What Is AMS Dating and How Does It Differ from Standard Radiocarbon Dating?

Updated

AMS dating (accelerator mass spectrometry dating) is a radiocarbon method that counts individual carbon-14 atoms rather than waiting for them to decay. The practical result: it needs milligrams of sample instead of grams, which makes it the default choice for small, rare, or valuable materials. It suits bone, teeth, shell, plant, seed, wood, and charcoal — but only when the sample still contains enough original carbon and hasn't been contaminated. SGS Beta, the lab behind radiocarbon.com, reports AMS results in 3–14 business days.

How AMS dating measures carbon-14

Both AMS and conventional radiocarbon dating rely on the same principle: living things absorb carbon-14 while alive, and that carbon-14 decays at a known rate after death. The difference is how the remaining carbon-14 is measured.

  • Conventional radiocarbon dating detects the radiation released as carbon-14 atoms decay. Because decays are infrequent, the method needs a relatively large sample and long counting times to get a statistically usable signal.
  • AMS dating uses an accelerator mass spectrometer to separate and count carbon-14 atoms directly. It measures atoms, not decay events, so far fewer atoms are needed for a result.

That single change — counting atoms instead of waiting for decays — is what drives nearly every practical advantage of AMS.

Why AMS needs far smaller samples

Because AMS counts atoms directly, the sample size drops from grams to milligrams. This matters in three common situations:

  1. The sample is small or precious. A single seed, a fragment of bone, a sliver of wood, or a few shell fragments may be all that survives. Conventional dating could consume or require more material than exists.
  2. The sample is rare or museum-held. When you cannot sacrifice a large piece, AMS lets you date a small subsample.
  3. You need to isolate a specific fraction. Small samples make it feasible to target a particular component (for example, a specific organic fraction) rather than dating bulk material.

The trade-off is that smaller samples are more sensitive to contamination. A tiny amount of modern carbon — from handling, storage, or the burial environment — can shift an AMS result more than it would a large conventional sample. Clean collection and honest disclosure of how a sample was stored are part of getting a reliable date.

Sample types commonly dated by AMS

SGS Beta lists technical support for selecting the most suitable samples and names several categories:

Sample type Notes for AMS suitability
Bones & teeth Often dated via a specific extracted fraction; small fragments can be viable
Shell & other carbonates Carbonate material; needs care because of environmental exchange
Plants & seeds Single seeds or small plant fragments are typical AMS targets
Wood Small wood fragments or individual growth features can be dated
Charcoal Frequently submitted; small pieces are well suited to AMS

The lab's guidance is to analyze the most suitable samples with expert technical support — meaning the right pretreatment and fraction choice depends on the material, not just its size.

Turnaround time and precision expectations

SGS Beta states AMS dating results are delivered in 3–14 business days. Precision depends on the sample, its carbon content, and how clean it is; AMS itself does not guarantee a tighter date than conventional dating — it mainly reduces the sample size required. Plan around the lab's closure schedule if your deadline is tight: the posted 2026 closures include November 19, November 26–27 (Thanksgiving), and December 24, 2026 through January 1, 2027 (all dates inclusive) for Christmas and New Year.

Key limits and pitfalls

  • Contamination. Small AMS samples are more vulnerable to introduced modern or old carbon. How a sample was collected, handled, and stored affects the result.
  • Calibration. A measured radiocarbon age is not a calendar date. It must be calibrated, and the calibration you use affects the final range.
  • Material-dependent pretreatment. Bone, shell, plant, wood, and charcoal each need different treatment; the wrong fraction can produce a misleading date.
  • Not everything can be dated. Material without sufficient original carbon, or that has exchanged carbon with the environment, may not yield a meaningful result.

Choosing between AMS and conventional dating

Choose AMS when the sample is small, rare, valuable, or when you need to isolate a specific fraction. Choose conventional radiocarbon dating when you have ample material and the larger sample size is not a constraint — it can be the more economical route for bulk material. If you are unsure, the deciding questions are: how much material can you spare, and how confident are you that it is uncontaminated? Those two answers usually settle the method.