Sample preparation is defined by the International Union of Pure and Applied Chemistry (IUPAC) as the process used to transform analytes into a measurable form. Several terms are used to describe the equipment used for sample prep, among which are sonicator baths.
For example, in Sample Preparation of Pharmaceutical Dosage Forms, “sonicators, also referred to as ultrasonic extraction, can be considered both particle size reduction techniques and an agitation technique. Because of its ability to disperse, mix and dissolve samples, it is widely used in the sample preparation of dosage forms.”
Sonicator baths, sonicators and ultrasonic cleaners are terms describing equipment commonly employed in drug design and development. Because the term “ultrasonic cleaner” is a bit misleading in this context (normally associated with thorough cleaning of parts) we’ll use sonicator bath in this post.
The Purpose of Sonicators
Sonication is one of the processes used in sample preparation to extract an active pharmaceutical ingredient (API) from its carrier, or excipient, prior to conducting content uniformity and potency assay tests.
Other methods of achieving desired results may be employed. These include shaking, stirring, vortexing, grinding, milling, blending and homogenizing.
Certain of these options, because of how they operate, can heat samples, which can degrade the API.
An ultrasonic cleaner employed as a sonicator bath can achieve rapid sample preparation while avoiding chemical degradation of the API due to excessive heat.
How Sonicator Baths Perform Pharmaceutical Sample Prep
Ultrasonic is defined as sound above the range of human hearing (nominally 20,000 cycles per second or 20 kHz). In sample prep applications it is used to create high-frequency energy in liquids.
This is accomplished in an ultrasonic cleaner (a sonicator bath) equipped with a tank to contain water and a surfactant.
When activated, the unit’s generators excite transducers bonded to the bottom of the tank that vibrate at ultrasonic frequencies. The tank bottom serves as a membrane; its vibration creates millions of microscopic vacuum bubbles in the bath.
When bubbles contact anything immersed in the tank they implode with violent force. This is termed ultrasonic cavitation. This energy also passes through glass beakers and flasks containing solvents and samples immersed in the tank. Energy acts on beaker contents to quickly perform the required function such as to disperse, mix and dissolve samples.
Equipment Employed to Carry Out the Process
The Elmasonic S150 ultrasonic sonicator bath available from Tovatech is ideally suited for USP methods that specify an ultrasonic bath for sample preparation.
It consistently gives reproducible sample prep results across a wide range of applications including solubilizing, dissolving, digesting, dispersing, emulsifying, homogenizing and mixing.
The efficiency of the S150 as a sonicator for sample preparation is due to a combination of its 37 kHz ultrasonic frequency, 300 watts average ultrasonic power, a relatively large surface area (20 inches x 12 inches) but a depth of only 4 inches.
The result is a significantly higher power per unit volume compared to conventional ultrasonic cleaner models. The sonication process is completed before heat buildup, a natural result of ultrasonic energy, can cause API degradation.
A caution: Simply reducing the liquid volume in a deeper model ultrasonic cleaner will not achieve the same result. Operating equipment with liquid substantially below the fill line can cause damage to the ultrasonic generator and tank.
Instead, the S150 is specifically tuned for optimum performance with a liquid depth of under 4 inches.
A Typical Sample Prep Cycle
- Prepare samples in accordance with established operating procedures and place them in flasks along with the recommended solvent.
- Volumetric flasks are placed in the stainless steel basket of the S150 and Erlenmeyer flasks are fitted into clamps affixed to the mesh bottom of the basket. The basket in turn is placed into the sonication bath solution consisting of water and a sonication amplifier. The flasks need to be only partially immersed.
- Set the timer for the sonication cycle.
- The set time and remaining time are exhibited on an LED display.
- Turn the unit on. Ultrasonic transducers powered by the ultrasonic generator will start the cavitation action producing the millions of minute bubbles in the sonication bath solution.
- The ultrasonic waves will penetrate the walls of the flasks causing cavitation to occur in the flask solution as well. These cavitation bubbles implode when contacting the samples, causing them to disperse and dissolve.
- At the end of the timed cycle the unit will shut off and sample analysis proceeds. Flasks should be thoroughly cleaned and inspected before reuse.
Suggested Pre-Prep Step
Fresh sonication solutions should be degassed before sample preparation steps begin. This process removes entrapped air that reduces the efficiency of cavitation.
The Elmasonic S150 has an auto-degas function that accomplishes the process in approximately 10 minutes. Note that ultrasonic units without a degas function will degas during normal operation, but this process can take 10-15 minutes or more depending on the amount of solution involved.
The degas function can also be used for degassing HPLC solvents and samples in flasks following the process described above.
In summary, sample preparation is an exacting science to achieve accurate quantitation when conducting assays for drug products. Sonication is widely used as a means of extracting APIs from solid matrices. Consult the scientists at Tovatech for help in specifying and using the correct ultrasonic cleaner for your assays.
*Sample Preparation of Pharmaceutical Dosage Forms: Challenges and Strategies for Sample Preparation and Extraction, Chapter 3 Agitation and Particle Size Reduction Techniques, Nickerson, Beverly & Lung, K.. (2011).