Tag Archives: Laser Diffraction

Particle Size Analysis – What You Need to Know

Particle size analysis has many important uses for many industries. While many people may not immediately recognise or understand the benefits of determining particle size and shape, processes and systems in a variety of industries are enhanced and made more efficient when this intelligence is known. Specialised equipment has been developed to assist with such analysis and is vital in ensuring quality control and product standards.

What is Particle Size Analysis used for?

Particle size analysis is used to learn more about the size and shape of grains and particles within a particular sample. This analysis is so sophisticated and versatile that it is applicable to solid materials and also suspensions, emulsions and aerosols.

As some particle size analysis methods can only be used for particular materials, it is important that the most appropriate method of analysis be used. Varying and inconsistent results can occur if an inappropriate method for determining size is used.

What are Particle Size Analysis results used for?

Quality control and efficient functioning of processes is better assured for many industries if particle size analysis testing is done. For any industry where milling or grinding is undertaken, it is important to know particle size and shape in order to maximise the efficient functioning of processes and the ultimate quality of products.

While an array of industries and products benefit from particle size analysis, some of the industries in which analysis is commonly and widely used are:

  • Pharmaceutical
  • Building
  • Paints and coatings
  • Food and beverages
  • Aerosols

What are some of the difficulties with Particle Size Analysis?

Problems can arise when particle size analysis attempts to reduce the size of particles to only one number. A two dimensional graph is usually used to report particle size and quantity. However, only the shape of a sphere can truly be expressed as a single number, as it is the only shape that has the same measurement across every dimension. This does not apply to shapes of other types and sizes; they do not consistently measure the same across all of their dimensions.

In light of this, a one dimensional property of a particle is related to the size of an ‘equivalent sphere’ in all particle sizing techniques. Commonly, the volume of each particle in a sample is measured and equated to the size of a sphere with the same volume as the measured particles. This is referred to as an ‘equivalent sphere’ and is often applied in laser diffraction methods.

What is Laser Diffraction?

One of the most often used particle sizing methods, laser diffraction operates from the principle that when a laser (beam of light) is broken and scattered by particles, the smaller the particle size, the larger the angle of light scattering will be.

Laser diffraction is so popularly used because of its application to many different sample types. Further advantages of this particle size analysis technique are that it is fast, reliable and a technique that can be reproduced. It is also possible to use this measurement technique over a wide size range.

Particle size analysis is vital for enhancing the processes used in a variety of industries. Modern, sophisticated equipment is specifically designed to provide accurate and reliable results pertaining to a range of materials. It is little surprise that particle size analysis is so popularly used when the specific information that it provides are so significant and important to companies and industries.

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A Guide to Modern HPLC

HPLC stands for high performance liquid chromatography. It is a chromatographic technique that can separate a mixture of compounds. This technique is used in biochemistry and analytical chemistry to identify, quantify and purify the individual components of the mixture, particularly in the separation of amino acids and proteins due to their different behaviour in solvents related to the amount of electronic charge of each one.
Like liquid chromatography, HPLC uses a liquid mobile phase to transport the sample mixture. However, HPLC is a step up from liquid chromatography in several ways.

  • Size matters: HPLC generally uses very small packing articles compared to liquid chromatography. A particle size analyzer can easily determine the size of these particles. Because the particles are smaller, there is greater surface area for interactions between the stationary phase and the molecules flowing past it, allowing for better separation of the components.
  • High Pressure: The solvent does not drip through the column under gravity in HPLC. Instead, it is forced through under high pressures of up to 400 atmospheres, quickening the entire process.
  • Stationary Phases: HPLC also utilises different types of stationary phases. The most common stationary phase is the hydrophobic saturated carbon chain but others such as a pump that moves the mobile phase and analyte through the column and a detector that provides a characteristic retention time from the analyte are also used.

How HPLC Works

  • The molecule of interest is held in the liquid state.
  • The sample is injected into the HPLC instrument.
  • The sample preparation passes through a column. Molecules are partitioned based on size and reasons of polarity interactions. Basically the column allows smaller molecules to pass through quickly and holds onto bigger molecules longer.
  • After each molecule is partitioned, it passes through the column and heads toward the detector. The sample is carried past the detector by the mobile phase.
  • The detector emits light in the range of 190-700nm. When the molecule of interest passes the detector it responds electronically with the light. The intensity of the response relates directly to the concentration of that molecule in the sample preparation.
  • The software plots the intensity of the molecule, on the y-axis. The software also records the time that the compound passed the detector. This is the “elution time,” or the characteristic time for that molecule, and represents the x-axis.

Four main types of HPLC

    • Partition

This was the first kind of chromatography that chemists developed. The partition method separates analytes based on polar differences.

    • Adsorption

Also known as normal-phase chromatography, this method separates analytes based on adsorption to a stationary surface chemistry and by polarity.

    • Ion-exchange

This is commonly used in protein analysis, water purification and any other technique that can be separated by charge

    • Size Exclusion or Gel

Size exclusion chromatography separates particles based on size. To determine the size of the particles, a commonly used technique is laser diffraction as it is able to measure the size of a wide range of particles from very fine to very coarse Size exclusion chromatography is generally a low-resolution technique that is usually reserved for the final “polishing” step of purification. This method is useful for determining the tertiary and quaternary structures of purified proteins.

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Scientific equipment, support and training

The chances are that every time you purchase a new piece of scientific equipment, for example a particle size analyser or something similar, there is much more involved than the purchase itself. You need to face up to the likelihood that staff training is going to be necessary and that’s why it’s important to choose your supplier carefully.

One of the most important elements in purchasing sensitive scientific equipment is to ensure that the support services provided by your supplier can meet your requirements. As a general rule, most overseas manufacturers choose their representatives carefully but in any case it is up to the local suppliers to perform to the high standards that are expected of them. More importantly, it is up to you as the purchaser to reassure yourself that you will not be left in the lurch and you can rely upon your supplier.

It goes without saying that investing in staff training is an absolute must. It is foolhardy to invest in highly sophisticated equipment and neglect the importance of using it to its optimum level. Not only will you get the best out of the your investment but also training stimulates employees and will boost morale. It’s important not to consider staff training as a one-off event. With staff turnover and the likelihood of staff being involved in the use of other pieces of equipment, ongoing training is the best way to keep skills at their optimum levels.

Apart from training, you should look for the following characteristics in the firm from whom you are purchasing your critical equipment.

  • Presales testing. All new equipment should fulfil certification requirements whether they be regulatory or otherwise. Your supplier should be able to ensure this.
  • Routine maintenance. Ideally, your supplier should offer a regular maintenance schedule so that that the equipment is maintained to perform at peak levels.
  • Consumables. If the equipment needs a regular supply of consumables, ensure that your supplier is able to supply these ex-stock so that you do have to wait for the arrival of supplies. This way you will avoid costly downtime.

Follow these simple tips on the purchase of your next piece of scientific equipment, whether it be laser diffraction equipment or not, and you can be assured that the support and training you have organised will maximise the benefits of your investment.

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Particle Sizing Techniques and Laser Diffraction

The ability to measure the size of very fine particles is an integral part of many industries today. For every material that goes through a grinding or milling process, the final particle size is usually the primary factor that governs product performance or process efficiency. This is why particle sizing has become essential in areas such as the pharmaceutical industry, foods and beverages, cement and building materials and for manufacturers of chemical compounds for industrial and residential uses.

One of the most popular and widely used particle sizing technologies available today is Laser Diffraction. Laser Diffraction has become so popular because it offers a very wide dynamic range; it can be applied to liquid suspensions, dry substances and aerosols and provides a very rapid and reliable measurement process.

The goal of all particle sizing technologies is to provide a systematic and reliable measurement for a range of different sized particles. However it is important to understand that there is a wide range of particle sizing technologies available and no one technology is suitable for every job. Each measurement technology has its own advantages and each is often best suited to specific industries or applications.

It is also important to understand that different measurement technologies can often give different particle results for the same sample. This is because each measurement technique measures a different aspect of the same material.

Sphere Approximations

When reporting particle size, we tend to display a graph showing “particle size” on one axis and “percent of material” on the other axis. However, it is very difficult to describe a multi-dimensional particle using one dimension. In fact there is only one shape that can be described by one dimension, and that is the diameter of a sphere.
For this reason, all particle sizing techniques measure some property of material and then relate this to an “equivalent sphere”.

Some common particle sizing techniques and there reporting methods are as follows:

  • Sedimentation techniques – measures the rate at which particles settle in a liquid column and reports the size of a sphere with the same settling rate.
  • Sieve technique – measures the mass of material retained on a series of screens and reports the amount of material between spherical hole sizes.
  • Aerodynamic sizing technique – measures the behaviour of particles in an airstream and reports the size of a sphere that has the same behaviour.
  • Laser diffraction – measures light scattering from a group of particles and reports the size of a sphere that produces the same scattering.

So, with so many different measurement techniques available. The question is: which techniques are the best?

Getting the Answer as Right as Possible

Like most questions in the world of science, the answer has multiple parts. In short, you need to choose the most suitable measurement technology for your product and your process. However, if you had to pick a single technology that can universally applied then it would have to be Laser Diffraction.

Laser diffraction is the best general technique as it can be used with a very wide range of particle sizes and also a very wide range of sample types. Laser diffraction works very well for sprays, dry powders, suspensions and emulsions. The results reported are also displayed in terms of a “volume” distribution, which is the most appropriate description for bulk material properties.

Apart from Laser Diffraction, another technique that is gaining popularity is Image Analysis. Image Analysis is also considered a particle sizing technique, but it does offer one significant difference. It is the only methodology that provides any information on particle “shape”! If particle shape is known to have an influence on product performance, then image analysis may be the most appropriate option.

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