RUŌOD Lab

Specialist track · Analytical Perfumery

Retention Indices and Identification

A retention index turns a retention time into a comparable number. Calculate one by hand, compare published values and use it to support an identification.

Specialist 25 min study By the RUŌOD Lab team Updated

In short

A retention index expresses when a compound elutes relative to a series of n-alkanes, so the number can be compared across instruments far better than a raw retention time. For a temperature-programmed run, you use the linear formula of van den Dool and Kratz. Combined with a mass spectrum, it can separate isomers that look alike, which makes it one of the most useful tools in essential oil analysis.

What you will learn

  • Calculate a linear retention index from retention times
  • Use published values while allowing for variation
  • Combine retention and spectral evidence

Before this lesson: Reading a Mass Spectrum

The calculation

Run a mixture of n-alkanes under the same method. An alkane with n carbons has an index of 100n by definition. For a peak eluting between alkanes with n and n+1 carbons:

RI = 100 × [ n + (tx − tn) ÷ (tn+1 − tn) ]

Worked example, with invented times. C10 at 10.00 minutes, C11 at 12.40 minutes, and a peak at 12.32 minutes.

  1. Fraction: (12.32 − 10.00) ÷ (12.40 − 10.00) = 2.32 ÷ 2.40 = 0.967.
  2. RI = 100 × (10 + 0.967) = 1097.

That's close to the value published for linalool on a low-polarity phase, so it would support that identification if the spectrum agrees.

Using published values

Published values vary. A single compound can show a range of several index units across laboratories on nominally the same phase. So:

  • Compare only with the same class of phase.
  • Use a tolerance, and state it. Many workers accept a few units, but the appropriate figure depends on your data.
  • Prefer a value run alongside authentic standards on your own system.
  • Use a reference that describes its method, such as Adams (2007) or the NIST WebBook.

Two columns, two numbers

A compound's position differs between phases by a useful amount, which relates to its polarity. Linalool at about 1100 on a low-polarity phase is reported at around 1550 on wax, a shift of some 450 units. A hydrocarbon of similar volatility shifts much less. So a peak's two indices together provide strong evidence, and they also expose some coelutions. Limonene and 1,8-cineole elute very close together on common low-polarity columns but separate well on wax.

Approximate index values, for practice. Confirm each one in a primary reference before using it.
CompoundLow polarity (approx.)Wax (approx.)
Limonene10301200
1,8-Cineole10301210
Linalool11001550

Notice that limonene and 1,8-cineole are close together in both of these rough figures, yet published work on polar columns generally reports them as resolved. Always check for a specific method, because the separation depends on the column and conditions. Rounded values can't show this.

Confidence levels

A simple scale for reporting identifications. It is our own summary of common practice.
EvidenceConfidence
Spectrum, retention index and authentic standard on your own systemHigh
Spectrum, plus retention index on two phases, against the literatureGood
Spectrum, plus one retention index against the literatureModerate
Library spectrum onlyTentative

Practical exercise: Five retention index problems

About 45 minutes · You will have: five calculated indices with published comparisons and a confidence note

This exercise needs no instrument. All retention times are invented.

  1. C9 8.00, C10 10.00, peak 9.00. Calculate.
  2. C10 10.00, C11 12.00, peak 11.20. Calculate.
  3. C11 12.50, C12 15.00, peak 13.00. Calculate.
  4. C13 18.00, C14 20.50, peak 19.25. Calculate.
  5. C15 24.00, C16 26.00, peak 25.50. Calculate.
  6. For a real compound from your own materials, look up its low-polarity index in the NIST WebBook. Note the range. State the tolerance you would use and why.

Answers: 950, 1060, 1120, 1350, 1575.

You have done this well if you got the answers by hand, and you can say why a published single number overstates how exact the value is.

Common mistakes

  • Comparing indices across different phases. Check the phase first.
  • Treating one matching index as proof. It's supporting evidence.
  • Using isothermal formulas for programmed runs. The linear form suits temperature programs.

Key terms

Retention index
A number that places a compound relative to n-alkane standards run under the same conditions, so results can be compared across laboratories.
Linear retention index
A retention index calculated with a straight-line formula, used for temperature-programmed runs, following van den Dool and Kratz.

Knowledge check

1. C10 elutes at 8.00 min and C11 at 10.00 min. A peak elutes at 9.50 min. What is its linear retention index?
Show the answer

1075. 100 × (10 + (9.50 − 8.00) ÷ (10.00 − 8.00)) = 100 × 10.75 = 1075.

2. Why not simply compare retention times across two labs?
Show the answer

Column length, flow and program differ, and retention times shift. An index removes much of that difference by reference to standards.

3. A peak's index on a low-polarity column is near 1100, and its spectrum matches linalool. What does a second run on a polar column add?
Show the answer

Independent evidence from a different separation. Agreement on two different phases gives stronger support.

Sources

  • van den Dool H, Kratz PD. A generalization of the retention index system including linear temperature programmed gas-liquid partition chromatography. J Chromatogr 11:463–471, 1963
  • NIST Chemistry WebBook: Gas Chromatographic Retention Data
  • Adams RP. Identification of Essential Oil Components by Gas Chromatography/Mass Spectrometry, 4th edition. Allured, 2007

Further reading and materials

Written and reviewed by the RUŌOD Lab team. This is general education, not legal, regulatory, medical or safety advice. Example dilutions and exercises are for learning, are not validated products, and have not been safety-assessed. Completing a lesson confers no qualification. How we check these lessons.