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STANDARD PROCEDURES
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The purpose of this section is to consider the principle steps of the active standard analytical
procedures emphasizing the function rather than the mechanics of the procedure. Each procedure will be
examined; then they will be compared. Bulk sample treatment will be considered first. There are no
certified procedures for bulk analysis of crystalline silica minerals, but there are many usable general
procedures described in
the literature.
The Analysis of Bulk Samples
The analysis of bulk samples for crystalline silica phases is essentially an extension of the usual
quantitative procedures reported in the literature. There is no certified procedure established by any of the
federal agencies because the samples to be analyzed have considerable variability in the associated phases,
although a very elaborate procedure for analyzing soils which include quartz has been presented by Raab
(1988). The NIOSH filter method has been used to analyze bulk materials, but it is not really suitable for
the low concentrations as required by OSHA (0.1% quartz) (Blount, 1989) because the sample is not large
enough to allow the detection and quantification of this amount. It should also be remembered that
crystallite statistics strongly favor the bulk sample because of the larger number of crystallites in the
sample.
An excellent review of quantitative procedures has been presented by Snyder and Bish (1989) and
a thesis by Cline (1986). The references in these articles cover most of the available literature, and the
paper discusses all the main techniques in use today. As mentioned above, there are essentially three
methods which are in use: the absorption correction method, the internal standard method and the external
standard method. There are also three ways to acquire and process the data in each method. Where
applicable, individual peaks may be integrated for each phase, Alexander and Klug (1948). If individual
peaks are not resolvable, then clusters of peaks may be used and the contribution of each phase to each
cluster may be treated as a matrix, Copeland and Bragg (1958), Karlak and Burnett (1966), Chung (1974a
and b, 1975), and Smith, et al. (1984). If the overlap is severe, the whole-pattern methods may be used,
Wiles and Young (1981), Smith et al. (1987, 1988a and b), Hill and Howard (1987), Bish and Howard
(1988), Madsen and Hill (1988), O'Connor and Rowan (1988) and Taylor (1991).
There are several methods mentioned in the literature that specifically involve the determination
of quartz in bulk samples. The first description is by Clark and Reynolds (1936), and Klug et al. (1948)
first apply modern theory to silica analysis. The reviews by McGlynn (1968) and Anderson (1975) cover
much of the earlier literature. More recent studies will be indicated here. The analysis of quartz and other
minerals in rocks is discussed by Pawloski (1985) and Bayliss (1986). Both use the external reference-intensity method. Fluerence et al. (1969) analyze raw materials and ceramics. Waersted (1986) has used
the RIR method to determine quartz in calcium sulfate preparations. Kamarchik (1980) describes the
determination of quartz, talc and clay in paints using the RIR method with TiO2 as the reference material.
Davis et al. (1986) has analyzed coal samples using the RIR method coupled with absorption
measurements to allow for the fact that not all the phases are determined and the coal component is
amorphous. Emig and Smith (1989) have quantified quartz in agricultural dolostone using direct
calibration methods because of the minimal interference from the other minerals in the rock. Corrections
were required when mica was present. Carter et al. (1987) have quantified both quartz and cristobalite in
bentonite products using an internal standard method with absorption corrections to allow for the presence
of other phases. McKee et al. (19__) and Hamilton and Peletis (1991) determined quartz in perlite using
spiking with quartz to prepare the calibration curves.
Whole pattern analysis has also been used for some quartz analyses, and as this technique
becomes better known, it will be employed more frequently. Smith et al. (1989) used whole pattern fitting
to analyze quartz and other minerals in sedimentary rocks. Jordan et al. (1990) applied the Rietveld
method to analyzing quartz specimens for the crystalline fraction. This study applies specifically to the
question of the presence of amorphous silica in quartz samples.
It is evident that there are many procedures which could be developed for the determination of
quartz in bulk samples. An extension of the above techniques is to employ additional external information
as constraints on the X-ray analysis. Renault (1987), Goehner (1982) and Garbauskas and Goehner (1982)
and Smith et al. (1989) have used chemistry as a constraint on the reduction of the X-ray data. This
addition leads to better reconciliation of the physical and chemical data which often causes doubts on the
validity of the X-ray results when the two do not agree.
Quartz Analyses using Membrane Filters
NIOSH Method 7500, OSHA/SLC ID-142 July 1989 This method is designed to quantify quartz
and cristobalite collected on membrane filters by air samplers or aerosol chambers. It is primarily used for
monitoring crystalline silica in atmospheric dust but has been applied to bulk samples as well. Specific
collecting conditions and apparatus are specified for the atmospheric samplers. The particles are collected
on 37 mm diameter polyvinylchloride, PVC, membrane filters which are then transferred to the analytical
laboratory. The PVC filters are then dissolved in tetrahydrofuran, THF, to release the particles which are
then transferred to a 25 mm silver membrane filter. This step concentrates the particles to a smaller area
and distributes them as an evenly-spaced particle layer for the X-ray diffractometer. This filter and its load
are then transferred to the diffractometer for measurement. The desired amount of sample is 2 mg or less.
The basis for this analysis have been discussed by Dollberg et al. (1980) and Abell et al. (1981a).
The X-ray measurement involves the integration of specific peaks for quartz, cristobalite and
silver. The process may be run in automated mode (Abell et al., 1978) both to collect the data from each
sample following specific steps and interrogate the information for deciding how to continue with the
sample. This program will also process many samples in sequence. The silver peak is scanned to
determine the intensity attenuation compared to a clean filter. The attenuation effect allows the thickness of
the sample layer to be calculated, and if the thickness is acceptable, at least three peaks for quartz (and/or
cristobalite) are scanned. The strongest peak from quartz is measured first to set the count time for all the
peaks.
The integrated intensities are converted to weights of quartz or cristobalite using calibration
curves from prepared filters with weighed amounts of silica deposited. Standard samples are interspersed
with the analytical samples to monitor instrument drift. The weight fractions are determined from the total
weight of the sample determined from pre- and post-weighings of the filter during preparation. The
detection limits are reported to be 5 æg quartz and 10 µg (*) cristobalite for qualitative determinations and
10 æg quartz and 30 µg (*) cristobalite for quantitative determinations.
Bulk samples are analyzed by crushing to pass 325 mesh and then taking an aliquot for further
processing. When the size range is right, the material is transferred to the silver filter in the same manner
as for the calibration procedures. It is now treated in the standard fashion.
HSE Method MDHS 5 1/2, March 1988 This method is reported only for the determination of
quartz in airborne dusts, but the principles could apply to the determination of cristobalite with appropriate
changes. The method uses the 25 mm diameter PVC or PVC-acrylonitrile copolymer filter in the
collection device which is transferred directly to the X-ray diffractometer. Sampling conditions are
prescribed. The amount of sample is determined by pre- and post-weighings of the filter. Dust loadings
should be kept below 2 mg to prevent loss of dust from the surface of the filter cake during handling and to
keep the intensity response versus weight of quartz in the linear range. If the sample is too large, or if there
are interfering phases present, an absorption correction may be necessary according to equation 1. The
four principle peaks of quartz are monitored depending on the interfering phases which are present as
determined by a fast diffractometer scan.
Calibration is performed by preparing standard samples in an aerosol chamber by creating a cloud
of airborne quartz dust with an air jet then letting the large particles settle. Four samplers are attached to
the cloud chamber. The suspended particles are then drawn through the samplers for prescribed times.
The amount of quartz deposited is determined by weight. Progressively longer sampling times create a
series of calibration samples. These samples are then measured in the same way that the analytical samples
are to be measured. The diffractometer system is monitored by a drift sample such as aluminum.
Dow Chemical Company This method (Henslee and Guerra, 1977) uses PVC filters both to collect
the sample and to support it in the X-ray diffractometer. A 25 mm diameter orifice (Gebhardt, 1975) is
used in the sampler to restrict the effective part of the filter, and the effective 25 mm circle is cut from the
37 mm filter and mounted on a circular holder that fits the diffractometer spinner. This approach
eliminates the need to transfer the sample.
Calibration samples are prepared by crushing quartz to pass a 400 mesh sieve and then settling the
particles in ethanol to separate the larger particles and create a sample with a mean size of 5µm. A
weighed amount of this sample was dispersed in water and then diluted to a fixed concentration. Aliquots
of this suspension were then diluted further to specific concentrations and the suspension then filtered
through a PVC filter. The actual amount deposited was determined by pre- and post-deposition weights.
The intensity calibration curve was linear over the range 0 - 100 µg/ml. Addition of 20% iron oxides did
not affect the magnitude or linearity of the intensity response per unit weight of quartz showing that the
iron oxide did not mask the quartz.
U. S. Bureau of Mines, IR-1021, 1975 This method (Freedman, 1972; Thatcher, 1975) collects the
dust on 37 mm PVC filters and then transfers the particles to silver membranes by a procedure very close
to the NIOSH method. A 1.5% solution of parlodion in amyl acetate may be used to cement heavy dust
deposits to the silver filter. The attenuation of the silver peak is used to determine the sample absorption
and correct the intensity measurement.
NIOSH Method 7501 for Amorphous Silica As mentioned above, the determination of the
amorphous content of a sample is not possible without special processing. Method 7501 uses the property
that most amorphous forms of silica will convert to cristobalite with heat treatment. The X-ray analysis
steps are similar to Method 7500, but the sample is ashed and fired to 1100 or 1500°C depending on the
type of amorphous silica suspected prior to final transfer to the silver filter. The sample is then analyzed
for cristobalite. The method depends on the quantitative conversion of the amorphous form to cristobalite
without reacting with anything else in the sample.
Other Reported Methods There are several other reports of the use of filter methods for the
quantification of quartz. Most of these methods describe the use of the filter method for bulk analysis or
analysis where sufficient dust is available for direct processing. Some allow the addition of an internal
standard by co-dispersing the phases before depositing on the filter. The methods include reports by
Murray and Merkl (19__), Malik and Viswanathan (19__), Clayton Environmental Consultants (19__),
Bumsted et al. (19__), and Davis and Johnson (1982a and b). Other methods are reported in papers by
Schliephake (1963), Schmelzer (1951, 1955), Plowman (1978), O'Connor and Joklevic (1981) and Kudo
(1982).
Several authors have been specifically interested in the determination of cristobalite. The NIOSH
Method 7500 does include cristobalite in the procedure and most of the other filter methods could be easily
modified to include cristobalite. Kupka (1967), and Stefanov (1972) describe methods to quantify
cristobalite in rocks. Carter et al. (1980) quantifies cristobalite in bentonite clays. Janko et al. (1989)
determine the cristobalite content of dusts in a ceramic plant where it occurs in mullite products and in the
atmospheres in the plant.
The determination of cristobalite is not as straightforward as the determination of quartz because
its structure is more variable than quartz. The true structure is based on a large unit cell which is a
supercell of the simple cubic arrangement usually used to describe cristobalite. The structure lends itself to
several types of disorder including distortions which create small domains and stacking faults which lead to
some tridymite character. The effect of these defects is to cause the peaks to broaden and shift and change
in intensity. Usually the intensity change of the principal peak (101) is less affected than the weak peaks,
so quantification is little affected when the peak is integrated by the usual techniques.
Comparisons of Filter Methods
Selection of the Filter Material There is considerable controversy concerning the most appropriate
filter for collecting and supporting the dust samples. The organic filters allow the recovery of the particles
where the procedure calls for the reconcentration of the particles because the filter is soluble in
tetrahydrofuran or other solvent. Where the filters are large, 37 mm, the particles usually require
processing to redistribute the particles over a smaller area for effective coverage by the X-ray beam. PVC,
polyvinylchloride; PVCA, polyvinylchloride acrylonitrile; Nucleopore polycarbonate, PC; and MCE,
mixed cellulose ester have been used for filters. These filters may also be used for direct analysis. MCE
filters has the advantage in the X-ray beam of being of low absorption and low scattered background, thus
allowing weak diffracted peaks from low particle loadings to be detected. The silver filters and the MCE
filters are reported to produce a lower background in the diffraction pattern than the PVC filters.
Evaluations of filters have been done by Mark (1974), Henslee and Guerra (1977), Altree-Williams et al.
(1977), Chung (1978), Dobreva et al. (1982), Davis and Johnson (1982a and b), Foster and Walker (1984)
and Knight (1984).
The silver filters are used both for collection when the sample is to be analyzed directly and as the
substrate when the particles are transferred from an organic filter. One advantage of the silver filters is that
they hold the particles better than the organic filters because of their structure, but particles are drawn into
the interior of the filter where they are masked from the X-ray beam by the surrounding silver. Thus, a
correction factor must be applied for this masking. Another advantage is the crystallinity of the silver
which allows the intensity of its diffraction peak to be monitored. If the sample is thicker than one particle
layer, the attenuation of the silver intensity provides the sample absorption coefficient. Where the PVC or
MCE filters are used for direct analysis, they can be placed over a crystalline support like a silver,
aluminum or zinc sample holder to provide the diffracted beam needed to measure the sample attenuation.
Greases may be used to hold the filters in place, but the grease may contribute to the diffraction pattern and
to the attenuation of the substrate peak intensity.
Direct-on-Filter Methods versus Transfer Methods Several laboratories have preferred to use
direct-on-filter methods because of the convenience of simple processing, more rapid analysis and a
concommitment improvement in the costs involved. The standard NIOSH Method 7500 is very time
consuming, but it is considered to be the accepted legal technique, so all other methods must be compared
with it. A very recent evaluation of the direct-on-filter methods was performed by Lorberau et al. (19__).
The results of this evaluation showed that the agreement between the NIOSH Method 7500 and the MCE
filter method agreed well, but that there was a slight bias in the comparison of the NIOSH method and the
silver filter. This bias was thought to be due to a mismatch in particle size between the samples tested, but
it might also be due to masking by the silver in the filter.
One of the main drawbacks of the use of direct-on-filter analysis is that the original distribution of
particles is spread over the full 37 mm diameter filter. This fact is the primary reason for the concentration
step incorporated in the transfer methods. Bradley (1967), Leroux and Powers (1969a and b), Crosby and
Hamer (1971), Knight et al. (1971, 1972), Leroux et al. (1972, 1973), Knight (1975, 1986), Altree-Williams et al. (1977), Frevel and Roth (1982) and Kohyama (1985) used direct deposition on silver filters.
Gebhardt (1975) modified the personnel samplers to concentrate the particles in the center of the filter by
using a aperture with a 25 mm diameter. Others have used this same modification. Chung (1978) and
Henslee and Guerra (1977) have examined by direct imaging the distribution of particles collected on
filters and showed that personnel samplers do lead to uniform dispersions if run for sufficient time and if
the atmosphere is not changing. There is some question whether liquid suspension filtration used to
prepare calibration standards yields as uniform a deposit as do the personnel samplers. Spinning the
sample during diffraction helps even out the irregularities which do occur. In theory, the particles should
be most likely to attach to the filter where the flow rate is the most active, i.e. where no other particles are
blocking the passages. This effect should tend to give a uniform distribution of particles as the loading
increases.
Calibration
All the methods of quantitative analysis require calibration standards and calibration procedures
and tests. For the on-filter methods the particle sizes as well as the minerals must correspond to the
samples under study. Obtaining such samples is not easy.
Procedures Because the on-filter methods do not usually use internal or external standards, the
standard approach for calibration is to prepare filters loaded with known amounts of silica and establish
calibration graphs of intensity response of the specific diffractometer versus weight of silica. The silica
should be deposited on the filters in essentially the same manner as for the analyte samples. Elaborate
aerosol chambers have been designed for creating uniform deposits by Davis and Johnson (1982b), Davis
(1986) and Carsey (1987). Most of the standard procedures prescribe the methods to follow for making the
reference standards.
Basically the technique is to disperse the calibrant in a suspension with either air or water and then
dilute the suspension to the desired level. The diluted aliquot is then forced through the filter either by
vacuum or by pressure. Weighing the filter before and after deposition confirms the loading. The main
purpose of the selected procedure is to produce a deposit which is uniform and crystallographically
random. Fortunately, the silica minerals show little tendency to orient except on very smooth filters
(Edmonds et al. 1977).
For bulk samples, the preparation of calibration samples is usually the problem of thoroughly
mixing a weighed set of ingredients. Both dry mixing and wet mixing may be used. Individual
components should be ground to the desired particle size prior to mixing because size reduction in a
mixture usually leads to unsatisfactory results. Except for purity, the materials to be used for the
calibrations are probably not as critical as for the filter standards, especially the particle size distribution.
Quartz Very high purity quartz is easily obtained in highly perfect crystals. The question is
whether this source is proper for use as a quantitative reference material. The difficulty is that the
perfection of the crystallites even after crushing leads to significant extinction effects. Where the quartz in
the analyte is natural and coarse grained, this reference is probably acceptable if the crystallite size is
equivalent. However, if the quartz is a recrystallization product like chert or produced by reaction, the
crystallites may be strained and imperfect altering the intensity response. Simply crushing coarse quartz
will not produce the same effect. Thus, the selection of the proper quartz is not always simple. Kacsmar
and Tomb (1984) have reviewed suggested materials for quartz calibration.
For respirable quartz, the problem of a suitable quartz is even more acute. Many studies have
shown that the particle size distribution affects the intensity response. It is probably more important that
the size range be similar to the analyte size range than the precise distribution. There is only one certified
quartz standard available for respirable particle analysis. It is distributed by the Office of Standard
Reference Data, OSRD, of the National Institutes for Standards and Technology, NIST, as SRM-1878. It
has a range of 0.33 to 5.0 µm with an average size of 1.62 µm. It is certified as to purity at >95% quartz.
The NIST SRM's are very expensive because of the effort required to certify them. The NIST/OSRD
programs for certifying standards have been described by Hubbard (1982) and Dragoo (1986). Chung
(1982) suggests Min-U-Sil (Pennsylvania Glass Sand Corp.) as a source of quartz. Currently, 1991, Min-U-Sil is produced in large quantities at many different places and is no longer sufficiently uniform to be an
off-the-shelf standard.
Cristobalite OSRD also supplies a cristobalite as SRM-1879 certified as >98.0% crystalline.
Chung (1982) reports that cristobalite can also be made in the laboratory by starting with a silica gel or
Min-U-Sil and firing it in a platinum crucible at 1450C for up to 48 hours. The product is usually >99%
converted to cristobalite. Diffraction patterns of cristobalite made in this manner usually show some
crystallite size broadening indicating that the average crystallite size is less than 0.2 µm.
Tridymite There is no certified sample available for tridymite. Chung (1982) reports that
tridymite may be made by fusing Min-U-Sil in NaCl at 1100°C for 72 hours and then washing the product
in water to remove the NaCl. This tridymite must contain substituted Na to stabilize it, but it is suitable as
an X-ray standard.
* These values
reported in the original reference are in error. It is not clear which value is wrong, but the detection limit
seems high.
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