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.
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.
A 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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