Selected Ion Monitoring SIM

Selected ion monitoring (SIM) is a technique widely used for trace analysis. In this technique, rather than the mass spectrometer being set to scan over a predefined mass range and record full mass spectra it is set to monitor the intensity of specific m/z values. SIM is used to introduce selectivity into an analysis and improve sensitivity. Sensitivity is enhanced over the full scan mode experiment since in the full scan experiment a large proportion of the scan time is spent recording areas of the spectrum where no ions of interest occur. Ions are still being produced in the ion source but are lost in the mass analyser as it brings others into focus on the detector. In SIM, in a 1 s duty cycle, only a few, i.e. 1-10, ions are selected. Hence, the mass analyser transmits these ions for a longer percentage of the time in which they are being produced and therefore more of the ions of the particular m/z values of interest are recorded.

SIM may also used to introduce selectivity into the experiment. This also has the effect of increasing sensitivity by decreasing the amount of 'chemical noise', i.e. real signal, but not from the compound of interest, observed when peaks of interest elute. The increase in selectively may also be achieved by the use of a double focusing mass spectrometer and high resolution and this may be enhanced by, for example, the use of negative chemical ionization.

An example of the increase in selectivity obtained by the use of SIM combined with NCI from our own laboratory can be seen in the determination of nit rated polycyclic aromatic hydrocarbons (nitro-PAH) in vegetation extracts. Nitro-PAH are absorbed on to vegetation from anthropogenic emissions, however their determination is made complex by the large amount of other compounds extracted from the vegetation by the sample preparation procedure. Figures 4A and B show a comparison between the chromatogram obtained from an extract of bark from a maple tree in an urban region using an ECD and the individual mass chromatograms obtained from the same extract using GC-MS in NCI-SIM mode. The

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Figure 4 A comparison of the chromatograms obtained from the analysis of a complex extract containing nitrated polycyclic aromatic hydrocarbons by gas chromatography (A) using an electron-capture detector and (B) by GC-MS employing negative chemical ionization and selected ion monitoring. Note the increase in specificity afforded by the use of GC-MS under these conditions.

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Figure 4 A comparison of the chromatograms obtained from the analysis of a complex extract containing nitrated polycyclic aromatic hydrocarbons by gas chromatography (A) using an electron-capture detector and (B) by GC-MS employing negative chemical ionization and selected ion monitoring. Note the increase in specificity afforded by the use of GC-MS under these conditions.

quadrupole mass spectrometer used in this case was set to monitor the M"'. ions obtained from 9 nitro-PAH. The complex chromatogram shown in Figure 4A does not allow simple identification of the peaks of interest and the possibility of interferences/peak overlap leads to difficulties when attempting quantification. This can be observed for peak 5 (tr = 34.1 min) in Figure 4A. This peak arises from the presence of 2-nitrofluorene in the bark extract. As can be seen, accurate and precise integration of this peak is made difficult by the presence of peaks with very similar retention time. In contrast the peaks from the nitro-PAH monitored by NCI-SIM can be seen clearly in Figure 4B. Each chromatographic trace in this figure represents the ion current observed from monitoring the m/z value of the M~' ion of a series of nitro-PAH. Peaks are readily integrated for quantification with the 2-nitrofluorene peak (peak 5) appearing well resolved on the m/z 211 trace.

One of many areas in which the use of resolution to introduce specificity into SIM experiments is important is the petroleum industry. Dibenzothiophenes (DBT) have been suggested as marker compounds for oil pollution. However, taking as an example diben-zothiophene itself, this has the same nominal molecular mass (184 Da) as the C4-alkylated naphthalenes which are also present in crude oil. Hence in order to specifically measure dibenzothiophene in crude oil it is necessary to monitor the accurate mass to charge ratio (m/z 184.0347) of its molecular ion at high resolution in a SIM experiment. Figure 5 compares the GC-MS-SIM analysis of dibenzothiophene in crude oil carried out using low and high resolution. As can be clearly seen the C4-naphthalenes are not observed in the high resolution data. The power of such analyses can be seen in Figure 6 which shows a comparison of the GC-HRMS-SIM data obtained from the analysis of methyl and C2 substituted diben-zothiophenes for three different crude oils obtained from two North Sea oil fields. The different crude oils can be clearly distinguished with such data.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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