7/29/2019 Bufi Poster March 2011
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XENON INTERFERENCE
Since ICP-MS differetiates by m/z of ions, 129I + is
indistinguishable from 129Xe +, acontaminant in the carrier gas.
Without correction for 129Xe, responses such as Figure 2ahave been
obtained: peaks occur which look similar to 129I but are actually
due to 129Xe. Inthis case the 129Xe peaks did not interfere with
the iodide/iodate regions, but did have thesame shape and size as
genuine iodine peaks and so could give misleading results.
By measuring a second isotope of Xe (131Xe), a correction factor
(CF) can be applied withinthe ICP-MS software, based on the
expected ratio of129Xe/ 131Xe. However this approach
has been shown to over-correct for 129Xe interference. Instead
the CF is now determinedfor each run, based simply on reducing the
background 129-signal to zero as shown inFigure 2b. If possible, it
would be preferable to reduce 129Xe interference within the
ICP-MSrather than correcting the data afterwards. This was
attempted by using reaction gasesintroduced into the hexapole
chamber between the cones and quadrupole (Figure 1).
OPTIMISING IODINE ANALYSIS BY ICP-MSSmith, H. E.1,2*, Ander, E.
L.2, Bailey, E. H.1, Crout, N. M. J.1, Watts, M. J.2, Young, S.
D.1
1 Division of Agricultural and Environmental Sciences, Biology
Building, University of Nottingham, University Park, Nottingham,
NG7 2RD.2British Geological Survey, Kingsley Dunham Centre,
Keyworth, Nottingham NG12 5GG. HES: [email protected]
Figure 3: Proposed charge transfer mechanism forremoval of xenon
interference
USING A REACTION GAS
It has been reported that using ICP-MS in collision/reaction
cell mode can improveiodine signal-to-noise ratios. Some authors
have used hydrogen and/or helium[1,2],which are commonly used
reaction gases, but more success has been reported whenusing
oxygen[3-6]. The suggested mechanism[4] by which oxygen could
remove xenoninterference from 129I analysis may be by charge
transfer to reduce Xe+ to Xe0,illustrated in Figure 3. Only charged
particles are transferred through the quadrupole tothe detector,
and reportedly xenon reacts much faster with oxygen than iodine
does.
H2 and He were tested as reaction gases for this project, but
neither removed the129
Xeinterference. Oxygen is being considered for future work but
there are important safetyfactors to be assessed before it is used.
Therefore the xenon CF based on removingbackground 129 counts will
continue to be applied to iodide/iodate chromatograms of129I tracer
in soil extracts as investigation into iodine behaviour in soil
continues.
REFERENCES
[1] St Remy, R. R. D. et al. (2004). Journal of Analytical
Atomic Spectrometry 19(9): 1104-1110[2] Becker, J. S. (2002).
Journal of Analytical Atomic Spectrometry 17(9): 1172-1185[3]
Brown, C. F. et al. (2007). Applied Geochemistry 22(3): 648-655
[4] Reid, H. J. et al. (2008). Talanta 75(1): 189-197[5] Izmer,
A. V. et al. (2004). Journal of Analytical Atomic Spectrometry 19:
1278 - 1280[6] Wang, K. E. and S. J. Jiang (2008). Analytical
Sciences 24(4): 509-514
Figure 2a: Effect of xenon interference
INTRODUCTION
Iodine is essential for healthy human life, and insufficient
intake can result in reduced mental and physicalwell-being, with
social and economic impacts for affected communities. Iodine
presence in foods relies uponsufficient uptake from the soil. In
some circumstances however, even when there is plenty of iodine
presentin soil, deficiency in food, crops and grazed pasture may
occur.
This PhD project focuses on understanding how soil properties
affect availability of iodine to plants, inparticular pasture
grazed by livestock. Thus the adsorption and transformation of
iodine in response to soilcharacteristics including composition and
pH are being studied in soils from Northern Ireland.
In order to model the behaviour of iodine in soil, the
long-lived radioisotope 129I (t0.5 = 15.7 million years) isbeing
added to soils and followed as a proxy for natural iodine.
Therefore accurate analysis of iodine species(e.g. iodide, I-, and
iodate, IO3
-) in soil extracts is required. This has involved development
of analysis usingHPLC-ICP-MS: high pressure liquid chromatography
separates iodine species before detection of specificisotopes by
ICP-MS. This poster concentrates on one aspect of analysis, and the
challenges investigated.
Figure 2b: Xenon correction factor applied
Iodine in soil
Iodine essential
in human diet
ICP-MS
Inductively coupled plasma mass spectrometry (ICP-MS)separates
and detects ions according to their mass to chargeratio, m/z. This
allows differentiation between naturally occurringiodine, 127I, and
129I which is added to samples as a tracer.
Samples are introduced to the ICP-MS as liquids via a
nebuliser,which creates an aerosol that is mixed with argon as a
carriergas. The fine mist is separated from larger liquid droplets
in thespray chamber before moving into the torch, where it is
ionisedin plasma at extremely high temperatures. The cones direct
andfocus the ion beam into the mass spectrometer, where ions
areseparated on the basis of their m/z ratio and finally detected
asnumber of counts per second.
Figure 1: Schematic diagram of inductively coupled plasma mass
spectrometer
Direction of sample flow
IodideIodate
NebuliserSpraychamber
Torch
ConesQuadrupole:massseparator
Detector
IodideIodate
129Xe+ 129Xe
O2
O2+
129I+
129I+
