by
tlahren » Thu Jul 07, 2011 12:44 pm
Here is a real basic definition of the differences.
http://wiki.answers.com/Q/What_is_the_d ... C_and_HPLC
Basically GC and LC both separate based on partitioning of a compound between a mobile phase (He carrier gas for GC and polar aqueous/organic liquid for HPLC). For HPLC compounds are mostly separated based on their polarity as the stationary phase is usually non-polar (often C18 coated SiO2 particles) and a polar mobile phase. Compounds partition between the two phases. The more polar the compound, the longer it stays on the "Stationary" phase and thus it takes longer for it to elute. For GC compounds are separated mostly by boiling point (although polarity can also be a factor). In GC a temperature gradient is used (increasing temperature versus time) to drive compounds into the gas phase from a liquid phase (if using a solvent trapping method). The hotter the system gets, the more gas-like the compounds become and more like the He carrier gas (thus more like the mobile phase) and are then carried to the detector by the carrier gas.
Because GC and LC separate compounds base on two different characteristics (volatility vs. polarity) it is expected that they will have different retention times (elution patterns). In other words they will come out in a different place in the chromatogram.
hope this helps a little. There are plenty of online help tools to learn more about HPLC and GC from basics to advanced troubleshooting.
http://www.chromacademy.com has some random free tutorials on GC and LC. Otherwise you can pay for a subscription and view all of them at any time.
Subscribe to LCGC magazine (online version or print) for free at
http://www.chromatographyonline.com. This has plenty of info about GC and LC as well as free webinars every once in a while.
