Validation of the Analysis of Respirable Crystalline ...

Ann. Occup. Hyg., Vol. 55, No. 4, pp. 357–368, 2011� The Author 2011. Published by Oxford University Press

on behalf of the British Occupational Hygiene Societydoi:10.1093/annhyg/meq093

Validation of the Analysis of Respirable CrystallineSilica (Quartz) in Foams Used with CIP 10-RSamplersCELINE EYPERT-BLAISON*, JEAN-CLAUDE MOULUT,THIERRY LECAQUE, FLORIAN MARC and EDMOND KAUFFER

Institut National de Recherche et de Securite (INRS), Centre de Lorraine, Departement Metrologie desPolluants, Laboratoire d’Analyses Inorganiques et de Caracterisation des Aerosols, Rue du Morvan,CS 60027, 54519 Vandoeuvre Les Nancy Cedex, France

Received 9 April 2010; in final form 10 November 2010; published online 21 January 2011

Sampling the respirable fraction to measure exposure to crystalline silica is most often carriedout using cyclones. However, low flow rates (<4 l min21) and continuing improvement in work-place hygiene means less and less material is sampled for analysis, resulting in increasedanalytical uncertainty. Use of the CIP 10-R sampler, working at a flow rate of 10 l min21, isone attempt to solve current analytical difficulties. To check the ability of the analysis of quartzsampled on foams, known amounts of quartz associated with a matrix have been injected intofoams. The results obtained show that the proposed protocol, with prior acid attack and ashingof the foams, satisfies the recommendations of EN 482 Standard [CEN. (2006) Workplaceatmospheres—general requirements for the performance of procedures for the measurementsof chemical agents. Brussels, Belgium: EN 482 Comite Europeen de normalization (CEN).],namely an expanded uncertainty of <50% for quartz weights between 0.1 and 0.5 times the8-h exposure limit value and <30% for quartz weights between 0.5 and 2 times the 8-h expo-sure limit value, assuming an exposure limit value equal to 0.1 mg m23. Results obtained showthat the 101 reflection line allows a quartz quantity of the order of 25 mg to be satisfactorilymeasured, which corresponds to a 10th of the exposure limit value, assuming an exposure limitvalue of 0.05 mg m23. In this case, the 100 and 112 reflection lines with expanded uncertaintiesof �50% would also probably lead to satisfactory quantification. Particular recommendationsare also proposed for the preparation of calibration curves to improve the method.

Keywords: CIP 10-R; foam; quartz; X-ray diffraction

INTRODUCTION

Methods of analyzing respirable crystalline silica arebased on X-ray diffraction (XRD) (AFNOR, 1995a, b;OSHA, 1996; NIOSH, 2003b; HSE, 2005) or infraredspectroscopy (NIOSH, 2003a; AFNOR, 2005; HSE,2005). The sensitivity of these methods depends on thequantity of material analyzed and this frequentlydepends on the sampling time and volume of airsampled. Moreover, use of cyclone samplers with flowrates such as the Dorr Oliver (1.7 l min�1) sampler

and improvements in workplace hygiene has lead to lessmaterial being available for analysis. The Frenchaverage exposure limit value (0.1 mg m�3) may be re-duced as a result of the most recent study led by NationalInstitute for Occupational Safety and Health (NIOSH,2002), which recommends a limit value of 0.05 mgm�3 and that of the American Conference of Govern-mental Industrial Hygienists (ACGIH, 2006), which rec-ommends an even lower limit value (0.025 mg m�3).The analytical uncertainty of the results is high becausethe quartz content in many samples is currently close to,or below, the quantification limit. The CIP 10 R sampler(Courbon et al., 1988) enables air to be sampled at a flowrate of 10 l min�1 because a lower backpressure is

*Author to whom correspondence should be addressed.e-mail: [email protected]

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This is a Description of the validation methodology for measuring crystalline silica using CIP 10R. Analysis of foam is described in article ASD# 050 the French standard NF X43-295

obtained with a foam and spinning cup than witha cyclone and filter.

Use of a cyclone (with a 25-mm diameter filter) al-lows the quantity of quartz sampled to be directly an-alyzed (HSE, 2005), whereas the CIP 10-R requiresindirect analysis with ashing of the sampling foam(AFNOR, 1995a). Concerning the preparation meth-ods used (direct or indirect), the following commentscan be made:

1. Comparison of direct and indirect methods car-ried out during earlier studies showed that the in-direct method associated with ashing at 600�C ofthe foam resulted in an underestimation in quartzcontent in the order of 13%, probably due to los-ses during preparation or to transformation of themineral compounds (Kauffer et al., 2005). If thesample contains some calcite, there is a risk thatsome of the silica would be transformed into cal-cium silicate (NIOSH, 2003b). It had also beennoted that the underestimation was more pro-nounced with filters containing calcite (23%)than for those in which none had been detectedduring qualitative analysis of XRD pattern (8%).

2. With large quantities of material and in order tocorrect matrix effects, one possible option is to fil-ter the calcined material onto a polycarbonate filterof low absorbency. This filter has the characteristicof being very smooth and it was noted that calibra-tion curves are not linear at low quartz weights,probably due to preferential orientation of thequartz particles (AFNOR, 1995a; Kauffer andMoulut, 1986; Edmonds et al., 1977; Smith,1997). This phenomenon decreases when theweight of deposited quartz increases. In this case,the surface becomes more uneven and the calibra-tion curves become linear after corrections for ab-sorption (in the case of high quartz loading).

On the other hand, with an indirect analysis method,calibration can be carried out by liquid filtration of sus-pensions containing known quantities of standardquartz using the usual methods available in any labora-tory. Unlike the indirect method, the direct method issampler specific and is more complicated insofar asthe standard filters must be prepared by samplingairborne reference quartz, in order to guarantee thatthe distribution of the particles will be the same on bothstandard filters and analytical filters.

Research to resolve difficulties linked to the sensi-tivity of quartz analysis methods is currently beingcarried out in several countries. The research relatesprimarily to increasing the flow rate of the differentcyclones used to collect the respirable fraction sam-

ple. However, this is a solution of limited potentialdue to the use of personal pumps, which seem, atpresent, for most of them, to be incapable of operat-ing at flow rates .4 l min�1. Use of the CIP 10-Rsampler at a flow rate of 10 l min�1 is investigatedas a solution to current analytical difficulties.

The aim of this study was to validate the use of theCIP 10-R for determining crystalline silica by XRD.This method, though described in several standards,still needs to be validated. In particular, the charac-teristics of the method must be compared to thegeneral performance requirements recommended inthe EN 482 Standard (CEN, 2006) when a measuredconcentration has to be compared to an exposurelimit value. In addition, some modifications aremade to the method in order to resolve analyticaldifficulties linked to possible transformation ofcertain minerals during ashing or to non-linearityof the calibration curves at low quartz loading whenparticles are deposited on a flat surface.

MATERIALS AND METHODS

Protocol

Respirable fractions of quartz, calcite, and iron ox-ide (hematite) dust obtained by liquid sedimentationwere used to make binary mixtures (quartz þ calciteand quartz þ iron oxide). These mixtures are com-posed of quartz (10 and 50%) and calcite or iron oxide(hematite). In the case of large quantities of dust sam-pling, elements in the sample may absorb the X-rayradiation. Mass absorption coefficients (for thecopper Ka radiation) for the substances involved areequal to 35 cm2 g�1 (quartz), 75.6 cm2 g�1 (calcite),and 230.4 cm2 g�1 (iron oxide), respectively. CIP10-R foams were spiked with various quantities ofthese mixtures (from 0.24 to 7 mg for quartzþ calcitemixtures and from 0.24 to 6 mg for quartz þ ironoxide mixtures). The quantity of quartz measuredby XRD was determined according to the followingprotocol:

1. Acid attack of the foam and the minerals withdilute hydrochloric acid (3N)

2. Filtration of the solution on a polyvinylchloride(PVC) filter (25-mm diameter, 0.4-lm pore size)

3. Ashing of the foam and PVC filter at 600�C ina platinum crucible

4. Addition of �1.5 mg of calcium fluoride (aver-age 5 1.56 mg, SD 5 0.18)

5. Filtration of the dust (ashed residue and calciumfluoride) on a preweighted polycarbonate filter(25-mm diameter, 0.4-lm pore size).

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The reason for the acid attack is to eliminate cal-cite to prevent the formation of calcium silicateduring foam ashing. The purpose of systematicallyadding 1.5 mg of calcium fluoride, whose mass ab-sorption coefficient is 96.8 cm2 g�1, is to correct thenon-linearity of the calibration curves, when onlya small mass of material is deposited on the filter(preferential orientation effect). The different com-binations between the weight of spiked dust, the na-ture of the substance, and the addition of calciumfluoride allowed deposits on the polycarbonate fil-ter with different mass absorption coefficient. Forthis study, the average measured absorption coeffi-cients varied from 52 to 173 cm2 g�1.

Two calibration lines were produced:

� For one, Cal1, the quantities of quartz were de-posited directly on the polycarbonate filter,without adding calcium fluoride to illustratethe preferential orientation effect at low quartzquantity.

� For the other, Cal2, �1.5 mg of calcium fluoridewas added to the quartz before being depositedon the polycarbonate filter.

To check the consistency of the analytical method,binary mixtures were also analyzed. In this case,a simplified protocol involving neither acid attacknor ashing was employed.

Materials

� Quartz QUIN1 (fraction ,5 lm, obtained byliquid sedimentation). The same quartz was usedto produce calibration lines and to prepare themixtures;

� Calcite (Prolabo� ref 22-296-294, fraction,5lm,obtained by liquid sedimentation);

� Iron oxide (Sigma-Aldrich�, ref. 310050, parti-cle size ,5 lm);

� Calcium fluoride (AlfaAesar�, ref 232-188-7,average diameter in volume 5 2.34 lm).

Instruments

The tests were carried using an X’Pert diffractom-eter (PANalytical) equipped with an automatic 42-station sample changer. The diffractometer operatingconditions were as follows:

� fine-focus Cu anode tube with a power of 2.2 kWused at 40 kV and 50 mA,

� proportional detector,� focal distance of 240 mm,� secondary graphite monochromator,

� programmable divergence and anti-scatteredslits,

� programmable analytical slit: 0.3 mm, and� sample spinner.

Data collection

Dust to be analyzed is deposited onto a polycar-bonate filter disposed on an aluminum support. Thequalitative analysis is carried out on a diffractionpattern recorded over a broad range (5–75� 2h, scanspeed 0.03� 2h s�1). The quantitative analysis is car-ried out on 101, 100, 112, and 211 reflections forquartz (angular width of 1.4� 2h, scan speed of0.002� 2h s�1). Measurements were also taken ondiffraction peaks 111 and 200 for aluminum to cor-rect for matrix effects (angular width of 1.5� 2h, scanspeed of 0.02� 2h s�1). Finally, measurement of 100diffraction line for a sample of polycrystalline sili-con was recorded at regular intervals to correct forany instrumental drift.

As detailed in Appendix 1, the analysis of quartz byXRD, using the conditions described in this part, al-lowed the bias, relative standard deviation (RSD) andexpanded uncertainty of the method to be determined.

Correction of matrix effects

When the weight of material to be analyzed is high(.1.5 mg, approximately), it cannot be assumed thatmass is a linear function of the intensity. Therefore,the matrix absorption effects have to be corrected byusing the technique described by Altree-Williamset al. (1977). The principle is as follows:

The dust to be analyzed is deposited on a polycar-bonate filter. During the measurement, the filter isplaced on an aluminum support. The ratio betweenthe intensity of the reflection of the aluminum supportcarryingeithera cleanpolycarbonate filterorapolycar-bonate filter with the mineral dust allows the averageabsorption of the matrix to be measured and thus tocorrect the matrix effect (obtaining, thereby, a linearrelationship between the measured intensity and themass of quartz). The correction to be applied dependson the weight of mineral dust to be analyzed, on its ab-sorption coefficient and on the reflection line used.

RESULTS

Calibrations

Table 1 gives the coefficients for the regressionlines linking intensity and quartz weight for thetwo calibrations made (Cal1, without addition ofCaF2 and Cal2, with addition of CaF2) and the four

Analysis of respirable quartz sampled by CIP10-R 359

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reflections used. The quartz weights vary from 0 to7 mg, approximately, for Cal1 and from 0 to 5 mg,approximately, for Cal2).

It is noted that the calibration without added cal-cium fluoride leads to a strongly positive Y-intercept(87.9 counts s�1 deg�1) for 101 reflection. In thiscase, the addition of calcium fluoride produces acalibration curve passing through the origin (Y-intercept not significantly different from zero). Fora better illustration of the phenomena observed,additional tests on quartz weights ,1 mg were alsocarried out. The calibration curves correspondingto 101 reflection are shown in Fig. 1, which illus-trates the fact that the intensity is not proportionalto the quantity of quartz, when no CaF2 is added.

Bias, RSD, and expanded uncertainty

Tables 2 and 3 give, for spiked foams, values forbias Bj;k and RSD Sj,k for the different weights ofquartz studied. Table 2 refers to quartz þ calcitemixtures, whereas Table 3 concerns quartz þ ironoxide mixtures. In both cases, the values obtainedfor calibrations Cal1 and Cal2 are presented for the

different diffraction lines studied. Tables 4 and 5present the same results obtained from the directanalysis of the binary mixtures used to spike thefoams. In this case, the dusts were analyzed withoutprior ashing or acid attack. Iron oxide interferes withthe 112 reflection for quartz, which explains why theresults for this reflection are absent from Tables 3and 5. Moreover, calcite interferes with the 211 re-flection for quartz, which explains why the resultsfor this reflection are lacking in Table 4.

Figures 2 and 3 illustrate the data from Tables 1and 2 for the analysis of spiked foams (101 reflec-tion). For reason of clarity, these figures are restrictedto quartz weights ,1 mg. Additionally, Figs. 4 and 5illustrate the data from Tables 4 and 5 for the analysisof the binary mixtures (101 reflection).

Table 6 gives the expanded uncertainty for the fourreflections studied as a function of quartz weight.The expanded uncertainty for the method was calcu-lated from the results obtained for spiked foams,bias, and RSD being calculated from all resultsobtained on both mixtures studied. The relative errordue to sampling was taken to be 5%, the relative

Table 1. Coefficients for the regression lines (slope 5 b Y-intercept 5 a) linking intensity (counts per second degree) to quartzweight (microgram) for the four reflections used (ns, not significant for a level of statistical significance of 0.05)

100 Reflection 101 Reflection 112 Reflection 211 Reflection

b a b a b a b a

Cal1 without addition of CaF2 0.23 �23.7 1.20 87.9 0.16 ns 0.12 ns

Cal2 with addition of CaF2 0.25 �19.4 1.26 ns 0.17 ns 0.12 ns

Fig. 1. Intensity measured as a function of deposited quartz weight for two calibrations Cal1 and Cal2 (101 reflection, depositedweight ,1000 lg).


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Table 2. Bias and RSD according to quartz weight. Analysis of spiked foams, quartz þ calcite mixture. Figures in normal typewere obtained with the Cal1 calibration (without added calcium fluoride) and those in bold type were obtained with the Cal2calibration (with added calcium fluoride)

Quartzweight (lg)


Bias (%) RSD (%) Bias (%) RSD (%) Bias (%) RSD (%) Bias (%) RSD (%)

25 62.1 20.5 �261.0 5.5 �22.3 17.2 113.4 71.2

25.7 18.9 21.2 5.4 224.2 16.8 109.9 70.1

51 41.4 11.3 �119.2 14.5 �13.5 12.2 72.7 60.2

3.6 10.3 15 14.6 215.7 11.9 69.9 59.2

104 23.7 3.5 �60.8 6.0 �11.3 4.6 6.4 2.9

1.2 3.0 1.9 3.5 213.6 4.4 4.7 2.8

312 9.3 1.2 �15.5 0.4 �2.6 1.0 �0.5 2.0

23.2 1.0 1.3 0.6 25.1 1.0 22.1 2.0

508 13.2 1.7 �6.8 3.4 �2.4 3.2 �1.4 2.8

2.1 1.6 1.1 3.0 �4.9 3.1 23.0 2.8

708 5.6 0.6 �6.8 0.7 �4.2 1.4 �1.7 0.7

24.2 0.5 22.6 0.7 26.7 1.4 23.3 0.7

1004 2.6 2.3 �5.5 1.7 �5.3 1.6 0.2 1.2

26.4 2.2 24.2 1.6 27.7 1.6 21.4 1.2

1502 4.5 2.4 �3.7 1.2 �3.5 1.0 �2.8 1.4

24.2 2.2 24.8 1.1 25.9 1.0 24.4 1.3

3493 �1.5 2.1 �5.2 1.6 �5.5 1.2 �5.1 1.0

29.2 1.9 28.8 1.5 27.9 1.1 26.7 1.0

Table 3. Bias and RSD according to quartz weight. Analysis of spiked foams, quartz þ iron oxide mixture. Figures in normaltype were obtained with the Cal1 calibration (without added calcium fluoride) and those in bold type were obtained with the Cal2calibration (with added calcium fluoride)

Quartzweight (lg)



24 97.9 21.4 �278.3 3.9 �31.9 39.6

26.4 19.3 9.9 1.7 233.0 39.0

51 52.3 5.1 �137.3 0.7 �4.4 60.8

14.0 5.0 23.6 0.7 25.9 59.8

99 40.4 8.5 �70.3 4.2 �12.8 26.4

16.1 8.1 24.2 3.0 214.2 26.0

301 23.4 6.4 �25.8 6.0 �15.3 3.9

9.6 5.9 27.6 5.6 216.7 3.9

510 8.7 1.8 �11.6 1.2 �3.5 6.4

22.1 1.7 23.4 1.1 25.0 6.3

601 5.8 4.9 �20.5 4.1 �12.7 6.9

24.4 4.6 213.9 3.9 214.1 6.8

1024 4.3 2.5 �9.1 1.9 �8.3 1.7

24.8 2.3 27.7 1.8 29.8 1.7

1499 0.7 2.5 �7.2 1.6 �9.5 1.7

27.7 2.3 28.1 1.5 210.9 1.7

2990 �1.0 3.9 �5.2 2.9 �3.6 2.9

28.9 3.6 28.4 2.8 25.1 2.9


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error due to the sampling convention 11% (CEN,2009), and the relative error due to comparison ofa secondary standard to a primary standard 3% [ex-act data for different secondary standards are givenby Stacey et al. (2009)].

Detection limits

Detection and quantification limits were calculatedfrom the standard deviation of the intensity measuredfor 10 polycarbonate filters onto which �1.5 mg ofCaF2 had been deposited. Detection and quantifica-

tion limits correspond to the quartz weight (Cal2 cal-ibration) calculated from the average measuredintensity plus 3 SD (detection limit) or plus 10 SD(quantification limit). The corresponding values aregiven in Table 7 for the four reflections studied.

DISCUSSION

Assuming a linear fit, Table 1 shows that the Y-intercept is significantly different from zero for the100 reflection whatever calibration is applied (with

Table 4. Bias and RSD according to quartz weight. Analysis of mixtures of quartz and calcite. Figures in normal type wereobtained with the Cal1 calibration (without added calcium fluoride) and those in bold type were obtained with the Cal2calibration (with added calcium fluoride)

Quartzweight (lg)



550 17.5 2.0 �6.6 2.0 �1.9 2.5

6.2 1.7 0.4 1.5 24.4 2.5

2700 3.6 2.5 �0.6 2.6 0.4 2.7

24.6 2.4 23.8 2.4 22.2 2.6

Table 5. Bias and RSD as a function of quartz weight. Analysis of mixtures of quartz þ iron oxide. Figures in normal type wereobtained with the Cal1 calibration (without added calcium fluoride) and those in bold type were obtained with the Cal2calibration (with added calcium fluoride)

Quartzweight (lg)



600 52.4 5.1 25.6 3.0 18.8 3.4

38.6 4.6 30.2 2.0 16.9 3.3

2900 24.9 2.9 16.3 2.5 11.3 1.8

15.1 2.7 12.0 2.4 9.5 1.8

Fig. 2. Percentage bias as a function of quartz weight. Analysis of spiked foams (101 reflection). Results are provided for the twomatrices studied (quartz þ calcite and quartz þ iron oxide) and for both calibrations made (Cal1 without added CaF2 and Cal2 with

added CaF2).


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or without addition of CaF2). This negative Y-interceptis due to the wide variation in the polycarbonate filterXRD pattern in the angular range concerned (Davisand Johnson, 1982). Adding calcium fluoride reducesthe effect but does not eliminate it. For the 101reflection, the Y-intercept is strongly negative whencalibration is applied without the addition of calciumfluoride, whereas the addition of calcium fluorideleads, in this case, to a calibration curve passingthrough zero (Y-intercept value is not significant).The phenomenon observed here is known: the

presence of flat surface on crushed grains of quartzwould tend to orient the particles where the substrateis very smooth (Edmonds et al., 1977; Smith, 1997).This phenomenon disappears progressively withincreasing weights deposited on the filter, the surfacebecoming more and more uneven. It is particularlypronounced when the mass deposited is low (,300lg), as shown in Fig. 1, which presents the relationshipbetween the measured intensity for the 101 reflectionand the mass of quartz deposits for weights ,1 mg.It is noted that the intensities corresponding to deposits

Fig. 3. Percentage RSD as a function of quartz weight. Analysis of spiked foams (101 reflection). Results are provided for the twomatrices studied (quartz þ calcite and quartz þ iron oxide) and for both calibrations made (Cal1 without added CaF2 and Cal2 with

added CaF2).

Fig. 4. Percentage bias as a function of quartz weight. Analysis of the mixtures (101 reflection). Results are provided for the twomatrices studied (quartz þ calcite and quartz þ iron oxide) and for both calibrations made (Cal1 without added CaF2 and Cal2 with

added CaF2).


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without added calcium fluoride are significantly higherthan the intensities corresponding to the deposits withadded calcium fluoride. This non-linearity in thecalibration curve, near the origin, was previouslymentioned but not completely explained for quartzdeposited on PVC filters (HSE, 2005) or on silverfilters (Altree-Williams, 1977). For 112 and 211reflections, the addition of calcium fluoride does notchange the parameters of their calibration curvesignificantly.

Thus, the addition of calcium fluoride to the quartzwhen preparing the calibration curve is essentialsince this permits reliable results to be producedwhen low weights of quartz are sampled by thefoam, when using the most intense reflection (101).To remove these preferential orientation effects,the solution retained for this work was to add a con-stant quantity of calcium fluorite when producing thecalibration curve and when treating the foams. Add-ing CaF2 at the time of calibration allows a linear

Fig. 5. Percentage RSD as a function of quartz weight. Analysis of the mixtures (101 reflection). Results are provided for the twomatrices studied (quartz þ calcite and quartz þ iron oxide) and for both calibrations made (Cal1 without added CaF2 and Cal2 with

added CaF2).

Table 6. Expanded uncertainty expressed as a percentage for the four reflections studied and for different quartz weights. Thesequartz weights are also expressed as a fraction of the exposure limit value, assuming an 8-h sampling period using the CIP 10-Rsampler (flow rate of 10 l min�1) and for an exposure limit value equal to 0.1 mg m�3

Quartzweight (lg)

Exposurelimit fraction

100 Reflection 101 Reflection 112 Reflection 211 ReflectionExpanded uncertainty Expanded uncertainty Expanded uncertainty Expanded uncertainty

25 0.05 59 41 63 199

51 0.11 35 40 46 161

102 0.21 35 25 37 49

307 0.64 29 27 26 34

509 1.06 24 24 26 27

643 1.34 26 33 27 34

1014 2.11 26 27 28 28

1500 3.13 27 27 26 30

3242 6.75 30 29 28 27

Table 7. Detection and quantification limits calculated for 100, 101, 112, and 211 reflections


Detection limit (lg) 17 2 18 37

Quantification limit (lg) 50 6 61 124


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relationship to be obtained between intensity andmass of quartz deposited.

However, two alternative solutions could be envis-aged to avoid the addition of calcium fluorite:

1. The first consists in producing a calibration curvewith forced zero intercept, as described in theOSHA method (OSHA, 1996), which can resultin a curve close to that obtained by addingCaF2 (Cal2) as far as standard filters with impor-tant masses of quartz (.1 mg) are taken into ac-count. This solution however has the drawbackof overestimating the mass of quartz for sampleswith low quartz and dust loadings. In this case,preferential orientation effects will not be cor-rected during the analysis, which can lead to anoverestimation of the mass of quartz that canreach �30%. In addition, choosing not to addcalcium fluorite and forcing the zero interceptis a device which can improve the results forthe 101 reflection but which cannot be appliedfor the 100 reflection regarding the origins ofits Y-intercept.

2. The second solution consists in fitting the cali-bration curve to a polynomial of an appropriatedegree in order to reproduce the non-linearityof this curve. This solution is also unsatisfactoryin that low quantities of quartz associated withhigh quantities of dust would lead to an underes-timation of quartz quantities. In this special caseof high dust quantity, no preferential effect is ex-pected on the analytical filter. Thus, choosing thecalibration curve that takes this effect into ac-count is inadequate.

The bias and to a lesser extent the RSD of the re-sults determined from foams spiked with knownquantities of quartz associated with a matrix (calciteor iron oxide) depends, as expected, strongly on thecalibration curve used. This is mainly true for lowquartz quantities (Tables 2 and 3 for all reflectionsand Figs. 2 and 3 for the 101 reflection). This effectis particularly pronounced for the most intense reflec-tion of quartz (101). It is also noted that the bias andRSD are strongly deteriorated for the 211 reflectionwhen the quantities of quartz analyzed are �50 lg.This is consistent with the limit of detection mea-sured for this reflection (37 lg, Table 7). Consideringonly the tests for which the quantity of quartz spikedinto the foams is .500 lg, the average bias for allreflections is negative, �6% (�4.7% for quartz þcalcite mixtures and �7.6% for quartz þ iron oxidemixtures). This bias is probably due to losses occur-ring during the different steps of sample preparation

(see Protocol section). Similar losses were alsoobserved in a previous study after recovery ofcalcined residue from PVC filters (Kauffer et al.,2005). The larger bias observed for lower quantitiesof quartz is probably explained by an imperfect cali-bration curve even when care is taken to add calciumfluoride during its preparation.

The results presented in Table 6 (expanded uncer-tainty expressed as a percentage) show that the gen-eral performance requirements of the EN 482Standard (CEN, 2006) are respected, namely an ex-panded uncertainty of ,50% for quartz weights be-tween 0.1 and 0.5 times the 8-h exposure limit value(assuming an exposure value limit equal to 0.1 mgm�3) and ,30% for quartz weights between 0.5and 2 times the 8-h limit value. The Table 6 data alsoindicate that the 101 reflection allows quartz quanti-ties of �25 lg to be satisfactorily measured, whichcorresponds to a 10th of the exposure limit value, as-suming a limit value equal to 0.05 mg m�3. The 100and 112 reflections having expanded uncertainties of�50% would probably also lead to satisfactoryquantification of low quartz weights. These resultswere obtained for two different matrices (quartzand calcite and quartz and iron oxide). The first mix-ture tested the effectiveness of the acid treatmentused to eliminate the calcite and to prevent therebythe formation of calcium silicate. The second testedthe effectiveness of the method used to correct ma-trix effects in the case of particularly absorbent ma-trices. Table 7, which gives the detection andquantification limits of the method, shows that thethree reflections (100, 101, and 112) allow quantifi-cation to the 10th of the value of the current exposurelimit. These results confirm the data previously ob-tained on spiked foams.

It is to be noted that the differences between per-formance of the proposed method and the require-ments of EN 482 being small, any modification ofthe protocol is likely to compromise the satisfactionof EN 482 requirements, especially for low quartzcontent.

In a study using WASP proficiency testing resultsfor quartz measurement, Stacey (2005) calculatedRSD for a direct analysis method dealing with XRD.It can be noted that respecting the requirement of EN482 (CEN, 2006) when using the direct on-filter anal-ysis approach (HSE, 2005) is not possible. A quartzloading of 15 lg represents one-fifteenth of a limitvalue of 0.1 mg m�3 when sampled with a cycloneoperating at 2.2 l min�1. When using the directon-filter analysis approach, a result of 15 lg wouldhave an estimated confidence level of –65% [Table 4from the report of Stacey (2005)]. The requirements


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of EN 482 (CEN, 2006) are that the expanded uncer-tainty at one-tenth of the limit value should be within50%. It can also be noted that for these sampling con-ditions, it is not possible to respect the requirements ofthe EN 482 Standard, especially since the confidenceinterval does not take into account all the uncertaintiescomponent involved in sampling.

Lastly, the binary mixtures prepared for this study(quartz þ calcite and quartz þ iron oxide at 10 and50% quartz) were also analyzed. For this, �5 mg ofdifferent mixtures were directly filtered onto a poly-carbonate filter and analyzed according to the methoddescribed in this paper. For this analysis, the dust wasnot calcined at 600�C since the purpose of ashing isonly to eliminate the foam. Similarly, no acid attackwas carried out since, without ashing, there is no riskthat the calcite would react with the quartz to formcalcium silicate. The results obtained show that ifthe RSD is comparable with that measured in the caseof spiked foams for the same quartz content (Figs. 5and 3, and Tables 2–5), then this is not true for bias(Figs. 4 and 2), mainly in the case of quartz þ ironoxide mixtures at 10%. It is noted that in this case,the bias is strongly positive (Fig. 4), whereas in thecase of spiked foams, it was fairly negative and ofweaker amplitude (Fig. 2). Figs. 6–8 (magnified�1500) display the images obtained by scanningelectron microscopy for non-calcinated particlesfrom a quartz þ iron oxide mixture at 10%. X-raypicture of the element silicon shows that the largestparticles are silica (Fig. 8), whereas the X-ray pictureof the element iron shows that the smallest particlesare iron oxide (Fig. 7). Using the electron back-scattered technique, where contrast is a function ofthe atomic number of the minerals, the particles ofiron are brighter than those of silica (Fig. 6). It is alsoseen that the silica particles are dissociated from theiron oxide particles and are larger. It is likely, there-fore, that the strongly positive bias measured duringanalysis of the mixtures is due to a micro-absorptioneffect (Jenkins and Synder, 1996). The incident elec-tron beam used in XRD is absorbed less by the silicaparticles than if it had passed through a medium hav-ing the same average absorption coefficient as themixture. The matrix effects are, therefore, probablyover-corrected, resulting in a strongly positive bias.The situation is different when the material is cal-cined, as can be seen in Figs. 9–11 (magnification�1500). In this case, the particles of iron oxide areclosely associated with silica particles, as seen inthe X-ray picture for the elements silicon (Fig. 11)and iron (Fig. 10). The XRD diagrams for calcinedmaterial show no evidence of new phases, whichdemonstrates that the iron oxide particles are proba-

bly joined together with the quartz particles. Theabsorption coefficient of these new entities is differ-ent from that of the quartz particles and closer tothe mean absorption coefficient of the mixture, result-ing in a different matrix effect correction and a lowerbias value.

Fig. 6. Non-calcined dust. BEI image.

Fig. 7. Non-calcined dust. Fe diagram.

Fig. 8. Non-calcined dust. Si diagram.


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CONCLUSIONS

Due to its high flow rate (10 l min�1), the use ofthe CIP 10-R sampler to measure silica concentra-tion in air allows larger quantities of dust to be sam-pled than most cyclones would allow. Whereas its

use implies an indirect analysis to recover the dustsampled by foam, the interpretation of the XRD pat-tern is simplified and permits more reliable analysisof the phases present.

The proposed analysis protocol, incorporatingprior acid attack and ashing of the foams, satisfiesthe requirements of the EN 482 Standard in termsof expanded uncertainty in a range from 0.1 to 2times the 8-h exposure limit value, for a limit valueequal to 0.1 mg m�3. The results can still be regardedas satisfactory for limit values twice as low. Thestudy made it possible to show the importance ofhaving a sufficient amount of matter on the analyti-cal filter, both at the time of calibration and duringthe analysis, in order to prevent deposits exhibitingpreferential orientation of the quartz particles.

Whereas this observation was made for a mixtureprepared from very different constituents in terms oftheir size and their absorption coefficient, this studyfinally demonstrated that, in an extreme case, prepa-ration of the sample substantially modified the ana-lytical bias.

Acknowledgements—The authors would like to thankMr Olivier Rastoix for his contribution in scanning electronmicroscopy.

APPENDIX 1

The bias Bij is given by:

Bij 5

�Qmij � Qtij

�Qtij

;

where Qmij is the measured quantity of quartz forsample i of matrix j, Qtij is the true quantity of quartzdetermined from the mixture constituents for samplei of matrix j.

For a given matrix j, n samples (usually 3) were pre-pared for different quantities k of quartz. In this way,the average bias� Bj;k and RSD Sj,k associated withthese different quartz quantities can also be determined.

Bj;k5

1

n

Xni5 1

Bij;

Sj;k 5

�1

n

Xni5 1

�Bij � Bj;k

�2�0:5

:

By grouping the results obtained for the two matri-ces studied, one can also determine the average biasBk and the RSD Sk for all matrices and for the differ-ent quartz quantities studied.

The expanded uncertainty, integrating the sam-pling uncertainty (Ssamp) due to flow rate variations,

Fig. 9. Calcined dust. BEI image.

Fig. 10. Calcined dust. Fe diagram.

Fig. 11. Calcined dust. Si diagram.


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uncertainty with regard to sampling convention(Sconv) and the uncertainty introduced when linkinga secondary calibration material to a primary calibra-tion material (Scal) for different quartz quantities kstudied is given by:

Uk 5 2 �ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi�B

2k þ S2

k þ S2samp þ S2

conv þ S2cal

q:

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