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The Environics, Inc. Post

The Environics, Inc. Post

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Gas Chromatography and Calibration Standards

 

Today, we will focus on gas chromatography (or GC), a technique used by many of our existing customers in a wide variety of fields.  To state it simply, GC is a method used to separate, identify and quantify chemical compounds.  A mobile phase containing the sample is passed over an unmoving and immiscible stationary phase.  The mobile phase is comprised of the sample and a carrier gas (typically helium, nitrogen, argon, hydrogen or air).  The purity of the carrier is critical and ultra-pure gases are normally purchased or, in the case of air, zero air can be generated on site for cost savings and high purity.   

GC separation resized 600

The time in which it takes components in the carrier to pass through the stationary phase, known as the retention time (tR), is determined.  The more soluble a component is in the stationary phase, the higher the retention time. 

Once a component passes through the stationary phase, a detector allows the user to identify each of them, determine their mass and quantify the concentration.  There are a wide variety of detectors, and the detector chosen depends on the components and the needs of the user. 

The most commonly used detectors are the flame ionization detector (FID) and the thermal conductivity detector (TCD). They share a common sensitivity and functional concentration range.  TCDs can be used to detect virtually any component other than the carrier gas and is non-destructive, while FIDs are sensitive primarily to hydrocarbons and incinerate the entire sample.

Most importantly, proper calibration of the GC is essential.  By calibrating GC, the various retention times for compounds of interest are detected. Also, the area under the peak can be used to determine the concentration of the sample components by comparison to a determined calibration curve. 

GC response time resized 600A calibration curve is generated by running various dilutions of the compound/s of interest and then plotting response time and against concentration.  These points represent the calibration curve. No two compounds will produce exactly the same calibration curve, and the user must construct a calibration curve for each analyte.  It is also best practice to rerun the calibration at frequent intervals.  Precise calibration standards can be generated by gas dilution systems, which offer the advantage of on-site gas blending of 100% pure gases cylinders, providing a solution to using numerous, costly premixed cylinders of gas.  

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Comments

very helpful content but can anyone tell me what is Thermal conductivity detector offset ?
Posted @ Friday, April 05, 2013 8:36 AM by Suel Ahmed
The zero-offset is the calibrated signal measured from the atmosphere within the chamber during which a reference gas is supplied. This correction signal is then subtracted during the measuring stage from the measuring side of the TCD. During each calibration, this offset is recalculated and the new value overrides the existing. I hope this helps!
Posted @ Friday, April 05, 2013 8:47 AM by Rachel Stansel
helpfull guide lines
Posted @ Friday, February 28, 2014 6:52 PM by lakshmi
The description and the figure is in a very simplified way, very easy o understand
Posted @ Thursday, May 22, 2014 2:59 AM by euracia l wahlang
GC - Headspace to be calibrated daily or what it the time gap
Posted @ Friday, July 25, 2014 2:26 AM by D. JEGANATHAN
Frequency of calibration is up to the user, but most users run a zero and 1-3 known samples before each run. If these do not verify, you would then need to repeat the calibration to get a corrected curve.
Posted @ Monday, July 28, 2014 8:48 AM by Rachel Stansel
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