What can Micromeritics Analytical Services (MAS) do to help me? The following questions are often asked, “What does MAS do?” “What techniques are available to help me better understand my material?” “What type of information does that technique generate?” “What are the basic assumptions associated with this technique?” and, ultimately, “Why should I use your services rather than using another lab service?” This document was created to answer those questions. If you have additional questions or comments, please contact us at
[email protected] or 770-662-3630.
Particle Size Accurately determining particle size has become essential in many industries, and is a fundamental physical characteristic that must be selected, monitored, and controlled from the raw material source to the finished product. Selecting the appropriate particle size technique is not as easy as it may at first seem because there is no single measurement that is appropriate for all materials and applications. Micromeritics Analytical Services currently has available 7 different techniques for measuring particle size. Each technique has specific advantages, which are discussed below.
Dispersion is probably the most important step in gathering accurate, reproducible particle size information. Wet Dispersion - The three basic steps of dispersion are wetting, agitation, and stabilization. Wetting involves replacing the solid-air interface by a solid-liquid one. Agitation involves breaking down agglomerated clusters of particles into individual particles by means of mechanical energy. Stabilization involves the instrument operator introducing the appropriate surfactants. Each material submitted to MAS for particle size analysis undergoes a dispersion study before any testing is conducted. Dry Dispersion - Some particles need to be dispersed in a dry form. A combination of mechanical forces produced by a vibratory feeder and sheer forces produced by air flow are used to separate agglomerations prior to the sample being introduced to the measurement zone of the analyzer.
Laser Light Scattering Technique Overview The great majority of particle sizing techniques measure specific parameters affected by particle size rather than measuring particle size directly. An example of this is the pattern of light scattered from a particle. Size is one of the characteristics of a particle that affects the scattering pattern, so size information can be inferred from measuring light intensity as a function of scattering angle. The technique of particle sizing by static light scattering is based on Mie theory (which encompasses Fraunhofer theory). Mie theory predicts the intensity vs. angle relationship of the scattering pattern as a function of the size of spherical scattering particles provided that other system variables are known and held constant. These variables include the wavelength of incident light and the relative refractive index of the sample material and suspension fluid.
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Advantages Laser light scattering is a very popular technique for measuring particle size primarily because of the speed of analysis. Almost any type of particle can be measured using this technique, but the accuracy is dependent on knowing other parameters such as the refractive index of the material and of the liquid. MAS recommends the use of a Saturn DigiSizer® 5200, which is the only commercially available particle sizing instrument to use a CCD detector. Use of the CCD allows the Saturn DigiSizer to capture a high resolution, digital representation of the scattering pattern produced by interaction of the laser light and the sample particles. This technique delivers exceptionally high levels of resolution, accuracy, repeatability, and reproducibility. These are important measurement qualities whether you are in research, product development, quality standards development, or production. MAS recently purchased a Malvern Mastersizer 2000, which is equipped with both a dry dispersion module and a liquid dispersion module. The Mastersizer provides dry dispersion sample testing capabilities and also allows comparison to historical data produced by other Mastersizers.
Typical Reports
Volume Frequency vs. Diameter Volume Frequency Percent
Cumulative Finer Volume Percent
100
2
0
0 0.1
0.5
1
5
10
Particle Diameter (µm)
Cumulative Finer Volume Percent
Volume Frequency Percent
Results typically are reported in graphical format such as below and also in tabular form with summaries of mean, median, and mode particle size values. As with all MAS analyses, an extensive set of raw and reduced data reports are available. The results below are from a sample of calcium carbonate.
Particle Size by Volume Distribution Geometric Statistics Mean = 0.870 Mode = 1.257 Median = 0.932
Electrical Sensing Zone Particle Size Overview The Electrical Sensing Zone technique commonly is referred to as the Elzone or “Coulter principle.” Each particle in a dispersion displaces a certain amount of conductive liquid, or electrolyte. An electrical circuit is created between two electrodes immersed in an electrolyte on opposite sides of a small orifice. An electrical signal proportional to the volume of the particle is produced as particles are swept through the orifice by a flow of the electrolyte.
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Advantages The Electrical Sensing Zone technique produces the highest resolution particle size data of any automated technique currently available. A major advantage of the Elzone is the particle size results are independent of physical properties such as sphericity or refractive index. The Electrical Sensing Zone technique is ideal for sizing and counting a wide variety of particulate materials, both organic and inorganic. Elzone particle size analyzers have the ability to measure low concentration samples and can produce accurate data where other techniques are limited. The results are reported as equivalent spherical diameter, but the technique is a direct measure of the volume of the particles.
Typical Reports The electrical sensing zone technique reports both a volume distribution and a number distribution. Results typically are reported in graphical format such as illustrated below and also in tabular form with summaries of the median and mode particle size values. Notice how the small particles which are present in the number distribution plot contribute almost nothing to the cumulative volume of particles depicted in the second illustration. The results below are from a sample of garnet.
Inc. Number% vs. Diameter Graph Cum. Number% vs. Diameter
100
2
0
0 3
4
5
6
7
8 9 10 Particle Diameter (µm)
20
30
Cumulative Number Percent
Incremental Number Percent
Incremental Number Percent vs. Particle Diameter Graph
Inc. Volume% vs. Diameter Graph Cum. Volume% vs. Diameter
100
2 0
0 3
4
5
6
7
8 9 10 Particle Diameter (µm)
Statistics (Volume Distribution) Mode = 11.59 Median = 11.34
20
30
Statistics (Number Distribution) Mode = 9.848 Median = 8.760
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Cumulative Volume Percent
Incremental Volume Percent
Incremental Volume Percent vs. Particle Diameter Graph
Particle Size by X-Ray Sedimentation Overview The SediGraph determines particle size by measuring the gravity-induced settling rates of different size particles in a liquid with known properties. The rate at which particles fall through the liquid is described by Stokes’ Law. The largest particles fall fastest while the smallest particles fall slowest, until all particles have settled and the liquid in the measuring zone is clear of any particles.
Advantages The SediGraph has established an outstanding reputation as a highly repeatable and reproducible particle sizing technique. The detection of X-ray absorption allows the SediGraph to measure directly the relative mass percent, which is a considerable advantage over other techniques in which the mass percent in each size fraction is inferred. The SediGraph allows for a variety of liquids to be used to disperse the sample and uses only 70mL of liquid per sample. If size determinations are needed to relate to the transport or deposition of particulates, then the SediGraph is the natural choice since the reported results relate directly to settling velocity.
Typical Reports Results typically are reported in graphical format such as illustrated below and also in tabular form with summaries of the mean, median, and mode particle size values. The results below are from a sample of tungsten carbide. Note that the results could be reported as mass fraction vs. particle size or mass fraction vs. settling velocity. Mass Frequency Percent
Mass Frequency Percent
Cumulative Finer Mass Percent
100
2
50
0
0 10
5
1
0.5
0.1
Particle Diameter (µm)
Mean = 3.246
Mass Distribution Arithmetic Statistics (um) Mode = 2.371
Cumulative Finer Mass Percent
Mass Frequency vs. Diameter
Median = 2.456
Other Particle Size Techniques Sieves are a popular and easy way to measure particle size. The disadvantage of Sieve analysis is the lack of resolution.
Dynamic Image Analysis rapidly takes images of articles as they are falling and uses a computer to classify the individual particles projected area. This technique works very well for particles larger than 200 micrometers and is much higher resolution than typical sieve analysis.
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Microscopy A variety of microscopes is available to measure particle size. Photomicrographs allow the shape and morphology to be captured. Particle identification and contamination identification are common problems solved using this technique. Identifying problems associated with dissolution of pharmaceutical tablets is another common application for this technique. The contaminant in this photo was relatively easy to identify. The black specs found on this stopper were from an insect imbedded in the resin.
Mayer-Stowe Method Using mercury porosimetry information, particle size information can be calculated using a special calculation method developed by Mayer and Stowe. This application works well for samples that cannot easily be separated, such as aggregates or magnetic particles.
Density Density is simply defined as mass per unit volume (g/cc). Density measurements guide the formulation process and allow the user to predict the performance of manufactured products. Knowing and understanding the density can help identify and solve problems in many industries. Some of the applications include medical, pharmaceutical, ceramic, agricultural, construction, glass, and plastic. Some of the more common definitions for density and volume are listed below. • • • • • •
Absolute Volume – The volume occupied by a material excluding all pores and voids. Absolute Density – Mass divided by the absolute volume. Envelope Volume – The external volume of a material such as would be obtained by shrinking a film to contain it. Envelope Density – Mass divided by the envelope volume. Bulk Density – The apparent powder density under defined conditions. Tap Density – The apparent powder density under stated conditions of tapping.
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Surface Area Knowing the surface area helps predict how materials will burn, dissolve, adsorb, or otherwise react with other materials. The method of Brunauer, Emmett and Teller (BET) is commonly used to determine the total surface area of materials. Micromeritics Analytical Services uses several different instruments designed to provide highly accurate and reproducible surface area measurements.
Sample Preparation (Degassing) As with particle size, sample preparation is a key component in obtaining accurate and reproducible surface area results. A sample not adequately cleaned of adsorbed contaminants will outgas during an analysis, thus causing erroneous results, or could cause some portion of the surface to be inaccessible to the probe molecules, another source of error. Samples are prepared by heating the sample while simultaneously evacuating the sample tube or by a flow of inert gas (N2) across the sample which sweeps away the liberated contaminants.
Typical Reports BET Surface Area Report BET Surface Area: 24.0504 ± 0.0394 m²/g Slope: 0.179822 ± 0.000291 g/cm³ STP Y-Intercept: 0.001181 ± 0.000055 g/cm³ STP C: 153.287194 Qm: 5.5248 cm³/g STP Correlation Coefficient: 0.9999882 Molecular Cross-Sectional Area: 0.1620 nm² Relative Pressure (P/Po) 0.052600838 0.072072452 0.098798966 0.123680923 0.148428326 0.173186937 0.197979188 0.222827493 0.247743101 0.272720356 0.297854608
Quantity Adsorbed (cm³/g STP) 5.2840 5.5039 5.7702 6.0037 6.2343 6.4685 6.7089 6.9570 7.2130 7.4758 7.7441
1/[Q(Po/P) - 1)] 0.010507 0.014112 0.018999 0.023508 0.027958 0.032382 0.036794 0.041213 0.045658 0.050160 0.054778
Porosity The distribution of pore area or volume by pore size (diameter or radius) is another important characteristic that differentiates materials and supplies additional information to predict a material’s performance. These pore size measurements can be made using either gas adsorption or mercury porosimetry techniques.
Gas sorption is capable of measuring pores as small as 0.4 nm and as large as 200 nm. This includes micropores, mesopores, and a small portion of the macropore size range as defined by IUPAC. Typical
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applications include catalysts and catalyst supports, fuel cell materials, and adsorbents. A variety of gases can be used, which includes, but is not limited to, nitrogen, argon, carbon dioxide, methane, butane, ethylamine, oxygen, carbon monoxide, and hydrogen. Typical reports include a graphical presentation of the pore size distribution as well as tabular reports. The following results were obtained from an extruded silica-alumina catalyst. BJH Adsorption dV/dD Pore Volume Faas Correction
Silica-Alumina Reference (N2)
0.005
Pore Volume (cm³/g·Å)
0.004
0.003
0.002
0.001
0.000 10
50
100
500
1,000
Pore Diameter (Å)
Mercury porosimetry is another popular technique used to determine pore size and pore volume. The pore size measurement range is 500 micrometers (µm) down to 3 nanometers (nm). Obviously, the dynamic pore size range is much broader than that of gas adsorption. The applications are broader as well. Applications exist in the following industries: paper and paper coatings, catalyst, pharmaceutical, filter, geological, and the semiconductor industry to name a few. Typical reports include a graphical presentation of the pore size distribution as well as a summary report of common statistical values. The following results were obtained from a sample of rock.
Intrusion Data Summary Total Intrusion Volume = Total Pore Area = Median Pore Diameter (Volume) = Median Pore Diameter (Area) = Average Pore Diameter (4V/A) = Bulk Density at 1.60 psia = Apparent (skeletal) Density = Porosity = Stem Volume Used =
0.0673 mL/g 4.055 m²/g 5.3174 µm 0.0049 µm 0.0663 µm 2.2993 g/mL 2.7198 g/mL 15.4629 % 74 %
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Log Differential Intrusion vs Pore size
Log Differential Intrusion (mL/g)
Intrusion for Cycle 1
0.04
0.02
0.00 100
10 1 Pore size Diameter (µm)
0.1
Chemical Adsorption Chemisorption is the interaction of an active gas and a solid surface, involving the sharing of electrons between the adsorptive molecule and the surface. Chemisorption is used to determine the percent metal dispersion, the active metal surface area, the number of reducible metal species present, as well as the number, type, and strength of active sites accessible to the probe gas or vapor. Traditionally, this technique has been used by the catalyst industry. Recently, other industries have begun to realize the benefit of understanding how their material reacts with different probe molecules under certain conditions. The reports are as varied as the number of experiments. The example below is from an analysis to determine the number of acid sites and the strength of the acid sites on a zeolite sample. TCD Signal (a.u.) vs. Temperature 10 C/min
5 C/min
20 C/min
2 C/min
15 C/min
7 C/min
300 350 Temperature (°C)
400
30 C/min
TCD Signal (a.u.)
0.10
0.05
0.00 150
200
250
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450
500
What Micromeritics Analytical Services offers that other labs cannot Micromeritics Analytical Services has a complete array of Micromeritics particle characterization products as well as instruments from other manufacturers. All of our instruments have been Installation Qualified (IQ), Operationally Qualified (OQ), and Performance Qualified (PQ). Micromeritics Analytical Services operates under cGMP/GLP conditions. We are licensed by the state of Georgia and United States Drug Enforcement Agency to handle class 2, 3, 4, and 5 controlled substances. We are also a registered analytical laboratory with the United States Food and Drug Administration and have been certified by an independent expert witness for the U.S. FDA. Our laboratory staff is composed entirely of degreed, experienced Scientists and, furthermore, we are supported by the Engineering and Scientific staff of Micromeritics Instrument Corporation. This support is called upon to solve the most challenging analytical problems including methods development. Our prices are competitive and the quality of data we produce is unsurpassed. Our mission is to satisfy your needs for particle characterization. We eagerly await your call for assistance.
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