Low mass cutoff

For molecules which can easily lose water or ammonia, the most abundant fragment observed in MS is M-18 or M-17, which is not very informative. To overcome this limitation, wide band excitation (range 20 m/z units) can be applied. Another difference compared to QqQ is that QIT have a low mass cutoff of about one-third of the mass of the precursor ion. However QIT is particularly attractive to follow fragmentation cascades as illustrated for bosentan in Fig. 1.22. It can clearly be concluded that the fragment at m/z 175 originated from the precursor at 202 and not from the precursor at m/z 311. 

Inherent low-mass cutoff limits Information rich tandem mass spectra... 

The quadruple Q1 is set to operate in wideband transmission mode with a low mass cutoff — ... 

Figure 9 Glow discharge mass spectra of NIST SRM 1103 Free Cutting Brass, demonstrating selective ion accumulation by using mass selective instability (a) low-mass cutoff, m/z 15 (b) low-mass cutoff, m/z 45. (From Ref. 36.)...

Xia et al. [328] demonstrated the utility of IDA approach to collect maximum amount of information with the minimum number of analytical runs during the course of identification of in vitro formed metabolites of gemfibrozil. The Q-Trap tandem mass spectrum contained fragment ions at mlz 113 and 85 and they were absent in the MS/MS spectrum generated using a 3D iontrap due to inherent low mass cutoff. With the 3D trap, fragment ions with mlz values lower than approximately one-third of the precursor ion were not detected. 

Recently introduced hybrid triple quadruple—linear ion trap (QTRAP) mass spectrometer has scan functions identical to those of classical triple quadruple (PI scan, NL scan, and MRM). It can be used for small-molecule quantification with MRM in both bioanalysis and in vitro ADME studies (Hopfgartner et ah, 2004). The QTRAP also has scan functions (full MS scan and MS/MS scan) similar to an ion trap mass spectrometer. Most importantly, MS, MRM, PI, and NL scans on QTRAP instruments can serve as survey scans to trigger the information-dependent acquisition (IDA) of enhanced product ion spectra (EPI) with polarity switching, which provide product ion spectra with rich fragments with no low-mass cutoff (Xia et al., 2003 Zheng et al., 2007 Wen et al., 2008b Yao et al., 2008 Jian et al., 2009). The information-dependent acquisitions have been widely applied to reactive... 

Note that the boundaries of the douhly stable region (Figure 6.18) are defined by the curves for = 0 or 1, for u = r and z. The = 1 curve intersects the q axis at q = 0.908 since in practice ion traps are usually operated without a E)C potential difference U, i.e., with a = a = 0, the point with q = 0.908 represents the ion of lowest mJz value that can be stored in the trap (the so-caUed low mass cutoff). 

An important quantity in all electric quadrupole devices (linear and three-dimensional) is the low mass cutoff, the minimum value of m z that has stable trajectories in the device under the stated operating conditions, i.e., the ion that has q just less than 0.908 (see discussion of Eignres 6.10 and 6.18). Then for this trap operated at Vg = 757 V, m/z) = 60.27/0.908 = 66.4, i.e., in practice the low mass cutoff for trapping of ions would be m z 67. The converse question, (i.e. what change would have to be made to the operating conditions to permit a specified low mass cutoff) amounts to determination of the required value of Vg since the RF frequency is considerably more difficult to adjust once fixed. Then the required value of Vg (in volts) for this particular hypothetical trap (i.e., rg, Zg and w all fixed) is given by Equation [6.36] as [(0.908/0.0796).( v z)iiiio] = H-41.(ffV z)imo-... 

Unfortunately, there is one major disadvantage of QFTs for fragment ion analysis in that they cannot simultaneously store ions over the full m/z range. It is a commonly accepted property of QITs to loose ions below an m/z value of about one third of the precursor ion. This phenomenon is known as low-mass cutoff (LMCO) [113]. 

Most ion activation analyses with the QIT and LQIT have used CAD however, photoinduced dissociation (PID) has shown promise as a means for high internal energy deposition. In addition, it is also a means to avoid the low-mass cutoff (LMCO) limitation since the precursor ion does not have to reside at a relatively high q for excitation as in the CAD process. PID has been demonstrated in ion cyclotron resonance (ICR) as well as in the QIT. Louris et al. used a fiber optic to introduce light from an Nd YAG laser... 

Fignre 14.6. ES mass spectra of apomyoglobin obtained by Q3 scan on a triple quadnqmle with Q1 set at miz 2000 in if-only mode (low mass cutoff < 1633) declustering in the ion source region was negligible, (a) No CID gas in Q2 (b) CID gas was added to Q2. (Reprinted from Ref. 79 with permission.)... 

Figure 16.7. ESI-MS/MS spectra of the N-terminally sulfonated peptide HSDAVFTDNYTR from (a) low-energy CID and (b) IRMPD. LMCO, low-mass cutoff , satellite peaks due to loss of H2O. (Adapted from Ref. 114 with permission from the American Chemical Society.)...

---