Selected Ion Monitoring Method

For quantification, the monitored ion intensity can be directly compared to that of the selected internal standard after direct infusion. In the case of LC-MS analysis, the reconstituted ion peak area of each m/z corresponding to a species of interest can be compared to either a standard curve of the molecular species or to the reconstituted ion peak area of an internal standard under identical experimental conditions. 

Unfortunately, the monitored m/z in the SIM technique could represent the combination of isobaric/isomeric species leaving us without unique identifiers. This is typical for the SIM analysis of a very complicated sample such as a serum or plasma sample where many compounds have the same m/z including multiply charged m/z of the species or stable isotopologues. Therefore, the technique is a kind of trading specificity for sensitivity. This technique is seldom used in modem lipidomics analysis due to its limitations. 

However, an alternative method of SIM is still commonly used in lipidomics, particularly in the platforms associated with LC-MS, where high duty cycle instruments such as Q-ToF-type mass spectrometers are employed. In this case, a product-ion analysis at any moment of elution time could be performed for certain ions above a preset threshold for identification of these species (i.e., data-dependent acquisition), while a mass spectrum in the full MS mode, which detects both miz values and intensities of the ions between the mass ranges of intoest at the eluent time, is acquired over the entire elution time period for quantification. Owing to the very high scan rate, high sensitivity, and very fast and efficient acquisition of full product-ion mass spectra with the Q-ToF-type instruments over QqQ-type mass spectrometers, multiple acquisitions can be recorded at an elution time for identification of the relatively abundant species. The combination of elution time, m/z value, and a number of product-ion mass spectra provides reasonably accurate information about the chemistry of lipid species. 

Importantly, the combination of ESI-MS detection with high-performance liquid chromatography (HPLC) separation makes this approach a potential choice for lipid quantification in lipidomics if one of the following conditions can be met. 

Among these three conditions, only the first approach or its variations was used widely in practice. Accordingly, a total ion current chromatogram of each individual molecular ion can be extracted (i.e., selected-ion monitoring) from the recorded mass spectra in the full MS mode from an HPLC run. Indeed, many ions of interest can be extracted simultaneously in this way [51]. To perform quantitative analysis of lipids, the linear dynamic range, limit of detection, and calibration curves of the molecular species of interest are generally predetermined. Thus, the reconstituted ion peak area of each species can be compared to a standard curve obtained under identical experimental conditions. 

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As mentioned previously, the formation of the methyl derivative is similar to several methods that already have been developed for organotin pesticides. The utilization of this derivative in conventional gas chromatographic residue procedures presents many challenges. Electron capture detection is sensitive enough but requires extensive clean-up while with FID and FPD the sensitivity is lower than desired. The development of a routine GC/MS procedure (single or multiple ion monitoring) was chosen then as the only possible alternative. A SIM (selective ion monitoring) method is specific for retention time and as well as for characteristic ions (m/e). 

Jacob, P., Ill Yu, L. Wilson, M. and Benowitz, N.L. Selected ion monitoring method for determination of nicotine, cotinine, and deuterium-labeled analogs. Absence of an isotope effect in the clearance of (S)-nicotine-3 3 -d2 in humans. Biol Mass Spectrometry 20 247-252, 1991. 

Zaylak M, -J., Curtius H-Ch, Leimbacher W., and Redweik U (1977) Quantitation of deuterated and non-deuterated phenylalanine and tyrosine in human plasma using the selective ion monitoring method with combined gas chromatography-mass spectrometry. Application to the m vivo measurement of phenylalanime-4-monooxygenase activity. / Chromatogr 142, 523-531... 

Ferretti, A. and Flanagan, V.P. A simple, selected ion monitoring method for the determination of prostagiandins E2 and F in human urine. Biomed. Mass Spectr. 6(10) 31- 3, 1979... 

A Simple, Rapid Selected Ion Monitoring Method for the Determination of Prostaglandins E2 and F2 in Human Urine... 

Leimbacher, W., and Redweik, U. Quantitation of Deuterated and Non-deuterated Phenylalanine and Tyrosine in Human Plasma Using the Selected Ion Monitoring Method with Combined Gas Chromatography-Mass Spectrometry. Application to the In Vivo Measurement of Phenyla1anine-4-monooxygenas e Activity... 

Thumhofer, S. and Vetter, W., A gas chromatography/electron ionization-mass spectrometry-selected ion monitoring method for determining the fatty acid pattern in food after formation of fattty acid methyl esters, J. Agric. Food Chem., 53, 8896-8903, 2005. 

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Figure 5.8 Electrospray and transformed electrospray spectra of the light- and heavy-chain antibody fragments of recombinant ritnximab obtained by LC-MS analysis. Reprinted from 7. Chromatogr., A, 913, Wan, H. Z., Kaneshiro, S., Frenz, J. and Cacia, J., Rapid method for monitoring galactosylation levels dnring recombinant antibody production by electrospray mass spectrometry with selective-ion monitoring , 437-446, Copyright (2001), with permission from Elsevier Science.

Quantitative methodology employing mass spectrometry usually involves selected-ion monitoring (see Section 3.5.2.1) or selected-decomposition monitoring (see Section 3.4.2.4) in which a small number of ions or decompositions of ions specific to the compound(s) of interest are monitored. It is the role of the analyst to choose these ions/decompositions, in association with chromatographic performance, to provide sensitivity and selectivity such that when incorporated into a method the required analyses may be carried out with adequate precision and accuracy. 

This selected ion monitoring (SIM) approach typically has greater applicability in cases where sensitivity is more of a concern. Kiehl and Kennington developed a swine liver confirmatory method for tilmicosin that confirmed structure based upon monitoring a parent ion and two additional structural fragment ions. A discussion of the validation requirements for confirmatory methods is provided in Section 6. 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

The most widely regarded approach to accomplish the determination of as many pesticides as possible in as few steps as possible is to use MS detection. MS is considered a universally selective detection method because MS detects all compounds independently of elemental composition and further separates the signal into mass spectral scans to provide a high degree of selectivity. Unlike GC with selective detectors, or even atomic emission detection (AED), GC/MS may provide acceptable confirmation of the identity of analytes without the need for further information. This reduces the need to re-inject a sample into a separate GC system (usually GC/MS) for pesticide confirmation. Through the use of selected ion monitoring (SIM), efficient ion-trap or quadrupole devices, and/or tandem mass spectrometry (MS/MS), modern GC/MS instruments provide LODs similar to or lower than those of selective detectors, depending on the analytes, methods, and detectors. 

LC/MS is used as a multi-residue analytical method. The recovery of imidacloprid from tomato was 90-105% for 0.05 and 0.5mgkg . The LOD for imidacloprid was < 10 pg kg in the full-scan mode and 1 pg kg in the selected-ion monitoring (SIM) mode. ... 

Greaves et al. [74] used a selected ion-monitoring assay method for the determination of primaquine in plasma and urine using gas chromatography-mass spectrometric method and a deuterated internal standard. After freeze-drying and extraction with trichloroethylene, the sample plus internal standard was reacted with Tri Sil TBT (a 3 3 2 by volume mixture of trimethylsilylimidazole, A/O-bis-(trimethylsilylacetamide and trimethylchlorosilane) and an aliquot injected to the gas chromatograph-mass spectrometer. The gas chromatographic effluent was monitored at m/z 403, and m/z 406, the molecular ions of the bis-tetramethylsilane ethers of primaquine and 6-trideuteromethoxy primaquine. 

The isotope dilution gas chromatography-mass spectrometry method described by Lopez-Avila et al. [16] and discussed in section 5.3.1.3 has been applied to the determination of Atrazine in soil. In this method known amounts of labelled Atrazine were specked into soil samples before extraction with acetone-hexane. The ratio of the naturally abundant compound and the stable-labelled isotope was determined by high-resolution gas chromatography-mass spectrometry with the mass spectrometer in the selected ion monitoring mode. Detection limits of 0.1-l.Oppb were achieved. Accuracy was >86% and precision better than 8%. 

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