# TOC limit calculation

Can anyone tell there is a problem in TOC limit calculation as below?

background:
19 L spinner flask for cell culture, working volume 15 L, flask area 3516 cm2; Cleaning acceptance criteria 10 ppm, detection method TOC, LOD: 0.34ppb, LOQ: 1.16ppb; swab method: 25 cm2, extract in 40ml water

Calculation for swab limit:
Allowable residue (TOC) for next batch: 10ppm x 15L = 150 mg
Average residue on the surface: 150mg/3516cm2 = 0.043mg/cm2
One swab contents in 25 cm2: 0.043 mg/cm2 x 25 cm2 = 1.07mg
Conc. of TOC in 40 mL water: 1.07mg/40mL = 0.026 mg/mL = 26ppm

I tried fermentors with different size, the limit will increase while the fermentor is larger!

Is it right I’m thinking in this way?

I think you are forgetting to account for the percentage of carbon in your residue (for purified proteins this is typically about 53% - not sure about cell culture broth). This should be calculated to reduce your final limit.
I’m assuming that the working volume of 15L is always used? If the process allows for smaller volumes to be used in the flask then the smallest possible volume should be used instead, as this presents the true worst-case risk and will yield a lower amount of allowable residue into the next batch.

Also, the reason a larger vessel could have a higher limit is because the ratio of working volume to surface area is probably greater. For example, trying to make a very small batch in a very large tank will yield low results (and vice versa).

[quote=Hanks]Can anyone tell there is a problem in TOC limit calculation as below?

background:
19 L spinner flask for cell culture, working volume 15 L, flask area 3516 cm2; Cleaning acceptance criteria 10 ppm, detection method TOC, LOD: 0.34ppb, LOQ: 1.16ppb; swab method: 25 cm2, extract in 40ml water

Calculation for swab limit:
Allowable residue (TOC) for next batch: 10ppm x 15L = 150 mg
Average residue on the surface: 150mg/3516cm2 = 0.043mg/cm2
One swab contents in 25 cm2: 0.043 mg/cm2 x 25 cm2 = 1.07mg
Conc. of TOC in 40 mL water: 1.07mg/40mL = 0.026 mg/mL = 26ppm

I tried fermentors with different size, the limit will increase while the fermentor is larger!

Is it right I’m thinking in this way?[/quote]

Hello Hanks,
Your calculations are perfect. I had replied to another question of yours with calculations. The limit depends on surface area of the equipment as well as the working volume (or batch size). In your example, you have considered 15 L as the working volume for 19 L flask (79%). When you go for higher size fermentors, working volume might be increased, which might have caused the increase in your MACO value. If you considered 79% as the working volume for any size of the flask / fermentor, the results will not change. Try yourself!

VEERRAJU
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