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Laser Diffraction – Why is it Such an Important Particle Sizing Technique?

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As a technique of particle size analysis, laser diffraction offers the ability to learn more about particle size and shape with a high level of accuracy. This information is incredibly useful to industries and for research and the information generated is influential in streamlining and enhancing processes used.

Particle sizing technologies are intended to provide a reliable measurement (that can be reproduced) for different sized particles. There are multiple technologies for particle size analysis and it is vital to appreciate that no one piece of technology is appropriate for every job. There are advantages and also drawbacks to each piece of measurement technology and different devices are best suited to particular industries or tasks.

Why is measuring particle size and shape important?

Today, many industries rely on the ability to use a particle size analyser to measure the size of particles of varying sizes, including those that are incredibly fine. We know that for all materials that are milled or ground, the resulting particle size is typically the factor that determines performance of the product and efficiency of the process.

As a result, analysis of particle size has become crucial to industries such as the pharmaceutical, food and beverage, building and chemical industries.

Why is laser diffraction one of the most important and used particle size analysis techniques?

As it can be used to determine particle size of liquid suspensions, dry substances and aerosols, laser diffraction is most popular for its dynamic nature and range.

However, different particle size analysis technologies can quite often produce different results for the same sample. There is a logical reason for this, being that each particle analysis measurement technique measures a different part of aspect of the same material. For this reason, all particle size analysis results must be considered as the best indications possible rather than definitive and exact measurements.

Why is ‘Equivalent Sphere’ Theory used?

Even the smallest particles are multi-dimensional and it is very hard, not to mention problematic, to describe a multi-dimensional particle using one dimension only.

As only one shape, a sphere, can be described by one dimension, all techniques that measure particle size relate this to an ‘equivalent sphere’.

What are the most common particle sizing techniques used?

  • Sedimentation techniques
  • Sieve technique
  • Aerodynamic sizing technique
  • Laser diffraction
  • Image analysis technique

With so many techniques for measurement available, which should be used?

Ultimately, there is no simple and definitive answer to this question. Because different products and processes can be measured, the most suitable measurement technology for the product and process needs to be chosen and applied. Having said this, of all of the technologies, the one that can be used most widely is laser diffraction.

The advantage of laser diffraction as a tool for determining particle size and shape is that it can be used to gain information about a wide range of particle sizes and sample types. This technique is suitable for materials such as sprays, powders, suspensions and emulsions and results are able to be delivered in the form of a ‘volume’ distribution, which is the most significant and logical description when bulk material properties are being analysed.

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Seven Facts About Laser Diffraction

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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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Particle Size Analysis – What You Need to Know

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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

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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

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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

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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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What is Nanotechnology?

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Although the range of scientific endeavours that involves the use or study of nanotechnology gets bigger every year, it is difficult to find a definition which covers every aspect. In simple terms the prefix nano indicates something with extremely small dimensions and this gives us an insight into what nanotechnology is really all about. A nanoparticle size analyser using Dynamic Light Scattering technology has already been developed which can measure particles as small as <1 nm to 6 μm.

When you consider that the measurement of a metre, as defined by the International Standards Organisation, as ‘the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second’ and that a nanometre is 10- 9 of a metre you will develop an idea of the dimension in which this particular science works.

Think of electrons and the scale at which they exist and then imagine scientists who work with physical products that can only be measured in these extremely small scales and you have an idea of what nanotechnology really is. It is working with matter at the molecular level and includes the scientific ability to manufacture items from these extremely small building blocks so that they eventually become high performing products.

You get a clearer understanding when you consider the types of products that are being developed.

Nanotechnology enables scientists to build machines which have the scale of molecules, they can be a few nanometres wide, in other words smaller than a cell, yet perform functions that would normally be expected of a computer. The science-fiction notion of submarines the size of a pinhead that can travel through the human body to perform complex medical operations are closer to the truth than might have been expected when this novel idea was dreamt up.

It is generally thought that nanotechnology has the capacity to provide improved efficiency for machines and processes in every facet of life. However there is also concern that making things smaller may also introduce dangers and risks that we dont yet understand. As more and more materials are being produced at nano sizes, the need for understanding nano-toxicology increases. A good example is titanium dioxide, which is an ingredient in sunscreens. Sunscreen manufacturers are using titanium dioxide powders with smaller particle size because it is more transparent and more effective at blocking UV radiation from the sun. However if the particles are too small then they can pass through the skin and get into the blood stream. As yet, scientists dont fully understand the negative effects of having these nanparticles in the human body.

Over the next 20 to 30 years the development of nanotechnology is likely to exert more and more influence in the world of science and the impact will become more apparent in a relatively short space of time.

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What’s Biotechnology?

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Biotechnology is a field of science that uses living organisms and bimolecular processes to make useful products. Applications span medicine, agriculture, food, energy and the environment.

Biotechnology draws on many specialist sciences such as genetics, microbiology, molecular biology, and biochemistry. In many instances it is also dependent on knowledge from outside biology such as chemical engineering.

Researchers often study biomolecular processes at the molecular level to gain an insight into small molecule structure and function. Protein interactions are widely studied, particularly their folding and refolding mechanisms. A wide range of analytical techniques are used from the established spectroscopic methods of UV-Vis, FTIR, and Circular Dichroism, through to the latest technologies such QCM-D (Quartz crystal microbalance) and Dual Polarisation Interferometry. Nano technology provides a whole new area of research and the possibility of new products. Particle size has a strong influence on the properties of nano materials. Nano particle size and Zeta potential are accurately measured by Dynamic light scattering.

What is new about biotechnology is that researchers can now take a single gene from a plant or animal cell and insert it into a different plant or animal cell. This is called transgenic technology. The gene chosen will contain code for a desired characteristic, for example plants that repel specific insect pests or disease.

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Particle Sizing Techniques and Laser Diffraction

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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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