Selectivity multiple reactions, analysis

The use of a mass spectrometer as a detector for LC analysis brings a number of benefits to mycotoxin analysis. There is no need for chromophores or fluorophores in the analytes so derivatization can be avoided. The chemical structure of the analytes can be confirmed from molecular mass and fragmentation information and the use of tandem MS (MS/MS) allows greater selectivity. Multiple reaction monitoring and selected ion monitoring modes mean that chromatographic separation of all analytes is not necessary, as differentiation is carried out by the different ion transitions measured, and many multiresidue mycotoxin LC-MS methods now exist. These data acquisition modes can also increase the sensitivity of the method as the background noise is often reduced. 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

Once the analyte has been identified and characterized, it is possible to determine its quantity. This is important information in a lot of fields and in cultural heritage in particular. There are specific experimental set-ups for quantitative analysis, such as selected ion monitoring (SIM) and multiple reaction monitoring (MRM). By considering that MS is highly sensitive, it is possible to carry out quantitative determinations of compounds at trace level.[7,8]... 

GC/MS(/MS) is also popular for quantifying DBFs. Selected ion monitoring (SIM) or multiple reaction monitoring (MRM) mode are used with GC/MS and GC/ MS/MS, respectively, to maximize the sensitivity and provide low detection limits. Some EFA Methods utilize GC/MS, including EFA Method 524.2, which uses GC/ EI-MS for THM analysis [155], and EFA Method 521, which uses for GC/CI-MS/ MS for nitrosamine analysis [55]. In addition, many priority unregulated DBFs have been measured using GC/MS in a U.S. Nationwide Occurrence Study [11,12]. 

Furthermore, the organic functionalization studies have indicated that multiple reaction products can form even for simple systems. Kinetic and thermodynamic influences must be considered in any analysis of the product distribution. Moreover, the studies have revealed differences in the dominance of kinetic vs. thermodynamic control between the silicon and germanium surfaces. The dissimilarity primarily stems from the fact that adsorbate bonds are usually weaker on Ge than on Si. This difference in energetics leads to observable differences in the degree of selectivity that can be achieved on the two surfaces. Another important motif is the significance of interdimer bonding in the products. Many molecules, even as small as ethylene, have been observed to form products that bridge across two dimers. Consequently, each analysis of adsorption products should include consideration of interdimer as well as intradimer species. 

Fig. 2.2.9 High-performance liquid chromatography-tandem mass spectrometric analysis of AdoMet and AdoHcy. The analytes were evaluated by multiple reaction monitoring with the following transitions m/z 399 -> 250 for AdoMet m/z 402 -> 250 for the internal standard, tridenterated AdoMet (AdoMet +3) m/z 385 -> 135 for AdoHcy m/z 390 ->- 135 for the internal standard, pentadenterated AdoHcy (AdoHcy+5). Mass spectrometric conditions are described in the text. TIC total ion current, SRM selected reaction monitoring (Figure courtesy of Dr. Ries Duran, Amsterdam)...

Formiminoglutamate (FIGLU), a marker for glutamate formimino-transferase deficiency, was recently also shown to be detectable by acylcarnitine analysis represented as a peak with m/z 287 (Fig. 3.2.3d) [64]. In poorly resolved acylcarnitine profiles, this peak may be confused with iso-/butyrylcarnitine (m/z 288). To avoid the incorrect interpretation of acylcarnitine profiles, we recommend performing the analysis in product scan mode as opposed to multiple reaction monitoring (MRM) mode. For example, the FIGLU peak at m/z 287 would not have been correctly identified in MRM mode because the transition of 287 to 85 is typically not selected. However, the 288/85 transition would reveal abnormal results, but in fact not represent either butyryl- or isobutyrylcarnitine, but another FIGLU related ion species. 

For the detection, a tandem mass spectrometer Quattro Micro API ESCI (Waters Corp., Milford, MA) with a triple quadrupole was employed. The instrument was operated in electrospray in the positive ionization mode (ESI+) with the following optimized parameters capillary voltage, 0.5 kV source block temperature, 130 °C nebulization and desolvation gas (nitrogen) heated at 400 °C and delivered at 800 L/h, and as cone gas at 50 L/h collision cell pressure, 3 x 1(F6 bar (argon). Data was recorded in the multiple reaction monitoring (MRM) mode by selection of the two most intense precursor-to-product ion transitions for each analyte, except for the ISs, for which only one transition was monitored. The most intense transition for each analyte was used for quantitative purposes. Table 2 shows MRM transitions, cone voltages and collision energies used for the analysis of the antidepressants included in the LC-MS/MS method. 

Selectivity is a concept that applies to processes with multiple simultaneous reactions. It is used to quantify the relative rates of the individual reactions. However, any discussion about multiple reactions and the analysis of these is beyond the scope of this chapter. Refer to the further reading material identified at the end of this chapter for more information. 

For trace analysis, in which full-scan data cannot be obtained and selected ion monitoring (SIM) or multiple reaction monitoring (MRM) is required, no criteria have been established by the OPCW. Rodriguez and Orescan (23) proposed the following criteria for confirmation of trace levels of pesticides by LC/API/MS under MS conditions that promoted in-source CID ... 

For purposes of quantitative analysis, selected ion monitoring (SIM) and selected reaction monitoring (SRM) are two commonly utilized approaches. The latter is also referred to as multiple reaction monitoring (MRM). In both modes, considerable structural information is lost nonetheless, these techniques are extremely powerful for target compound quantihcation in biological matrices, if the compound of interest is known. 

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