We undertake all the steps required to transform raw sample material into graphite targets that can be analysed by accelerator mass spectrometry for radiocarbon measurement/dating.
First, we perform a pretreatment on samples to remove potential contaminants and isolate the fraction required for dating. We select the best pretreatment for a particular sample based on the sample type and its characteristics. The pretreatment usually involves chemical (e.g. acid-base-acid washes) and/or physical (filtering, sieving) procedures. This takes place across our seven individual analytical suites, each with specialised capabilities for physical processing, wet chemistry, and instrumentation.
Samples are next converted to carbon dioxide gas either by combustion (organic materials) or acid hydrolysis (carbonates).
The next stage is to recover the produced carbon dioxide and purify it. It is fundamental that we avoid introducing contamination to the sample, and therefore, processing is undertaken under high vacuum in custom-built vacuum rigs to prevent introducing atmospheric carbon dioxide to the sample. Cryogenic techniques, such as freezing to liquid nitrogen temperatures (-196 deg C) are used to purify the carbon dioxide.
Aliquots of purified sample carbon dioxide are reserved for off-line δ13C measurement on our in-house Isotope Ratio Mass Spectrometer (Thermo Fisher Delta V).
We convert the sample carbon dioxide to graphite (solid carbon) using the Fe:Zn reduction method. The resulting graphite powder is pressed into an aluminium sample holder for AMS measurement.
Providing the highest quality radiocarbon measurements is our number one priority. All the methods that we use are thoroughly tested to verify their reliability using international (e.g. IAEA) and in-house radiocarbon standards. Many of our methods and approaches are published in peer-reviewed international scientific journals, and we participate in international intercomparison exercises within the worldwide radiocarbon community.
Every batch of samples processed in the laboratory includes quality assurance standards that are treated identically to the samples, with around 1 in 10 measurements we perform representing a quality assurance standard. Numerous other standards are used to verify every step of our procedures to assess cleanliness, and accuracy and precision of the AMS measurements.
Following pretreatment, we have a range of combustion methods to transform organic samples into carbon dioxide: combustion in sealed quartz tubes, Costech ECS 4010 elemental combustion system and a custom-built high pressure combustion ‘bomb’. The latter is applied in specialised projects for very large samples, usually with ultra-low carbon content.
CARBONATES (Shells etc)
Carbonates, such as shells and foraminifera are hydrolysed to carbon dioxide which is purified cryogenically. The outer layer of carbonate can be removed using an etching pretreatment to remove potential contaminants.
CARBON IN WATERS
Water can contain carbon in several different forms which we routinely analyse. Particulate organic carbon (POC) is removed by filtration and subsequently processed like other organic materials. We recover dissolved organic carbon (DOC) as solids using rotary evaporation and/or freeze drying, which is then combusted. Dissolved inorganic carbon (DIC) samples are processed using an acid hydrolysis method that we have modified in-house. Finally, dissolved carbon dioxide and dissolved methane can be recovered for dating using headspace equilibration techniques that we have pioneered for radiocarbon analysis.
CARBON IN GASES
We routinely analyse carbon dioxide gas and methane gas for radiocarbon concentration/age. Samples can be submitted in various storage vessels which can be tailored to individual project needs. Over the years we have developed a range of novel field sampling methods for carbon dioxide and methane gas sampling (see below). In the laboratory, carbon dioxide samples are cryogenically purified before converting to graphite. Methane samples are purified using chemical and cryogenic techniques, then converted to carbon dioxide using an ultra-clean platinum catalytic combustion technique, and then processed as for carbon dioxide.
Hydrogen pyrolysis (HyPy) is a technique for separation of polyaromatic compounds from other organic molecules in a mixed sample. This allows us to isolate a whole class of compounds from fire-derived biomass (charcoal, Pyrogenic Carbon, Black Carbon), even when these compounds exist in a highly heterogeneous sample, such as a soil or sediment. This means the fire-derived material is extracted for 14C measurement without interference from other forms of organic carbon, that would not be separated with other routine methods. HyPy also offers a means to quantify the proportion of Pyrogenic carbon in a sample, with or without 14C measurement. The non-aromatic compounds in the sample are retained on silica, and are also available for other analyses, such as GC-MS or 14C measurement.
Molecular sieve sampling of carbon dioxide
We have pioneered the use of cartridges containing zeolite molecular sieve for sampling carbon dioxide for radiocarbon analysis. These cartridges are highly portable and easy to use – the gas being sampled (e.g. chamber headspaces gases) simply needs to be pumped through the cartridge whereupon the carbon dioxide is adsorbed and concentrated on the zeolite. This carbon dioxide is recovered from the sieve using purpose-built equipment in our laboratory. Sample carbon dioxide can be stored safely in the sieve cartridge for many months, which combined with the high portability, makes them particularly suitable for field sampling.
Radiocarbon analysis of methane
We have several decades of experience in radiocarbon dating / analysis of methane gas and continue to develop and improve our field sampling and laboratory analytical techniques. Recent innovations include novel methods to extract methane from aquatic environments for dating and determination of source. We can provide advice and training on sample collection and have developed and verified the use of several different sample collection and storage approaches.
Design and build of custom sampling equipment
We have designed, built, and tested a range of different sampling kits to meet the demands of different sampling applications. For carbon dioxide sampling we can provide novel and highly portable sampling kits that enable the collection of sample from chambers or incubation vessels, as well as the ambient atmosphere. These kits can be pump-based, enabling the rapid collection of sample carbon dioxide. We have also developed passive carbon dioxide samplers for radiocarbon analysis of soil respiration, atmospheric carbon dioxide, and carbon dioxide dissolved in water – these passive samplers have the advantages of being extremely easy to deploy and having the ability to collect time-integrated samples (i.e. performing representative sample collection over days to many weeks).
Not sure about the best sampling approach for your project? Please contact us and we will be happy to provide more details on our existing sampling methods and to discuss the possibility of building bespoke equipment to meet your projects needs.