ISOCYANATES: METHOD 5521, Issue 2, dated 15 August 1994 - Page 3 of 6 SAMPLE PREPARATION: 7.
Add 25 µL acetic anhydride to acetylate the excess 1-(2-methoxyphenyl)piperazine remaining in the sample solution, to provide for efficient chromatography. NOTE: The acetylation reaction is: CH3OC6H4NC4H8NH + CH3C(=O)OC(=O)CH3 → CH3OC6H4NC4H8NC(=O)CH3 + CH3C(=O)OH
8. 9. bath
Evaporate the acetylated sample to dryness under a gentle stream of nitrogen while warming to 60 °C on a hotplate. Redissolve the residue in 5.0 mL methanol, while agitating the sample in an ultrasonic water for 15 min.
CALIBRATION AND QUALITY CONTROL: 10.
11.
Prepare at least six working standards containing 0.01 to 4.0 µg/mL of the appropriate urea(s) (TDIU, HDIU, and/or MDIU) and 100 µg/mL of 1-(2-methoxyphenyl)piperazine by adding aliquots of calibration stock solutions to 2 mL methanol in a 10-mL volumetric flask. Add 10 µL acetic anhydride to each standard. Mix and dilute to the mark with methanol. NOTE: The standard solutions need include only ureas derived from the diisocyanates expected in the air samples. Analyze working standards together with samples and blanks (steps 13 through 15). Prepare a calibration graph for the urea in terms of quantity of isocyanate, M (ECHD area vs. mmol of isocyanate group per sample). Molecular weights of typical ureas are: TDIU = 558.7 g/mol; MDIU = 634.8 g/mol; HDIU = 552.7 g/mol; NDIU = 594.7 g/mol.
Where: M is the quantity of isocyanate per sample (µg) C is the concentration of urea in the standard solution (µg/mL) 5 is the liquid volume of a sample (mL) MWu is the molecular weight of the urea MWI is the molecular weight of the isocyanate 12
Prepare control samples by adding 0.1, 1.0 and 10.0 µg of urea to 15 mL sampling medium. Prepare these samples for analysis (steps 7 through 9).
MEASUREMENT: 13. 14.
Set the HPLC system according to manufacturer's recommendations and to the conditions given on page 5521-1. Inject a 10-µL aliquot of the sample solution from step 10. Capacity factors for the urea derivatives are:
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
