From the viewpoint of a biochemist, there are a number of powerful methods that one can use to decipher the contents of this potion using standard analytical chemistry techniques.
This is an extremely powerful technique, and modern mass spectrometers can be used in ways that will make it extremely difficult for non-scientists to confound them. For example, Green's answer suggests that one spike it with many different chemicals. A modern analytical chemist would easily defeat this by passing the mixture through a tandem GC-MS (gas chromatography mass spectrometer) which will separate out the spiked contaminants.
The gas chromatography machine will be able to separate out the various spiked-in chemicals that are interfering with the setup by the time they take to pass through the machine. By using different column coils (which bind chemicals with different affinities), chemically different compounds can be easily separated from each other. Once the unknown (new) secret ingredient is found, it can then be studied further.
Chromatography can also be used to filter out other irrelevant ingredients by fractionating the love potion into various subsets. Tools such as the Äkta series of chromatography machines can be used to further narrow down the possible chemicals by testing the fractions which retain love potion activity, discarding those that do not have significant activity.
There are a few standard assumptions that biochemists make, and violating these assumptions during the process of manufacture can completely mess up their compound libraries.
Using isotopically unusual mixtures.
The natural abundance of carbon-13 is around 1.1%, but if you use an isotopically pure carbon-12 or 2.2% carbon-13 mixtures, your adversaries would not be able to get matches on their compound libraries, making it far more difficult for them to isolate your compound of interest.
Conjugating your active compound to a mixture of proteins
While mass spectrometry can measure small molecules easily, it still has difficulties in proteomics, due to the large and varied structure of proteins. By conjugating your chemical of interest to different proteins, its MS signal will be mixed into the protein, making it far harder for ad adversary to determine the chemical you are using. It also makes it very difficult for chromatography, since the different proteins will fractionate into every fraction, preventing them from easily finding out your secret ingredient.
NMR (nuclear magnetic resonance) allows one to determine the exact structure of a chemical once it has been purified. As a result, NMR alone is unlikely to reveal the nature of the chemical, but once the chemical is purified, it will be used to find the structure, and then allow organic chemists to synthesise your compound.
Using isotopically pure, NMR-insensitive nuclei (such as carbon-12 and oxygen-18) will make it impossible to detect the absorbances of these nuclei. While proton NMR is still possible, the impossibility of getting NMR spectra from such chemicals will make it extremely difficult for the analytical chemists to determine the compound.
Overall, given enough time and resources, the secret will be broken by a sufficiently determined adversary. However, these techniques will force them to undertake a long period of R&D, and greatly extend the time period during which you can make money.