Environmental Engineering Reference
In-Depth Information
ejected.electrons,.also.is.useful.for.elemental.composition.information..Paired.with.
these.different.detectors,.SEM.can.provide.information.on.the.size.and.shape.of.a.
particle,.three-dimensional.topographic.information.on.surface.features.and.texture,.
crystalline. or. amorphous. structure,. and. elemental. composition.. The. technique. is.
most. useful. for. measurements. of. particles. in. the. range. of. 50. nanometers. (nm). or.
higher,. although. stronger. electron. sources. can. achieve. spatial. resolution. of. 1. nm..
More. advanced. detectors. are. available. now. that. can. charactize. the. difference. in.
chemistry.between.the.top.2.nm.of.a.particle.and.its.interior.
With.TEM,.the.measurements.are.taken.underneath.the.material..The.portion.of.
the.electron.beam.that.passes.through.the.particle.can.be.projected.onto.a.luorescent.
screen.to.form.a.two-dimensional.image.of.the.particle..Resolution.of.less.than.0.1.
nm.can.be.achieved,.making.it.a.primary.tool.for.characterization.of.the.smallest.
nanoparticles..As.with.SEM,.a.variety.of.detectors.can.be.used.to.detect.scattered.
electrons.and.x-rays.released.by.the.interactions.of.the.electron.beam.with.the.atoms.
of.the.particles..TEM.analyses.can.be.designed.to.determine.the.elemental.composi-
tion.of.the.particle.and.the.chemical.bonding.environment,.particle.shape.and.size,.
and.its.crystalline.or.amorphous.structure..TEM.also.can.be.conducted.in.a.scanning.
mode.(STEM),.where.the.narrowly.focused.electron.beam.scans.over.the.particle.for.
maximum.sensitivity.and.resolution..A.more.detailed.introduction.to.TEM.is.avail-
able.on.the.Internet.[6].
Researchers.frequently.use.SEM.and.TEM.to.characterize.nanoparticles.before.
their.use.in.laboratory.experiments.and.to.monitor.progress.or.results..TEM.has.been.
used.to.characterize.TiO
2
.and.fullerene.for.inhalation.and.aquatic.toxicity.studies.[7,.
8].. Rothen-Rutishauser. et. al.. [9]. used. TEM. techniques. to. visualize. TiO
2
. and. gold.
nanoparticles. absorbed. into. red. blood. cells;. and. Sipzner. et. al.. [10]. monitored. the.
dermal.absorption.of.TiO
2
.nanoparticles.using.TEM.
Reported.environmental.applications.include.the.use.of.SEM.and.TEM.to.charac-
terize.ine.and.ultraine.particulates.present.in.ambient.air..In.an.urban.air.study.[11],.
Utsunomiya.et.al..conducted.analyses.using.several.TEM.techniques.to.characterize.
the.particulate.size.associated.with.heavy.metals.and.to.speciate.the.metals.detected..
Metals. of. particular. interest. for. engineered. nanomaterials. —. titanium,. iron,. and.
silver.—.were.all.detected.in.nanoparticles..Titanium.and.iron.were.present.at.com-
paratively. high. concentrations. and. were. attributable. to. fractal. rock. and. numerous.
natural.and.anthropogenic.sources,.highlighting.the.dificulty.of.determining.poten-
tial.air.sources.from.the.manufacture.or.use.of.zero-valent.iron.or.titanium.dioxide.
nanoparticles.against.naturally.high.backgrounds..Silver.was.present.at.low.levels,.
primarily. associated. with. soot. particles,. and. tentatively. attributed. to. background.
combustion.sources.
SEM. and. TEM. provide. invaluable. information. for. many. purposes.. They. do,.
however,. have. several. limitations. for. environmental. applications.. Although. SEM.
has.a.larger.ield.of.view.than.TEM,.both.SEM.and.TEM.can.analyze.only.a.rela-
tively.small.number.of.particles.at.a.time..Representativeness.for.a.nonhomogeneous.
sample.is.dificult.to.achieve..The.instrumentation.is.costly.and.requires.a.high.level.
of.technical.expertise.to.operate.properly..The.sample.preparation.and.analysis.are.
time-consuming..The.particles.must.be.deposited.on.a.support.ilm,.and.the.differ-
ent. ways. of. achieving. this. deposition. may. allow. some. nanoparticles. to. aggregate.
