Annals of Occupational Hygiene Advance Access originally published online on September 18, 2007
Annals of Occupational Hygiene 2007 51(7):648-649; doi:10.1093/annhyg/mem042
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LETTER TO THE EDITOR |
Reply
Mr Walker makes several points regarding the paper by Creely et al. (2006). Some of these points are best answered by the authors of this paper, others concern the Health and Safety Laboratory (HSL) method MDHS 25/3 (HSE, 1999). I will reply on these points and leave the others to Creely et al. in their accompanying reply.
As a general comment, HSL has, with the support of the Health and Safety Executive (HSE), carried out extensive validation and experimental work on MDHS 25 over a period of 
30 years. MDHS 25/3, in addition to being the method of choice for HSE/HSL, is an official International Standards Organisation (ISO, 2001) method for the determination of isocyanates in air.
HSE METHOD MDHS 25
Both MDHS 25/3 (HSE, 1999) and my paper (White, 2006) describe the procedure for quantification of isocyanates (NCO) as their 1-(2-methoxyphenyl)piperazine (MP) derivatives.
Monomeric NCO-MP, for which a suitable standard exists or can be prepared, may be measured by ultraviolet/visible (UV/vis) or electrochemical (EC) detection. UV/vis detection is simpler to use and gives a less variable response than EC detection but is less sensitive.
MDHS 25/3 (paragraph 70 onwards) states that monomeric and oligomeric NCO-MP derivatives are quantified by the EC detector using the response factor obtained from the relevant MP-derivatized monomer. My paper states the reasoning behind this (White, 2006, p. 17) and gives the results of an extensive validation exercise (p. 20). Both MDHS 25/3 (HSE, 1999) and White (2006) give the references for the original research on which this procedure is based.
In MDHS 25/3 peaks are identified as being derivatized isocyanates by using the ratio of the response of EC and UV/vis detectors for the derivatized polymer and monomer and comparing this ratio with an empirically derived range. This range was modified slightly in White (2006) in the light of further experiments. The use of this EC/UV ratio approach predates the widespread availability of liquid chromatography/mass spectrometry (LC/MS) and I discussed the validity of the EC/UV ratio as a ‘poor man's MS’ in White (2006, p. 17 and p. 20). I intend to write up my work using LC/MS as a detector for MP-derivatized NCO and submit it to this journal in the near future.
Mr Walker describes in his letter the quantification of NCO-MP derivatives using UV/vis detection. As stated above this is not the procedure as described in MDHS 25/3. However, UV/vis detectors are widespread and relatively easy to use and so HSL has carried out research to see in what circumstances NCO-MP derivatives may be quantified using them.
Firstly, as stated in White (2006, p. 17), the UV/vis detector may be used when monomeric NCO-MP derivatives are being measured as it will usually be possible to obtain (prepare or purchase) a standard of the relevant NCO-MP monomer derivative.
For oligomeric NCO the situation is slightly more complicated as standards of the MP-derivatized material are not commercially available. This is particularly important for 1,6-diisocyanatohexane (aka hexamethylene diisocyanate, HDI) formulations, which are as Mr Walker states, usually a mix of oligomeric HDI species with a very small monomeric HDI content. For a compound to be amenable to UV/vis analysis, it must have a good chromophore that absorbs UV/vis light (i.e. with a wavelength of 190–800 nm). For the MP derivatives of the common aliphatic isocyanates, i.e. HDI-MP and isophorone diisocyanate-MP, the major chromophore is the aromatic substituted methoxyphenyl ring of the MP reagent. The aliphatic parts of these compounds are very poor chromophores. The UV absorbance of the methoxyphenyl ring of the MP-derivatizing reagent remains the same irrespective of the NCO species it is attached to and so the UV/vis response of MP-derivatized oligomeric HDI has been found to be similar to that of MP-derivatized monomeric HDI. This means that the quantification of oligomers of aliphatic NCO-MP using a UV/vis response factor obtained from the relevant NCO-MP monomer does not in practice lead to major errors (White et al., 2005). For NCO where a significant chromophore apart from the methoxyphenyl ring of the MP reagent exists, e.g. oligomers of aromatic NCO such as MDI and TDI, the use of UV/vis for quantification would be expected to give rise to larger errors.
For the reacting paint mixes Mr Walker mentions that, e.g. paint aerosols produced during spraying 2K HDI-based paints, the rate of reaction of the NCO with the MP-derivatization reagent has been found to be much faster than the rate of reaction (curing) of the NCO with the other paint components such as polyols. This is the reason the preparation of a bulk formulation MP derivative can assist in identifying NCO-derived peaks (MDHS 25/3 paragraphs 76 and 77), i.e. gives similar chromatograms to those obtained during airborne sampling. For the faster reacting aromatic NCO, the agreement between bulk derivatives and airborne samples will be less good, e.g. the commonly observed inversion in the abundances of 2,4 and 2,6 isomers of TDI seen throughout a process (Rando and Hammad, 1985). In confirmation of the comments above, I cite that the dual filter method (ISO, 2006) for airborne isocyanates also allows the use of UV/vis detection to quantify the MP derivatives of oligomeric NCO.
Incidentally, in regard to HDI-based paints, Mr Walker states ‘we all know that the amount of free monomer in commercial systems is negligible'. I wish that it were true that everyone knew this—and also the subsequent requirement to measure the oligomeric NCO!
SAMPLING PROCEDURES
In relation to when or not to use the impinger/filter sampling train, I would draw Mr Walker's attention to the discussion and references cited in White (2006, p. 16). Regarding his comments on the vapour pressure of MDI, it is true that the vapour pressure above solid MDI should be very low but for liquid MDI and TDI the figures quoted in Allport et al. (2003) are informative.
The major reference for the validation of MDHS 25/3 for NCO aerosol sampling is, as quoted in MDHS 25/3 (paragraph 48) and White (2006, p. 16), the work on polymeric MDI aerosols carried out by Hext et al. (2003). For workplace monitoring, i.e. validation using ‘real’ aerosols, I refer Mr Walker to the several papers comparing methods I cite in White (2006, p. 16) and to the comparison of alternative personal sampling methods in Unwin et al. (2005).
Health and Safety Laboratory, Harpur Hill, Buxton SK17 9JN, UK
E-mail: John.HSL.White{at}hsl.gov.uk
Received June 20, 2007;
REFERENCES
Allport DC, Gilbert DS, Outterside SM, eds. MDI and TDI: safety, health and the environment, a source book and practical guide; tables 5.4.5 to 5.4.7 and 5.4.13 to 5.4.14 (2003) Chichester, UK: J Wiley and Sons Ltd.
Creely KS, Hughson GW, Cocker J, et al. Assessing isocyanate exposures in polyurethane industry sectors using biological and air monitoring methods. Ann Occup Hyg (2006) 50:609–21.
Hext PM, Booth K, Dharmarajan V, et al. A comparison of efficiencies of a range of atmosphere samplers when collecting polymeric diphenylmethane diisocyanate (MDI) aerosols. Appl Occup Environ Hyg (2003) 18:346–57.[CrossRef][Medline]
HSE. Methods for the determination of hazardous substances #25/3; organic isocyanates in air (MDHS 25/3) (1999) Sudbury, UK: Health and Safety Executive. Available at http://www.hse.gov.uk/pubns/mdhs/pdfs/mdhs25-3.pdf. Accessed 3 September 2007.
ISO. Workplace air quality. Determination of total isocyanate groups in air using 1-(2-methoxyphenyl)piperazine and liquid chromatography. ISO method 16702:2001 (2001) Geneva: International Standards Organisation.
ISO. Workplace air quality. Determination of total isocyanate groups in air using the dual filter method. ISO draft CD 17736 (2006) Geneva: International Standards Organisation.
Rando RJ, Hammad YY. Modified Marcali method for the determination of total toluenediisocyanate in air. Am Ind Hyg Assoc J (1985) 46:206–10.[Web of Science][Medline]
Unwin J, Dabill DW, Keen C, et al. Investigation of complex harmful substances call-off contract 2003–2005. HSL report OMS/2005/11 (2005) UK: Health and Safety Executive. Available at http://www.hse.gov.uk/research/hsl_pdf/2006/hsl06106.pdf. Accessed 4 September 2007.
White J. MDHS 25/3 revisited; development of MDHS 25/3, the determination of organic isocyanates in air. Ann Occup Hyg (2006) 50:15–27.
White J, Corns H, Jones K. New instrumental techniques for toxic and harmful substances call-off contract 2003–2005. HSL report OMS/2005/17 (2005) UK: Health and Safety Executive. Available at http://www.hse.gov.uk/research/hsl_pdf/2007/hsl0718.pdf. Accessed 4 September 2007.
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