What is the theoretical aspect of using subtraction of mass spectrum in Agilent Chemstation or Mass Hunter in regard of obtaining the complex matrices (Blood and urine)? What are the criteria of using it? What does it mean by minus (-) sign in Mass spectrum sheet? is it necessary to do subtraction for complex matrices for the sake of getting cleaning mass spectrum as acquired by the library reference and to get higher quality of that particular drug,metabollite and poison? Please explain in detail.
Dear Shayheful Bakar,
Your questions are very important for the proper use of a Mass spectrometer and normally are teach in the basic courses of practical MS.
Briefly when you take a peak never collect only one scan, it is more representative collect the average of the apex of the peak and the background of the base previous or after the peak. Unless you are doing de-convolution analysis. In the case of Phenytoin you use scan 637 minus scan 639. (-639)
What you saw was that a lot of small MS signal were eliminated from the mass spectra. Because contribution of noise or matrix were eliminated.
But take care that if your compound has a low abundase ion, it will be also eliminated if you did a wrong subtraction!!
This concepts are very important when you analyze biological examples where there are a lot of substances that can co elute in one peak.
Best regards
Eleuterio
