Task Group 1 : Methods
Task Group Co-Chairs
Debra Kaiser, NIST
Aleks Stefaniak, NIOSH
Contributing Task Group Members (to date)
Keana Scott, Tinh Nguyen, and Rick Davis, NIST
Jurg Schutz, CSIRO, Australia
Frank von der Kammer, University of Vienna, Austria
Dermont Bouchard, EPA
Technical Experts Consulted (to date)
Robert Cook, Frank DelRio, Jeffrey Fagan, Justin Gorham, Angela Hight-Walker,
Elijah Petersen, Keana Scott (NIST); Jeff Simpson (Towson University)
NanoRelease Consumer Products: Multi-Wall Carbon Nanotube (MWCNT) in Polymers
Steering Committee Workshop
May 16-17, 2013
Work Flow
Materials + Products
Release Processes
MWCNTs + polymer
Release
Forms of released
material
MWCNT-polymer
composites
Driving
forces
Sampling methods
Products
Gaps and Needs
New and improved
measurement methods
Interlaboratory studies
Standardized methods
Sample preparation
Methods and
evaluation
Detection
Quantification
Characterization
Measurement of
Released Material
Methods:
• Generation of released material
• Representative sampling
TG 1: Methods
Release
scenarios
• Sample preparation for measurement
• Measurement of released material
NanoRelease Steering Committee
May16-17, 2013
2
Materials and Consumer Products
SEM images: A. Vladar, NIST
forming
+ polymer
2 µm
500 nm
Raw MWCNTs
MWCNT−polymer
composites
Sporting goods
Considerations:
• Polymers identified by TG2: epoxy, polyamide (PA), polyurethane
(PU), polyethylene (PE), and polycarbonate (PC)
• MWCNTs only form of carbon nano-objects
• Products too complex
• Release from MWCNT-polymer composites: two scenarios
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
3
Measurement Concepts
Measurement methods include instrument specification , procedures or
well-defined protocols, data analysis and representation, and data
compilation in a common format
Measurement “Hierarchy”*
• Detection: presence (yes or no) of MWCNTs; detection limit
• Quantification: number or mass concentration of MWCNTs in released
material per unit volume or area of composite
• Characterization: determination of characteristics and properties of
MWCNTs and fragments
* Adapted from von der Kammer et al., Trends Anal. Chem. 2011, 30, 425-436 (note: identification combined with detection)
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
4
Measurement Concepts
Qualitative vs. Quantitative Measurements
• Qualitative
• ranges from, e.g., “the sample does or does not contain MWCNTs” to “the sample
contains about 50% MWCNTs” per unit area or volume examined
• relative uncertainty in the estimate is large
• Semi-Quantitative
• measurement of, e.g., number of MWCNTs as “counted” in a sample
• not all MWCNTs present may be measured (e.g., encased MWCNTs)
• difficult to perform measurements that are statistically significant (e.g., tedious,
representative sample)
• Quantitative
• measurement produces a numerical result, e.g., the diameter ranges from 100 nm
to 200 nm
• most MWCNTs present are measured
• sufficient number of measurements to be statistically significant (can report
uncertainty)
 What degree of quantitation is required?
* Adapted from von der Kammer et al., Trends Anal. Chem. 2011, 30, 425-436 (note: identification combined with detection)
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
5
Release Scenario 1
• Polymer is not degraded (i.e., remains cross-linked)
• Mechanical “driving force”: high energy process
• Abrading, sanding, drilling…
fragments: may or may not
contain MWCNTs
MWCNTs may protrude from
fragment surface, be
encased in fragment, or both
fragment sizes: 100 nm to 1 mm
MWCNT-polymer
composite
all studies report the
presence of fragments
only in released material
fragments and
unbound MWCNTs
some studies report the
presence of MWCNTs in
released material
TEM image of MWCNTs protruding from fragment
Cena et al. J. Occup. Env. Hygiene. 2011, 8, 86-92.
TG 1: Methods
NanoRelease Steering Committee
June 21-22, 2013
6
Release Scenario 1: Sampling Methods: Fragments
Real-time instruments
• Instantaneous measures of number, mass, size, or surface area
concentration
• Not chemical-specific
Time-integrated samplers
• Collection particles onto substrate for off-line analysis
• Size-selective samplers
• Separate particles by aerodynamic or other size
• Can be dichotomous or multi-stage samplers: 10’s of nm to 10’s of
μm
• Collect particles with sizes well above the nanoscale;
agglomerates
• ‘Total’ (non-size-specific) samplers
• Plastic cassette and conductive cowl samplers that hold filters
• Precipitators (some instruments can be size-selective)
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
7
Release Scenario 1: Sampling Methods, MWCNTs
Real-time instruments are problematic
• Estimate ‘equivalent’ diameter assuming spherical shape
• Problems with fibers (multiple charging effects, etc.)
Time-integrated samplers as described above
• Conductive cowl sampler designed for fibers
• Precipitators have good efficiency in nanoscale
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
8
Release Scenario 1: What to Measure?
fragments only
fragments and unbound MWCNTs
 Presence of MWCNTs in fragments (detection)
 Number or mass concentration of MWCNTs in
fragments (quantification)
 Physico-chemical characteristics or properties
of MWCNTs in fragments, e.g., average size
(diameter and length), size distribution, and
surface composition
 Relative amounts of fragments that contain
MWCNTs vs. fragments that do not contain
MWCNTs (by number or mass)
 Average size and size distribution of the
fragments
 Shape of fragments
 All of the same to the left
 Presence of unbound MWCNTs in sample
(detection)
 Relative amounts of fragments vs. unbound
MWCNTs in sample
 Number or mass concentration of unbound
MWCNTs in sample
 Physico-chemical characteristics or properties of
unbound MWCNTs in sample, e.g., average size
(diameter and length), size distribution, and
surface composition
 Prioritization and selection of what to measure is the essential first step
 More than one measurement method is required for quantification and characterization
 Sample large enough to yield a statistically relevant result (quantitative measurements)
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
9
Release Scenario 1: Measurement Methods
12 published studies
• fragments only (7); fragments and unbound MWCNTs (5) studies
• Polymer: epoxy (7); PA (2), PC (2), PU (2), POM* (2), PMMA* (1)
Measurement method
Frequency of method
SEM (scanning electron microscopy)
10
TEM (transmission electron microscopy)
7
TEM-EDX (TEM-energy dispersive X-ray spectroscopy)
4
XPS (X-ray photoelectron spectroscopy)
2
AUC (analytical ultra-centrifugation)
2
LD (laser diffraction)
2
TOF-SIMS (time-of-flight secondary ion mass spectroscopy)
1
ICP-MS (inductively-coupled mass spectrometry)
1
AFM (atomic force microscopy)
—
RS (Raman spectroscopy)
—
UV-VIS (ultra-violet visible spectroscopy)
—
SEM and TEM most widely used measurement methods
Methods for sizing fragments were not considered, except for AUC
* Polymers not considered by TG2: POM = polyoxymethlene, PMMA: Poly(methyl methacrylate)
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
10
Release Scenario 1: Method Specifications
Table completed with input from NIST experts; additional input is welcome
Measurement
method
Media
Spatial
resolution
Information
depth
Maximum sample
or scan area
Detection limit
Type of
Information
SEM
V
1 nm to
10 nm
near-surface
to few μm
1 mm x 1 mm
one MWCNT per
sample area
semi-quantitative
TEM
UHV
sub-nm
100 nm
maximum
10 μm x 10 μm
one MWCNT per
sample area
semi-quantitative
AUC
LS
10 nm
NA
unlimited volume
unknown
quantitative
AFM
A
10 nm
0.1 nm
50 μm x 50 μm
one MWCNT per
scan area
semi-quantitative
XPS
UHV
10 nm
3 nm to 10 nm
700 μm x 2 mm
10 wt % MWCNTs
in compositeb
semi-quantitative
Raman
spectroscopy
A
1 μm
1 μm to 5 μm
1 μm
5 wt % MWCNTs
in compositeb
qualitative to
semi-quantitative
UV-VIS
A
100 nm to
1 μm
1 μm to 5 μm
1 μm
unknown
qualitative
Measurement media: A = ambient; LS = liquid suspension; V = vacuum (10-6 torr); UHV = ultra-high vacuum (10-9 torr)
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
11
Release Scenario 1: Measured Characteristics
Measurement
method
Concentration of
MWCNTs
Surface
composition
Relative
concentrations
of fragments
and unbound
MWCNTs
Average size and size distribution
in
fragments
Unbound
MWCNTs in
fragments
Unbound
MWCNTs
Fragments
SEM
SQ
SQ
SQ
SQ
SQ
NA
SQ
TEM
QL
QL
QL
QL
QL
NA (EELS)
QL
AUC
NA
NA
NA
QN
QN
NA
QN
AFM
SQ
SQ
SQ
SQ
SQ
NA
SQ
XPS
SQ – total MWCNTs
NA
NA
NA
QN
NA
RS
QL – total MWCNTs
NA
QL
NA
NA
NA
UV-VIS
QL – total MWCNTs
NA
QL
NA
NA
NA
QL , qualitative: yes/no or rough estimate
SQ, semi-quantitative: can get a numerical result that is a good estimate,
uncertainty is medium to high, dependent on numerous factors
QN, quantitative: get a numerical result with low uncertainty
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
12
Release Scenario 1: Method Evaluation
Measurement
method
Representative
sample amount
Ease of sample
preparation
Availability of
instruments
Practicality of
Measurements
D
Q
C
SEM
M
H
H
M
H
L
M
TEM
L
H
M
M
H
L
M
AUC
H
H
M
M
M
H
H
AFM
M
M
H
H
H
L
M
XPS
H
H
M
M
M
L
M
Raman
H
H
H
M
M
L
L
UV-VIS
H
H
H
M
M
L
L
High: statistically relevant sample size; minimal sample preparation; broadly available commercial instrument;
measurement requires minimal expertise and time
Medium: unlikely that sample size is statistically relevant; moderate sample preparation; moderate availability
of instruments, may contract measurements; skilled expertise and significant measurement time
Low: sample size not statistically relevant; difficult sample preparation; few instruments available at e.g., user
facilities; exceptional expertise and measurement time
For detection (D), quantification (Q), and characterization (C)
High: easy to detect, quantitative result for Q and C
Medium: difficult to detect; semi-quantitative result for Q and C
Low: not used for detection; qualitative result for Q and C
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
13
Conclusions for Scenario 1
Measurement methods for released material
• Must first prioritize and select key characteristics and properties of
MWCNTs, in fragments and unbound, and fragments
• More than one method is required to determine a characteristic or
property
• Numerous methods for detection of MWCNTs, unbound and in
fragments
• Most methods for quantification (concentration of MWCNTs) are semiquantitative at best, i.e., may get a numerical result that is a reasonable
estimate, uncertainty is medium to high
• Most methods for characterization are semi-quantitative at best:
• Tedious to measure a large enough amount of material for statistically
relevant results
• For many methods, cannot measure MWCNTs encased in a fragment
• Validated protocols and reference materials essential for accurate
measurements
• Validation of methods and data are difficult and time-consuming
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
14
Release Scenario 2
• Polymer is chemically degraded in a binding or cross-linking sense
• “Weathering”: optical (UV) and hydrolytic (humidity) “driving forces”: low
energy process
• Accelerated weathering by long-term exposure or by accelerated processes (e.g.,
the NIST “SPHERE”)
MWCNT-polymer
composite
oligomers
tangled network of
MWCNTs on the surface
of the composite
200 nm
Peteren et al., submitted
to ACS Nano
potential subsequent release
of unbound or tangled
MWCNTs by agitation, wear,
chemical reaction, or fluid flow
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
15
Release Scenario 2: What to Measure?
Tangled network of MWCNTs
Potential release of unbound MWCNTs
 Presence of MWCNTs (detection)
 Number or mass concentration of MWCNTs on
surface (quantification)
 Physico-chemical characteristics or properties
of MWCNTs , e.g., average size (diameter and
length), size distribution, spatial distribution
(degree of dispersion), and surface composition
 Presence of MWCNTs in release media (detection)
 Number or mass concentration of unbound MWCNTs
per volume of media
 Physico-chemical characteristics or properties of
unbound MWCNTs in media, e.g., average size (diameter
and length), size distribution, and surface composition
Release media (dependent on lifecycle stage and mode of consumer use):
• Environmental media: air, water, sludge, soil…
• Biological media: saliva, blood, tissue
 Prioritization and selection of what to measure is the essential first step
 More than one measurement method is required for quantification and characterization
 Sample large enough to yield a statistically relevant result (quantitative measurements)
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
16
Release Scenario 2: Measurement Methods
8 published studies
• Polymer: epoxy (2); PA (3), PU (2), POM* (1)
• Methods identified below considered only tangled network resulting from polymer
degradation (not subsequent release of MWCNTs or fragments by further action)
Measurement method
Frequency of method
SEM (scanning electron microscopy)
5
TEM (transmission electron microscopy)
2
SEM-EDX (SEM-energy dispersive X-ray spectroscopy)
1
XPS (X-ray photoelectron spectroscopy)
2
TOF-SIMS (time-of-flight secondary ion mass spectroscopy)
1
AFM (atomic force microscopy)
—
SEM most widely used measurement method
* Polymers not considered by TG2: POM = polyoxymethlene
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
17
Release Scenario 2: Measured Characteristics
Table completed with input from NIST experts; additional input is welcome
Measurement
method
Concentration
of MWCNTs
Average size and size
distribution
Degree of
dispersion
Surface
composition
SEM
SQ
SQ
SQ
NA
TEM
QL
QL
QL
NA
AFM
SQ
SQ
SQ
NA
XPS
QL
NA
NA
QN
QL , qualitative: yes/no or rough estimate
SQ, semi-quantitative: can get a numerical result that is a good estimate,
uncertainty is medium to high, dependent on numerous factors
QN, quantitative: get a numerical result with low uncertainty
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
18
Release Scenario 2: Method Evaluation
Table completed with input from NIST experts; additional input is welcome
Measurement
method
Representative
sample amount
Ease of sample
preparation
Availability of
instruments
Practicality of
Measurements
D
Q
C
SEM
M
M
H
M
H
M
M
TEM
L
L
M
L
H
L
L
AFM
H
M
H
M
H
M
M
XPS
H
H
M
M
M
L
M
High: statistically relevant sample size; minimal sample preparation; broadly available commercial instrument;
measurement requires minimal expertise and time
Medium: unlikely that sample size is statistically relevant; moderate sample preparation; moderate availability
of instruments, may contract measurements; skilled expertise and significant measurement time
Low: sample size not statistically relevant; difficult sample preparation; few instruments available at e.g., user
facilities; exceptional expertise and measurement time
For detection (D), quantification (Q), and characterization (C)
High: easy to detect, quantitative result for Q and C
Medium: difficult to detect; semi-quantitative result for Q and C
Low: not used for detection; qualitative result for Q and C
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
19
Conclusions for Scenario 2
Measurement methods for tangled networks of MWCNTs
• Must first prioritize and select key characteristics and properties of
MWCNT networks
• More than one method is required to determine a characteristic or
property
• Numerous methods for detection of MWCNTs
• Most methods for quantification (concentration of MWCNTs) are semiquantitative at best, i.e., may get a numerical result that is a reasonable
estimate, uncertainty is medium to high
• Most methods for characterization are semi-quantitative at best:
• Tedious to measure a large enough amount of material for statistically
relevant results
• Difficult to separate tangled MWCNTs
• Validated protocols and reference materials essential for accurate
measurements
• Validation of methods and data are difficult and time-consuming
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
20
Recommendations
Pilot Testing and Interlaboratory Studies (ILS’s)
• Start with a pilot study involving a few labs with great expertise in the
topic
• Possible to design a pilot test and eventually an ILS for:
• Generation of released material in a controlled manner
• Representative sampling of released material
• Very difficult to design a pilot test for measuring MWCNTs in polymer
composites that would yield reproducible results
• Start with protocol development
• Generation of released material by one or more specific methods
• Sampling or sample preparation protocols
• Protocols for qualitative or semi-quantitative measurements
Standardization of Methods
• Too early!
• Requires well-defined, validated protocols for any method
TG 1: Methods
NanoRelease Steering Committee
May16-17, 2013
21