Selecting Product Ions and CID Conditions

The ideal product ion for use in MRM is one that is both structurally characteristic of the analyte (good selectivity) and can be observed at good relative abundance in 

For both chemical and physical reasons, product ions of low mass can be problematic for MRM detection. The practical interpretation of low mass in this context varies with circumstances. Thus, the chemical disadvantage is mainly concerned with the observation (Aebi 1996) that chemical noise (background) from API sources is considerably more intense at lower m/z values. While this observation directly affects SIM rather than MRM detection, it can also apply to MRM to some extent. The physical problem with low mass product ions for MRM concerns the low mass cut off that exists for all RF-quadrupolar field devices (Section 6.4.2). This problem is extremely important for 3D ion traps, as expressed in the so-called one third rule whereby the low mass (really low m/z) cut off for product ions in MS/MS is 30% 

In practice, for quadrupole collision cells operated with typical values of rg and Vg, the low mass cut off problem is not often a major concern. In such cells that are operated with a sufficiently high pressure of collision gas to be in the collisional focusing regime (Section 6.4.3), the loss to the quadrupole rods of ions with q 0.908 and mjz ijnlz) (predicted for collision free conditions) is countered to a large degree by the collisional focusing of ions on the main axis of the device. 

Just as for the analyte, the choice of product ion for MRM detection of the SIS requires the same considerations. In addition, usually the m/z value of the SIS precursor ion is different from that of the analyte, so there is no problem if the best choice of product ion turns out to be the same for both. This conclusion does not apply if there is appreciable overlap between the isotopic distributions of anal5h e and SIS, i.e. cross-contributions between the precursor ions (Section 8.5.2c), since the latter circumstance requires uniquely different product ions for there to be any hope of distinguishing the two. It is good practice to make a choice of product ion as the primary candidate for the MRM method, but also to choose one or two others as back-up candidates that should be independently optimized (collision energy etc.). Then, during subsequent stages of the overall method development, continue to use aU of these candidate ions until the final optimization, as an insurance against unforeseen selectivity problems with the primary candidate for the real-world samples. 

For multi-analyte method development, it is possible to use one tuning solution containing several different compounds for MRM optimization. However, there is always a possibility that impurities in any of the reference standards being used could result in the optimization being done for some solute other than the anal5h e of interest. For this reason it is safer to use independent timing solutions for the initial optimization of Qj and especially for product ion optimization. 

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