Environmental Engineering Reference
In-Depth Information
table 4.1
Typical Precision of Mercury Isotope Ratio Measurements
Using Different ICP/MS Systems
ratio mass spectrometry (Nier and Schlutter, 1986; Obolenskii
and Doilnitsyn, 1976). They were later viewed with some skep-
ticism (Koval et al., 1977; Nier and Schlutter, 1986) because
the achievable analytical precision with this method was less
than 20‰, and observed variations were typically within this
range as well. Instrumental neutron activation analysis (INAA)
provided results on Hg isotope ratio variations in extraterres-
trial materials (Lauretta et al., 2001). However, INAA is appli-
cable only to measure
196
Hg and
202
Hg, which complicated
data confi rmation. In fact, the values were later called into
question by high-precision mass spectrometry (MS) measure-
ments (De Laeter et al., 2003; Lauretta et al., 2001).
Instrument
RSD (‰)
Magnetic-sector high resolution
1.7
Time of fl ight
0.8
Collision cell
2.8
Multicollector
0.020
unique solutions to address and overcome element-specifi c
challenges in sample preparation and sample introduction.
As with conventional ICP/MS analysis (Hintelmann and
Ogrinc, 2003), introduction of the Hg analyte into the MC-
ICP/MS requires careful consideration. First and foremost,
the analytical strategy must take into account the mini-
mum mass of Hg required for highly precise isotope ratio
measurements. Based on the experience in my laboratory, it
is desirable to have at least 10 ng of Hg available for the mea-
surement. This is in stark contrast to the picogram or even
femtogram detection limits achievable nowadays with the
most sensitive equipment used for quantitative measure-
ments. It should be stressed that the absolute sensitivity of
the MC-ICP/MS is on a par with, if not better than, the most
sensitive atomic fl uorescence spectrometers (AFS) or single
collector ICP/MS. However, sensitive ion-counting detec-
tors typically found in single detector ICP/MS instruments,
and now also available for MC-ICP/MS, are not suitable for
ultraprecise isotope ratio measurements, and less sensitive,
but more precise, Faraday detectors must be used instead.
Second, the counting statistic theory predicts that the preci-
sion of isotope ratio measurements improves with increas-
ing signal strength. Therefore, nanogram quantities of Hg
must reach the ICP source to obtain a precision that is suf-
fi cient to distinguish natural isotope abundance variations.
Measurement of Mercury Isotopes by Mass
Spectrometry
Technological advances in MS technology have led to sub-
stantial improvements in the precision of Hg isotope ratio
measurements. Inductively coupled plasma-time-of-fl ight-
mass spectrometry (ICP-ToF-MS) offers simultaneous detec-
tion of multiple isotopes using a detector array. Reaction
and collision cell techniques dramatically improved sen-
sitivity and precision through the reduction of the ion
energy spread, and magnetic-sector instruments generate
fl at-topped peaks, which are desirable for precise isotope
ratio determinations. However, each of these technologies
has also some inherent disadvantages for very precise iso-
tope ratio determinations. While ICP-ToF-MS suffers from
low sensitivity, the other technologies are restricted to hav-
ing a single detector, which necessitates the measurement
of multiple isotopes sequentially in time. Effects such as
plasma noise or minute variations in sample uptake and
transmission quickly compounded uncertainties. Ultimately,
a multidetector arrangement following the MS separation
of isotopes is required to obtain the necessary precision.
Thermal ionization mass spectrometry (TIMS) is a tech-
nique widely applied to determine very precise isotope
ratios. Unfortunately, the high ionization potential of mer-
cury makes TIMS unsuitable for measuring this element.
Only mating multicollector MS with the inductively
coupled plasma (ICP) ionization source greatly improved
the precision of isotope ratio measurements for heavy ele-
ments in general and Hg in particular. A comparison of
typical precisions obtained by the various MS techniques
is presented in Table 4.1. To be consistent with the nomen-
clature in this chapter relative standard deviations (RSDs)
are given in‰. Clearly, multicollector-ICP/MS (MC-ICP/
MS) offers precision that is at least one order of magnitude
better than the other instruments. This power is required,
since natural variations are too small to be resolved by sin-
gle collector instruments, and today, Hg isotope fraction-
ation studies are conducted exclusively using MC-ICP/MS.
Gold Trap Amalgamation and Preconcentration
Gold trap amalgamation is a convenient preconcentra-
tion method that is widely used for quantitative Hg deter-
minations and relatively easy to interface with ICP/MS
(Hintelmann and Ogrinc, 2003; Schauble, 2007). In princi-
ple, this technique offers several advantages: (a) When used
in combination with a cold vapor generation system, there
is theoretically no limit to the amount of sample and ana-
lyte mass that could be processed. Hence, in principle, even
background air and aqueous samples with low Hg concen-
trations could be analyzed, but they would require reduc-
ing, purging, and trapping Hg from up to several liters of
water or several cubic meters of air. (b) When used in com-
bination with a sample combustion system (Biswas et al.,
2008; Evans et al., 2001; Xie et al., 2005) the use of any
chemical reagents required for wet digestions is avoided,
reducing the risk of sample contamination. (c) For certain
Sample Introduction
Similar to the quantitative determination of Hg in environ-
mental samples, Hg isotope ratio measurements also require
