Here is a possible alternative for HPLC or GC analysis for your aspartame to methanol/benzaldehyde project.
Since you are a senior, I’m sure you have had a chemistry class. Here is another chemistry lesson.
The Wikipedia article on aspartame includes its molecular structure. Please note that aspartame has a benzene ring (C6H6), which means that it will absorb UV light at 280 nm. You could quantitate the molecule by using a spectrophotometer or microplate reader that can read samples at 280 nm. You would set up a standard curve with increasing concentrations of aspartame and then measure the UV absorbance of your standards and samples, and interpolate the aspartame concentration of the samples from the standard curve. http://en.wikipedia.org/wiki/Aspartame
Here is a website that lists methods for measuring methanol in biological samples. Methanol can be quantitated by oxidizing it to formaldehyde, and then reacting it with chromotropic acid and sulfuric acid and measuring the optical density at 570 nm. A standard curve would be run to give quantitative results.http://www.inchem.org/documents/pims/ch ... 35.htm#8.2
I would try the above method with an unoxidized sample to quantitate the benzaldehyde, but if for some reason it did not work, here is a website that describes a method for measuring benzaldehyde using a titration of the hydrochloric acid, which is released from a solution of hydroxylammonium chloride. This analysis would require using a pH indicator such as phenolphthalein or a pH meter. http://pubs.acs.org/doi/abs/10.1021/i560102a025
It might be a lot easier to find a spectrophotometer or microplate reader and the various chemicals needed for the analysis compared to an HPLC or GC system. If you think you want to pursue these options, try looking for references that give a little more detail about the analytical method. The websites I found give general descriptions only.
Also, I recommend using a solution of pure aspartame in water for one of your samples. You also need to check the labels for the Tab, Coke, and Sprite Zero to make sure they don’t contain any ingredients that would interfere with the spectrophotometric analysis of your analytes.
I hope this makes sense to you. Please let me know if you have any questions.