Investigating the use of an octupole ion guide for ion storage and high-pass mass filtering to improve the quantitative performance of electrospray ion trap mass spectrometry.

An octupole radio frequency (rf) ion guide was evaluated for storage and filtering of ions generated by electrospray prior to introduction into an ion trap mass spectrometer. The control of the rf potential on the ion guide enabled its operation as a high-pass mass filter, removing low-mass chemical noise that would normally fill the ion trap and result in reduced sensitivity, mass resolution, and dynamic range. Also, the ion guide can serve to store the high-pass filtered ions during the ion trap mass analysis, enabling nearly 100% duty cycle and reduction of the cycle time by a factor of two. The linearity, precision, and detection limits of the liquid chromatography (LC) electrospray - ion guide-ion trap MS/MS system were evaluated for the determination of ceftiofur in milk. A linear calibration (linear correlation coefficient of 0.986) from 2-200 ppb was obtained with a relative standard deviation for replicate analysis of less than 8%. A quantitation detection limit of 100 pg of ceftiofur on-column (2 ppb) was achieved from a milk extract. This detection limit is nearly a factor of 10 lower compared with the determination on the same electrospray ion trap system not using a rf ion guide for high-pass mass filtering and ion storage. Copyright 1999 John Wiley & Sons, Ltd.