LASER-INDUCED BREAKDOWN SPECTROSCOPY OF ROCK
POWDERS PERFORMED AT VARIABLE ANGLES OF ABLATION AND
COLLECTION
K. H. Lepore , E. A. Breves , M. D. Dyar , S. C. Bender , and R. L. Tokar , Mount Holyoke College, Planetary Science Institute
2 1
2
Results and discussion
Relative importance of ablation and collection angles
- Changes in the ablation angle
are the predominant cause of
observed spectral variability.
- Depending on geochemical
composition, some samples
are more susceptible to differences in collection geometry
than others (Fig. 6, Fig. 4).
- The effects of variable collection angle in the VV configuration are diminished because
smaller plasmas form at large
ablation angles and are collected efficiently despite poor collection alignment (Fig. 2).
- Smaller plasmas are less
dense and less likely to suffer
from self-absorption [5].
400000
400000
600000
350000
350000
500000
400000
300000
200000
2
DH4911
0
40
45
50
55
400000
300000
200000
60
65
70
75
0°
13°
5.35
7°
20°
5.31
VF
27°
40°
4.77
47°
60°
3.65
Collection angle
Variable Ablation/
Fixed Collection
Atomic emission
Continuum emission
-13°
13°
5.22
0°
13°
5.35
FV
Fixed Ablation/
Variable Collection
-13°
0°
5.22
-13°
13°
5.22
20
-10
150000
W1
SRM688
GBW07313
200000
AGV2
GBW07104
150000
GBW07110
VF Atomic Emission
-60
-50
-40
-30
-20
-10
-60
0
350000
350000
300000
200000
GBW07113
-50
-40
-30
-20
-10
0
Ablation Angle (o)
600000
400000
JA1
50000
Ablation Angle (o)
500000
60
DH4911
100000
VF Continuum Emission
50000
0
40
250000
400000
80
-13°
20°
5.22
-13°
40°
5.22
-13°
60°
5.22
Figure 2. Geometric configurations and estimated ablation beam energy densities
used for variable angle LIBS measurements.
0 º:60 º Intensity Ratio
-13°
0°
5.22
Ablation angle
Ablation:
Collection:
Energy Density (GW cm2):
0
GBW07105
400000
300000
250000
200000
150000
100000
FV Total Emission
0
dacite
SiO2 (wt.%)
-20°
13°
5.03
200000
0
JA1
Standard surface
-40°
13°
4.10
250000
DH4911
GBW07105
300000
SRM688
250000
W1
GBW07313
200000
AGV2
10
20
30
40
50
60
0
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
470
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
470
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
470
FV Atomic Emission
20
30
40
50
0
60
Collection Angle (o)
VV
570
10
670
770
VF
GBW07313
-13°
-20°
-40°
-60°
3
2
1
0
470
520
570
4
620
670
720
GBW07313
3
770
13°
20°
40°
60°
2
1
0
470
520
570
620
4
670
720
SRM688
770
-13°
-20°
-40°
-60°
3
2
1
0
470
520
570
620
4
670
720
770
SRM688
3
13°
20°
40°
60°
2
1
10
20
30
40
Figure 6. Ratios of continua intensities in
the VNIR from 0o to 60o.
JA1
GBW07113
50000
50000
4
GBW07104
150000
100000
FV Continuum Emission
Collection Angle (o)
Variable Ablation/
Variable Collection
Ablation:
-60°
Collection:
13°
Energy Density (GW cm2): 2.68
-20
-20
300000
700000
AGV2
andesite
-20°
-7°
5.03
300000
100000
VF Total Emission
-30
Summed Intensity
500000
-40
-40
o
350000
-50
GBW07113
-60
60
350000
VV
-40°
-27°
4.10
40
600000
GBW07113
Collection Geometries
- LIBS spectra were acquired under Mars atmospheric conditions using a q-switched Nd:YAG laser at 1064 nm.
- Fixed angle mounts and rotating collection optics were coordinated to achieve three configurations (Fig. 2).
Ablation:
-60°
Collection:
-47°
Energy Density (GW cm2): 2.68
20
JA1
o
Ablation Angle (o)
W1
basalt
0
400000
100000
GBW07110
GBW07104
basaltic
andesite
-20
GBW07110
50
60
Collection Angle (o)
Dependency on standard composition
- Standards that exhibit the least
amount of variability in signal with
changing ablation or collection angle
(DH4911, SRM688, GBW07105, and
W1) contain the largest Fe2O3T and
MgO and lowest SiO2 abundances.
- Standards that exhibit the greatest
variability have the highest total alkalinity (GBW07313, AGV2, GBW07110,
JA1,and GBW07113) (Fig. 4).
0
470
520
570
620
670
720
Wavelength (nm)
770
As the collection angle increases, the self-absorption of
minor elements and those with many emission lines (Fe and
Mg) may decrease. Therefore, spectra of more geochemically
diverse standards are less susceptible to changes in collection
angle (Fig. 4, Fig. 6).
5.0
4.5
4.0
RMSE
picrobasalt
-40
GBW07104
GBW07110
4
SRM688
-60
AGV2
3.5
VV
0°
13°
20°
40°
60°
3.0
2.5
2.0
1.5
1.0
4.5
0.5
4.0
0.0
3.5
Al2O3 CaO Fe2O3 K2O MgO MnO Na2O SiO2
TiO2
FV
0°
13°
20°
40°
60°
3.0
2.5
2.0
1.8
570
670
770
FV
570
DH4911
GBW07105
SRM688
W1
GBW07313
AGV2
GBW07104
GBW07110
JA1
GBW07113
670
Wavelength (nm)
770
VV
1.7
DH4911
GBW07105
SRM688
W1
GBW07313
USAGV2
GBW07104
GBW07110
JA1
1.6
1.5
1.4
1.3
1.2
1.1
1.0
-60
-40
-20
0
20
40
Ablation Angle (o)
1.8
60
VF
1.7
Figure 4. Ratio of spectral intensity at 0 ablation/collection
to 60o at each pixel in a portion of the VIS/near-IR spectrum for continuum emission.
1.5
1.4
1.3
1.1
-60
-50
-40
-30
-20
-10
Ablation Angle (o)
1.8
0
FV
1.7
DH4911
GBW07105
SRM688
W1
GBW07313
USAGV2
GBW07104
GBW07110
JA1
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0
10
20
30
40
50
Collection Angle (o)
4.0
3.5
3.0
0.5
VF
0.0
0°
-13°
-20°
-40°
-60°
2.5
2.0
1.5
1.0
0.5
0.0
DH4911
GBW07105
SRM688
W1
GBW07313
USAGV2
GBW07104
GBW07110
JA1
1.6
1.0
1.0
4.5
Al2O3 CaO Fe2O3 K2O MgO MnO Na2O SiO2
1.2
o
Plasma dynamics and changes in collection and ablation angles
- The intensity ratio of Mg(II) to Mg(I) is
used as a proxy for plasma temperature.
- The minimum energy threshold required
to produce emitting species was achieved,
even at reduced energy densities (Fig. 5,
Fig. 2).
1.5
RMSE
6
60
Mg (II) / Mg (I)
GBW07105
basaltic
trachytrachy- andesite
basalt
40
Mg (II) / Mg (I)
tephrite/
basanite
GBW07313
o
Mg (II) / Mg (I)
8
20
Summed Intensity
trachyandesite
phonotephryte
0
400000
0º:60ºIntensity Ratio
Na2O + K2O (wt.%)
10
-20
GBW07313
150000
700000
-60
tephryphonolite
foidite
-40
SRM688
200000
50000
0
rhyolite
W1
50000
0º:60ºIntensity Ratio
12
trachyte/
trachydacite
150000
250000
0
Summed Intensity
phonolite
200000
GBW07105
100000
100000
Summed Intensity
Figure 1. Standard compositions plotted as wt. % SiO2 vs. total alkalis.
14
250000
DH4911
300000
100000
Summed Intensity
Summed Intensity
Standards analyzed include eight igneous rocks, one forsteritic olivine (DH4911), and one marine sediment (GBW07313) (Fig. 1).
300000
100000
-60
Experimental setup
Summed Intensity
700000
Summed Intensity
Summed Intensity
LIBS has become a popular tool in geochemists’ toolkits
Continua and atomic emission intensities increase for all standards as ablafor terrestrial and planetary exploration [1]. Quantitative tion and collection angles approach normal to the standard surface (Fig. 3).
analysis of LIBS spectra is plagued by variabilities in colFigure 3. Sum of intensities at all spectrometer pixels for total, continuum, and atomic emission.
lected spectra that cannot be attributed to differences in
elemental composition [2,3]. These are likely exacerbated
during field work, when the angles of ablation and collection are constantly changing [4]. In this study, multiple
sampling geometries are used to 1) quantify spectral variVV Continuum Emission
VV Atomic Emission
VV Total Emission
ability due to changes in LIBS configurations, and 2) isolate
Ablation Angle ( )
Ablation Angle ( )
Ablation Angle ( )
the variability in LIBS spectra due to changes in laser ablation angle from those due to changes in the collection
angle.
VV/VF
2
VV/FV
1,2
VV/VF
Introduction
1
VV/FV
1
60
Figure 5. Mg(II)/Mg(I) ratio of each standard
measured in all three configurations.
TiO2
Al2O3 CaO Fe2O3 K2O MgO MnO Na2O SiO2
TiO2
Figure 7. RMSE of elemental analyses of the
variable ablation/collection angle data from
this presentation, predicted using spectra
collected under identical Mars conditions
but with a normal collection angle and a 13o
angle of ablation.
Implications
Elemental compositions of our data predicted using a larger
but conventionally-collected calibration suite show that accuracy generally suffers when the ablation and collection angles
are high, even when spectra are normalized to total counts for
each detector (Fig. 7). Optimal quantitative analysis of LIBS
spectra must include some knowledge of the angle of ablation
and collection so the approximate increase in uncertainty introduced by a departure from normal angles can be accurately
reported.
Acknowledgments: This work was supported by NSF grants CHE-1306133 and CHE-1307179
and NASA grants NNX14AG56G, NNX12AK84G, and NNX15AC82G.
References: [1] Wiens R. C. et al. (2013) Space Sci. Revs., 170, 167-227. [2] Clegg S. M. et al.
(2009). Spectrochim. Acta, Part B, 64, 79-88. [3] Anzano J. M.et al. (2006). Analytica Chimica
Acta, 575, 230-235. [4] Multari R. A. et al. (1996) Appl. Spectrosc., 50, 1483-1499. [5] Shabanov V. (2008) Applied Optics, 47, 1745-1756.