The amount of Sample needed for mass spectrometry is a common question, but is not easily answered.   I prefer being able to see the compound, but the amount of sample needed depends on the nature of the sample and the cleanliness of the sample.  A clean sample of easily ionized compounds such as peptides can be done in the low femtomole levels. However, a dirty sample can reduce the sensitivity significantly, especially, if the sample is contaminated by salts.