6
Using EA IsoLink IRMS System for CNSOH
Measuring Sulfur Isotopes
108
EA IsoLink IRMS System for CNSOH Operating Manual
Thermo Scientific
Measuring Sulfur Isotopes
This section provides the instructions for measuring sulfur isotopes.
Introduction
In comparison to isotope ratio measurements of nitrogen (
15
N) and carbon (
13
C) in organic and
inorganic matter, the analysis of sulfur (
34
S) has always been more challenging.
Analyzing biological sulfur makes experimental difficulties caused by the low abundance of sulfur in
organisms (for example 0.2 wt% [mg/mg] in plants), as well as by the fact that sulfur is present as a
mixture of organic and inorganic compounds.
Difficulties in using the Elemental Analyzer arise due to a large amount of carbon in the same samples,
which quickly exceeds the capacity of combustion reactors. This causes incomplete combustion.
Thermo Fisher Scientific has developed a technique for precise, accurate and fast sulfur measurement,
which puts it on a par with carbon and nitrogen in terms of ease of use and sample size.
Due to the high natural abundance of the heavier isotope
34
S, less amplification is required and may
become necessary for all Delta IRMS before Delta
plus
XP (since 2002).
It is recommended to use a smaller resistor (1 * 10
10
Ω
) on the cup for mass 66 (usually 3 * 10
10
Ω
).
Procedure
Sulfur measurements are performed using a specially equipped Elemental Analyzer. Combustion and
reduction are carried out in a single reactor filled with tungsten oxide (WO
3
) and copper (Cu) as
reducing agent.
The technique used for sulfur determination is based on the quantitative
Dynamic Flash Combustion
method. The samples - sometimes together with vanadium pentoxide (V
2
O
5
) - are wrapped in tin
capsules and placed into the autosampler. Then they are continuously purged with helium to remove
any traces of water and nitrogen. When a sample is dropped into the reactor, the helium stream is
temporarily enriched with pure oxygen.
The sample and its container melt as the tin promotes a violent reaction flash combustion). Under
these favorable conditions, even thermally resistant substances are completely oxidized.
In the reactor, for example barium sulfate (BaSO
4
) is thermally decomposed within a tin capsule.
The following reactions can then take place. See
Bailey, S.A. and Smith, J.W., 1972
):
Although the process does not require oxygen (O
2
), better combustion has been experienced when
oxygen (O
2
) is injected and vanadium pentoxide (V
2
O
5
) is added to the sample. If either the oxygen
(O
2
) pressure is low or a bad catalyst is selected or the reactor has too much ash, combustion will
proceed slowly.
Note
Bailey, S.A. and Smith, J.W. (1972
): An improved method for the preparation of sulfur
dioxide from barium sulfate BaSO
4
) for isotope ratio studies.
Anal. Chem.
44
, 1542-1543.
BaSO
4
BaO
SO
2
1
2
---O
2
+
+
→
BaSO
4
BaO
SO
3
+
→
SO
3
SO
2
1
2
---O
2
+
→
