Evaluation of QXRPD as a Tool for Crystalline Silica Analysis
Compared with the alternative techniques for analyzing crystalline silica in bulk and respirable
samples, XRD is unique in its sensitivity to the specific crystalline phase or phases that may be present in
the sample. For identification applications to detect quartz and cristobalite and to determine the interfering
minerals, diffraction techniques are fast and easy to apply. For quantification applications, diffraction
methods have proved as accurate as any of the other methods available.
Recent comparisons with infrared by Pickard (1985) indicates that the performances of both
methods in terms of detection limits and precision is equivalent and acceptable for occupational hygiene
surveys at the present exposure limits. Anderson (1975) came to the conclusion that the X-ray diffraction
method was unacceptable because there were too many interferences. Other comparisons by Swallow
(1978, 1980) and Groff (1980) (quoted in Chung, 1982) using round-robin tests on samples prepared by
one laboratory and analyzed by 61 other labs indicate precisions that are generally unacceptable for all
methods. Huggens et al. (1985) describe another interagency test. No one method stands out as superior
to any other method. Precisions, å/x in %, are reported from 28 to 50 % for respirable quartz and 22 to 24
% for cristobalite in a bulk sample. These figures echo earlier reports by Freedman et al. (1974), Peters
(1976) and Nagelschmidt (1956). Chung (1982) summarizes the situation as follows: "[XRD] needs
validated procedures to attain precision; it needs certified primary standards to achieve accuracy." NIOSH
Method 7500 is now a validated procedure, and NIST SRM's 1878 and 1879 are certified primary
standards. However, this procedure and these standards are for only one specific analysis
that is for the
determination of respirable crystalline silica collected in a specific manner.
The objection to the NIOSH Method 7500 is its time-consuming preparation which precludes fast
and economic analyses. Many laboratories have proposed and tested alternate procedures which are more
economical and which compare favorable to Method 7500. It is apparent that further efforts are needed to
locate a more generally acceptable method. Until then, Method 7500 is the only method with any legal
significance. The precision and accuracy limits are related to the crystallite statistics discussed below. It
may be that the methods are already at the limit.
Specific Problems in QXRPD Analysis
Regardless of the relative accuracy of the XRD methods, its sensitivity to the specific silica phase
requires its use in many situations. This section will consider some of the experimental problems that have
been recognized in recent years.
Problems Specific to Filter Techniques The many studies involving membrane filters as the
collecting and sample supporting method have identified several problems. Different filters have different
efficiencies at collecting and retaining particles (Mark, 1974). The organic filters, PVC and MCE, have
one side which is smooth and one side which is rougher. The rough side retains particles better, but the
smaller particles may be drawn into the interior of the filter and partially masked from the X-ray beam
during the analysis. The smooth side is reported to lead to more reproducible results when used for
calibration mounts (Edmonds et al. 1977). Because of the heavy element, chlorine, the PVC filters have a
significant background. MCE filters have considerably lower backgrounds. Silver filters have low
backgrounds because of their crystalline nature, but they also exhibit masking effects. The great advantage
of silver is that it produces a diffraction peak that does not interfere with quartz or cristobalite. The
attenuation of this peak can be used to measure the absorption coefficient of the sample.
Some preferred orientation of quartz has been reported by Edmonds et al. (1977). Quartz does
have a tendency to fracture subparallel to (101) and may have a cleavage parallel to (100) (Frondel, 1962).
The presence of flat surfaces on crushed grains would tend to orient the particles where the substrate is
very smooth. This effect is more prominent with large particles, > 15 µm, and with freshly crushed quartz
than with small particles and precipitated quartz. The effect is strongest with small amounts of quartz on a
smooth filter.
The particle size distribution has proved to be very critical because of the amorphous layer on the
quartz particles. Lorberau et al. (1991) have illustrated the need to match the size distribution in the
calibrating material and the analyte. Even the SRM 1878 needs to be sieved to remove the >10 µm
particles. For samples prepared by dispersing small amounts of respirable-sized calibrant as a thin film on
a filter and then determining both the sample weight and the intensity response, a significant fraction of
amorphous component will strongly diminish the intensity response and alter the calibration curve.
Problems with Bulk Samples The problems of processing bulk samples containing quartz is
essentially the same as processing bulk samples in general, and there are many discussions of the
difficulties encountered. The two major problems are preparing a sample that is representative of the bulk
material and eliminating preferred orientation of the particles if they have a tendency to orient. The first is
common to all analytical procedures. An aliquot must be obtained that is equal in composition to the
material being analyzed whether it is a bin of raw materials or a small block of a product. This aliquot
must then be pulverized to < 5 æm particle sizes and split a into smaller aliquot for the diffraction analysis.
The preparation must not allow any particle separation because of different physical properties of the
different phases.
Eliminating orientation may be more difficult. There are several suggestions in the literature.
Smith and Barrett (1979), Jenkins et al. (1986) and Bish and Reynolds (1989) have reviewed sample
preparation for diffraction applications in general. Calvert et al. (1982) have compared several methods of
orientation reduction, some simple, some complex. The side-rifted method of loading sample holders is
shown to be the simplest effective method that works well in many cases. In severe cases spheroidizing
will be necessary. Smith et al. (1979a and b) describe the spheroidizing process in detail which is
patterned after a ceramic process called "spray drying". The concept is to create clusters of particles in a
spherical shape and then to pack the spheres randomly into a cavity mount. This technique is effective
regardless of the shape of the individual particles.
The potential accuracy obtainable from a bulk sample is related to the effective number of
particles in the X-ray beam. As seen in the section on crystallite statistics, the number of particles is
controlled by the absorption coefficient. The discussion considered a pure quartz sample, but if the quartz
is mixed with iron oxide, the effective number of particles is considerably reduced, and the accuracy is also
affected. Under ideal conditions using modern diffractometers with high resolution, the maximum
achievable accuracy is around 2% absolute.
Detection limits which are usually quoted are for conditions which are ideal, i.e. when there are no
interferences for the strongest peak of quartz or cristobalite. Fortunately, this situation does exist for many
analyses. Unfortunately, the most common contaminants such as mica and the clay minerals do interfere
with the (101) quartz peak and degrade the detection limit significantly as well as affect the quantification
accuracy for most analyses. It is difficult to determine a detection limit where interferences exist.
The amorphous surface phase on quartz particles is probably of little significance in most bulk
samples. The main reason is that it is really difficult to reduce the particle size to the range where the
surface layer becomes dominant. Also, freshly broken p
articles may not develop the layer immediately,
and the diffraction experiment may be completed before the layer forms.
Comments on Standardized Procedures for Respirable Silica
The standard NIOSH Method 7500 is the current legally accepted procedure for determining
respirable quartz and cristobalite, and all other methods which are used must be compared with this
method. What is the ultimate accuracy of the method? One of the factors in setting the limit of accuracy is
the crystal statistics. The analysis for the bulk sample showed that the effective volume of the sample is
around 20 mm3. For a respirable dust sample of 2 mg distributed evenly over a filter, the effective volume
of sample is 0.75 mm3. This volume is only 4% of the bulk volume, and the crystallite statistics are
affected accordingly. Assuming a 2 µm average particle size, the number of effective particles in the
sample is 1.8 x 108 and the number in diffraction is around 4000. At the 2.3 level, the accuracy is around
4% for this number of particles. This figure is an absolute accuracy regardless of the percentage of quartz
in the sample. As the percentage of quartz decreases, the relative accuracy increases significantly.
With this estimate of absolute accuracy, it is apparent that the X-ray diffraction method is already
near its limit with the present diffractometers. Several of the alternative procedures proposed to increase
the economics of running many samples agree with accuracies close to this 4% figure. How then can
accuracy be improved? The obvious direction is to increase the effective number of crystallites in
diffraction. Use of a larger sample leads to problems in particle retention and masking which requires
absorption corrections. It should be more effective to increase the range of diffraction to include more
crystallites. Most diffractometers are set up and aligned for maximum resolution. If a broad-focus X-ray
tube were used with coarse sollar slits and focal slits there would be a wider angular range for crystallites to
diffract. If the sample is also rocked a few degrees while it is spinning, the angular range would be
increased considerably. Because quantitative measurements are based on intensity, the loss in resolution
would be inconsequential, and the quantification accuracy could be improved considerably.
Bulk Sample Analysis
Standardized procedures for quantifying crystalline silica in bulk samples do not exist in the sense
of having a single recognized and accepted procedure which has legal status. However, many analytical
service laboratories have established procedures specific to their local needs depending on the types of
samples to be processed. The biggest single problem is the variability of the matrix in which the silica
forms occur and the interfering effects of the matrix compounds. If there are no interferences,
quantification down to the detection limit is possible by direct peak integration. If interferences are
present, then the procedure must be tailored to this interference. Two main approaches may be used, either
a correction is applied for the interfering phase of the interfering phase is removed from the sample prior to
the diffraction analysis.
Correction procedures for interfering phases are difficult to apply to diffraction data unless a
considerable amount of data is acquired. The interfering phase concentration must be determined by
diffraction effects in parts of the diffraction trace that are free of their own interferences and then the
contribution of this concentration to the silica peaks must be subtracted to obtain the silica quantification.
Propagation of errors in the individual abundances makes this approach undesirable. The use of whole-pattern fitting methods alleviates some of the difficulties of pattern overlap but not all. Profile fitting of
specific peaks also shows some promise but cannot resolve peaks that are directly superimposed.
Pre-concentration methods applied to eliminate the non-silica phases in a sample shows
considerable promise. Physical separations are not effective, but chemical methods are. Quartz is
sufficiently inert for many phases to be removed in its presence by chemically methods. In fact, the
chemical methods for silica analysis rely on the quantitative removal of non-silica phases. For diffraction
analysis, only the interfering phases need to be removed, and if 90% of the bulk sample can be eliminated,
the silica concentration is increased an order of magnitude. The detection of 0.1% silica becomes the
detection of 1% silica which considerably simplifies the analysis. If the silica phase is indeed inert, the
concentration factor is simply the weight ratio of the bulk sample pre- and post-treatment. No
determination of phases in the matrix is required. Because the samples are bulk with an adequate quantity
of material available for an infinitely thick sample, any of the three methods: absorption correction, internal
standard or external standard may be employed.
There is considerable doubt that a "standard procedure" for bulk analysis is feasible or desirable.
Because of the variability of the matrixes, no single procedure is applicable to all situations. Because no
single procedure is feasible, the establishment of any procedure as a "standard" could present legal
difficulties in those situations where the "standard' procedure is not appropriate as it might preclude the
acceptance of a more accurate appropriate procedure. Whenever legal status of an analysis is required, it
would be more suitable to establish a set of criteria for creating and measuring calibration samples and for
simulating tests of comparable bulk samples than to define and approve a single "standard" analytical
procedure.
The Cristobalite Problem
The quantification of cristobalite requires considerably more study before quantification can be
done with confidence. Studies must determine the applicability of diffraction techniques to all situations
where cristobalite is encountered in both respirable dust and in bulk samples. It is already established that
cristobalite may vary from the well-crystallized form encountered when the temperature of formation is
high (~1500°C) to very poorly crystalline forms when the formation temperature is low (~50°C). There
are many physical differences which may occur that alter the diffraction pattern, either affecting the shape
of the diffraction peak or its angular position. The crystal structure of cristobalite is actually very complex,
and there are several polymorphs which may occur. Usually only the low temperature form is encountered,
but slight trace of an alkali element may stabilize one of the high-temperature forms. The structure
transitions on cooling may lead to considerable twinning and domain structures which decrease the
effective crystallite size. Low temperature growth may lead to very small coherent domains also.
Structural defects mostly stacking faults lead to considerable tridymite character. This latter phenomena is
common in the devitrification of opal.
Several approaches are available for the quantification of cristobalite. If the full range of the
characteristic diffraction peaks are included in the integration, the area which is determined is theoretically
proportional to the amount of cristobalite present. However, the possibility of amorphous surface layers as
in quartz is very real, and the effect of stacking faults and twinning needs to be evaluated. One possibility
exists that may be universally applicable. Heating the sample to 1500°C does convert even the most poorly
crystalline silica to well-crystallized cristobalite, so a heat treatment may result in a phase consistent
enough for quantification. As long as there is no reaction with other matrix phases, the conversion should
be quantitative.
In bulk samples which are the result of firing processes where cristobalite is one of the products,
the physical state of the cristobalite may prevent any special treatments. If the cristobalite crystallites are
incorporated in ceramic products at grain boundaries or within the other grains, the cristobalite cannot be
isolated for additional treatments. Fortunately, the cristobalite is usually well crystallized, but it might be
in very tiny crystallite sizes producing peak broadening. X-ray diffraction will detect this cristobalite
where other methods may not be as sensitive to it. It would be difficult to establish calibration samples for
this situation.
A serious interference problem in the determination of cristobalite is due to the similarity of the
diffraction patterns of opal-C, opal-C and cristobalite as illustrated in Figure 3. Even the peak position of
the amorphous band in opal-A occurs at the same diffraction angle. Consequently, whenever there is the
potential for both phases to be present in the sample being monitored, cristobalite will be overestimated
when using any of the standard methods based on the integration of the characteristic peaks.
Distinguishing the two phases is not easy, but shape of the diffraction peak is indicative. Where the peak
width at half height is larger than usual for the diffractometer in use, opal should be suspected. All opal is
paracrystalline, and the diffraction peaks, if any, always show broadening due to the effective crystallite
size. Very little crystobalite should show crystallite-size broadening effects in the diffraction pattern.
Thus, whenever peak broadening is encountered in the diffraction analysis, it should be reported, and the
phase should probably be interpreted as opal. An analysis for water in the sample would support the opal
designation. Profile-fitting or pattern-fitting methods of diffraction analysis will help resolve this problem.
The Importance of Tridymite and Other Silicas
There is little to no evidence that tridymite is a significant phase in industrial environments.
Based on the SiO2 phase diagram, it is logical to worry that tridymite might be present in silica products,
but the evidence is that tridymite is difficult to form and may not be a stable phase without the presence of
an alkali metal to stabilize it. Regardless, there is little justification at this time to spend any effort
establishing procedures for its quantification.
The same may be said for the other forms of silica. Coesite and stishovite are so rare in nature
that it is a challenge just to concentrate enough to detect in a X-ray pattern. The other crystalline forms
would have to be a product of a reaction. Where the material is being produced for manufacture, e.g. a
clathrasil, then the material should be monitored. Otherwise, there is no need to set up a general procedure
for its quantification. Opal on the other hand, may be a more serious problem that is under estimated. It
may be abundant in some environments without recognition, as its amorphous character makes it difficult
to detect. More studies should be made on opal, both on how to detect it and on its potential toxicity.
The technique reported for quantifying opal, NIOSH Method 7501, is a viable approach to the
detection and quantification of amorphous silicas. The procedure is based on the same method of sample
treatment as is recommended for preparing standard samples of cristobalite from a silica precursor (Chung,
1978). Firing at 1500°C quantitatively converts any silica form to cristobalite, even fine-grained quartz.
Unfortunately, clay particles andther dust components could react with the silica to form compounds other
than cristobalite and reduce the amount of cristobalite formed. The conversion steps including ashing the
filter and firing in a platinum crucible may prevent reactions by keeping the particles separated. The major
problem is quantitative recovery and transfer of the particles to a substrate for the diffraction analysis.
The Need for Standard Silica Samples
In order to achieve accuracy in any analytical procedure, proper calibration standards are required.
At present, 1991, there are only two certified standards available, both supplied by the NIST/OSRD as
quartz, SRM-1878, and cristobalite, SRM-1879. The certification for both is phase and chemical
composition and average particle size. Altree-Williams et al. (1981a) have described the preparation of
quartz standards. Chung (1982) has suggested Min-U-Sil as a standard for quartz and preparation
procedures for cristobalite and tridymite. Min-U-Sil is now available from from many manufacturers and
varies considerably from batch to batch. Before it could be used, a large quantity of a single sample would
have to be set aside, homogenized and characterized. A cristobalite prepared by the Chung method is
available from The Gem Dugout, State College, PA. Unfortunately, none of these materials are technically
certified. Obviously, a program should be initiated somewhere to provide additional certified standards.
Unfortunately, the expense of certification is high, adding significantly to the cost of the material. Only a
few laboratories would have the legal status for the certification to be acceptable in court.
Proper certification involves careful physical measurements on aliquots of a sample of sufficient
size to provide reference material for many years of supply. The sample should be examined for phase
composition, chemical purity, particle size and size distribution, effective crystallite size and crystallite
perfection, uniformity of the large mass of material for each parameter, long term stability of the sample,
and the feasibility of supplying small aliquots to users at a reasonable cost. For a project such as
certification, the cost must be amortized through the sales of the certified samples. Unfortunately, the high
cost of each aliquot prevents many users from purchasing the samples and forces the unit cost to be set
even higher. It is a "Catch 22" situation.
Samples of cristobalite are much more difficult to obtain than samples of quartz because there is
no commercial product available nor is it common in nature. There is a small layered deposit in Eastern
Oregon which is 96% SiO2 and only shows a cristobalite-like pattern by diffraction; however, the percent
cristobalite (or opal) has not been determined. This natural material could be homogenized and
characterized as a reference sample. Alternatively, a large batch of synthetic cristobalite could be prepared
by a method similar to that described by Chung (1982) then homogenized and characterized. The high
temperature of the synthesis, > 1500°C, precludes its routine synthesis in readily available laboratory
furnaces. Also, because it needs to be fired in a platinum container, only small batches may be processed
at any one time. There are ways to fire larger batches at temperatures up to 1600°C, but they would require
special arrangements with ceramic processors and might "contaminate" a production line with silica.
Tridymite would have to be synthesized because there is no adequate natural source. First, it
would have to be established that tridymite was indeed required. Its synthesis temperature, 1100°C, is a
more reasonable laboratory temperature, but it would still have to be prepared in small batches.
It is feasible for individual laboratories to synthesize small batches, but not to carry out the full
characterization. Although the properties would be reasonably consistent to other batches synthesized in
the same manner, such a product would not have the legal status of a certified standard. Particle size could
vary considerably depending on how the material was treated after recovery from the crucible. Proper
crushing could be tailored to provide specific size and size distributions.
Calibration Curves and Correction Factors
The most common procedure for calibrating the intensity response for the filter methods is to
prepare a series of filters with uniformly deposited, weighed amounts of the crystalline silica phase. The
intensity response is then measured and plotted versus weight of silica on the filter. As long as the
response is linear, there is no particle interference, and no correction is necessary unless masking occurs in
a highly absorbing filter such as silver. A departure from linearity usually implies particle masking which
then requires an absorption correction. However, theoretical absorption corrections assume a continuous
thin layer of sample which is not appropriate to thin particle dispersions which are discontinuous layers.
Applying a correction for the absorption effect is difficult with discontinuous samples until the samples are
many layers thick to average out the individual particle variations. Thus, it is wise to avoid samples with
more than 2 mg on a filter which is where the particles tend to overlap significantly. However, where the
absorption is measured directly on the analysis sample by transmission, the averaging effect of the X-ray
beam compensates somewhat for the discontinuous distribution. In spite of the inherent difficulties,
Casciani and Ripanucci (1984) do report nomograms for the determination of quartz content where the
intensity response is not linear.
For bulk samples, absorption corrections are appropriate, but they are more difficult to measure.
Because of the thickness of the samples, transmission methods on the same sample are impossible, and a
second preparation must be made for the attenuation measurement. Most diffractionists prefer to avoid the
absorption correction and to employ either the internal-standard or external-standard method which are
feasible for bulk samples.
Detection Limits
The minimum detection limit, MDL, of crystalline silica in very small samples is as important as
its quantification at any level. Most health regulations define a personal exposure level, PEL, or a
maximum concentration above which action must be taken. What is critical is that the MDL of the
analytical procedure be well below the PEL.
Tolerance limits are quoted in mg•m-3 for atmospheres and weight percent for bulk samples. X-ray diffraction measures weight of a sample on a filter or weight percent of a bulk sample. The weight
measurement must be converted to weight per unit volume by correcting for the amount of air sampled.
Typical samplers filter 1.7 1•min-1 which amounts to about 800 l in an 8 hour working shift. Thus, for
respirable silica at the PEL, the amount would be 25 µg of quartz. Most procedures are reporting MDL
values of 3 - 5 µg which is within the PEL. If the total mass of the sample is 2 mg, the MDL is below 1%
with an accuracy around 4%.
For bulk samples the MDL is reported to be around 0.03 weight percent (Emig and Smith, 1989)
without interferences using present equipment. Another report by Schreiner (1990) claims a detection limit
of 0.01%. This value is insufficient to assure the PTL of 0.1 weight percent for bulk products because the
accuracy of 2% is outside the acceptance range. Bulk samples may also be treated as respirable samples by
depositing on a filter substrate, but Blount (1989) has shown that the MDL is insufficient to be meaningful.
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