A Comparison of International Silica (a-Quartz) Ann Occup Hyg-2001-Verma-429-35

Ann. occup. Hyg., Vol. 45, No. 6, pp. 429–435, 2001 2001 British Occupational Hygiene Society

Published by Elsevier Science Ltd. All rights reservedPrinted in Great Britain.

0003-4878/01/$20.00PII: S0003-4878(00)00083-1

A Comparison of International Silica (a-Quartz)Calibration Standards by Fourier Transform–Infrared SpectrophotometryDAVE K. VERMA* and DON S. SHAWOccupational and Environmental Health Laboratory, McMaster University, 1200Main Street West,Hamilton, Ontario L8N 3Z5, Canada

Seven international silica (a-quartz) standards were examined for relative purity to the USNational Institute of Standards and Technology (NIST) Standard Reference Material (SRM)1878 Respirable a-quartz by Fourier Transform–Infrared Spectrophotometry (FT–IR). Thestandards examined have been used in North America, the UK, Australia and Germany. The189 samples analyzed included NIST-SRM 1878, Min-U-Sil 5, Ottawa Silica Sand, Sikron F-600, A9950 (AUST 1), DQ12-Robock, DQ12-Bergbau. Size distributions of the standards weredetermined by Coulter Counter to be broadly similar with equivalent spherical volumemedian diameter ranging between 1.2 and 3 mm.

The results showed the standards to differ by as much as 30% in relative purity. Conse-quently, an internationally agreed upon calibration standard is urgently needed. Min-U-Sil5 based NIST-SRM 1878 or Sikron F-600 are the two most likely candidates. Any agreedstandard must have a well characterized size distribution and closely match the respirabledust criteria. It should also be studied by both infrared spectrophotometry and X-ray diffrac-tion techniques. 2001 British Occupational Hygiene Society. Published by Elsevier ScienceLtd. All rights reserved

Keywords: respirable silica; silica analysis; calibration standard; Fourier transform–infrared spectrophotometry;particle size

INTRODUCTION

Occupational exposure to crystalline silica (α-quartz)occurs in mining, manufacturing and constructionindustries. It is well known that prolonged inhalationof dust containingα-quartz can cause silicosis — afibrotic lung disease (Anon, 1975). The InternationalAgency for Research on Cancer (IARC) has recentlyrevised the classification of silica to Group 1 HumanCarcinogen (IARC, 1997). The American Conferenceof Governmental Industrial Hygienists (ACGIH) hasalso classified silica as a category A2 carcinogen, i.e.a suspected human carcinogen (Anon, 1999).

It has been reported that the variable health effects,in terms of fibrogenesis and carcinogenesis, arerelated to the inherent characteristics of crystallinesilica determined by the origin of the sample and

Received 20 March 2000; in final form 12 September 2000.*Author to whom correspondence should be addressed. Tel.:+1-905-525-9140; fax: +1-905-528-8860; e-mail:[email protected]

429

external factors, i.e. association with or contaminationby substances other than silica which might activateor blunt its fibrogenicity and carcinogenicity (Fubini,1998; Donaldson and Borm, 1998). It has been earliersuggested by Altree-Williams (1982) that variablecrystallinity may be related to fibrogenicity and hehypothesized that the Crystallinity Index (CI) basedon an X-ray powder diffraction measurement may berelated to the specific toxicity and fibrogenesis ofquartz. The epidemiological evidence of silica, sili-cosis and lung cancer have also been recentlyreviewed (Soutaret al., 2000; Hesselet al., 2000; Fin-kelstein, 2000). Finkelstein concluded that the risk ofsilicosis following a lifetime exposure at the currentOSHA (USA) standard of 0.1 mg/m3 is likely to beat least 5-10% and the lung cancer risk is likely tobe increased by 30% or more. On the other hand,Hesselet al. (2000) dispute the validity of IARC’sdesignation of silica as a Group 1 carcinogen, statingthat data on humans demonstrates a lack of associ-ation between lung cancer and exposure to crystal-line silica.

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430 D. K. Verma and D. S. Shaw

A dose–response study of silica and silicosisamong Ontario hardrock miners was conducted(Verma et al., 1989) using the potassium bromidedisc method of silica analysis using Infrared Spectro-photometry (IR) as described by Dodgson and Whit-taker (1973). The method, with minor modifications,was also issued as NIOSH Method 7602. As part ofthe epidemiological study an investigation was under-taken to compare the measurement of α-quartz by X-ray diffraction (XRD) and IR (Verma et al., 1992). Inthat study, it was noted that there may be significantdifferences in the quantified results obtained by usingvarious α-quartz calibration standards in these ana-lytical techniques. At the time of the study, in the lateeighties, there was no universally accepted inter-national α-quartz standard for use with these analyti-cal methods.

Various methods of silica analysis have been criti-cally reviewed. An important early review discussedthe issues related to the commonly used method(Anderson, 1975). Since then, another major reviewupdated this work and identified the present andfuture needs (Madsen et al., 1995). The issues andcontroversy in the measurement of crystalline silicawas a topic of an international conference, summar-ized by Miles (Miles, 1999). For risk estimates, meta-analysis of dose–response studies are often conductedwhere dose measured in various studies are com-bined. The silica dose measured in different studiescould be different for several reasons, one being thepurity of calibration standards. As noted earlier, nointernationally accepted consensus for an α-quartzstandard exists. If there were such a standard, inter-method variability due to different standards couldbe eliminated.

A few studies, all of which were conducted in theearly 1980s, compared the then available α-quartzstandards by XRD and or IR (Dewell and Ambidge,1980; Altree-Williams et al., 1981; Biggins, 1982).These studies recommended the need for further com-parative studies and advocated the development of aconsensus international standard. In Canada, OttawaSilica Sand (a product of Ottawa, Illinois, USA) wasused as the α-quartz standard in the Province of Onta-rio, whereas in the rest of Canada and across NorthAmerica, generally Min-U-Sil 5 was used (Verma etal., 1992).

This study was undertaken to compare seven inter-national α-quartz standards by FT–IR for their rela-tive purity.

The ‘purity’ of quartz calibration standard isrelated to crystallinity, particle size, presence andabsence of an amorphous layer and other impurities.Ideally, an absolute ‘pure’ standard will be 100%crystalline with no amorphous layer and impuritiesand of the appropriate particle size. However, no suchmaterial exists. An α-quartz calibration standard maynot be 100% crystalline silica (α-quartz) nor freefrom amorphous layers and impurities, but its analyti-

cal response can be assumed to be from a 100% pureα-quartz. For this investigation NIST-SRM 1878, for-merly known as NBS-SRM 1878, was assumed to be100% pure. The analytical purity of all other α-quartzstandards have been measured compared to NIST-SRM 1878 and referred to as relative purity.

The specific objectives were:

1. To determine the size distribution of seven inter-national α-quartz standards and;

2. To compare the relative purity of the standards byusing an FT–IR.

MATERIALS AND METHODS

Agencies and researchers involved in silica analy-sis in the USA, Canada, the UK, Australia, Germanyand Scandinavian countries were contacted and askedfor details of the standards they use, and if possible,to supply samples of the standards. Our own labora-tory has performed silica analysis since 1978 and hada supply of a number of standards used in NorthAmerica. We were able to obtain seven standardsused in various countries. These α-quartz standardsand their sources are listed in Table 1. Although someof these standards may have come from the samesource (i.e. Sikron F-600 and A9950), they weretreated as separate standards for this investigation.

For the purpose of a more complete characteriz-ation of all seven standards, particle sizing was per-formed. Particle sizing was carried out by using aCoulter Counter Model TAII, (Coulter Corp., Miami,Florida) equipped with a 50-µm diameter aperture,permitting particle sizing in the range of 1–20 µm. ACoulter Counter Channelyzer, Model C1000 (CoulterCorp., Miami, Florida) was also used in conjunctionwith the Model TAII. These instruments were cali-brated using L2 (2 µm) and L5 (5 µm) latexmonospheres (Coulter Corp., Miami, Florida).

All standards were analyzed using a Nicolet Model510P FT–IR, (Nicolet Instruments Corporation,Madison, Wisconsin, USA), using NIOSH Method7602 (Anon, 1994). NIST-SRM 1878 was used as thesilica calibration standard for the instrument, conse-quently all the bulk standards were analyzed withrespect to the NIST standard. The bounds of theinstrument’s calibration curve ranged from 5 to 500µg. No sample preparation of the standards wasrequired prior to silica analysis and particle sizingexcept for Ottawa Silica Sand. Ottawa Silica Sand, asreceived, was a coarse river sand-like material. Thecoarse Ottawa Silica Sand was mixed and ground ina Spex Mixer Mill (Spex Industry Inc., Metuchen, NJ,USA) then sieved to a less than 10 µm diameter pre-cision sieve (ATM Corporation of Milwaukee, Wis-consin, USA). The less than 10 µm fraction of sandwas used for both particle sizing and silica analysis.

Nominal weights selected for each quantified silica



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431International silica calibration standards

Table 1. List of international α-quartz standards

Name Origin Where Used Source

Min-U-Sil 5 USA USA, Canada Pittsburgh Sand and Glass Company,USA and NIOSH in USA

NIST-SRM 1878 USA USA, Canada and other AIHA National Institute of Science andaccredited laboratories Technology, MD, USA

Ottawa Silica Sand USA Ontario, Canada Elliott Lake Laboratory, Energy Minesand Resources Canada (GeofferyKnight)

Sikron F-600 (also labelled Germany UK and Germany Health and Safety Executive, UK (Drby HSE as A9950) P. A. Elwood)A9950 (AUST) No 1 (subset Germany Australia Stephen Altree-William, The Australianof UK A9950 derived from National University, and BernardSikron F-600) Jordan, Deakin University, AustraliaDQ12-Robock Germany Germany British Cast Iron Research Association,

UK (Dr P. D. Biggins)DQ12-Bergbau Germany Germany British Cast Iron Research Association,

UK (Dr P. D. Biggins)

standard were 20, 50, 100, 150, 200, 250, 300, 350and 400 µg. Each standard was mixed and groundwith 250 mg of potassium bromide (KBr) in a Vibro-mill (Beckman Coulter Inc., Fullerton, California),and pressed in an evacuable 13-mm diameter die(Beckman Coulter Inc., Fullerton, California) at 8 tpressure to form a pellet. Three individual quantitat-ive FT–IR analyses were performed by rotating eachpellet in 120° increments through 360°. The meanvalue of the three rotations was used for each datapoint. Primary quantification of the characteristiccrystalline α-quartz doublet was based upon the peakheight absorbance at 798 cm�1, with the less sensitivesecondary singlet located at 696 cm�1 used as a con-firmatory tool. Three pellets were prepared from eachstandard and analyzed at the appropriate nominalweights. The order in which the series of nominalweights were analyzed was randomized such that theseries of 20-µg nominal weight standards did notnecessarily precede the 50-µg nominal weight series.A total of 189 samples were analyzed.

RESULTS

The results of size distribution by Coulter Counterof the standards are given in Table 2 and showngraphically in Fig. 1. The volume median diametersof all seven standards were found to be between 1.2and 3 µm. The analytical results of all seven cali-bration standards obtained by FT–IR, shown in Fig.2, are plotted on the Y axis with NIST-SRM 1878 asthe Gold Standard on the X-axis. The equation of theline for each standard, based on 27 pairs of obser-vations are also shown in Fig. 2.

DISCUSSION

Analytical methods for silica such as IR and XRDare particle-size dependent. The particle-size distri-bution of samples to be analyzed and the calibration

standards are therefore important. It has been shownthat as particle size increases XRD peak height alsoincreases but IR peak height decreases (Bhaskar etal., 1994). This effect was also reported by Addison(1991) with coal mine dust samples. It should benoted that mineralogical qualities other than crystal-linity and size distribution also have an effect.Amorphous silica, probably as a disordered shellaround a crystalline core of comminuted quartz, hasthe effect of reducing the absorbance coefficientwhile the normal effect of particle size reduction isto increase it (Duyckaerts, 1959). The ideal particle-size distribution of the calibration standard shouldmatch the sample being analyzed as closely as poss-ible. In the case of the respirable dust sample, whichconsists predominantly of fine particles less than 10-µm aerodynamic diameter with a median aerody-namic diameter cut point of 3.5–4 µm, the calibrationstandards should also be composed of fine particles.The particle-size distributions of the seven standardswere found to be approximately log-normally distrib-uted with median equivalent spherical volume diam-eter ranging between 1.2 and 3 µm, as shown in Table2 and Fig. 1.

The volume median diameter and mass mediandiameter of the log-normally distributed particle sizedistribution can be considered approximately thesame (Reist, 1984). The volume median diametergiven by the Coulter Counter can also be convertedto aerodynamic mass median diameter. Ogden et al.(1983) have shown that for coal particles, equivalentspherical volume diameter obtained by the CoulterCounter is equivalent to aerodynamic diameter. Byknowing the density and the shape factor, the equival-ent spherical volume diameter can be converted toaerodynamic diameter as follows:

da = �ss0

×d2

s

K



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Table 2. Cumulative particle size distribution of silica standards, analyzed by Coulter Counter (particle-size as equivalentspherical volume diameter)

Normalized cumulative % total volume

Particle Ottawa Silica Min-U-Sil 5 NIST SRM Sikron F-600 A9950 DQ12- DQ12-diametera Sand 1878 Bergbau Robock

0.6 1 0 0 1 0 0 10.8 5 6 7 4 5 7 161.0 10 16 17 9 12 17 381.3 17 32 33 15 23 31 581.6 25 49 51 23 37 45 662.0 35 68 70 34 51 60 702.5 45 83 82 47 65 72 733.2 56 92 88 62 76 82 754.0 70 96 91 74 84 89 785.0 82 98 92 83 90 95 796.4 90 99 93 88 93 97 838.0 92 99 93 90 95 97 8410.1 96 99 93 92 96 98 8912.7 98 99 98 93 97 98 9216.0 99 100 99 99 99 99 9920.2 100 100 100 100 100 100 100

aEquivalent spherical volume diameter.

Fig. 1. Cumulative particle size distribution (as equivalent spherical volume diameter) by Coulter Counter.

where:

da is aerodynamic diameter,ds is equivalent spherical volume diameter,K is shape factor,s is density of the particle, ands0 is 1 g/cm3 (unit density sphere for aerody-

namic particle).

By substituting a K value of 1.4 and density of 2.6mg/cm3 for silica from the literature (Mercer, 1973)in the equation above, the median equivalent spheri-cal volume diameter of the NIST-SRM 1878 stan-

dards of 1.6 µm would convert to aerodynamicmedian diameter of 2.18 µm as:

da = �2.61

×(1.6)2

1.4= 2.18

Likewise Sikron F-600’s median diameter of 2.7µm would convert to an aerodynamic median diam-eter of 3.68 µm. These standards provide a goodmatch for the respirable dust curves (50% acceptanceat 3.5 µm old ACGIH curve, 50% at 5 µm BMRCcurve and 50% at 4 µm currently recommended by



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Fig. 2. Relationships between NIST-SRM 1878 and other standards as analyzed by FTIR: (a) Min-U-Sil 5 (b) Ottawa SilicaSand (c) Sikron F-600 (d) A9950 (AUST 1) (e) DQ12-Robock (f) DQ12-Bergbau.

the European and international Committee’s as wellas the ACGIH). In fact, Sikron F-600 would providea closer match to the newly agreed European Com-munity and ACGIH’s respirable dust criteria of 50%at 4 µm.

It should be noted that the size distribution ofNIST-SRM 1878 by the Coulter Counter gave anequivalent spherical volume median diameter of 1.6µm, which could also be considered as the massmedian diameter for the log-normally distributedaerosol. Size distribution information provided withthe NIST-SRM 1878 standard, as determined by asedimentation method, was found to be between 0.33and 5 µm with a mass mean spherical diameter of1.62 µm (Anon, 1983). The Coulter Counter result of1.6 µm was thus remarkably similar to that obtainedby the Sedimentation method. The volume mediandiameter of other standards given in Table 2 couldthus be compared to each other with assurance.

Biggins 1982 examined the particle size distri-bution of eight α-quartz standards namely; X-7488,Bahai, Fyle, Min-U-Sil 5, Min-U-Sil 10, Min-U-Sil15, DQ 12, DQ 120. The mass median diameter of

these standards ranged from 1.85 to 5.15 µm. How-ever, many of the standards examined by Biggins areno longer in use or not available.

NIST-SRM 1878 is nearly equivalent to Min-U-Sil5, as shown in Fig. 2(a), which is not surprising sinceNBS-SRM 1878 was likely made from the latter. Theground Ottawa Silica Sand standard shows a markeddeviation in purity, relative to NIST-SRM 1878, ofapproximately 30% [Fig. 2(b)]. Sikron F-600 had adifference of approximately 20% [Fig. 2(c)] relativeto NIST-SRM 1878. A9950 (Aust 1) was also nearlyequivalent to Sikron F-600 [Fig. 2(d)], probably dueto the fact that A9950, used in UK, was likely derivedfrom Sikron; this was the standard A9950 (Aust 1)used in Australia. DQ12-Robock is equivalent toNIST-SRM 1878 [Fig. 2(e)] while DQ12-Bergbau[Fig. 2(f)] was nearly equivalent to the NIST stan-dard.

Addison (1991) investigated the accuracy and pre-cision of coal mine dust sample analysis within theEuropean Community with the aim of harmonizationof standard reference materials (i.e. α-quartz cali-bration standard) and analytical methods. Thirty dif-



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ferent samples from three UK coal mines, with widelydifferent mineral compositions and particle sizeranges, were analyzed by XRD and IR in five Euro-pean laboratories, one French, two German, oneBelgian and one from the UK. Three laboratoriesemployed DQ12 as the calibration standard, one usedFrench IR87 and another used A9950. The study con-cluded that particle size and mineralogical inter-ferences can contribute to differences of up to 30%in quartz content, despite efforts to control for boththese factors.

Since the early studies of Dewell and Ambidge(1980), Altree-Williams et al. (1981) and Biggins(1982), to the best of our knowledge, only one studyhas been published comparing Sikron F-600 to NIST-SRM 1878 (Jeyaratnam and Nagar, 1993). In thatstudy, Jeyratnam and Nagar reported that Sikron F-600, which was assumed to be 100% α-quartz incomparison to NIST-SRM 1878, should be taken ascontaining 92.5% α-quartz for the bulk sample and95.5% α-quartz for on-filter analysis.

Our results indicate the Sikron F-600 to be about80% of NIST-SRM 1878, when analyzed by FT–IR.This difference could be because of the use of the analternate technique (i.e. IR as opposed to XRD) byJeyaratnam and Nagar (1993). It was demonstratedby Dewell and Ambidge (1980) and Biggins (1982)that the crystallinity of α-quartz standards could infact be different and vary significantly in their XRDand IR responses, as subsequently observed by Vermaet al. (1992). It is therefore important that calibrationstandards should be evaluated for their relative purityby both IR and XRD methods.

High inter-laboratory variability exists in the USProficiency Analytical Testing (PAT) program for sil-ica, where as many as 12 different α-quartz cali-bration standards have been used by the participatinglaboratories. The same silica exposure sample givento two different laboratories could result in silica con-centrations which may differ by a factor of 2 or more(Eller et al., 1999a,b). A similar high inter-laboratoryvariability has been observed in the UK’s WorkplaceAnalysis Scheme for Proficiency (WASP) programfor silica. A well characterized and commonlyaccepted silica calibration standard would be of sig-nificant help in reducing some of the variablility. Theresults of our study indicate that the two possible can-didates are Min-U-Sil 5 based NIST-SRM 1878 orSikron F-600. The size distribution of Sikron F-600matches the respirable dust criteria more closely thanNIST-SRM 1878. However, it remains to be seenwhether Sikron F-600 can be obtained in sufficientquantity to satisfy this need.

Acknowledgements—We are grateful to our colleagues in NorthAmerica, Australia, the UK and Europe who generously pro-vided us with the information and samples. In particular, wethank Dr P. D. Biggins and Dr P. Elwood of UK, Mr StephenAltree-Williams and Mr Bernard Jorden of Australia, and Dr

Peter Eller of USA. We thank Lorraine Shaw and Tracey Tuttlefor their help in preparing this manuscript.

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A Comparison of International Silica (a-Quartz) Ann Occup Hyg-2001-Verma-429-35