Simultaneous Measurement of Diffraction and Image Analysis with Solvents as Sample Preparation Suspension Fluid

There are many materials that can interact or be suspended in an aqueous environment. There are others, however, which are not compatible with measurement in water, as a result of impaired reactivity, solubility or other issues. The importance of using solvents other than water for measuring such materials in a rapid and efficient way was recognized by the Microtrac Particle Analysis Laboratory (PAL) as far back as the beginning of the 1980s.

As a result, they developed new Microtrac instruments that allowed the experimental environment to be switched from aqueous to organic solvent without the need to alter any other component in the fluid path. The materials in the fluid path, which is the site of contact, are all compatible with organic solvent, and include:

  • Glass
  • Kalrez, a fluoropolymer
  • Stainless steel
  • Teflon

The Microtrac Sync continues the same tradition of organic solvent compatibility. The solvent keeps the particles being measured in the intact state. This article describes several materials which require the use of solvent.

Only a transfer fluid is necessary in order to shift the setup from aqueous to organic solvent. The most commonly used transfer fluid is isopropyl alcohol, IPA, which mixes in both water and organic fluids, and can be used to rinse any system to remove water from it. This also allows the subsequent use of the final solvent without any issues, as IPA mixes effectively with many organic fluids as well. The presence of water would create droplets within the organic solvent due to immiscibility issues.

The SLAN-28 Sample Preparation Guide contains a wealth of information on how to prepare samples for many types of materials, and can be downloaded from Microtrac.com which also provides values for viscosity, wetting or dispersal chemicals and refractive index.

In this situation, image analysis and laser diffraction are performed on the same sample at the same time, using a single sample cell. This prevents the splitting of samples using internal diversion tubing, and enable the dual measurement concept that is operative in the Microtrac Sync.

The list below shows some of the materials that dissolve in water and others that require organic solvent.

Food – Peanut Butter

The major constituents of peanut butter are roasted peanuts, sugar and salt. Sometimes, hydrogenated oil and stabilizing agents may be present. The process involves grinding roasted peanuts to about 800 microns followed by a second grinding to 250 microns, which gives it a smooth feel. This sample was first prepared with the help of acetone, which, however, promoted coating of the sample cell, preventing the optical conditions necessary for the instrument to work properly.

When Isopar was used along with lecithin, the sample was optimally dispersed and wetted, without cell contamination, and under good operating settings, as shown in Figure 1 which displays the difference caused by the two methods of preparation. The beaker walls remained much cleaner with the use of Isopar in sample preparation,

Figure 2 displays the Sync diffraction measurements using Isopar as the solvent. The two peaks in the distribution have been calculated in volume amount. The two preparations are compared in Figure 3, showing finer particles to be present in the Isopar distribution. This indicates that Isopar use promotes a more even and realistic dispersion than the use of acetone does.

The peanut butter was subjected to image analysis and diffraction simultaneously, using samples prepared with Isopar. As seen in Figure 4, the Sync diffraction data showed 3, 12, and 161 µm (D1, D50 and D99), and the imaging data showed 6, 32 and 262. In other words, the data results were quite similar. Any difference found is attributed to the variation in calculation methods.

The image analysis is conducted on the basis of photographs which allow the measurement or calculation of any area or geometric feature of each separate particle. Following this, these values are further subjected to other calculations to yield data on particle shape and form.

The Da is a calculated value which describes the particle diameter, using the measured particle area to draw a circle representing the area, and then finding the circle diameter. Light scattering uses the defined pattern produced by the scattering of light by a cloud of particles – the ensemble approach. Diffraction takes the angles and intensities of light scattering as the basis for further measurements.

Image files give further information containing data on particle shape with a potential effect on the final quality and consistency of the product. The graph below shows that the W/L Aspect Ratio remains relatively unchanged from 15 to 150 µm, but then increases with larger particle size.

The Krumbein parameters are inversely related to particle size, showing that the large particles have sharper corners and edges. When there is an increased percentage of coarser particles, the mix is less likely to contain mostly spherical or round particles. Round particles roll over each other smoothly and such a mix feels smooth.

On the other hand, a gritty mix is one which contains larger, jagged particles. There may be individual variation on which type is felt to be more delicious. The shape variation between the largest and smaller particles is shown in Figure 5 and 6 for the distributions in the graph. Only some images are shown as the complete file involved the measurement of more than 200 000 particles.

Energetic, High Reactivity Chemicals

A host of chemicals are incorporated into reactions to act as oxidizers in controlled reactions, explosives in very rapid ones, and colorant agents. Some of the common oxidizers include:

  • Ammonium perchlorate(AP), used in motors for solid rocket motors (SRM) as in the NASA Shuttle, which uses aluminum or magnesium as fuel
  • Strontium nitrate, used for fireworks colored red
  • Potassium nitrate, used in airbags to scavenge the very pure and highly reactive sodium metal that is released from the ignition of sodium azide, in turn producing more nitrogen gas which further expands the airbag.
  • Ammonium nitrate (AN)

Many of these promote fuel burning, often of a hydrocarbon such as the ANFO explosive, used to clear passages in mining. In all these applications, particle size is very important, as the reaction must proceed uniformly to be effective. The reaction stability, particle packing and mixing, and the homogeneous nature of the reaction, as well as the kinetics, all depend upon particle shape and size.

In Figure 7 and 8 the Sync diffraction results are displayed, including an interesting comparison of three production samples of strontium nitrate. The Sync tracks the changes in shape and size that occur as milling proceeds, using laser diffraction and digital image analysis.

 

 

Figure 9 shows the data and graphs that indicate the stability within limits of sphericity and L/W ration with changing particle size. The size range is equivalent to the data obtained by diffraction despite the use of different calculations. The reported Sync diffraction values are 11 μm, 70 μm and 298 μm at D1, D50 and D99 respectively.

The image analysis values are 15 μm, 70 μm and 201 μm. The shape of the particles changes with the appearance of more angular aspects to the particles. Larger particles are more likely to have higher L/W values. The distribution width is quite high, indicating a considerable range of particle sizes. While this would be likely to affect particle segregation while the particles were in transit, the sphericity does not change, and nor does the the L/W ratio.

The sphericity is represented by the ratio of the area equivalent diameter to the area equivalent perimeter. The Krumbein Roundness value goes down as the size increases, showing that larger particles are not as round. This type of particle array ensures homogenous material transport and packing. The image analysis results are shown in Figures 10 and 11.

These compounds can all act as explosives or to facilitate energetic reactions. They are often water-soluble but must be measured in organic solvents. Microtrac diffraction systems are highly sensitive to particle size variations, a very useful property. They can also observe solids dissolving in a sample very easily.

This is also of immense use as fine particles, which have the greatest energy, dissolve first and are not visualized during the measurement. The Sync device allows for measurement of size and shape of materials in organic solvent, at the same time, thus yielding information on particle packing. The following information concerns ammonium perchlorate which is used to provide oxygen for propellants.

The Da diameter in Figure 13 is determined by the measuring the particle area and then drawing a circle or sphere with the same area, whose diameter is the Da. The form factors used include the Sphericity, Convexity, L/W ratio, Roundness, and cumulative and differential distributions, which allow for various other calculations to be performed automatically.

The size of the particles determines the L/W aspect ratio, with smaller particles being rounder and more spherical. The present sample shows that L/W ratio changes significantly with particle size until the size reaches about 70 microns, so that the particles retain the same Sphericity values but on the lower side. This prevents segregation and promotes safety which could be jeopardized by the concentration of “fines”.

Monitoring Sample Preparation and Assuring Dispersion

Ammonium and calcium nitrates are used in fertilizer to provide nitrogen, which is the base for the synthesis of amino acids, DNA and other biological nitrogenous compounds. Heme is one compound of this sort which is found to have a similar structure in both plants and animals. The only difference is in the metal atom incorporated into it.

When found in blood the metal is iron, and the compound is called heme. In plants, magnesium is the metal atom at the center and the compound is called chlorophyll, the substance which uses solar energy to run the energy conversion chain that stores chemical energy in the form of ATP and NADPH. These energetic molecules are used to make glucose using carbon dioxide.

Glucose is the go-to source of energy for plant growth processes. Nitrogen could be obtained from the fixation of natural nitrogen in the air or from added fertilizer, which has to pass through several chemical reactions to reduce the nitrogen to usable nitrate form. This is then used to make proteins and other biomolecules which in turn make up chlorophyll, requiring four nitrogen atoms for the formation of each molecule.

Plant health and vegetative growth therefore depend upon nitrate availability, since this is the usable form for plants, though other nitrogen compounds may also be used. Both calcium and ammonium nitrate are readily water-soluble and yield nitrate ions. The need to measure the material in a fluid environment, as well as the high water-solubility make an organic solvent necessary. Dry powder could be measured, of course, using the TurboTrac or TurboSync instead.

Figure 14 shows a structural representation of a chlorophyll molecule, with the four nitrogen atoms highlighted. These come from nitrate ions. In blood, hemoglobin looks quite similar, except that iron replaces the central magnesium.

The ionic nature of calcium and ammonium makes them indissoluble in organic solvents including Isopar, hexane or heptane. These are hygroscopic and may agglomerate due to the formation of van der Waals attraction, moisture bridges or capillary forces of attraction. The data in the following figures shows how calcium nitrate can be treated in steps after being suspended in Isopar G with lecithin as a surfactant.

In Figure 15 the sample is transferred straight into the circulation system which is agitated, but not dispersed in any way, which allows the distribution of data to shift to smaller size modes. During measurement, in other words, the material is being dispersed with intervention. When ultrasonic energy is passed through the sample, the size changes, and the longer the period of ultrasound exposure, the smaller the size becomes. Image analysis and other tests show that the best effect was produced with 30 seconds of ultrasound treatment at 50% amplitude.

 

The image analysis leads to similar patterns, as the Summary Data shows, through the calculated average using a set of different shape and form values. No parameter shows any difference between the two technologies, including Mean Convexity, W/L ratio, Krumbein Roundness and Angularity. Of course, averaged data from any source can conceal small differences in shape as well as particle size, which is an undesirable but normal consequence.

 

Figures 17 and 18 show the presence of sharp corners and edges on particles. This surprising finding was evaluated further using the Search feature, which confirmed it. This shows that the Search feature can yield more specific results compared to the averaged results with the Summary Data.

In this experiment, since a wide range of values was available, the calculated Krumbein Roundness value was limited to less than 0.4, this being chosen by inspecting the shapes through the View Particles and Scatter Diagram features of the Sync software.

The use of organic solvent helped in two ways, by avoiding particle dissolution and keeping the shape intact for measurement purposes. The image analysis not only confirmed the data from diffraction study, but explained the change in particle size during sample preparation by revealing the effects of dispersion on the particles, while also showing how particles could be damaged by supraoptimal ultrasonic exposure.

The energy needed to disperse the particles must be applied wisely. When calcium nitrate in this form is used, limited amplitude and duration are essential to prevent particle disintegration while breaking up agglomerated particles at the same time. Early tests showed immediate initiation of sample dissolution when it was placed into the Isopar. The settings were adjusted for proper sample preparation, avoiding sample destruction. High energy could damage the particles, while mild energy disperses the particles more rapidly under properly controlled conditions.

As ultrasonic treatment increases, the Krumbein values are reduced. An overall increase in these values or a higher percentage of particles exceeding a set value may show that the particles are more round with decreased irregularity, and vice versa, as Figure 19 shows. If ultrasound is used at 100% amplitude for 30 seconds, the particles are damaged and more round particles result. This yields inaccurate results in such instances because the particles are read as being smaller than expected.

Summary

The concepts in this article can help measure materials which dissolve in water or organic solvents preferentially. The Microtrac fluid circulation system can handle any organic chemical without having to change the materials within the flow system that come into contact with the fluid. Based on the type of material measured, either the TurboTrac or TurboSync can be used for dry powder.

The use of diffraction and image analysis in a single instrument allows for the samples to be simultaneously measured and a final report on conditions of preparation to be made while preserving the materials and achieving dispersion. In some conditions, dispersal is undesirable, while agglomeration or aggregation of particles is necessary. Here again, the combination of diffraction and image analysis is highly useful to ensure that particle measurement is for intact materials.

This information has been sourced, reviewed and adapted from materials provided by Microtrac, Inc.

For more information on this source, please visit Microtrac, Inc.

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