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).

7

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