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Bulk samples of building materials taken for asbestos identification are first examined for homogeneity and preliminary fiber identification at low magnification. Positive identification of suspect fibers is made by analysis of subsamples with the polarized light microscope.
The principles of optical mineralogy are well established. 1 2 A light microscope equipped with two polarizing filters is used to observe specific optical characteristics of a sample. The use of plane polarized light allows the determination of refractive indices along specific crystallographic axes. Morphology and color are also observed. A retardation plate is placed in the polarized light path for determination of the sign of elongation using orthoscopic illumination. Orientation of the two filters such that their vibration planes are perpendicular (crossed polars) allows observation of the birefringence and extinction characteristics of anisotropic particles.
Quantitative analysis involves the use of point counting. Point counting is a standard technique in petrography for determining the relative areas occupied by separate minerals in thin sections of rock. Background information on the use of point counting 2 and the interpretation of point count data 3 is available.
This method is applicable to all bulk samples of friable insulation materials
submitted for identification and quantitation of asbestos components.
The point counting method may be used for analysis of samples containing from
0 to 100 percent asbestos. The upper detection limit is 100 percent. The lower
detection limit is less than 1 percent.
Fibrous organic and inorganic constituents of bulk samples may interfere with
the identification and quantitation of the asbestos mineral content. Spray-on
binder materials may coat fibers and affect color or obscure optical
characteristics to the extent of masking fiber identity. Fine particles of other
materials may also adhere to fibers to an extent sufficient to cause confusion
in identification. Procedures that may be used for the removal of interferences
are presented in Section 1.7.2.2.
Adequate data for measuring the accuracy and precision of the method for
samples with various matrices are not currently available. Data obtained for
samples containing a single asbestos type in a simple matrix are available in
the EPA report Bulk Sample Analysis for Asbestos Content: Evaluation of the
Tentative Method.
A low-power binocular microscope, preferably stereoscopic, is used to examine the bulk insulation sample as received.
• Microscope: binocular, 10-45X (approximate).
• Light Source: incandescent or fluorescent.
• Forceps, Dissecting Needles, and Probes
• Glassine Paper or Clean Glass Plate
Compound microscope requirements: A polarized light microscope complete with polarizer, analyzer, port for wave retardation plate, 360° graduated rotating stage, substage condenser, lamp, and lamp iris.
• Polarized Light Microscope: described above.
• Objective Lenses: 10X, 20X, and 40X or near equivalent.
• Dispersion Staining Objective Lens (optional)
• Ocular Lens: 10X minimum.
• Eyepiece Reticle: cross hair or 25 point Chalkley Point Array.
• Compensator Plate: 550 millimicron
retardation.
Sample preparation apparatus requirements will depend upon the type of insulation sample under consideration. Various physical and/or chemical means may be employed for an adequate sample assessment.
• Ventilated Hood or negative pressure glove box.
• Microscope Slides
• Coverslips
• Mortar and Pestle: agate or porcelain. (optional)
• Wylie Mill (optional)
• Beakers and Assorted Glassware (optional)
• Certrifuge (optional)
• Filtration apparatus (optional)
• Low temperature asher (optional)
• Distilled Water (optional)
• Dilute CH • Dilute HCl: ACS reagent grade (optional)
• Sodium metaphosphate (NaPO Refractive Index Liquids: 1.490-1.570, 1.590-1.720 in increments of
0.002 or 0.004.
• Refractive Index Liquids for Dispersion
Staining: high-dispersion series, 1.550, 1.605, 1.630 (optional).
• UICC Asbestos Reference Sample Set: Available
from: UICC MRC Pneumoconiosis Unit, Llandough Hospital, Penarth, Glamorgan CF6
1XW, UK, and commercial distributors.
• Tremolite-asbestos (source to be determined)
• Actinolite-asbestos (source to be determined)
Note: Exposure to airborne asbestos fibers is a health hazard. Bulk
samples submitted for analysis are usually friable and may release fibers during
handling or matrix reduction steps. All sample and slide preparations should be
carried out in a ventilated hood or glove box with continuous airflow (negative
pressure). Handling of samples without these precautions may result in exposure
of the analyst and contamination of samples by airborne fibers.
Samples for analysis of asbestos content shall be taken in the manner
prescribed in Reference 5 and information on design of sampling and analysis
programs may be found in Reference 6. If there are any questions about the
representative nature of the sample, another sample should be requested before
proceeding with the analysis. Bulk samples of building materials taken for the identification and
quantitation of asbestos are first examined for homogeneity at low magnification
with the aid of a stereomicroscope. The core sample may be examined in its
container or carefully removed from the container onto a glassine transfer paper
or clean glass plate. If possible, note is made of the top and bottom
orientation. When discrete strata are identified, each is treated as a separate
material so that fibers are first identified and quantified in that layer only,
and then the results for each layer are combined to yield an estimate of
asbestos content for the whole sample. Bulk materials submitted for asbestos analysis involve a wide variety of
matrix materials. Representative subsamples may not be readily obtainable by
simple means in heterogeneous materials, and various steps may be required to
alleviate the difficulties encountered. In most cases, however, the best
preparation is made by using forceps to sample at several places from the bulk
material. Forcep samples are immersed in a refractive index liquid on a
microscope slide, teased apart, covered with a cover glass, and observed with
the polarized light microscope.
Alternatively, attempts may be made to homogenize the sample or eliminate
interferences before further characterization. The selection of appropriate
procedures is dependent upon the samples encountered and personal preference.
The following are presented as possible sample preparation steps.
A mortar and pestle can sometimes be used in the size reduction of soft or
loosely bound materials though this may cause matting of some samples. Such
samples may be reduced in a Wylie mill. Apparatus should be clean and extreme
care exercised to avoid cross-contamination of samples. Periodic checks of the
particle sizes should be made during the grinding operation so as to preserve
any fiber bundles present in an identifiable form. These procedures are not
recommended for samples that contain amphibole minerals or vermiculite. Grinding
of amphiboles may result in the separation of fiber bundles or the production of
cleavage fragments with aspect ratios greater than 3:1. Grinding of vermiculite
may also produce fragments with aspect ratios greater than 3:1.
Acid treatment may occasionally be required to eliminate interferences.
Calcium carbonate, gypsum, and bassanite (plaster) are frequently present in
sprayed or trowelled insulations. These materials may be removed by treatment
with warm dilute acetic acid. Warm dilute hydrochloric acid may also be used to
remove the above materials. If acid treatment is required, wash the sample at
least twice with distilled water, being careful not to lose the particulates
during decanting steps. Centrifugation or filtration of the suspension will
prevent significant fiber loss. The pore size of the filter should be 0.45
micron or less. Caution: prolonged acid contact with the sample may alter the
optical characteristics of chrysotile fibers and should be avoided.
Coatings and binding materials adhering to fiber surfaces may also be removed
by treatment with sodium metaphosphate. 7 Add 10 mL of 10g/L sodium
metaphosphate solution to a small (0.1 to 0.5 mL) sample of bulk material in a
15-mL glass centrifuge tube. For approximately 15 seconds each, stir the mixture
on a vortex mixer, place in an ultrasonic bath and then shake by hand. Repeat
the series. Collect the dispersed solids by centrifugation at 1000 rpm for 5
minutes. Wash the sample three times by suspending in 10 mL distilled water and
recentrifuging. After washing, resuspend the pellet in 5 mL distilled water,
place a drop of the suspension on a microscope slide, and dry the slide at 110
°C.
In samples with a large portion of cellulosic or other organic fibers, it may
be useful to ash part of the sample and view the residue. Ashing should be
performed in a low temperature asher. Ashing may also be performed in a muffle
furnace at temperatures of 500 °C or lower. Temperatures of 550 °C or higher
will cause dehydroxylation of the asbestos minerals, resulting in changes of the
refractive index and other key parameters. If a muffle furnace is to be used,
the furnace thermostat should be checked and calibrated to ensure that samples
will not be heated at temperatures greater than 550 °C.
Ashing and acid treatment of samples should not be used as standard
procedures. In order to monitor possible changes in fiber characteristics, the
material should be viewed microscopically before and after any sample
preparation procedure. Use of these procedures on samples to be used for
quantitation requires a correction for percent weight loss. Positive identification of asbestos requires the determination of the
following optical properties.
• Morphology
• Color and pleochroism
• Refractive indices
• Birefringence
• Extinction characteristics
• Sign of elongation Table 1-1 lists the above properties for commercial asbestos fibers. Figure
1-1 presents a flow diagram of the examination procedure. Natural variations in
the conditions under which deposits of asbestiform minerals are formed will
occasionally produce exceptions to the published values and differences from the
UICC standards. The sign of elongation is determined by use of the compensator
plate and crossed polars. Refractive indices may be determined by the Becke line
test. Alternatively, dispersion staining may be used. Inexperienced operators
may find that the dispersion staining technique is more easily learned, and
should consult Reference 9 for guidance. Central stop dispersion staining colors
are presented in Table 1-2. Available high-dispersion (HD) liquids should be
used.
Asbestos quantitation is performed by a point-counting procedure or an
equivalent estimation method. An ocular reticle (cross-hair or point array) is
used to visually superimpose a point or points on the microscope field of view.
Record the number of points positioned directly above each kind of particle or
fiber of interest. Score only points directly over asbestos fibers or
nonasbestos matrix material. Do not score empty points for the closest particle.
If an asbestos fiber and a matrix particle overlap so that a point is
superimposed on their visual intersection, a point is scored for both
categories. Point counting provides a determination of the area percent
asbestos. Reliable conversion of area percent to percent of dry weight is not
currently feasible unless the specific gravities and relative volumes of the
materials are known.
For the purpose of this method, "asbestos fibers" are defined as having an
aspect ratio greater than 3:1 and being positively identified as one of the
minerals in Table 1-1.
A total of 400 points superimposed on either asbestos fibers or nonasbestos
matrix material must be counted over at least eight different preparations of
representative subsamples. Take eight forcep samples and mount each separately
with the appropriate refractive index liquid. The preparation should not be
heavily loaded. The sample should be uniformly dispersed to avoid overlapping
particles and allow 25-50 percent empty area within the fields of view. Count 50
nonempty points on each preparation, using either
• A cross-hair reticle and mechanical stage; or
• A reticle with 25 points (Chalkley Point Array) and
counting at least 2 randomly selected fields. For samples with mixtures of isotropic and anisotropic materials present,
viewing the sample with slightly uncrossed polars or the addition of the
compensator plate to the polarized light path will allow simultaneous
discrimination of both particle types. Quantitation should be performed at 100X
or at the lowest magnification of the polarized light microscope that can
effectively distinguish the sample components. Confirmation of the quantitation
result by a second analyst on some percentage of analyzed samples should be used
as standard quality control procedure.
The percent asbestos is calculated as follows:
% asbestos=(a/n) 100% where a=number of asbestos counts,
n=number of nonempty points counted (400).
If a=0, report "No asbestos detected." If 0< a≦3, report "<1%
asbestos".
The value reported should be rounded to the nearest percent. 1. Paul F. Kerr, Optical Mineralogy, 4th ed., New York, McGraw-Hill,
1977.
2. E. M. Chamot and C. W. Mason, Handbook of Chemical Microscopy, Volume
One, 3rd ed., New York: John Wiley & Sons, 1958.
3. F. Chayes, Petrographic Modal Analysis: An Elementary Statistical
Appraisal, New York: John Wiley & Sons, 1956.
4. E. P. Brantly, Jr., K. W. Gold, L. E. Myers, and D. E. Lentzen, Bulk
Sample Analysis for Asbestos Content: Evaluation of the Tentative Method,
U.S. Environmental Protection Agency, October 1981.
5. U.S. Environmental Protection Agency, Asbestos-Containing Materials in
School Buildings: A Guidance Document, Parts 1 and 2, EPA/OPPT No. C00090,
March 1979.
6. D. Lucas, T. Hartwell, and A. V. Rao, Asbestos-Containing Materials in
School Buildings: Guidance for Asbestos Analytical Programs, EPA
560/13-80-017A, U.S. Environmental Protection Agency, December 1980, 96 pp.
7. D. H. Taylor and J. S. Bloom, Hexametaphosphate pretreatment of insulation
samples for identification of fibrous constituents, Microscope, 28, 1980.
8. W. J. Campbell, R. L. Blake, L. L. Brown, E. E. Cather, and J. J. Sjoberg.
Selected Silicate Minerals and Their Asbestiform Varieties: Mineralogical
Definitions and Identification-Characterization, U.S. Bureau of Mines
Information Circular 8751, 1977.
9. Walter C. McCrone, Asbestos Particle Atlas, Ann Arbor: Ann Arbor
Science Publishers, June 1980.
The principle of X-ray powder diffraction (XRD) analysis is well established.
1 2 Any solid, crystalline material will diffract an impingent beam
of parallel, monochromatic X-rays whenever Bragg's Law, λ = 2d sin &thetas;, is satisfied for a particular set of planes in the crystal lattice, where λ = the X-ray wavelength, A d = the interplanar spacing of the set of reflecting lattice planes, A &thetas; = the angle of incidence between the X-ray beam and the
reflecting lattice planes. By appropriate orientation of a sample relative to the incident X-ray beam, a
diffraction pattern can be generated that, in most cases, will be uniquely
characteristic of both the chemical composition and structure of the crystalline
phases present.
Unlike optical methods of analysis, however, XRD cannot determine crystal
morphology. Therefore, in asbestos analysis, XRD does not distinguish between
fibrous and nonfibrous forms of the serpentine and amphibole minerals (Table
2-1). However, when used in conjunction with optical methods such as polarized
light microscopy (PLM), XRD techniques can provide a reliable analytical method
for the identification and characterization of asbestiform minerals in bulk
materials.
For qualitative analysis by XRD methods, samples are initially scanned
over limited diagnostic peak regions for the serpentine (∼7.4 A Accurate quantitative analysis of asbestos in bulk samples by XRD is
critically dependent on particle size distribution, crystallite size, preferred
orientation and matrix absorption effects, and comparability of standard
reference and sample materials. The most intense diffraction peak that has been
shown to be free from interference by prior qualitative XRD analysis is selected
for quantitation of each asbestiform mineral. A "thin-layer" method of analysis
5 6 is recommended in which, subsequent to comminution of the bulk
material to ∼10 μm by suitable cryogenic milling techniques, an accurately known
amount of the sample is deposited on a silver membrane filter. The mass of
asbestiform material is determined by measuring the integrated area of the
selected diffraction peak using a step-scanning mode, correcting for matrix
absorption effects, and comparing with suitable calibration standards.
Alternative "thick-layer" or bulk methods, 7 8 may be used for
semiquantitative analysis.
This XRD method is applicable as a confirmatory method for identification and
quantitation of asbestos in bulk material samples that have undergone prior
analysis by PLM or other optical methods. The range of the method has not been determined.
The sensitivity of the method has not been determined. It will be variable
and dependent upon many factors, including matrix effects (absoprtion and
interferences), diagnostic reflections selected, and their relative intensities.
Since the fibrous and nonfibrous forms of the serpentine and amphibole
minerals (Table 2-1) are indistinguishable by XRD techniques unless special
sample preparation techniques and instrumentation are used, 9 the
presence of nonasbestiform serpentines and amphiboles in a sample will pose
severe interference problems in the identification and quantitative analysis of
their asbestiform analogs.
The use of XRD for identification and quantitation of asbestiform minerals in
bulk samples may also be limited by the presence of other interfering materials
in the sample. For naturally occurring materials the commonly associated
asbestos-related mineral interferences can usually be anticipated. However, for
fabricated materials the nature of the interferences may vary greatly (Table
2-3) and present more serious problems in identification and quantitation.
10 Potential interferences are summarized in Table 2-4 and include
the following:
• Chlorite has major peaks at 7.19 A • Halloysite has a peak at 3.63 A • Kaolinite has a major peak at 7.15 A • Gypsum has a major peak at 7.5 A • Cellulose has a broad peak that partially
overlaps the secondary (3.66 A • Overlap of major diagnostic peaks of the amphibole
asbestos minerals, amosite, anthophyllite, crocidolite, and tremolite, at
approximately 8.3 A A. Insulation materials Chrysotile
"Amosite"
Crocidolite
*Rock wool
*Slag wool
*Fiber glass
Gypsum (CaSO Vermiculite (micas)
*Perlite
Clays (kaolin)
*Wood pulp
*Paper fibers (talc, clay, carbonate fillers)
Calcium silicates (synthetic)
Opaques (chromite, magnetite inclusions in serpentine)
Hematite (inclusions in "amosite")
Magnesite
*Diatomaceous earth B. Spray finishes or paints Bassanite
Carbonate minerals (calcite, dolomite, vaterite)
Talc
Tremolite
Anthophyllite
Serpentine (including chrysotile)
Amosite
Crocidolite
*Mineral wool
*Rock wool
*Slag wool
*Fiber glass
Clays (kaolin)
Micas
Chlorite
Gypsum (CaSO Quartz
*Organic binders and thickeners
Hyrdomagnesite
Wollastonite
Opaques (chromite, magnetite inclusions in serpentine)
Hematite (inclusions in "amosite")
*Amorphous materials__contribute only to overall scattered radiation and
increased background radiation. • Carbonates may also interfere with
quantitative analysis of the amphibole asbestos minerals, amosite,
anthophyllite, crocidolite, and tremolite. Calcium carbonate (CaCO • A major talc peak at 3.12 A The problem of intraspecies and matrix interferences is further aggravated by
the variability of the silicate mineral powder diffraction patterns themselves,
which often makes definitive identification of the asbestos minerals by
comparison with standard reference diffraction patterns difficult. This
variability results from alterations in the crystal lattice associated with
differences in isomorphous substitution and degree of crystallinity. This is
especially true for the amphiboles. These minerals exhibit a wide variety of
very similar chemical compositions, with the result being that their diffraction
patterns are chracterized by having major (110) reflections of the monoclinic
amphiboles and (210) reflections of the orthorhombic anthophyllite separated by
less than 0.2 A If a copper X-ray source is used, the presence of iron at high concentrations
in a sample will result in significant X-ray fluorescence, leading to loss of
peak intensity along with increased background intensity and an overall decrease
in sensitivity. This situation may be corrected by choosing an X-ray source
other than copper; however, this is often accompanied both by loss of intensity
and by decreased resolution of closely spaced reflections. Alternatively, use of
a diffracted beam monochromator will reduce background fluorescent raditation,
enabling weaker diffraction peaks to be detected.
X-ray absorption by the sample matrix will result in overall attenuation of
the diffracted beam and may seriously interfere with quantitative analysis.
Absorption effects may be minimized by using sufficiently "thin" samples for
analysis. 5 13 14 However, unless absorption effects are known to be
the same for both samples and standards, appropriate corrections should be made
by referencing diagnostic peak areas to an internal standard 7 8 or
filter substrate (Ag) peak. 5 6 Because the intensity of diffracted X-radiation is particle-size dependent,
it is essential for accurate quantitative analysis that both sample and standard
reference materials have similar particle size distributions. The optimum
particle size range for quantitative analysis of asbestos by XRD has been
reported to be 1 to 10 μ m. 15 Comparability of sample and standard
reference material particle size distributions should be verified by optical
microscopy (or another suitable method) prior to analysis. Preferred orientation of asbestiform minerals during sample preparation often
poses a serious problem in quantitative analysis by XRD. A number of techniques
have been developed for reducing preferred orientation effects in "thick layer"
samples. 7 8 15 However, for "thin" samples on membrane filters, the
preferred orientation effects seem to be both reproducible and favorable to
enhancement of the principal diagnostic reflections of asbestos minerals,
actually increasing the overall sensitivity of the method. 12 14
(Further investigation into preferred orientation effects in both thin layer and
bulk samples is required.) The problem of obtaining and characterizing suitable reference materials for
asbestos analysis is clearly recognized. NIOSH has recently directed a major
research effort toward the preparation and characterization of analytical
reference materials, including asbestos standards;1617 however, these
are not available in large quantities for routine analysis.
In addition, the problem of ensuring the comparability of standard reference
and sample materials, particularly regarding crystallite size, particle size
distribution, and degree of crystallinity, has yet to be adequately addressed.
For example, Langer et al. 18 have observed that in insulating
matrices, chrysotile tends to break open into bundles more frequently than
amphiboles. This results in a line-broadening effect with a resultant decrease
in sensitivity. Unless this effect is the same for both standard and sample
materials, the amount of chrysotile in the sample will be underestimated by XRD
analysis. To minimize this problem, it is recommended that standardized matrix
reduction procedures be used for both sample and standard materials. Precision of the method has not been determined.
Accuracy of the method has not been determined. Sample preparation apparatus requirements will depend upon the sample type
under consideration and the kind of XRD analysis to be performed.
• Mortar and Pestle: Agate or porcelain.
• Razor Blades
• Sample Mill: SPEX, Inc., freezer mill or
equivalent.
• Bulk Sample Holders
• Silver Membrane Filters: 25-mm diameter,
0.45-μ m pore size. Selas Corp. of America, Flotronics Div., 1957 Pioneer Road,
Huntington Valley, PA 19006.
• Microscope Slides
• Vacuum Filtration Apparatus: Gelman No. 1107
or equivalent, and side-arm vacuum flask.
• Microbalance
• Ultrasonic Bath or Probe: Model W140,
Ultrasonics, Inc., operated at a power density of approximately 0.1 W/mL, or
equivalent.
• Volumetric Flasks: 1-L volume.
• Assorted Pipettes
• Pipette Bulb
• Nonserrated Forceps
• Polyethylene Wash Bottle
• Pyrex Beakers: 50-mL volume.
• Desiccator
• Filter Storage Cassettes
• Magnetic Stirring Plate and Bars
• Porcelain Crucibles
• Muffle Furnace or Low Temperature Asher
Sample analysis requirements include an X-ray diffraction unit, equipped
with:
• Constant Potential Generator; Voltage and mA
Stabilizers
• Automated Diffractometer with Step-Scanning
Mode
• Copper Target X-Ray Tube: High intensity,
fine focus, preferably.
• X-Ray Pulse Height Selector
• X-Ray Detector (with high voltage power
supply): Scintillation or proportional counter.
• Focusing Graphite Crystal Monochromator; or
Nickel Filter (if copper source is used, and iron fluorescence is not a
serious problem).
• Data Output Accessories:
• Sample Spinner (optional).
• Instrument Calibration Reference Specimen:
α-quartz reference crystal (Arkansas quartz standard, #180-147-00, Philips
Electronics Instruments, Inc., 85 McKee Drive, Mahwah, NJ 07430) or equivalent.
The reference materials listed below are intended to serve as a guide. Every
attempt should be made to acquire pure reference materials that are comparable
to sample materials being analyzed.
• Chrysotile: UICC Canadian, or NIEHS
Plastibest. (UICC reference materials available from: UICC, MRC Pneumoconiosis
Unit, Llandough Hospital, Penarth, Glamorgan, CF61XW, UK).
• Crocidolite: UICC
• Amosite: UICC
• Anthophyllite: UICC
• Tremolite Asbestos: Wards Natural Science
Establishment, Rochester, N.Y.; Cyprus Research Standard, Cyprus Research, 2435
Military Ave., Los Angeles, CA 90064 (washed with dilute HCl to remove small
amount of calcite impurity); India tremolite, Rajasthan State, India.
• Actinolite Asbestos Tape, petroleum jelly, etc. (for attaching silver membrane filters to sample
holders). 1 percent aerosol OT aqueous solution or equivalent. ACS Reagent Grade. Samples for analysis of asbestos content shall be collected as specified in
EPA Guidance Document #C0090, Asbestos-Containing Materials in School
Buildings. 10 All samples must be analyzed initially for asbestos content by PLM. XRD
should be used as an auxiliary method when a second, independent analysis is
requested.
Note: Asbestos is a toxic substance. All handling of dry materials
should be performed in an operating fume hood.
The method of sample preparation required for XRD analysis will depend on:
(1) The condition of the sample received (sample size, homogeneity, particle
size distribution, and overall composition as determined by PLM); and (2) the
type of XRD analysis to be performed (qualitative, quantitative, thin layer or
bulk).
Bulk materials are usually received as inhomogeneous mixtures of complex
composition with very wide particle size distributions. Preparation of a
homogeneous, representative sample from asbestos-containing materials is
particularly difficult because the fibrous nature of the asbestos minerals
inhibits mechanical mixing and stirring, and because milling procedures may
cause adverse lattice alterations.
A discussion of specific matrix reduction procedures is given below. Complete
methods of sample preparation are detailed in Sections 2.7.2.2 and 2.7.2.3.
Note: All samples should be examined microscopically before and after
each matrix reduction step to monitor changes in sample particle size,
composition, and crystallinity, and to ensure sample representativeness and
homogeneity for analysis.
2.7.2.1.1 Milling -- Mechanical
milling of asbestos materials has been shown to decrease fiber crystallinity,
with a resultant decrease in diffraction intensity of the specimen; the degree
of lattice alteration is related to the duration and type of milling
process.19,&thnsp≧22 Therefore, all milling times should
be kept to a minimum.
For qualitative analysis, particle size is not usually of critical
importance and initial characterization of the material with a minimum of matrix
reduction is often desirable to document the composition of the sample as
received. Bulk samples of very large particle size (>2-3 mm) should be
comminuted to ∼100 μm. A mortar and pestle can sometimes be used in size
reduction of soft or loosely bound materials though this may cause matting of
some samples. Such samples may be reduced by cutting with a razor blade in a
mortar, or by grinding in a suitable mill (e.g., a microhammer mill or
equivalent). When using a mortar for grinding or cutting, the sample should be
moistened with ethanol, or some other suitable wetting agent, to minimize
exposures.
For accurate, reproducible quantitative analysis, the particle size of
both sample and standard materials should be reduced to ∼10 μm (see Section
2.3.3). Dry ball milling at liquid nitrogen temperatures (e.g., Spex Freezer
Mill, or equivalent) for a maximum time of 10 min. is recommended to obtain
satisfactory particle size distributions while protecting the integrity of the
crystal lattice. 5 Bulk samples of very large particle size may
require grinding in two stages for full matrix reduction to <10 μm. 8,
16
Final particle size distributions should always be verified by optical
microscopy or another suitable method.
2.7.2.1.2 Low temperature ashing --
For materials shown by PLM to contain large amounts of gypsum, cellulosic,
or other organic materials, it may be desirable to ash the samples prior to
analysis to reduce background radiation or matrix interference. Since chrysotile
undergoes dehydroxylation at temperatures between 550 °C and and 650 °C, with
subsequent transformation to forsterite, 23, 24 ashing temperatures
should be kept below 500 °C. Use of a low temperature asher is recommended. In
all cases, calibration of the oven is essential to ensure that a maximum ashing
temperature of 500 °C is not exceeded.
2.7.2.1.3 Acid leaching -- Because
of the interference caused by gypsum and some carbonates in the detection of
asbestiform minerals by XRD (see Section 2.3.1), it may be necessary to remove
these interferents by a simple acid leaching procedure prior to analysis (see
Section 1.7.2.2). 2.7.2.2.1 Initial screening of bulk
material -- Qualitative analysis should be performed on a representative,
homogeneous portion of the sample with a minimum of sample treatment.
1. Grind and mix the sample with a mortar and pestle (or equivalent method,
see Section 2.7.2.1.1.) to a final particle size sufficiently small (∼100 μm) to
allow adequate packing into the sample holder.
2. Pack the sample into a standard bulk sample holder. Care should be taken
to ensure that a representative portion of the milled sample is selected for
analysis. Particular care should be taken to avoid possible size segregation of
the sample. (Note: Use of a back-packing method 25 of bulk sample
preparation may reduce preferred orientation effects.)
3. Mount the sample on the diffractometer and scan over the diagnostic peak
regions for the serpentine (∼67.4 A 4. Submit all samples that exhibit diffraction peaks in the diagnostic
regions for asbestiform minerals to a full qualitative XRD scan (5°-60° 2 5. Compare the sample XRD pattern with standard reference powder diffraction
patterns (i.e., JCPDS powder diffraction data 3 or those of other
well-characterized reference materials). Principal lattice spacings of
asbestiform minerals are given in Table 2-2; common constituents of bulk
insulation and wall materials are listed in Table 2-3.
2.7.2.2.2 Detection of minor or trace
constituents -- Routine screening of bulk materials by XRD may fail to
detect small concentrations (<5 percent) of asbestos. The limits of detection
will, in general, be improved if matrix absorption effects are minimized, and if
the sample particle size is reduced to the optimal 1 to 10 μm range, provided
that the crystal lattice is not degraded in the milling process. Therefore, in
those instances where confirmation of the presence of an asbestiform mineral at
very low levels is required, or where a negative result from initial screening
of the bulk material by XRD (see Section 2.7.2.2.1) is in conflict with previous
PLM results, it may be desirable to prepare the sample as described for
quantitative analysis (see Section 2.7.2.3) and step-scan over appropriate 2 The proposed method for quantitation of asbestos in bulk samples is a
modification of the NIOSH-recommended thin-layer method for chrysotile in air.
5 A thick-layer or bulk method involving pelletizing the sample may
be used for semiquantitative analysis; 7,8 however, this method
requires the addition of an internal standard, use of a specially fabricated
sample press, and relatively large amounts of standard reference materials.
Additional research is required to evaluate the comparability of thin- and
thick-layer methods for quantitative asbestos analysis.
For quantitative analysis by thin-layer methods, the following procedure is
recommended:
1. Mill and size all or a substantial representative portion of the sample as
outlined in Section 2.7.2.1.1.
2. Dry at 100 °C for 2 hr; cool in a desiccator.
3. Weigh accurately to the nearest 0.01 mg.
4. Samples shown by PLM to contain large amounts of cellulosic or other
organic materials, gypsum, or carbonates, should be submitted to appropriate
matrix reduction procedures described in Sections 2.7.2.1.2 and 2.7.2.1.3. After
ashing and/or acid treatment, repeat the drying and weighing procedures
described above, and determine the percent weight loss; L.
5. Quantitatively transfer an accurately weighed amount (50-100 mg) of the
sample to a 1-L volumetric flask with approximately 200 mL isopropanol to which
3 to 4 drops of surfactant have been added.
6. Ultrasonicate for 10 min at a power density of approximately 0.1 W/mL, to
disperse the sample material.
7. Dilute to volume with isopropanol.
8. Place flask on a magnetic stirring plate. Stir.
9. Place a silver membrane filter on the filtration apparatus, apply a
vacuum, and attach the reservoir. Release the vacuum and add several milliliters
of isopropanol to the reservoir. Vigorously hand shake the asbestos suspension
and immediately withdraw an aliquot from the center of the suspension so that
total sample weight, W 10. Attach the filter to a flat holder with a suitable adhesive and place on
the diffractometer. Use of a sample spinner is recommended.
11. For each asbestos mineral to be quantitated select a reflection (or
reflections) that has been shown to be free from interferences by prior PLM or
qualitative XRD analysis and that can be used unambiguously as an index of the
amount of material present in the sample (see Table 2-2).
12. Analyze the selected diagnostic reflection(s) by step scanning in
increments of 0.02° 2 13. Determine the net count, I 14. Normalize all raw, net intensities (to correct for instrument
instabilities) by referencing them to an external standard (e.g., the 3.34 A
1. Mill and size standard asbestos materials according to the procedure
outlined in Section 2.7.2.1.1. Equivalent, standardized matrix reduction and
sizing techniques should be used for both standard and sample materials.
2. Dry at 100 °C for 2 hr; cool in a desiccator.
3. Prepare two suspensions of each standard in isopropanol by weighing
approximately 10 and 50 mg of the dry material to the nearest 0.01 mg.
Quantitatively transfer each to a 1-L volumetric flask with approximately 200 mL
isopropanol to which a few drops of surfactant have been added.
4. Ultrasonicate for 10 min at a power density of approximately 0.1 W/mL, to
disperse the asbestos material.
5. Dilute to volume with isopropanol.
6. Place the flask on a magnetic stirring plate. Stir.
7. Prepare, in triplicate, a series of at least five standard filters to
cover the desired analytical range, using appropriate aliquots of the 10 and 50
mg/L suspensions and the following procedure.
Mount a silver membrane filter on the filtration apparatus. Place a few
milliliters of isopropanol in the reservoir. Vigorously hand shake the asbestos
suspension and immediately withdraw an aliquot from the center of the
suspension. Do not adjust the volume in the pipet by expelling part of the
suspension; if more than the desired aliquot is withdrawn, discard the aliquot
and resume the procedure with a clean pipet. Transfer the aliquot to the
reservoir. Keep the tip of the pipet near the surface of the isopropanol. Filter
rapidly under vacuum. Do not wash the sides of the reservoir. Leave the vacuum
on for a time sufficient to dry the filter. Release the vacuum and remove the
filter with forceps. 1. Mount each filter on a flat holder. Perform step scans on selected
diagnostic reflections of the standards and reference specimen using the
procedure outlined in Section 2.7.2.3, step 12, and the same conditions as those
used for the samples.
2. Determine the normalized intensity for each peak measured, I For each asbestos reference material, calculate the exact weight deposited on
each standard filter from the concentrations of the standard suspensions and
aliquot volumes. Record the weight, w, of each standard. Prepare a calibration
curve by regressing I Determine the slope, m, of the calibration curve in counts/microgram. The
intercept, b, of the line with the I Using the normalized intensity, I
Table 1-1_Optical Properties of Asbestoc Fibers
----------------------------------------------------------------------------------------------------------------
Refrac- tive indices \b\
Mineral Morphology, color ----------------------------- Birefring- Extinction Sign of
\a\ [alpha] [gamma] ence elonation
----------------------------------------------------------------------------------------------------------------
Chrysotile Wavy fibers. Fiber 1.493-1.560 1.517-1.562\f\ .008 [verbar] to +
(asbestiform bundles have (normally fiber length. (length
serpentine). splayed ends and 1.556). slow)
``kinks''. Aspect
ratio typically
>10:1. Colorless
\3\, nonpleochroic.
Amosite Straight, rigid 1.635-1.696 1.655-1.729 .020-.033 [verbar] to +
(asbestiform fibers. Aspect \f\ (normally fiber length. (length
grunerite). ratio typically 1.696-1.710. slow)
>10:1. Colorless
to brown,
nonpleochroic or
weakly so. Opaque
inclusions may be
present.
Crocidolite Straight, rigid 1.654-1.701 1.668- .014-.016 [verbar] to -
(asbestiform fibers. Thick 1.717\3e\ fiber length. (length
Riebeckite). fibers and bundles (normally fast)
common, blue to close to
purple-blue in 1.700).
color. Pleochroic.
Birefringence is
generally masked by
blue color.
Anthophyllite- Straight fibers and 1.596-1.652 1.615-1.676 .019-.024 [verbar] to +
asbestos. acicular cleavage \f\. fiber length. (length
fragments.\d\ Some slow)
composite fibers.
Aspect ratio
<10:1. Colorless
to light brown.
Tremolite- Normally present as 1.599-1.668 1.622-1.688 .023-.020 Oblique +
actinolite- acicular or \f\. extinction, (length
asbestos. prismatic cleavage 10-20° slow)
fragments.\d\ for
Single crystals fragments.
predominate, aspect Composite
ratio <10:1. fibers
Colorless to pale show[verbar]
green. extinction.
----------------------------------------------------------------------------------------------------------------
\a\ From reference 5; colors cited are seen by observation with plane polarized light.
\b\ From references 5 and 8.
\c\ Fibers subjected to heating may be brownish.
\d\ Fibers defined as having aspect ratio >3:1.
\e\ to fiber length.
\f\ [verbar]To fiber length.
Table 1-2_Central Stop Dispersion Staining Colors \a\
----------------------------------------------------------------------------------------------------------------
Mineral RI Liquid [eta] [eta][verbar]
----------------------------------------------------------------------------------------------------------------
Chrysotile............................... 1.550 \HD\ Blue....................... Blue-magenta
Amosite.................................. 1.680 Blue-magenta to pale blue.. Golden-yellow
1.550\HD\ Yellow to white............ Yellow to white
Crocidolite \b\.......................... 1.700 Red magenta................ Blue-magenta
1.550\HD\ Yellow to white............ Yellow to white
Anthophyllite............................ 1.605\HD\ Blue....................... Gold to gold-magenta
Tremolite................................ 1.605\HD c\ Pale blue.................. Gold
Actinolite............................... 1.605\HD\ Gold-magenta to blue....... Gold
1.630\HD c\ Magenta.................... Golden-yellow
----------------------------------------------------------------------------------------------------------------
\a\ From reference 9.
\b\ Blue absorption color.
\c\ Oblique extinction view.
SECTION 2, X-RAY POWDER DIFFRACTION
Table 2-1_The Asbestos Minerals and Their Nonasbestiform Analogs
------------------------------------------------------------------------
Asbestiform Nonasbestiform
------------------------------------------------------------------------
SERPENTINE ............................
Chrysotile Antigorite, lizardite
AMPHIBOLE ............................
Anthophyllite asbestos Anthophyllite
Cummingtonite-grunerite asbestos Cummingtonite-grunerite
(``Amosite'')
Crocidolite Riebeckite
Tremolite asbestos Tremolite
Actinolite asbestos Actinolite
------------------------------------------------------------------------
Table 2-2_Principal Lattice Spacings of Asbestiform Minerals [SU]a[/SU]
----------------------------------------------------------------------------------------------------------------
Principal d-spacings (A) and relative
intensities JCPDS Powder diffraction file
Minerals ---------------------------------------- \3\ number
----------------------------------------------------------------------------------------------------------------
Chrysotile............................... 7.37[INF]100 3.65[INF]70 4.57[INF]50 21-543[SU]b[/SU]
[/INF] [/INF] [/INF] 25-645
7.36[INF]100 3.66[INF]80 2.45[INF]65 22-1162 (theoretical)
[/INF] [/INF] [/INF]
7.10[INF]100 2.33[INF]80 3.55[INF]70
[/INF] [/INF] [/INF]
``Amosite''.............................. 8.33[INF]100 3.06[INF]70 2.756[INF]7 17-745 (nonfibrous)
[/INF] [/INF] 0[/INF] 27-1170 (UICC)
8.22[INF]100 3.060[INF]8 3.25[INF]70
[/INF] 5[/INF] [/INF]
Anthophyllite............................ 3.05[INF]100 3.24[INF]60 8.26[INF]55 9-455
[/INF] [/INF] [/INF] 16-401 (synthetic)
3.06[INF]100 8.33[INF]70 3.23[INF]50
[/INF] [/INF] [/INF]
Anthophyllite............................ 2.72[INF]100 2.54[INF]10 3.480[INF]8 25-157
[/INF] 0[/INF] 0[/INF]
Crocidolite.............................. 8.35[INF]100 3.10[INF]55 2.720[INF]3 27-1415 (UICC)
[/INF] [/INF] 5[/INF]
Tremolite................................ 8.38[INF]100 3.12[INF]10 2.705[INF]9 13-437[SU]b[/SU]
[/INF] 0[/INF] 0[/INF] 20-1310[SU]b[/SU] (synthetic)
2.706[INF]10 3.14[INF]95 8.43[INF]40 23-666 (synthetic mixture
0[/INF] [/INF] [/INF] with richterite)
3.13[INF]100 2.706[INF]6 8.44[INF]40
[/INF] 0[/INF] [/INF]
----------------------------------------------------------------------------------------------------------------
[SU]a[/SU] This information is intended as a guide, only. Complete powder diffraction data, including mineral
type and source, should be referred to, to ensure comparability of sample and reference materials where
possible. Additional precision XRD data on amosite, crocidolite, tremolite, and chrysotile are available from
the U.S. Bureaus of Mines.\4\
[SU]b[/SU] Fibrosity questionable.
TABLE 2-3 -- COMMON CONSTITUENTS IN INSULATION AND WALL
MATERIALS
Table 2-4_Interferences in XRD Analysis Asbestiform Minerals
------------------------------------------------------------------------
Primary
diagnostic
peaks
Asbestiform mineral (approximate Interference
d-spacings,
in A)
------------------------------------------------------------------------
Serpentine ............
Chrysotile 7.4 Nonasbestiform
serpentines
(antigorite,
lizardite)
Chlorite
Kaolinite
Gypsum
3.7 Chlorite
Halloysite
Cellulose
Amphibole ............
``Amosite'' 3.1 Nonasbestiform
Anthophyllite [rcub3] amphiboles
Crocidolite (cummingtonite-
Tremolite grunerite,
anthophyllite,
riebeckite,
tremolite)
Mutual interferences
Carbonates
Talc
8.3 Mutual interferences
------------------------------------------------------------------------
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