GC Analysis – Part VII. Hydrosol Analysis
Alexis St-Gelais, M. Sc., chimiste – Popularization
We have previously seen a worked example of essential oil analysis. Hydrosols can be analysed in about just the same way as essential oils. Yet, there is one huge difference between a pure essential oil and an hydrosol: the latter is always a solution. This implies some specific care in its analysis.
For a given volume of hydrosol, the amount of organic volatile constituents is very small compared to a pure essential oil. Hydrosol analysis thus often implies a preconcentration step. The aqueous solution is thus brought into contact with a non-miscible organic solvent. Most volatile constituents of the hydrosol have more affinity for the organic liquid, and leave the water during this liquid-liquid extraction. The organic solvent is evaporated gently, leaving a concentrated “extract” of the hydrosol, devoid of water.
Since essential oils are (in optimal conditions, of course) undiluted, we consider that the sum of all peaks seen in GC-FID corresponds to all of the sample. Even if the oil is diluted, this is done with an organic solvent, which is also detected by FID. The signal=sample approximation therefore can still stand to some extent – at the very least, you can get an idea of how badly the oil was diluted.
Yet, FIDs react to organic compounds only. Water, lacking any carbon, is not of those, and produces little to no analytical response. This implies that there is no way to directly know the concentration of organic compounds in solution in the hydrosol. Using only relative peak areas, you would get exactly the same analytical result for a given hydrosol and the same hydrosol diluted in 5 times its volume of water – you would just have a little more trouble analysing the latter due to weak peak intensities. Monitoring the concentration of an hydrosol is thus very important. This is true not only because of dilution, but also since the production process may have a huge impact on its concentration. While the organic compounds origin from the plant material distilled, the volume of water depends on the total distillation time, the rate at which vapor passes through the system and the efficiency of the cooling coils. Batch to batch consistency is thus at times harder to achieve, and should be closely monitored if the hydrosol is to be sold.
In order to get an idea of the concentration of organic compounds in the hydrosol, Laboratoire PhytoChemia uses an internal standard method. Once the hydrosol sample is extracted with organic solvent, we add to it a known amount of tetradecane. This compound is not soluble in water and is usually not found in noticeable amounts in original hydrosols. When injecting on GC-FID, we can tie the peak area of tetradecane to a known mass of compound in the organic solvent extract. With that at hand, we can convert all other peak areas to a mass using a rule of three. We can then back-calculated the concentration of a given compound in the hydrosol if the amount extracted by the organic solvent is known. This allows us to provide reports with compounds quantified in mg (equivalent tetradecane*) per L of hydrosol.
Going a step further, one can bypass the intrinsic influence of production process on the hydrosol concentration to look for the yield of water-soluble volatile compounds obtained from the plant material. In this case, the total volume of hydrosol collected must be known, as well as the mass of plant distilled. The yield can then be obtained as mg compound / kg of plant, or as a mass/mass percentage. With such information, one could theoretically tune its distillation apparatus to obtain a relatively fixed concentration of hydrosol. We also use this calculus when performing lab-scale hydrodiffusions, where we are often using a large water-to-plant ratio given the size of our apparatus. This implies that we obtain relatively diluted hydrosols with values that are of little significance for large-scale applications if analysed as is.
In general, hydrosols are rich in oxygenated compounds. Small volatile organic acids are often found in it, which implies that hydrosols usually have an acid pH, which can also be monitored for batch to batch consistency. It should also be known that hydrosol analysis is younger than essential oils studies, and that there are higher odds of encountering unknown constituents that have not been included in scientific databases or publications. Any strongly water-soluble constituent which is found only in minute quantities in essential oils but much more abundant in the aqueous media could produce such a result. The percentage of total volatile constituents identified will thus likely be lower than for an essential oil, should you ask for a report, and this should not be seen as negative. It only shows that science still has more discoveries to make.
All in all, commercial hydrosol analysis is highly relevant, but require some special care. Laboratoire PhytoChemia can perform these on a routine basis and with great pleasure. Going a step further, one should also be aware that hydrosols, being aqueous, are prone to bacterial development. It can be a good idea to double-check the sterility of the unopened product once in a while. Should you need such testing, we can also make proper arrangements for you.
*As discussed earlier, there are some differences between volatile compounds in the way they produce an analytical response, based on their structure. If we quantified each compound against a calibration curve prepared precisely for itself, we would calculate an absolute quantity, but that would be extremely long and costly. Since we use tetradecane to normalize the peak-area-to-mass relationship, all other compounds will thus be quantified as tetradecane equivalent units. The analysis is thus an approximation, but one that is perfectly fit for comparisons between suppliers and batches.