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Learn · Part Two — Learning to resolve · Lesson 9 of 10

Retention time means something only if you know the column

The minutes on an analysis are meaningless without one line of information that many analyses leave out.

Many analyses have a column giving retention time, in minutes. It tells you at what moment the compound left the chromatography column.

That figure can be compared only between samples run on the same type of column and the same temperature programme. Without those two pieces of information it is of no use for anything.

The column decides the order

A chromatography column is a thin tube coated on the inside with a sticky material. The nature of that coating decides which compounds are held back longest.

A non-polar column — usually labelled DB-5 or HP-5 — holds compounds by volatility. The order in which they come out is close to the order of boiling points, which is why an analysis on this column can be read like a fragrance’s notes.

A polar column — usually labelled WAX or PEG — holds compounds by polarity. An alcohol can come out after a heavier hydrocarbon, because its hydroxyl group sticks to the stationary phase.

The same sample run on the two types of column gives two different orders.

The temperature programme

The chromatography oven does not stay at a fixed temperature but rises steadily during the run, usually by a few degrees per minute.

So distances along the time axis are not linearly proportional to differences in boiling point. Two compounds thirty degrees apart may be two minutes apart at the start of the programme and forty seconds apart at the end.

The order is right. The spacing cannot be read to scale.

Retention index

To make comparisons between laboratories possible, a retention index is used — converting a compound’s retention time to a standard scale based on a series of n-alkanes run under the same conditions.

The retention index is much more stable and is used alongside the mass spectrum to confirm a compound’s identity. An analysis that gives retention indices is one done seriously.

What to check

When you hold an analysis with a retention-time column, look for three lines: column type, temperature programme, and instrument.

If the column type is missing, the minutes in that analysis have meaning only inside the laboratory, and you cannot compare them with any other analysis.

This is the first thing to look for when someone hands you an analysis, and it is on the list of seven things to check in Part Four.

Further reading for this part

A shared list for the whole of Part Two — Learning to resolve, not a note for every sentence. We state what each source was used for.

  1. Ernest Guenther, 1948–1952. The Essential Oils Six volumes, still the foundation reference for classifying distillation methods. The three-way division we use — water, water and steam, direct steam — comes from here.
  2. International Organization for Standardization. The ISO standards for individual essential oils Each commercial essential oil has an ISO standard setting the ranges for its main components. It is the basis for every statement about a “normal range” in Part Two. The standards must be bought; they cannot be consulted for free.
  3. Robert Tisserand and Rodney Young, 2013. Essential Oil Safety The trade’s standard reference for dilution rates, photosensitisation and contraindications. The figures in this site’s Safety section follow it.

One thing must be said plainly. We have not read every work above in the original; most of the knowledge comes through secondary literature and reviews. We say so rather than let the list suggest a depth of research we have not reached.

Last updated: September 2026