Most samples require chemical pre-treatment to ensure their purity or to recover particular components of the material.
The objective of pre-treatment is to ensure that the carbon being analyzed is native to the sample submitted for dating.
These so-called "solid-carbon" dates were soon found to yield ages somewhat younger than expected, and there were many other technical problems associated with sample preparation and the operation of the counters.
Gas proportional counters soon replaced the solid-carbon method in all laboratories, with the samples being converted to gases such as carbon dioxide, carbon disulfide, methane, or acetylene.
Many laboratories now use liquid scintillation counters with the samples being converted to benzene.
All of these counter types measure the C-14 content by monitering the rate of decay per unit time.
Pre-treatment seeks to remove from the sample any contaminating carbon that could yield an inaccurate date.
Acids may be used to eliminate contaminating carbonates.
The first measurements of radiocarbon were made in screen-walled Geiger counters with the sample prepared for measurement in a solid form.
A more recent innovation is the direct counting of c14 atoms by accelerator mass spectrometers (AMS).
The sample is converted to graphite and mounted in an ion source from which it is sputtered and accelerated through a magnetic field.
Libby calculated the half-life of c14 as 5568 ± 30 years.
This means that half of the c14 has decayed by the time an organism has been dead for 5568 years, and half of the remainder has decayed by 11,136 years after death, etc.