Single quadrupole

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 

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G. J. Opiteck, J. W. Jorgenson, M. A. Moseley III and R. J. Anderegg, Two-dimensional mia ocolumn HPLC coupled to a single-quadrupole mass spectrometer for the elucidation of sequence tags and peptide mapping , 7. Microcolumn Sep. 10 365-375 (1998). 

The instrument scan mode called selected reaction monitoring (SRM) is generally used for quantitative applications. SRM is similar to selected ion monitoring (SIM) in single quadrupole MS. The difference is that a product ion from the decomposition reaction in the collision cell is measured instead of a single ion formed in the... 

Voyskner, R. D. Pack, T. Investigation of collisional-activation decomposition process and spectra in the transport oregion of an electrospray single-quadrupole mass-spectrometer. Rapid Comm. Mass Spectrom. 1991, 5, 263-268. 

LC/MS liquid chromatography/mass spectrometry (single quadrupole)... 

Recently, comparatively inexpensive, very reliable, and stable single quadrupole mass spectrometers have entered the market. These spectrometers can be coupled to GC, LC, and CE separation methods simply by modifying the sampling interfaces. Although these detectors are more expensive than most conventional detectors including the versatile electron capture and diode array absorbance detectors used for GC and LG respectively, the reduction in sample preparation effort and their increased specificity can often rapidly... 

Fig. 1.6 (A) Electron impact spectrum obtained on a single quadrupole mass spectrometer of a compound with Mr = 355. (B) Product ion spectrum after atmospheric pressure ionization obtained on a triple quadrupole instrument. Chemical ionization and atmospheric pressure ionization give in both cases protonated precursor ions, which is ideal for tandem mass spectrometry.

Fig. 1.13 Negative mode single quadrupole MS spectra of sulfuric acid monoester of 3-hydroxy retinoic acid (A) electrospray, (B) atmospheric pressure chemical ionization.

Undoubtedly, mass spectrometric detection has a substantial role to play in condensed-phase chromatographic analyses of toxic impurities. As in GC/MS, it can be highly sensitive, although this is probably more analyte-specific than in GC/MS. Selectivity can be gained by SIM on single quadrupoles or, if necessary, SRM on MS/MS instruments. What must be considered is the appropriate ionisation mode to be used in LC/MS. Most modern instruments use atmospheric pressure ionisation sources, including electrospray ionisation (ESI), atmospheric pressure chemical ionisation (APCI) and more recently atmospheric pressure photoionisation (APPI). 

Unfortunately, this technique is not selective and all components are affected in the same way so that if exact co-elution of sample components occurs, this may not be detected. Using a single quadrupole instrument limits the user to using in-source fragmentation, but in many cases this can provide enough information to identify unknowns. 

The mass spectra of mosapride and the impurity generated by LC-MS were compared (Figure 6.14). The impurity shows MH" at m/z 404, and comparing this with MH of mosapride at miz 422, the mass difference observed is 18 Da. From the single quadrupole experiments, fragments at m/z 198 and 170 with the characteristic chlorine isotopic abundance can be seen in both mosapride and its impurity. This indicated that the 4-amino-5-choro-2-ethoxy-benzamide moiety was unmodified. 

An in-process sample when examined by LC-MS on a single quadrupole showed three impurities, all with the same MH at the nominal mass of miz 402. This equated to an extra 14 Da higher than the parent mass. From the structure... 

Although low-resolution difference Fourier maps for oxy-Hr and deoxy-Hr show little change in the protein structures, some of the iron center properties are significantly altered in deoxy-Hr. The differences provide a rationale for an oxygen-binding mechanism. The Mossbauer spectrum for deoxy-Hr has a single quadrupole doublet with an isomeric shift typical of high-spin ferrous iron (8 = 1.14 mm/sec AEq = 2.76 mm/sec) (Clark and Webb, 1981). As for met-Hr the two iron environments are similar, yet differ in coordination number for exam-... 

Notes AA, acetic acid AAc, ammoniuin acetate ACN, acetonitrile AH, airnnonium hydroxide FA, formic acid MeOH, methanol TFA, trifluoroacetic acid THF, Tetrahydrofuran APCI, atmospheric pressure chemical ionization APPI, atmospheric pressure photoionization ESI, electrospray ionization IS, ion spray IT, ion trap Q, single quadrupole QqQ, triple quadrupole TOF, time of flight. 

Mullen, W., Lean, M.E.J., and Crozier, A., Rapid characterization of anthocyanins in red raspberry fruit by high-performance liquid chromatography coupled to single quadrupole mass spectrometry, J. Chromatogr. A, 966, 63, 2002. 

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