Mass isolation window

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. 

Fig. 5. Mass spectra from dissolved, particulate and colloidal particles isolated from coastal surface water off La Jolla, California, June 1993. Identical 50-360 atomic mass units windows were chosen for the detail (reproduced from Vernonclark et al. [29]...

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. 

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. 

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. 

Since the analytical point of view most of current analytical methods are based on LC-MS/MS, but for some classes of pesticides GC-MS continues being the technique of choice. The use of quadrupole ion trap (QIT) to analyze multiple pesticide residues is limited to several multiclass pesticides in fruit [162], because of the limited number of ions that can be isolated at the same time. For this reason, the use of several time windows is required and this is indeed a strong limitation in practice. The use of hybrid triple quadrupole linear ion trap (QqLlT) mass spectrometer has provided significant contribution to the development of high-sensitive multiresidue analytical methods for pesticide control. An example of application is the method reported by Hernando et al. for the analysis of pesticide residues in olive oil [65]. 

MS-MS can provide enhanced information on the individual peptide contained in a proteolytic digest, facilitating the identification of the proteins and offering the possibility of de novo sequencing (Kinter and Sherman, 20(X)) when no representative entry is in a database. Typically in a first pass, the introduced peptides are separated according to m/z by the mass spectrometers (MS spectrum). A list of peptides with signals above a pre-established threshold is created. In a second pass, a mass window centered on a selected peptide is isolated by the mass spectrometer and the kinetic energy of the selected peptide is increased. The collision of the peptide with small gas molecules (CID) transfers... 

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