Mass-filtering mode

Further benefits can be realized by the implementation of mass-selective external accumulation. The dynamic range and sensitivity of the instrument are improved. Mass-selective external accumulation can be achieved by interfacing a quadrupole mass filter with the FT-ICR mass spectrometer. The quadrupole can be operated either in RF/DC mass filtering mode, in which one m/z region traverses the quadrupole, or in RF-only resonant dipolar excitation mode. The latter allows selective removal of multiple m/z peaks. For example. Smith and co-workers showed that this mode could be applied to remove the [M -i- 16H] +and [M + 14H] +ions of myoglobin from the charge-state envelope + 13 through -i-18 [49]. 

MALDI-QTOF instraments basically follow an ion optical arrangement similar to that shown in Figure 2.16. The first quadrupole, Qo, is simply used to cool the ions kinetic energy and focus them onto the quadrupole axis so that they enter Qi with a well-defined initial trajectory, and is always used in RF-only mode. Qi is a mass-filtering quadrupole which can either be used in RF-only mode to pass a wide range of mass/charge values, or in mass-filtering mode to select particular... 

The hexapole cannot act as a mass filter by applying a DC field and is used only in its all-RF mode, in which it allows all ions in a beam to pass through, whatever their m/z values. In doing so, the ion beam is constrained, so it leaves the hexapole as a narrow beam. This constraint is important because the ion beam from the inlet system tends to spread due to mutual ion repulsion and collision with residual air and solvent molecules. By injecting this divergent beam into a hexapole unit, it can be refocused. At the same time, vacuum pumps reduce the background pressure to about 10 mbar (Figure 22.1). The pressure needed in the TOF analyzer is about 10 ... 

Quadmpole mass spectrometers (mass filters) allow ions at each m/z value to pass through the analyzer sequentially. For example, ions at m/z 100, 101, and 102 are allowed to pass one after the other through the quadmpole assembly so that first m/z 100 is transmitted, then m/z 101, then m/z 102, and so on. Therefore, the ion collector at the end of the quadmpole unit needs to cover only one point or focus in space and can be placed immediately behind the analyzer (Figure 30.1). A complete mass spectram is recorded over a period of time (temporally), which is set by the voltages on the quadmpole analyzer. In this mode of operation, the ions are said to be scanned sequentially. The resolution of m/z values is dependent solely on the analyzer and not on the detector. The single-point collector is discussed in detail in Chapter 28. 

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.

Due to their comparatively low costs and easy operation, quadtupole instruments are the most common instruments used for hyphenation in CEC analyses. However, these instruments only operate at low mass resolution. Sensitivity can be enhanced by operating in selected ion monitoring mode instead of full scanning acquisitions. Unfortunately, this leads to the loss of structural information. The expansion of biological applications has been largely accommodated by the TOE, quadtupole mass filter, and ion-trap instruments. The major advantage of TOE is its potential for speed, resolution, and good mass accuracy. 

A triple quadrupole instrument (QqQ) is a combination of two mass quadrupole mass filters (tandem mass spectrometry) separated by a collision cell which is also a quadrupole operating in RE-only mode (Pig. 1.18). A common nomencla-... 

Daughter ion scan. In this mode, an ion is selected with the first mass filter and enters the collision cell where it collides with a target gas (argon or N2). The fragments formed in the collision cell are then analysed by scanning the second mass analyser. In this way, the mass spectrum of a selected ion is obtained. 

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... 

Ions exiting the drift tube are mass analyzed in mass spectrometer MS2, an important feature if reactions are occurring in the drift cell. Ions are generally detected after MS2 by ion counting techniques. The mass spectrometers MSI and MS2 are typically quadrupole mass filters, and either one or the other can be run in RF-only mode for better signal but without mass selection, if desired. 

---