Choosing the Right Laboratory Stirrer

Magnetic stirrers are the most common laboratory stirrers.

They consist of a base with an electric motor and a plate on which the container with the solution is placed, as well as a magnetic bar that is inserted into the container. In the base, an electric motor rotates a magnet which, in turn, rotates the bar inside the sample, stirring it.

2mag magnetic stirrer

These are the criteria for choosing a magnetic stirrer:

• With or without a heating plate: Some magnetic stirrers have a heating plate that can reach up to 600°C.

• Stirring plate material: Stirrer plates can be made of plastic, aluminum, ceramic, stainless steel, etc. The material of the plate must therefore be adapted to the intended use of the stirrer:

– Aluminum is more resistant to thermal shocks

– Ceramic is not affected by chemicals

– Stainless steel is extremely resistant

• Number of positions: Many magnetic stirrers process one sample at a time, but there are also multi-position stirrers, designed to stir up to 60 samples simultaneously, depending on the model.

• Volume of the samples: The size of the plate and the power of the motor must be appropriate for the volume of the solutions to be stirred.

• Stirring speed: This is determined by the rotation speed of the motor and can be adjusted by the user. Note that the speed range of the stirrers varies from model to model.

Overhead stirrers are designed for stirring, mixing, and homogenizing liquids, even those with a high viscosity. These stirrers are placed on top of the container to be stirred as opposed to other stirrers that operate from below (such as magnetic stirrers). These stirrers consist of a motor and a stirring tool that is inserted into the solution.

DLAB overhead stirrer

These are the criteria for choosing an overhead stirrer:

• The stirring tool: This attaches to the mandrel. Stirring tools are available in various materials (stainless steel, Teflon, or glass) and shapes. The choice depends on the substance and mixture required.
  – Propeller stirrers provide well-homogenized mixtures.
  – Interchangeable blade stirrers mix solutions more slowly.
  – Turbine stirrers provide dispersion effects.
  – Anchor stirrers bring the substances on the walls of the container toward the interior of the container for greater homogeneity.

• Stirring capacity: Manufacturers specify a maximum solution volume for their stirrer models, which generally corresponds to a solution with a viscosity equivalent to that of water.
• Viscosity and motor torque: Torque is the rotational force that the motor is capable of generating. The higher the viscosity of the solution to be stirred, the higher the motor torque must be. Manufacturers therefore specify the maximum torque of the motor and the maximum viscosity supported.
  
• Stirring speed: There are some fixed-speed stirrers, but most offer a variable speed range. Make sure you choose a model with a speed range that meets your needs. Stirrers with digital speed control allow you to adjust this parameter with high precision and repeatability.

VELP Scientifica vortex stirrer

Vortex stirrers are widely used, particularly in molecular biology, to rapidly mix solutions directly in test tubes or microtubes.

They consist of a heavy base with a rubber receptacle on top. When the operator presses down on the receptacle with the bottom of a test tube, the electric motor starts up, imparting a low-amplitude, high-speed orbital movement to the test tube and the solution it contains.

It is also possible to stir several tubes simultaneously, or microplates with stirrers whose receptacle is replaced by a specific support.

These are the criteria for choosing a vortex stirrer:

• The amplitude of orbital movement: This can vary by a few millimeters at most and must correspond to the requirements of the intended applications.
• How it works: Stirrers with touch mode are activated by pressing the tube into the receptacle. Models with microplate or multi-tube holders work continuously and are activated by a button. Some stirrers have both modes and offer different receptacles and supports as accessories.
• Fixed/variable speed: The simplest (and cheapest) models have a fixed rotation speed, but most agitators allow you to adjust the rotation speed.
• Analog/digital interface: Digital stirrers enable more precise speed adjustment, which can be important when applications require reproducible parameters.
• Timers: Stirrers equipped with a timer can be used to set the stirring duration.

FALC Instruments rotary stirrer

Rotary and roller stirrers are suitable for fragile or viscous samples packed in closed tubes and are frequently used for mixing blood samples.

There are several different configurations for rotary stirrers, but they all work according to the same principle: sample tubes are placed on a support that rotates at a fairly slow speed (less than 50 rpm) in order to gently homogenize the contents of the containers.

In a roller stirrer, test tubes are placed on rollers that are subjected to a double rolling and oscillating movement.

These are the criteria for choosing a rotary or roller stirrer:

• Capacity (number of tubes): The number of rollers and the size of the support dictate the number of tubes the equipment can process simultaneously.
• Rotating speed: As with other stirrers, speed is an important parameter and it is useful to be able to adjust it. Models with a digital speed indicator guarantee greater precision and repeatability.
• The angle of rotation axis: This has a direct effect on the movement of the liquid inside the tubes and therefore on the mixture obtained. Rotating the tubes horizontally provides minimal agitation, while vertical rotation causes the liquid to move inside the tube, thus promoting vigorous agitation. There are stirrers with a fixed rotational angle and stirrers that offer the possibility of adjusting this angle as required. Being able to set the angle precisely is a great advantage for applications that require good repeatability of results.
• Timers: An integrated timer for programming the stirring time is also very useful.

DLAB roller stirrer

These stirrers are equipped with a tray that fits most containers, such as microcentrifuge tubes, Petri dishes, microtiter plates, Erlenmeyer flasks, etc. The tray can be bare, fitted with a non-slip mat, or with specific supports to better hold the container. Several trays can also be stacked to increase the number of containers.

The difference between these stirrers lies in the type of tray movement.

Orbital stirrers move the tray in a flat circular orbit. This creates a swirling movement for the liquid inside the container, which is ideal for promoting sample aeration.

Reciprocating or linear stirrers linearly use an alternating back-and-forth movement. This linear action is more aggressive than orbital movement.

Rocking stirrers offer less aggressive shaking for the samples, as the platform performs a lateral, oscillating movement, generating ripples in the sample. A variant of the rocking stirrer is the 3D stirrer, whose tray moves in a three-dimensional motion around its central point, causing the sample to swirl gently, which is ideal for applications on fragile cells.

These are the criteria for choosing one of these stirrers:

• The nature of the sample: Especially when samples contain living cells, which are more or less fragile depending on their type and origin. The fragility of the samples must be taken into account when choosing the speed and range of movement of the stirrer.
• Speed adjustment range
• The number, type, and volume of containers to be used: These will largely determine the type of tray and equipment required, as well as the weight and height of the samples. Due to the type of movement of these stirrers, heavy or tall elements require certain precautions, such as reducing the stirring speed.

Eppendorf orbital stirrer