Certification of 10 �m Diameter PolystryreneSpheres (“Space Beads”)

There are numerous examples in trade and commercein which small particles play an important role, either asthe commodity of trade itself, as unwanted contami-nants in a product or an environment, or as a basis forcomparison between a “normal” particle and an“abnormal” one. Many of the products we buy come inthe form of small particles, powders, or particle suspen-sions, including medicines, cosmetics, food products,paints, talcum powder, cements, photocopier toners, andmilk (which is essentially a suspension of microdropletsof fat in water). In other cases, small particles areunwanted, for example in cleanrooms for microelectron-ics or pharmaceutical manufacturing, in lubricating oilsin motor vehicles, and in the air we breathe and thewater we drink. There are also situations where onewould like to compare a known, standard particle toan unknown, test particle as in the case of blood-celltesting. In all of these instances, particle standards playa key role, enabling quality control, product uniformity,conformance to standards, traceability to NIST,interchangeability of instrumentation, uniformity ofmeasurement, or some combination of these goals.

With these benefits in mind, the National Bureau ofStandards set out, in the early 1980’s, to develop a rangeof particle-sizing Standard Reference Materials (SRMs)for use in calibrating and certifying instruments thatmeasure particle size, whether as products, by-products,or contaminants. In cooperation with ASTM CommitteeE-29 on Particle Size Measurement, researchers in thePrecision Engineering Division (PED) at NBS devel-oped a series of five SRM’s consisting of monosizedpolystyrene microspheres with diameters of 0.3, 1, 3,10, and 30 �m [1-4]. The three smallest particles,donated by commercial vendors, were certified first,since such small microspheres were comparativelyeasy to grow using conventional polymer emulsiontechniques. However, for the two larger diameters,10 �m and 30 �m, the techniques that existed at thetime did not yield rigid particles of the required spheri-city because of the detrimental effects of gravity on themicrosphere growth process. (At the time, there weretechniques to make large microspheres, but these wererelatively soft and unsuitable for use as SRMs, whichmust be rigid and stable.)

At about the same time that NBS was certifying itsseries of particle-sizing SRMs, a group of researchersled by John Vanderhoff of Lehigh University and

Dale Kornfeld of NASA Marshall Space Flight Centerwas conducting experiments aboard the space shuttleto determine the effect of microgravity on chemicalreaction rates of emulsion polymerization, as well as onthe morphology (shape) of polymer microspheres grownin microgravity [5]. The first experiments of theMonodisperse Latex Reactor (MLR) were conducted in1982 aboard space shuttle flight STS-3 and resulted inmicrospheres as large as 5 �m in mean diameter. Asubsequent experiment on a later shuttle flight, STS-6,produced particles of 10 �m mean diameter. In 1984,the NBS group obtained samples of the 5 �m and the10 �m materials and did a detailed intercomparisonbetween the space-made particles and earth-made parti-cles of similar composition [1,2]. In both cases, thespace-made materials were found to be superior interms of individual particle sphericity, narrowness ofsize distribution, and, importantly, in particle rigidity.An agreement was then made between NASA and NBSfor NASA to provide a sufficient quantity of the 10 �mmaterial to make up 600 5 mL vials containing liquidsuspensions of the polystyrene microspheres for use asan SRM. The agreement also called for NBS to receivethe 30 �m polystyrene spheres to be grown on a sub-sequent shuttle flight.

To certify SRM 1960, the so-called “space beads”(Figs. 1 and 2), researchers at NBS, led by Tom Lettieriof the PED, developed three new particle-sizingtechniques, which are described in the publicationCertification of SRM 1960: Nominal 10 �m DiameterPolystyrene Spheres (“Space Beads”) [1]. These tech-niques were center distance finding (CDF), resonancelight scattering (RLS) from a liquid suspension ofmicrospheres, and metrology electron microscopy(MEM). The primary certification technique forSRM 1960, center distance finding, was developed byIke Hartman of the PED. CDF uses a conventionaloptical microscope and relies on the fact that the micro-spheres act like tiny lenses when placed on a glass slidein the microscope. The microspheres were spread ontoa slide such that they formed long chains of contactingspheres, rather than the regular hexagonal arrays formedin conventional array sizing. This ensured that the parti-cles were in close contact, compared to conventionalarray sizing where the non-zero diameter distribution ofthe microspheres leads to voids, cracks, gaps, and otherflaws in the arrays. These flaws lead to uncertainties in

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the mean sphere diameter measured with conventionalarray sizing, uncertainties that are avoided when particlechains are used. If the microspheres are illuminatedfrom below and the microscope is focused just abovethem, then the tiny focal spots can be photographedand the distance between the spots determined veryaccurately. Using CDF, the mean sphere diameter forSRM 1960 was determined to be 9.89�0.04 mm: thiswas the certified mean diameter for the SRM.

Supporting measurements were made using RLSand MEM. In the RLS technique, developed by TomLettieri, a tunable dye laser was used to generateresonance light-scattering intensity patterns from themicrospheres as they were suspended in water. As thelaser was tuned through its wavelength range, relativelysharp peaks in the light-scattering intensity appeared atcertain wavelengths. Although this technique had beenused before by others to investigate sharp resonancesfrom single microspheres, this was the first time thatsuch resonances were detected in a suspension of manymicrospheres: this could be done because the sphereshad a very narrow size distribution (<1 % standarddeviation). By comparing the experimental resonancewavelengths with those calculated by Egon Marx of the

PED using Mie scattering theory, the NBS researcherswere able to use RLS to arrive at a mean diameter of9.90�0.03 �m for the SRM 1960 microspheres. Thisresult was in excellent agreement with those from bothCDF and MEM.

The other supporting metrology technique wasMEM. Gary Hembree of the PED used MEM tomeasure the microspheres in a scanning electron micro-scope (SEM). With MEM, the spheres are mounted ontothe microscope stage as in a conventional SEM. How-ever, in a conventional SEM the electron beam is raster-scanned past the stationary spheres, whereas in theMEM technique developed at NBS, the electron beam isheld stationary while an individual particle is movedthrough the beam via a piezoelectric scanning stage.The scattered electron intensity is measured versus stageposition and, from this, a particle profile is generated.These profiles could then be used to determine thediameter of the individual particles. Using MEM, amean diameter of 9.89�0.06 �m was obtained for theSRM 1960 spheres, in excellent agreement with theother two metrology techniques. Indeed, the remarkableagreement among the three metrology techniques, andthe relatively small uncertainty in the diameter, likelymade SRM 1960 the best characterized particle-sizestandard in the world at the time. An important pointin these measurements was that the three metrologytechniques were independent of each other, in that noneof the measurements relied in any way on measurementsfrom the other techniques. This is always good practicewhen certifying an SRM.

In 1985, SRM 1960 was first offered for sale to thepublic through the NBS Office of Standard ReferenceMaterials (OSRM), making the SRM the first commer-cial product to be manufactured in space (SRM-1961,the nominal 30 �m spheres, was the second space-madeproduct). The sale of the space beads was reported inhundreds of newspapers, magazines, and television/radionews stories around the world, from the New York Timesto the CBS Nightly News to ABC’s Good MorningAmerica. The work was also featured on a NationalPublic Radio program and on an Australian scienceshow, Beyond 2000. Later on, SRM 1960 won anIR-100 award from Research & Development magazinefor being among the top 100 products of 1985.

Over the years, SRM 1960 has proven to be a valuabletool for the calibration of particle-sizing instruments inthe United States and around the world. Samples havebeen purchased by dozens of U.S. and foreign compa-nies for use as primary particle-sizing standards. Thesecompanies include not only the makers of particle-sizing standards and instrumentation, but also “everyday” users who need to maintain accuracy and traceabil-ity of their measurements. Among the primary users are

Fig. 1. Scanning electron photomicrograph of Standard ReferenceMaterial 1960 microspheres.

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major pharmaceutical companies, Fortune 500 petro-chemical and chemical companies, small-to-mid-sized biomedical instrumentation companies, particlestandard supply houses, and numerous firms, of allsizes, involved in the measurement of particle size.Several U.S. Government and non-profit labs, includingNASA Ames, Battelle Northwest, EPA, FDA, LosAlamos, Sandia, and the USGS have also purchasedSRM 1960. In the international arena, the material hasbeen used by research laboratories in Australia, Austria,Brazil, Canada, England, France, Germany, India, Italy,Japan, Korea, Mexico, Norway, Spain, Switzerland, andThailand. Sales of SRM 1960 average about 25 vials peryear through the NIST OSRM. The OSRM also offersfor sale another version of the space beads, SRM 1965,which are standard microscope slides with smallpatches of particles in both regular arrays and micro-sphere chains. These have been used throughout theworld for educational and training purposes, as well asto satisfy the needs of those who wish to own something

“made in space.” In addition to OSRM, the micro-spheres are also sold through the EuropeanCommunity’s Bureau Communautaire de Reference(BCR).

The demand for SRM 1960 is spurred, in part, by itsincorporation into several document standards in theUnited States. For example, the U.S. Pharmacopeia, inits test entitled “Particulate Matter in Injections,” speci-fies the use of SRM 1960 for calibrating the liquid-borne particle counters used in the test. As a water-quality standard, SRM 1960 is listed by the NationalOceanic and Atmospheric Administration (NOAA) inits compilation of Standard and Reference Materials forMarine Science as a physical standard for the assessmentof water and sediment quality. The particles have alsofound application in the monitoring and assessment ofair quality, especially with regard to the EnvironmentalProtection Agency (EPA) PM10 standard that specifies acutoff of 10 �m for the aerodynamic diameter of partic-ulate emissions from motor vehicles, smokestacks, and

Fig. 2. Photograph of SRM 1960 showing a vial of the SRM, the certificate, and the package.

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other industrial emission sources. In the medical field,SRM 1960 has been valuable as a calibration standardfor blood-cell counting and sorting, as the mean dia-meter of the SRM is very near that of human red bloodcells. Thousands of hospitals and medical testinglaboratories throughout the U.S. use blood-cell countersto check for sickle cell anemia, Tay-Sachs disease,and other blood abnormalities, and SRM 1960 helps toensure the quality and accuracy of such tests.SRM 1960 will, undoubtedly, find more applications inthe medical field as biotechnology and medical diagnos-tics become more pervasive in our daily lives.

In addition to the above applications of SRM 1960,other areas of scientific research where the materialhas found use include: electron microscopy; chemicalchromatography; powder metallurgy; ceramics; foodprocessing; photographic films; and basic particleresearch, among many others. The rigid demands ofISO9000 will likely increase the importance of particle-sizing standards such as SRM 1960, as companiesbecome more concerned with quality control, confor-mance assessment, and reliability issues.

Of the four authors of the space beads paper, two,Tom Lettieri and Egon Marx, are still at NIST. Tomstarted at NBS in 1978 after graduate school at theUniversity of Rochester, joining the Pressure Group todo high-pressure optical studies of liquids. Less thantwo years later, he joined the PED to conduct opticalmetrology studies of small particles, rough surfaces, andnoncontact methods for dimensional measurement. Tomis now a Program Manager with the NIST AdvancedTechnology Program, where he helps select andmanage industrial technology projects in the area ofoptics/photonics. Egon Marx did his graduate studiesin nuclear physics under the direction of MurrayGell-Mann at the California Institute of Technology.After a few years of teaching at Drexel University,

Egon went to work at Harry Diamond Laboratories,conducting theoretical investigations in electromagnetic(EM) interference. His interests at NIST have includedEM scattering, surface roughness, linewidth metrology,and quantum electrodynamics. Two authors, GaryHembree and Ike Hartman, left NIST not long afterthe space beads project was finished. Gary came toNBS after completing graduate studies in electronmicroscopy at Arizona State University (ASU). Afterseveral years at NBS, he returned to ASU to work withthe world-renowned electron microscopy group there.Ike Hartman came to NBS after spending a number ofyears at General Electric doing optics and imagingresearch. At NBS, he worked in various areas of opticalmetrology, including microscopy, particle sizing,electro-zone particle counting, and linewidth metrology.He retired from NIST after a long and satisfying careerin optics, both in his native Netherlands and in theUnited States.

Prepared by Tom Lettieri.

Bibliography

[1] T. R. Lettieri, A. W. Hartman, G. G. Hembree, and E. Marx,Certification of SRM 1960: Nominal 10 �m DiameterPolystyrene Spheres (“Space Beads”), J. Res. Natl. Inst. Stand.Technol. 96, 669-691 (1991).

[2] G. Mulholland, G. Hembree, and A. Hartman, Sizing ofPolystyrene Spheres Produced in Microgravity, NBSIR 84-2914,National Bureau Standands, Gaithersburg, MD (1985).

[3] T. R. Lettieri, Optical Calibration of Accurate Particle SizingStandards at the U.S. National Bureau of Standards, in OpticalParticle Sizing: Theory and Practice, Gerard Gouesbet andGerard Grehan (eds.), Plenum Press, New York (1988).

[4] Nancy M. Trahey (ed.), NIST Standard Reference MaterialsCatalog 1998-1999, NIST Special Publication 260, NationalInstitute of Standards and Technology, Gaithersburg, MD (1998).

[5] D. Kornfeld, Monodisperse Latex Reactor (MLR), NASA TM-86847, NASA, Washington, DC (1985).

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