Two-Step Resonance-Enhanced Desorption Laser Mass

46th Lunar and Planetary Science Conference (2015)
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TWO-STEP RESONANCE-ENHANCED DESORPTION LASER MASS SPECTROMETRY FOR IN SITU
ANALYSIS OF ORGANIC-RICH ENVIRONMENTS. S. A. Getty 1, X. Li2, A. Grubisic3, K. Uckert 4, T. Cornish 5, J. E.
Elsila1, M. P. Callahan 1, and W. B. Brinckerhoff1, NASA/GSFC, 8800 Greenbelt Rd., Greenbelt, MD 20771 (Stephanie.A.Getty@nasa.gov), 2University of Maryland, Baltimore County, 3University of Maryland, College Park, MD,
4
New Mexico State University, Las Cruces, NM, 5C&E Research, Inc., Columbia, MD.
Introduction: The interrogation of solid-phase samples by laser desorption/ionization mass spectrometry
(LDMS) is a powerful analytical technique that can
elucidate inorganic and organic composition from an
unprepared planetary surface material. LDMS will be
used as part of the Mars Organic Molecule Analyzer
(MOMA) investigation on the 2018 ExoMars rover mission. Here we report efforts, using a time-of-flight mass
spectrometer prototype, to advance laser desorption/ionization (LDI) beyond single-color, broadband
analysis, to provide further confidence in the identification of sample constituents.
In two-step laser mass spectrometry (L2MS), one
laser wavelength is dedicated to a desorption step,
while a second laser wavelength is optimized for ionization of the resulting neutrals. With L2MS, the fragmentation of the resulting molecular ions can be minimized,
and selectivity to certain organics in the presence of an
inorganic matrix can be enhanced [1-3].
In a laboratory prototype, we have demonstrated
the use of a tunable infrared (IR) laser to effect res onance-enhanced desorption for those species having a
vibrational mode within the accessible range. We
demonstrate the technique for the case of Murchison
meteorite powder.
Instrument and Methodology: The L2MS is a compact prototype, as shown in Figure 1, with the mass
analyzer measuring only 30 cm in length and 5 cm in
diameter [3-5]. The sample is held at the focal plane of
the instrument, approximately 2-3 mm from the ion inlet.
The IR desorption laser (Opotek 2731) is focused at the
plane of the sample. Its wavelength can be tuned (via
an optical parametric oscillator) between 2.7 µm and 3.1
µm, and its output is pulsed with repetition rate up to
20 Hz and pulse width as short as 4 ns. The ultraviolet
(UV) ionization laser is oriented parallel to the sample
plane, and it is focused at some distance (< 1 mm)
above the plane of the sample. The UV laser is operated at the 266 nm harmonic of a pulsed Nd:YAG with
pulse duration as short as 4 ns. The UV pulse is triggered at a delay after the IR laser pulse to intersect and
ionize the neutral plume generated by the desorption
step. The resulting ions are accelerated into a time-offlight mass analyzer with curved-field reflectron. The
time-resolved ion packets are detected by a microchannel plate.
The sample preparation was minimal, consisting
simply of affixing a layer of powdered meteorite onto a
layer of double-sided tape that has been shown to impart negligible background signal to the mass spectrum
at the energies used in this study.
IR laser
UV laser
L2MS prototype
Figure 1. A compact prototype has been developed
that enables two-step laser mass spectrometry on planetary analog and meteorite samples in an in situcompatible instrument volume.
Results: The Murchison meteorite is well known to
contain a wide variety of organic molecules and carbonaceous macromolecular material in its composition [6].
Here we examine spectra from a powdered sample of
Murchison meteorite with the L2MS technique and
with single-color LDMS for comparison. LDMS can be
used to infer the broadband composition of the ino rganic and organic constituents of the sample. Twocolor L2MS mode with the use of 266 nm ionization
wavelength lends specificity to the fraction of organic
material that is aromatic in nature.
An LDMS spectrum of Murchison meteorite powder is shown in Figure 2. This single-color measurement used a 266 nm laser pulse to desorb and ionize the
sampled species in a single step. No data processing
was used here, for example, to amplify the higher molecular weight peaks to compensate for the dominant
Na and K peaks (seen at m/z 23 and 39/41, respectively). Despite the low relative intensity of the organic
species, a familiar envelope of mass peaks can be seen
at higher m/z, corresponding to the masses of acenaphthylene, phenanthrene and/or anthracene, fluoranthene
46th Lunar and Planetary Science Conference (2015)
and/or pyrene, and a series of their methylated derivatives.
Murchison, LDMS
UV 266nm
0.10
0.01
0.08
0.06
0.00
0.04
0.02
100
150
200
250
300
350
0.00
0
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100
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m/z
Figure 2. A single-color UV LDMS spectrum of the
Murchison meteorite is dominated by the Na and K salt
peaks and shows low-intensity mass signatures of organic composition at m/z > 150.
A series of L2MS spectra are shown in Figure 3 for
the same Murchison meteorite powder, using varying
IR wavelength to desorb neutrals from the sample. The
IR and UV laser powers were held constant as the
wavelength was varied. A maximum in signal strength
is observed to occur at an intermediate wavelength,
between 2.9µm and 3.0 µm. The signal is significantly
reduced at wavelengths outside of this range; lower
wavelength of 2.8 µm and higher wavelength of 3.1 µm
is shown in Figure 3.
Murchison, 40shots Average
2800nm
0.016
0.014
0.012
0.010
0.008
0.016
2900nm
0.014
0.012
0.010
0.008
0.016
3000nm
0.014
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0.016
3100nm
0.014
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0.008
0
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100
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500
Mass (Da)
Figure 3. L2MS signal intensity is seen to be maximized
at IR wavelength between 2.9 µm and 3.0 µm.
The wavelength at which these spectra exhibit a
maximum coincides with a range of vibrational res onances of polycyclic aromatic hydrocarbons known to
occur in the Murchison meteorite. The pattern of mass
peaks is, specifically, consistent with contributions
from phenanthrene and/or anthracene, fluoranthene
and/or pyrene, and their corresponding methylation
series. Anthracene and fluoranthene, in particular, ex-
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hibit an absorption feature seen in the vibrational spectra in the range 2.9-3.1 µm [7]. This feature is smaller
than the well-known C-H stretch mode at 3.2-3.3 µm.
Taken together, these data suggest that the IR desorption laser at particular wavelengths can couple strongly
to the vibrational resonances of specific sample analytes. For the Murchison meteorite, the dual specificity
offered by the IR vibrational resonance and resonanceenhanced ionization at 266 nm provides high confidence in the peak assignments of methylated PAHs
discussed here.
The measurements reported here illustrate the utility
of exploiting the effects of resonance-enhanced desorption in two-step laser mass spectrometry. This
technique enables enhanced specificity in analyses of
complex samples and can improve confidence in the
identification of mass peaks in L2MS spectra.
References:
[1] Zenobi R. et al. (1989) Meteoritics 24, 344. [2] Elsila
J. E. et al. (2005) Geochim. Cosmochim. Acta 69, 1349.
[3] Getty, S. A. et al. (2012) Rapid Communications in
Mass Spectrometry 26, 1. [4] Brinckerhoff W. B. et al.
(2000) Rev. Sci. Inst. 71, 536. [5] Cornish T. et al. (2000)
Rapid Comm. Mass Spec. 14, 2408. [6] Botta O. and
Bada J. L. (2002) Surveys in Geophysics 23, 411. [7]
Coblentz Society, Inc., "Evaluated Infrared Reference Spectra" in NIS T Chemistry WebBook, NIS T S tandard Reference Database Number 69, Eds. P.J. Linstrom and W.G.
M allard, http://webbook.nist.gov.