Resolving Power of Linear Quadrupoles

Quadrupole analyzers generally are operated at so-called unit resolution normally restricting their use to typical low resolution (LR) applications. [111,112] At unit resolution adjacent peaks are just separated from each other over the entire m/z range, i.e., R = 20 at m/z 20, R = 200 at m/z 200, and R = 2000 at m/z 2000 (Fig. 4.37). 

The resolution as adjusted by the U/V ratio cannot arbitrarily be increased, but is ultimately limited by the mechanical accuracy with which the rods are constructed and supported ( 10pm). [Ill] Above an m/z value characteristic of each quadrupole assembly, any further improvement of resolution can only be achieved at the cost of significantly reduced transmission. High-performance quadrupoles allowing for about 10-fold unit resolution have only recently been developed. [112] 

Theoretically, each electrode should have a hyperbolic cross section for optimized geometry of the resulting quadrupole field, and thus for optimized performance. [103,104] However, cyclindrical rods are often employed instead, for ease of manufacture. By adjusting the radius of the rods carefully (r = 1.1468ro), a hyperbolic field may be approximated. [113] However, even slight distortions of the ideal quadrupole field either from interference with external fields or due to low mechanical precision or inadequate shape of the device cause severe losses of transmission and resolution. [114] The expected advantages of hyperbolic rods [115] have been demonstrated by ion trajectory calculations [110,116] circular rods cause a reduction in macromotion frequency because of an increased residence time of the ions in close vicinity to the rods this in turn means reduced resolution. 

2 High-Resolution with Quadrupole Mass Analyzers 

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