DogMom2JoeAndWillie, you sound like an expert - can you explain azeotropes, and do they have anything to do with the finding of residual chloroform, absent acetones or esthers?
Thanks for the compliment, but I am far from being an expert; I only have a BS in Chemistry. I've just been around analytical chemistry labs a good bit because my Mom owns/operates one and has for as long as I can remember. I focused on the use of GC-MS for the identification and quantification of mescaline in ancient peyote samples for my undergraduate research project, but I really only know the basics.
The best explanation that I can give you is that distillation is carried out on a mixture that is in the liquid phase. You separate the components of a mixture by boiling point when you employ simple distillation. Boiling point is a physical property, so separating a mixture by boiling point only requires heat; no chemical reactions take place in the separation process. You begin with a mixture of two chemicals and are (in theory) able to separate the mixture into two parts that consist of one pure chemical that remains unchanged (there are no chemical reactions). A classic example is the distillation of a mixture of benzene and toluene because it best illustrates the "ideal" conditions required for the separation of a two component mixture by boiling point (if you want more info, read up on Raoult's law and Dalton's law... I don't wanna put everyone to sleep... if i haven't already!). In reality, it is nearly impossible to COMPLETELY separate a mixture into separate, purified components without taking advantage of the chemical reactivity of the different chemical components in the mixture. For example, a mixture of ethanol and water can't be separated by simple distillation; it is necessary to create a change in pressure OR add another component that will create a CHEMICAL (not physical) change within the mixture so that the two components may be separated.
Regardless of whether or not the mixture is azeotropic, the separation of a liquid system into its pure chemical components can be done without destroying any of the sample. If you have a two component mixture of volume equal to 10mL, you can separate it into component A of volume equal to 2mL and component B of volume equal to 8mL (volume A+volume B=10mL, so there is no significant "loss"). Evaporation is used WITH re-condensation when simple distillation is performed, so both the sample and the end products are in the liquid phase.
The main point is that the azeotrope is a mixture of TWO SEPARATE CHEMICALS and the different chemicals still have DIFFERENT CHEMICAL PROPERTIES from one another. Don't think of it as A+B=C; think of it as A+B=A+B. Each chemical species maintains it's OWN IDENTITY even though the boiling point of the mixture makes it APPEAR as if an entirely different chemical has been formed.
When you perform analysis with GC-MS, you inject a VERY small volume of your sample into the injection port. The sample is COMPLETELY vaporized upon injection (so it's in the gas phase) and the mobile phase (also a gas) moves the sample through the column where it CHEMICALLY interacts with the stationary phase (the coating on the walls inside the column). The evaporation of the liquid sample is a physical change and the less volatile (lower molecular weight) compounds will move through the column more quickly, BUT the CHEMICAL reactions between these compounds and the stationary phase are also involved in the separation process. Not only are you using separation by boiling point with GC-MS, but you are also using separation based on POLARITY (a chemical property) and based on retention time (also related to chemical property).
Another difference is that since the volume injected is so small with GC-MS, it is nearly impossible to separately collect each individual eluent when it comes off the column. GC-MS detects chemical species in the ppm and ppb ranges... you can imagine how difficult it would be to collect 10ppb methanol in a vial when it is eluted from the column.
I really hope this has helped out a little bit without getting too confusing. I am not very good at explaining things, but am happy to try. Let me know if there is anything that doesn't make sense or if I need to clarify.
