6-1.6 Secondary Ion Mass Spectrometry Methods
A general description of SIMS techniques is given in Volume I, Part 1, Chapter 30. A recent review on the use of SIMS technique for measuring light stable isotopes, including O isotopes, on silicate samples was presented by Valley et al. (1998).
Older general publications on SIMS techniques and the possible applications of SIMS were given in Shimizu et al. (1978: general and basic discussion) Liebl (1980: include historical overview and explanation on the basic working of the SIMS), Shimizu & Hart (1982a: applications), Reed (1984, 1989Reed (1984)1989: applications), Lorin (1992): discussing double sector MS machines for O-isotope measurement), Hervig et al. (1992): discussion on SIMS technique for 16O, 17O, and 18O measurement on insulators), Hinton (1995: SIMS in geology, procedures and applications) and MacRae (1995: SIMS and micro-analysis in geology).
The great advantage of the SIMS technique is its high spatial resolution (about 10nm) that enables measurement of within-grain isotopic variations. Required amount of sample material is in the picomole range (Saxton et al., 1995). Diffusion, for instance of 18O through silicate minerals or of 7Li through silicate glasses, thus can be studied.
Samples are usually analyzed as polished or thin sections. The polished surface of the sample generally is coated with carbon, gold or Au–Pd alloy (60–40% ratio) to make the surface electrically conductive (e.g. Mc Keegan et al., 1985; Reed, 1989; Valley & Graham, 1991aMcKeegan et al., 1985Reed, 1989Valley and Graham, 1991a). Sometimes a thin line of high conductivity Ag colloid paint, connecting the grains for analysis in the sample holder, is added to increase conductivity (Valley & Graham, 1991a). Instead of a conductive coat, also a conductive grit can be applied (Hervig et al., 1992). Small samples also can be pressed into Au foil (Mc Keegan et al., 1985; Fahey et al., 1987a; Virag et al., 1991; Hervig et al., 1992)McKeegan et al., 1985Fahey et al., 1987aVirag et al., 1991Hervig et al., 1992). To start analysis, a short ‘burn-in’ period to remove the coating is required.
Oxygen isotope measurement by SIMS is generally carried out with a 133Cs+ primary beam (Reed, 1989; Valley & Graham, 1990, 1991a; Hervig, 1992; Hervig et al., 1992; Riciputi & Paterson, 1994; Lyon et al., 1995; Engrand et al., 1997; Macaulay et al., 2000Reed, 1989Valley & Graham, 1990Valley & Graham, 1991aHervig, 1992Hervig et al., 1992Riciputi & Paterson, 1994Lyon et al., 1995Engrand et al., 1997Macaulay et al., 2000).
Hervig et al. (1989) also used a primary 39K+ beam with success. For both 133Cs+ and 39K+ beams primary currents ranged from 0.1 to 5nA (beam ø from 10 to 30μm).
The use of F− and CN− primary beams were tested by Hervig et al. (1989), with negative results.
Sample charging was compensated by a normal-incident electron gun producing a cloud of low-energy electrons. The use of an electron multiplier enhanced flat topped peaks. Peaks for 18O and H216O were clearly resolved. Extreme energy filtering (Giletti & Shimizu, 1989; Schauer & Williams, 1990; Hervig, 1992)Giletti & Shimizu, 1989Schauer & Williams, 1990Hervig, 1992 eliminates hydride or ‘cluster ion’ interferences on 18O (or 17O) and eliminates the need for high mass resolution. An alternative way to achieve this is by lowering the accelerating voltage from 4500V, called voltage offset (often in the range of 30–100V: Giletti & Shimizu, 1989).
High mass resolution, possibly separating fragment ions from single-atom ions, however, causes a decrease in signal intensity (Giletti & Shimizu, 1989).
Reed (1984) stated that: ‘Ideally, isotope ratios are measured by repeated switching between flat-topped peaks. In this way the drift is minimized, while flat peak-tops eliminate errors caused by inaccuracy in field setting, at least within certain limits’ (see also Giletti & Shimizu, 1989).
Each analysis generally consists of 200 comparisons of 18O/16O with 104 counts on 18O. Valley et al. (1998) reported counting times of 1s for 16O and 5s for 18O, resulting in approximately 102 more counts for the major beam (e.g. 108 counts for 16O if 106 counts were required for 18O). Gradients in δ18O near-grain margins were explained by sample topography and poorer charge compensation effects (Valley & Graham, 1990). Background measurement procedures were described by Lyon et al. (1995).
Isotopic fractionation occurs during the sputtering/ionization process (e.g. Lyon et al., 1994a, 1995; MacRae, 1995Lyon et al., 1994aLyon et al., 1995MacRae, 1995), and therefore it is important to have reference material that is close in isotopic composition to the sample of interest and analyzed under similar analytical conditions in the session (Shimizu & Hart, 1982a). Lighter isotopes are sputtered in preference to the heavier isotopes, causing measured isotopic ratios which are too light in the start of sputtering while the surface layer of the sample is enriched (Steele et al., 1976; Lorin et al, 1977; Bradley et al., 1978; Macdougall & Phinney, 1979; Slodzian et al., 1980; Klossa et al., 1981; Shimizu & Hart, 1982a, b; Hervig et al., 1992Steele et al., 1976Lorin et al, 1977Bradley et al., 1978Macdougall & Phinney, 1979Slodzian et al., 1980Klossa et al., 1981Shimizu & Hart, 1982aShimizu & Hart, 1982bHervig et al., 1992). Slodzian et al. (1980), in analyzing isotopic compositions of Li, Mg, Si, K, Ca and Zr, observed that:
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secondary ion production of the lighter isotope was always enhanced relative to the heavy isotope
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the magnitude of the isotopic fractionation was proportional to the mass difference
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the degree of fractionation was a function of the secondary ion energy.
Additionally, matrix effects can change the magnitude of the instrumental isotopic mass fractionation (e.g. Reed, 1989; Eiler et al., 1997)Reed, 1989Eiler et al., 1997. Lyon et al. (1998) discussed oxygen isotopic fractionation caused by different orientation of the crystal structure in the prepared sample. This effect was detected in magnetite, but not in minerals such as quartz, olivine or carbonates. Moreover, instrumental isotopic fractionation for oxygen can be variable in time by up to tens of permil (Jull, 1982; Shimizu & Hart, 1982b; Engström et al., 1987; Lyon et al., 1995)Jull, 1982Shimizu & Hart, 1982bEngström et al., 1987Lyon et al., 1995.
A discussion on secondary ion intensities in polycomponent materials was given by Shimizu & Hart (1982a).
Measurements of oxygen isotopes on different terrestrial oxide standards (spinel and hibonite) showed repeatability within 2‰ (2σ), if measurements on many individual grains are averaged. Individual measurements on single grains may vary typically 5–8‰ from the average value (Zinner, 1989). Corrections for dead time in the counting system must be applied.
High resolution analysis of quartz, with a spot size of 10–30μm, resulted in a precision of ±1–2‰ and sometimes better (Giletti & Shimizu, 1989; Hervig et al., 1989, 1992; Valley & Graham, 1990; Lyon et al., 1995)Giletti & Shimizu, 1989Hervig et al., 1989Hervig et al., 1992Valley & Graham, 1990Lyon et al., 1995.
To increase the beam spot (30–40μmø) a defocused beam can be applied, accepting only oxygen ions from the central 8μm diameter of the beam spot for isotopic analysis (Valley & Graham, 1990, 1991aValley & Graham, 1990Valley & Graham, 1991a: on magnetites).
Studies including O isotopic measurement on silicate materials can be found in Fahey et al. (1987a), McKeegan (1987a, b), Zinner & Tang (1988), Giletti & Shimizu (1989), Valley & Graham (1991a, 1996), Arbey et al. (1993), Lyon et al. (1994b, 1995), Saxton et al. (1995) Graham et al. (1996), Valley & Graham, (1996), Engrand et al. (1997) and Williams et al. (1997) and Girard et al. (2001)Fahey et al. (1987a)McKeegan, 1987aMcKeegan, 1987bZinner & Tang (1988)Giletti & Shimizu (1989)Valley and Graham, 1991aValley and Graham, 1996Arbey et al. (1993)Lyon et al., 1994bLyon et al., 1995Saxton et al. (1995)Graham et al. (1996)Valley & Graham, (1996)Engrand et al. (1997)Williams et al. (1997)Girard et al. (2001).
Lyon & Turner (1992) described a multicollector SIMS machine, capable of doing SIMS, RIMS (resonance ionization mass spectrometry) and TIMS analyses. Lyon & Turner (1992) reported preliminary results on O isotope (16O, 17O and 18O) measurements. The multicollector detection system eliminates the need for peak switching between the different isotopes. 18O/16O ratios were obtained with an internal consistency of ±0.3‰ on a single spot and in ∼10min blocks counting time.
17O considerations – Zinner (1989) reviewed the use of SIMS for measuring O isotopic compositions in silicate materials. Cs+ bombardment of a silicate material is producing a high yield of O− secondary ions. Interference with 16OH− (separated from 17O by 3.6×10−3amu) might give problems when measuring low abundance 17O and 18O samples (Zinner, 1989; Hervig et al., 1992)Zinner, 1989Hervig et al., 1992.
Hervig et al. (1992) stated that a high mass resolution (m/Δm) of about 8000 is needed (Lyon et al., 1995: 6000; Engrand et al., 1997, used ∼7000), and even then the interference on the 17O− is so strong (tailing) a small correction is still needed. Hervig et al. (1992) preferred to use a low mass resolution (m/Δm ≈ 500) and apply extreme energy filtering (see above) technique to remove the 16OH− hydride interference.
The source slit, normally wide open, was narrowed to 30–40μm to achieve a flat-topped ‘17O peak’ (Lyon et al., 1995).
Precision for 17O/16O ratios of ±0.1‰ can only be reached with long counting times. With reasonable shorter counting times selected a precision of ≈±0.2‰ can be obtained (Hervig et al., 1992; Saxton et al., 1995)Hervig et al., 1992Saxton et al., 1995. A precision of ±1–1.5‰ was reported for measurements of δ17O and δ18O on micro-size stony cosmic spherules (with a Cameca ims-1270; Taylor et al., 2005).
A review on 17O analyses is given in Chapter 6-9.
6-1.6.1 NanoSIMS
A relatively new device for stable isotope measurement on small samples (ng size) with high resolution is the NanoSIMS (Cameca NanoSIMS 50), described in Chapter 12-0.2.4.1. Oxygen isotopes (16O, 17O, 18O) analyses are mostly reported on meteorite or planetary materials, but potential to extent the field of analysis, such as in rock samples or minerals and organic compounds, strongly exists. For references of oxygen isotopes with NanoSIMS, see Chapter 6-9.6.1.
Labotka et al. (2004) studied oxygen-isotope exchange between alkali feldspar (albite) and aqueous chloride solution by NanoSIMS.