The Retention Index System in Gas Chromatography: McReynolds Constants

Importance of the Topic

The retention index system based on McReynolds constants provides a standardized way to characterize and compare gas chromatography stationary phases by polarity. This approach facilitates the selection of appropriate columns, prediction of elution orders for related analytes, and efficient method development across diverse chemical families and column types.

Objectives and Study Overview

This bulletin aims to explain the McReynolds retention index system, demonstrate how to measure and calculate these values using five probe compounds, and supply a comprehensive table of McReynolds constants for over 200 commercially available phases. It enables chromatographers to classify phases, predict retention behavior, and interchange equivalent phases without conducting extensive experiments.

Methodology and Instrumentation

Methodology:

• Probe Compounds: Benzene (x'), n-butanol (y'), 2-pentanone (z'), nitropropane (u'), and pyridine (s').
• Reference Phase: Nonpolar squalane assigned zero polarity; normal n-alkanes indexed at 100× carbon number.
• Procedure: Inject a mixture of reference n-paraffins and the probe; record retention times; correct for dead time; plot log(corrected retention time) vs. known indices for n-hexane, n-heptane, n-octane; interpolate to derive the retention index for each probe.
• Calculation: I = 100·Z + 100·[log t'R(i) – log t'R(Z)] / [log t'R(Z+1) – log t'R(Z)], where Z is the carbon number of the preceding n-alkane.

Instrumentation:

• Gas chromatograph equipped with temperature control capable of isothermal and programmed operation.
• Capillary columns coated with squalane or alternative stationary phases at specified film thickness.
• Detection system typically a flame ionization detector (FID) or equivalent.

Main Results and Discussion

Classification of Phase Polarity:

• Nonpolar Phases: McReynolds I values 0–100.
• Intermediate Polarity: Values 100–400.
• Highly Polar Phases: Values above 400.

Key Findings:

• Phases with McReynolds constants within ±4 units can be substituted interchangeably.
• Elution Predictions: Knowledge of probe elution allows estimation of relative retention shifts for analytes in the same chemical family (e.g., aromatics elute as if shifted by a fractional carbon number).
• Instrumental Tables: Over 200 phases listed, including methylsilicone, phenylsilicone, cyanopropylsilicone, polyethylene glycol, and numerous polar polymers, with their operating temperature ranges and five McReynolds constants.
• Comparative Examples: Dinonylphthalate vs. squalane (Ibenzene = 84) and SP-2340 vs. squalane (Ibenzene = 520) illustrate increasing polarity and retention for aromatic probes.

Benefits and Practical Applications

The McReynolds system offers:

• Rapid Phase Selection: Simplifies cross-vendor comparison and replacement of stationary phases.
• Method Development Aid: Predicts elution order and resolution for target analytes across polarity ranges.
• Quality Control: Facilitates routine checks on column performance and batch-to-batch consistency.
• Research Utility: Supports the design of separation strategies in industrial and academic laboratories, including QA/QC and environmental analysis.

Future Trends and Applications

Emerging directions include:

• Digital Databases: Integration of McReynolds values into chromatographic software for automated column recommendations.
• Predictive Modeling: Use of machine learning to extend retention index predictions to novel phases and compounds.
• Advanced Techniques: Application of McReynolds constants in two-dimensional GC and comprehensive GC×GC separations.
• New Probes and Phases: Development of additional probe compounds to characterize modern polar and specialty phases under varied temperature programs.

Conclusion

The McReynolds retention index system remains a fundamental tool in gas chromatography for quantifying phase polarity, guiding column selection, and predicting analyte behavior. The extensive compilation of constants provided eliminates the need for routine experimental determination and supports efficient method development across applications.

References

1. Budahegyi MV, et al. Journal of Chromatography. 1983;271:213–307.
2. Ettre L. Analytical Chemistry. 1964;36(8):31A.
3. Káplár L, et al. Journal of Chromatography. 1972;65:115.
4. McReynolds WO. Journal of Chromatographic Science. 1970;8:685.
5. Rohrschneider L. Journal of Chromatography. 1966;22:6.
6. Rohrschneider L. Journal of Chromatography. 1969;39:383.
7. Supina WR, Rose LP. Journal of Chromatographic Science. 1970;8:214.
8. Takács J, et al. Journal of Chromatography. 1972;65:121.