Triple quadrupole scan modes

A further extension of the DFG S19 method was achieved when polar analytes and those unsuitable for GC were determined by LC/MS or more preferably by liquid chromatography/tandem mass spectrometry (LC/MS/MS). Triple-quadrupole MS/MS and ion trap MS" have become more affordable and acceptable in the recent past. These techniques provide multiple analyte methods by employing modes such as time segments, scan events or multiple injections. By improving the selectivity and sensitivity of detection after HPLC separation, the DFG S19 extraction and cleanup scheme can be applied to polar or high molecular weight analytes, and cleanup steps such as Si02 fractionation or even GPC become unnecessary. 

Fig. 11.16. Representation of three tandem mass spectrometry (MS/MS) scan modes illustrated for a triple quadrupole instrument configuration. The top panel shows the attributes of the popular and prevalent product ion CID experiment. The first mass filter is held at a constant m/z value transmitting only ions of a single mlz value into the collision region. Conversion of a portion of translational energy into internal energy in the collision event results in excitation of the mass-selected ions, followed by unimolecular dissociation. The spectrum of product ions is recorded by scanning the second mass filter (commonly referred to as Q3 ). The center panel illustrates the precursor ion CID experiment. Ions of all mlz values are transmitted sequentially into the collision region as the first analyzer (Ql) is scanned. Only dissociation processes that generate product ions of a specific mlz ratio are transmitted by Q3 to the detector. The lower panel shows the constant neutral loss CID experiment. Both mass analyzers are scanned simultaneously, at the same rate, and at a constant mlz offset. The mlz offset is selected on the basis of known neutral elimination products (e.g., H20, NH3, CH3COOH, etc.) that may be particularly diagnostic of one or more compound classes that may be present in a sample mixture. The utility of the two compound class-specific scans (precursor ion and neutral loss) is illustrated in Fig. 11.17.

Fig. 9. Precursor ion scan on an electrospray triple quadrupole mass spectrometer. From all the peptides present of the digested protein only those that are phosphorylated are detected in a precursor ion scan for the phosphate ion (P03, mass 79 Da) in negative ion mode. From the TPX protein three phosphorylated peptides could be detected Ml, AQLTM PSTPTVLK M2, LSETSVNTEQNSK and M3, VQPVQTTPSKDDVSNSATHVC DVK. M, Oxidized methionine C, carbamidomethylated cysteine.

Scan Modes for Tandem MS with Triple Quadrupole Instruments... 

Table 1.3 Settings of the Ql and Q3 quadrupoles for the various scan modes of a triple quadrupole mass spectrometer.

In tandem MS mode, because the product ions are recorded with the same TOF mass analyzers as in full scan mode, the same high resolution and mass accuracy is obtained. Isolation of the precursor ion can be performed either at unit mass resolution or at 2-3 m/z units for multiply charged ions. Accurate mass measurements of the elemental composition of product ions greatly facilitate spectra interpretation and the main applications are peptide analysis and metabolite identification using electrospray iomzation [68]. In TOF mass analyzers accurate mass determination can be affected by various parameters such as (i) ion intensities, (ii) room temperature or (iii) detector dead time. Interestingly, the mass spectrum can be recalibrated post-acquisition using the mass of a known ion (lock mass). The lock mass can be a cluster ion in full scan mode or the residual precursor ion in the product ion mode. For LC-MS analysis a dual spray (LockSpray) source has been described, which allows the continuous introduction of a reference analyte into the mass spectrometer for improved accurate mass measurements [69]. The versatile precursor ion scan, another specific feature of the triple quadrupole, is maintained in the QqTOF instrument. However, in pre-... 

A Q-TOF spectrometer is similar to a triple quadrupole but Q3 is replaced by an orthogonal TOF mass spectrometer. Using a Q-TOF instrument only the product ion scan mode can be collected, but because of its high resolving power, accurate masses for both the precursor ion and product ions can be obtained. (See the section below on accurate mass measurements.)... 

Different MS MS experiments of product ion scan, precursor ion scan, and neutral loss scan modes of selected flavonoids can be carried out in order to confirm the structure of flavonoids previously detected by the full-scan mode. In the product ion scan experiments, MS MS product ions can be produced by CID of selected precursor ions in the collision cell of the triple-quadrupole mass spectrometer (Q2) and mass analyzed using the second analyzer of the instrument (Q3). However, in the precursor ion scan experiments, Q1 scans over all possible precursors of the selected ion in Q3 of the triple quadrupole. Finally, in neutral loss... 

The analytical performance of four modes of LC-MS, multiple MS (MS"), and tandem MS operation (APCI and ESI with positive and negative ionizations) has been compared for two mass spectrometers, a triple-quadrupole and an ion-trap instrument.With 15 flavonoids as test compounds, the use of APCI in the negative-ion mode gave the best response, with the signal intensities and the mass-spectral characteristics not differing significantly between the two instruments. Under optimum conditions, full-scan limits of detection of... 

A new generation of linear ion trap mass spectrometers has been developed and exhibits increased performance compared to traditional three-dimensional (3D) ion traps (Hopfgartner et al., 2003 Douglas et al., 2005). A further evolution of the triple-quadrupole family and ion trap class of instruments is the production of the hybrid triple-quadrupole/linear ion trap (QQQ/LIT) platform. Hybrid instruments of this nature allow for operation in space and not just in time when performing MS/MS analysis. This feature allows for increased performance compared to classical ion traps. A powerful combination possible on a hybrid LIT/QQQ instrument is the ability to use highly sensitive and selective precursor ion, constant neutral loss, and multi-MRM as a survey scan for dependent LIT MS/MS. Compared to a simple MS experiment, these comprehensive triple-quadrupole and LIT modes can be more complex to setup. 

Several scan modes are unique to the triple-quadrupole instrument, and most of these modes are superior in duty cycle versus an ion trap, Fourier transform (FT), or time-of-flight (TOF) mass spectrometers. Different elements of the triple-quadrupole perform different operations for each scan mode. These scan modes, each of which will be described in detail, are single-reaction monitoring (SRM) or multiple-reaction monitoring (MRM), precursor ion scanning (PIS), and constant-neutral-loss scanning (NLS). These scan modes and applications for structural elucidation have been described in detail (Yost and Enke, 1978, 1979). 

A triple-quadrupole MS consists of three sets of quadrupole analyzers. As ions drift through the space between quadmpoles, pairs of rods of opposite polarity, the voltage can be varied so that only certain ions of specific m/z-ratios pass through the filter, whereas others are diverted. The applied voltage can be varied over time (scanned) and the number of ions exiting the filter can be analyzed as a function of a selected m/z value. In a triple-quadrupole MS, the second quadrupole is run in RF (transmitting all the ions present in the mixture) mode only, without preselection of m/z ratios, to induce fragmentation of ions by collisions with inert gas (CID, collision-induced dissociation). 

The quadrupoles in an MS instrument serve as selective mass filters to isolate ions with m/z-values specific for the analytes of interest. The triple quadrupole MS/MS instrument is typically operated by a pneumatically assisted electrospray source with an additional heated auxiliary gas flow for higher flow rates. There is a trade-off between resolution (favored by lower flow) and sensitivity (favored by higher flow) of the quadrupole analyzers. The biological molecules can be proto-nated or deprotonated at multiple sites to produce ions of n charged states [M nH]n . The MRM-MS/MS scan mode has a high duty cycle for the detection... 

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