Transmission quadrupole mass analyzers

LIT The linear ion trap (LIT) (also referred to as a two-dimensional, or 2D, trap) is a variation on the transmission quadrupole mass analyzer. In the LIT, the quadmpole is constructed such that either ions can be analyzed immediately or, ions can be trapped and held in the quadrupole region and then analyzed (Hager, 2002 Schwartz et al., 2002). Various types of MS/MS can be performed, as described in Chapter 3. 

Describe two methods for ionizing liquid samples for mass spectral analysis. Diagram a transmission quadrupole mass analyzer. What are the advantages and disadvantages of this mass analyzer compared with a double focus mass spectrometer ... 

The single quadrupole is one of the earliest mass analyzers to become widely available. A quadrupole separates ions according to their m/z ratio as a function of their trajectory through an oscillating electric field. With a transmission quadrupole mass analyzer, direct-current (DC) and radiofrequency (RF) voltages are applied to four parallel rods (Fig. 6.7a). As ions are accelerated through... 

Figure 3.9 Conceptual view of tandem mass spectrometry with a tandem-inspace triple quadrupole mass analyzer." The first mass analyzer (Ql) selects the precursor ion of interest by allowing only it to pass, while discriminating against all others. The precursor ion is then fragmented, usually by energetic collisions, in the second quadrupole (q2) that is operated in transmissive mode allowing all fragment ions to be collimated and passed into the third quadrupole (Q3). Q3 performs mass analysis on the product ions that compose the tandem mass spectra and are rationalized to a structure.

In a quadrupole mass analyzer, only a single mass-to-charge ratio m/z) value is transmitted to the detector for any given combination of radio frequency (RF) and direct current (DC) potentials. Typically, the RF/DC ratio is held constant and scanned to provide a mass spectrum. If, for example, a quadrupole is scanned from m/z 1 to 1000 in 1 second, then any particular m/z is transmitted to the detector for only 1 millisecond, representing a duty cycle of 0.1 %. Thus, a quadrupole mass analyzer has a low transmission duty cycle in the full-scan mode, which results in limited full-scan sensitivity. In contrast, ion-trap and TOF mass analyzers have the theoretical potential to transmit all ions that enter the mass analyzer and yield far better sensitivity across the entire mass spectrum. In reality, the pulse sequences associated with these analyzers devote significant time to functions such as ionization and detection. The actual duty cycles are generally between 10 and 25%, still far better than a scanning quadrupole mass spectrometer. 

Instrumental Aspects. The early developments of static SIMS (largely driven by polymer surface analysis requirements in terms of practical application) were carried out with noble gas ion sources and quadrupole mass analyzers (QMS). The latter had the benefits of compactness, ready availability, and relatively straightforward adaptability to SIMS use in UHV systems. Although all the essential features of polymer surface analysis were introduced using this technology, the QMS has major limitations for static SIMS. Firstly, it is a serial device, so that only one mass at a time can be detected. Secondly, it has a limited mass range (<1000 Da) and the transmission (ie, sensitivity) decreases with mass (by at least m ). Thirdly, it is only possible to achieve a uniform mass resolution... 

Figure 3. Dependence of analyte ion intensity on concentration. (A) Dependence of total current / on concentration M = mol/L of analyte ion, (Morphine)H+, i.e., MorH+, in solution. (B) Dependence of mass-analyzed MorH+ ion current in counts/s on MorH+ concentration. At low MorH+ concentrations, [MorH+] < 1CT6 M, the dominant electrolyte in the solution are impurity ions Na+ and NH4. In this region MorH+ intensity is proportional to [MorH+] in solution. Mass-analyzed ion intensity was corrected for mass-dependent transmission Tm, of quadrupole. Concentration of morphine hydrochloride given in mol/L (M). From Kebarle, P. Tang, L. Anal. Chem. 1993, 65, 973A, with permission.

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