Only MS gives direct molecular structure information of the analyte of interest among the analytical tools, making it a vital analytical method in the detection and quantification of a wide range of compounds in all fields of science and technology. The ion source, mass analyzer, and detector are the three major components of MS. Although all three components contribute to an MS's performance, coupling and/or decoupling the new ion sources with the MS appears to be a simple operation. As a result, substantial effort has gone into developing new ion sources; yet, a single ion source may not be suitable for all kinds of compounds. The ESI, for example, is very successful for ionization of polar molecules, whereas glow discharge (GD) techniques such as APCI, DBDI, APPI etc are suited for non-polar compounds.