We have analysed fish and rapeseed oils for secondary oxidation products (AOCS, 1998) using the direct method protocol from Current Protocols in Food Analytical Chemistry (2001). We weighed off 25 and 100 mg of fish oil and rapeseed oil, respectively, and added 25 mL of butanol in a volumetric flask. The rest was performed according to the protocol; the same goes for the standard curve. However, now we are struggling to calculate the concentration of TBA in mg/kg oil. The formula described in the protocol gives us extremely high concentrations: K = (1/slope) × 72.03 g/mol × (25 ml/5 ml) × 10e6 ]/m = (3.60 × 10e8 )/(slope × m)
Can someone help out?! Thanks in advance
it is 10^6 not 106 and your m value is incorrect as the formula is different between the two parts.
Dear Kylian Manon Eggink Let me first express that to me the protocol looks hopelessly complicated and confusing, and I definitely would use another method as well.
Anyway, I noticed in your calculation sheet, concentrations expressed in mM while I read in the protocol (when it comes to the standard curve): “A typical standard series should consist of five samples, ranging from 1 × 10−6 to 1 × 10−5 M malonaldehyde.”
I notice in the (last) calculation step always a high number like 2.88x1010 in 2.88 × 1010/(slope × % recovery). Meaning that the slope should be high or the 1/slope very small in order to ‘normalize’ the TBA value. Could it be the concentration of MA that should be in the range of 10-5 or 10-6 instead of mM?
Kind regards.
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