Lab Manager's Independent Guide to Purchasing a GC System

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Gas chromatography (GC) is a common technique used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition. GC is typically used for separating the different components of a mixture, improving the purity of a particular substance, or identifying a particular compound. GC is a ubiquitous technique, and the various GC instruments available are designed to achieve every requirement of the technique. This guide is designed to help direct the purchaser through a range of instruments and help him or her choose the most appropriate option. Once you've read the guide, check out LabWrench.com to find the latest models that fit your needs.

Gas Chromatography-Mass Spectrometry (GC-MS)

GC-MS is considered the gold standard for many analytical processes, including forensic substance identification. Other applications include drug detection, fire investigation, environmental analysis, explosives investigation and identification of unknown samples. In addition, it can identify trace elements in disintegrated materials.

A. GC-MS -Time of Flight

GC-MS-TOF delivers faster results and greater sensitivity compared with quadrupole systems. GC-TOF instruments are found in environmentally focused and material analysis labs for organic analysis and large molecule screening applications. They are also used in advanced life science research.

B. GC-MS (quadrupole and Ion Trap)

The quadrupole gC-mS is a particularly popular instrument due to its reliability, cost advantages, ease of use, versatility and high sensitivity. However, this technique is not typically used for unknown or new compound identification, due to its nominal unit mass resolution and lack of tandem MS capabilities.

C. GC-MS Magnetic Sector

High-resolution GC-MS allows for the highest sensitivity, making it the ideal choice for ultra-low level trace detection applications such as monitoring for digoxins and related compounds.

D. GC-MS-MS

GC-MS-MS, also known as tandem GC-MS, is achieved by adding a second MS to the output of the first system. This technique reduces sample prep time, shortens analysis cycles, eliminates false results and simplifies data review for improved productivity.

Integrated GC Systems

Integrated GC systems feature an integrated detector that cannot be substituted. These systems are ideal for simple analytical tasks or for situations in which one type of detection is required for all tasks.

A. Basic Programmable

These systems have a programmable temperature ramp. This allows the user to change the oven temperature during the analysis.

B. Basic Isothermal

Basic isothermal systems hold the column oven at a set temperature throughout the entire analysis.

Micro Systems

Micro systems tend to be small and compact and are ideal for use in the field or in other situations where space is limited.

A. Laboratory use only

Some micro GC systems are designed for use in a laboratory where space is limited. Although these systems may lack some of the functionality of their larger counterparts, they offer efficient and accurate operation.

  • Accommodates single detector only: A single detector is appropriate for many GC applications, and a number of GC base units accommodate only one detector at any one time.
  • Accommodates autosampler: The autosampler provides the means to introduce a sample automatically into the inlets. Automatic insertion provides better reproducibility and time optimization.
  • Accommodates multiple detectors: More than one kind of detector can be used to achieve selective and/or highly sensitive detection of specific compounds encountered in particular chromatographic analyses.

B. Field use

A number of micro systems are designed to be portable so they may be moved from one location to another or even used in the field.

Modular Systems

Many GC systems are available as basic units that can accommodate different detectors and different sample injectors. This allows users to customize the GC system according to their own particular needs, providing full flexibility. The purchaser should first choose a base system, then a detector type and finally a method of sample introduction. Compatible combinations are indicated graphically in the diagram below.

A. Choose your base system

The base system is the fundamental part of the modular GC unit, accommodating the GC column and basic hardware.

  • Accommodates 1 detector: A single detector is appropriate for many GC applications, and a number of GC base units accommodate only one detector at any one time.
  • Accommodates >1 detector simultaneously: More than one kind of detector can be used to achieve selective and/or highly sensitive detection of specific compounds encountered in particular chromatographic analyses.
  • Accommodates autosampler: The autosampler provides the means to introduce a sample automatically into the inlets. Automatic insertion provides better reproducibility and time optimization.

B. Choose your detector type

A number of detectors are used in gas chromatography. the most common are the flame ionization detector (FID) and the thermal conductivity detector (TCD). Other detectors are sensitive only to specific types of substances, or work well only in narrower ranges of concentrations.

Thermal Conductivity Detector (TCD)
Able to measure almost all molecules, including inorganic and permanent gases as well as organic vapors. A good non-destructive detector for general organic and analytical testing, atmospheric monitoring, and bulk measurements. Helium is the recommended carrier.

Flame Ionization Detector (FID)
Commonly used for organic analyses, solvent and material characterization, environmental samples, TPH and petroleum analyses. Air and hydrogen are support gases, using helium as a carrier gas.

Electrolytic Conductivity Detector (ELCD)
Ideal for environmental applications. Provides cleaner and more linear response of DELCD compared to ECD. Works best using helium as a carrier, with air and hydrogen as supplemental gases.

Electron Capture Detector (ECD)
Uses a radioactive beta particle (electron) source to measure the degree of electron capture. The ECD is the most sensitive detector available for the environmental monitoring of halocarbons and pesticides, and it is usually run independent of other detectors.

Nitrogen Phosphorus Detector (NPD)
Responds to nitrogen-bearing compounds with phosphorus-containing materials. As part of an FID assembly, it needs the air-hydrogen jet to burn the eluting “peak” with a helium carrier.

Helium Ionization Detector
Provides “universal” detection and, similar to TCD, is non-destructive; for gaseous species with an ionization. Less expensive than similar detectors. Suited for special tests, such as transformer oil gases, road bed emissions and other non-organic vapor measurements. High-quality helium is required as the carrier.

Flame Photometric Detector (FPD)
Used for the detection of sulfur- or phosphorus-containing compounds. The device uses the chemiluminescent reactions of these compounds in a hydrogen/air flame as a source of analytical information.

Photoionization Detector (PID)
The photoionizaton detector is the most efficient and inexpensive type of gas detector and measures volatile organic compounds (VoCs) and other gases in concentrations from sub ppb to 10,000 ppm. The PID is capable of giving instantaneous readings and monitoring continuously.

Catalytic Combustion Detector (CCD)
A unique and economical alternative to the FID for the detection of hydrocarbon compounds, the CCD has a typical detection limit of about 500 ppm when run with a helium carrier. It can also run at lower temperatures (<200°C (392°F)) when run on air, creating a truly self-contained GC.

Discharge Ionization Detector (DID)
Uses a high-voltage electric discharge to produce ions. The ions produce an electric current, which is the signal output of the detector.

C.Choose your sample induction type

The column inlet (or injector) provides the means to introduce a sample into a continuous flow of carrier gas. The inlet is attached to the column head.

Split/splitless injector
A sample is introduced into a heated chamber via a syringe and then volatilized. The carrier gas then either sweeps the entire sample (splitless mode) or a portion split mode) into the column. Split injection is preferred when working with samples with high analyte concentrations (>0.1%), while splitless injection is best suited for trace analysis with low concentrations of analytes (<0.01%).

On-column inlet
A sample is introduced in its entirety without heat.

Multimode inlet
The multimode inlet offers capabilities in a single inlet that would typically require two or more inlets. For example, increasing system sensitivity, analyzing thermally labile compounds, or injecting dirty samples.

D. GC Sample Extraction

A couple of common techniques are employed to treat samples on the GC prior to introduction into the carrier gas stream.

Headspace Analysis
A sample is heated gently in a closed container and the resulting vapor above the sample is then injected into the GC.

Purge and Trap
A purge gas is run across a sample and onto an adsorbate, like charcoal, which traps the volatile components of interest. The adsorbate is then heated to release the sample's volatile components into the GC.

E. Multidimensional GC

GC x GC systems allow the detection of more compounds than is possible with conventional GC by dividing the usual tens or hundreds of peaks into thousands of individual components from complex mixtures. Like conventional GC systems, GC x GC systems are available with different types of detectors.

Categories: Purchasing Guides

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Scientists & The Social Media

Published: April 1, 2011

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