Document 3eQZ2LGajJGeYqZzDRyG2G7DD

FILE NAME Talc TALC DATE 1973 Dec 3 DOC TALC137 DOCUMENT DESCRIPTION Letter to the FDA from Walter McCrane Associates with Comments on Asbestos Proposal to * * . : fi& ] walter c.mccrone associates inc CONSULTING ULTRAMICROANALYSIS . MICROSCOPY SMALL PARTICLE PROBLEMS STATE CHEMISTRY 3 December 1973 Hearing Clerk Food and Drug Administration Room 6-86 5600 Fisheries Lane Rockville Maryland 20852 Gentlemen We are writing to comment on the Commissioner's proposal covering Asbestos Particles in Food and Drugs as published in the Federal Register Vol 38 No. 188 pages 27076-27081 28 September 1973. We are interested in helping the FDA choose a practical analytical procedure for the detection and identification of asbestos Our laboratory has analyzed many hundreds of asbestos and containing samples by most of the various analytical procedures polarized light microscopy ray diffraction scanning electron microscopy with dispersive ray analysis transmission electron microscopy with selected area electron diffraction electron microprobe analysis and electron microscope microprobe analysis We would like to begin by giving you our evalu- ation of each of these methods The methods to consider are 1. Morphology by light and electron microscopy 2. Optical properties by light microscopy 3. Diffraction by ray or electron diffraction 4. Elemental analysis by electron or ion microprobe The various instruments available are capable of a variety of combinations of these methods See Table 1 attached to this letter The advantages and disadvantages of each area are discussed individually The polarizing light microscope LM requires great skill and background obtained only with long practice A microscopist trained in optical crystallography and familiar with the serpentines and amphiboles still requires hours of time sample to find and identify each of the possible asbestos types The problem is complicated by the presence of interfering fibers 3 aspect ratio e.g. paper talc on edge diatoms quartz mineral wool plant hairs and fibers etc. Finally all of the asbestos types show considerable variation in all of their properties and these variations must be familiar to the microscopist 75P.0261 FDA00002496 Page Two @ @ FDA a Dispersion staining DS greatly speeds up the detection and identification of asbestos fibers It is the only practical way to apply optical crystallography to samples containing small percentages of asbestos The technique can be learned much more rapidly than optical crystallography yet it is securely based on optical crystallographic properties ray diffraction XRD also based on crystal properties is somewhat less sensitive than dispersion staining but requires no more background and skill The sensitivity can be increased from 0.5-1.0 to 0.1-0.5 by scanning One advantage over LM and DS is smaller size limit - about 50 nm compared with about 5,000 nm 5 ...m Scanning electron microscopy with an energy dispersive ray analyzer is an overrated tool Many substances besides asbestos are fibrous and it is nearly impossible to obtain a good chemical analysis of submicrometer particles in a particulate matrix with EDXRA in the SEM there is too much interference Fitting the SEM with a wavelength dispersive ray analyzer greatly improves it usefulness The transmission electron microscope TEM with selected area electron diffraction is an ideal tool when the asbestos fiber size is 0.2 ...m The shape of fibers down to 0.005 mis immediately apparent on scanning a sample and the diffraction pattern is immediately discernible The electron microprobe analyzer EMA with its wavelength dispersive ray analyzer is better than the SEM fitted with EDXRA but is surpassed by an SEM fitted with wavelength dispersive ray analyzer The better morphological image of the SEM tips the balance EMMA the combination transmission electron microscope microprobe analyzer is by far the most useful of the analytical tools for asbestos It may not be needed if the fiber size is large enough about 0.5 ...min smallest dimension but smaller particles can be characterized by morphology diffraction and chemical analysis in this one instrument It is ideal for water samples containing suspended asbestos fibers The ion microprobe although a highly sensitive trace analytical tool is not usually used for asbestos fiber analysis It does however detect fine fibers and identifies them by qualita- tive chemical analysis quantitative when compared with standards Summarizing then - if the size of the asbestos fibers is very small as it nearly always is for water samples LM is useless and a diffraction method must be used If the size is large enough say 1 x 3 ...mand the percentage high enough say /0.01 /0.01then LM can do the job If high sensitivity is not needed say about 0.2 or greater then XRD is the best method Very fine particles require the TEM with selected area diffraction It should not be necessary to emphasize that no method is better than the analyst who makes the analysis Few laboratories have all of the above instruments and highly skilled analysts for each Instead each tends to use the tools available and the methods with which their personnel are more or less familiar In a situation as important as this the choice should not be based on availability of tools and skills Fortunately the tools for detection and identification of asbestos are neither the most expensive nor the most sophisticated FDA00002497