Mass spectrometry is based on identifying molecules based on their molecular weight and weights of fragments formed from the molecules as measured by using electric and magnetic fields to measure masses of ions generated in the gas phase. It is a powerful analytical technique that can used for identification and quantification of the compounds in a mixture with the help of separation techniques such as gas chromatography (GC)  and liquid chromatography(LC). Mass spectrometry is one of the most sensitive techniques that can be used for identification of compounds. 

Mass spectrometry is such a powerful technique that gas chromatography-mass spectrometry (GC-MS) is often portrayed in media as the ultimate technique powerfully with the power to answer any question about a compound.  While GC-MS is a powerful technique there are some limitations that are not often mention in these media presentations.  Two such limitations are the compounds have to be in a database and be volatile. 

To get around the volatility issue, Liquid chromatography mass spectrometry (LC-MS) has increased in popularity.  One limitation with LC-MS for identification purposes is that the ionization techniques compatible with LC-MS will produce very little fragmentation, which is useful for determining its molecular weight, but does not provide any structural information on the molecules. 

To obtain structural information with the ionization techniques compatible with LC-MS, tandem mass spectrometry is often used. Tandem mass spectrometry is accomplished by selecting out the ion of interest with a mass spectrometer,fragmenting the ion. and measuring the masses of the fragment ions using another mass spectrometer.    Again, one limitation of this method is the limited databases available for the tandem mass spectra at the present time.