Scanning with Triple Quadrupole Analyzers

Linked Scanning with Triple Quadrupole Analyzers 

Linked scanning techniques by which fragmentation reactions can be examined are particularly easy to apply with QqQ instmments. The ease with which RF and DC voltages can be changed rapidly means that the scanning can be done very quickly. Three common and popular types of linked scan are briefly described here and serve to illustrate its principles. 

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Linked Scanning with Triple Quadrupole Analyzers... 

Scanning techniques are carried out differently with such hybrid instruments as the triple quadrupole analyzer, the Q/TOF (quadrupole and time-of-flight), and double magnetic-sector instruments. 

FIGURE I (A) An ESI source attached to a single quadrupole analyzer. The Instrument can be interfaced to an HPLC, CE or a syringe pump. The ionization is done at atmospheric pressure and the mass analysis in vacuum. This configuration is used for molecular weight determination. (B) An ESI source attached to a triple quadrupole analyzer. In a typical MS - MS experiment the precursor ion (Mp) is selected by the first analyzer (Ql), reacts with an inert gas in Q2 to yield the product ions (MI, M2, M3), and the product ions are analyzed by scanning Q3. This yields the product (fragment) ion spectrum. 

For optimal selectivity, particularly for quantitation with ion trap or triple quadrupole analyzers, MS/MS scanning techniques can be utilized in GCMS. With these techniques, instruments are operated to perform one of three basic experiments including product ion analysis, precursor ion analysis or neutral loss analysis. Ion traps are normally limited to product ion scans in which a particular ion of interest is isolated in the ion trap, subjected to collisional activation and its fragments are detected. Triple quadrupoles can additionally be operated to detect all precursors that generate a common fragment or the analyzer can be set to detect a specific neutral loss characteristic of an analyte functional group. All these methods add specificity to mass spectral detection. 

Tandem mass spectrometry (MS/MS) is a method for obtaining sequence and structural information by measurement of the mass-to-charge ratios of ionized molecules before and after dissociation reactions within a mass spectrometer which consists essentially of two mass spectrometers in tandem. In the first step, precursor ions are selected for further fragmentation by energy impact and interaction with a collision gas. The generated product ions can be analyzed by a second scan step. MS/MS measurements of peptides can be performed using electrospray or matrix-assisted laser desorption/ionization in combination with triple quadruple, ion trap, quadrupole-TOF (time-of-flight), TOF-TOF or ion cyclotron resonance MS. Tandem... 

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 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... 

A 10% aliquot of the digestion mixture was analyzed on a Vydac C18 250 im ID fu silica cap aiy column connected on-line to ESI-MS (4). A linear gradient of 2% B to %% B in 45 minutes using solvent A (0.1% TFA) and solvent B (90% acetonitrile, 0.07% TFA) was used with a flow rate of 2 jul/min. Sample elution was monitored by UV detection at 200 nm. Mass spectra were recorded in the positive ion mode using a TSQ-700 triple quadrupole instrument (Fiimigan-MAT, San Jose, CA) with an electrospray ion source opo ating at atmospheric pressure. Scans were continuously acquired every three seconds between la/z 500 and 2(XX) in the centroid mode. 

About 1 jul of each fraction was analyzed by SIMS using a thioglycerol (3-mercapto-l,2-piopanediol) matrix. Mass spectra were recordt in the positive ion nude using a TSQ-700 triple quadrupole instrument (Finnigan-MAT, San Jose, CA) equipped with an 8 keV cesium ion gun (Phrasor Scientific, Inc, Duarte, CA). Scans were continuously acquired every seven seconds between m/z 400 and 4000 in the centroid mode. 

Tandem mass spectrometers are either triple quadrupole systems (the collision cell is also a quadrupole) or quadrupole ion-trap spectrometers. A triple quadrupole mass spectrometry system is shown in Figure 32F-2. Here, the first quadrupole acts as a mass filter to select the ion of interest. This ion is then fragmented by collision with an inert gas in the collision cell. The final quadrupole mass analyzes the fragments produced. The triple quadrupole system can be operated in other modes. For example, if the first quadrupole is operated as a wide mass filter to transmit a wide range of ions and no collision gas is present in the collision cell, the instrument is operating as an LC/MS system. The instrument can be operated by scanning one or both quadrupoles to produce mass spectra of the fragments of ions selected by the first quadrupole as that quadrupole is scanned. 

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