2.2 Ion sources
As noted above, the traditional start of mass spectrometric
analysis is to heat the sample under vacuum, leading to thermal ionisation. With the exception of the rare gases,
thermal ionisation is normally achieved by loading a solid deposit of the
sample onto a metal filament, which can then be subjected to resistive heating.
However, the manner by which the sample is deposited and then heated has a
major effect on the efficiency of the analysis. Therefore these procedures will
be discussed in some detail.
2.2.1 Thermal ionisation
For some elements such as Sr,
stable emission of metal ions is achieved from a salt deposited directly onto a
single metal filament (Fig 2.5a), usually tantalum (Ta). The loading procedure
involves evaporating the salt solution onto the filament before insertion into
the vacuum system. The sample is often loaded in phosphoric acid, which seems
to a) displace all other anion species to yield a uniform salt composition, b)
destroy organic residues (such as ion exchange resin) mixed with the sample,
and c) glue the sample to the filament. During mass spectrometric analysis, the
filament current is raised by means of a stabilised power supply to yield a
temperature where simultaneous volatilisation and ionisation of the sample
occurs.
Fig. 2.5. The arrangement of filament ribbons
on commonly used single- and triple-filament bead assemblies. Note that only one side filament is shown attached to the ‘triple’
bead.
However,
for many elements, stable volatilisation and ionisation of metal species does
not occur at the same temperature. This problem was noted by Ingram and Chupka (1953), who first proposed the use of multiple
source filaments (Fig. 2.5b). In this configuration, one or more filaments
bearing the sample load can be heated to the optimum temperature for stable
volatilisation, while another hotter filament can be used to ionise the atomic
cloud by bombarding it with electrons.
This
method is particularly effective for REE analysis, where the sample is usually
loaded onto one or both of the Ta side filaments of a triple-filament bead.
These are held at a moderate temperature (ca. 1400 oC)
where REE volatilisation is most stable. The centre filament (usually Re) is
held at a much higher temperature (ca. 2000 oC),
which promotes ionisation of the metal vapour. To some extent the ratio of
metal to oxide species can be controlled by the centre filament temperature,
which may help to suppress REE isobaric interferences. The properties of the
REE under such conditions vary from light to heavy rare earths. La and Ce tend to form oxides unless extremely high centre
filament temperatures are used, while heavier REE tend to form the metal
species (Hooker et al., 1975; Thirlwall, 1982).
Uranium
and thorium may also be analysed by the triple-filament technique. Again, the
temperature of the centre filament controls the metal/oxide ratio of the
emitted ions (Li et al., 1989). The
triple-filament method was also used in the first successful analysis of Hf (Patchett and Tatsumoto, 1980). However, Hf
analysis is now exclusively performed by ICP-MS (section 2.5.2).
An
alternative to TIMS analysis with multiple filaments is to use special conditions
to control the evaporation)ionisation behaviour from a single filament.
For example, in the case of Pb, the sample is usually
loaded on a rhenium filament in a suspension of silica gel (Cameron et al., 1969). This is thought to form a
blanket over the sample which effectively retards Pb
volatilisation so that the filament can be raised to a higher temperature
(where Pb fractionation is more reproducible) without
burning off the sample uncontrollably.
Early
Nd isotope determinations (Lugmair
et al., 1975; DePaolo
and Wasserburg, 1976) were made using NdO+ ions, whose emission from a single filament
source was promoted by relatively high oxygen pressures in the mass
spectrometer source housing. Some workers have continued to use this method,
rather than the more popular multiple-filament method, since it can yield
higher efficiency. Oxygen may be bled into the source in minute amounts to
increase oxide emission. Alternatively, loading with silica gel may achieve the
same objective without degrading source vacuum (Thirlwall,
1991a). Uranium may also be analysed as the oxide by mixing with TaO2
powder on a tungsten filament.
A
different approach to single filament U and Th
analysis is to use graphite to promote the formation of metal ions (Edwards et al., 1987). In this procedure it is
critical to prevent oxidation during sample loading by avoiding oxidising acids
and by maintaining temperatures below the point of visible glowing. Noble et al. (1989) applied a similar
technique to Nd isotope analysis. The use of platinised graphite was argued to give greater thermal
stability to the reducing agent.
2.2.2 Plasma source mass
spectrometry
Technically, plasma source mass spectrometry is
a kind of thermal ionisation mass spectrometry, because it relies on heating in
a gas plasma to achieve ionisation of the sample.
However, for practical purposes, the term ‘TIMS’ is usually restricted to the
case where thermal ionisation achieved by a heated filament under vacuum.
Therefore, plasma source mass spectrometry is regarded as a distinct technique.
Plasma
source mass spectrometry was invented in the late 1970s when the inductively
coupled plasma (ICP) was first attached to a mass spectrometer (MS) to produce
the ICP-MS (Houk et
al., 1980). The ICP source consists of a plasma torch made by using a radio
frequency (RF) generator to induce intense eddy currents in a stream of ionised
argon gas. The RF generator transmits about a kilo-watt of power into the
plasma, raising its temperature to about 5000 oC
and causing very efficient ionisation of most elements (Houk,
1986). Furthermore, the extreme temperature of the plasma ensures that nearly
all of these ions are monatomic ions, ideal for mass spectrometric analysis.
When
the ICP-MS was first conceived, it was found most convenient to use a quadrupole mass spectrometer (
Fig. 2.6. Schematic
illustration of an ICP-MS instrument with quadrupole
analyser.
The
main technical breakthrough in the development of the ICP-MS was the physical
feat of actually feeding a plasma at 5000 oC and atmospheric pressure into a mass
spectrometer whose analyser pressure is ca. 108 times lower (10-5 mbar). This was achieved by firing
the plasma at a two-stage water-cooled orifice, with continuous pumping of the
intermediate space by a mechanical pump (Fig. 2.6). Subsequent technical
developments in ICP-MS over the next 20 years have mainly involved greatly
increased efficiency in the sampling of the plasma by the mass spectrometer.
This has allowed ICP-MS to reach remarkable sensitivity, with detection limits
as low as parts per trillion (pico grams per gram).
However, the precision of isotope ratio measurements by conventional ICP-MS is
limited by instabilities in the plasma. By rapidly scanning the ‘mass
spectrum’, the quadrupole analyser can achieve
precision of around 1%, but this is a practical limit for single collector
analysis. This level of precision was found to be useful in early development
of the Re-Os method (section 8.1), but is not useful for most radiogenic
isotopic systems. Hence, to apply ICP-MS to these systems it was necessary to
introduce multiple collection techniques to cancel out the instability of the
source.
To
perform multiple collection mass spectrometry (MC-MS) with a plasma ion source
it was necessary to link the ICP source with a magnetic sector mass spectrometer
of the type normally used in TIMS instruments (section 2.3). This involved two
main technical challenges. The first problem is that magnetic sector mass
spectrometry requires a large accelerating voltage to raise ions to the high
velocities where magnetic separation is efficient. However, in order to keep
the whole analyser assembly at electrical ground, the ion source must be at up
to 8000 v positive. For MC-ICP-MS, this means isolating the plasma at up to
8000 v positive, with all of the attendant engineering problems. The second
problem is that magnetic sector mass spectrometry requires an ion beam with a
very small range of ion energy, whereas the ICP source generates ions with a
relatively large energy range. This was overcome in the first MC-ICP-MS
instruments by using both quadrupole and
electrostatic pre-filters to smooth the energy distribution in the ion beam
before it entered the magnetic sector (Fig. 2.7). Subsequent instruments have
used a variety of other filtering devices to achieve similar objectives.
Fig. 2.7. Simplified plan view of the VG
Elemental Plasma 54 instrument. After Halliday
et al. (1998).
2.2.3 Mass fractionation
The process of volatilisation and ionisation
during mass spectrometry requires the breaking of chemical bonds, but the
strength of these bonds is mass dependent. Therefore, excitation of the sample
leads to mass-dependent fractionation, which can be understood by approximating
the chemical bond between two atoms as a harmonic oscillator.
The
energy of a molecule (or part of an ionic lattice) decreases with decreasing
temperature, but at absolute zero it has a certain finite value called the zero
point energy, equal to 0.5 h< (where h is Plank’s constant and < is the vibrational
frequency). A bond involving the light isotope of an element has a higher
vibration frequency and hence a higher zero point energy than one involving a
heavier isotope, as illustrated in Fig. 2.8. The difference in bond energies
diminishes as temperature rises, but still persists. Because the potential
energy well of the bond involving the lighter isotope is always shallower than
for the heavier, the bond with the lighter isotope is more readily broken.
Hence it is preferentially released from the hot filament, causing isotopic fractionation.
Fig. 2.8. Schematic diagram of potential
energy against bond length for a hypothetical molecule made of two isotopes,
based on the ‘harmonic oscillator’ model.
In
a plasma source mass spectrometer (section 2.2.2), fresh sample is continually
fed into the plasma torch. Hence, mass fractionation produces a fairly constant
(but large) discrepancy between the isotopic composition of the solid sample
and the ion cloud. In contrast, solid source TIMS analysis produces smaller
fractionation effects, but the continual process of fractionation starts to
‘use up’ the lighter isotope on the filament so that the isotopic composition
of the sample gets progressively heavier (the ‘reservoir effect’). Eberhardt et al.
(1964) showed that this process follows a Rayleigh
fractionation law (Fig. 2.9). The magnitude of this effect could yield totally
unacceptable errors of up to 1% in measured isotope ratio. However, for
elements with two or more non-radiogenic isotopes, an internal normalisation
for such mass-dependent fractionation can be performed.
Fig. 2.9. Effect of
within-run fractionation, over time, on a sample of natural rubidium undergoing
isotopic analysis. Points are observed ratios; dashed line schematically
indicates actual composition of Rb on the filament. Data from Eberhardt et al. (1964).
In
the case of strontium, the fractionation of 87Sr/86Sr can
be monitored using the 88Sr/86Sr ratio, since 88Sr
and 86Sr are both non-radiogenic (i.e. produced only by nucleosynthetic processes in stars). The ratio 86Sr/88Sr
is constant throughout the Earth and is taken to be 0.1194 by international
convention. This value cannot be measured absolutely, but was originally
estimated from the average beam composition half-way through very many TIMS
runs. The deviation of observed 86Sr/88Sr from 0.1194 at
each point through the run is divided by the difference between the two masses
()mass = 2.003) in order to calculate a
fractionation factor per mass unit:
(86Sr/88Sr)obs
)))))))))
! 1
0.1194
F =
)))))))))))))))) [2.1]
)mass
This fractionation factor can then be used to
correct the observed (raw) 87Sr/86Sr ratio, for which )mass = 1.003:
(87Sr) (87Sr)
()))) = ()))) @ ( 1 + F )mass) [2.2]
(86Sr)true (86Sr)obs
This has the effect of improving the within-run
precision of the 87Sr/86Sr ratio from ca. 1% to better
than 0.01%. Neodymium metal analyses are similarly normalised for fractionation
(Fig. 2.10) using an internationally agreed value of 146Nd/144Nd
= 0.7219 (O’Nions et
al., 1979). However, Nd oxide analyses are
normalised to different values (Wasserburg et al., 1981) which are incompatible
with the Nd metal normalising value.
Fig. 2.10. Plot of raw 146Nd/144Nd
ratios and fractionation-corrected 145Nd/144Nd ratios
(normalised to 146Nd/144Nd = 0.7219) for a single mass
spectrometer run. Each point is a mean of 10 scans of the mass spectrum, while
horizontal lines are grand means. After Noble (pers. comm.).
The
fractionation correction described above is usually called the linear law, but
the power law (Wasserburg et al., 1981; Thirlwall, 1991b) is
effectively identical. Both of these laws assume that fractionation is
proportional to mass difference only, and is independent of the absolute masses
of the fractionating species. In other words, fractionation per mass unit is
constant. However, this is an approximation to the real evaporation process,
where fractionation per mass unit must vary inversely with the absolute masses
of the evaporating species. Russell et
al. (1978) first observed a break)down of the linear law in isotopic analysis of the
‘light’ element Ca. To remedy this, they introduced an ‘exponential’ law, where
the fractionation factor depends also on the mass of the evaporating species.
This gave a better fit to Ca isotope data than the linear law (Fig. 2.11).
Fig. 2.11. Plot of measured/true 44Ca/48Ca
versus 40Ca/44Ca
ratios showing fit of linear and exponential fractionation laws to typical data
from two runs. After Russell et al. (1978).
These
problems are much less severe for Sr and Nd isotope analysis because of their heavier masses.
However, Thirlwall (1991b) found small deviations
from linear law behaviour in a large data set of Sr
standard analyses. This is revealed by a correlation between normalised 87Sr/86Sr
and average observed 86Sr/88Sr ratios for complete runs
(Fig. 2.12). Thirlwall found that he could eliminate
the correlation by retrospectively applying an exponential law correction to
the data. This is described as follows:
| (87Sr/86Sr)norm |ln(86/88) | (86Sr/88Sr)obs |ln(87/86)
| ))))))))) | = |
))))))))) | [2.3]
| (87Sr/86Sr)corr | | .1194 |
where ‘norm’ and ‘corr’
refer to products of linear normalisation and exponential correction, and where
86, 87 and 88 are the actual masses of evaporating ions.
Fig. 2.12. Plot of
fractionation-corrected 87Sr/86Sr ratios against mean
un-corrected 86Sr/88Sr ratios for measurements of the
SRM987 standard over a period of several months. Error bar indicates
average within-run precision. After Thirlwall
(1991b).
This
model assumes that strontium evaporates from the filament as the species ‘Sr’. On the other hand, Habfast
(1983) suggested that a strontium sample on a rhenium filament might evaporate
as a species such as SrReO4 rather than atomic Sr. Hence the
‘apparent mass’ of 88Sr which should be
used in the exponential correction would be ca. 330 rather than 88. Under such
conditions the exponential law might actually produce a worse fit to the true
fractionation behaviour of the sample than the linear law. However, Thirlwall’s data suggest that evaporation from a tantalum
filament does occur as the metal species, so the exponential model is an improvement
over the linear model.
At
the limits of analytical precision, fractionation errors have also been
observed with the exponential law. However, Thirlwall
(1991b) argued that between-bead precision of Nd
isotope analyses could be improved to a level comparable with within-run
precision by using a secondary normalisation against 142Nd/144Nd.
This is based on an observed correlation between fractionation-corrected 143Nd/144Nd
and 142Nd/144Nd ratios from standard runs over a period
of months (Fig. 2.13). Subsequent work on Nd
fractionation during MC (multi-collector) - ICP-MS analysis of Nd showed similar behaviour (Vance and Thirlwall,
2002). However, because mass fractionation in the plasma source is an order of
magnitude larger, the effect is much bigger (Fig. 2.13).
Fig. 2.13. Plot of (exponential law) fractionation-normalised
143Nd/144Nd against 142Nd/144Nd for
analyses of a laboratory standard over a period of several months using
normalisation to 146Nd/144Nd = 0.7219. ( " ) = TIMS data; ( ! ) = MC-ICP-MS data.
After Vance and Thirlwall (2002).
Vance
and Thirlwall showed that this problem could be
solved using two alternative approaches. One is to apply a post-analysis
correction to 143Nd/144Nd ratios using the information
from the 142Nd/144Nd ratio, as in Fig. 2.13. An
alternative approach is to choose masses for the fractionation monitor that
bracket the masses to be corrected. For example, the mean mass of 143Nd
and 144Nd is 143.5, which is the same as the mean mass of 142Nd
and 145Nd. Therefore, 142Nd/145Nd represents
the ideal monitor ratio for fractionation correction of 143Nd/144Nd
ratios. This approach is only possible if Ce
interferences on 142Nd are low, and is therefore not feasible for TIMS analysis using the popular ‘reverse phase’ Nd separation (section 2.1.2). However, for most TIMS analysis the fractionation problem can be avoided by
collecting data near a 146Nd/144Nd ratio of 0.7219. For Nd analysis by MC-ICP-MS it is necessary to take account of
this problem in order to achieve reproducibilities
comparable with TIMS.
However, Ce isobaric interferences can be accurately
corrected in MC-ICP-MS analysis, so 142Nd can be used as one of the
fractionation monitors.
Internal
fractionation correction is only possible where there are two or more isotopes
present in a constant ratio. This is not the case for Pb
isotope analysis by TIMS, or in the isotope dilution analysis of Rb, so an external correction must be used. This depends on
achieving uniform fractionation behaviour between standards and different
samples, so that an across-the-board correction can be made to all runs. In the
case of Pb, the use of a silica gel blanket on the
filament achieves this objective by reducing the magnitude of fractionation
processes drastically, from a previous between-lab. variation
of ca. 3% to a present variation of ca. 0.3% . This improvement is partly due
to the higher filament temperatures possible using silica gel. Because bond
energy levels become closer with increasing temperature, the magnitude of
isotope fractionation falls with increasing temperature. However, for this
technique to work well, the Pb sample has to be well
purified during the chemical separation (section 2.1.4). In the analysis of
uranium, fractionation effects may similarly be reduced by running at high
temperature as the oxide. Alternatively, analysis as the metal ion produces
larger but relatively consistent degrees of fractionation, which can be
corrected by comparison with standard runs.
Where
the relevant nuclides exist, double spiking with two artificial isotopes may be
used to apply an internal fractionation correction to elements such as Pb with only one natural non-radiogenic isotope (section
2.4.2). In MC-ICP-MS analysis the two isotopes that form this double spike do
not even have to be the same element as the isotope ratios being corrected. For
example, the two isotopes of thallium can be used to correct Pb isotope data (section 2.5.4).
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