Tag Archives: Particle Size Analyser

The Best Ways to Measure and Analyse Particles

For many people, the thought of measuring and sizing particles (that are often too small to be visible) can seem completely baffling. Just the thought of how this could possibly be done can seem daunting.

These days there are several types of particle size analyser instruments available that operate using different measurement techniques. laser diffraction is one such technique that can give us great insight into the properties of different materials.

Of course, there is no single measurement technique that can be used to measure all materials and all particle sizes. As such, some industries use a range of different techniques to measure different materials.  The problem with this is that different techniques measure different dimensions of particles, so when they often yield different results.

In order to understand more about the different results that are obtained through particle measurement and analysis, it is useful to be aware of some commonly used methods. These include:

Sieves:

This is one of the oldest and most traditional of all the methods of measuring particle size and is so often used because it is cheap and reasonably effective when measuring larger particles. The measurement of large particles occurs in a number of industries, including the mining industry.

However, the use of a sieve makes it impossible to measure particles within sprays or emulsions.  Even dry powders become very difficult to measure when particle are very fine. A further disadvantage is that the ability to reproduce results is difficult, particularly when the technique of wet sieving is applied.

In order for particle size results from sieves to be useful and informative, the operating methods and measurement times need to be standardised.

Sedimentation:

This is another technique which has been around for a long time and has historically been used in certain industries.  Soil scientist have long used sedimentation to measure course grained soils (ie.sand) and it has also been widely used in the ceramics industry. While some industries have a long history with sedimentation, it has also provided these industries with some problems.

For a start, the calculation of size from sedimentation rate is only valid for spherical particles.  If the particles are not spherical then the results reported can be very different from reality.  It is also necessary to know the density of a material.  If density is not known, or if a sample contains a mixture of materials, it is difficult to obtain accurate results. It is impossible to determine the size of emulsions particles, where the material does not settle. Similarly, it is difficult to get accuratete measure of large and very dense materials where the material quickly settles.

Electrozone Sensing:

This technique is particularly useful for measuring the size of blood cells but it poses some problems as a technique for measuring industrial materials. Samples must be suspended in a salt solution so it is not possible to measure emulsions and many dry powders.  Measuring the particle size within sprays simply cannot be done.

Laser Diffraction:

A popular and preferred measurement technique, laser diffraction also has the advantage of being one of the most accurate ways of measuring particle size. Other advantages of laser diffraction include:

  • It provides a great degree of flexibility for different materials
  • Results and answers can be provided quickly (in less than one minute)
  • It is an absolute method of particle analysis
  • There is no need to calibrate instruments against a standard
  • It is possible to measure dry powders, suspensions, emulsions, sprays and many other materials
  • The technique provides a wide and dynamic range of measurement
  • The measurement of an entire sample is possible
  • The technique can be repeated easily.

While there are a range of ways to measure and analyse particle size, the most appropriate way of measuring any given material will and should depend on the type of material that is being measured.

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Facts About Laser Diffraction

As a technique of particle size analysis, laser diffraction is widely used and has many advantages. It offers a high level of precision, a fast speed of response, high potential for the repetition of results and a wide measurable particle diameter range. Particle size analysers offer a sophisticated way of measuring particle size and enable incredible insight and understanding of the particles that make up materials used in a range of industries.

Laser diffraction is an important technique for measuring particle size. Here we offer a range of facts about this technique:

  • It is also known as Low Angle Laser Light Scattering (LALLS).
  • Considered a standard method in many industries because of its ability to characterise particles and for reasons of quality control.
  • Huge advances have been made in instrumentation over at least the last two decades.
  • The method is founded on the fact that the angle of diffraction is inversely proportional to particle size.
  • The instrument consists of a laser (as a source of coherent intense light that has a fixed wavelength), a detector (typically an array of photo sensitive silicon diodes) and a means of passing the sample through a laser beam.

Laser diffraction is so popularly and extensively used because it offers a number of advantages. These advantages include:

An absolute method grounded in fundamental scientific principles – In this method it is not necessary for an instrument to be calibrated against a standard. However, validation of equipment is possible to prove that it is performing to a standard that can be traced.

Wide and dynamic range – In measuring particle size, good equipment will allow the user to measure particles sized between approximately 0.1 and 2000 microns.

Flexibility – Laser diffraction offers new possibilities for measuring materials. It is even possible to measure the paint that is sprayed from a nozzle in a paint booth. The pharmaceutical and agricultural industries are two of the many industries that have benefited greatly from such advances.

Dry powders – Even dry powders can be measured through the technique of laser diffraction. Although this may result in a poorer level of dispersion than if a liquid dispersing medium was used, it is an advance that dry powders can be directly measured. In combination with a suspension analysis, it can support the assessment of the amount of agglomerated material in a dry state.

Liquid suspensions and emulsions – It is possible to use a recirculating cell to measure liquid suspensions and emulsions. This technique promotes a high level of reproducibility and facilitates the use of dispersing agents and surfactants to determine the primary particle size. If it is possible to do so, it is preferable to take measurements in a liquid suspension.

Sample measured – This technique allows for the whole of the sample to be measured. As the sample passes through the laser beam, diffraction is measured for all particles.

Rapid – This technique is so rapid that results can be derived in one minute or less. Feedback can therefore quickly be provided to plants and repeat analyses can also be made quickly.

Repeatable – This technique is highly repeatable and knowing that the results can be relied upon provides peace of mind.

A sophisticated method for determining particle size, laser diffraction is widely used and offers distinct advantages that are beneficial for many industries.

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Particle Size Analysis Basics

In many industries, the ability to determine the size of particles is not only useful but can be very important. A Laser diffraction particle size analyser is commonly used to provide this sort of information but the processes used to obtain particle size can be quite complex. Sampling, dispersion processes and the shape of materials all contribute to the complexity of particle size analysis.

Here we provide key information for understanding particle size analysis.

What does it mean to describe a particle?

When dealing with a three dimensional shape, one unique number cannot accurately be given to describe its size. While this remains true in so many situations and circumstances, it is particularly relevant when seeking to describe complex shapes such as a grain of sand or even a particle within a can of paint.

While many of us may question why this information is so important, it is imperative and influential for people such as Quality Assurance Managers within particular industries and organisations. For example, a Quality Assurance Manager may need to know whether the average size of particles has increased or decreased since the last production run.

The equivalent sphere

So often we seek to describe a shape by only one number. As has been mentioned, this is problematic as there is only one shape – a sphere – that can be accurately described by one number.

In order to arrive at a particular number to explain the size of a shape, equivalent sphere theory is frequently used. Using the equivalent sphere theory, some property of the particle is measured and it is then assumed that this refers to the diameter of a sphere to describe the particle. Essentially, this means that three or more numbers do not have to be used to describe a three dimensional particle. Although it is more accurate to describe three dimensional particles with three or more numbers, it is also inconvenient and can become unmanageable.

Using different techniques

When a particle is examined under a microscope, a two dimensional image is seen and the information that is deduced relates to the two dimensional shape. Consequently, there are a number of different diameters that can be reported. If the maximum length of the particle is used, then the result relates to a sphere with diameter of the maximum dimension. Conversely, if the minimum length is used for the calculation, then the result pertains to a sphere with diameter of the minimum dimension.

It is therefore important to note that each technique used to measure a particle will provide a unique result because it measures a different property of a particle.

In light of this, there is no absolute right or wrong answer. All answers are correct for the techniques that are used and the dimension of the particle that they are measuring.

What does this all mean?

For those to whom particle size analysis is important, different techniques for measuring particle size mean that there can be no standard size for particles such as grains of sand. If meaningful comparisons are to be made between different techniques, it must be done with standards containing spherical particles. Further to this, characterisation of particle size standards is only possible when the same technique has been used and this allows comparison between instruments that use the same technique.

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Seven Facts About Laser Diffraction

Laser diffraction is one type of particle size analysis and is a technique known and respected across many applications for its ability to provide fast and reliable particle size data. In this type of particle size analysis, a cloud (or ‘ensemble’) of particles that is representative of the greater collection, travels through a broadened beam of laser light which scatters the light on to a specialised lens. Information about particle size and shape can then be deduced from the scattered pattern of light.

The laser diffraction technique assumes that the particles that pass through a laser beam will scatter light at an angle that directly corresponds with their size. It then follows that as the size of particles decreases, the scattering angle that is observed will increase. Essentially, light scattered at narrow angles with high intensity indicates large particles and particles scattered at wider angles and with low intensity suggest smaller particles.

A laser diffraction system requires the following:

  • A laser – this is necessary as a source of intense and coherent light that is of a defined wavelength
  • A sample presentation system – this ensures that the material being tested successfully travels through the laser beam as a stream of particles that have a known state of dispersion that can be reproduced
  • Detectors – specialised detectors are applied to measure the light pattern produced across a range of angles.

Facts About Laser Diffraction:

  1. Over the last twenty years, laser diffraction has, to a large extent, replaced traditional methods of particle size analysis, such as sieving and sedimentation.
  2. Laser diffraction has replaced microscopy (including optical and electron) for particles that are larger than tens of nanometres.
  3. Laser diffraction offers many advantages, including: efficient and fast operation and ease of use; the capacity to reproduce results; a vast size range that spans up to five orders of magnitude.
  4. Laser diffraction analysers do not only measure simple diffraction effects. Light sources that do not make use of lasers are sometimes used to enhance the primary laser source to reveal extra information about particle size and shape.
  5. Particles that relate to or are measured for particular industries commonly resemble spheres and corners and edges of these particles are generally smoothed as a result of the rolling and turning motion originating from sample circulation as particle size and shape is measured.
  6. While modern equipment can give quite precise results, it can never be assumed that the size of particles (produced through laser diffraction or any other type of particle sizing measurement) will not differ from their true dimension.
  7. The spherical modelling theory remains the only accepted and logical choice used in a commercial device intended to analyse a wide range of samples, regardless of the real particle shape and size.

Laser diffraction is a particle size analysis technique that generates results that are incredibly useful for processes used for research and in various industries. Providing details about particle size and shape, this technology can be used to provide fast and accurate results.

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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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The Fundamentals of Liquid Chromatography

Chromatography is the collective term for a set of techniques used to separate mixtures. These techniques include gas chromatography (GC), thin layer chromatography (TLC), Size exclusion Chromatography (SEC), and high performance liquid chromatography (HPLC).

The Two Phases

Chromatography involves passing a mixture dissolved in a “mobile phase” through a “stationary phase”. The mobile phase is usually a liquid or a gas which transports the mixture to be separated through a column or flat sheet which has a solid stationary phase.

Liquid Chromatography

Liquid chromatography (LC) is a separation technique in which the mobile phase is a liquid. It can be carried out in either a column or a plane. LC is particularly useful for the separation of ions or molecules that are dissolved in a solvent.

Simple liquid chromatography consists of a column with a fritted bottom that holds a stationary phase in equilibrium with a solvent. Commonly used stationary phases include solids, ionic groups on a resin, liquids on an inert solid support and porous inert particles. The mixture to be separated is loaded onto the top of the column followed by more solvent. The different components in the mixture pass through the column at different rates because of the variations in the partitioning behaviour between the mobile liquid and stationary phases.

Liquid chromatography is more widely used than other methods such as gas chromatography because the samples analysed do not need to be vaporised. Also, the variations in temperature have a negligible effect in liquid chromatography, unlike in other types of chromatography.

High Performance Liquid Chromatography (HPLC)

Present day liquid chromatography that generally utilises tiny packing particles and a fairly high pressure is known as HPLC. It is basically a highly improved form of column chromatography often used by biochemists to separate amino acids and proteins due to their different behaviour in solvents related to the amount of electronic charge of each one.

Instead being allowed to drip through a column under gravity, the solvent is forced through under high pressures of up to 400 atmospheres, making the process much faster. Because smaller particles are used, with their sizes being determined by a particle size analyser, there is greater surface area for interactions between the stationary phase and the molecules flowing past it. This in turn allows for much better separation of the components in the mixture.

There are many advantages of HPLC. For one, it is an automated process that only takes a few minutes to produce results. This is a vast step up from liquid chromatography, which uses gravity instead of a high-speed pump to force components through the densely packed tubing. HPLC produces results that are of a high resolution and are easy to read. Moreover, the tests are easily reproduced via the automated process.

Unfortunately, there are also disadvantages of this technique. It is difficult to detect coelution with HPLC and this may result in inaccurate compound categorisation. The equipment needed to conduct HPLC is also costlier and its operation can be complex.

Thanks to rapid advances in technology, analytical instrumentation such as HPLC are increasing in popularity. For the most part, the efficiency of these techniques outweighs their disadvantages making them a popular choice particularly in the pharmaceutical and medicinal industries.

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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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