Determination of sorbic acid in urine by gas chromatography-mass spectrometry

The average daily uptake of the common food preservative sorbic acid is est imated to range from 0.01 to 1.1 mg kg(-1) Sorbic acid mainly is metabolise d to carbon dioxide. Minor amounts are converted to trans,trans-muconic aci d (ttMA) as well as excreted unchanged into the urine. Since urinary ttMA i s a biomarker for the occupational and environmental exposure to benzene, t here is an additional need for monitoring the uptake of sorbic acid, partic ularly at low, environmental benzene exposure levels. For this purpose, a s imple, robust and rapid method for the determination of sorbic acid in urin e at trace levels was developed. After addition of 10 ml of water and 5 ml of 8 M hydrochloric acid to 10 ml of the thawed urine, the sample was water steam distilled using an automated distillation device. A total of 100 ml of the distillate were solid-phase extracted. After washing, the sorbic aci d was eluted with 4 ml methanol. The eluate was reduced under a stream of n itrogen to a volume of 300 mu l. After addition of 500 mu l boron trifluori de in methanol and incubation for 1 h at 60 degrees C, the resulting sorbic acid methyl ester was extracted three times with 1 ml heptane. To the comb ined heptane layers, sorbic acid ethyl ester was added as an internal stand ard. After reducing to a volume of 100 yl in a stream of nitrogen, the fina l analysis was performed by GC-MS using the fragment ions m/z 126 for the a nalyte and m/z 140 for the internal standard. The limit of detection was 0. 7 ng ml(-1) urine and the R.S.D. of 69 duplicate determinations was 7.5%. I n a controlled, experimental study and in a held study, we were able to sho w that urinary sorbic acid is a marker for the dietary uptake of sorbic aci d and that sorbic acid is converted to ttMA. On average, 0.1% of the dietar y sorbic acid is excreted unchanged into the urine. Excretion is complete w ithin 24 h. We found that, on average, 0.23% of the oral dose of sorbic aci d is excreted as urinary ttMA. There was a significant correlation between urinary excretions of sorbic acid and ttMA (r=0.74, n=69). We conclude that urinary sorbic acid can be used to correct the urinary ttMA level in order to determine the portion related to benzene exposure. This appears to be n ecessary particularly at low, environmental benzene levels. (C) 1999 Elsevi er Science BN. All rights reserved.