Page:NIOSH Manual of Analytical Methods - 5525.pdf/6

 ISOCYANAT ES, TOTA L (MAP): METHO D 5525, Issue 1, dated 15 March 2003 - Page 6 of 17 NOTE 2:

For isocyanate products that essentially contain only monom er, the gradient can be truncated after the monomer elutes. After truncation, a two minute hold using pH 1.6 mobile phase before return ing to initial co nditions will ens ure b ase line stability for the ne xt sa m ple run. NOTE 3: If there is an interfering peak in the area of interest that is not influenced by pH, changing the length of hold at the beginning of the run should move the interference peak away from the area of interest. NOTE 4: For multi-component monom ers such as TDI and HMDI and for multi-component isocyanate products that contain monom er and oligom er, ch anging the hold at the beginning, changing the gradient ramp, placing a hold in the middle of the ramp, or a combination of all of the abo ve m ay help in sep aration of the individual co m pon ents for a s pec ific prod uct. 15. Injec t a 30-µL sam ple aliqu ot. 16. Mo nom er m eas urem ent: Isoc yanate spe cies for which pure ana lytical stand ards are a vailable, suc h as diisocyanate monom ers, can be measured by comparing the response of the peak at the correct retention time with the calibration curve generated by analyzing standards. Either UV peak height or area or fluorescence peak height or area can be us ed for quantification of m ono m er. T he fluo rescence detec tor is m ore sensitive, s o it is usually better for measuring monom ers at low levels. Peak height is usually preferable to peak area, especially in the presence of closely-eluting interfering peaks. Confirmation of the identity of the monom er is achieved by comparing the FL/UV response ratio of the sample peak to that of the standard giving sim ilar resp ons e. NOTE: The FL/UV ratio may change somewhat at low levels. The sample peak should give a FL/UV response ratio within 15% of the ratio for the standard peak of comparable size. 17. Oligom er m easurem ent (total isocyanate): Isoc yanate species for which pure analytical standards are not available, such as oligomeric isocyanates, must be measured by using the UV are a of the sam ple peak(s) and the slope of the linear portion of the calibration curve generated by analyzing monom er standards. Frequently, numerous isocyanate species elute as an envelope of poorly res olved peaks. In this case, rather than attempting to integrate peaks individually, the entire chromatogram is integrated over the area o f interest. Because the fluorescence baseline is not disturbed by the gradient and the fluorescence detector is selective for MAP-derivatized compounds, the sample fluorescence chromatogram can be very useful for determ ining when to begin and end integration of peaks in the UV chromatogram. The fluorescence chromatogram is used qualitatively to confirm the presence of MAP gro ups in the eluting species. The fluorescence response varies too much from com pound to compound to quantify isocyanate species for which standards are unavailable. However, the excitation and emission wavelengths that have been chosen make the detection very selective for MAP derivatives. Experience has shown that MAP-derivatized isocyanates will have a FL /UV ratio of approxim ately 0.33 to 2 times that of the M AP-derivatized mo nom er.

CALCULATIONS: 18. Monom ers: Ge nera te a calibration graph usually using fluorescence, plotting peak area or peak height as a function of concentration (no rm ality, isocyanate milliequivalents per milliliter) of monom er standards. Determine the normality of the analyzed sample aliquot from the calibration graph.

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition