Restek manufactures several different residual solvent standards. These can be used for testing according to USP <467>, the EP monograph, or other residual solvent assays, such as medical cannabis testing. The calculations involved for the use of these standards has led to questions for our technical team. This post should help provide guidance for their usage. I’ll use USP <467> in my example calculations, but they can be applied to other methodology.
Since USP <467> is a limit test, each residual solvent has a concentration limit based upon its potential risk to human health. For that reason, not all compounds are at the same concentration in our reference standards. As an example, Benzene has a limit of 2 ppm for the Class 1 Residual Solvents. Catalog #36279 contains benzene at 10 mg/mL. This solution would be what USP refers to as Residual Solvent Mixture RS.
According to the monograph, you are to do the following:
Class 1 Standard Stock Solution— [Note—When transferring solutions, place the tip of the pipet just below the surface of the liquid, and mix.] Transfer 1.0 mL of USP Class 1 Residual Solvents Mixture RS to a 100-mL volumetric flask, previously filled with about 9 mL of dimethyl sulfoxide, dilute with water to volume, and mix.
Using Catalog #36279
10,000 µg/mL x 1.0 mL / 100 mL = 100 µg/mL
Transfer 1.0 mL of this solution to a 100-mL volumetric flask, previously filled with about 50 mL of water, dilute with water to volume, and mix.
100 µg/mL x 1.0 mL / 100 mL = 1.0 µg/mL
Transfer 10 mL of this solution to a 100-mL volumetric flask, previously filled with about 50 mL of water, dilute with water to volume, and mix.
1.0 µg/mL x 10 mL / 100 mL = 0.1 µg/mL Class 1 Standard Stock Solution
Class 1 Standard Solution— Transfer 1.0 mL of Class 1 Standard Stock Solution to an appropriate headspace vial containing 5.0 mL of water (place the tip of the pipet just below the surface of the liquid for dispensing), apply the stopper, cap, and mix.
0.1 µg/mL x 1.0 mL / 6.0 mL = 0.0167 µg/mL
Note: Final volume = 6 mL (1 mL of Class 1 Stock Solution + 5 mL Water)
This solution is the one analyzed by headspace GC/FID. It is less important to consider the 0.0167 µg/mL concentration of benzene than it is to know that there is 0.1 µg of benzene in the vial. This 0.1 µg of benzene is the amount partitioned into the gaseous headspace and injected into the GC
1 mL of 0.1 µg/mL = 0.1 µg (benzene)
With your drug substance or excipient, you are to do the following according to USP:
Test Stock Solution— Transfer about 250 mg of the article under test, accurately weighed, to a 25-mL volumetric flask, dissolve in and dilute with water to volume, and mix.
250 mg / 25 mL = 10 mg/mL
Test Solution— Transfer 5.0 mL of Test Stock Solution to an appropriate headspace vial, add 1.0 mL of water, apply the stopper, cap, and mix.
Since this is the solution analyzed by headspace GC/FID, it is more important to know how much article is present and not the concentration.
10 mg/mL x 5 mL = 50 mg of article tested
The limit for benzene is 2 ppm (0.002 µg/mg), which happens to be the concentration you would find by having 0.1 µg of benzene in your 50 mg of article.
0.1 µg (benzene) / 50 mg (article) = 0.002 µg/mg (2 ppm)
You can do the same math for any of the other compounds in the Class 1 Mix.
The dilution scheme for Class 2 Solvents is similar to the Class 1 Solvents. Since Class 2 Solvents are considered not as toxic as the Class 1, the limits are much higher. Since the allowable limit is so much greater, Restek has accounted for the first dilution with the Class 2 Solvents. This is in Catalog #36012. For the Class 2 Solvents, I will use acetonitrile as an example.
Following USP and the calculations outlined above, the procedure would be:
2050 µg/mL x 1.0 mL / 100 mL = 20.5 µg/mL
20.5 µg/mL x 1.0 mL/ 6.0 mL = 3.417 µg/mL or 20.5 µg of acetonitrile
20.5 µg (acetonitrile) / 50 mg (article) = 0.41 µg/mg (410 ppm) = limit for acetonitrile
I hope you find this helpful.