The collagen fraction usually yields more reliable dates than the apatite fraction (see Dates on bones).
In addition to various pre-treatments, the sample must be burned and converted to a form suitable for the counter.
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.
Bases may be used to remove contaminating humic acids.
Some types of samples require more extensive pre-treatment than others, and these methods have evolved over the first 50 years of radiocarbon dating.
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.
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.
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.