Industries Information

Laboratory mixers are used to mix, emulsify, homogenize, disintegrate and dissolve samples. There are several basic types of products. Dual-shaft mixers use a three-wing or helical anchor to generate flow and remove mixed materials from the vessel wall. Double-planetary mixers use a rectangular or finger-shaped blade to feed material to an orbiting high speed dispenser (HSD). Single-stage rotor/stator devices use a stationary stator to turn an immersed rotor at high speeds. The blades pass each port in the stator and expel material at a high velocity into the surrounding mix. Multi-stage rotor/stator mixers increase shearing to produce smaller particle sizes and more homogenous batches. When two to four rotor/stator pairs are nested concentrically, the mixed material that moves outward from the center of the unit is subjected to a rapid, sequential shearing. Some mixers can be operated at variable speeds. Others are designed for continuous speeds.

Selecting laboratory mixers requires an analysis of performance specifications. Speed range and accuracy are usually measured in revolutions per minute (rpm). Viscosity range is measured in cycles per second (cps). Capacity, the size of the mixing vessel, is expressed in either liters (L) or gallons (gal). Operating temperature and operating range are measured in degrees Fahrenheit (F) or degrees Celsius (C). There are two sample introduction methods for laboratory mixers. Continuous devices accept a continuous flow of the sample. By contrast, batch mixers accept only a measured flow or volume. In terms of temperature control, laboratory mixers with an internal heating or cooling elements are commonly available. Heating elements are usually made of ceramic materials, powered electrically, and measured in watts (W). Smooth cooling shovels that rotate within a drum are used to ensure maximum contact without inter-particle friction.

Laboratory mixers vary in terms of user interface options, display types, and special features. Analog front panels include potentiometers, dials, switches or other manually-actuated inputs that allow operators to adjust ranges and control the output. Digital front with numeric keypads or menus are used to perform these same functions. Video displays consist of a cathode ray tube (CRT), liquid crystal display (LCD), flat panel display (FPD) or other multi-line display. Intrinsically safe (IS) laboratory mixers will not produce sparks, electrical energy or other thermal effects that would cause an explosion under normal or abnormal conditions. Some laboratory mixers are handheld, but not necessarily portable. Others mount on an overhead stand. Programmable mixers and devices that use a timer are used in many applications.

Laboratory filters are used to remove particulates from samples in laboratory-scale applications. They consist of a filter medium and housing or holder that constrains and supports the filter media in the sample’s path. There are several basic types of laboratory filters. Membrane filters are thin, polymeric films that contain thousands of microscopic pores. The size of the pores determines the size rating of the membrane. Typically, membrane filters are used in the quantitative separation or filtration of suspended matter from liquids and gases. Pre-filters are often placed upstream from membrane filters to reduce particulate loading and allow the membrane to operate more efficiently. Centrifugal filters are suitable for small-scale laboratory separations such as protein or nucleic acid desalting and concentration. These devices use centrifugal force to drive liquid through the filter. Increasing the centrifuge speed (G force) increases the pressure across the filter. Fouling is minimized by design features that cause the G force to reach the filter at an angle, sweeping accumulated molecules from the surface. Syringe filters consist of a filter element and housing assembly and are used in applications where a sample must be filtered before entering a syringe. Specialized and proprietary filters that are made from a variety of porous papers are also available. 

Laboratory filters vary in terms of configuration, sample type, filter paper measurement type, filter medium material and housing material. Configurations usually consist of a filter element, a housing element, or both a filter element and housing assembly. Filters that are used with solid, liquid or gaseous sample types are commonly available. There are two filter paper measurement types: qualitative and quantitative. Qualitative filter papers have an ash content that is ten times higher than quantitative filter papers. Common filter medium materials include cellulose, cellulose acetate, nitrocellulose, and regenerated cellulose; ceramic, carbon, and glass fiber materials; polytetrafluoroethelene (PTFE), polyvinylidene fluoride (PVDF), and polyvinylidene chloride (PVDC); and polypropylene (PP), polysulfone (PSU), and polyethersulfone (PES). Housing materials for laboratory filters include acrylics, plastic acrylics, modified acrylics, and polypropylene (PP).

Selecting laboratory filters requires an analysis of both physical specifications and performance specifications. Physical specifications include sample size, pore size, filtration area, and filter shape. Sample size, the maximum amount that filters can accept, is especially important for centrifugal filters and syringe filters. Pore size indicates whether particles of a specific size are retained with an efficiency rate less than 100%, typically 90% – 98%. Rating methods vary widely among manufacturers. The effective filtration area (EFA) is the total, usable filter area. As a rule of thumb, the larger the filter area, the faster the flow rate at a given pressure differential and the larger the throughput volume prior to clogging. Most filters are circular or rectangular in shape. Performance specifications for laboratory filters include particle retention size, flow rate, porosity, hold-up volume, and maximum pressure. Flow rate determines the volume of a liquid that flows through the filter at a fixed pressure and temperature. Hold-up volume is the maximum volume of a sample that a filter can retain.

Laboratory centrifuges are used for separating particles from solutions according to their size, shape, density, viscosity of the medium and rotor speed. The theoretical basis of this technique is the effect of gravity on particles (including macromolecules) in suspension. Two particles of different masses will settle in a tube at different rates in response to gravity. Centrifugal force (measured as xg, gravity) is used to increase this settling rate depending on the rotation to induce centrifugal force, which accelerates the separation of the liquid from the solids. Centrifugal force causes the solid phase to move through the liquid phase in a straight line and away from the center of rotation. Solid-phase characteristics such as particle density, size, shape, and consistency and the rotation speed for the laboratory centrifuges’ chamber (a basket or bowl, depending on the centrifuge type) of a given diameter also influence how fast the solid phase moves away from the center of rotation. The higher the rotation speed, the higher the G force exerted on the solid phase and the faster the solids accumulate.

The two most common types of laboratory centrifuges are analytical and preparative; the distinction between the two is based on the purpose of centrifugation. 

Preparative laboratory centrifuges are used to isolate specific particles. This classification is divided into two types: differential and density. Differential centrifuges are used to separate particles from a liquid medium or to separate particles of different masses into separate fractions of the supernatant. Density centrifuges work by spinning two fluids of different densities within a rotating container or rotor the heavier fluid is forced to the wall at the inside of the rotor while the lighter fluid is forced toward the center of the rotor.

Analytical laboratory centrifuges measure the physical properties of particle, such as sedimentation coefficient or molecular weight. Optimal methods are used in analytical ultra centrifugation. Molecules are observed by optical system during centrifugation, to allow observation of macromolecules in solution as they move in gravitational field. The samples are centrifuged in cells (tubes with quartz windows) having windows that lie parallel to the plane of rotation of the rotor head. As the rotor turns, the images of the cell (proteins) are projected by an optical system on to film or a computer. The concentration of the solution at various points in the cell is determined by absorption of a light of the appropriate wavelength (Beer’s law is followed). This can be accomplished either by measuring the degree of blackening of a photographic film or by the pen deflection