The classic method for determination of molecular formula of a substance involves three major steps. Qualitative analysis, quantitative analysis and molecular mass determination. Molecular mass or molecular weight is determined by some methods like vapor density method, cryoscopic method or by vapor pressure osmometry. But, in modern laboratory, molecular mass is determined through use of mass spectrometry.
Mass spectrometer is an instrument, which is used to determine mass of atoms/molecules, relative abundance of isotopes or even for structures of more complex molecules. It performs in some stages.
First stage: Ionization :
substance to be analyzed should be taken in gaseous state. Gaseous particles released into ionization chamber. Particles are bombarded with high energy electrons and ionized.
Second stage: Acceleration:
Ionized particles are accelerated to uniform speed by electric field.
Third stage: Deflection:
Accelerated ions are deflected by magnetic field, depending on mass to charge ratio. More heavy particles,less deflection and vice versa. In most cases, charge is (+1), so deflection mainly depends on mass only.
Fourth stage: Detection:
Deflected particles are detected using detector. More particles landing at a single point of detector, peak will be larger. Magnitude of peak gives relative abundance of isotopes.
Finally, a plot of relative ion abundance versus m/z ratio is obtained. This is called Mass spectrum.