Overall Efficiency of an Electron Ionization Ion Source

The term sensitivity specifies the overall response of any analytical system for a certain analyte when run under well-defined conditions of operation. 

Deflnition The sensitivity is the slope of a plot of analyte amount versus signal strength. In mass spectrometry, sensitivity is reported as ionic charge of a specified m/z reaching the detector per mass of analyte used. The sensitivity is given in units of C pg for solids. [27] For gaseous analytes, it can be specified as the ratio of ion current to analyte partial pressure in units of A Pa . [28,29] 

According to the above definition, sensitivity does not only depend on the ionization efficiency of El or any other ionization method. Also relevant are the extraction of ions from the ion source, the mass range acquired during the experiment, and the transmission of the mass analyzer. Therefore, the complete experimental conditions have to be stated with sensitivity data. 

Example Modem magnetic sector instruments are specified to have a sensitivity of about 4 X 10 C pg for the molecular ion of methylstearate, m/z 298, at R = 1000 in 70 eV El mode. The charge of 4 x 10 C corresponds to 2.5 x lO electron charges. One microgram of methylstearate is equivalent to 3.4 x 10 mol or 2.0 X lO molecules. For a molecule entering the ionization volume one there- 

The term detection limit is almost self-explanatory, yet it is often confused with sensitivity. 

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