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GC Separations are a Function of: 1. Temperature 2. Selectivity of the Stationary Phase 3. Mobile Phase Flow Rate 4.

GC Separations are a Function of: 1. Temperature 2. Selectivity of the Stationary Phase 3. Mobile Phase Flow Rate 4. Amount of Stationary Phase Present. Choosing a Stationary Phase for a Specific Application 1. Retention Index 2. Rohrschneider Constant 3. McReynolds Constant.

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GC Separations are a Function of: 1. Temperature 2. Selectivity of the Stationary Phase 3. Mobile Phase Flow Rate 4.

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  1. GC Separations are a Function of: 1. Temperature 2. Selectivity of the Stationary Phase 3. Mobile Phase Flow Rate 4. Amount of Stationary Phase Present

  2. Choosing a Stationary Phase for a Specific Application 1. Retention Index 2. Rohrschneider Constant 3. McReynolds Constant

  3. Retention Index (Kovats) Based on the log-linear relationship between number of carbons (n) in an n-alkane and retention time.

  4. Retention Index (Kovats) Based on n-alkanes where: t’N = Net retention time = tr – t0 and the analyte elutes between Cn and Cn+1

  5. Retention Index (Rohrschnieder Constant) Reports ΔI for different test solutes ΔI = Isp – Inon-polar s.p. From a table of ΔI values, one may choose the best stationary phase (s.p.) for a given class of solutes

  6. Retention Index (McReynolds Constant) Reports ΔI for a specific stationary phase (squalane), and 5 different reference compounds: benzene, n-butanol, 2-pentanone, nitropropane, pyridine ΔI = Isp – Isqualane. From a table of stationary phase ΔI values, one may choose the biggest ΔI value for the reference compound most like the solute of interest.

  7. Rules for Retention Index • R.I. increases 100 points for every CH2 group in a molecule • ΔI for 2 isomers can be calculated from boiling points: ΔI ≈ 5 Δbp • R.I. for non-polar compounds is constant for any stationary phase. • R.I. for ANY compound is constant for ALL non-polar stationary phases. • ΔI for a solute between a polar and a non-polar stationary phase is a characteristic of the solute and can be predicted.

  8. DB-5 slightly more polar than DB-1 C thickness > A E thickness > D

  9. GC Limitations • Solutes must be: • Thermally Stable • Relatively Volatile • MW < 400

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