Industries Information

Titration instruments are used to determine the concentration of dissolved substances. Titration is based on a complete chemical reaction between the analyte and a reagent or titrant of known concentration that is added to the sample. The analyte is the substance which a laboratory test is designed to measure. The reagent or titrant is the substance that produces a chemical reaction in order to determine the presence of the analyte. There are two basic titration methods: manual and automatic. Manual titration is performed with a burette, a piece of laboratory glassware that has volumetric graduations along its length and a tap or stopcock on its bottom. Automatic titration is performed with an auto-titrator, an electrically-powered laboratory instrument that can be interfaced to a personal computer (PC). Typically, automatic titration instruments are used for repetitive titrations.

Titration instruments differ in terms of addition and indication methods. There are two titrant addition methods: volumetric and coulometric. Volumetric devices add the titrant directly to the sample. Coulometric devices generate the titrant electromechanically. There are many different titration indication methods. Voltametry, a technique also known as the Karl Fischer water determination method, measures the concentration-dependent potential of a solution against a reference potential. Potentiometry, redox and precipitation measure the potential at a constant electric current. Photometry, complexometry, and turbidimetry measure the light transmission of a colored or turbid solution with a photometric sensor. With amperometry, the current flowing in a sample is measured at a constant potential. Titration instruments that measure conductivity include a conductivity meter while devices that measure temperature include a sensor.

Selecting titration instruments requires an analysis of product specifications, features, display types, and computer interfaces. Product specifications for titration instruments include concentration range and reaction time. Some instruments provide data storage and temperature-compensation features. Others measure solids or gases. In terms of display types, titration instruments with analog or digital front panels are commonly available. Analog user inputs include potentiometers, dials and switches. Digital front panels can be setup or programmed using a digital keypad or menus. Titration instruments that can be controlled or monitored via a computer interface use serial or parallel communications and may include application software. RS232, RS485 and universal serial bus (USB) are common serial interfaces. The general-purpose interface bus (GPIB) is a common parallel interface.

Solid phase extraction (SPE) is an extraction method that uses a solid phase and a liquid phase to isolate one, or one type, of analyte from a solution. It is usually used to clean up a sample before using a chromatographic or other analytical method to quantify the amount of analyte(s) in the sample. The general procedure is to load a solution onto the SPE phase, wash away undesired components, and then wash off the desired analytes with another solvent into a collection tube. Solid-phase extractions use the same type of stationary phases as are used in liquid chromatography columns. The stationary phase is contained in a glass or plastic column above a frit or glass wool. The column might have a frit on top of the stationary phase and might also have a stopcock to control the flow of solvent through the column. Commercial SPE cartridges have 1-10 mL capacities and are discarded after use. Separation techniques covers a wide range of instrumentation used to separate samples into their component parts for analysis and measurement. Centrifuges, capillary electrophoresis, laboratory filters, titration, Solid-phase extraction (SPE), and chromatography are major groupings within this family. Chromatography instruments are divided into two main areas, high performance liquid chromatographs (HPLC) and gas chromatographs (GC), and their related columns and detecting devices.

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

High performance liquid chromatograph (HPLC) detectors pass a beam of light through a column effluent as the fluid passes through a low-volume flow cell. Variations in light intensity are recorded and a chromatograph is generated. HPLC detectors use several detection methods. Ultraviolet (UV) detectors measure the ability of a sample to absorb light at one or more wavelengths. Light scattering detectors nebulize the effluent, vaporize the solvent, and then detect droplets in a light scattering cell. Electrochemical detectors measure the current from the oxidation/reduction reaction of an analyte at a suitable electrode. Radiochemical detectors use tritium or carbon-14 to detect the fluorescence associated with beta-particle ionization. Mass spectroscopy detectors ionize a sample and use a mass analyzer to detect the ion current. Nuclear magnetic resonance detectors irradiate nuclei that are placed between the poles of a strong magnet. The radiation is absorbed, the parallel nuclei enter a higher energy state, and each atom produces a spectra specific to its location and chemical composition. Some HPLC detectors measure the change in the refractive index of the column effluent passing through the flow cell. Others detect the fluorescence that occurs when compounds are excited by shorter wavelength energy and emit higher wavelength radiation.

Important specifications for HPLC detectors include dynamic range, detector wavelength, response time, flow cell volume and flow cell rate. Dynamic range allows for the quantification of unknown samples using a multi-point calibration curve. It indicates when the peak area is directly proportional to the solute amount so that, for example, doubling the solute amount results in a doubling of peak area. Detector wavelength indicates the emission wavelength. Response time is the amount of time between a solute’s entry into an HPLC detector and the generation of a chromatogram. Flow cell volume and flow cell rate are measurements of capacity and efficiency, respectively. Maximum pressure rating is another important specification to consider when searching for HPLC detectors.

HPLC detectors vary in terms of general features, display options, and interface options. General features include self-calibration, self-test diagnostics, data storage, and programmability. Devices with a built-in chromatograph and application software are commonly available. Some HPLC detectors are suitable for extreme temperatures. Others output an analog voltage, analog current, frequency, or pulse. Examples include amplitude modulation (AM), frequency modulation (FM), pulse width modulation (PWN), sine waves and pulse trains. Analog devices usually include an analog meter and a front panel with switches and knobs. Digital devices often provide a serial or parallel computer interface and a video display. Serial protocols include universal serial bus (UBS), RS232, RS422, and RS485.

 High performance liquid chromatography (HPLC) is a separation technique that uses liquid mobile phase to separate the components of a mixture. The sample is first dissolved in a solvent, then forced to flow through a chromatographic column under high pressure. Within the column, the mixture is resolved into its component parts. Individual HPLC columns may use any one of a number of different phases or processes to induce this resolution including normally bonded phase, reversed phase, size exclusion, ion exclusion, and ion exchange.

Normally bonded phase HPLC columns are composed of a stationary bed, which is strongly polar in nature, and a mobile phase that is non-polar. Polar samples are thus retained on the polar surface of the column packing longer than less polar materials. This technique works well with water-sensitive samples, analytes that are insoluble in water, certain geometric isomers that are difficult to separate by reversed-phase, and class separations. Organic solvents used in normal-phase separations are more Mass Spectroscopy (MS) friendly than some of the typical buffers used in HPLC. In reversed-phase HPLC, operating conditions sometimes must be dramatically modified to allow compatibility with MS detection.

Reversed phase HPLC columns are the inverse of normal phase chromatography. The stationary bed is non-polar (hydrophobic) in nature, while the mobile phase is a polar liquid, such as mixtures of water and methanol or acetonitrile. The lower the polar differential, the longer the material is retained.

In size exclusion the HPLC column is filled with material having precisely controlled pore sizes, and the sample is screened or filtered by the stationary phase according to its solvated molecular size. Larger molecules are rapidly washed through the column, and smaller molecules penetrate the pores of the packing particles and elute later.

In ion exclusion the anions are retained longer on the stationary phase than on anion-exchange columns. Quantification is therefore simplified in the event large amounts of cationic contaminants are present in the sample.

In ion exchange, the stationary bed with the HPLC column has an ionically charged surface of opposite charge to the sample ions. This technique is used almost exclusively with ionic or ionizable samples. The stronger the charge on the sample, the stronger it will be attracted to the ionic surface and thus, the longer it will take to elute. The mobile phase is an aqueous buffer, where both pH and ionic strength are used to control elution time.

High performance liquid chromatographs (HPLC) use a liquid mobile phase to separate the components of a mixture. The components are dissolved in a solvent and forced to flow through a chromatographic column under high pressure. In the column, the mixture is resolved into its components. The components flow through a detector and a chromatogram is generated. Most high performance liquid chromatographs (HPLC) use high-pressure pumps to force solvents through packed stationary beds. Isocratic pumps use constant mobile phase composition to elute compounds. By contrast, gradient pumps vary the strength of the organic solvent to elute different compounds. Pumps with flow control are well-suited for high performance liquid chromatographs (HPLC) that deliver gradients automatically. Specifications such as flow rate, maximum pump pressure, and pump accuracy vary among devices. In simple systems, the sample is introduced with an injection valve. More complex high performance liquid chromatographs (HPLC) incorporate an autosampler with a microprocessor.

High performance liquid chromatographs (HPLC) use several detection methods. Ultraviolet (UV) detectors measure the ability of a sample to absorb light at one or more wavelengths. Light scattering detectors nebulize the column effluent, vaporize the solvent, and then detect droplets in a light scattering cell. Electrochemical detectors measure the current from the oxidation/reduction reaction of an analyte at a suitable electrode. Radiochemical detectors use tritium or carbon-14 to detect the fluorescence associated with beta-particle ionization. Mass spectroscopy detectors ionize a sample and use a mass analyzer to detect the ion current. Nuclear magnetic resonance detectors irradiate nuclei that are placed between the poles of a strong magnet. The radiation is absorbed, the parallel nuclei enter a higher energy state, and each atom produces a spectra specific to its location and chemical composition. Some high performance liquid chromatographs (HPLC) measure the change in the refractive index of the column effluent passing through the flow cell. Others detect the fluorescence that occurs when compounds are excited by shorter wavelength energy and emit higher wavelength radiation.

High performance liquid chromatographs (HPLC) vary in terms of general features, display options, and interface options. General features include self-calibration, self-test diagnostics, data storage, and programmability. Devices with a built-in chromatograph and application software are commonly available. Some high performance liquid chromatographs (HPLC) are suitable for extreme temperatures. Others output an analog voltage, analog current, frequency, or pulse. Examples include amplitude modulation (AM), frequency modulation (FM), pulse width modulation (PWN), sine waves and pulse trains. Analog devices usually include an analog meter and a front panel with switches and knobs. Digital devices often provide a serial or parallel computer interface and a video display. Serial protocols include universal serial bus (UBS), RS232, RS422, and RS485.

Gel electrophoresis equipment is used to separate macromolecules, either nucleic acids or proteins, on the basis of size, electric charge, and other physical properties. Gel electrophoresis equipment allows researchers to maintain a uniform electric field across a gel. It also provides for a cooling effect that prevents thermal artifacts and allows access to the gel for convenient loading and monitoring.

An essential piece of gel electrophoresis equipment is the gel electrophoresis apparatus, which uses a positive and a negative charged pole generated by electrical currents to separate molecules. These apparatuses come in horizontal or vertical forms. Horizontal gel electrophoresis units are boxes divided into two compartments by a middle platform and are generally used for separating, visualizing samples, and analyzing restrictions. Vertical gel electrophoresis products are typically used for DNA sequencing and consist of negative and positive electrode chambers, sample wells, gel plates, and cooling plates.

Gel electrophoresis equipment uses an electrophoresis power supply attached to a running tank to provide a regulated electric current to separate the charged molecules. Gel electrophoresis tanks, or running tanks, comprise of a flat bed, column, and slab. Another important component to gel electrophoresis equipment are electrophoresis DNA sequencers, which are made of a slab of gel with electrophoresis migration paths sandwiched between two planes of glass. Different DNA samples are inserted into each migration path and moved under the influence of electrophoresis.

Acrylic electrophoresis equipment, such as an acrylic tank, is used to ensure that there are no leaks during the electrophoresis process. The acrylic material allows for durability and full view of contents due to its transparent material. Platinum electrodes for gel electrophoresis provide gel electrophoresis equipment with a uniform, corrosion resistant, electrical field and are embedded in acrylic tanks for an even current flow. Nucleic acid electrophoresis equipment is used for analytical and preparative studies of nucleic acids and usually comprises of a removable UV-transparent gel-casting tray, end gaskets that allow the gel to be cast directly in the running tank, and buffer recirculation ports.

Gas chromatography detectors (GC detectors) identify solutes as they exit the chromatographic column. As solutes are eluted from the gas chromatography column they interact with the detector. The GC detector converts this interaction into an electrical signal that is sent to the data system. The magnitude of the signal is plotted versus time (from the time of injection) and a chromatogram is generated.  GC detectors use one of several technology types to identify solutes as they exit the column.  These methods include thermal conductivity, flame ionization, atomic emission, electron capture, photoionization, flame photometric, chemiluminescence spectroscopy, and nitrogen phosphorous.

Thermal conductivity GC detectors consist of an electrically heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature to rise in the element, which is sensed as a change in resistance.

Flame ionization GC detectors consist of a hydrogen / air flame and a collector plate. The effluent from the GC column passes through the flame, which breaks down organic molecules and produces ions. The ions are collected on a biased electrode and produce an electric signal.

Atomic emission GC detectors simultaneously determine the atomic emissions of many of the elements in analytes that elute from the GC capillary column. As the eluant comes off the capillary column it is fed into a microwave powered plasma (or discharge) cavity where the components are destroyed and their atoms are excited by the energy of the plasma. The light that is emitted by the excited particles is separated into individual lines via a photodiode array.

Electron capture detectors use a radioactive Beta emitter (electrons) to ionize some of the carrier gas and produce a current between a biased pair of electrodes. When organic molecules contain electronegative functional groups, such as halogens, phosphorous, and nitro groups pass by the detector, they capture some of the electrons and reduce the current measured between the electrodes.

Photoionization GC detectors use ultraviolet light as a means of ionizing an analyte exiting from a GC column. Electrodes collect the ions produced by this process. The current generated is therefore a measure of the analyte concentration.

Chemiluminescence spectroscopy detectors use quantitative measurements of the optical emission from excited chemical species to determine analyte concentration. Usually the emission is measured from energized molecules rather than excited atoms. The bands of light determined by this technique emanate from molecular emissions and are therefore broader and more complex than bands originating from atomic spectra.

Nitrogen phosphorous GC detectors burn the compound in plasma surrounding a rubidium bead supplied with hydrogen and air. Nitrogen and phosphorous containing compounds produce ions that are attracted to the collector. The number of ions hitting the collector is measured and a signal is generated.

In specifying a GC detector the detection limit or sensitivity, dynamic range and temperature range are all important parameters.  GC detectors also have important parameters that are specific to each technology type.

Gas chromatograph columns (GC columns) are tubes through which solutes being measured pass at a rate primarily determined by their physical properties and the temperature and composition of the column. The speed at which solutes move through the column can be weighed against a number of variables to determine its specific properties.  As each solute elutes from the column, it enters a heated detector. An electronic signal is generated upon interaction of the solute with the detector. This signal is recorded and plotted.

GC columns are either packed or capillary style. Packed GC columns are typically of glass or stainless steel coil construction. They are generally one to five meters in total length with an inner diameter of five millimeters.  These devices are filled with a stationary phase, or packing coated with the stationary phase. Capillary GC columns, also known as open tubular columns, are composed of two major parts: tubing and stationary phase. A thin film (0.1-10.0 micro meters) of high molecular weight, thermally stable polymer is coated onto the inner wall of small diameter tubing, usually 0.05-0.53 mm I.D.

Among capillary columns, there are three sub types to choose from. Support coated open tubular (SCOT) columns have an inner wall of the capillary that is lined with a thin layer of support material, such as diatomaceous earth, onto which the stationary phase has been adsorbed. Porous layer open tubular (PLOT) columns are capillary GC columns in which the stationary phase is based on an adsorbent or a porous polymer. Wall-coated open tubular (WCOT) columns consist of a capillary tube whose walls are coated with liquid stationary phase.

One of the most important specifications for GC columns is the material. Typically they are made of stainless steel, glass or a silicate glass such as alkali-borosilicate glass, borosilicate glass or alumina silicate glass. Length of the columns is very important as well, as are the inner diameter and the film thickness. Another factor to consider is the rated operating pressure required for the application.