2 Mass Spectrometry
In order to use radiogenic isotopes as dating
tools or tracers, they must be separated from non-radiogenic isotopes in a
‘mass spectrometer’. The concept of separating positive ions according to their
mass was conceived by Thompson, who proposed the use of this technique in
chemical analysis (Thompson, 1913). The first type of mass spectrometer to be
invented was a ‘magnetic sector’ instrument, and one of the earliest such
instruments was described by Dempster (1918). A cloud
of positive ions was accelerated and collimated, then passed through a
sector-shaped magnetic field, which separated the ions by mass. Aston (1927)
used such an instrument to make the first isotope ratio measurements on lead.
Other
methods of mass separation are also possible, such as the ‘quadrupole’
instrument (section 2.2.2). A fore-runner of this design was actually the first
instrument to be named a Mass Spectrometer (Smyth and Mattauch,
1932). However, the first high-precision mass spectrometer (Nier,
1940), was based on the magnetic sector approach. Nier’s
design pioneered so many of the features of modern mass spectrometers (Fig.
2.1) that these have often been called ‘Nier-type’
instruments.
Fig. 2.1. Schematic
illustration of the basic features of a ‘Nier type’ magnetic
sector mass spectrometer. Solid and open circles represent the light and
heavy isotopes of an element. After Faul
(1966).
The
normal method of ionising a sample in a mass spectrometer is simply to heat it
under vacuum, either from a gaseous source, as in the case of the rare gases
He, Ne, Ar and Xe, or from a purified solid sample loaded on a metal
filament. Hence the method is termed Thermal Ionisation Mass Spectrometry
(TIMS). However, thermal ionisation is only effective if the sample is first
purified by extraction from the matrix that makes up most of the rock sample.
Therefore, the normal starting point of precise isotopic measurements by TIMS
is chemical separation of the element to be analysed.
More
recently, the use of an Inductively Coupled Plasma (ICP) source has relaxed
some of the requirements for chemical purification before mass spectrometry
(section 2.5). In particular, minerals with relatively high concentrations of
radiogenic elements may be analysed by in
situ laser ablation. However, in matrices such as whole-rocks, where the
concentration of radiogenic isotopes is often low, it is still necessary to
pre-concentrate most samples, after which they are introduced into the plasma
in aqueous solution. Therefore, methods of dissolution and chemical extraction
will now be examined.
2.1 Chemical separation
Geological samples, which are commonly
silicates, are routinely dissolved in concentrated hydrofluoric acid (HF),
although some laboratories use perchloric acid as
well. Most rock-forming minerals will dissolve in hot concentrated HF at
atmospheric pressure. However, certain resistant minerals such as zircon must
be dissolved under pressure in a bomb, in order to achieve the temperatures of
up to 220 oC necessary for decomposition.
The bomb liners and beakers used for dissolution are almost universally made of
poly-fluorinated ethylene (PFE).
The
conventional bomb dissolution technique for zircons is described by Krogh (1973). A more recent technique, pioneered by Krogh and described by Parrish (1987), is to place several
‘micro-capsules’ carrying different samples into one larger bomb. The
micro-capsules are open to vapour transfer of HF, but Pb
blanks are very low (less than 50 pg), showing that volatile transfer of Pb between samples does not occur.
A
major problem which may be encountered after HF dissolution is the formation of
fluorides which are insoluble in other ‘mineral’ acids (e.g. hydrochloric acid,
HCl). Refluxing with nitric acid (HNO3)
helps to convert these into soluble forms. Experiments by Croudace
(1980) suggested that this process was promoted if additional nitric acid was
added before complete evaporation of the HF stage.
If
at some stage complete dissolution is not achieved, it may be necessary to
decant off the solution and return the undissolved
fraction to the previous stage of the process for a second acid attack (Patchett and Tatsumoto, 1980).
When complete dissolution has been achieved the solution may need to be split
into weighed aliquots, so that one fraction can be ‘spiked’ with an enriched
isotope for isotope dilution analysis (section 2.4) while another is left ‘unspiked’ for accurate isotope ratio analysis (section
2.3). Following dissolution, the sample is often converted to a chloride for
elemental separation, which is normally performed by ion exchange between a
dilute acid (eluent) and a resin (stationary phase)
contained in quartz or PFE columns.
2.1.1 Rb)Sr
The separation of Rb
and Sr, and the preliminary separation of Sm)Nd, is often performed on a cation exchange column eluted with dilute (e.g. 2.5M) HCl. Columns are normally calibrated in advance of use by
means of test solutions. Finally, before separation of the sample, the column
resin is cleaned by passing sequential volumes of 50% acid and water.
A
small volume of the rock solution is loaded into the column, washed into the
resin bed carefully with eluent, and then washed
through with more eluent until a fraction is
collected when the desired element is released from the resin. For TIMS analysis, the sample is evaporated to dryness, ready to
load onto a metal filament. Elements are eluted from the cation
column by HCl in roughly the following order: Fe, Na,
Mg, K, Rb, Ca, Sr, Ba, REE (Crock et al.,
1984). This series is defined by increasing partition coefficient onto the
solid phase (resin), requiring increasing volumes of eluent
to release successive elements.
It
is very important to remove the ‘major elements’ of the rock, e.g. Na, K and
Ca, from the Sr cut. Nitric acid is not effective for
this purpose because it does not separate Sr from Ca
(Fig. 2.2). Rb must also be eliminated from Sr because 87Rb is a direct isobaric
interference onto 87Sr. Small levels of Rb
are not a problem in an otherwise clean sample because the Rb
burns off before Sr data collection begins. However,
the presence of significant Ca in the Sr cut prevents
Rb burn-off, causing major interference problems.
2.1.2 Sm)Nd
Rare earth elements (REE) may be separated as a
group on a cation exchange column eluted with dilute
mineral acid, but because the chemical properties of individual rare earths are
so similar, more refined techniques must be used for separations within the REE
group. This is necessary for TIMS analysis because
there are several isobaric interferences (e.g. 144Sm interferes onto
144Nd). Ba must also be kept to a minimum
in the REE cut as it suppresses the ionisation of trivalent REE ions. This can
be done by switching to a dilute (e.g. 2M) nitric acid medium after collection
of the Sr cut, whereupon Ba
is rapidly washed off the column ahead of the rare earths (Crock et al., 1984). The REE can be stripped
most quickly as a group in ca. 50% HNO3 (Fig. 2.2).
Fig. 2.2. Elution curves for various elements
from cation exchange columns: a) with hydrochloric
acid; b) with nitric acid. Modified after Crock et al. (1984).
Several
methods of separation between the rare earths have been used:
1) Hexyl di-ethyl hydrogen phosphate
(HDEHP)-coated Teflon powder (stationary phase) with dilute mineral acid eluent, (Richard et
al., 1976). In this technique, which may be called the ‘reverse phase’
method, light REE are eluted first, whereas in the other techniques, heavy REE
are eluted first. The reverse phase method yields sharp elution fronts but long
tails (Fig. 2.3). It is very effective for removing the Sm
interference from 144Nd, and presently most popular. However,
substantial Ce is usually present in the Nd cut, so 142Nd cannot be measured accurately
(section 2.2.3). Similarly, the separation between light REE is not good enough
for Ce isotope analysis.
2) Cation exchange resin with Hydroxy isobutyric acid
(HIBA) eluent, (Eugster et al., 1970; Dosso
and Murthy, 1980). This method requires more work in preparation of eluent, whose pH must be carefully controlled. It is
therefore less popular than (1) for Nd, but very
effective for Ce (Tanaka and Masuda, 1982; Dickin et al.,
1987).
3)
Anion exchange resin with Methanol)dilute acetic acid)dilute nitric acid eluent (Hooker et al.,
1975, O’Nions et
al., 1977). This method is least popular at present, but may be better than
(1) if an analysis of 142Nd is required.
4) An
alternative approach described by Cassidy and Chauvel
(1989) is to use high-pressure liquid chromatography (HPLC).
Fig. 2.3. Elution profiles of light REEs from a reverse phase HDEHP column showing sharp peak
fronts and long tails.
2.1.3 Lu)Hf
Precise Hf isotope
analysis requires a high quality separation from Zr,
because the presence of Zr strongly suppresses Hf ionisation. However, separating Hf
and Zr is difficult because of the similar chemistry
of these elements. In addition, Ti also has similar chemistry to Hf, and because of its high abundance in mafic rocks, this must be efficiently removed. Hence, the
method developed by Patchett and Tatsumoto
(1980) involved three stages of ion exchange column separation. It was also
necessary to carry out much of the procedure in an HF medium, due to the risk
of deposition of insoluble fluorides.
Although
Hf isotope analysis is now performed by MC–ICP–MS, it
is still necessary to purify Hf before analysis in
order to get good data. A revised method by developed by Barovich
et al. (1995) has been used quite widely. This also involves three stages of
(anion) column chemistry, but only the first is eluted with HF. The second and
third columns were eluted principally with H2SO4. A
modification of this process by David et al. (1999) was able to use only two
columns by having a multi-step elution process with several changes in acid
type (e.g. H2SO4–H2O–HF–HCl).
2.1.4 Lead
Lead (Pb) and uranium
(U) have normally been separated from zircons by elution with HCl on anion exchange columns (Catanzaro
and Kulp, 1964). However, this method is not able to
separate Pb from the large quantities of Fe in
whole-rock samples, which then causes unstable Pb
emission during mass spectrometry. A widely used alternative is to elute all
elements except Pb from a miniature anion column with
dilute hydrobromic acid (Chen and Wasserburg,
1981). The distribution coefficient for Pb onto the
resin has a maximum value at just under 1M HBr, and falls off sharply on either side (Fig. 2.4).
Elution with HBr effectively strips most elements,
including Fe, from the column. However, an alternative is to elute with a
mixture of 0.5M HBr and 0.5M HNO3, which
removes Zn more efficiently than HBr alone (e.g. Kuritani and Nakamura, 2002). Finally, Pb
is collected with 6M HCl or water. (Pb also has a distribution coefficient maximum onto the
solid phase at 2)3 M HCl which falls off in more dilute and
more concentrated acid).
Fig. 2.4. Plot showing the distribution
coefficient (KD) of Pb from dilute HBr onto anion
exchange resin as a function of molarity. The curve
represents a best fit to the data points. After Manton
(1988).
The
purity of Pb samples is improved by a second pass
through an ion exchange column. This could involve a second pass through the
same column (after cleaning). However, a smaller clean-up column gives better
results. For example, Kuritani and Nakamura (2002)
used a clean-up column with only 10ul of anion resin. Alternatively, Manton
(1988) showed that the distribution coefficient for Pb
onto anion resin in dilute HBr is large enough for
the absorption of small Pb samples onto a single
large resin bead. The Pb is subsequently
back-extracted into water. Yields from zircon samples were about 50%, after an
equilibration period (with stirring) of about 8 hours. Alternatively, the
process can be accelerated by agitation in an ultrasonic bath. The somewhat low
efficiency may be outweighed by the high purity of the product.
An
alternative method of Pb separation from rocks is to
use a two-stage electro-deposition method by which Pb
is first deposited on the cathode at an accurately regulated potential, then redissolved and redeposited in a
miniature cell on the anode (Arden and Gale, 1974). This method yields a high
quality of separation, but is cumbersome to operate. Nevertheless, the anion
stage may be useful alone for purifying galena Pb.
Levels of environmental contamination
introduced during laboratory procedures are determined by the analysis of
‘blanks’. These are measured by taking an imaginary sample through the whole
chemical separation procedure, after which the amount of introduced extraneous
contamination is measured by isotope dilution (section 2.4). Blank levels must
be minimised in all of the chemical procedures described above, but
particularly strenuous efforts are necessary to limit Pb
contamination due to its relatively high concentration in the environment
compared to normal rocks.
The
minimum laboratory requirements to maintain low blanks would be an overpressure
air system, sub-boiling distillation of all reagents in quartz or PFE stills,
and evaporation of all samples under filtered air. For typical terrestrial
whole-rock samples, acceptable total chemistry blanks would normally be less
than a nanogram (ng = 10!9g) for Pb, Sr and Nd. This is necessary
because the samples to be analysed often contain less than 1 microgram (:g = 10!6g) of the element of interest
. However, in the analysis of very small samples (e.g. single zircons)
blanks of a few picograms (pg = 10!12g) are necessary (e.g. Roddick
et al., 1987), since the Pb sample itself may weigh less than 1 ng.
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