Industries Information

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.

Gas chromatographs separate organic chemical mixtures according to physical properties. Instruments consist of a flowing mobile phase, an injection port, a separation column with a stationary phase, and a detector. Differences in partitioning behavior between the mobile phase and the stationary phase cause the separation of compounds. Gas chromatographs use many different detection methods. Choices include: thermal conductivity detectors (TCD), flame ionization detectors (FID), atomic emission detectors (AED), electron capture detectors (ECD), photo-ionization detectors (PID), flame photometric detectors, and nitrogen phosphorous detectors. Gas chromatography that uses chemiluminescence spectroscopy is also available. Some gas chromatographs are used with agricultural chemicals and pesticides, cosmetics and fragrances, drugs and pharmaceuticals, electronics, energy and environmental applications, food and beverage processing, forensics and narcotics, industrial gases, medical and biological applications, or metals and welding. Others are used with organic and inorganic chemicals; paint, castings, and inks; paper, pulp and wood; petroleum products; plastics, polymers, and resins; or soaps and cleaners.  

Oven operation, pump specifications, and injection operation are the main parameters for gas chromatographs. Devices with a column oven carry specifications such as operational method and oven temperature range. There are two operational methods for columnar ovens: isothermal or constant-temperature, and variable or temperature-programmable. Flow control and inlet pressure range are common pump specifications. In terms of injection operation, parameters include number of injection ports, injection method, and column injection. For optimum efficiency, a sample should not be too large for a column, and is best introduced as a "plug" of vapor. There are two choices for column injection method: split/split-less and packed column. With split or split-less column injection, the injector has an additional set of gas lines, providing another path for the vaporized sample. This extra line is sometimes called a split line or vent. Importantly, the amount of gas that exits the split line controls the amount of sample that enters the column. With packed column injection, all samples enter the column - including the gaseous sample, solvent, and analyte.

Specifications for gas chromatographs may include user interface options, display options, additional output options, and general features and functionality. Local interfaces use an analog front panel, a digital front panel, or a computer interface with parallel or serial communications. Analog meters, digital readouts, and video displays are common choices for display options. Additional output options include: analog voltage, frequency or modulated frequency, analog current, and switch or relay contacts. Some gas chromatographs are programmable, self-calibrating, or include a built-in chromatogram. Others provide data storage options and are designed for extreme environments. Gas chromatographs that accept liquid samples and include self-test diagnostics are also available. 

Chromatographs are used to perform chromatographic separation and produce chromatograms. Chromatography separates complex mixtures of chemicals or proteins into their various constituents. A chromatograph takes a chemical mixture carried by a liquid or gas and separates it into its component parts as a result of the differential distribution of the solutes as they flow around or over a stationary liquid or solid phase. The basic principal of chromatographs is that when a stream of chemical compounds flows over a surface that attracts some of the compounds more than others, the less strongly attracted compounds move faster than the compounds that stick more tightly to the surface, and the mixture becomes separated. Most chromatographs consist of five different parts: the mobile phase supply system, the injection or sampling system, the column and column oven, the detector, and the data acquisition and data processing system.

There are two basic types of chromatographs: liquid chromatographs and gas chromatographs. A liquid chromatograph is used to separate ions or molecules that are dissolved in a solvent. If a sample solution contacts a second solid or liquid phase, the different solutes interact with the other phase to differing degrees due to differences in adsorption, ion-exchange, partitioning, or size. These differences allow the mixture components to be separated by using the differences to determine the transit time of the solutes through a column. A gas chromatograph can be used to identify air pollutants or chemicals based on volatility and/or evaporation. For example, researchers in the fragrance industry can use a gas chromatograph mass spectrometer to identify the chemicals that make up a particular odor or taste. The gas chromatograph separates the chemical mixture into pulses of pure chemicals. The mass spectrometer identifies and quantifies the chemicals.

Chromatographs are used in a variety of industries and applications such as the drug discovery industry, organic and inorganic chemistry, and the food industry. A gas chromatograph is often used for determine the nutritional quality and safety of food. For best results, chromatographs must provide fast data acquisition speeds with high resolution/exact mass measurements. A chromatograph must also feature a high dynamic range for both qualitative and quantitative analysis.

electrophoresis (CZE), Capillary electrophoresis (CE) is a group of related separation techniques that use narrow-bone fused-silica capillaries to separate a complex array of large and small molecules. Capillary electrophoresis (CE) is used for the analysis of ions, and both basic and chiral pharmaceuticals. There are many types of capillary electrophoresis (CE) depending on the type of capillaries and the electrolytes used. Examples include micellular electrokinetic chromatography (MEKC), capillary zonecapillary gel electrophoresis (CGE), and capillary isoelectric focusing (CIEF). Micellular electrokinetic chromatography (MEKC) is used for separating mixtures containing both ionic and neutral species. Capillary zone electrophoresis (CZE) is known as free solution CE and it provides constant field strength throughout the length of the capillary. A capillary gel electrophoresis (CGE) is used to separate the proteins and nucleic acids. Capillary isoelectric focusing (CIEF) separates amphoteric molecules such as proteins by electrophoresis in a pH gradient that is generated between cathode and the anode. Other capillary electrophoresis (CE) are commonly available.

There are several ways in which capillary electrophoresis (CE) function. In micellular electrokinetic chromatography (MEKC), the surfactants are added to a buffer solution and the separation is done by differential partition between the solvent and micelle. Capillary zone electrophoresis (CZE) fills the capillary tube with aqueous buffer solution that contains both positive and negative ions. The surface of the capillary has a net negative charge because of Si-O chemical groups present in the glasslike material. Capillary gel electrophoresis (CGE) uses gel which builds heat and is detrimental to separation. It is used for DNA sequencing and genotyping. Capillary isoelectric focusing (CIEF) combines with capillary reversed-phase liquid chromatography (CRPLC) using a micro injector as the interface for performing the 2-D protein separations from complex protein mixtures. Capillary electrophoresis (CE) is designed and manufactured to meet most industry specifications.

Capillary electrophoresis (CE) is used in many applications. Some examples include genetic analysis, analysis of pharmaceuticals (containing nitrogenous bases), gunshot residue analysis, and counter-ion analysis in drug discovery. Capillary electrophoresis (CE) adheres to ICC standards.

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.

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 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 titrationamperometry, 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.