AMS SERVICES UNIT
Accium's AMS Services Unit provides high throughput, high capacity measurement of 14C/12C ratios in biological samples. The AMS Unit accepts samples from Accium's Bioanalytical Unit or directly from customers. Samples that are suitable for direct submission include plasma, whole blood, urine, HPLC fractions, processed feces, homogenized tissue and any other sample that requires no other processing other than graphitization.
A small volume of each sample (<20 uL of plasma, whole blood, urine or milligram-sized tissue homogenate) is dried, combusted and converted to graphite in Accium's Graphitization Laboratory. The ratio of 14C/12C in each graphite sample is then measured by Accium's AMS instrument. The 14C/12C ratio is multiplied by the total carbon concentration (either a reference value or a measured value) to produce the total 14C concentration in the sample.
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The AMS Services Unit provides the
following services:
- Sample receiving
- Screening by liquid scintillation
- AMS batch creation
- Sample drying
- Combustion and graphitization
- AMS measurement
- Total carbon measurement
- Data review and approval
- Standard reporting of results
The results are thoroughly reviewed by the AMS Unit Study Director. GLP service includes additional independent review of all documents and calculations by Accium's Quality Services Unit. AMS results are submitted to the originator of the samples (either our customer or Accium's Bioanalytical Services Unit) in a standard reporting format.
The turnaround time from sample receipt to report submission is typically 7 to 14 days. To ensure speedy service, we encourage customers to contact Accium in advance if they intend to submit greater than 250 samples in a single service request.
Customers who do not wish to perform any front-end sample processing may request Accium's Bioanalytical Services Unit to provide expert sample processing services. Customers may also seek guidance from Accium prior to setting up their own front-end bioanalytical capability.
AMS Unit Workflow
1. Combustion and graphitization
Biological samples such as plasma, urine, fecal and tissue homogenates are first aliquotted into individual quartz tubes and dried in a vacuum centrifuge. The dried sample is flame-sealed inside a quart glass tube containing a small amount of copper oxide. Sealed tubes are placed in a combustion oven where the organic material is converted to CO2. The CO2 gas is then reduced to fullerene (known as graphite) over an iron catalyst to obtain solid graphite. Samples that contain an insufficient amount of total carbon (usually less than 0.5 mg carbon) received carrier carbon at the beginning of the process to facilitate formation of graphite.
2. AMS Measurement
Graphite samples are placed onto an AMS instrument where a beam of C- ions is produced by bombarding the cool cesiated surface of the graphite sample with about 5 keV Cs+ ions. The C- beam produced by the sputtering of the sample by the Cs+ beam is accelerated, focused and mass analyzed into mass 14, and 13 amu beams. By recording the 13C current and 14C counts as known and unknown samples are sputtered, the amount of 14C present in a biological sample is determined to high accuracy. Importantly, nitrogen (also 14 amu) does not make a negative ion and is effectively removed as a confounding atomic isobar.
3. Reporting
- Standard reports are submitted to the Bioanalytical Unit Study or directly to the client.
- Results are reported in DPM/g carbon
- If the total carbon content is known, the results are converted to DPM/mL sample
- If the specific activity of the 14C radiotracer is known, the results are also converted to ng-equivalent/mL
- If the sample was an HPLC fraction of the unmetabolized 14C radiotracer, the results are reported in ng tracer/mL
Validation of Sample Processing and AMS Measurement
The performance specifications of the Accium AMS instrument and Graphite Preparation Laboratory were assessed in a series of validation studies carried out in accordance with the "Good Laboratory Practice for Nonclinical Laboratory Studies" (GLP) quality system (U.S. Food and Drug Administration, 21 CFR58).
Precision and accuracy were first determined for the AMS instrument by measuring IAEA-C6 and IAEA-C7 certified reference standard (Le Clercq,1998), as well as HOxI, and HOxII standards (Stuiver, 1993) prepared at the Australian Nuclear Science and Technology Organization (ANSTO) (Fink, 2004). For each standard, the graphite was pooled and thoroughly mixed before being loaded into different cathode holders. Three IAEA-C7 cathodes were each measured 3 times for 120 seconds and normalized against HOxI and HOxII standards and their results combined into a single mean value. The whole measurement sequence was repeated on 3 consecutive days. Please follow this link for a full report or the peer-reviewed published manuscript.
Parameters evaluated
- Accuracy
- Linearity
- Reproducibility
- Sensitivity
- Dynamic range
Sample Preparation QualitySamples were readily measured with 0.3% precision and accuracy, machine background levels were consistently in the low 10^–16 (14C/12C), and chemical background was approximately equivalent to a fraction of modern of 0.004. In addition, when 100-times-modern samples were processed, no increase in background was observed, either during sample processing or during AMS measurement. This corresponds to a dynamic range for 14C analysis of 6 orders of magnitude.
