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 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. 

Digesters break down samples into their basic constituents for analysis.  While they function in a similar fashion to biological digestion, the constituents are not used for food, rather for study.  Sample digestion is often involved in hunts for trace metals, to prepare for such analyses as atomic absorption or inductively coupled plasma spectroscopy.  This classification of devices includes those used for laboratory research, sample testing, and process study.  Large-scale troughs, tanks, or silos used for sewage treatment, treatment of agricultural waste, or methane abatement are not covered. The two most common types of digesters are those that function via acid digestion or microwave digestion (although custom, specialized and proprietary technologies are available).

In acid digestion the sample is digested under elevated pressure (100-500 psi) and temperature (50-180 degrees C) conditions using conventional acids, such as nitric and hydrochloric acids.

Microwave digesters heat microwave-absorbing reagents containing a sample inside a pressurized, microwave-transparent container. Pressurization allows higher temperatures to be achieved, thus increasing the speed of digestion. Pressure and temperature inside the control vessels may be monitored and used to determine the amount of microwave energy applied.

Laboratory digesters are all of a batch-type design.  Sample material, generally organic matter, is placed within the device and is allowed to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.  Installation of a fume hood is helpful to remove gases produced by the digestion process.

One of two methods is used to process samples: open flask or closed flask digestion.  As the name implies, in open flask digestion, the reaction takes place in an open container.  This is the more commonly used method; calling for lower pressure to perform the procedure, although gas build up is more likely.  Closed flask reaction takes place in a closed container.  The elevated pressures that can be achieved in closed reaction vessels decrease digestion time.  Generally, high-pressure vessels are used for biological and organic chemical samples, while lower pressure vessels are used for soil/sediment, environmental samples, and catalysts.

Digesters can be preset to process a batch of samples over a specific period of time. Automated operation frees key laboratory personnel from managing the long and tedious digestion processes and provides a head start on analysis.

Degassers are used to filter or remove gases from solvents and samples. The primary methods employed include vacuum degassing, flow degassing and helium degassing. Flow degassers function via a gentle flow of dry gas over the surface of the sample particles.  This function serves to quickly wick moisture and other contaminants away from the sample.  In many cases, needle valves are employed, which allow careful control of flow rate while avoiding elutriation (blowing out) of fine powders.

Vacuum degassers remove atmospheric interferences and diffused gases from solvents, mobile phases, and reagents during use.  Within these degassers, water trickles through the system where its flow is interrupted up by a packed filtration medium. The water flow is broken down into a very thin film, which allows gases to escape at an enhanced rate. An internal vacuum system inside the filter increases the rate at which gases can be extracted from the sample. 

Solvent or membrane degassers remove gaseous components such as O₂, N₂, and CO₂ from solvents by passing the solvent through a special fluoropolymer membrane tube with the pressure outside the tube reduced. This is a popular degassing method, because membrane separation is carried out under such mild conditions, that the process has virtually no effect leaving the composition of the solvent relatively unchanged. In addition, it is highly efficient, yielding results similar to that of degassing by the helium sparging method.

In helium degassers, the dissolved air within the solvent is expelled by continuously passing helium through the solvent. This is the most effective method of degassing and is particularly recommended for trace analysis, and for avoiding the fluorescence quenching due to the affect of dissolved oxygen on the analytes. The disadvantages associated with helium degassing are the high price of helium, and the fact that the helium can release more volatile solvent components, like tetrahydrofuran, over a period of time. This can give rise to changes in retention times.

Other important configurations to consider include the vacuum rating of the degasser, the number of input channels in the device, and the maximum flow rate per channel.  The more channels that degassers have, and the higher the flow rate, the more rapidly the devices can dissipate gases from a sample.

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.