Informations regarding ultrasonicator Rahul N 02-05-2015
Ultrasonication ? Sonication is the act of applying sound energy to agitate particles in a sample, for various purposes. Ultrasonic frequencies (>20 kHz) are usually used, leading to the process also being known as ultrasonication or ultra-sonication. The ultrasonic power supply (generator) converts 50/60 Hz voltage to high frequency electrical energy. This alternating voltage is applied to disc-shaped ceramic piezoelectric crystals within the converter, causing them to expand and contract with each change of polarity. These high-frequency longitudinal mechanical vibrations are amplified by the probe (horn) and transmitted into the liquid as alternating high and low pressure waves. The pressure fluctuations cause the liquid molecule cohesive forces to break down, pulling apart the liquid and creating millions of micro-bubbles (cavities), which expand during the low pressure phases, and implode violently during the high pressure phases. As the bubbles collapse, millions of microscopic shock waves, micro jet streams, eddies and extremes in pressures and temperatures are generated at the implosion sites and propagated to the surrounding medium. Microtips and probes amplify and radiate the ultrasonic energy into the sample. Smaller diameter tips produce greater intensity of cavitation, but the energy released is restricted to a narrower, more concentrated field. Conversely, larger diameter tips produce lower intensity, but the energy is released over a greater area permitting larger volume to be processed.
Relationship between Sample Volume and Probe Size Selecting the proper size probe is extremely important. Each probe has a recommended sample volume range. For example the ½” probe can process approximately Tip Processin Diameter g Volume 20-250ml. Depending on the vessel size and shape, the ½” Range probe may have difficulty fitting inside a 20ml volume and a microtip may be a better option. Sample vessel size and 1/16" (2mm) 200ul - 2ml shape is a factor when selecting a probe. 5/64" (2mm) 200ul - 2ml Small volumes require a small tip to fit inside the sample 1/8" (3mm) 1ml - 15ml tube. Small tips (microtips) are recommended for processing samples inside small, thin vessels and never 1/4" (6mm) 5ml - 50ml samples larger than 50ml. 1/2" (12mm) 20ml - Microtips are high intensity and made for short processing 250ml times. Microtip will generate a considerable amount of heat 3/4" (19mm) 100ml - in small volumes and therefore should be used in the pulse 500ml mode to prevent heat buildup. 200ml - Larger volumes require a larger probe for effective 1" (25mm) 1,000ml processing. For example a 1” probe will process 1 liter more 1” with 500ml - quickly than a ¾” probe. Using the proper size probe will not booster 1,500ml only reduce the processing time but increase the lifespan of Continuous the probe. Using a stir bar can increase a probe s maximum ʼ Flocell fmow processing volume.
Replaceable vs. Solid Tips Replaceable tip probes are used with aqueous samples. Replaceable tip probes have threaded ends and when the tip is worn out it can be unscrewed and replaced. If you sonicate a solution that contains organic solvents, alcohols or any low surface tension liquid, the liquid will seep inside this threaded tip (regardless of how tight the connection is attached). Once liquid gets inside the tip, it will loosen and cause the Sonicator to overload. If you are processing a sample containing solvents or low surface tension liquids you must use a solid tip probe. Solid tip probes can be used for any type of sample. Nanomaterials and Probe Size Certain applications such as processing nanoparticles, often require long processing times. Using a larger probe will speed up processing and larger probes will not erode as quickly as smaller ones.
Tip Depth / Foaming Issue Probes/tips must be submerged properly. If the tip is not submerged enough the sample will foam or bubble. If the tip is too deep it will not circulate the sample effectively. Both conditions will end up with poor results. Foaming often occurs with samples volumes below 1ml. Foaming can also be caused when the amplitude setting is too high.
Vessel Shape and Size • A narrow vessel is preferable to a wide vessel. The ultrasonic energy is generated from the tip and is directed downward. As a sample is processed the liquid is pushed down and away in all directions. If the vessel is too wide it will not mix effectively and some sample will remain untreated at the periphery. Twice the volume in a narrow vessel takes a shorter time to process than the same volume in a wider vessel. In addition, the probe should never touch the Amplitude and Time Settings sides or bottom of a vessel. All applications require optimization of amplitude and time settings. Test your probe/tip with water using the same volume and sample vessel that you will use for actual sample processing. Observe how the liquid moves during sonication at different amplitude (intensity) settings. You should see and hear the cavitation and the entire sample volume should mix and flow well. Choose an amplitude that does not create foam or splashing as your starting point. Smaller volumes will require lower amplitude settings and shorter pulses of sonication. Larger volumes can be sonicated at 100% if necessary, to speed up processing times. After choosing an amplitude setting, perform a time study by testing a sample at several time intervals and comparing results. Adjust amplitude and time as needed to obtain the desired result.
Controlling Temperature There are many options for keeping samples cool during sonication: Use the pulse mode to reduce heat buildup. Put samples on ice along with the pulse mode. Coolracks chill samples and prevent movement (when ice melts tubes may shift). Chillers provide additional cooling capacity and can be used with cup horns or as an accessory with a cooling zz jacket or tank. Cooling the Converter Sonicating for long time periods can cause heat to transfer up the probe to the converter. Overheating can severely damage the converter and Sonicator system. Larger samples that require continuous processing for over 20 minutes must utilize air cooling of the converter.
Power vs. Intensity Power is the measure of the electrical energy that is being delivered to the convertor. It is measured in watts and displayed on the sonicator s screen. At the convertor, the electrical energy is transformed into mechanical energy. It ʼ does this by exciting the piezoelectric crystals causing them to move in the longitudinal direction within the convertor. This change from electrical into mechanical energy causes a motion that travels through the horn/probe causing the tip to move up and down. The distance of one movement up and down is called its amplitude. The amplitude is adjustable. Each probe has a maximum amplitude value. For example, with a ½” diameter probe at setting 100%, the probe will achieve an amplitude of approximately 120μm.At setting 50% the amplitude is approximately 60μm. Note this is approximate and not perfectly linear. Amplitude and intensity have a direct relationship. If you operate at a low amplitude setting, you will deliver low intensity sonication. If you operate at a high amplitude setting, you will have high intensity sonication. In order to be able to reproduce results, the amplitude setting, temperature, viscosity and volume of the sample are all parameters that need to remain consistent. The amplitude, not the power, is most critical when trying to reproduce sonication results. Power has a variable relationship with amplitude/intensity. For example, sonicating water at setting 50% requires less wattage when compared to a viscous sample (such as honey). For both samples the amplitude/intensity is the same but the power/wattage will differ because the viscous sample will require more watts in order to drive the horn. The viscous sample puts a heavier load on the probe so they system must work harder to vibrate up and down at the same intensity. Small fluctuation in the wattage display during sonication is normal. Major swings in wattage (+/-20 watts) may indicate a problem with the sample, setup or the sonicator itself.
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