Detection and The Principle of Scan Analysis

Outline

1. Typical scan analysis
2. Scan analysis principle
3. The difference between mass spectrum and mass chromatogram
4. Ions detection
5. The use of mass spectra

Typical scan analysis

1. Detector measure ions over the mass range m/z 50 - m/z 600 every 3 seconds ( i.e over 500 ions in 3 seconds)
2. Each peak elutes from the GC yields a distribution of fragment ion masses. Thus, a time-slice of the ions is detected every 3 sec.

Scan analysis principle

            Using dimensional diagram, which x,y and z axis represent = the time/scan number (total number of scans per 3 sec; related to GC retention time), the mass to charge ratio of the ions (m/z), and the detector response.

The difference between mass spectrum & mass chromatogram

—  Mass spectrum = plotting m/z VS response at constant scan number (related to time)

—  Mass chromatogram = plotting scan number VS response at a constant m/z

            The difference between them is critical for understanding the section that follow.

Ions detection

—  Detection of the ions separated by the mass analyzer is achieved using a multistage electron multiplier giving output either recorded directly onto an analog recorder OR digitize and acquired by a data system.

—  How does it work?

            Relying on electron multiplier (discrete dynode) detection, where the electron beam is amplified by a cascade of collision w/ special metal surface.

The use of mass spectra

—  Mass spectra stands for each compound eluting from the GC yields a particular distribution of fragment ion masses.

—  Why does it useful?

            Because, they usually show the molecular mass (some of the ionize, but don’t fragment further) and characteristic fragmentation pattern (that can be used to infer structure).

The use of mass spectra

—  Its ideal condition (?)

            Each GC represents a separate compound resulting in a unique mass spectrum that can be used for identification.

—  In reality (?)

            most peaks are unresolved mixtures of two or more compounds, thus complicating interpretation.

—  To show a series of peaks that represent the relative amount of eluting compounds by plotting the magnitude of the total ion current for all mass spectra in each sample VS retention time on a RIC (reconstructed ion chromatogram) or TIC.

—  RIC and GC traces of petroleum are essentially identical, except an RIC requires MS detection while GC uses the more conventionalflame ionization detector (FID)