Laser desorption/ionization time-of-flight mass spectrometry (LD-TOF-MS) is a versatile, low-complexity instrument class that holds significant promise for future landed in situ planetary missions that emphasize compositional analysis of surface materials. Here we describe a 5kg-class instrument that is capable of detecting and analyzing a variety of analytes directly from rock or ice samples. Through laboratory studies of a suite of representative samples, we show that detection and analysis of key mineral composition, small organics, and particularly, higher molecular weight organics are well suited to this instrument design. A mass range exceeding 100,000 Da has recently been demonstrated. We describe recent efforts in instrument prototype development and future directions that will enhance our analytical capabilities targeting organic mixtures on primitive and icy bodies. We present results on a series of standards, simulated mixtures, and meteoritic samples

Brinckerhoff, W. B.

Callahan, M.

Chanover, N.

Cornish, T.

Ecelberger, S. A.

Elsila, J. E.

Floyd, M. A. Merrill

Getty, S. A.

Glenar, D.

Li, X.

Tawalbeh, R.

Uckert, K.

Voelz, D.

Xiao, X.

NASA Technical Reports Server

Laser Time-of-Fllght Mass Spectrometry for Future In Situ Planetary Missions. S. A. Getty,' W. B. Brin~ker­
hoff,' T. Cornish,' S. A. Ecelberger,' X. Li,' M. A. Merrill Floyd,' N. Chanover,' K. Uckert,' D. Voelz,' X. Xiao,' R. 
Tawalbeh,' D. Glenar,' J. E. Elsila,' and M. Callaban' 'NASA Goddard Space Flight Center (Mailstop 699.0, 
Greenbelt, MD 20771; Stephanie.A.Getty@nasa.gov), 2C&E Research, Inc., Columbia, MD 21045, 'University of 
Maryland, Baltimore County, Baltimore, MD 21250, "New Mexico State University, Las Cruces, NM 88003. 
Introduction: Laser desorption/ionization time-of-
flight mass spectrometry (LD-TOF-MS) is a versatile, 
low-complexity instrument class that holds significant 
promise for future landed in situ planetary rnissions 
that emphasize compositional analysis of surface mate-
rials. Here we describe a 5kg-class instrument that is 
capable of detecting and analyzing a variety of analytes 
directly from rock or ice samples. Through laboratory 
studies of a suite of representative samples, we show 
that detection and analysis of key mineral composition, 
small organics, and particularly, higher molecular 
weight organics are well suited to this instrument de-
sign. A mass range exceeding 100,000 Da has recently 
been demonstrated. We describe recent efforts in in-
strument prototype development and future directions 
that will enhance our analytical capabilities targeting 
organic mixtures on primitive and icy bodies. We pre-
sent results on a series of standards, simulated mix-
tures, and meteoritic samples. 
Current Instrument Design: The core instrument 
is housed in a high vacumn chamber, as shown in Fig-
ure I, and includes an ultraviolet (UV) laser to desorb 
and ionize aoalytes from 'a solid surfuce, an electrostat-
ic ion extraction lens assembly. a curved field ion re-
flectron, and back-to-back linear- and reflected-mode 
rnicrochannel plate detectors [1-3]. A pulsed (4-7 ns), 
frequency tripled (355 om) or quadrupled (266 om) 
Nd:YAG laser is focused at the sample to generate ions 
from a - I 00 micron diameter spot. The ions then sep-
arate according to their masses based on the TOF tech-
nique. 
Complementary· data can be acquired through the 
close integration of other in situ sample interrogation 
techniques, such as infrared (JR) point spectroscopy. 
This is illustrated by the recent integration of an acous-
to-optic tunable filter (AOTF) spectrometer developed 
at New Mexico State University [4) with a LD-TOF-
MS prototype. The illumination source of the JR spec-
trometer is wavelength-tunable in the near-JR using an 
AOTF module, and the sample reflectance is recorded 
using an HgCdTe detector (Teledyne Judson Technol-
ogies Company). The resulting miniature AOTF spec-
trometer measures approximately 4.5" x 5" x 1.6" and 
was designed to fit into a 8" Kimball Physics vacuum 
chamber. To truly integrate the two instruments, the 
focal planes are designed with a common boresight, 
such that the same region on the sample surface can be 
analyzed simultaneously by both techniques. This re-
quired that the LD-TOF-MS ion inlet be made narrow 
enough to penetrate a bore-hole in the AOTF mirror 
assembly, as shown in Figure 2. The sample focal 
plane is located - 5 mm below the LD-TOF inlet and 
AOTF housing. 
Figure 1. The LD-TOF-MS is a 5 kg-c1ass instrument meas-
uring - 30 cm long. Capabilities include po.sitive and nega-
tive ion modes, advanced MS techniques such as tandem MS 
and L2MS, and demonstrated complementarity to other in 
situ analytical instruments, such as IR point spectros~opy. 
Figure 2. The AOTF IR point spectrometer is housed in a 
volume of - 4.5" x 5" x 1.6". Integration with the LD-TOF-
MS will enable coordinated, coincident sample analyses, 
https://ntrs.nasa.gov/search.jsp?R=20120013609 2019-08-30T21:50:54+00:00Z
Recent Instrument Improvements: Traditiooally, 
mass spectrometers for planetary missions have fo-
cllsed on positive ion detection, but, because both posi-
tive and negative ions are produced by the laser, the 
LD-TOF-MS is also naturally suited to negative ion 
analyses [5). The laser pulse produces photoelectrons 
through interactions with the solid surface, and nega-
tive ions are fonned through electron attachment to the 
desorbed neutrals. In negative mode, the detector volt-
age bias is held constant, but the other instrument volt-
ages are adjusted to reverse polarity, to be optimized 
for the transmission of negatively charged species to 
the detector plane. 
The ability to acquire both positive and negative 
mass spectra point-by-point represents a significant 
enhancement in our analytical capabilities. In positive 
mode, the mass spectra are often dominated by the om-
nipresent signatures ofNa and K cations. Because the 
ionization potential of these species is relatively low, 
these ions and molecular adducts are formed easily 
from most sample surfaces. The dominsnt peaks that 
appear at mlz of 23 and 39 are often much higher inten-
sity than the analytes of interest. Na and K are not seen 
in negative mode and the obscuring effects are there-
fore avoided. 
Some classes of molecule are especially well suited 
to negative ion fonnation. For example, we have re-
cently observed that negative ions are preferentially 
fonned for carboxylic acids that are mixed with clays 
and other mineral matrices in laboratory simulants. 
The intact negative molecular ion is readily observed, 
whereas positive mode shows little evidence of the 
parent molecule. Negative ion mode is also particular-
ly useful for detecting electronegative atomic and mo-
lecular species that may be present in planetary surface 
samples, such as described for the case of chlorine and 
perchlorate below. 
Future Directions: Time-<>f-flight mass analysis is 
well suited to high molecular weight mass identifica-
tion. In principle, the time-of-f1ight technique is uulim-
ited in mass range, i.e. , the heavier ions simply take 
longer to reach the detector, without additional re-
quirements on the power supplies. For this reason, the 
LD-TOF-MS is a promising instrument technology for 
advanced in situ organics analysis from solid samples. 
New proof-of-concept modifications to the core proto-
type bave demonstrated advanced spectrometric capa-
bilities that will allow, not only the mass assignment 
from a complex mixture of organics, but also the pos-
sibility of molecular structure assignments. 
Tandem mass spectrometry. In commercial instru-
ments, tandem mass spectrometry, or MSIMS is used to 
isolate a particular mass of interest and intentionally 
induce fragmentation of that species to produce a type 
of molecular fingerprint. The resulting fragmentation 
pattern can then be compared to published product ion 
data to assign a molecular structure to the mass peak. 
In practice, the parent molecular ion must be first se-
lected by ion gating before the fragmentation step. [on 
gating has recently been demonstrated in the miniature 
prototype shown in Figure 1. The fragmentation pat-
tern is ShOWD in Figure 3 for two peptide species that 
fragment spontaoeously: angiotensin II and polypro-
line (P 14R). The ability to focus both the parent ion 
and the fragments simultaneously relies on our unique 
curved-field reflectron. Although this technique is 
widely used in commercial MALDi mass spectrome-
ters, to our knowledge, this is the first report of pseudo-
tandem MS on a miniature instrument. 
'l,K 
~rodu,t tonI G.U, ~elerted -
, 
-,--,- IJ.; 
,,-
_. 
Gate ""~cted -1, 
"g/o~"" " Prnd"'.IM' . I I 
' J~ __ 
, 
Matrix Ions MIxtUt.: ~o II + Pull I I 111 1. JG~OFF l ~, Anliotensln III t·R 
200 
"'" 
600 800 llJOO 1400 
Figure 3 . A critical first step towards tandem MS capabili-
ties in a miniature LD-TOF-MS is this demonstration of ion 
gating. For a mixture of angiotensin II and polyproUne 
(Pl~)' a pulsed ion gate allows the isoLation of each patent 
ion and its post-source decay product ions. 
In the near term, we will be integrating an induced 
fragmentation functionality to the miniature instrument. 
In addition to species that spontaneously decay, as 
shown in Figure 3, this induced fragmentation will en-
able generalized fragmentation analysis of a wide range 
of parent molecules. 
Two-step laser mass spectrometry. Another ad-
vanced MS technique that represents a new advance for 
these prototypes is post-ionization, or two-step laser 
mass spectrometry (L2MS). In some cases, use of a 
single laser to desorb and ionize in one step leads to an 
excess of energy in the resulting molecular ion, which 
can produce undesirable fragmentation. By decoupling 
the desorption and ionization processes, we are better 
able to tune the laser energies to operate below the 
fragmentation threshold. This is accomplished by us-
ing a lower-intensity infrared laser pulse to first desorb 
neutrals "intact" from the sample surface, followed by 
an orthogonal UV laser pulse to · intersect the neutral 
plume to produce ionization. L2MS offers the ad-
vantage of selectivity to aromatic compounds, as has 
been thoroughly demonstrated on laboratory-scale 
L2MS instrumentation [e.g. , 6-9]. In the miniature 
prototype, we have conducted L2MS studies of a mod-
el P AH, pyrene, to show that fragmentation can be 
minimized by operating the IR laser below the ioniza-
tion threshold and the UV laser just above the ioniza-
tion threshold [10]. 
Terrestrial Planets: The mineralogy of solid 
planetary bodies is of primary interest as part of a land-
ed investigation. Understanding the mineralogy of a 
sampling site gives valuable geologic and petrologic 
context for any compositional analyses that are under-
taken. The insights enabled by spatially correlated LD-
TOF-MS and AOTF analyses produces clues to miner-
aID gy by revealing the vibrational signatures of key 
functional groups, as well as the overall inorganic 
composition. As a low-power screening tool, the 
AOTF capability offers information that supports sam-
ple composition, mineralogy, and organic content. 
Detailed analyses can then be conducted on a sample 
of interest using the LD-TOF-MS to produce in-depth 
compositional data from a solid or powdered sample. 
Prior to integration of the AOTF spectrometer 
with the LD-TOF-MS prototype, a suite of mineral 
samples were analyzed in parallel by each instrument. 
The sample suite was selected to span major classes of 
mineral that would have implications for an aqueous 
geologic history: sulfates, carbonates, iron oxides, and 
clays. A carbonate series is shown in Figure 4, as 
measured in LD-TOF-MS positive ion mode. 
Ali can be seen for siderite, magnesite, calcite, and 
dolomite, the cation is readily detected using LD-TOF-
MS. With corroborative AOTF measurements, as 
shown in Figure 5, the -CO, group is identified by a 
series of characteristic features in the IR spectrum, as 
indicated. Taken together with LD-TOF measure-
ments, the two data sets strongly suggest source miner-
alogy and can reveal mass peaks originating from or-
ganic content. Although not shown here, we have 
found that the population of bigher order oxides in the 
LO-TOF spectra is found to correlate with degree of 
hydration of the particular mineral, as verified by 
AOTF hydration signatures and the known source min-
eral [4]. 
~--'-'-'--'-'- ~-~·-~~"'--"-'1 
0.35 ;-...... -.. Carbonates J-l 
0.30 r 
-Co""" 
. r . I 
. i .~ \ ,- _ .. . Dolomite I 
Q.I 0.25 r -c'») '. / \. .. -- Sid..-,At • 1 ~ 0,20 ~-.. -. -.-. .. -..... -.... ... ,~ I.,. .... \. /" J 
tJ , ....... \ \. / 1 
.!! ' I '. \, . __ .. __ .. --..... . 
~ 0.15 ,. - -- .... / /i 
0.10 ~ ,./ .. , 
0.05 ~ 11 
0.00 I'--~":"::~~~'--'-.....:...._:"--" __ (o'-=-...JI 
2.0 2.5 3.0 3.5 
Wavelength (jim) 
Figure 5, AOTF poiDt spectrometer analysis reveals vibra-
tional signatures at IR wavelengths that identify carbonate 
groups in calcite, dolomite, and siderite. 
Perchlorate salt is an important species known to be 
present at the Martian surface. We have conducted 
negative ion mode studies of perchlorate salt as a 
standard and in the presence of a clay mineral matrix 
with organic species added. An example of this labora-
tory simulant sample is shown in negative mode in 
Figure 6. With kaolinite clay as the mineral matrix, a 
1 % NaC10, solution was added to the clay and allowed 
to dry. Finely ground phthalic· acid powder was also 
added to the clay sample and mixed thoroughly. In 
negative ion mode, the CI isotopes are seen at m/z 35 
and 37, and the perchlorate anion is a dominant feature 
in the spectrum at mlz 99. and 101. Eveo in the pres-
ence of perchlorate salts, the parent molecular ion of 
phthalic acid is observed at mlz 166. This distin-
guishes LD-TOF-MS from other regolith analyzers, 
such as pyrolysis-mass spectrometry, which evolves 
mnple volatiles and decomposition products using 
prolonged application of heat. This thennal decompo-
sItion process may promote the degradation of organics 
that are co-located with perchlorate salts, thereby de-
Slroying the structural information of the parent organic 
compound. In contrast, LD-TOF-MS does not impart 
a?preciable heat into the mixed sample and therefore 
preserves the organic parent ion. 
These classes of mineral 'samples are relevant ana-
logs for the surface of Mars and perhaps Venus. It is 
worth noting that MgSO, is also suspected of contrib-
uting to the salty deposits on the surface of Jupiter's 
icy moon Europa. Direct measurements of epsomite 
are planned for the near future. 
0.03 
Figure 6. LD-TOF-MS mass s~rum of kaolinite clay 
spiked with NaCl04 and phthalic acid shows clear signatures 
of Cl isotopes, the perchlorate anion, and the phthalic acid 
parent ion. 
Primitive and ley Bodies: 10 a complex mixture, 
such as that seen for carbonaceous chondrite meteor-
ites, a mixture of organic compounds is present, which 
can lead to challenges in peak identification and the 
ability to resolve isomolecular interferences. The selec-
tivity of L2MS to aromatics effectively separates the 
aromatic composition from the mixture without the 
need for front-end chromatography. This is shown in 
Figure 7 for a Murchison meteorite extract. The total 
organic extract (top) was first analyzed by LD-TOF-
MS (data from a commercial Bruker Autoflex Speed is 
shown here) to reveal the complexity of the mixture. 
Using GSFC liquid chromatography facilities, the non-
polar fraction was isolated and analyzed by the com-
mercial Bruker LD-TOF-MS (center). The bottom 
spectrum shows the total organic extract, as measured 
by L2MS. The mass spectrum shows striking similari-
ties with the LC-separated non-polar extract, suggest-
ing that L2MS effectively accomplishes this separation 
through the specificity of the two-laser pulse technique. 
IOCOOOr-------------------~M-u'~ch~j~~"----~ 
Total Organic Carbon 
5l1OOO j. 8t~ Autoflex 
0~·,~~~~~~~$+· ~·~~1~~~ 
1110 180 200 220 2-40 260 leD 300 320 340 
~ l~r-------------------~No~"~~-'~F-~~·~~ 
.e ..... 218 _.. 8ruker Autoflex 
.!!. 50000 ~ £ .... 
1110 I . ~~6 ~o 27" 288 300 314 328 i " I ":,, , II . ..' I' -:]; ,' , 
is 160 1eo 200 220 240 2GO 280 300 3:) 340 
0.02 ,--------------------=-.,..,..,-...,...7"~ 
Total Org8n1e Carbor 
L GSFC Mini l2MS 
J Ii ,'\ L~v'l. .~ J. .!. . .OO ~~~~,,~~~·~~OW~~~~~~·~~,~O~,.~,.~~~~~~ 
160 110 200 220 240 2&0 2&0 300 320 340 
mfz (Oa) 
Figure 7. L2MS affet'S enhanced specificity to m:omatic 
species within a complex mixture, such as is shown here for 
a Murchison meteorite total organic extract (LD-TOF-MS, 
top). The L2MS spectrum (bottom) produces comparnble 
information to that seen in a single-laser LD-TOF-MS analy-
sis of an LC-separated non-polar fraction (center). 
The Murchison meteorite is a representative sam-
ple, but only the beginning of analyses, relevant to 
primitive and icy bodies. In future work, we plan to 
incorporate the capability to analyze icy surfaces to-
wards future mission opportunities to the icy satellites 
and primitive, icy bodies. 
References: [I] Brinckerhoff W. B. et aJ. (2000) 
Rev. Sci. Insl. 71, 536. [2] Cornish T. et al. (2000) 
Rapid Comm. Mass Spec. 14, 2408. [3] Getty el al. 
(2012) Concepts and Approaches for Mars Explora-
lion, Abstract #4302. [4] Chanover et a1. (2011) IEEE 
Aerospace Con[. DOl: 10.1109/AERO.2011.5747295. 
[5] Getty et al. (2011) Lunar Plan. Sci. Con! Abstract 
#2490. [6] Zenobi el al (1989) Science 146, 1026-
1029. [7] Pizzarello el al. (2001) Science 193, 2236-
2239. [8] Clemert el al (2003) Science 261, 721-725. 
[9] Elsila el al. (2005) Geochim. Cosmochim. Acta 69, 
1349-1357. [10] Getty el al. (2012) Rapid Commun. 
Mass Spec. in review. 


Laser Time-of-Flight Mass Spectrometry for Future In Situ Planetary Missions

Laser Time-of-Flight Mass Spectrometry for Future In Situ Planetary Missions

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