Isolation window

Figure 4.20 Chromatograms recorded in resonant (signal m/z 195 +197) and not-resonant (signal m/z 167 + 169) mode in the analysis of a 1 ppt spiked red wine. Precursor ion m/z 211.9, isolation window 5u.m.a. excitation amplitude 80 V...

The three steps of an MS/MS experiment are performed by DIT using an approach substantially different from that employed in 3D IT or linear ITs (Ding, 2004). In those cases, the precursor ion isolation is performed by applying one or more dipole excitation waveforms, with a maximum isolation resolution of -1300 (expressed as the isolation mass divided by the baseline width of the isolation window). In the case of DIT, ion isolation is performed by sequential forward and reverse scans, so as to eject all ions with m/z values lower and higher than that of interest, respectively. This method can provide precursor ion isolation with a resolution >3500. 

C/min to 210 °C held for 15 min El (70 eV), ion source temperature 200 °C Precursor ions isolation window 3amu, scan time at... 

GC column 5% diphenyl-95 % dimethylpolysiloxane 30 m x 0.25 mm i.d. 0.25-pm. Oven temperature from 80 to 300 °C at 5°C/min ionization type El isolation window 3.0m/z MS/MS waveform resonant mode excitation amplitude 0.40 V excitation time 20mse. 

FIGURE 15.18 Top, a pseudo-product ion mass spectrum of m/z 264 of PFTBA but without collision-induced dissociation (CID) the mass spectrum was acquired with default isolation parameters and isolation window of 1 Th. Bottom, a full scan mass spectrum of PFTBA obtained with the same duration of ionization and showing both the m/z 264 peak and the C-isotopomer peak at m/z 265. By comparison, loss of the m/z 264 ion during the isolation process is essentially zero for this chemically-stable ion. For chemically-unstable ions, loss of precursor ion during mass-selective isolation can be minimized by using an isolation window of 3-5 Th. 

Figure 9.5 Isotopic distributions of intact Ub ions (charge state +10) before (a) and after (b, c, and d) precursor ion isolation (under exchange-out conditions). Panel (a) shows the extent of H retention on partially exchanged Ub (Ub ) by overlaying its spectrum with spectra of unlabeled (Ub), completely exchanged Ub (endpoint) and fully deuterated (Ub ) protein ions on a zoom-out scale. Panel (b) shows broadband isolation of Ub ions, and panels (c) and (d) illustrate Isolation of Ub ions representing con-formers C-l and C-2, respectively (the isolation windows are shown In panel (a)). Reproduced with permission from [25]...

An example of the application of the wide isolation window in tandem mass spectrometry is the detection and quantitative imaging of cocaine in postmortem human brain tissue (Reich et al., 2008 Reich et al., 2010). In this instance, it was determined necessary to develop an MS wide isolation window method because of interfering background ions. Cocaine detected in brain tissue was confirmed by matching six MS product ions with those from a cocaine standard. For the quantitation of cocaine analyzed from the tissue, the trideuterated ( Hs) internal standard was spiked beneath the tissue at known concentrations before matrix application to develop a calibration curve. The MS wide isolation method was then employed for the analysis of cocaine and cocaine-fi 3. Cocaine was analyzed successfully and quantified using this approach. 

Reich RF, Cudzilo K, Levisky JA, Yost RA. Quantitative MALDI-MS" analysis of cocaine in the autopsied brain of a human cocaine user employing a wide isolation window and internal standards. J Am Soc Mass Spectrom 2010 21 564-571. 

For this analysis, splitless injections (1 jlL) were performed on a Varian Star 3400CX (Varian, Walnut Creek, CA) gas chromatograph (GC) with helium carrier gas. CLA were separated using a CP-Sil 88 capillary column (100 m x 0.25 mm x 0.2 Llm Chrompak, The Netherlands) with the injector held at 225°C while the head pressure was maintained at 35 psi. The GC program was as follows start at 80°C and hold for 2 min, ramp to 120°C at 10°C/min then ramp to 220°C at 2.5 C/min and hold for 30.8 min. All mass spectrometry was performed with a Saturn 2000 ion trap (Varian, Walnut Creek, CA). CLA FAME were analyzed with an isolation window of 3 mass to charge units and a storage level of miz 90. Excitation amplitudes used were 0.35 V with the ion trap held at 175°C. 

Relative MALDI quantitation has been shown for lipids (14) and small molecules in the MALDI linear ion trap. Good precision in MALDI quantitation requires normalizing for an internal standard. MS and MS improve precision by providing specificity (15). Including a deuterated internal standard in the isolation window for MS has also been shown to improve precision (16). 

Resolution is usually assessed by examination of patterns under an electron microscope. Confusion can occur here because the measured resolution is different for different types of feature as a result of radiation scattering. Consequently isolated lines are the most difficult features to define in positive resists and isolated windows in negative resists. Assessment of resolution should therefore take account not only of processing details but of the feature being defined. 

Single (or selected) ion monitoring (SIM) Park Q1 RF/DC amplitudes at one (m/z)i with an ion isolation window of 1-10 Da. Q3 is set to RF-only mode to transmit all ions. Q1 and Q3 functions are interchangeable in SIM while q2 is RF-only, always. Multiple ions may be analyzed by jumping the parked RF/DC to various mass windows, but each jump will decrease the duty cycle for each ion accordingly. 

Since the use of a QMF is not practical for a pulsed MALDI technique and because ESI often produces multiply charged ions, thus decreasing the ion s m/z, there has been little attempt commercially to increase the mass range of the QMF beyond miz 4000. However, several researchers have increased the QMF to mIz 9000 and 45,000. ° Sobott and co-work-ers ° showed that it was possible to analyze miz 22,000 within an isolation window of 22 Th on a quadrupole time-of-flight (QTOF) tandem mass spectrometer by lowering the QMF to 300 kHz. 

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