Determination of Complex Particle Size Distributions Using Packed

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Chapter 11 Determination of Complex Particle Size Distributions Using Packed Column Hydrodynamic Chromatography E. Meehan and K. Tribe Polymer Laboratories Ltd., Essex Road, Church Stretton, Shropshire SY6 6AX, United Kingdom

Packed column hydrodynamic chromatography (HDC) is a well established separation technique which fractionates particles according to their size. The practical range of particle size which can be separated by this technique is 53000nm. Using appropriate calibration procedures it has been shown that packed column HDC can be used to measure particle size distributions for a wide range of particle types. Commercial instrumentation for the application of this technology has recently been introduced. This article describes the design and operation of such instrumentation and illustrates the capabilities of the technique using a variety of particles which differ in composition, density and particle size.

© 2004 American Chemical Society In Particle Sizing and Characterization; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Particle size can have a fundamental effect on the physical properties of colloidal dispersions. For many systems the measurement of average particle size is not sufficient, the presence of different size populations will have a strong influence on properties and could be related to the production process. Hydrodynamic chromatography (HDC), a technique for separating small particles by flow through a packed bed of non-porous particles, was invented by Small (1) and first described in 1974 (2). HDC provides a method for the separation of particles in suspension based on their size, eluting in the order largest to smallest. Based on this method offractionation,HDC can then be applied to the determination of particle size for colloidal dispersions. Fractionation techniques, including packed column HDC, can offer advantages over non-fractionation techniques for particle sizing in that the method produces information abouc the average particle size and the distribution of particle size. Non-fractionation techniques are less well suited for the analysis of multi-modal samples or samples with broad particle size distributions due to the low resolution of the method. This paper describes the application of a new instrument, the PL-PSDA (Polymer Laboratories, UK), which can be used to determine the particle size distribution of a variety of colloidal dispersions based on the principle of packed column HDC.

Experimental The PL-PSDA is an integrated, automated system in which the major components consist of a solvent delivery system, automated sample handling, separating cartridge and concentration detector. Instrument control, data acquisition, data analysis and reporting are all accessed from one PC based graphical user interface operating in a Windows environment. In the PL-PSDA, a proprietary eluent (water based and containing a mixture of salts and surfactants at a controlled pH) is continuously pumped through the system at a constant flow rate of around 2.0 ml/min. A carousel based autosampler enables multiple sample vials to be loaded and sampled for continuous, unattended operation. Samples are prepared in 2 ml vials as a dilute slurry in the eluent after pre-filtration with 0.45μιη or 2.0μπι membrane filters, depending on the cartridge type employed. The sample under investigation, and a small molecule marker solution (3-nitrobenzenesulfonic acid), are introduced into the system via a two position, electrically actuated valve such that the eluent flow is not interrupted. At the heart of the system is a separating "cartridge", which consists of columns packed with non-porous polymeric particles of controlled particle size distribution, and the choice of cartridge dictates the particle size measuring range of the instrument with options available to cover the rangefrom5nm to 3um. In the HDC technique described here, a suspension of particles flows through the cartridge which contains a packed bed of non-porous beads. The inter-particle spaces between the beads of the packed bed can be visualized as a

In Particle Sizing and Characterization; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.


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series of small capillaries, within each capillary a velocity gradient exists described by a parabolic flow profile. The larger particles spend a greater proportion of time near the center of the capillary, experience the greater flow and are eluted from the column first The fractionation is therefore based on the different eluent velocity experienced by particles of different size due to the velocity gradient in the capillaries and solute particles elute from the system in order of decreasing size. Following the separation, sample components eluting from the cartridge are detected by a U V detector (λ=254ηηι) and in this technique detection occurs predominantly because the particles scatter the incident U V light, although there will be some contribution from absorption i f the particles contain a U V chromaphore. The total analysis time is less than 10 minutes. The detector response is used to calculate the concentration of particles of different size present in the sample and is corrected for Mie scattering using a detector response calibration curve. The subsequent computation of particle size also requires a calibration procedure which is specific to the separating cartridge employed. The primary system calibration procedure correlates particle size with retention factor, Rf, where R f = Retention time marker / Retention time particle. The calibration plot is normally generated using 6 to 9 individual particle size standards and the data is fitted using a quadratic equation. Using a suite of specifically designed software algorithms, which include peak shape fitting and band broadening corrections, the instrument thus provides a complete particle size distribution for a sample as well as a calculated mean diameter and coefficient of variation (CV).

Universal Calibration Cartridge calibration is usually performed using a series of certified polystyrene latex standards (Duke Scientific, Palo Alto C A , USA) to correlate R f and particle size. Other types of certified particles have been analysed to illustrate the universality of the technique. Silica and melamine particles (G Kisker, Steinfurt, Germany), albeit in a more limited range of sizes, were analysed and the results for all three types of particle plotted on a "universal" calibration. The resultant calibration data in figure 1 shows that all types of particles lie on the same calibration line indicating that the separation method is independent of particle density (polystyrene = 1.05g/ml, melamine = 1.51g/ml, silica = 2.00g/ml). This observation suggests that the H D C technique described here has wide applicability for the determination of particle size distribution of many different particle chemistries.


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Figure 1. Particle size calibration data for different particle types with different particle densities.

Resolution in packed column HDC The ability to separate particles of different size is predominantly controlled by the size of the beads in the packed bed of the separating cartridge. According to classical liquid chromatography theory, by decreasing the packed bed bead size, the chromatographic efficiency of the cartridge is increased (3). Also, with smaller packed bed bead size, the effective diameter of the inter-particle "capillaries" is reduced. Assuming a simple exclusion model, the ratio of solute particle radius to effective capillary radius, λ, determines to what extent the average particle velocity differs from the average solvent velocity (4). Therefore the packed bed bead size effectively controls the selectivity of the separation, or the ability to separate solute particles of similar sizes. The effect of packed bed bead size on HDC resolution is illustrated in figure 2 where the raw data chromatograms obtained for a series of five polystyrene latex particle size standards are overlaid and compared for two cartridges, Type 1 (smaller bead size) and Type 2 (larger bead size).


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Figure 2a. Chromatograms of polystyrene latex standards (21nm, 50nm, I02nm, 204nm, 304nm) obtained using a smaller bed bead size Type 1 cartridge

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Figure 2b. Chromatograms of polystyrene latex standards (21nm, 50nm, 102nm, 204nm, 304nm) obtained using a larger bed bead size Type 2 cartridge


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One limitation however is that as the inter-particle capillary diameter is reduced, the maximum size of particle that can pass through it is also reduced. Hence a small bead size cartridge Type 1 offers much greater resolution but over a more limited operating range (5-300nm) compared to a larger bead Type 2 cartridge (20-1500nm). As the upper size limit is approached, larger particles may not be completely recovered from the packed column and there is evidence of particle deposition in the cartridge.

Applications Packed column HDC has been used to characterize a variety of particle types covering a wide range of size and distribution. The basic requirement for sample preparation is that the sample must be fully dispersed in the PL-PSDA eluent. Sample concentration is typically in the range 0.1-10.0mg/ml depending on the particle type and particle size range of interest. Polymer latex particles, covering a wide range of size distribution and chemistry, have been extensively analysed. In many cases commercial samples of polymer latex contain a mixture of particles of different sizes and the high resolution of the HDC method is ideal for studying multi-modal distributions. This is illustrated in figure 3 which shows the particle size distribution determined for a mixture of equal quantities of three narrow dispersity polystyrene latex samples with nominal sizes of 102nm, 304nm and 519nm measured using the PL-PSDAfittedwith a Type 2 cartridge.

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Figure 3. Particle size distribution obtained for an equal mixture of three polystyrene latexes (102nm, 304nm, 519nm).


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The composition of the mixture of these three latex standards was varied in order to evaluate the ability of the technique to quantify the relative amounts of discrete populations in a multi-modal distribution. Table 1 compares the theoretical composition of four different mixtures with the results obtained by the H D C method.

Table I. Comparison of theoretical and calculated compositions of mixtures of three polystyrene latex standards (102nm:304nm:519nm) Theoretical composition (%) 33 : 33 : 3 3 3 50:34:16 16:34:50 25 : 50 : 25

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Calculated composition (%) 3 2 : 3 2 51:33:16 19:33:48 27 : 50 : 23

With this method it is possible to quantify with confidence the presence of a secondary species down to around 2-4% of the total volume of the sample. A further example of the effectiveness of this high resolution method is given in figure 4 which shows the particle size distribution measured for a commercial sample of P V C latex indicating the presence of three components varying in size, distribution and volume fraction in the sample.

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Figure 4. Particle size distribution determinedfor a commercial sample of PVC latex.


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A further example of the application of packed column HDC is in the measurement of colloidal gold particles which are used extensively in clinical and diagnostic applications. The particle size is generally

Determination of Complex Particle Size Distributions Using Packed

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