It's common to express results in μL/mL when dealing with liquid extracts, as this reflects the volume of the extract used per volume of the solvent. However, when you're interested in the concentration of active compounds, expressing the results in terms of weight (e.g., μg/mL) can be more informative.

When dealing with liquid extracts, if the concentration of the active compounds is unknown, researchers typically rely on volume-based dilutions (e.g., adding more or less of the liquid extract to the solvent).

μL/mL (microliters per milliliter) is a volume-to-volume ratio. It tells you how many microliters of a liquid (e.g., a liquid extract) are present in each milliliter of the total solution.

If the extract is used directly and the active compound concentrations are unknown, reporting in μL/mL is acceptable, but it should be clearly stated that the concentration refers to the volume of the extract used, not the concentration of any specific compound.

If possible, analyzing the extract to determine the concentration of key bioactive compounds would allow you to express the results in μg/mL, which is more informative for comparing bioactivity across different studies or extracts.

For your study on herbal teas, if you're working with liquid extracts and don't know the exact concentration of active compounds, you can start by using volume-based dilutions (e.g., 1 mL extract in 10 mL of solvent) and report your results based on the volume of extract used. Alternatively, you can concentrate the extract by evaporating the solvent, then dissolve the residue in a known volume of solvent to create a stock solution.

In the context of bioactivity studies using aqueous plant extracts in liquid form, it is imperative to consider the appropriate expression of concentrations and the preparation of test solutions. It is common practice to process plant extracts without evaporating the solvent, thereby maintaining the extract in a liquid state. In such cases, it is appropriate to express concentrations in microlitres per millilitre. This unit represents the volume of extract added to a given volume of medium, which is common practice when testing liquid extracts directly without isolation of individual bioactive components.

A major challenge is the accurate preparation of these extracts for experimental use, particularly when concentrations need to be expressed in units such as µg/µL or µg/mL. Regardless of the liquid state of the extract, dilution to specified concentrations can be achieved without converting the extract to a powdered form. This is known as serial dilution, where the initial extract is treated as a concentrated solution and diluted progressively with water or an alternative solvent. Such dilutions are straightforward when working with volume-to-volume (v/v) ratios, commonly expressed as microlitres of extract per millilitre of solvent.

It is of utmost importance to quantify the dry weight of bioactive compounds in the liquid extract in order to express the concentration in µg/µL or µg/mL. This can be achieved by chemical analysis methods such as gravimetric analysis, spectrophotometry or chromatographic techniques that determine the concentration of specific compounds in the original extract. In the absence of precise knowledge of the mass of bioactive compounds, the concentration can be estimated on the basis of the total soluble solids or active constituents, which can be measured directly in the liquid extract.

For example, if chemical analysis indicates that 1 mL of the liquid extract contains 1000 µg of bioactive compounds, subsequent dilutions can be made accordingly. To prepare a solution with a final concentration of 100 µg/mL, 0.1 mL of extract must be mixed with 0.9 mL of solvent. This approach ensures that accurate and reproducible concentrations can be achieved for bioactivity assays, even without drying the extract.

In summary, when working with liquid extracts, it is both appropriate and widely accepted to express results in µL/mL. However, conversion to µg/ml or similar units requires prior quantification of the active components in the extract. To ensure accurate and consistent experimental conditions, especially when dealing with complex herbal infusions or tea extracts in bioactivity studies, it is essential to employ proper chemical analysis and to follow careful dilution protocols.

Thank you very much, Ankit Negi and Phil Käding for your detailed and informative explanation.

In this case, I want to ask a question like this:

1) Let's say I will extract 2 grams of plant sample in 200 ml of hot water. Can I accept the concentration of the liquid extract I obtain as 10 mg/mL from 2000 µg/200 mL?

2) Is it correct to calculate how much extractive is in the extraction liquid with the following method?

For example, weigh 1 gram of a plant leaf dried at room temperature and extract it in 100 ml of ethanol. Let the moisture value of 1 gram of plant be 8.50%. In this case, the full dry weight of our plant is 0.915 g. At the end of extraction, the complete dry weight of the plant leaf is 0.892 g. In this case, when we subtract the full dry leaf weight (0,892 g) at the end of extraction from the initial full dry weight (0.915 g), the result we find (0,023 g) will give us the amount of extract in 100 ml. Is this method correct to find out how much extract is in 100 ml?

At final steps of my extraction procedure, i rota vapored the solvent and lyophilized the concentrated extract solution to get the powder form and resuspended the powder in water with known weight and volume to define my concentration.

No ,activity should be expressed activityper weight of original plant (dry or wet0

Yes, bioactivity results of aqueous extracts from plants can be expressed in μL/mL when used directly without removing the solvent. This measurement allows for standardized comparisons of extract concentration in relation to its bioactivity.

When expressing findings, it's usually preferable to use a unit that represents the impact of the extract on a certain concentration of the bioactive ingredient, such as mg/mL, or to base it on the concentration of active chemicals. Instead of the extract's actual volume, the activity of the extract is usually correlated with the concentration of its active ingredients. A direct correlation between the amount of extract and its concentration might be implied by reporting data in μL/mL.