Chemical Geology 225 (2006) 30 – 39
www.elsevier.com/locate/chemgeo
High precision analysis of all four stable isotopes of sulfur
( S, 33S, 34S and 36S) at nanomole levels using a laser fluorination
isotope-ratio-monitoring gas chromatography–mass spectrometry
32
Shuhei Ono a,*, Boswell Wing b, Douglas Rumble a, James Farquhar b
a
b
Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington DC 20015, United States
Earth Systems Science Interdisciplinary Center and Department of Geology, University of Maryland, College Park, MD 20742, United States
Received 15 September 2004; received in revised form 31 July 2005; accepted 2 August 2005
Abstract
The discovery of mass-independent isotope effects observed in Archean rocks, certain classes of meteorites, and atmospheric
aerosols has had profound implications to our understanding of ancient and present atmospheric sulfur chemistry. We present a new
technique that takes advantage of continuous He flow isotope-ratio-monitoring gas chromatography–mass spectrometry to achieve
precise analysis of all four stable sulfur isotopes (32S, 33S, 34S, and 36S) at nanomole level samples. The technique involves
fluorination of sulfide (silver sulfide or pyrite), and separation of product gas by gas chromatography and the removal of mass-131
interference by a liquid-nitrogen ethanol slush at 110 8C. This technique works with an optimum sample size of 100 to 200 nmol
with precision for D33S and D36S at 0.1 and 0.5x (2r). Samples, as small as tens of nanomole, can be analyzed using this new
method. One of the major sources of error in irm-GCMS is found to be tailing of the major ion beam (32SF5+) onto minor beams
(33SF5+ and 36SF5+), which results in contraction of the measured d 33S and d 36S scales. This effect is corrected by measuring a series
of reference sulfide samples with mass-dependent sulfur isotope compositions. This methodology increases the spatial resolution of
the laser ablation in situ analysis and considerably reduces the analysis time as compared with conventional dual inlet methods.
D 2005 Elsevier B.V. All rights reserved.
Keywords: S-33; S-36; Sulfur isotope; Laser; Isotope analysis; Continuous flow; Peak tailing; Mass-independent fractionation; Archean
1. Introduction
Sulfur has four stable isotopes, 32S, 33S, 34S, and 36S
with fractional abundances of approximately 95.04%,
0.75%, 4.20%, and 0.015%, respectively (Ding et al.,
2001). Measurements of three isotope ratios of sulfur,
33
S / 32S, 34S / 32S, and 36S / 32S, have applications to
studies of meteorites (Farquhar et al., 2000b; Gao and
* Corresponding author. Fax: +1 202 478 8901.
E-mail address: s.ono@gl.ciw.edu (S. Ono).
0009-2541/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemgeo.2005.08.005
Thiemens, 1991), ancient rocks and minerals (Farquhar
et al., 2000a; Mojzsis et al., 2003; Ono et al., 2003;
Runnegar et al., 2002), polar ice (Savarino et al., 2003),
and atmospheric samples (Romero and Thiemens,
2003). These materials are known to exhibit nonmass-dependent isotope effects that can only be seen
by precise measurement of more than one sulfur isotope
ratios.
Multiple isotope ratios of sulfur have been measured
by a multi-collector secondary ion mass spectrometer
(SIMS) (Farquhar et al., 2002; Greenwood et al., 2000;
Mojzsis et al., 2003) and a conventional gas-source
S. Ono et al. / Chemical Geology 225 (2006) 30–39
isotope-ratio mass spectrometer (IRMS) (Gao and Thiemens, 1991; Hoering and Prewitt, 1988; Hu et al.,
2003; Hulston and Thode, 1965; Rumble et al.,
1993). With the SIMS, polished samples are sputtered
by a primary beam of Cs+ ions and the resulting secondary ions of 32S, 33S and 34S are collected simultaneously by multiple faraday cups. Precision for
D33S (see Notation section for definition) can be as
good as 0.2x to 0.3x (2r). SIMS has the best spatial
resolution for in situ analysis (~25 Am diameter) for
measurements of two isotope ratios of sulfur (d 33S and
d 34S).
Most laboratories measure d 34S by gas source mass
spectrometry in the form of SO2 but analysis of D33S is
less precise compared to SF6 method because of mass
interferences due to three isotopes of oxygen (16O, 17O
and 18O) (Hulston and Thode, 1965; Rees, 1978). The
advantage of the SO2 method in application of analysis
of d 33S and d 34S, however, is high sample throughput
when performed by combustion of sulfur by means of
elemental analyzer and isotope analysis by isotoperatio-monitoring gas chromatography–mass spectrometer (irm-GCMS) (Baublys et al., 2004).
Because fluorine has only one isotope (19F), SF6
method has been the preferred method for high precision multiple sulfur isotope analysis. It involves fluorination of silver sulfide (or sulfide minerals) by either
BrF5 or F2 and purification of SF6 by gas chromatography (Gao and Thiemens, 1991; Hoering and Prewitt,
1988; Hu et al., 2003; Rumble et al., 1993) or a
cryogenic trap (Beaudoin and Taylor, 1994). The accuracy and precision of the method is typically 0.2x for
d 34S and better than 0.1x and 0.2x for D33S and D36S
respectively (Gao and Thiemens, 1991; Hoering and
Prewitt, 1988; Hu et al., 2003). The SF6 technique can
be coupled with an IR laser (Beaudoin and Taylor,
1994) or a UV laser (Hu et al., 2003) for in situ analysis
of sulfide minerals. However, a conventional dual inlet
IRMS typically requires 1 to 5 Amol SF6 for routine
analysis. For pyrite, this corresponds to a ca. 300 to 500
Am diameter cylindrical pit of ca. 300 to 500 Am depth.
Because modern laser sampling systems can achieve pit
sizes more than ten times smaller than this, spatial
resolution of the dual-inlet technique is limited not by
the optics of the laser system but by the sample size
requirement for dual inlet isotope analysis (Hu et al.,
2003). This is also the case for the laser-sampling
technique for oxygen isotopes analyses of minerals
(Wiechert et al., 2002; Young et al., 1998).
Here, we describe the first application of an irmGCMS to the laser ablation SF6 technique to measure
all four stable isotopes of sulfur. The advantage of irm-
31
GCMS is analysis of small samples (nanomole to picomole level) by introducing sample gas in a stream of
He. First, the precision and accuracy of the continuous
flow SF6 method is evaluated and discussed independent of assessment of errors associated with laser sampling. Overall precision and accuracy of the method for
in situ analysis are tested by cross-comparison with
conventional dual inlet methods and irm-GCMS. Significant improvement was made for measurements of
36
S that had been laborious and required careful analyses (Gao and Thiemens, 1991; Hoering and Prewitt,
1988).
2. Notation
Conventional delta notation is used to describe the
isotope composition as:
dx S ¼
x
Rsa =x Rref 1Þ 1000;
ð1Þ
where, x is 33, 34, or 36, and x R = x S / 32S for the sample
(R sa) and the reference material (R ref) such as Cañon
Diablo Troilite. Capital delta notation is used to describe the deviation of the isotopic composition of a
given sample from a reference mass fractionation line.
We define (Hulston and Thode, 1965; Miller, 2002;
Ono et al., 2003):
D33 S ¼ ln d33 S=1000 þ 1
33 k ln d34 S=1000 þ 1 1000 and
ð2Þ
D36 S ¼ ln d36 S=1000 þ 1 36 kln d34 S=1000 þ 1Þ
1000:
ð3Þ
The values of 33k and 36k are approximately 0.515 and
1.91, respectively (Farquhar and Wing, 2003; Hulston
and Thode, 1965). Exact values vary by a few percent
depending on the sulfur species, temperature, and fractionation mechanism (Farquhar and Wing, 2003; Hulston and Thode, 1965; Young et al., 2002).
3. Instrumentation
A laser fluorination SF6 system was developed to
measure four isotopes of sulfur (32S, 33S, 34S, and 36S)
at the Geophysical Laboratory (Hu et al., 2003). We
modified the earlier version of the system as described
below. The system consists of three parts: a laser fluorination manifold, a gas chromatography purification
system, and a gas-source isotope-ratio mass spectrometer (Finnigan MAT 253) (Fig. 1). The mass spectrometer has an acceleration voltage of 10 keV, and is
32
S. Ono et al. / Chemical Geology 225 (2006) 30–39
Fig. 1. A schematic diagram of LA-irm-GCMS at Geophysical Laboratory. RC: reaction chamber; M: capacitance manometer; CT: cold traps, V1-3:
multiport valves.
equipped with faraday cups that allow simultaneous
determination of mass 127, 128, 129, and 131 (32SF5+,
33
SF5+, 34SF5+, 36SF5+, respectively). Registers (R f) are
3 108, 3 1010, 1 1010, and 1 1012 (V) for mass
127, 128, 129, and 131, respectively, and the amplifiers
have dynamic a range of 50 V.
The gas purification system consists of a gas chromatograph and three multiport valves (Fig. 1). The
column is 1 / 8 in. OD, 8 feet length, packed with
Hayesep Q (80–100 mesh), and operated at 80 8C
with flow rate of 23 mL He/min. A thermal conductivity detector (TCD) is used to monitor fluorination products. A six-port valve (V1) is used to interface the
laser fluorination manifold and irm-GCMS. A collection loop is used to trap contaminants causing a mass131 interference for irm-GCMS analysis (described in
detail below). A four-port valve (V3) is added to vent
contaminants trapped in the collection loop. The GC
system is interfaced to a mass spectrometer via an open
split, which has a 0.3 mL/min flow rate from the open
split to the mass spectrometer.
4. Analytical procedure
4.1. Fluorination and sample injection
For in situ analysis, samples are prefluorinated at
b30 Torr F2 without laser sputtering. This pre-fluorina-
tion is an important step for in situ laser analysis
because compounds reactive with fluorine at room
temperature (hydrocarbons, water, hydrous minerals,
etc.) often interfere with fluorination reaction by producing S–O–F compounds (Beaudoin and Taylor,
1994; Hu et al., 2003). The prefluorination also minimizes a system blank that derives from spontaneous
fluorination of fine-grained sulfide minerals, and thus
become critical in analysis of small samples by irmGCMS. The sample and reaction chamber are repeatedly prefluorinated and evacuated. Fluorination products are monitored by a capacitance manometer and
GC until blank levels are below 1 nmol. This prefluorination does not apply for analysis of silver sulfide
international reference materials (IAEA S-1, S-2, and
S-3) because silver sulfide reacts with fluorine at room
temperature. These silver sulfides were loaded one at a
time to prevent cross contamination.
Sulfur hexafluoride is produced by reaction of sulfide minerals (pyrite, and silver sulfide) with ~30 Torr
of elemental fluorine by either a CO2 laser to heat
physically separated grains and silver sulfide reference
materials, or an excimer laser (KrF: 248 nm) to ablate
parts of grains for in situ analysis. After the reaction,
excess fluorine is converted to KF by reaction with
heated KBr and product Br2 was trapped in cold traps
(CT2 and CT3 in Fig. 1). SF6 that is remaining in the
reaction chamber and the vacuum line is transferred to
S. Ono et al. / Chemical Geology 225 (2006) 30–39
CT1 during this process. After evacuating non-condensables, the SF6 in CT1 is expanded into injection loop
of the GC system (Fig. 1).
Samples were introduced into the irm-GCMS system by two methods. For large samples (ca. 2 Amol S),
SF6 is simply expanded and the injection loop is isolated by closing valve 11 in Fig. 1. Approximately 10%
of the sample are introduced in the injection loop. The
sample is injected into the irm-GCMS by changing the
position of multiport valve (V1 in Fig. 1). After analysis, He in the injection loop is evacuated from valve
12 with valve 11 closed. The next aliquot of SF6 is
expanded into the injection loop with valve 11 open and
valve 12 closed. By repeating this process, SF6 produced by a single fluorination can be analyzed multiple
times. In this way, we evaluated the irm-GCMS system
by avoiding errors associated with fluorination. For in
situ analysis, less than 200 nmol SF6 are cryofocused in
the injection loop at liquid nitrogen temperature, and
introduced into the irm-GCMS. The SF6 is thawed and
injected into irm-GCMS by changing the position of
V1. Approximately one third of the sample is lost in the
dead volume between V1 and valves 11 and 12. After
injection, the peak of SF6 comes at approximately 2
min at the TCD and 2.8 min at the mass spectrometer.
Two reference gases are introduced; one from an open
split and the other from an adjustable volume of the
mass spectrometer (Fig. 2). We find no systematic
33
difference between two reference gas injections. Injection of reference gas through an open split, however, is
relatively costly.
5. Mass 131 interference
The relatively low abundance of 36S (ca. 0.015%)
and interferences on mass 131 (36SF5+) make measurement of d 36S difficult. The mass interference on 131 is
suggested to be C3F5+, which may be produced by
fragmentation of fluorinated hydrocarbons (Rumble et
al., 1993). Some protocols describe multiple GC purification or flushing GC column for hours between
samples (Gao and Thiemens, 1991; Hoering and Prewitt, 1988; Hulston and Thode, 1965).
One of the advantages of irm-GCMS technique is
real-time monitoring of mass-131 signal. Experiments
with the irm-GCMS showed that the compound responsible for the mass interference has a long elution time in
the GC column, and causes an unstable baseline over
the course of analytical session. The 131 baseline is
stable within F 1 mV (i.e., 1 fA with 1012 V register)
before the analysis of the first sample in an analytical
session but increases to about 20 mV after a single
analysis, and becomes as large as 300 mV during the
course of the analysis. It was found that an ethanol and
liquid nitrogen slush ( 110 F 3 8C) on the collection
loop of the GC line effectively removes the 131 interference and stabilizes the baseline within F 1 mV over
typically less than a few mV signal. No change was
observed for the isotopic ratios d 33S and d 34S by using
ethanol–liquid N2 trap.
6. Factors controlling precision and accuracy for
#33S and #36S
For dual inlet SF6 analysis, the major source of error
is associated with fluorination reactions and incomplete
transfer of the sample gas in the vacuum line. In the
irm-GCMS system, shot noise error and ion scattering
also introduce uncertainties. These can have a large
effect on both d 34S and D33S, as evaluated in the
following section.
7. Shot noise limited performance
Fig. 2. Typical GC and MS traces for LA-irm-GCMS for injection
of 170 nmol SF6. The first and the third peaks in MS trace are
the reference gas from an open split and an adjustable volume,
respectively.
We evaluate the irm-GCMS technique for shot noise
error because this is the factor that ultimately limits the
precision and accuracy of isotope ratio measurements.
The shot noise error originates from the random distribution of SF6 molecules in a capillary flow, and is
estimated from counting statistics of the signal. The
34
S. Ono et al. / Chemical Geology 225 (2006) 30–39
Table 1
Shot noise errors (3r) calculated for Geophysical Lab irm-GCMS as a
function of sample size
m (nmol)
1
5
10
50
100
200
300
d (Am)
29
50
63
107
135
170
195
Shot noise errors (3r)
d 33S
d 34S
d 36S
0.85
0.38
0.27
0.12
0.09
0.06
0.05
0.36
0.16
0.12
0.05
0.04
0.03
0.02
5.18
2.32
1.64
0.73
0.52
0.37
0.30
The d values are the diameter/depth of pyrite corresponding to sample
size. Split ratio is 0.013.
shot noise limited performance for sulfur isotope system can be written as (Merritt et al., 1994):
x
1
2
6 1þ R
rd ¼ 2d10
ð4Þ
xR
32 aEmN
a
where, x is 33, 34 or 36, 32a is 32S abundance (ca. 0.95),
E is ionization efficiency (SF5+ ions/SF6 molecules), m
is the amount of SF6 introduced into the ion source
(mol), and N a is the Avogadro’s number (6.023 1023
molecules/mol). The term 32aEN a represents the overall
sensitivity of the system, and equals the number of
32
SF5+ ions counted by the faraday cup per moles of
sample (m). For Geophysical Laboratory irm-GCMS, E
is ~0.5 10 3 (SF5+ ions/molecules SF6) with the mass
spectrometer source conductance window open (6
turns) (E is ~1 10 3 when it is fully closed). The
current GC open split configuration is not fully optimized for analysis of small samples because the actual
amount of sample introduced to the ion source (m) is
only 1.3% of the total SF6 introduced into the gas
chromatograph. The open split is where most of the
GC flow (23 mL/min) is vented and only small fraction
(0.3 mL/min) enters to the mass spectrometer. Thus,
reducing the GC flow rate and/or increasing flow from
the open split to the MS will substantially increase the
sensitivity. For example, the maximum flow rate that
can be accepted by the MS is ca. 2 mL/min. Thus,
sensitivity can be increased by a factor of seven by
maximizing the He flow to the mass spectrometer. It is
described in the Next section, however, that a high He
flow rate may affect peak tailing effects, which also will
affect precision of the method. Optimum condition may
be determined by a combination of factors depending
upon application. Such factors may include sample size,
precision and sample throughput.
Calculations show that the shot noise error (3r) of
the system for d 33S, d 34S and d 36S is better than 0.1x,
0.04x, and 0.5x, respectively, for 100 nmol S. The
relatively large error for d 36S is due to its low abundance (Table 1). This analysis indicates that the irmGCMS system has the potential to achieve precision
comparable to conventional dual inlet analysis for larger than 100 nmol samples; smaller samples (b 100
nmol) may be analyzed at lower precision.
8. Peak tailing and abundance sensitivity correction
It was found that the major source of error in irmGCMS analysis for SF6 is from peak overlap of the
major (127) beam and minor ion beams (128 and 131).
All four SF5+ peaks are well resolved for dual inlet
analysis. However, upon introduction of He into the
source (1.6 10 6 mbar source pressure) all peaks
become broader and tailing occurs (Fig. 3).
Taking the 127 and the 128 beams as examples,
when tailing of 127 beam overlaps onto 128, the measured isotope ratio 33S / 32S (33R m) is biased as,
33 m
R ¼
128
I þ128 i
127 I
ð5Þ
where, 128I is an ion current from 33SF5+, and 128i is an
ion current due to 127 beam tailing contribution. The
measured d 33S value (d 33Sm) with respect to the working reference gas is:
33 abs 128
Rsam þ csam
33 m
d S ¼ 33 abs 128
1 1000
ð6Þ
Rref þ cref
where, 33R abs is the absolute ratio of 33S / 32S that is free
from peak tailing contribution, and 128c sam and 128c ref
Fig. 3. Signal intensity as a function of acceleration voltage. The
effect of introduction of He into the ion source is shown. The signal
size is balanced to have 10 nA signal for 127 ion beam for each
analysis (i.e. 3V on 127 cup). The measured source pressures are
7.7 10 8 and 1.6 10 6 mbar with and without He, respectively.
The scan shows tailing of 127 ion beam onto 128 and onto 131 when
He is introduced to the mass spectrometer. There also is a peak of
mass-132 on the left of 131 that is a contaminant in reference gas.
S. Ono et al. / Chemical Geology 225 (2006) 30–39
35
and evaluate 33SC in the Following section. The second
term in Eq. (7) indicates there will be an error originated from variation in 128c sam. Because 128c sam is a
function of 127I sam, this results in sample size dependence for irm-GCMS analysis. We note that the sample
size dependent term should be irrelevant for dual inlet
analysis because 128c sam and 128c ref cancel each other
when the reference and sample signals are balanced.
The mathematics for the correction is similar to H3+
correction in D/H analysis by irm-GCMS, although the
physical origin of the effect (formation of H3+) is different from that of SF5+ peak tailing. Our correction
method is essentially the same as that of peak-wise
correction applied by Sessions et al. (2001a,b). In this
work, we have not explored point-wise correction (Sessions et al., 2001a), which may be advantageous and
can be readily applied for SF6.
Fig. 4. Repeated analyses of IAEA S-3 produced from a single
fluorination showing sample size dependence of the Geophysical
Laboratory irm-GCMS. The lines are linear regressions. Note that
regression lines curve because of the log scale used for the x-axis.
Error bars represent shot noise errors (3r) calculated based on sample
sizes.
are 128i / 127I for sample and reference, respectively. Eq.
(6) can be rearranged to:
d33 Sm ¼
33 abs
Rref
33 Rabs þ128
ref
cref
d33 Sabs
128
þ
csam 128 cref
1000
33 Rabs þ128 c
ref
ref
ð7Þ
where, d 33Sabs is the d 33S value that would be measured if there was no peak tailing contribution. Eq. (7)
is essentially the same as abundance sensitivity corrections for 44CO2 peak tailing on 45CO2 and 46CO2 except
the definition of abundance sensitivity is reciprocal of
the c term used in here (Deines, 1970).
Eq. (7) indicates two consequences of 127-beam
peak tailing on d 33Sm. The first term indicates that
the d 33Sm scale will be contracted such that all delta
values will be closer to that of the reference gas. We
define scale contraction factor for d 33S (33SC) to be:
33
SC ¼
33 abs
Rref
þ128
33 Rabs
ref
cref
¼
33 abs
Rref
33 Rm
ref
;
ð8Þ
Fig. 5. D33S and D36S as a function of d ?4S for IAEA reference
materials and Permian pyrite. The D33S and D36S are calculated by
using 33k = 0.515 and 36k = 1.91 without scale contraction correction.
Error bars are standard deviation (1r) for three irm-GCMS analyses.
The lines are linear regressions, and show systematic bias in the
values of D33S and D36S introduced by mass 127 beam peak tailing
contribution.
36
S. Ono et al. / Chemical Geology 225 (2006) 30–39
9. Sample size dependence
The sample size dependence of the system was
evaluated by injection of various sizes of SF6. The
SF6 was produced by a single fluorination of a reference silver sulfide (IAEA S-3, Fig. 3) in order to avoid
errors associated with fluorination reaction. Linear sample size dependence is found; 0.16, 0.04, and 0.38 x/
100 nmol, for d 33S, d 34S, and d 36S, respectively (Fig.
4). This linear sample size dependence is consistent
with the expectation that the peak tailing contribution
(128i and 131i) is proportional to the square of the major
ion beam current, perhaps due to a coulombic effect
(Craig, 1957).
The data in Fig. 4 illustrates the two factors in irmGCMS analysis that control variability with sample
size. The first is the shot noise error (error bars in
Fig. 4) that increases with decreasing sample size,
resulting in lower precision for smaller samples. The
second factor is a peak tailing contribution that is
linear to the sample size. For operational reasons,
optimum sample size for Geophysical Lab irmGCMS is determined to be between 100 to 200
nmol. The error associated with sample size becomes
large above 200 nmol and shot noise error becomes
large below 100 nmol.
was analyzed by sub-sampling SF6 three times by
expanding SF6 to the injection loop, and the data for
three analyses were averaged. First, the values of D33S
and D36S were calculated by Eqs. (2) and (3) without
applying peak tailing corrections (Fig. 5). The data in
Fig. 5 indicates that systematic error in the reference
fractionation line was caused by the bias introduced by
peak tailing contributions. Assuming scale contraction
on d 34S is negligible, the scale contraction factors for
33
S and 36S were determined such that D33S and D36S
values were minimized while the values for 33k and 36k
were held constant at 0.515 and 1.91 respectively
(Table 2). This analysis yields 0.983 and 0.953 for
33
SC and 36SC, respectively. There are minor offsets
(non-zero intercepts) in the regressions of 0.03x and
0.97x for D33S and D36S, respectively (Fig. 5). Part of
this is due to the sample size dependent term (i.e., the
second term in Eq. (7)). Part of this is also due to
natural variation of D33S and D36S in post-Archean
mass-dependently fractionated sulfur. IAEA reference
materials as well as our working reference gas have
analytically resolvable variations of D33S (and D36S),
which is a subject of considerable research (Ono et al.,
2005).
11. Application to Laser Ablation (LA) in situ
analysis
10. Defining terrestrial fractionation line
Three IAEA reference materials (IAEA S-1, S-2 and
S-3) and two pyrite separates from Permian ash beds
(MD-99-33u and MZ-99-28B) were measured with the
irm-GCMS in order to define reference fractionation
line for the irm-GCMS system. Measurements were
undertaken for fluorination of 0.4 mg silver sulfide or
0.1 mg of pyrite to produce 1.5 Amol of SF6. The SF6
11.1. LA-irm-GCMS analysis of pyrite with
mass-dependent sulfur isotope composition
The Geophysical Lab irm-GCMS system was tested
for in situ analysis of a pyrite sample that was previously analyzed in our laboratory for d 33S and d 34S (Hu
et al., 2003). For most analyses, laser pits of 150 Am
diameter and 150 Am depth were produced with a UV
Table 2
Measured and corrected multiple sulfur isotope compositions for reference materials
Sample
33
IAEA S-1
IAEA S-1
IAEA S-2
IAEA S-3
IAEA S-3
MD-99-33u
MD-99-33u
MZ-99-28B
MZ-99-28B
Correcteda
Raw data
34
36
33
36
d S
d S
d S
D S
D S
d 33S
d 36S
D33S
D36S
0.20
0.33
11.26
16.81
16.72
8.55
8.40
21.63
21.62
0.35
0.51
22.33
33.00
32.64
16.70
16.41
42.21
42.16
2.1
1.3
40.1
59.6
59.5
31.3
30.3
75.7
76.3
0.02
0.07
0.18
0.33
0.23
0.09
0.09
0.34
0.33
1.4
0.3
2.9
2.7
2.1
0.4
0.9
3.7
2.9
0.17
0.31
11.48
17.07
16.97
8.67
8.51
21.97
21.96
1.2
0.4
42.9
61.4
61.3
31.8
30.7
78.2
78.9
0.01
0.04
0.04
0.07
0.03
0.03
0.03
0.00
0.02
0.6
0.6
0.2
0.8
0.1
0.1
0.4
0.9
0.1
All delta values are with respect to working reference SF6.
a
d 33 S and d 36 S are corrected for peak tailing using 33 SC and 36 SC of 0.983 and 0.953, and offset of 0.03x and 0.97x for d 33 S and d 36 S
respectively.
S. Ono et al. / Chemical Geology 225 (2006) 30–39
37
Fig. 6. Comparison of laser ablation irm-GCMS and conventional dual inlet measurements for laboratory pyrite reference material. The numbers in
left figure are d 34S values (in x) reported previously in Hu et al. (2003). The area analyzed by irm-GCMS is shown in a box with a dashed line
(note for scale difference), and the data are shown in the right figure with SF6 yields and their delta values (in x).
laser (KrF), and yielded ca. 170 nmol SF6. Smaller laser
spots were made to test the analytical capability for
small (b100 nmol) samples. No systematic variations
were observed between sample size, laser pit shape
(depth/diameter), and measured values of D33S. Small
sample sizes (27 to 76 nmol) gave large positive D36S
values that are likely due to incomplete removal of
interference on mass 131. The measured D33S and
D36S for 12 analyses of 123 to 201 nmol samples
average to 0.05 F 0.1x and + 0.37 F 0.44x (2r),
respectively. These values represent overall precisions
for in situ laser sampling of sub-micromole samples.
The values of d 34S range from + 1.9x to +3.5x.
Because dual inlet analyses of the same pyrite yield
considerable range of d 34S from + 2.6x to +5.6x (Fig.
6) (Hu et al., 2003), this range of d 34S is likely due to
natural isotopic heterogeneity. The analysis shows LAirm-GCMS system is capable of routine analysis of
Fig. 7. A sketch of a rock chip (2.5 3.0 cm) of Archean carbonaceous shale that contains two pyrite bands analyzed for their multiple sulfur
isotope compositions. The dots represent locations of laser pits for LA-irm-GCMS analysis and ovals are areas sampled by a dental drill, followed
by conventional chemical extraction and dual inlet analysis. Averages for LA-irm-GCMS data for each band are shown in italic, and the data by
dual inlet analysis are shown in bold.
38
S. Ono et al. / Chemical Geology 225 (2006) 30–39
100–200 nmol samples with precision comparable to
dual inlet analysis. Another advantage of the LA-irmGCMS is high sample throughput (as fast as 20 min per
analysis) compared to dual inlet analysis (ca. 1 h per
analysis).
11.2. LA-irm-GCMS analysis of Archean pyrite
The analysis of a 2.5 3.5 cm rock chip of Archean
age (the 2650 Ma Jeerinah Formation in the Hamersley
Basin, Western Australia) was undertaken by LA-irmGCMS to test analysis of samples with non-mass-dependent isotope ratios. Approximately 1.5 mg of the same
pyrite was also sampled by a dental drill and pyrite sulfur
was extracted by a conventional Cr reduction technique,
and converted into silver sulfide. The silver sulfide was
analyzed with a conventional dual inlet system at the
University of Maryland. The LA-irm-GCMS data show
these two pyrite bands have distinct isotopic compositions (average to + 4.9x and 0.4x for D33S) and much
less variation (b 1x for D33S) within each band (Fig. 7).
The data obtained by dual inlet analysis agree well with
the average of the LA-irm-GCMS measurements (Fig.
7). There are small but measurable heterogeneity in D33S
within each pyrite band. Such small-scale variations in
multiple isotope ratios (d 34S, D33S and D36S) can be
resolved easily and efficiently with the LA-irm-GCMS
system described in this paper.
12. Conclusion
The LA-irm-GCMS described in this paper allows in
situ analysis of sulfide minerals at laser spots of ~150
Am diameter. Analysis of smaller spot size is currently
possible with lower precision. Comparable precision is
likely possible for smaller pit size upon optimization of
the GCMS system (i.e. by increasing the split ratio).
The technique offers an alternative to multi-collector
SIMS for in situ analysis of sulfide minerals. Advantages of LA-irm-GCMS include relatively fast analysis
and the capability of analyzing d 36S. In addition, the
LA-irm-GCMS can be fully devoted to analysis of the
sulfur isotope system. With combination of improved
wet chemical sulfur extraction techniques, the irmGCMS system may be capable of analyzing nanomole
quantity of sulfur in many geological, cosmochemical,
and meteorological materials.
Acknowledgement
We thank R. Husted of Thermo Finnigan for technical support for mass spectrometer, G. Hu and P-L
Wang for construction and various tests for initial SF6
dual inlet system, M. Fogel, S. Shirey, and C. Henning
for various inputs about irm-GCMS, and J. Eigenbrode
and S. Bowring for providing samples. We also thank
anonymous reviewers for their helpful comments. We
acknowledge financial support from Carnegie Institution, NSF EAR-0125096 (Rumble) and NSF EAR025953 (Rumble) and JPL Grand Challenge Program
(contract 1213932). [PD]
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