Hydrosol Analysis

“First, we need to have analyses, and then we need to learn how to read them.

Most aromatherapists have a general understanding of how to read a GC/MS report of an essential oil. However, you must understand that reading the GC/MS report of a hydrosol is not the same as reading that of an essential oil. GC/MS is an abbreviation for Gas Chromatography–Mass Spectrometry. Currently the GC/MS is our least expensive analysis to measure the percentage of volatile components in a hydrosol.

A GC/MS report of a hydrosol differs from the GC/MS report of an essential oil because it identifies a percentage of the volatiles, which are a fraction of a percent of the hydrosol. Hydrosols contain only a small percentage of volatile components; less than 1% of a hydrosol solution is volatile. The chemist must first extract the volatile components from the aqueous, non-volatile sample. He then runs that sample for analysis. Therefore, it takes two extractions for one hydrosol analysis. This is one of the reasons that a hydrosol is more expensive to analyze than an essential oil and less accurate.” 

Reprinted with Permission from Harvest to Hydrosol, 2015

It is the GC/MS analysis that gives us information about what volatile components are in our hydrosol.  For more information on this subject please read Phytochemia’s excellent blog series GC Analyses, especially Part VII, Hydrosol Analysis.

Analysis Report

The following image is a partial GC/MS report from Pyrenessences Lab in France. You will notice six (6) columns in this report. The first column “Peaks” refers to the peaks on the GC (Gas Chromatography) part of the report which is not shown. The second column “RT”  is the  Retention Time, the third column “Compound Name” refers to the individual volatile components. Now we have the fourth column “%,” which is the percentage of the component (in relation to the other volatile components). The fifth column is the one we are most interested in; it is the quantifying number (mg/L) which tells us the actual amount of the component in the given hydrosol.

Please note that this is only a partial report, the full report is several pages long. This image is used as a learning tool.

Why do we prefer mg/l over percentages in a hydrosol analyses?

A hydrosol is an aqueous distillate and contains various concentrations of volatile components from a low of 50 mg/l (0.005 %) to over 2000 mg/l (0.2%).  Different plants and different distillation techniques will yield hydrosols that may vary in total volatile concentration.

For example, a Calendula hydrosol (Calendula officinalis) may only contain 36 mg/l (0.004%) total volatiles, whereas a Bay Laurel hydrosol (Laurus nobilis) will be much more concentrated with as many as 1300 mg/l (0.13%) of total volatiles. You can see from the chart below a hydrosol from the same plant can have very different concentrations of volatiles. I will talk more about why this happens in a future post.

Botanical Name

 Total Volatile Concentration (TVC)

 in mg/l

Number of Samples
Bay Laurel (Laurus nobilis)1143-1372 mg/l2
Tulsi (Ocimum sp)160-860 mg/l8
Frankincense (Boswellia sp)105-718 mg/l9
Lemon Thyme (Thymus x citriodorus)163-588 mg/l10
Thyme (Thymus vulgaris)574-584 mg/l2
Lavender (Lavandula angustifolia)313-639 mg/l3
Rose (Rosa sp)57-481 mg/l11

Hydrolates-What have we learned about them?, AromaKongress, Munich 2019

Percentages are misleading without a knowing the total volatile organic component (VOC).

If you read a hydrosol report that gives you only percentages without VOC, you will only know what the components are in relationship to each other. You will not know how much of each component is in the hydrosol, this is measured in mg/l.

Let’s take a look at an example:

Here I have two (2) German Chamomile hydrosol (Matricaria chamomilla) analyses, both showing relatively the same percentage of oxides.

Bisabolol oxide BBisabolol oxide A
(Sample 1) 6.01%(Sample 1) 72.35%
(Sample 2) 5.88%(Sample 2) 77.77%

Passion and Research, AquaAromatica, Germany, 2017

If you saw these on a report – without knowing the total volatile concentration (TVC) – you could believe that the two hydrosols contained relatively the same amount of oxides. This would be a wrong assumption in this case, since Sample 1 has a TVC of 6 mg/l and Sample 2 has a TVC of 110 mg/l.

Let’s take a closer look:
Bisabolol oxide BBisabolol oxide A
(Sample 1) 6.01% * 6 = 0.4 mg/l(Sample 1) 72.35% * 6 = 4.3 mg/l
(Sample 2) 5.88% * 110 = 6.5 mg/l(Sample 2) 77.77% * 110 = 85.5 mg/l

Passion and Research, AquaAromatica, Germany, 2017

Here is another example of two (2) Rosemary hydrosols (Salivia rosmarinus Syn. Rosmarinus officinalis) where the percentages are misleading.

(Sample 1) 6.79%
(Sample 2) 15.36%

If you were to assume that Sample 2 has more than twice as much Borneol as Sample 1 you would be incorrect. Sample 1 has 1220 mg/l of VOC and Sample 2 is much less concentrated with only 301 mg/l of VOC. So Sample 1 actually has nearly twice as much Borneol as Sample 2.

(Sample 1) 6.79% * 1220 = 83 mg/l

In Conclusion

It is far more accurate to use mg/l when describing the quantity of components in a hydrosol rather than percentages, which can be misleading.

If a report only lists percentages and does not additionally list either total volatile organic concentration or mg/l you will only know what components are in the hydrosol, you will not know the amount.

  • mg/l tell you specifically how much of any given component is in a hydrosol
  • percentages tell you the relationship of volatile components to each other; they are in fact are a percentage of a percentage.

References and Further Reading

“Circle H Institute.” 2020. GCMS Analyses. www.circlehinstitute.com.

Harman, Ann. 2015. Harvest to Hydrosol. Botannicals.

St-Gelais, Alexis. 2015. “GC Analysis – Part VII. Hydrosol Analysis,” March. https://www.phytochemia.com/en/2015/03/15/gc-analysis-part-vii-hydrosol-analysis/.

———. 2019. “Phytochemia.” An Algorithm to Solve Them All (Almost) (blog). June 7, 2019. https://www.phytochemia.com/en/2019/06/07/an-algorithm-to-solve-them-all-almost/.