Improving Performance of Ultrasonic Cleaners for Sieve Cleaning, Solvent Degassing, and Sample Prep

Ultrasonic energy, specifically ultrasonic cavitation, is widely accepted in research laboratories to accomplish multiple functions. Three typical applications of ultrasonic energy in the lab are sieve cleaning, solvent degassing, and sample prep. These, along with applying ultrasonic cavitation to the cleaning of lab instruments, have until now generally required equipment specifically designed for the purpose.

Figure 1 – The 37-kHz Elmasonic S50R Lab Technology ultrasonic cleaner with a sieve holder, sieves, flask support assembly, and other accessories.

Acoustic R&D engineers at Elma in Singen, Germany, manufacturers of Elmasonic ultrasonic cleaning equipment, worked with end users to develop a single 37-kHz unit optimized for these three tasks. What sets apart the Elmasonic S50R Lab Technology ultrasonic cleaner (Figure 1) (available from Tovatech, South Orange, NJ) is its programming capability, which allows scientists to select the operating mode on the control panel, i.e., sieve cleaning, solvent degassing, or sample prep (also for instrument cleaning); the unit adjusts its operating parameters accordingly. Specially designed baskets, sieve racks, and flask and beaker supporting devices are provided as options.

Overcoming the challenges of cleaning laboratory sieves

The main challenge in cleaning laboratory sieves is ensuring the complete removal of previous samples from the screen. Fine-mesh screens are especially challenging. Further, the finer the screen, the more susceptible it is to damage from brushing.

Although conventional ultrasonic cleaners can be used for sieve cleaning, analysis revealed that these cleaners can have what are called “dead zones,” where cavitation is weak or absent. These zones are typically located near the drains of square or rectangular cleaning tanks and can result in unacceptable cleaning levels.

A successful solution to the challenge is found in round ultrasonic cleaning tanks without drains. Through this design ultrasonic cavitation is evenly distributed across sieves positioned in specially designed sieve holders.

Development testing also revealed that insoluble particles are removed by intermittent sharp peaks or pulses in ultrasonic power, whereas water-soluble and water-swollen particles respond best to moderate power without peaks. Elma engineers developed a sieve cleaning program that automatically alternates between what are called pulse and sweep modes.

“Pulse” is defined as intermittent high-intensity bursts, or peaks, of ultrasonic power to 600 W or 450 W more than the unit’s 150 W of effective power. This helps dislodge recalcitrant particles. “Sweep” provides a homogeneous distribution of ultrasonic energy throughout the cleaning bath to avoid what are called “standing waves” or areas of relatively high and low ultrasonic energy. Sweep works best on water-soluble and water-swollen particles in sieves.

The technique is highly effective in removing water-soluble and swelling, as well as water-insoluble, particles from 100- and 200-mm laboratory sieves as well as 200-mm sieves with transponders. The program is activated simply by selecting the “sieve clean” key on the unit’s control panel.

Several ultrasonic cleaning solutions, generally biodegradable concentrates, are available for cleaning laboratory sieves. Users should consult their ultrasonic equipment provider for recommendations.

Ultrasonic degassing of HPLC solvents

Ultrasonic cleaners equipped with a degas mode are also widely used to degas HPLC solvents placed in Erlenmeyer flasks or solvent bottles and suspended in a water bath containing a surfactant to improve cavitation action. Cavitation energy penetrates the glass walls of the container and acts on the HPLC solvent to drive off air.

Degassing works in an on-off mode that respectively causes air bubbles to form and then rise and burst. If the off phase is too short, the bubbles may not rise and burst but instead remain in the node of the standing wave. Elma scientists approached this challenge by adding a special frequency modulation program to the degas mode that is activated by selecting the “degas” key on the unit’s operating panel. This mode is also used to degas fresh ultrasonic cleaning solutions and water baths.

As a further refinement, the unit is fitted to accept an optional laboratory stand and clamp to hold Erlenmeyer flasks and solvent bottles, and adjust their position in the water bath. This option has the added advantage of reducing noise because the containers are not resting and vibrating on wire mesh support baskets.

Fast, safe sample prep

 Figure 2 – An orifice-equipped cover can be used when suspending glass beakers in the sonication tank.

Speed and thoroughness are important to avoid heat buildup when mixing, dissolving, dispersing, or otherwise preparing samples for further analysis. This procedure is accomplished in much the same way as HPLC solvent degassing, but in this instance the unit operates in the stronger pulse mode. Sample prep can be conducted in glass beakers fitted into the orifice of a special cover, or in flasks suspended at least 1 cm in the water bath (see Figure 2). The sample prep key is activated to start the process.

Routine instrument cleaning can be accomplished using the sample prep mode or simply by pressing the main on-off key. In this case, an ultrasonic cleaning solution is used instead of a water bath. Equipment is placed in a stainless steel mesh basket and lowered into the cleaning solution.

Control panel functions and operation

The Elmasonic S50R Lab Technology ultrasonic cleaner is programmed to handle sieve cleaning, solvent degassing, sample prep, and general lab instrument cleaning. These functions are activated by three mode keys plus an on-off key all equipped with a light-emitting diode (LED). The control panel has an LED timer indicating set and remaining time, and can be programmed from 1 to 30 min with automatic shutoff (see Figure 3). Continuous operation can also be selected, but the unit will shut off after 12 hr.

Figure 3 – The easily understood microprocessor-controlled operating panel with the timer and switches for (from the top) sieve cleaning (activated) degas, sample prep, and on-off (activated).

The stainless steel ultrasonic cleaner has an outside diameter of 10.25 in. and a height of 13.75 in. It has a minimum capacity of approximately 0.75 gal and a maximum capacity of approximately 1.5 gal. Fresh water baths and cleaning solutions should be degassed for 5–10 min by activating the degas mode to drive off trapped air that interferes with cavitation action.

Ultrasonic cleaning solutions will discolor and lose their effectiveness with repeated use. Users should empty the tank and dispose of the contents in an approved manner. It is important to take the time to remove cleaning residues, in particular hard particles and films, by using methods recommended by the manufacturer. New cleaning solutions and water baths must be degassed before proceeding.

Bob Sandor, Ph.D., is Director, Tovatech LLC, 11 Harrison Ct., South Orange, NJ 07079, U.S.A.; tel.: 973-913-9734; e-mail: rsandor@tovatech.com.

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