particle characterization in ceramic processing was presented by Amal and Raper (B14). Finsey (B15) covered the application of photon correlation spectroscopy for determining particle size distributions. Zorll (B16) discussed international and German standards for quality control of powder paints with reference to powder properties such as particle size determination. Mintz and Zeiri (B17) reported on the application of singleparticle analysis models in kinetic measurements of gadsolid reactions, showing the dependence upon particle shape and size distributions. The application of fiber-optic photon correlation spectroscopy to on-line and in situ characterization of concentrated dispersions was reviewed by Wiese and Horn (B18).Azzopardi (B19)discussed the accuracy, limitations, and future of instrumentation for particle size analysis by far-field dfiaction. Schumacher and Bartels (B20) reviewed optical particle counters and their use in the control of particulate contamination of electronic chemicals. Photon Correlation Spectroscopy. Photon correlation spectroscopy (PCS) , also reported as quasi-elastic light scattering (QELS) or dynamic light scattering @E), is a convenient, frequently used technique to determine the effective size and size distribution of particles suspended in solution. The translational diffusion coefficient of the particles is measured from the intensity autocorrelation function determined experimentally by digital correlation techniques. An average effective size, or hydrodynamic diameter, is calculated for the particles using the StokesEinstein relation between the diffusion coefficient and particle diameter. Previous reviews (As) have covered developments in this field, such as diffusion wave spectroscopy (DWS), dual-color, crosscorrelation techniques, and fiber-optic methods to measure particle size in concentrated solutions. Van der Meeren et al. (C1) reviewed fiber-optic quasi-elastic light scattering (FOQELS) as a fast and reproducible characterization method. They discussed proper operating conditions and limitations of the technique. Buloenko et al. (CZ) described a new correlator for use in PCS measurements. Extensions of the PCS technique were considered by Harada and Asakura (C3) and by Stasiak and Cohen (C4), where the former examined the effect of radiation pressure on 258R
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the particles and the latter described Brownian particles in a solvent with internal relaxation. Both situations require modification to the usual calculations. Similarly, Killmann et al. (C5) investigated electrokinetic and electroviscous effects on PCS due to polarization of an electrostatic double layer on colloidal particles. A number of theoretical analyzes of PCS results were p u b lished. Fmsy et al. (C6) reported on a comparative study to resolve bimodal distributions. Ostrowsky (C7)explained various data analysis procedures used to look at liposome distributions. Finsy and co-workers (C8) presented a review of the use of single-value analysis and reconstruction in PCS. Another review was given by Finsy et al. (C9) on mono- and bimodal PCS analysis. Two analysis programs were used by Bashurova and co-workers (C10) to demonstrate the difficulties in interpreting a PCS experiment. Van der Meeren et al. ( C l l ) compared the results obtained with Malvern software and with the CONTIN program. It was concluded that the latter yielded much more accurate results for distribution widths. Grantham and McNeil-Watson (C12) used a new algorithm to allow PCS measurements that are relatively independent of solution concentration. A number of comparisons between PCS and other techniques were reported. The validity of a chronoamperometric method for measuring the electrophoretic mobilities of colloid particles was established by comparison with PCS by Rosseinsky et al. (C13). Finsy and co-workers (C14) compared PCS particle size results for polymer colloids with static light scattering, sedimentation field flow fractionation (SFFF), disk photosedimentometry,and electron microscopy. Test emulsions of fat droplets were investigated using PCS and conventional light microscopy by Mueller and Heinemann (C15). Results of the two measurements were similar, with those of PCS being more quantitative. McNeil-Watson and Parker (C16) used PCS to validate an incremental sizing method involving dielectric sedimentation sensors for measuring solid/ liquid dispersions. PCS was used with a variety of other techniques to determine the nature of materials in solutions. Sastry (C17) used PCS, static light scattering, and viscosity measurements to interpret micellar properties of ethylene oxide-styrene block copolymers in methyl alcohol. Specific viscosities of potassium zirconium titanate suspensions suggesting small particles were confirmed by Schulze Bergkamen et al. (C.18) with PCS and TEM. The colloidal stability of very fine gold suspensions was studied with PCS and microelectrophoretic measurements by Chen and co-workers (C19). Bartsch et al. (C20) used PCS and forced Rayleigh scattering to look at the dynamics of highly concentrated polystyrene micronetwork spheres. Maeda and Armes (C21) investigated colloidal dispersions of polypyrrole-silica nanoparticles using PCS, TEM, disk centrifuge photosedimentometry, and other techniques. GdCeCuO particles, prepared by sol/gel techniques, were studied by Mahia et al. (C22) with PCS, X-ray d ~ a c t i o nand , TEM. PCS was used in conjunctionwith electrophoresis by Polverari and van der Ven (C23) to look at layer thicknesses of polyethylene oxide adsorbed onto carboxylated butadiene-styrene particles. Levin et al. (C24 used Sedimentation field flow fractionation (SFFF) to characterize pharmaceutical emulsions. Good agreement was obtained with PCS values for the size of various particle fractions. Li and Caldwell (C25) characterized commercial fat emulsions using a combination of PCS, SFFF, and freeze-fracture electron microscopy. Westesen and Wehler (C26) looked at a model intravenous emulsion with PCS, TEM, and NMR. In
contrast to the latter two techniques, the PCS results showed no particle sizes under 140 nm. It was concluded that the broad size distribution of the sample precluded measurement of smaller particles by PCS. Mueller and Heinemann (C27-C29) analyzed fat emulsions using PCS, microscopy, and laser diffractometry. During the period covered by this review there was considerable use of PCS to characterize biological materials. Siekmann and Westesen (C30) demonstrated that stabilized tripalmitate particles were less than 100 nm. Bume and Sellen (C31) determined that gellan gels are largely stationary at a molecular level, unlike gels of flexible polymers such as polyacrylamide. The calculated molecular size and shape of vinculin in aqueous solution were compared to experimental results of PCS by Eimer et al. (C32). The decrease in size of casin micelles during the initial stages of renneting was shown by Home and Davidson (C33). Nordmeier (C34) studied the conformational and dynamic behavior of fractionated samples of pullan and dextran with static and dynamic LS. Aggregate formation of lysozyme was studied as a prototype of early protein crystallization by Georgalis et al. (C35). The particle size distribution of a submicrometer-sized soybean oil-in-water emulsion was determined by Westesen and Wehler (C36). Griftin and co-workers (C37) measured the extent of aggregation of bovine ,L-lactoglobulinupon heat quenching. The interactions between liposomes and chitosan were studied by Henriksen et al. (C38). A variety of other materials were examined using PCS. Ledin et al. (C39) characterized the size and size distribution of colloidal matter in deep groundwater. The size distribution of colloidal quartz and hematite as a function of pH, ionic strength, and fulvic acid was determined by Ledin and co-workers (C40). The migration of colloids through a shallow sandy aquifer was studied under controlled conditions of groundwater flow by Higgo et al. (41). The effective pore size of screen filters for solid particulate phases serving as carriers of heavy metal trace elements in surface waters was found by Karlsson and co-workers (C42). Garcia and colleagues (C43) studied the coulometric initiation of acrylamide latex particles. Richter et al. (C44) determined the particle size of siosomes of a number of alkanoyloxysilanes. Colloidal dispersions of surfactant-stabilized polypyrrole in aqueous media were analyzed by various techniques including PCS by DeArmitt and Armes (C45). Yin et al. (C46) determined the diffusion coefficients of divinylbenzene-styrene copolymer particles in good and 0 solvents. Flocculation of latex particles containing high levels of surface carboxylation was studied by Husband and Adams (C47). The stability of monodisperse polystyrene latexes was determined in the early stages of flocculation by Einarson and Berg (C48). A particularly interesting study using PCS was by Bronk et al. (C49), who used the technique to measure 0.5pm subparticles held in micrometersized droplets of saturated sodium chloride solution, which were captured in an electrodynamiclevitator and maintained at constant diameter for several days at a time. A flowing aerosol was analyzed for concentration, aperture, velocity, and particle size effects by Weber and co-workers (C50). Cloake (C51) reviewed controlled reference methods of particle size determination including homodyne detection, frequency spectrum, and scattering efficiency. PCS was used to study molecular interactions in crystallizing lysozyme solutions by Eberstein et al. (C52). Weber (C53) reported experiences in the determination of particle size in the nanometer range using PCS. Ross and Dimas
(C54) considered noise and distortion in correlation data. Ruf (C59 discussed data accuracy and resolution in particle sizing by PCS. The application of diffusing-wavespectroscopy to particle sizing in concentrated dispersions was described by Home and Davidson (C56). Angermann et al. (C57) studied basic aluminum chloride solutions using PCS. Problems in working with highconcentration submicrometer particle size distributions using correlation techniques were discussed by Trainer at al. (C58). Dellago and Horvath (C59) discussed the accuracy of size distribution information obtained from inversion analysis of optical data. Classical Light Scattering. Time-averaged, static, or classical light scattering (CLS) is a technique that has been used for many years to analyze materials in solution for weight-average molecular weight, z-average mean-square radius of gyration, particle shape, second virial coefficient, and interaction of particles with size ranges from submicrometer to tens of micrometers. A number of theoretical studies of this type of scattering were presented during this review period. Ma (01) calculated the total radiative flux emitted from an isothermal gadparticle mixture with anisotropic scatterers. It was found that the normalized difference in total emittance between no scattering and scattering solutions can be greater than 25% and thus must be included in the radiative heat-transfer analysis of a gas mixture. A theoretical analysis of particle-scattered radiation from surfaces restricted by a gas volume was made by Vasil'eva (02). Makino and Kurata (03) found that, in the case of particles larger than the wavelength of the radiation considered, the scattering characteristics can be affected strongly by the surface structure. Buitenhuis et al. (04) calculated the singleparticle scattering properties of cylinderical particles using the coupled dipole method and concluded from these results the range of validity of the Raleigh-Gans-Debye (RGD) theory. The particle scattering factors for suspensions of flat, tabular colloids were calculated by Watson and Jennings (05)using RGD theory. Yu and Fissan (06)discussed the effect on scattered light from a particle by an adjacent surface. In experimental work, Helmstedt and Schaefer (07)characterized particle masses, radii of gyration, and hydrodynamic radii of spherical poly(methy1 methacrylate) particles by combined CLS and PCS. Gamini and Mandel (08) investigated the secondary structure of xanthan in solutions of low salt concentration at room temperature using CLS. The conformation of poly(4vinylpyridine) in aqueous solutions of sulfuric acid was studied by light scattering and viscosity measurements by Yoshida et al. (09).Andersson and Ribbing (010) compared the scattering of light from surface irregularities and bulk voids in ceramic materials to particulate scattering. Light scattering measurements on particles of Be0 and Sic codrmed their interpretation. A qualitative experimental study of the effects of spherical particle scattering on the polarization of linearly polarized He/Ne laser beams was reported by Look and Chen (011). Chae and Lee (012) performed light scattering experiments on polymer particles to determine radiative transport properties of the suspension. Veefkind (023) constructed a light scattering measurement system to determine atmospheric aerosol content after tests with laboratory salt aerosols. Holve (014)correlated light scattering particle sizing results with sieving measurements. Vlasov et al. (015) used a scattered light optical method to measure particle content in bulk solutions. Analytical Chemistry, Vol. 67, No. 72,June 15, 1995
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A light scattering method for optical determination of size and concentration of particles below 100 nm was proposed by Mavliev (016). Kaolinite particle sizes of (2 pm were measured by Mackinnon and co-workers (017) using laser light scattering. Childers and Hieftje (018) described a laser light scattering instrument for the measurement of solute evaporation rates in analytical flames. The partition of sodium dodecyl sulfate into stratum comeum lipid liposomes was studied by Downing et al. (019) using freeze-fracture electron microscopy and laser light scattering particle size analysis. Witriol and Sidoni (020) studied forward polarized light scattering in layered spherical aerosol particles. The feasibility of using a pulsed copper vapor laser in light scattering particle detection and sizing was demonstrated by Grant et al. (021). The measurement of radii of sulfur hexduoride clusters generated by condensation in a supersonic nozzle was proposed by Kawahashi and co-workers (022) using a twocolor light scattering method. Hansen (023)used low-angle laser light scattering to measure particulates in a kinetic method for simultaneous assays for multiple analytes in a fluid sample. Charalampopouloset al. (024) explored the role of metal additives in light scattering from flame particulates. Light scattering particle size measurements were used in conjunction with ultracentrifugation/FT-IRtechniques by Papke (025)to study irreversible adsorption of dispersants onto colloids. Peters and Rudolph (026') performed filter tests of bis(2-ethylhexyl) sebacate aerosols with a new laser aerosol particle size spectrometer. Wiedemann (027) used IR laser scattering to determine the particle size of pyrotechnic granules that were not amenable to conventional sieve analysis. Reardon et al. (028)discussed the use of laser light scattering instruments over a 10-year period for controlling the quality of thermal spray powders. Fenvom and co-workers (029) studied the formation and growth of asphaltene particles from heavycrude petroleums by using a laser particle analyzer. Zege and Kokhanovsky (030) discussed optical particle sizing of coarse-dispersed aerosols under multiple light scattering in a medium. The suitability of a spectrodissymmetry method for size distribution analysis of polystyrene latexes was investigated by Antalik et al. (031). Jerkovic and Fissan (032) described a scattered light photometer for on-line monitoring of size distribution parameters and particle concentration. Bowen and co-workers (033) discussed a computer program that corrects for small particle light scattering in a commercial particle size analyzer. Van der Meeren et al. (034 examined the effect of light reflection at sample cuvette walls upon the results of both static and dynamic light scattering measurements. Turbidimetry. As a light beam passes through a medium, it can be both adsorbed and scattered by the materials of the medium. The scattered light can be analyzed to provide information about the scattering material. Irache et al. (El) studied the capacities and limits of the technique for determining the particle concentration in latexes. They showed the results were similar to gravimetrically measured concentrations. Wang et al. (E2) presented a turbidimetric method for particle size determination. A computer simulation showed that their method is capable of determining both an average particle diameter and size distribution in a very broad size range. An integral scattering cross section and the turbidity wavelength were calculated for fractal clusters by Khelbtsov and Melnikov (E3). Coumil and Crawley (E4)developed a single-wavelength turbidity sensor to evaluate 260R
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processes of agglomeration and crystal germination of solids from dilute liquid solutions. A photoelectron flow microscope was used by Akopov et al. (E5) to automatically determine water turbidity. Emulsion stability is often expressed in terms of the ratio of two turbidities measured at different wavelengths; Gunji and coworkers ( E a calculated an expression for this ratio and tested it experimentally using oil-in-water emulsions. Turbidimetric methods are frequently used to determine properties of biomedical material. Corradini et al. (E7) used a turbidometric assay to monitor cholesterol crystal growth in human gallbladder bile. Kraus (Es)used a turbidity measurement in a clinical analyzer as an indicator of analyte formation in a biological material. Rus and co-workers (E9)studied the effect of poly(ethy1ene glycol) upon the turbidity results from lipemic sera and from intralipid solutions. The treatment caused the coalescence of light scattering complexes. A turbidometric latex agglutination/inhibition assay was developed for the estimation of smooth lipopolysaccharide content in Brucella cells (El0). Wang and Broda ( E l l ) found that hydrolysis of a stable xylan suspension lowered its turbidity due to the formation of soluble fragments. A simple immunoturbidmetric method for quantifying lipoprotein(a) serum, based upon latex-enhanced particle agglutination, was described by Borque and co-workers (E12). Levinson and Wagner (E13) compared total cholesterol and D L cholesterol of blood serum as markers for coronary artery disease using turbidimetric and other methods. Rodgers et al. (E14 used turbidimetric measurements and PCS to show that urinary macromolecules are promoters of calcium oxalate nucleation. Jin and co-workers (El5) monitored recombinant inclusion body recovery in an industrial disk stack centrifuge by measurement of tubidimetric ratios. The inclusion bodies scatter light more effectivelyat 600nm than smaller debris particles when compared to the light scattered at 420 nm. Simo et al. (E16)described a simple and rapid particle-enhanced turbidimetric immunoassay suitable for routine application in a commercial centrifuge with commercially available reagents. Beaufays and co-workers (E13 reported a kinetic turbidimetric study of a model of calcium oxalate iethogenesis. Nonbiological studies using turbidimetric methods were also reported. Giordano-Palmino et al. (E18) studied the adsorption of the nonionic surfactant 1x100 in two silica suspensions to relate the structure of the adsorbed layer to the stability of the suspension. Durrer and co-workers (El9) compared a Fourier transform IR spectroscopy-attenuated total reflection technique with turbidimetry for direct quantification of adsorbed polystyrene latexes on rat intestinal mucosa; the methods agreed well. Pazourek et al. (E201 reported the separation and characterization of silica gel particles using gravitational field flow fractionation with a turbidimetric detector. The formulation and polymerization of microemulsions containing a mixture of cationic and ionic monomers were monitored by Corpart and Candau (E2l) using surface tension and turbidimetric methods. Apfel and co-workers (E22) described a turbidimetric analysis of particle interactions in latex suspensions. Cai et al. (E23) describe a particle sizer based upon the principle of light extinction. Light transmission measurements of submicrometer alumina particles in rocket plumes were made by Vaughn (E24and by Kim (E25).Horvath and Dellago (E26') reported on the accuracy of the particle size distribution information obtained from light extinction and scattering measurements.
Shah and Roy (E27) estimated the average particle size in a particulate system from light absorption coefficient measurements. Huve et al. (E28)used turbidity measurements to determine poly(lactic acid) nanoparticle concentration. Cai and Wang (E29) reported the determination of particle size distribution using a light extinction method. Akers et al. (E30)used light obscuration for particle size analysis of flocs. Particle size distributions obtained by contact filtration were compared to results from turbidity measurements by Clark and co-workers (E31). Diffraction. Sugrue and Row (F1,F2) discussed the true resolution in laser diffraction particle sizing. Wolfanger (F3) explored particle size determination by laser diffraction analysis as one aspect of quality assurance. A multitasking particle characterization system for laboratory and process control using laser diffraction was described by Parsons (F4).Frias et al. (F5) show that measurements of the particle size distribution as a function of time by a laser granulometer can be used to follow the reaction between pozzolan and lime. JSanerva and co-workers (3’6)evaluated laser light diffraction in the determination of the size distribution of sphericalparticles. The question of resolution in laser diffraction particle size analysis was addressed by Schoofs (F7). Simultaneous measurements of velocity and equivalent diameter of nonspherical particles were obtained by Morikita and co-workers (F8)using light diffraction. The effect of multiple scattering on the performance of a commercial Malvern particle sizer was determined by Paloposki and Kankkunen (Fs).A commercial light dfiction particle sizer was modified by Schuchmann and Schubert (FlO)for in-line particle size determination for jet agglomeration processes. Harfield (FII)studied particle size analysis of pigments using an enhanced laser diffraction analyzer. Agrawal and Pottsmith (F12) discussed optimizing the kernel in the theory for laser diffraction particle sizing. Friedrich et al. (F13)described progress in the characterization of sludge particles using laser diffraction measurements. Kuga et al. (F14)compared flaky graphite particle sizes measured by a laser diffraction method and by sedimentation;the ration of the two methods was used to obtain information about the particle shape. Olofsson and Nilsson (F15)showed that the laser light diffraction method could replace screening in the determination of particle sizesfor dry sugar products. Oshchepkov and Dubovik (F16)considered the problem of using laser diffraction spectrometry when the phenomenon of anomalous diffraction dominates the scattering. Grantham (F17)presented an introduction to particle sizing by laser diffraction and its advantages to the coating industry. The particle size determination of some pharmaceutical fillers by laser light diffraction was discussed by Merkku et al. (F18). Hitchen (F19)considered the effect of suspension medium refractive index on the particle size analysis of quartz by laser diffraction. Diffracted light was suggested by Noguchi and Kembo for analyzing optical noise in a system for detecting h e particles on silicon wafers (F20).A laser diffraction method was used by Caramella and co-workers (F21)for polymer particle swelling characterization. Bouvet (FZZ) showed that laser diffraction determination of the particle size of molding sand was rapid, reliable, and precise. Elhaus et al. (F23)compared laser diffraction sizing and phase Doppler anemometry as noninvasive measurements of spray particles in spray-drying towers. Both could be satisfactory
methods, but neither was foolproof. Ulrich and Luehmann (F24) described the adaptation of a laser diffraction spectrometer for particle size determination in industrial crystallization. Visible light diffraction was used by Lubetkin and Edser (F25)to measure interparticle separations in concentrated silica dispersions. Roberts and Durst (F26)used laser diffraction particle sizing to study liposome immunoaggregation-induced increases in liposome size over time and to determine optimal conditions for the application of the technique. ,Hanson and Theliander (F27) determined the outer dimensions of lime particles using a laser diffraction particle sizer. Particle breakup in shock waves was studied by single-particle light scattering by Strecker and Roth (FZS).Heffels et al. (F29)describe the use of azimuthal intensity variations in diffraction patterns for particle shape characterization. Morikita and co-workers (F30)evaluated the performance of a laser-based optical technique to measure simultaneously the velocity and diameter of nonspherical particles. Ge and co-workers (2731)used a combination of X-ray diffraction and other methods to analyze additive-coated silicon nitride powder. Nanocrystals of gallium arsinide were characterized by Hagan et al. (F32)using X-ray diffraction and a variety of other analysis techniques. Eifert (F33)reviewed laser granulometry in the monitoring of nanometer-size metal powder suspensions. Cohn (2734)described a new laser diffraction particle analyzer. Bondars et al. (F35)developed a computer program for the simulation of diffraction patterns of small particles. A new sample cell for use in difli-acting or scattering light for determining particle size distributions was discussed by Togawa and co-workers (F36). Wolff (F37)described the construction and application of a laser diffraction particle size analyzer. Sommer et al. (F38)compared results of particle size distributions obtained with a Fraunhofer diffraction instrument and an optical single-particle-sizing instrument. Optical Particle Counters. Recent legislation in the United States requiring water treatment plants to use particle size measurements, as well as turbidity measurements, to ensure the removal of pathogenic and toxic materials from water supplies has stimulated much new work in this area. Anderson and Pohl (GI)reviewed recent improvements in optical particle counters for use in high-purity water treatment systems. Katagiri and Ehara (G2) reported a comparative study of the performance of 28 commercial particle counters, 24 of which were laser instruments. Some counters were found to have very low counting efficiencies, typically less than 1%.The fact that the sequence of sample analyzes can affect the results is demonstratedby Srivastava (G3), who presented a protocol that avoids such problems. Boyko et al. (G4) consider problems of on-line sampling and real-time particle size measurement of process-generated dust. Allen (GS) discussed calibration of sieves against volume diameter using “tacky dots”. Satisfactory results were obtained in comparison with an optical counter for particles greater than 325 pm. The development of a number concentration standard for micrometer-sized particles was described by Horton and Mitchell (G6). Morgan and Bukauskas (G7) described a reliable, inexpensive method for the generation of streams of latex microspheres used in the calibration of optical particle counters. A method using the sizeclassifying properties of partially penetrating filters was developed by Liebhaber and Willeke (G8)for quickly and easily calibrating optical counters. Analytical Chemistry, Vol. 67, No. 72, June 75, 7995
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Sachweh (G9)described the measurement of size, concentration, and velocity of aerosol particles using an optical particle counter. The theory and mathematics for predicting the counting efficiency of optical particle counters based on light scattering were presented by Sommer (G10).Blesener and Halvorsen (G11) described an in situ vacuum particle detector designed for installation in the exhaust line of vacuum process tools. The physical properties of an optical aerosol particle counter were simulated by Jaenicke and Hanusch (G12)with a numerical algorithm that showed the iduence of monochromatic light scattering on the results of size distribution measurements. Schenek et al. (G13)described the operating principles of a system for electrooptical measurement of visible foreign matter in cotton where size distribution is reported. A method of particle monitoring in aseptic production laboratories using particle counters, which can be switched from one measuring location to another, was discussed by Muehlbacher (G14).The basic features of particle counters and a stepby-step approach for evaluating particle counter capabilities was discussed by Lewis and co-workers (G15).Nguyen and Draina (G16)described a molecular restricter suitable for use in optical particle counters. An electrostatic single-particle counter was introduced by Rossner (G17)which allows the detection of particles to 1.5 ym at various flow rates. Good agreement was obtained in comparison with an optical particle counter. Lieberman (G18)summerized calibration and correlation procedures and problems for operation of liquid-borne optical counters. Quant et al. (G19)discuss the performance characteristics of condensation particle counters using three continuousflow designs. Monte Carlo simulations of capture efficiencies of micrometer-sized particles from liquid flow by nonwoven filter media were compared to experimental laser particle counter values by Choi and co-workers (G20). Many applications of optical particle counters have been published. Jacksier et al. (G21)described techniques to quantify both particulate- and vapor-phase impurities in electronic-grade chlorine. The effect of different refractive indexes on particle sizing accuracy was investigated by Chae and Lee (G22).Lewis et al. (G23)demonstrated the usefulness of particle counters in assessing Giardia cyst penetration through a water treatment process. Latex sphere retention by microporous membranes was studied by Lee et al. (G24)by measuring upstream and downstream particle concentrations. A proposal for determining impurity particles in ULSI-grade chemicals was presented by Itano and co-workers (G25).Kratel particle counter techniques were used along with electron microscopy, tensiometry, and PCS by Muller et al. (G26)to determine amphiphilic properties of a prodrug of phenytoin. Quali and Pefferkorn (G27)reported the use of a particle counter technique for studying the rate of aggregate fragmentation induced by polymer adsorption. Several commercial dionized water particle monitors were evaluated by Fardi (G28)using suspensions of latex particles of differing size. Gurav et al. (G29)analyzed the generation of nanometer-size fullerene particles via vapor condensation. Field evaluations of the size distributions of outdoor submicrometer particles and selected combustion sources of indoor particles were made by Li and co-workers (G30).Kerkmann (G31)proposed a new monitoring system to determine evaporation residues of ultapure water using a condensation particle counter. 262R
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Nicoli et al. (G32)described a single-particle sensing method used in conjunction with an automatic dilution system. Sommer (G33)claimed to show how one can determine the true particle size distribution from highly concentrated samples of powders in suspensionby using an instrument that uses single-particle sizing and counting. The problem of pulsed recording of light scattering particles in a flux was studied by Oshchepkov (G34).Peng and Williams (G35)demonstrated the use of an on-line scanning laser microscope for measuring the size distribution of silica particle aggregates in concentrated suspensions. Haese (G36)discussed the use of particle counters based upon light scattering for monitoring and characterizing test aerosols. Holve and Harvill (G37)compared liquid and dry particle size distribution measurements using in-line and off-line optical instruments. The results of tests of a number of commercial instruments were reported. Baltog et al. (G38)compared results of their instrument with those of a Cilas-Alcatel device. An EPCS laserbased ensemble particle sizing instrument was tested with gasatomized zinc powders by Boyko and co-workers (G39). Allen and Bakker (G40)employed a Malvern Instruments particle sizer in tests for industrial spray drying control. Sparks and Dobbs (G41)tested a Partec laser backscatter particle size analyzer under a variety of conditions. The use of a PAMAS particle size analyzer was described by Drissen et al. (G42). Pietsch et al. (G43)discussed corrections for phase distortion in particle size measurements made with a Malvern spray analyzer. Measurement methods and experiences with the use of an Analyzette instrument in particle size determination were described by Svehlova (G44). Velocimetry. Particle image velocimetry (I",particle tracking velocimetry 0, laser Doppler velocimetry (LDV), multipoint number fluctuation laser velocimetry (MNFLV), and other velocimetry methods are all variations of a technique for measuring the properties of moving particles from their light scattering behavior. Many publications investigating these techniques were made during this review period because of their relevance to commercial applications. Cole (HI) patented a method for analyzing particles using LDV. Kojima et al. (H2)dealt with measuring the velocity of flow fields. A particle image technique in PIV is described by Huang and coworkers (H3).Malik et al. (H4) discussed PTV techniques with a three-camera system used for three-dimensional flows. Maas and co-workers (H5) also discussed three-dimensional PTV methods. The steps necessary in the application of PIV techniques to the measurement of fluid velocities were described by Farrell (H6). Rockwell et al. (H7)described laser scanning in high-imagedensity PIV. Yamamoto and co-workers (H8)proposed a binary crosscorrelation method using an algorithm for particle identification. A new two-stage algorithm for particle tracking in flows was derived by Yagoh et al. (H9).A comparison was made between laser diffraction and phase Doppler velocimeter techniques in highturbidity, smalldiameter sprays by Cossali and Hardalupas (H10). Liu and Adrian (H11)described combined velocimetry optical imaging systems for two-phase flow. A laser three-focusvelocimeter using an array of laser diodes for simultaneous measurement of particle size and velocity was proposed by Nakatani et al. (H12). Coil and Farrell (H13),using diffraction-based diagnosis, reported particle size measurements in sprays. Takagi (H14)
claimed that electrophoretic light scattering based on Doppler velocimetry can be taken as a nanometer-scale version of traditional electrophoresis. Durst and co-workers (H15) discussed the use of laser Doppler measuring systems for flow and particle measurements. Bauckhage (HIS)considered the possibility of particle size determination in fine-disperse systems of high concentrations or coarse-disperse systems with fine-disperse components using phase Doppler anemometry. The electromagnetic scattering problem of laser Doppler anemometry, where the scattering particles act as tracers of fluid flow, is discussed by Raszillier and Durst (H17). The analysis of swirling particulate two-phase flow using phase Doppler anemometry is reported by Sommerfeld and Qiu (H18, H19).Calibration of a phase Doppler particle analyzer with monodisperse droplets is described by Ceman et al. (H20). Albrecht and co-workers (H21) presented a generalized theory for the simultaneousmeasurement of particle size and velocity using laser Doppler and laser two-focus methods. The dependence of particle size distribution on the usercontrolled settings of a phase Doppler anemometry signal processor was discussed by Wriedt (HZZ). Qiu and Sommerfeld (H23) claimed a reliable method for determining measurement volume size and particle mass flux using phase Doppler anemometry. Naqwi and Durst (H24) extend the phase Doppler technique to sizing and material recognition of submicrometer particles. Rudoff et al. (H25) used the phase Doppler particle analyzer for in-situ sizing of fine spherical particles of polystyrene. The application of the phase Doppler technique to optically absorbent liquids was discussed by Manasse and co-workers (HZS). Naqwi and Ziema (H27) report an extended phase Doppler anemometer for sizing particles smaller than 10 pm, Naqwi et al. (H28) considered the measurement of particle size and refractive index using an extended phase Doppler system. Prangishvili and co-workers (H29) proposed a method to analyze moving particles using time-averaging of coherent radiation. Naqwi et al. (H30)claim that diode-pumped Nd/YAG lasers provide new opportunities for miniaturizing laser Doppler and phase Doppler anemometers. Neutron and X-ray Scattering. Neutron scattering and X-ray scattering, although more complicated experimentally than light scattering, are used for analysis in the case of samples where optical wavelength radiation cannot penetrate or is absorbed by the material. Cosgrove et al. (Ill used small-angle neutron scattering (SANS) to determine the structure of layers of polystyrene of various molecular weights terminally grafted onto surface-modified silicas. Pathmamanoharan and Philipse (12) described the preparation of alkane-grafted silica particles that are stable in nonpolar solvents; these particles were said to be suitable for SANS and X-ray scattering studies. A method was proposed by Plavnik and Troshkin (13) for determining size distributions of colloid particles from limited portions of smallangle neutron or X-ray scattering curves. Kim et al. (14)used a direct emulsification technique to create an artificial polystyrene latex with a narrow molecular weight distribution and uniform particle size; the films were analyzed by small-angle neutron scattering. Methods for the free-form determination of particle size distributions by small-angle scattering for systems with hardsphere interactions were described by Skov Pederson (15). Krauthaeuser et al. (IS)determined particle size distributions of mesoscopic metallic systems by small-angleX-ray scattering using
an indirect transformation method. Zhu and cc-workers (17) discussed factors involved in determining the size distribution of nanometer particles using small-angle X-ray scattering. Zhu et al. (18) concluded that a combination of small-angle X-ray scattering and TEM is the most rapid, accurate, and comprehensive method to evaluate and measure nanoparticle size. Small-angle X-ray scattering was used by Shinohara and co-workers (19) to determine the particle size of colloidal silica and ferrite materials. SIZE EXCLUSION AND HYDRODYNAMIC CHROMATMRAPHY As in previous years, there have been few reported studies on the use of size exclusion chromatography (SEC) and hydrodynamic chromatography (HDC) for determining the distribution of particles. Although SEC and HDC require only conventional HPLC instrumentation,appropriate software is needed to correct for band broadening and detector response. Since these methods are secondarytechniques,calibration with standards is necessary. Furthermore, as in any chromatographic system, mobile-phase conditions must be optimized to avoid particle-packing interactions. Lastly, loss of sample via entrapment of aggregatesor larger particles by the column is a concern. However, a number of chemical industries do use SEC/HDC for quality control or for monitoring relative particle size distributions among samples. In general, SEC and packedcolumn HDC are applicable for submicrometer particles, and capillary column HDC can typically handle samples in the range of 0.7-50 pm. HDC has also been used for the separation of macromolecules in solution, in which relevant papers are given below. Schauer and Dulong V I ) compared the use of HDC and field flow fractionation for particle size analysis. Although the emphasis was on macromolecules, Revillion v.2) discussed the use of HDC, and other chromatographic methods, as alternative techniques to SEC. Tijssen and Bos v3) compared the separation mechanisms of SEC and HDC. Stegeman et al. v4) investigated the use of HDC, using columns packed with 1.5pm nonporous particles, for the separation of polymers in solution. Elution behavior in these columns could be explained using migration theories developed for open tubes. The mechanism of capillary HDC, in terms of retention time, resolution, and peak width, was reported by Shiragami (Is). Capillary HDC was also applied to polymer latexes v6). SEC was used to determine the particle size distribution of aggregated hydrophobized polysaccharide particles v7), colloidal silica (40-1000 A) US), colloidal gold (3-20 nm) v9), and colloidal cadmium sulfide and zinc sulfide (2-20 nm) (/IO),Pille and Solomon (111) used SEC with an on-line light scattering detector to study the formation of microgels in which living poly(4tert-butylstyrene) was reacted with 1,4divinylbenzeneas the cross-linker. FIELD FLOW FRACTIONATION Clearly, of all the particle separation techniques, field flow fractionation (FFF) is the most versatile in terms of separation range, selectivity, and resolution. FFF comprises a family of methodologies in which an external field is applied perpendicular to a flow stream generated in a thin, open channel of defined geometry. The external cross-field causes particles (or dissolved macromolecules) to partition among velocity streamlinesthat are formed by the parabolic velocity profile in the carrier fluid. The AnalyticalChemistty, Vol. 67, No. 12, June 15, 1995
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velocity profile is close to zero near the walls of the channel and reaches a maximum at the center of the channel. When an external cross-field is applied, particles or macromolecules are forced or focused into different velocity streamlines, depending on the nature of interaction with the external field. For example, in the case of sedimentation FFF, the most commonly used form of FFF, the channel is coiled into a centrifuge, and the sedimentation cross-field forces the more massive or dense particles closer to the wall. As a result, these particles sample the lower velocity streamlines and are more retained than less massive or dense particles, which elute first. Retention is controlled by the applied centrifugal force. One of the attractive features of sedimentation FFF,is that no calibration is required-the relationship between retention and particle size is determined from first principles. Cross-fieldsthat have been used are sedimentation, electrical, flow, thermal, magnetic, and dielectric (K1).Furthermore, there can be different operating modes for each cross-field, such as steric and hyperlayer sedimentation FFF. The steric mode is applicable to larger particles ('1 pm) which physically make contact with the wall. Hyperlayer FFF occurs at higher flow velocities in which hydrodynamic lift forces cause the particle to move away from the wall. The nomenclature and theory of FFF is covered in depth by Giddings (KI),the originator of FFF. The most commonly used FFF techniques are sedimentation (for particles), thermal (for macromolecules), and flow (for both particles and macromolecules). (If higher centrifugal forces are used, high molecular weight polymers can also be separated by sedimentation FFF.) The particle separation range of FFF is typically in the order of 0.01-100 pm, depending on the operating mode. As in previous years, we fhd that FFF is still an active area of research and there is a wealth of published applications. Unfortunately, the major reason why FFF has not achieved the popularity and widespread usage it deserves, is, until recently, the lack of commercial instrumentation. In this section, the use of FFF for the characterization of dissolved macromolecules, as well as particles, is covered. Reviews and General Theory. Proceedings of a symposium on SEC and FFF held August 1991 at the American Chemical Society National Meeting were published (K2). The Third International Symposium on FFF was reviewed in ref K3. A number of reviews on FFF have appeared (K4-K10) including a review on hyperlayer FFF (K1I), Giddings (K12)examined the theoretical and practical aspects of miniaturizing FFF channels, and Martin (K13) reviewed advances in miniaturized separation methods including FFF. Giddings (K14)discussed the theory of field-programmedFFF with corrections for steric effects, which is caused by the finite size of particles undergoing migration in the channel. The retention (steric) inversion phenomenon, and its practical implications in particle size, density, and shape analysis, was also examined (K15).Janca (K16,K17) described a generalized isoperichoric focusing theory based on coupling a primary field with a secondary field. (See refs K21 and K22 for applications.) Anbdreev and Stefanovich (K18)developed a model to predict peak broadening resulting from reversible adsorption on the channel wall. Electrical FFF. Caldwell and Gao (K19, K20) described the construction and evaluation of an electrical FFF unit and applied 264R
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it to the separation of polystyrene latex standards. Janca and Audebert (K21,K22) introduced a new concept of isopycnic focusing in which a primary electric field is coupled with a secondary gravitational field. Examples are given for the separation of model polymer latexes and cross-linked polymers. Chmelik and Thormann (K23) compared isoelectric focusing FFF to capillary isoelectric focusing in terms of protein separation; the latter approach gave higher resolution than the former. Nakagama and Hobo (K24) used electrical FFF to separate low molecular weight organic compounds and investigated the influence of a packed channel on separation. Semenov (K25) studied particle separation in electrical FFF by monitoring current pulses generated by the passage of particles between planar electrodes. Stevens (K26) was issued a patent for applying an electric field that is periodically reversed in polarity. Flow FFF. Ratanathanawongs and Giddings (K27) compared flow FFF to sedimentation FFF using different types of particles. These investigators (K28)used flow/hyperlayer FFF to determine the particle sue distribution of HPLC silica supports. Giddings et al. (K29) described and evaluated a peak breakthrough technique for measuring the void volume of FFF channels, which was found to be particularly useful for flow FFF. Liu and Giddings (K30) used flow FFF to separate and measure the translational diffusion coefficients of linear and circular DNA chains. Giddings and co-workers (K31) used both flow FFF and thermal FFF to characterize model styrene- (ethylene-propylene) diblock copolymer micelles. Carlshaf and Joensson (K32, K33) described and evaluated the use of hollow fibers as separation channels in flow FFF. Liken et al. (K34) experimentally evaluated the theory of the asymmetrical flow FFF channel and applied this technique to the separation of monoclonal antibody aggregates (K35) and acid phosphatase (K36). Dean et al. (K37) described a fused-alumina frit with uniform pore structure for flow FFF. Dixon et al. (K38) used flow FFF to characterize the effluents from pulp and paper mills. Sedimentation FFF. General reviews on sedimentation FFF are given in refs K39 and K40. Giddings and co-workers (K41) empirically derived an expression to describe the lift forces in sedimentation/steric FFF and applied this mode of sedimentation FFF to chromosomes (K42), starch granules (K43), and nom spherical particles (K44). Hoyos and Martin (K45) described a retention theory for sedimentation FFF of concentrated particle suspensions in which particle/particle interactions are taken into account. Giddings et al. (K46) studied the performance of sedimentation FFF at elevated temperatures and found an improvement in separation power and speed. Mori took into account particle/wall interactions (K47) and zone broadening caused by secondary relaxation (K48)in sedimentation FFF. The technique of split flow (SPLITI) separation fractionation is described in refs K49-K51, in which the inlet and outlet ends of the flow channel have flow splitters to collect fractions. The use of gravitational FFF is described in detail by Chmelik and co-workers (E20,K52-K54). Other applications of sedimentation FFF include particle size studies of polybutadiene latexes (K55),poly(methy1methacrylate) latexes (K56), polystyrene latexes coated with triblock polymeric surfactants (K57), fluorocarbon (K58, K59) and other emulsions (C24, C25), and bacteria (K60).
Thermal FFF. Myers et al. (K61)presented a comprehensive review on the theory and applications of thermal FFF. Thermal FFF was also reviewed by Schimpf (K62).Giddings (K63) demonstrated that a type of universal calibration can be applied to thermal FFF. Calibration constants are based on ordinary and thermal diffusivities. Nguyen and Beckett (K64)developed and tested a calibration procedure which uses broad molecular weight standards. Liu and Giddings (K65)investigated the thermal diffusion phenomenon underlying the separation mechanism in thermal FFF and obtained thermal diffusion coefficients for aqueous suspensions of silica particles and different types of latexes. Schimpf et al. (K66,K67) used thermal FFF to study thermal diffusion coefficients of random and block isoprene-styrene copolymers and found that they were a linear function of copolymer composition. Jeon et al. (K68)studied the retention behavior of poly(ethy1eneco-vinyl acetates) with respect to effective separation and thermal diffusion coefficients; the latter was dependent on copolymer composition. Stegeman et al. (K69)developed a general theory for comparing the molecular weight separation capabilities of thermal FFF, packed-column HDC, capillary HDC, and SEC. This group also investigated factors affecting the separation speed of thermal FFF (K70)and computed the velocity profiles for 12 solvents at different temperature gradients (K71).Furthermore,they studied the retention behavior, using different solvents, of polytetrahyrofuran (K72),and polybutadiene (K73). Other applications of thermal FFF are the separation of polysaccharides in DMSO (K74),and the determination of the gel content of electron beam-irradiated high molecular weight poly(methy1 methacrylate) (K75). ELECTROLONE SENSING Using the Coulter technique, Berge et al. (L1)studied singleparticle dynamics over long periods of time by reversing the pressure gradient shortly after a particle exits the pore such that the same particle reenters the pore at regular intervals. Bezmkov et al. (U) introduced an interesting technique for counting single macromolecules. This was accomplished by incorporating natural channel-forming peptides into a bilayer lipid membrane to detect the passage of single molecules with gyration radii as small as 5-15 8. Figueiredo and cc-workers (L3)discussed a calibration method for electrozone sensing with emphasis on the mass integration method, especially the equations used to calculate the mass calibration constant. Merkus et al. (LI)described the need of a mass balance to analyze materials that contain a signiscant amount of submicrometer particles. Michoel (L5)compared results obtained between electrozone sensing and laser diffraction. Ryall and colleagues (L6) used the Coulter counter to obtain rates of calcium oxalate crystal growth and aggregation. A review was presented for determining cell volume in tissue cultures by the Coulter counter (L7). Electrozone sensing was used to determine emulsion stability (Ls,L9). SEDIMENTATION AND CENTRIFUQATION Sedimentation. Allen (MI) reviewed sedimentation methods of particle size analysis including gravitational sedimentation, centrifugal sedimentation, and linestart centrifugal sedimentation techniques. Particle size distribution errors occurring with
sedimentation analysis are discussed in refs M2 and M3. Leschonski (M4) presented a review on two-phase flow techniques for particle size analysis which include measurement of settling rate distributions in cross-flow systems. Malghan et al. (M5)discussed the statistical analysis of parameters affecting particle size distribution measurements of submicrometer silicon nitride powders using a Sedigraph. Schultz (M6)evaluated and intercompared results obtained using the Analyzette 22 particle sizer, the Micrometics Sedigraph, and the Sympatec Helos instrument. Centrifugation. Maechtle (M7) described a high-resolution ultracentrifugation method for determining particle size distributions in the submicrometer range. A method was derived by Gropsian et al. (M8) for evaluating the size distribution of colloidal particles in magnetic liquids which consisted of correcting ultracentrifugation measurements for concentrationeffects using viscosity data. A simple capillary centrifuge with a UV/visible detector for determining particle size distribution of suspensions was presented by Schauer and Dulog (M9).The chemical heterogeneity of latexes was determined using zonal centrifugation in density gradients coupled with scanning of the centrifuge tubes to determine light scattering (MlO). Bowen et al. (M11)used a cuvette photocentrifuge (Horiba CAPA-700) for submicrometer particle size measurements of alumina and quartz powders. A Brookhaven disk centrifuge photosedimentometer was used to determine the particle size distribution of polybutadiene latexes having a particle size range of 50-300 nm (M12). Li et al. (M13) employed a disk centrifuge to determine the particle size distribution of lubricant powders and to study the effects of particle shape, extinction coefficient, and dispersity. Allen (M14)evaluated a commercial X-ray scanning sedimentometer (DuPont/Brookhaven scanning X-ray disk centrifuge) that can operate in either the gravitational or centrifugal mode. The particle size range of the instrument is
