Sample Preparation Details

Pre-acid Etch

Treating carbonates with a mild acid leach prior to submission is not recommended because this procedure is best done immediately prior to full acidification. Over time, an etched surface may adsorb CO₂ from the air. If there is enough material, and an acid etch/leach is required to remove the outer surface of the material, this will be performed upon request at no additional cost.

Vial-in-Vessel Method

For small carbonate samples that are difficult to completely transfer from the sample container, consider submitting samples in a vial that can be directly placed into our acidification vessel (vial-in-vessel method), bypassing sample transfer and minimizing the loss of sample mass. With this method, the entire glass vial containing the carbonate sample is dropped into the acidification vessel which gives increased recovery of the sample carbon. Please see this document for the details of submitting samples for this method.

Coral/Mollusk/Otolith

Many corals, otoliths, and mollusks are like tree-rings, in that these may have annual growth bands. Keep this in mind when submitting a large sample that requires subsampling. When there is the luxury of excess material, use it to prepare a clean sample avoiding outer portions that are most susceptible to recrystallization. Consider sampling specific years or across growth-bands for a representative sample or using the inner or outer year growth parts only for a more age-constrained sample. In cases where the sample has been milled and likely to stick to the walls of the sample vial or container, please consider using the vial-in-vessel method described above. Sample definition is important and best done by the investigator. Submit only what you would like dated.

Foraminifera

Foraminifera ages may be affected by sediment that is not removed if it contains carbonate of a different age than the tests. We recommend sonicating in distilled water and not using solvents that contain carbon. Some submitters sonicate forams in a 3% peroxide and sodium hexametaphosphate solution and rinse with DH₂O to clean tests. Take care to inspect the sample immediately prior to submitting and to remove anything that is not a foram, including fibers, hair or quartz grains. The smaller the sample, the larger the potential impact of small bits of contamination on the isotope concentration.

Water Dissolved Inorganic C (DIC)

For ocean, lake, pond, ground or pore water, post-collection biological activity can alter the carbon isotopic concentration of the samples. Addition of 50-100 microliters of a saturated mercuric chloride solution will kill all bacteria (typically used for seawater samples), but this may not be appropriate for your sampling situation. Other poisons should be discussed with us if you choose not to use HgCl₂. If the sulfur content in the water is greater than that of standard seawater, then you may need additional poison. Chilling samples is acceptable but not as effective as poisoning.

Our preferred DIC sample collection vessel is a Wheaton 29/22, 100 ml borosilicate glass bottle (VWR bottle #W216015) with a high-quality ground-glass stopper (54 x 110 mm). The stopper is lubricated prior to collection with Apiezon grease to prevent exchange with the atmosphere. We may be able to supply bottles for your sample collection - please contact us for more information and refer to the collection protocols for full specifications. A shipping account number will be required to send sample bottles.

The DIC concentration must be supplied or estimated for us to extract an appropriate mass of carbon from a water sample on our vacuum system.

Sediment - Carbonate Minerals

The inorganic carbon content of sediments can vary considerably. We need at least an approximate idea of the %CaCO₃ of sediment before we can treat it. Without this information, we will not be able to process the samples. Submit dry sediment samples in well-labeled, clean glass or plastic containers. We assume samples are homogeneous. Some submitters dry and grind sediment samples prior to submission to ensure homogeneity.

Plant/Wood

It’s best to date clean, pure materials rather than a mixture of components. Separation or purification of the component you want analyzed may be performed by gently cleansing samples with distilled water, sieving and using a tool like a wet "picking" brush or tweezers under microscopic or magnified examination to remove unwanted components. If extraneous material is included with your sample that contains organic carbon, the radiocarbon result can potentially be contaminated by younger or older carbon.

Clean all non-plant/wood material from the sample using distilled water and thoroughly dry in a low temperature oven (50 degrees C). Weigh and place in a clean, labeled glass vial. A general rule-of-thumb is to estimate 40% carbon by weight for plant/wood or charcoal. Some mass is removed during pre-treatment of plant/wood which involves the base extraction of mobile humic and fulvic substances and acid removal of inorganic carbon. If stable isotope analysis is requested, we will take approximately a 10% split of the CO₂ produced for in-line analysis.

Sediment (OC)

The organic carbon content of sediments varies considerably. We must know the measured % organic carbon content to gauge how much sample is required for AMS analysis. Too much material may cause the combustion tube to explode and sample loss; too little material will result in an insufficient amount for AMS analysis. Please indicate on the submittal form whether you have measured the %OC or not. If not, we can determine it during analysis.

Sediment (C_org) samples typically undergo the acid pretreatment only, to remove inorganic carbon before combustion. We also offer vapor-phase acidification when requested. If your sediment contains plant material like peat, then we recommend the same treatment as plant/wood samples to remove any mobile humic or fulvic components. If the sediment contains a peat component, please indicate your preference for acid only or an acid-base-acid series of leaches on the submittal form.

Submit dry sediment samples in well-labeled, clean glass or plastic containers. We assume samples are homogeneous. Some submitters dry and grind sediment samples prior to submission to ensure homogeneity.

Charcoal

The carbon content of charcoal can vary significantly. However, under good conditions, 40% is a reasonable estimate of the organic carbon by weight.

Often, charcoal becomes broken and disseminated in surrounding soil matrixes. Separation or purification of the charcoal component may be as simple as scraping away the outer surfaces, but if disseminated, can be performed by gently cleansing samples, sieving and using a wet "picking" brush under a microscopic or magnifier to collect the charcoal fragments. Then, if further cleaning is necessary, gently sonicate samples with distilled water and carefully draw off the water. Repeat until clean. Dry, weigh and transfer to clean glass vials. If a mixture of disseminated material is submitted, then the mixture will be combusted; our procedures do not include hand-picking out the fragments of charcoal.

The pretreatment of charcoal is the same as that of plant/wood and involves a series of acid-base-acid leaches to remove the mobile phases of organic carbon (fulvic and humic acids) and any inorganic carbon.

Compound-specific (CSRA)

Compound Specific Radiocarbon Analysis (CSRA) samples are those collected by submitters with a PCGC, or preparative capillary gas chromatographic system (supplemented with a liquid-chromatographic system). Compound-specific samples are usually small samples (less than 100 µg C) requiring a solvent transfer to a combustion tube. Techniques to handle such small samples were initially developed at NOSAMS specifically to aid the development of compound-specific radiocarbon analysis of compounds like fatty acids, sterols, plant waxes, lignin phenols, PAHs, PCBs, and lipids. Training is available for the PCGC technique. Compound-specific samples may be prepared and isolated by NOSAMS staff with fees determined upon request. (contact NOSAMS)

Water DOC

Analysis of Dissolved Organic Carbon (DOC) by UV extraction of CO₂ is available in our Sample Prep Lab. Typically investigators filter samples prior to submission. DOC samples can be acidified with high purity hydrochloric or phosphoric acid (to pH of ~2.5 +/- 0.3) or sent frozen (filtered, non-acidified) to the facility. Only glass bottles are recommended for storing DOC samples. We will not accept DOC samples preserved with mercury (II) chloride as the mercury absorbs light from the UV lamp and may contaminate the quartz reactor. Unless prior arrangements and a shipping account number are provided, remaining sample and the containers will be disposed of after analysis.

The rate of sample analysis for DOC is one per day so expect longer turnaround times than for other processes. Samples containing less than 100 micrograms of carbon will be analyzed only by arrangement with the staff chemist, Roberta Hansman (rhansman@whoi.edu).

Proteinaceous Corals

Unless otherwise requested, proteinaceous corals are considered "ready to burn" and do not require pretreatment.

POC or Aerosols on Filters

Particulates collected on quartz filters can be combusted at high temperature (850 deg C) together with the sample. If pyrex filters are used, the temperature must be reduced (550 deg C) which could result in incomplete combustion of total organic carbon. Please provide the carbon loading (in milligrams C) and filter type in order for us to assess how much of the sample/filter to pack into a combustion tube and the temperature to use. It's best to receive the filters flat rather than packed or rolled to fit into vials or containers. Past clients have used cleaned/baked aluminum foil to double wrap the sample and filter, freeze and send to us. Pre-combust foil at 550 degrees for one hour before using. Aerosols are kept cool and dry to avoid bacterial growth. Unless pretreated by the submitter, POC samples will be acid fumigated, neutralized and thoroughly dried prior to combustion. If submitted pre-treated, samples must be completely neutral and dry or an additional fee may apply.

Pure CO₂

Pure CO₂ should be submitted in 6 or 9 mm (outside dimension) flame-sealed Pyrex or quartz tubes with a targeted sample size of 80 micromoles of gas and a flame-sealed tube length of 6-7 inches to fit our standard gas crackers. Two milliliters of pure CO₂ gas yields approximately 1 mg carbon which is what we target for ¹⁴C analysis. Small samples are in the range of 2 - 9 µmol. Larger amounts of CO₂ must have a sealed pressure of less than 2 atmospheres to avoid over-pressurizing the tubes. Label the tube with attached tape or paper (tightly attached) or use a permanent black marker. We need to know the amount of gas submitted, preferably in µmol, before we can accept CO₂ samples.

For submissions of CO₂ in batches of 10 or more paid samples, we offer free analysis of one blank and one modern standard, and we charge half price for the standards submitted with batches of less than 10 paid gas samples. It is good practice for submitters to include a modern and ¹⁴C-free or blank sample to assess the reliability of lab techniques and process blank for methods used to prepare a sample to CO₂. Fees are not assessed for these standards and blanks as long as the number submitted matches criteria above and they contain > 8 µmol (100 µg) carbon. If the sample is between 2 and 8 µmol of CO₂ (24 - 100 µg C) it can be analyzed as a small sample and will bear a fee. If a blank contains < 2 µmol of CO₂ and is not analyzed, a partial processing fee will be assessed to cover the cost of quantifying the CO₂.

Methane in Gas or Water

For pure CH₄, 2 ml of gas at 1 atm yields approximately 1 mg of carbon which is what we target for ¹⁴C analysis. Submit in flame-sealed tubes (6 or 9 mm diameter, 15-20 cm in length), or pre-flushed/evacuated septa-sealed serum vials.

Dissolved methane samples should be submitted in crimp-top serum bottles (up to 1 L) sealed with Chemglass CLS-4209-14 rubber stoppers. This type of stopper is superior to most that are currently available for this purpose. Samples should be poisoned with mercuric chloride to prevent biological activity, and if not poisoned then kept cold to minimize biological activity. In general, at least 1% of headspace should be left for expansion of water samples over a range of 30°C. Fees are subject to change if once we receive the samples we determine they require extra processing. Minimum sample requirement for extracted methane is 25 µg carbon. Samples yielding between 25 and 100 µg of carbon incur an additional small sample processing fee.

CO₂ or Methane in a Gas Mixture

NOSAMS has the capability to extract CO₂ and methane from a mixture of gases. The minimum sample requirement is 25 µg C. Please contact the staff chemist (rhansman@whoi.edu) to discuss details.

Graphite

Potential submitters of graphite should first contact us for details on packing, fees and turnaround time before sending graphite samples to NOSAMS for coordination and supplied cathodes.

Generally, as an initial test, we ask for 10 to no more than 15 graphite samples in a batch to determine whether we can successfully analyze your graphite. These should all be primary or secondary standards and blanks of known ages. We routinely analyze graphite prepared using Fe catalyst with a minimum of 2.5 mg Fe. Our suggested target weight is 1 milligram carbon on 2.5-3.5 mg Fe. A client is allowed up to 30 free test samples to establish that we can successfully handle and analyze them, after that, the normal fees apply. Based on the performance of the test batches, we will decide whether we can accept unknowns for a fee. For every five unknowns, one standard (OXI or OXII) is required. So, n/5 + 1 additional standards are requested to bracket all unknowns where n is the number of unknowns. We typically analyze 2 process blanks per process type without fee. A unique graphite number should be assigned to each graphite sample submitted and each sample should arrive labelled accordingly.

When we can successfully analyze loose graphite from a client, we can progress to receiving pressed samples if so desired, again by starting with test batches of 10-15 targets.

Contamination/swipe

Swipe samples are used to check surfaces for above natural levels of ¹⁴C. Swipes and other materials suspected of elevated radiocarbon content are analyzed in a separate preparation lab using a single-step graphitization procedure to prevent cross-contamination. We provide swipe analysis as a service to our clients who submit natural samples to us for radiocarbon analysis. This service is provided in conjunction with the submission of regular samples or for clients who are moving to a new space and want to confirm it is radiocarbon free prior to sending us samples. Please contact Kathryn Elder (kelder@whoi.edu) for setting up analysis of contamination checks and refer to this protocol for collecting swipe samples.

Single Step Reduction for Organic Carbon Samples

Single Step graphitization offers a lower-cost, high-throughput method for converting some types of organic carbon samples into graphite for radiocarbon measurements by accelerator mass spectrometry (AMS). The method works well for high %OC materials like plant or wood and pure compounds, but not for complex mixtures like sediments. Sample combustion and reduction to graphite is combined into a single procedure making it much less time consuming than traditional reduction methods. The analytical precision and accuracy approaches that of traditional two-step combustion and hydrogen reduction methods. δ¹³C by IRMS is not available for samples analyzed using this method.