Single-stage Accelerator Mass Spectrometer Research Articles

14C AMS at Suerc: Improving QA Data with the 5MV Tandem and 250KV SSAMS

In 2003, a National Electrostatics Corporation (NEC) 5MV tandem accelerator mass spectrometer was installed at SUERC, providing the radiocarbon laboratory with 14C measurements to 4–5‰ repeatability. In 2007, a 250kV single-stage accelerator mass spectrometer (SSAMS) was added to provide additional 14C capability and is now the preferred system for 14C analysis. Changes to the technology and to our operations are evident in our copious quality assurance data: typically, we now use the 134-position MC-SNICS source, which is filled to capacity. Measurement of standards shows that spectrometer running without the complication of on-line δ13C evaluation is a good operational compromise. Currently, 3‰ 14C/13C measurements are routinely achieved for samples up to nearly 3 half-lives old by consistent sample preparation and an automated data acquisition algorithm with sample random access for measurement repeats. Background and known-age standard data are presented for the period 2003–2008 for the 5MV system and 2007–2008 for the SSAMS, to demonstrate the improvements in data quality.

Improved SSAMS performance

We operate a new NEC 250 kV single-stage accelerator mass spectrometer (SSAMS) next to our established 5 MV tandem. This permits good comparison of 14C-AMS and challenges SSAMS performance. Initial SSAMS ion-optical deficiencies have been addressed by shimming the injection magnet and 3‰ 14C/ 13C measurement with background limited by sample chemistry is routine. Higher-precision analysis is also achievable. A problematic measurement interference remains, however. The small spectrometer was procured to add measurement capacity, and for experiments with positive ions, but is now the preferred instrument for both convenience and measurement quality.

Attempted positive ion radiocarbon AMS

The SUERC single-stage accelerator mass spectrometer is capable of generating and accelerating-from-ground positive ions. Having previously shown suppression of molecular interferences we have proceeded to study positive ion radiocarbon AMS. Potential benefits are convenient and efficient ion production. This depends on overcoming the large 14N atomic isobar which we have suppressed by seven orders of magnitude in the accelerator terminal. A remaining interference is more problematic, however.

A late Pleistocene record of aeolian sedimentation in Blanche Cave, Naracoorte, South Australia

We provide geochemical analyses and grain size data for a clearly layered, 80 cm thick sedimentary deposit close to a roof collapse in Blanche Cave near Naracoorte in SE South Australia. This deposit contains aeolian material deposited between ∼40 ka and 14 ka cal BP and which yields airborne sediments spanning the Last Glacial Maximum, a period of time with little information for the Australian continent. The deposit also contains abundant vertebrate fossil material derived from owl pellets, accumulation and pitfall entrapment. Below the studied profile, large vertebrate remains are found but are not discussed here. No Holocene sedimentation occurred at the site examined in the cave, and the top of the sequence is capped with a layer that has been anthropologically disturbed and contains exotic Pinus pollen. Chronologies of the deposit were obtained using two dating techniques: single stage accelerator mass spectrometer (SSAMS) 14C analysis of 23 charcoal samples and optically stimulated luminescence (OSL) dating of quartz from 6 sediment samples. The 14C chronology is preferred to describe the history of the deposits since the OSL chronology, which consistently overestimates the associated radiocarbon dates, may be inaccurate due to complexities in calculating dose rates, and may in addition represent the timing of sediment deposition through the cave opening rather than sediment transport to the deposit site. Morphological analysis of single quartz grains and grain size analysis indicate different provenance that is confirmed through the geochemical analyses of bulk sediment. Major elements were measured by X-ray Fluorescence (XRF), trace and rare earth elements by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA ICP-MS), and Neodymium isotopic ratios were obtained using a Thermal Ionisation Mass Spectrometer (TIMS). Our results indicate that the aeolian material deposited in Blanche Cave over the 40–14 ka cal BP period originated from different sources across South Australia, although Nd isotopes clearly indicate a close association with sediments of the Kanmantoo Group outcropping along the eastern portion of the Mount Lofty Ranges and the Padthaway Ridge further south, both being located to the NW and NNW of the cave. During the latter part of Marine Isotope Stage 3, conditions were wetter and windblown sediment came from the coastal region just north and south of the Coorong Lagoon, with winds originating from the north-northwest. At that time, woody taxa appear to have vegetated the landscape. During the drier phases, especially the Last Glacial Maximum, sediment came from further inland, thus suggesting a different predominant wind direction, more from the northwest. The deglaciation saw material originating from a more northerly direction.

Comparison of a 250 kV single‐stage accelerator mass spectrometer with a 5 MV tandem accelerator mass spectrometer – fitness for purpose in bioanalysis

The introduction of 'compact' accelerator mass spectrometers into biomedical science, including use in drug metabolism and bioanalytical applications, is an exciting recent development. Comparisons are presented here between a more established and relatively large tandem accelerator which operates at up to 5 MV and a conventional laboratory-sized 250 kV single-stage accelerator mass spectrometer. Biological samples were enriched with low levels of radiocarbon, then converted into graphite prior to analysis on each of the two instruments. The data obtained showed the single-stage instrument to be capable of delivering comparable results, and thus able to provide similar study support, with that provided by the 5 MV instrument, without the significant overheads and complexities which are inherent to the operation of the larger instrument. We believe that the advent of these laboratory-sized accelerator mass spectrometry (AMS) instruments represents a real turning point in the potential for application of AMS by a wider user group.

Positive ion AMS with a single-stage accelerator and an RF-plasma ion source at SUERC

The new single-stage accelerator mass spectrometer at SUERC can also accept positive ions. Potential benefits of positive ion measurement with suitable ion sources are more convenient sample form, smaller sample size, a variety of available charge states and maybe new applications. We have begun with oxygen isotope analyses to demonstrate destruction of interfering molecules.

Performance of the new single stage accelerator mass spectrometer at the SUERC

A NEC 250kV single stage accelerator mass spectrometer (SSAMS) has been installed along-side our five year old 5MV spectrometer. The new instrument is intended to take over routine radiocarbon measurement, thereby providing more capacity for our rapidly growing cosmogenic nuclide programme. Initial tests show that although flawed the SSAMS sufficiently mimics the good performance of the first machine; high-precision 14C/13C measurement is being achieved with a background limited by sample preparation.