Process Gas Chromatographs. Tony Waters

gas.

       The sample must be a gas or a volatile liquid that quickly vaporizes and enters the column as a vapor.

       In the laboratory, sample injection is by glass syringe, either manual or by an autosampler.

       Process gas chromatographs (PGC) use an automatic injection valve to inject the sample.

       The carrier gas carries the sample into the column where it contacts the stationary phase.

       The stationary phase may be a solid adsorbent or an immobilized non‐volatile liquid.

       Contact with the stationary phase delays some peaks more than others, so separation occurs.

       Special routing valves may direct the peaks into different columns to finish the desired separation.

       PGCs can now use either packed columns or capillary (open‐tubular) columns.

       The separation process takes time; typically one to ten minutes, sometimes longer.

       The carrier gas elutes peaks from the column into a chosen detector for measurement.

       The detector responds to a property of analyte molecules that differs from carrier gas molecules.

       The TCD, FID, and FPD are popular detectors in process gas chromatographs.

       The detector output signal forms a chromatogram display when plotted against elapsed time.

       Analyte molecules cluster together at different times to form separate chromatogram peaks.

       The PGC may measure peak area or peak height to compute the concentration of an analyte.

       The PGC uses stored calibration factors to calculate the concentration of each analyte.

       The chromatogram is a most valuable source of information, but one must learn to read it.

       To the expert troubleshooter, all chromatographic faults are visible on the chromatograms.

Did you get it?

      Self‐assessment quiz: SAQ 01

      1 Q1. In gas‐liquid chromatography, what are the gas and liquid doing?Select the one correct answer:They are both moving.The gas is moving, and the liquid is stationary.The gas is stationary, and the liquid is moving.They are both stationary.

      2 Q2. In a gas chromatograph, which one of the gases listed below would not suitable as the carrier gas?Select the one correct answer:OxygenNitrogenHydrogenHelium

      3 Q3. In a gas‐liquid chromatograph, which one of the materials listed below might be the stationary phase?Select the one correct answer:An inert gasA granular adsorbent solidA volatile liquidA non‐volatile liquid

      4 Q4. In a gas‐liquid chromatograph, what really causes the separation?Select the one best answer:The carrier gas causes the separation.The mobile phase causes the separation.The stationary phase causes the separation.The chromatogram causes the separation.

      5 Q5. In a gas chromatograph, what do the columns do?Select the one best answer:They convert all the components into peaks.They separate the analytes from all other components and from each other.They separate all the components of the sample.They allow only measured components to enter each detector.

      6 Q6. In a gas chromatograph, what does a detector do?Select the one best answer:It provides a continuous flat baseline as a reference for measuring the peaks.It generates a signal proportional to the instantaneous number of component molecules leaving the column.It measures either the area or the height of each peak.It converts each component of the sample to a concentration.

      7 Q7. Why is the chromatogram so important?Select the one best answer:It shows the baseline used for measuring the peaks.It shows the shape of each peak.It shows the separation between peaks.All of the above.Check your SAQ answers with those given at the end of the book.

Student evaluation test: SET 01

      Your instructor will provide the answers to these test questions.

      1 S1. In gas‐liquid chromatography, what is the physical state of the mobile phase?Select the one best answer:GasGas or liquidLiquidSolid

      2 S2. In gas‐liquid chromatography, what is the physical state of the stationary phase?Select the one best answer:GasGas or liquidLiquidSolid

      3 S3. In gas‐liquid chromatography, what is the physical state of the injected sample?Select the one best answer:GasGas or liquidLiquidSolid

      4 S4. According to the chapter text, what is a typical number of analyses done per day by a process gas chromatograph, relative to the number done by a laboratory gas chromatograph?Select the one best answer:About 80 times as manyAbout 120 times as manyAbout 240 times as manyAbout 720 times as many

      5 S5. According to the chapter text, what is the main function of a housekeeping column?Select the one best answer:To separate all the measured components.To separate only the unmeasured components.To allow strongly‐retained components to quickly exit the column system.To permanently absorb one or more unmeasured components.

      6 S6. Imagine you are changing a PGC from liquid sample injection to gas sample injection and the gas sample will be at atmospheric pressure. If the previous liquid sample volume was 1 μL, what new gas sample volume would you install to get about the same peak heights?Select the one best answer:0.001 mL0.3 mL0.9 mL3.0 mL

      7 S7. PGC detectors may not respond to every analyte. Consider the statements below; which statements are correct?Select all of the correct statements and none of the incorrect statements:No detector can respond to hydrogen sulfide, H2S.The thermal conductivity detector does not respond to hydrocarbons.The flame ionization detector is very sensitive and can measure low concentrations of carbon dioxide, CO2.The flame photometric detector does not respond to carbon monoxide, CO.

      8 S8. What is a chromatogram, and why is it important?From the list below, select all the correct statements and none of the incorrect statements:The chromatogram is a graphical plot of the detector signal against elapsed time.On a chromatogram, the horizontal axis represents elapsed time, which always increases from left to right, with the zero‐time marker on the far left.On the chromatogram from a modern process gas chromatograph, the vertical axis represents the concentration of the analytes and is scaled from zero to 100 %.Chromatographic faults produce symptoms visible on the chromatogram that an expert user can diagnose.

      9 S9. How is each analyte peak on the chromatogram measured?From the list below, select all the correct statements and none of the incorrect statements:Most modern PGCs measure the peak height, but a few measure the peak area.To calculate the concentration of an analyte, the PGC multiplies the analyte peak height or peak area by a stored calibration factor.The calibration factor depends on the detector in use but is the same for each analyte in the sample.The calibration factors come from the analysis of a calibration sample containing known concentrations of the analytes.

References

      Cited

      1 Ettre, L.S. (2008). Chapters in the Evolution of Chromatography (ed. J.V. Hinshaw). London, UK: Imperial College Press.

      2 Harvey, D. (2017). Gas chromatography. In: LibreTexts, Section 2.4 (updated July 28, 2017), accessed October 23, 2018 at https://chem.libretexts.org

      3 Rahman, M.M., El‐Aty, A.A., Choi, J., Shin, H., Shin, S.C., and Shim, J. (2015). Basic overview on gas chromatography columns. In: Analytical Separation Science (eds. J.L. Anderson, A. Berthod, V. Pino, and A.M. Stalcup), 823–834. Verlag, Germany: Wiley‐VCH. doi:10.1002/9783527678129.assep024

      4 Waters, T. (2017). The fine art of chromatogram reading. Proceedings of the 2017 Analysis Division Symposium, Pasadena, California (April 24–26, 2017). Research Triangle Park, NC, USA: International Society