Analysis of naturally occurring isotopic variations
is a promising tool for investigating Zn transport and
cycling in geological and biological settings. Here, we
present the recently installed double-spike (DS) technique
at the MAGIC laboratories at Imperial College London.
The procedure improves on previous published DS methods
in terms of ease of measurement and precisions obtained.
The analytical method involves addition of a 64Zn–67Zn
double-spike to the samples prior to digestion, separation of
Zn from the sample matrix by ion exchange chromatography,
and isotopic analysis by multiple-collector inductively
coupled plasma mass spectrometry. The accuracy and
reproducibility of the method were validated by analyses
of several in-house and international elemental reference
materials. Multiple analyses of pure Zn standard solutions
consistently yielded a reproducibility of about ±0.05‰
(2 SD) for δ66Zn, and comparable precisions were obtained
for analyses of geological and biological materials. Highly
fractionated Zn standards analyzed by DS and standard
sample bracketing yield slightly varying results, which
probably originate from repetitive fractionation events
during manufacture of the standards. However, the δ66Zn
values (all reported relative to JMC Lyon Zn) for two less
fractionated in-house Zn standard solutions, Imperial Zn
(0.10±0.08‰: 2 SD) and London Zn (0.08±0.04‰), are
within uncertainties to data reported with different mass
spectrometric techniques and instruments. Two standard
reference materials, blend ore BCR 027 and ryegrass BCR
281, were also measured, and the δ66Zn were found to be
0.25±0.06‰ (2 SD) and 0.40±0.09‰, respectively. Taken
together, these standard measurements ascertain that the
double-spike methodology is suitable for accurate and
precise Zn isotope analyses of a wide range of natural
samples. The newly installed technique was consequently
applied to soil samples and soil leachates to investigate the
isotopic signature of plant available Zn. We find that the
isotopic composition is heavier than the residual, indicating
the presence of loosely bound Zn deposited by atmospheric
pollution, which is readily available to plants.