Organic materials, which require the most processing, are limited to younger ages by their corresponding process blank.

Due to counting and measurement errors for the blanks and samples, statistical errors are higher for very old samples.

Thus, ages are limited by the age of the process blanks (more on that below) and by the statistical uncertainty of the C measurement.

For small samples, blank contribution as a fraction of sample mass becomes a more important term, so a mass balance blank correction is applied.

This correction is performed as follows: $$Fm_{mbc} = Fm_{corr} + ( Fm_{corr} - Fm_b)\frac{M_b}{(M - M_b)}$$ Where $$M$$ is sample mass, and $$M_b$$ and $$Fm_b$$ are the mass and Fm of the blank.

Fraction Modern is a measurement of the deviation of the C is also affected by natural isotopic fractionation.

In AMS, the filiamentous carbon or "graphite" derived from a sample is compressed into a small cavity in an aluminum "target" which acts as a cathode in the ion source.

The surface of the graphite is sputtered with heated, ionized cesium and the ions produced are extracted and accelerated in the AMS system.

After acceleration and removal of electrons, the emerging positive ions are magnetically separated by mass and the C counts per second are collected.

It is expected then, for a 5,570 year (1 half-life) or 11,140 year old (2 half-lives) sample that 125 or 63 counts per second would be obtained.

Although one can simply measure older samples for longer times, there are practical limits to the minimum sample activity that can be measured.

At the present time, for a 1 milligram sample of graphite, this limiting age is about ten half-lives, or 60,000 years, if set only by the sample size.

However, limiting ages or "backgrounds" are also determined by process blanks which correspond to the method used to extract the carbon from the sample.

» NOSAMS General Statement of C from contamination introduced during chemical preparation, collection or handling.

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