Gas chromatography, an introduction.pdf
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The document provides an introduction to gas chromatography (GC), describing its principle as a separation technique using a gaseous mobile phase and a stationary phase. It details the sample preparation, injection techniques, and the significance of the carrier gas's purity, alongside the types of GC columns, their characteristics, and the effects of temperature on analysis. Additionally, it explains retention time, chromatographic terms, and calculation methods for column efficiency and resolution.
![• The commonly used stationary phase for capillary GC column is
poly dimethyl siloxane [PDMS]. The separation, in this case, will
depend upon dispersive interactions (van der Waals) with
nonpolar part of the analytes.
• The polarity of PDMS stationary phases can be increased using;
cyano, or trifluoro functional groups, each of these additives leads
to different separation results.
• Using PDMS with cyano, trifluoro, or (especially) hydroxyl
functional groups, analyte separation performed based on
hydrogen bonding interactions (the strongest intermolecular force
in the capillary gas chromatography (GC).
GC-COLUMN
https://www.chromatographyonline.com/view/
pragmatic-rules-gc-column-selection](https://image.slidesharecdn.com/gaschromatographyanintroduction-221208103936-f0403db1/85/Gas-chromatography-an-introduction-pdf-11-320.jpg)
Gas chromatography, an introduction.pdf
- 1. Gas chromatography, an introduction A. Prof. Sherif M. Taha Tel: 01004724944 sherif2taha@gmail.com للهامسب يح رلانمح رلا م
- 2. • Is a separation technique based on analytes partitioning between two immiscible phases: a) gaseous mobile phase (Carrier gas) b) a stationary solid or immobilized liquid phase (packed or hollow capillary column). Gas chromatography • A sample is first vaporized inside a heated liner to be passed through a gaseous carrier into GC column. • The passed analytes condensed/ adsorbed on the stationary phase of the GC column, the initial temperature of the used column (equal to the oven temperature) being much lower than that of the liner (being lower than the boiling point of the sample solvent. Stauffer E, Dolan JA, and Newman R (2008) Fire Debris Analysis, p. 246. Burlington, MA: Elsevier Academic Press.
- 3. Gas chromatography • The adsorbed analytes elaborated (from the condensed spot) by applying heating program (the stronger adsorption of an analyte, the higher heat required for its desorption). Therefore, GC is a suitable technique for thermally stable compounds. Sample evaporation inside the liner Compounds desorption Sample condensation https://www.crawfordscientific.com/chromatography-blog/post/gc-carryover-problems A B C Solvent A, B, and C target compounds A B C
- 4. Carrier gas • A carrier gas is responsible for carrying the sample molecules into the column and finally to the detector. • Carrier gas must be: inert with the stationary phase, of high purity 99.999 %. (for lower residue analysis 99.9999 % is preferred). Agilent, fundamental gas chromatography, G1176-90000 It is better to be vertically
- 5. GC Injection Techniques • Split/split-less • Programmed temperature vaporization (PTV) injection. • On-column injection
- 6. Split injector • Sample enters the hot liner and volatilized rapidly. • Vaporized sample is mixed with a carrier gas (diluted/spread). • Finally, a large part of the diluted vaporized sample is split away from the Colum, while a small part will enter the column. Agilent, fundamental gas chromatography, G1176-90000 This mode of injection is used for analysis of samples of high analyte concentrations.
- 7. Split-less injector • Sample enter the hot liner and volatilized rapidly. • Vaporized sample is mixed with a carrier gas (diluted/spread). • Finally, all the diluted vaporized sample enter the column. Then after, the split valve is opened to remove residual vapors. Agilent, fundamental gas chromatography, G1176-90000 This mode of injection is used for the analysis of trace analyte concentrations (residue analysis).
- 8. • Sample is injected in a cooled liner (enable sample and analyte adsorption). While, the solvent will be split away. Therefore, injection in PTV should be carried out slowly. • After solvent evaporation, closing the split valve, and beginning ramping temperature increment (ramp PTV) to vaporize adsorbed sample matrix and analytes to be carried into the column. Baffled Liner Vent Syringe needle Column Sample Analyte Solvent Program temperature vaporized (PTV) injector This mode of injection is used for the injection of large sample volume, especially with solvents of large expansion volumes. Agilent, fundamental gas chromatography
- 9. • The used solvent (containing the analytes) shall have an expansion volume that not exceed 75 % of the liner volume (especially when using split-less mode of injection). • Inner-surface of the used liner shall be highly inert, especially for residue analysis. • Testing samples of dirty matrices the selected liners preferred to have a glass wool. • The temperature of the liner should be enough to evaporate the desired analytes and avoid its degradation. • A bottom tap liner is more favorable for split-less injection. Selecting the right liner http://www.restek.com/Supplies-Accessories/GC- Accessories/Inlet-Liners-Liner-Supplies https://www.chromservis.eu/c/gc-liners
- 10. GC-COLUMN Capillary columns Open tubular (OT) K. Maštovska, S.J. Lehotay, J. Chromatogr. A, 1000 (2003) 153 Packed columns Agilent, fundamental gas chromatography, G1176-90000 Agilent, fundamental gas chromatography, G1176-90000
- 11. • The commonly used stationary phase for capillary GC column is poly dimethyl siloxane [PDMS]. The separation, in this case, will depend upon dispersive interactions (van der Waals) with nonpolar part of the analytes. • The polarity of PDMS stationary phases can be increased using; cyano, or trifluoro functional groups, each of these additives leads to different separation results. • Using PDMS with cyano, trifluoro, or (especially) hydroxyl functional groups, analyte separation performed based on hydrogen bonding interactions (the strongest intermolecular force in the capillary gas chromatography (GC). GC-COLUMN https://www.chromatographyonline.com/view/ pragmatic-rules-gc-column-selection
- 12. GC-COLUMN
- 13. • A longer column will increase the resolution (selectivity), but it will also increase analysis time, and cost. • Reduced column internal diameter double the efficiency and leads to better selectivity. This will increase retention time when using isothermal separations. such columns easily contaminated and suffer from peak broadening after routine work. • Changes in film thickness effects retention of analyte species, interaction with the silica tubing increased with increasing film thickness. Usually, thin films (0.10-0.25 µm) are used for trace analysis. GC-COLUMN
- 14. A common GC-COLUMN for residue analysis/ HP 5 MS Internal diameter 0.25 mm Length 30 m Film 0.50 µm Temperature Limits -60 - 325/350 °C • Stationary phase coating; 5 % Phenyl-methyl poly siloxane
- 15. Column Temperature • Every capillary column has its working temperature range that maintains the state of the used coated stationary phase. • Working on GC column should be below its maximum Temperature by 20 ºC (if possible).
- 16. Column starting temperature • The starting temperature for the column (oven temperature) should be much lower than the boiling points of the target compounds. • For analytes of low boiling points, the adsorption of both solvent and these analytes is required at the front column inlet (starting column temperature of 20 C lower than the used solvent) which will subsequently enhance its peak shape. • While, for highly boiling points analytes (late eluting peaks), adsorption of the used solvent isn’t required (it cost a longer run time) only start with a temperature below the boiling points of these analytes but that give enough adsorption at the front column inlet (no peak tailing).
- 19. Retention time, related terms and definitions • Chromatogram : A plot of the detector response related to the effluent time. • Mass spectrum: A plot of the intensities versus m/z of specific peak. • Retention time tR : Time taken by the carrier gas from the analyte injection to its completely detection. • Dead time (holdup time) tM : Is the time taken by the carrier gas from the point of injection to the detector. • Adjusted retention time : = tR - tM • Distribution constant K: The ratio of analyte concentration in the stationary phase to its concentration in the mobile phase 𝐾 = 𝐶𝑠 𝐶𝑚
- 20. Column Resolution, related terms and definitions • Separation factor (selectivity factor): is the relative retention times of two adjacent eluted peaks, α= tR1 tR2 . • Peak resolution based on their peak widths Rs= 2(tR1− tR2) Wb1 + Wb2 . • Peak resolution based on their peak widths Rh= h1−hw h1 • The number of theoretical plates, N = 16 ( tR W ) 2 • Efficiency (N) ,for thin film column, is related to column length (L) and internal diameter (dc), N= 𝐿/𝑑𝑐 https://doi.org/10.1016/B978-0-12-409547-2.14348-3
- 21. • Yehua Han, Yanfen Zhang, Huwei Liu https://doi.org/10.1016/B978-0-12-409547-2.14348-3 • A.I. Ruiz-Matute, S. Rodrı´guez-Sa´nchez, M.L. Sanz and A.C. Soria, https://doi.org/10.1016/B978-0-12-814264- 6.00012-8 • Alejandra Garcia Piantanida , Andrew R. Barron, Principles of Gas Chromatography, 2014. http://cnx.org/content/m50228/1.2/ • Fundamentals of Gas Chromatography, Agilent Technologies, Inc., 2002. G1176-90000 • https://www.restek.com/pdfs/GNBR1724-UNV.pdf • http://www.chromatographyonline.com/pragmatic-rules-gc-column-selection • https://chem.libretexts.org/Textbook_Maps/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/I nstrumental_Analysis/Chromatography/Gas_Chromatography. References
- 22. Thank you A. Prof. Sherif M. Taha Tel: 01004724944 sherif2taha@gmail.com