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Do’s & Don’ts Regarding Operation of Gas Chromatography


Do’s
  • Before starting the instrument, verify the volume of gas in cylinder.If the primary gas pressure is less than 4 psi, replace it.
  • Purity of carrier gas should be maintained to prevent degradation of chromatographic hardware.
  • Purity of gases should be 99.999% and cylinder should be supplied with a purity certificate.
  • Gases should be passed through a gas panel containing Hydrocarbon and Oxygen traps to ensure no impurity or moisture enters the Inlet or Column.
  • Immediately change the expired oxygen and moisture traps (when blue colour changes to white) to avoid gas contamination.
  • During cartridge replacement, check valves and close off the system to the atmosphere, further minimizing the entry of contaminants.
  • Start the Instrument (GC-HS) first then Load the software from the system.
  • Once the Instrument is “On”, always start the machine by running ‘Conditioning’ method for half hour.
  • Samples and Standards should be clearly marked and labelled, to avoid error.
  • Always use fresh, clean and sterile vials for sample preparation and storage.
  • Septa with a PTFE face down, should be used to seal the headspace vial to eliminate bleed from the septa.
  • Shaking or vibrating the vial containing high viscosity sample matrices during heating can assist in achieving equilibrium faster.
  • Adjust the temperature of the sample to change the solubility of the analyte as well as to drive the equilibrium in towards the gaseous phase.
  • To minimize matrix problems and prevent water condensation from aqueous samples, use a higher transfer line temperature, usually 100 higher than sample heating temperature.
  • Heat the syringe to a temperature comparable to the sample vial temperature to minimise pressure differences, when using gas tight syringes.
  • Ensure the septum of the GC injection port is well maintained to decrease the possibility of a leak.
  • In Headspace, septa can be changed after 500 injections.
  • Use injection port liners of small internal diameters and lower buffer volumes to maintain a narrow bandwidth.
  • Use an on-column syringe when injecting into an on-column inlet so that the injector, syringe and column are not damaged.
  • Inject standards and samples in order from low to high concentrations to help minimize carryover.
  • For high concentration samples in a sequence of samples, run a blank after the suspected samples to reduce carryover contamination.
  • Increase the oven temperature after the samples are completely transferred to the column to increase the movement of compounds inside the column.
  • Built sufficient time into the sample cycle to achieve constant state of equilibrium.
  • GC instrument maintenance should include checking fittings and connections with a gas leak detector.
  • To prevent stationary phase decomposition, the oven and inlet should be at room temperature when not in use and when changing the septum.
  • After running the samples, condition the system at least 30min before shut down.
  • Always ensure Detector temperature reaches 500°C and lower, before switching “Off” the GC Instrument.
Don'ts
  • Constant exposure of capillary columns to oxygen and moisture should be avoided especially at high temperatures as it may produce rapid and severe column damage.
  • Improper handling or installation of gas lines should be avoided as moisture introduced by this can be a common cause of column stationary phase degradation.
  • Do not use sample matrices containing high molecular weight compounds to avoid incomplete or inefficient transfer into the GC injection port.
  • Transfer line temperature should not be kept low than sampler oven as water from the sample matrix can cause problems by condensing in the transfer line.
  • Avoid using high-concentrated samples lest they produce ghost peaks in subsequent analyses due to carryover of sample from previous injections.
  • Do not use reuse unclean or unpacked vials.
  • Septa with punctured PTFE face should not be reused.
  • Do not use transfer line having smaller internal diameter than the injection port liner to avoid broader peaks, tailing peaks, lower sensitivity, and loss of resolution.
  • Use company recommended regulator materials and tubing. These should only be changed by an company engineer.
  • Non-metallic types of tubing such as polyethylene and Teflon are not recommended for GC applications due to their gas permeability and difficulty in cleaning.
  • Unclean or improperly cleaned tubing can lead to contamination of the system with disastrous results.
  • Never open the GC door if oven temperature is more than 1000°C or during running conditions.
  • Do not inject air into the vials to prevent the vacuum. This often damages the cap seal.
  • Avoid cleaning agents that are alkaline, contain phosphates, or are strongly acidic for syringes or glassware.

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