Iron Data using the Nu Plasma 1700
Introduction
In order to illustrate the high resolution performance of the Nu Plasma 1700 mass spectrometer
the isotope ratios of iron have been measured using Aldrich and Alfa ICP iron standard solutions.
There are well known polyatomic interferences of the iron peaks namely, argon oxide, nitride and
hydroxide formed in the plasma expansion process which require a resolving power of >2600 to
separate them. These interferences can be greatly reduced by using a desolvating nebuliser as the
sample introduction system. However we have decided enhance the contribution of these
interferences to demonstrate the instrument performance, by using a “wet” plasma with shielded
torch and running the samples in nitric acid (rather than HCl). In this manner we have been able
to increase the level of the ArO peak, which lies adjacent to the 56Fe peak, to a level normally not
encountered when using our instruments. By altering the intensity of the iron solution itself, we
can also easily illustrate the resolution performance of the instrument. This is shown in Figures 1
and 2, where the transmission of the system was reduced to approximately 40% in order to record
these traces.
There are also isobaric interferences at masses 54 and 58 due to chromium and nickel,
respectively. Although these cannot be resolved from iron, their contribution to the 54Fe and 58Fe
intensities should be correctable, by measuring the 60Ni and 52Cr beams. There is a further spectral
overlap of 52Cr by ArC, which thus requires the use of high resolution at the extreme wing of the
detector array.
Analysis
Initially the instrument was set to give 0.25 a.m.u. collector spacing with only 57Fe, 56Fe and 54Fe
beams being measured, as shown in Figure 3. This configuration however does not permit the
measurement of the iron 58 and Nickel 60 peaks. By changing the zoom and using a 0.33 a.m.u.
collector spacing we are also able to measure these beams, as shown in Figure 5. The sensitivity
for these runs was approximately 6-8v/ppm, which considering the reduction in sensitivity due to
the closing of the source slit is comparable to the sensitivity of other instruments, under the same
operating conditions. (Note that we have not optimised for sensitivity here since we wish to
demonstrate performance with high interference levels.) To allow for any mass bias drift of the
instrument during the experiments, alternate Aldrich standard / Alfa sample solutions were
measured, the measurements are given in delta format. The data below was taken over several
months and shows a high level of precision. By altering the iron concentrations for the third set of
data taken on 13/7/01, we hoped to highlight if peak overlap is a problem, and from the data
shown here, it is seen that this is not the case. It is also interesting to note that these analyses were
done using approximately 10 times weaker solutions than previous studies on low-resolution
instruments, where the more intense iron beams are used to ensure that any residual spectral
interference is swamped. Scans of the mass spectrum in the region around 51 to 58 a.m.u. showed
that no other interferences were present, but nickel was seen to be present as a minor constituent
in both solutions, at different levels.
To correct for this nickel, we used the known 54/57 ratio of the Aldrich standard to calculate the
mass fractionation factor from the standard runs. This factor was then used in the subsequent
analysis to correct for the overlap on 58Fe using the measured 60Ni peak. Although this decreased
the observed precision of the measurement by approximately 50%, ignoring this contribution even
in these supposedly “pure” solutions produced a nonsensical result of -16‰. We show a scan of
the set up used for these measurements in Figure 6, 60Ni being monitored in collector H7 at low
resolution.
Page 1 of 6
Iron Data using the Nu Plasma 1700
Note : The data obtained on 13 July was run starting with an Alfa solution, rather than Aldrich. The delta values have
been inverted to allow for this, to allow them to be directly compared with the other data in the table.
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Iron Data using the Nu Plasma 1700
In an alternative approach the mass bias was obtained using a copper solution, in a separate
analysis, to give absolute isotopic ratios. In these experiments the conditions were again
optimised to give large molecular peaks, and the ArO intensity being approximately 1/3 of the
iron intensity in the more dilute runs.
These data can be compared with the measured values for this standard (from Beard and Johnson
1999, Geochimica et Cosmochimica Acta):
54
Fe / 56Fe = 0.063683, 0.1 ‰
Fe / 56Fe = 0.023087, 0.1 ‰
54
Fe / 57Fe = 2.75839, 0.1 ‰
±
57
±
±
To further illustrate the effects of varying the solution concentrations, we show below data
obtained by altering both the sample and standard concentrations by a factor of 5. The intensity of
the ArO in these runs was 1.1v, and the resolution employed was 2900.
The poorer internal and external precision of this data set is a direct result of the smaller beam
intensities and overlap corrections employed. The large intensity changes highlight the correction
for 58Ni as not being very successful, due to its moderate contribution (3 to 5% of the 58amu
signal) and the unknown isotope ratio of the source(s). To study this effect a bit further, we doped
some 1.5ppm Alfa iron solution with 10ppb with nickel NBS986 standard, so that the 58Ni
intensity was ~1.5x the 58Fe. This produced a measured 58/56 delta of -0.71±0.6‰, a more
acceptable, but still incorrect, result, which does indicate that this approach can be used, provided
that the isotopic composition of the nickel impurity is known, and its contribution is not too large.
Obviously further studies are required here. No problems were encountered with a doped
chromium solution, due undoubtedly to its smaller contribution under the iron peak, as had been
illustrated by us earlier.
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Iron Data using the Nu Plasma 1700
Resolution = 2700
10% peak height
Figure 1: 56Fe resolution at approx 40% transmission (1ppm solution)
56
Fe
ArO
Figure 2: The increase in resolution from 2700 to 3000 was achieved by narrowing
the adjustable collector slit.
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Iron Data using the Nu Plasma 1700
54
Fe
57
56
Fe
Fe
ArO ArN ArOH
Figure 3: Scan showing coincidence of 57Fe, 56Fe & 54Fe isotopes (at 0.25 a.m.u.
collector spacing) easily resolved from their respective argide interferences.
100 ppm
Figure 4: Scan showing at least 100 ppm peak flat with coincidence of 57Fe, 56Fe &
54
Fe isotopes (at 0.25 a.m.u. collector spacing ) at 2770 resolution.
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Iron Data using the Nu Plasma 1700
57
54
Fe
Fe
58
Fe
56
Fe
Figure 5: Scan showing coincidence of 58Fe, 57Fe, 56Fe & 54Fe isotopes (at 0.33
a.m.u. collector spacing) with the axial collector, receiving 56Fe, set to its 20v
range. Dry Plasma running conditions.
52
Cr
ArC
Figure 6: Scan showing coincidence of Iron & Chromium peaks (at 0.33 a.m.u.
collector spacing) with isobaric overlap of the 54Fe & 54Cr beams.
Page 6 of 6