What are the reasons for failing residual solvent analysis?

Failures for residual solvents are typically granted retest permission except in extreme cases of solvent misuse. The current state limits for solvents are as follows:

Solvent*
ppm
Acetone
5,000
Benzene
2
Butanes
5,000
Cyclohexane
3,880
Chloroform
2
Dichloromethane
600
Ethyl acetate
5,000
Heptanes
5,000
Hexanes
290
Isopropanol
(2-propanol)
5,000
Methanol
3,000
Pentanes
5,000
Propane
5,000
Toluene
890
Xylene**
2,170
*
And isomers thereof.
**
Usually 60% m-xylene, 14% p-xylene, 9% o-xylene with 17% ethyl benzene.

Possible reasons for failing residual with well-maintained equipment:
The 3-dimensional shape and viscosity of a product affects how well and how quickly it can purge. A product with consistency typically termed “honeycomb wax” (which has very high surface area per gram) will purge relatively quickly. In contrast, products with consistency like “shatter” or “sugar wax” (which have much less surface area per gram) will require a little more aggressive purge program: a little more heat, deeper vacuum, or more time.

Variance between batches can affect outcomes as well. Even two runs from the same harvest on different days can produce different results as far as the precise chemical make-up of the extract. This can be caused by differences in starting material or differences in other variables such as temperature, pressure, solvent makeup, steep time, etc.

We do not test for every compound in the extract, and picking up more or different impurities can affect product viscosity – which in turn affects behavior during purge. Moisture level and terpene content can also affect impurity makeup and purge time, as both terpenes and water can act as co-solvents. Even the physical structure of the buds, and how they are physically packed into the extraction chamber, can affect the outcome of the run. No two extraction runs are identical, but we can help you mitigate the variability in your extraction process – resulting in a more consistent product.

Even the most refined and consistent extraction and purge protocol cannot account for minor differences between batches of naturally variable product. A skilled oil maker is aware of these nuances, and adjusts protocol slightly to meet the needs of the starting material. Some protocols are inherently more robust than others, but the key message here is that there are more variables in extraction than first meet the eye. Many of these nuances are trade secrets, and so it is up to you to discover what extraction technique works best for you. As always, your lab is here to help.