Multi-Sector Instruments

Example The FAB-CID-MS/MS spectrum of thymosin-Tl [M-hH] ions, m/z 1427.7, as obtained from a magnetic four-sector instrument [81] shows numerous fragment ions due to TV-terminal, C-terminal, and internal fragmentations (Fig. 9.16) [82]. It would be almost inpossible to obtain sequence information from such a spectrum without generalizing rules to follow [83-88]. The most abundant ions obtained from the fragmenting peptide ion usually belong to six series named a, b, and c if the proton (charge) is kept in the TV-terminus or x, y, and z, respectively, where the proton is located in the C-terminal part. Within each series the mass difference should only be one amino acid. Ideally, one can then count down the amino acids. 

Note One of the advantages of MS/MS techniques is that they do not require the full isolation of all conpounds of interest, because the precursor ion selection of MSI excludes accompanying ions from contributing to the CID spectrum of the actually selected precursor ion as acquired by MS2. 

Note Collision cells of simple short tube design like in tandem TOE or magnetic sector instmments (Chp. 9.3.1) cannot be combined with quadmpoles since the ions are rather dispersed at the exit and would be lost upon further 

In MS/MS operation, the mass-selected ions emerging from Qi are accelerated into the ion guide collision cell by a potential offset of typically 5-50 V where the collision gas (N2, Ar) is provided at a pressure of 0.1-0.3 Pa Careful optimization of all parameters allows for major improvements of CID efficiency and resolution [97]. If MS/MS is not intended, either Qi or Q3 may be set to RF-only mode, thereby reducing its function to that of a simple flight tube with ion-guiding capabilities. The instmment then behaves as though it was a single quadmpole mass spectrometer. 

Note At first sight, there is no difference whether Qi or Q3 is switched to RF-only for MS mode. However, for El it seems better to operate Q3 in RF-only mode. Otherwise, the ion source would effectively extend up to the entrance of Q3 making fragment ions more abundant due to elongated time for dissociations. Soft ionization methods do not show such differences. 

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An accurate determination of copper and zinc traces in human serum samples from the International Measurement Evaluation Programme-17 launched by IRMM (Geel) has been made by isotope dilution TIMS.38 An analytical method for the multi-element determination of metals (Ti, V, Cr, Co, Ni and Mo) potentially released from dental implants and prostheses into human body fluids (in blood and urine) by ICP-MS (double-focusing sector field instrument and quadrupole instrument with octopole collision cell) for medical studies was developed in Sanz-Medel s group.39 The Cr and Co concentrations found in blood samples of patients with chromium-cobalt based alloy varied in the sub-p,gl 1 range and were not significantly higher than the basal levels found by other authors.40... 

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