Unexpected detector counts/ bad blank

If blank targets from previous measurements are available, remeasure these blanks to rule out issues with sample preparation (e.g., contamination, swapped sample labels, etc.):
- A question that may generally be useful to ask is: What changed between the time when the blanks were still fine and when they were no longer acceptable?
- If you are convinced that the issue lies with the sample preparation and not with the AMS:

Problem Solving

To distinguish the source of the contamination, it is helpful to examine how the ¹⁴C/¹²C ratios evolve over the course of the measurement.

If the ¹⁴C/¹²C ratios are initially high and then decrease (“clean up”), this typically indicates surface contamination, which can result from pressing or touching the sample. This is usually easy to remedy by ensuring that all tools are clean and by avoiding contact with the cathode surfaces.

If all blanks are consistently elevated, this may point either to a problem within the AMS (e.g., source cleanliness, stripper, detector, etc.) or to a constant source of contamination during sample preparation - for example, from the EA gases or from uniformly contaminated sample materials, cups, or boats. In this case, the following checks are recommended:
1. How do the EA runins (empty combustions) look? Does the baseline become truly flat when zooming in on the carbon peak area? If not, clean the ash finger and, if necessary, replace the combustion and reduction tubes, including their fillings.
2. Has the gas cylinder recently been replaced? Contaminated gases can cause problems even if the stated purity is correct. This is often indicated by larger carbon peaks in runins after longer intervals between them. If possible, test a different gas cylinder.
3. Check the pump of the AGE system and ensure that the resulting pressure is at most 1 mbar.
If only certain blanks appear elevated and show consistently high ¹⁴C/¹²C ratios throughout the measurement, this suggests contamination of specific samples with modern or even overmodern carbon. In this case, ensure that sample preparation is performed with great care and in a consistent, reproducible manner.

Check the helium feed pressure and compare the “LE accelerator pressure” with previous values. If the pressure is too low, this may indicate insufficient stripper gas density and incomplete molecular dissociation. Molecules with an atomic mass of 14 amu that survive the stripping process can enter the detector and increase count rates well into the kHz range.

Problem Solving

If this appears to be the problem:
- Ensure the correct helium feed pressure (replace the empty He cylinder if necessary).
- Perform a stripper scan and set the appropriate stripper control value.

If blank values deteriorate over time and the ion source has not been cleaned recently:

Problem Solving

Perform ion source maintenance (see the user manual or instructional videos for guidance).

Problem Solving

If the pressure has changed significantly, follow the procedure for evacuating and refilling the detector with detector gas (typically isobutane). Ensure that the lines are thoroughly flushed to prevent contamination with air.

Ions entering the detector volume generate a voltage pulse. If this pulse exceeds a certain threshold (the “discriminator threshold”), the system registers the event as a ¹⁴C particle.

Problem Solving

Ensure that the discriminator threshold is set correctly:
1. Measure the ¹⁴C/¹²C ratio using a blank at different discriminator settings, ranging from 0.5 to 4.0.
2. Repeat the same procedure with a ¹⁴C standard (e.g. OX2)
3. Select a discriminator setting that results in a low blank value (threshold high enough) while still allowing the complete detection of all real ¹⁴C ions (threshold not too high)

Close the detector valve and monitor the count rate. If counts are still detected while the valve is closed, check for:

Problem Solving

Compare the ion source vacuum pressure with previous values. If the pressure is significantly higher than in previous measurements (e.g. > 1•10^-6 mbar):

Problem Solving