7.1.1.b  Gas Flow:  As noted in the GC chapter, all gases in GC-MS, and in most GC applications, must be 5-nine quality (99.999 percent pure).  These typically include a He or H2 as a carrier gas, Ar/CH4 for makeup gas for electron capture detectors, and H2 and compressed air (lower grade) for flame ionization detectors in GC.  For GC-MS, only He is required for the carrier gas, with CH4 being commonly used in chemical ionization mode.  Even at this purity, the gas must be purified further by passing it through a resin trap that has a high affinity for specific contaminants, including water, atmospheric oxygen in some cases, and hydrocarbons.  Although the presence of these contaminants would result in a high detector background, the main reason such high purity gases are needed is due to the use of temperature programming.  Two contrasting examples will demonstrate the need for high purity gas purifiers.  For the first case, imagine running the GC-MS with a high-temperature isothermal oven setting.  At this temperature all contaminants in the gases will pass freely through the system unretained in the separation column and a high, but steady, background detector signal would result.  For the second case, imagine a temperature-programmed analysis where initially the column oven is at a temperature lower than the boiling point of any contaminants in the carrier gas.  As carrier gas passes through the analytical column, contaminants would be adsorbed to the stationary phase at the beginning of the column.  As the temperature program progresses, these contaminants would volatilize and appear as peaks in the chromatogram.  The height of the contaminant peaks (concentration) would be inconsistent since it would depend on the time and the amount of carrier gas passing through the column between runs.  The contaminants would result in additional problems if they co-eluted with an analyte of interest. 

The gas pressure in the supply tank is usually between 2000 and 2500 psi (up to 17000 kPa).  Instruments require that this pressure be reduced with step-down or secondary regulators that drop the pressure to 100 psi (700 kPa) or less, depending on the instrument and gas.  Integrated regulators or mass-flow controllers further reduce the pressure to 5 to 20 psi at the head of the capillary column, resulting in a flow of 1 to 5 mL/minute depending on the internal diameter of the capillary column. 

 

 

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