Improvements in LA-ICP-MS for Trace Forensic Analysis of Common Household Products Using a
Short Pulse Ultraviolet Laser
Megan MekoliJonna Berry, Maria Bock, Jeneé Jacobs, Stan
Bajic and R. Sam Houk
Iowa State University
Overview• Develop a method for use in forensic evidence analysis
using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)
• Compare nanosecond-LA-ICP-MS to femtosecond-LA-ICP-MS
– Duct and electrical tape• Used in bombs and explosive devices
• Binding during a crime
– Copper speaker wire• Binding during a crime
• Used in bombs
– Galvanized steel pipe• Used to make pipe bombs and other explosive devices
LA-ICP-MS
Photo courtesy of Dr. R.S. Houk, www.thermo.comwww.cetac.com
3
LA-ICP-MS
Photo courtesy of Dr. R.S. Houk, www.thermo.comwww.cetac.com
4
LA-ICP-MS and Forensic Science• Low detection limits
• Isotope abundances measureable
• Sample destruction limited
• Range of sample sizes
• Homogeneous1
• Quantification without matrix matched standards is difficult.2
1Anal. Bioanal. Chem. 2003, 376, 1255.2J. Anal. At. Spectrom., 2003, 18, 872.
Why Femtosecond Ablation?
• fs shorter pulse duration (~100 fs vs. 10 ns)
• Ablated particles from fs smaller, more consistent– ns ablated matter can contain large particles
• Large spikes in signal, worse stability
• fs less heat transfer to sample– Less melting
• Good for tapes especially
– Less fractionation problems
Inte
nsi
ty (
cps)
Mass (u)
Ti46Ti47
Ti48
Ti49Ti50 Mn55
Fe56
Ni58
Ni60
Ni61Ni62Cu63
Zn64
Cu65 Zn68Ga69
Zn66
Duct- 3M, Backing Side
Duct- Nashua, Backing Side
Mass (u)
Ti48
Ti47 Ti49Ti50
Ti46
Mn55
Fe56 Ni58
Ni60
Ni61 Ni62Cu63
Cu65
Zn64 Ga69
Zn68
Inte
nsi
ty (
cps)
Can You Tell the Difference Between These Mass Spectra?
While you may be able to spot some differences by eye, what really matters are the analytical differences in the data. To quantify these differences, I’m going to use Principle Component Analysis (PCA).
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PCA
• Multi-variable statistical technique– First consider variables (intensities) with the most
variance, then second, third, etc.
– Score
– Plot scores
– Eigenvector Solo 4.1 software (Eigenvector Technologies)
• Approach meant for pairwise “head-to-head” comparison analysis
• Not intended to be for attribution studies
Tapes• Duct and Electrical Tapes
– Often used for binding during a crime
– Can be used in bomb making
3 mm
Duct- 3M, Backing Side Duct- 3M, Adhesive Side
3 mm
Method
• 1 revolution-waste
• Sample on large steel washer
– cut off excess
• Ablate backing side
• Flip, ablate adhesive side.
• 3 different samples per roll (0 ft., 3 ft., 9 ft.)
“9 ft.” “3 ft.” “0 ft.”
3 ft.6 ft.
5
1.75 in.
Instrumental Conditions
• Laser: CETAC LSX 500 nanosecond Nd:YAG (~10 ns)– 9.1 mJ/Pulse (100% power)
– Frequency: 20 Hz
– Wavelength: 266 nm
– Spot diameter: 100 μm
– 350 μm/s rastering• This is fast to avoid ablating through the tape.
• Thermo Element 1 ICP-MS:– Cones: Ni
– Medium resolution (m/Δm=4000)
– Dual (counting and analog) mode
• Analysis: Principal Components Analysis (PCA)– Eigenvector Solo 4.1 Software (Eigenvector Technologies)
1.75 in.
Instrumental Conditions
• Laser: COHERENT LIBRA femtosecond Ti:Sapphire(~100 fs)– 140 μW (100% power)
– Frequency: 1000 Hz
– Wavelength: 266 nm
– Spot diameter: 50 μm
– 300 μm/s rastering• This is fast to avoid ablating through the tape.
• Thermo Element 1 ICP-MS:– Cones: Ni
– Medium resolution (m/Δm=4000)
– Dual (counting and analog) mode
• Analysis: Principal Components Analysis (PCA)– Eigenvector Solo 6.2 Software (Eigenvector Technologies)
1.75 in.
0.00
1.00
2.00
3.00
4.00
5.00
6.00
-6.00 -4.00 -2.00 0.00 2.00 4.00 6.00
Sco
res
on
PC
1 (
98
.08
%)
x 107
Scores on PC 2 (1.78%) x 105
Homogeneous? PCA Plot- Duct Tape, Backing Side, 3M Brand
0 ft.
3 ft.
9 ft.
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
0.00 1.00 2.00 3.00
x 105
x 107
FemtosecondAblation
0.90
0.95
1.00
1.05
1.10
1.15
1.20
-1.00 -0.50 0.00 0.50 1.00 1.50 2.00 2.50
Sco
res
on
PC
1 (
99
.9%
)
x 108
Scores on PC 2 (0.00%)x 106
PCA Plot- Duct Tape, Adhesive Side, Duck Brand
0 ft.
3 ft.
9 ft.
FemtosecondAblation
-8
-6
-4
-2
0
2
4
6
8
10
-10 -8 -6 -4 -2 0 2 4 6
Au
tosc
ale
d S
core
s o
n P
C 2
(1
2.5
3%
)
Autoscaled Scores on PC 1 (14.51%)
PCA Plot-Duct Tape, Backing Side
3M
Duck
Nashua
Staples
Nanosecond Ablation
1
10
100
1000
10000
100000
1000000
Ave
rage
Q-R
esi
du
al
Model
Q-Residual Plot- Duct Tape, Backing Side
3M
Duck
Nashua
Staples
m_3M m_Duck m_Nashua m_Staples
95% C.I.
Electrical tape
1.75 in.
-5.00
-4.00
-3.00
-2.00
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00
Sco
res
on
PC
3 (
0.1
2%
)
x 105
Scores on PC 1 (96.92%)
x 106
PCA Plot-Electrical Tape, Adhesive Side, UL Brand
0 ft.
3 ft.
9 ft.
FemtosecondAblation
-4
-2
0
2
4
6
8
10
-8 -6 -4 -2 0 2 4 6 8 10 12
Sco
res
on
PC
1 (
95
.35
%)
x 106
Scores on PC 2 (2.77%)
x 105
PCA Plot- Electrical Tape, Adhesive Side
Old Super 33
Plymouth
Super 33
UL
Nanosecond Ablation
1.00E+08
1.00E+09
1.00E+10
1.00E+11
1.00E+12
1.00E+13A
vera
ge Q
-Re
sid
ual
Model
Q-Residual Plot Adhesive Side
Old Super 33
Plymouth
Super 33
UL
m_Old Super 33 m_Plymouth m_Super 33 m_UL
95% C.I.
m_Old Super 33 m_Plymouth m_Super 33 m_UL
Copper Speaker Wire
• Can be used for binding in a crime
• Can be used to make bombs and explosive devices
Method• Remove plastic coating
• Separate individual strands within the bundle
• Mount on device
• Single line ablation
• ns ablation
2 cm
0.1
1
10
100
1000
10000
0 1 2 3 4 5 6 7 8 9 10
Ave
rage
Q R
esi
du
al
Model for Wire #
Q-Residual Plot for 10 Wires
Wire 1
Wire 2
Wire 3
Wire 4
Wire 5
Wire 6
Wire 7
Wire 8
Wire 9
Wire 10
95% C.I.
Nanosecond Ablation
Galvanized Steel Pipe• Commonly used for making pipe bombs and
other explosive devices
• Representative for bulk metal analysis
• Steel is mostly Fe, coating is Zn
– Coating varies by manufacturer conditions
• Number of times dipped
• Cooling time, temperature
Method• Cut into pieces
• Line depth profile, ns laser3.81 cm
0
10
20
30
40
50
60
70
a b c d e f
Inte
nsi
ty (
cps)
X 106
Line
Depth Profile: Outside of Galvanized Pipe
Fe(56)
Zn(64)
Zn(66)
Conclusions
• Electrical and Duct tape homogeneous within roll
• Tape brands may be distinguished from one another
• Bulk copper speaker wire bundles are homogeneous within a roll
• Individual copper speaker wire strands can be distinguished from one another– Could allow us to do attribution studies
• Galvanized pipe coating depth may be sampled– Layer by layer analysis
Acknowledgments
• NIJ for the funding of this grant
• Dr. R. Sam Houk
• Dr. Stan Bajic
• Jonna Berry (copper wire)
• Maria Bock (tape)
• Jeneé Jacobs (galvanized pipe)
• Dan Zamzow