Multiple ion counting application note added to the downloads page (T10412)

TIMS analysis of very small (picogram) sample loads results in ion signals that are small and short lived. Whilst the ion-counting Daly can be used in peak jumping mode for such samples, the constantly varying signal intensity really calls for a multi-collector approach and in these circumstances multiple ion-counting becomes the method of choice.

The largest contribution to reducing the accuracy and precision of MIC is poor cross calibration of the ion-counters. The best way to achieve a calibration is to use a multi-dynamic approach allowing efficiencies to be cancelled but the transient nature of the signals precludes using the MIC in this way – after all, we’ve moved to MIC primarily because we can no longer peak jump!

The next best option is to calibrate the ion-counters prior to the acquisition using a stable ion beam and simply apply the calibration to subsequently acquired data. This requires the MIC’s to be extremely stable with respect to each other for at least for the duration of the subsequent acquisition but preferably over a considerably longer period.

The stability of the multiple ion-counting on a Phoenix TIMS instrument was tested by performing 10 individual acquisitions of U500 over the course of one hour collecting the data on four adjacent ion-counters. The external precision for 235U/238U over the 10 acquisitions was 0.05% (%1RSD n=10, ion beams ~100,000 cps) highlighting the high stability of the Phoenix MIC’s over a one-hour period. To further challenge the instrument these tests were repeated another six times to give a complete analysis period of seven hours. The results over seven hours were comparable to the results acquired over the first hour indicating that the MIC on Phoenix TIMS is extremely stable over an extended period.

The multiple ion-counting on Phoenix represents the current state of the art for stability, robustness and versatility. The performance demonstrated here supports our contention that this design is the best currently available as demonstrated by the fact that the most demanding MIC applications in the world are carried out using Isotopx instruments.

A PDF of the application note is available on our downloads page.


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