Gas Chromatography - Tandem Techniques - Gas Chromatography Mass Spectroscopy (GC-MS) Systems > Ion Generation > Page 64

 

Figure 51. The Ryhage Concentrator

 

Each subsequent jet has a smaller aperture than the previous one. Helium from the column (used as the carrier gas) passed through the center channel and diffused away from the main stream between each jet and was removed by appropriate pumps (normally one pump for each jet). The solute vapor, having greater momentum, was carried into the next jet and finally, after the last jet into the mass spectrometer. The concentration was normally increased by about a factor of ten and depended somewhat on the jet arrangement. The sample recovery was usually significantly in excess of 25%. Even today, the Ryhage concentrator is still used in GC/MS systems when packed columns are employed.

The Bieman Concentrator

 

The Bieman concentrator worked on an entirely different principle and was somewhat simpler but equally effective. A diagram of the Bieman concentrator is shown in figure 52. It consisted of a heated glass jacket surrounding a sintered glass tube. Helium was again used as the carrier gas which, after leaving the column, passed directly through the sintered glass tube and the helium diffuses radially through the porous walls and was pumped away. The helium stream enriched with solute vapor then passed on to the mass spectrometer.

Figure 52. The Bieman Concentrator

 

The concentration factor and sample recovery was similar to the Ryhage device. The apparatus, however, was more bulky but easier to operate. The sintered tube could be replaced by a porous polytetrafluorethylene (PTFE) and would function in a similar manner. The introduction of the open tubular columns with their much smaller flow rates eliminated the need for concentrators. In addition, the mass spectrometer pumping system could easily dispense with such gas flows and peak concentrations were high enough for direct injection into the mass spectrometer.

 

Ion Generation

 

Before a molecule can be deflected electrostatically or magnetically, it must be ionized and/or broken up into charged fragments. There are two common methods of ion production used in GC/MS systems, namely, electron impact ionization (EI) and chemical ionization (CI). More recently, a third method has become popular and that is inductively coupled plasma ionization (ICPI). This ionizing method was largely employed for liquid or solid samples but has now been applied to GC/MS systems for certain types of inorganic analysis.