Why Choose Programmable Temperature over Isothermal Operation for Gas Analysis?

Gas mixtures can be rapidly analyzed with a Micro GC, which uses multiple GC modules for the analysis of multiple samples in parallel. The isothermal operation of conventional Micro GCs arercapable of rapidly analyzing light hydrocarbons and fixed gases. However, both isothermal and temperature programmable operations can be performed effectively by the next generation Micro GC, the Micro GC Fusion.

Isothermal and Temperature Programmable Operation

Isothermal Operation

In isothermal operations the column temperature is constantly maintained throughout all of the analysis. However, this operation has its own drawbacks especially in the presence of heavy (C4 and above) components in the gas sample. The major disadvantages of this set up are:

  • A decreased sample throughput
  • Run times are longer than temperature ramping
  • Broad peaks for late eluting components
  • Carryover effect from heavier components

Temperature Programming

In temperature programming, the ramp rate and the temperature profile can be adjusted during the run. The elution of desired components can be accelerated by optimizing the temperature profile of each module. Quick cycle times can be achieved (often below 2 minutes) through optimization of cool down times in Micro GC Fusion.

The temperature programmable operation is advantageous as it allows:

  • A higher sample throughput
  • Less carryover
  • Extended application range on a single column
  • Sharper peaks for late eluting components

Experimental Procedure

Experiment 1 used an 8m Rt-Q-Bond column to isothermally analyze a natural gas calibration standard and Experiment 2 used a temperature programmed method for the analysis of a natural gas calibration standard on the same 8 m Rt-Q-Bond column.

Table 1 lists down the concentrations of the components.

Table 1. Natural gas calibration standard concentration information

Component Mole %
nitrogen 1.53
methane 88.7
carbon dioxide 1.21
ethane 2.99
propane 2.01
isobutane 1.00
n-butane 0.996
isopentane 0.300
n-pentane 0.300
hexanes 0.603
heptanes 0.320
octanes "plus" 0.0750

Experimental Results

In the isothermal operation, the Rt-Q-Bond column is restricted to methane through propane analysis (Figure 1).

The column faces the problem of sticking of heavier components, meaning it is necessary to extend the run time for another 2-3 minutes to allow the heavier components to elute from the column.

Chromatogram of the natural gas calibration standard-isothermal

Figure 1. Chromatogram of the natural gas calibration standard-isothermal

Conversely, the application range of the Rt-Q-Bond column is extended in the temperature-programmed run, allowing it to analyze heavier components such as C4-C8+. The elution of these heavier components during the run minimizes carryover, as shown in Figure 2.

Chromatogram of the natural gas calibration standard-temperature programmed

Figure 2. Chromatogram of the natural gas calibration standard-temperature programmed

The retention time and the propane peak shape are compared in both experiments to demonstrate the ability of temperature programming to provide a sharper peak shape. The elution of propane takes place at 161 secondswhen using an isothermal operation, whereas the elution time of propane in temperature programming is 49.4 second.

The faster elution of propane makes the peak sharper as there is less time for longitudal diffusion. The shaper peak facilitates a more accurate integration.

Component retention times are listed in Table 2.

Table 2. Retention time comparison for the natural gas calibration standard

Number of Component Name of Component Retention Time (s) Isothermal Retention Time (s) Temperature Programmed
1. nitrogen 19.2 18.5
2. methane 20.5 19.5
3. carbon dioxide 25.2 22.9
4. ethane 40.8 30.6
5. propane 161 49.4
6. isobutane N/A 64.7
7. n-butane N/A 68.2
8. isopentane N/A 82.1
9. n-pentane N/A 84.4
10. hexanes N/A 86.0 to 100
11. heptanes N/A 101 to 119
12. octanes "plus" N/A 120 to 240

Conclusion

Users have a number of benefits with temperature programming on Micro GC Fusion over isothermal operation. Improved sample throughput, a sharper peak shape and the ability to analyze multiple components on a single column are some of the advantages of performing temperature programming on Micro GC Fusion.

This information has been sourced, reviewed and adapted from materials provided by INFICON Inc..

For more information on this source, please visit INFICON Inc.

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