1) Volcanoes are not a major source of CO2. (They are a major source of SO2, but that's a completely different compound. http://www.temis.nl/aviation/so2.php)

2) CO2 is made of carbon and oxygen. They both have isotopes - atoms with the same number of protons (6 for Carbon, 8 for Oxygen) but different numbers of neutrons. These isotopes have different properties and their ratios in a sample of gas can give lots of information - like whether the carbon was recently part of the atmosphere or whether it was locked up in the Earth for millions of years. The ratio changes because one of the carbon isotopes is radioactive, so the ratio of C14 to C12 changes over time.

http://en.wikipedia....sotopes_of_carbon
http://en.wikipedia.org/wiki/Oxygen-16
http://www.physicsfo....php/t-29375.html

All of this is known and accepted science. There is no controversy about the physics.

Where it gets complicated is in the details. How are the samples taken? How are they protected from contamination? How are they analyzed? What are the error bars (how accurately are the numbers known, and what is the real precision of the value)?

Cheers,
Scott.