Tandem mode

M. R. Guasch Jane, M. Ibem Gomez, C. Andres Lacueva, O. Jauregui and R. M. Lamuela Raventos, Liquid chromatography with mass spectrometry in tandem mode applied for the identification of wine markers in residues from ancient Egyptian vessels, Analytical Chemistry, 76, 1672 1677 (2004). 

A well-known method for peak tracking is based on the phase-system switching idea, " which was developed to solve problems of mobile phase incompatibility in LC/MS target compound analysis. An analytical column is usually connected to a trapping column in tandem mode. A switching valve is placed after the UV detector, and the flow of nonvolatile eluents is directed through the trapping column to waste. When the peak of interest elutes from the analytical column it is... 

The use of two consecutive Claisen rearrangements has been accomplished in both the sequential and tandem modes. A noteworthy example of the former method has been applied creatively to syntheses in the vitamin D area by Lythgoe and his coworkers (Scheme 9). (/()-Methyl orthodihydrocitronellate... 

Table 6.1 Comparison of some figures of merit for tandem mass spectrometers used in quantitative analysis. Double focusing magnetic instruments are not included here as their range of plicability is somewhat limited and they are never used in tandem mode for quantitation. The stand-alone linear quadrupole is not included in this Table (no MS/MS capabilities) although it is widely used in quantitative applications by GC/MS. The stand-alone TOF analyzer also has potential in this regard. 

If the energy of the extracted particle is sufficiently high, occasionally a series of targets (tandem mode) is irradiated (cf. Ruth 1985). It is then possible to produce two or more radionuclides simultaneously. However, this technique has found only limited application. 

Gauasch-Jane, M. R., et al. "Liquid Chromatography with Mass Spectrometry in Tandem Mode Applied for the Identification of Wine Makers in Residues from Ancient Eg)q tian Vessels." Analytical Oiemistry. 76 2004,1672-1677. 

Consequently, analysis of PAHs is mainly accomplished by GC-MS in simple or tandem mode (GC-MS/MS) and HPLC coupled with fluorescence detector (HPLC-FLD) since most of the PAHs are strong fluorophores. Multiple reaction monitoring (MRM) in a triple quadrupole GC-MS/MS is inherently more selective and sensitive than either scan or selected ion monitoring (SIM) as many matrix interferences are minimized or even removed. For this reason various groups have used tandem MS (Matozzo et al. 2010 Xia et al. 2012), however sufficient detection was accomplished also with GC-MS-SIM (Webster et al. 2006). HPLC-UV has also been used with less sensitivity as expressed by LOD values, although in one work LOD values with UV detection are reported quite low (Cravo et al. 2012). PAHs as in almost all UV or FLD methods were identified on the basis of retention time and quantification on an external standard method. 

This notion can be illustrated by example in Scheme 16.3. Intermolecular [4-1-2] cycloaddition between nitroethylene (8) and chiral vinyl ether 9 provides nitronate 10 with very high diastereoselectivity [23]. This intermediate is sufficiently stable to survive an aqueous work up howevCT, it cannot withstand silica gel chromatographic purification. Nevertheless, the nitronate 10 reacts in a tandem mode with dimethyl ftimarate at ambient temperature and the final product, nitroso acetal 12 is isolated in 40% yield. This sequence is used in the total synthesis of (-l-)-macronecine (13). 

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