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Published byFrederick O’Neal’ Modified over 9 years ago
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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
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Choosing a Stationary Phase for a Specific Application 1. Retention Index 2. Rohrschneider Constant 3. McReynolds Constant
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Retention Index (Kovats) Based on the log-linear relationship between number of carbons (n) in an n-alkane and retention time.
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Retention Index (Kovats) Based on n-alkanes where: t’ N = Net retention time = t r – t 0 and the analyte elutes between C n and C n+1
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Retention Index (Rohrschnieder Constant) Reports ΔI for different test solutes ΔI = I sp – I non-polar s.p. From a table of ΔI values, one may choose the best stationary phase (s.p.) for a given class of solutes
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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 = I sp – I squalane. 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.
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Rules for Retention Index 1.R.I. increases 100 points for every CH 2 group in a molecule 2.ΔI for 2 isomers can be calculated from boiling points: ΔI ≈ 5 Δbp 3.R.I. for non-polar compounds is constant for any stationary phase. 4.R.I. for ANY compound is constant for ALL non-polar stationary phases. 5.ΔI for a solute between a polar and a non-polar stationary phase is a characteristic of the solute and can be predicted.
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DB-5 slightly more polar than DB-1 C thickness > AE thickness > D
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GC Limitations Solutes must be: 1. Thermally Stable 2. Relatively Volatile 3. MW < 400
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