Separation of Chrysotile Asbestos from Minerals That Interfere with Its Infrared Analysis Robert P. Bagioni 13 Logan S t . , New Britain, Conn. 06051
The persistence of interferences in chrysotile asbestos determinations has rendered the use of infrared methods totally unsuitable for analytical analysis. Interferences that show a stretch at 2.72 p will invalidate most infrared techniques. Where it was previously possible to remove organic interferences through ashing techniques, it is also possible to remove mineral interferences through centrifugation in a high density liquid. The removal of mineral interferences has helped to improve the infrared methods. Some of the more common methods of airborne asbestos analysis are neutron activation ( I ) , X-ray diffraction (Z ), electron microscopy (3), and infrared analysis. In view of recent asbestos regulations ( 4 ) , electron microscopy has been established as the only method consistently sensitive in the required ranges. The procedure described here is an attempt to improve on the infrared methods of asbestos analysis. The technique employed is an outgrowth of work done by Crable et al. ( 5 ) on coal dust analysis, and Gadsen et al. (6) on chrysotile asbestos analysis. The infrared method is based on a study of the 0-H stretch at 2.72 p of a chrysotile asbestos molecule (Figure 1). The main limitation of the infrared method is the presence of interferences that also possess a stretch a t 2.72 p . Serpentine would be an example of this. Serpentine and chrysotile asbestos both possess 0-H stretches at 2.72 p . The method described here removes interferences through selective separation in a high-density liquid.
Experimental Apparatus. A Perkin-Elmer 257 Grating Infrared Spectrophotometer and attenuator attachment were used. Potassium bromide disks (13 mm) were pressed using a Carver laboratory press Model B (20,000 psi). Filtration was carried out using a Millipore filter apparatus (Pyrex ZxxlO 047 OD) and Millipore filters (0.45 p Type HA). It is necessary to use Pyrex filtration equipment. The reagents used will damage the Millipore plastic equipment. Reagents. Prepare a mixture of 35 ml of 1,1,2,2-tetrabromoethane (reagent grade) and 15 ml of carbon tetrachloride (reagent grade). Adjust the density to 2.45-2.50 g/ml 20°C. Place the mixture into a dark bottle and stopper tightly. Dry a spectrophotometric grade of potassium bromide overnight at 110°C. Procedure. Samples .of asbestos and/or interfering minerals were prepared as shown in Table I. Each sample is then placed into 3 ml of the density mixture in a centrifuge tube. Completely wash all of the sample into the tube with an additional 2 ml of the density mixture. Centrifuge the sample for 15 min at 2000-2500 rpm. The asbestos will be found floating in the upper 3 ml of the mixture. Carefully remove the upper 3 ml of the mixture with a micropipet and place it into the Millipore filter apparatus. Wash the pipet clean with carbon tetrachloride collecting all washings in the filter apparatus. Apply aspira262
Environmental Science & Technology
tor suction to the apparatus, and wash the sample with two 10-ml portions of carbon tetrachloride. Remove the filter and put it into a porcelain crucible. Place the crucible on a hot plate for a few minutes to allow any .remaining liquid to evaporate. Then ash the filter a t 600°C for l hr. (If possible, low-temperature ashing techniques should be used in determinations of airborne asbestos samples.) Prepare a potassium bromide disk by mixing 100 mg of KBr with the sample and concentrating it into the center of the disk. The remaining 200 mg of KBr are then used to fill in the disk. Place the pressed disk into the Perkin-Elmer instrument. Adjust the transmittance to 60-7070 with the attenuator attachment, and then scan the pellet from 2.5-16 1.1. Instrument settings are a scan speed of medium and a slit opening of normal.
Results Each spectrum was analyzed and the incident (IO)and emergent (I) radiations measure as shown in Figure 1. The first five samples were used to plot a standard curve. The curve is reasonably linear. The next four samples were analyzed, and the weights of asbestos were determined from the standard curve. The results were then compared with the actual known weights of asbestos placed into the samples. The percentage of error is low enough to ensure a qualitative use for the separation of chrysotile from interferences (Table 11). With refinement of the recovery technique the procedure may have a quantitative application. Table I1 also shows that the weight of asbestos determined is generally below that of the known weight placed in the sample. This suggests that agglomeration of fibers or particles of interfering minerals with the asbestos is not a significant source of error. Only when the weight of asbestos reaches the milligram range does agglomeration seem a possible source of error. But the milligram weight range is far above the determination range requirements. The last four samples studied contained only mixtures of interfering minerals. As can be seen in Figure 2, the spectra show an absence of peaks in the 2.72-p range. This indicates that the separation achieved is minimizing any chance for an 0-H stretch caused by interfering minerals. Discussion If the infrared techniques are to become a practical method of asbestos analysis, it is necessary to ensure that the samples to be analyzed are free of interfering minerals. The removal of interferences is also desirable when using X-ray or electron microscopy techniques. In the described method, the interferences are removed in the first step. All employed techniques should benefit from the separation of interferences. Literature values ( 5 ) have shown infrared analysis of chrysotile to have a lower limit of about 20 p g . However, this value was arrived at through an extrapolation on a standard curve. Preparation of samples containing 1 total
... ... ...
... ... ... =:
230-
> 1 total
..*
Mineral mixture
.-
...
2.5
4.0
amphibole, talc, serpentine, phylite.
microns
Table I I . Determination of Percentage Recovery of Asbestos Sam-
’le no.
Weight abestor;,
6 7 8‘ 9
1299.0 718 503 598
I
10
26 40 41 42
56 58 56 58
Log
1/10
0.3324 0.1614 0.1335 0.1399
Weight asbestos std. Error, curve, pg %
1350 650 505 560
3.94 9.51 0.40 6.35
‘iZO/ , , ,
,
L
c
4000
3500
3000
2500
2000
wavenumber cm-’ Figure 2. Spectrum for asbestos/mineral separation
It should be emphasized that infrared techniques seem best suited for risbestos analysis in areas of high asbestos use. With a practical handling limit of
