Ion selection for

An improved specificity was observed when FIA-MS-MS in product or parent ion mode was used to perform quantification of the surfactants in the methanolic mixtures. The ions selected for quantitation in product or parent ion mode were C13-AE m/z 71, 85, 99, 113, and 127 from alkyl chain together with 89, 133, and 177 from PEG chain generated from parent ions m/z 394, 526, 658, 790 and 922 alkylbenzyl dimethyl ammonium quat m/z 91 and 58 generated from parent ion m/z 214 FADA m/z 88, 106 and 227 generated from parent ions m/z 232, 260, 288, 316, 344 and 372 while the alkylamido betaine was quantified generating the parent ion m/z 343 obtained from product ion at m/z 240. 

LITs capable of scanning, axial or radial excitation of ions, and precursor ion selection for MS/MS experiments [118,134-136] have lately been incorporated in commercial mass spectrometers (Fig. 4.39). The replacement of Q3 in a QqQ instrument with a scanning LIT, for example, enhances its sensitivity and offers new modes of operation (Applied Biosystems Q-Trap). Introduction of a scanning LIT [118,135] as MSI in front of an FT-ICR instrument (Thermo Electron LTQ-FT) shields the ultrahigh vacuum of the FT-ICR from collision gas and decomposition products in order to operate under optimum conditions. In addition, the LIT accumulates and eventually mass-selects ions for the next cycle while the ICR cell is still busy with the previous ion package. 

Table I. MID Ions Selected for Each Semi>Volatile Brominated Compound...

In spite of the simple theoretical treatment of the electrochemical alkali ion selectivity for liquid membranes incorporating electrically neutral ligands, nothing comparable has been achieved for negatively charged ligands. As shown in Table 6 for this type of membrane, two completely different expressions for the selectivity constant can be derived (27, 25, 26), depending on which assumptions are made ... 

Fig. 17. Experimental ion selectivity for macroheterobicyclic ligand III (Table 4) and nonactin (Table 2)...

Table 11 Representative Ion Selectivities for Ionophorous Agents and Crown Ethers...

Quantitative results were produced for each compound on the basis of internal standard method calculations. A three-point calibration curve was generated for each compound by using peak areas of a quantitation ion extracted from the mass spectrum of the compound. The ion was selected on the basis of it being a uniquely characteristic mass of the compound. The use of extracted ion quantitation produces more accurate results than total ion-current quantitation in cases in which two or more components are not completely resolved chromatographically. This situation is generally the case in complex mixture analysis. The quantitation ions selected for each of the compounds in the mix are listed in the box. 

Determination of MS/MS conditions using the MALDI ion source coupled to the triple-quadrupole mass spectrometer takes a different approach (Kovarik et al., 2003) which at present is not as easily automated. As mentioned earlier, some product ion spectra will contain additional fragment ions not related to the analyte due to isobaric matrix peaks. However, MS/MS conditions previously obtained by using an ESI-MS/MS system can be directly ported over to the MALDI triple-quadrupole mass spectrometer. These conditions provide the advantage of product ion selection for SRM of the analyte. In the typical high-throughput environment, individual methods for each chemical entity are not normally utilized. Rather, the semioptimized template-style methods referred to above are often used wherein a few values of collision energy are combined with the appropriate SRM and polarity. These methods are ported to the MALDI triple quadmpole mass spectrometer, and no further... 

The ions selected for such recording can be one, two, three, or more (multiple ion recording). In fact, through judicious choice of ions, the method can be so selective that a chosen component can be identified and quantified even though it could not even be observed in the original total-ion-current chromatogram. This very powerful technique is frequently used to examine extremely complex mixtures when the goal is to identify small amounts of a particular substance in a mass... 

For the six sulfonylurea herbicides included in Figure 1, we monitored the protonated molecular ion fen each herbicide in addition to one or two major fragment ions. Table 1 shows the ions selected for each of the sulfonylurea and Figure 4 shows the ion traces for each compound. The table shows two ions which are common for some of these sulfonylurea herbicides. HARMONY, ALLY and GLEAN contain the same triazine urea ion at m/z 184 while ALLY, OUST and EXPRESS contain the same sulfonamide ions at m/z 233. Selecting these common ions for quantitation will increase the overall sensitivity for multiresidue analysis. 

This method relies on the mass selectivity of the mass spectrometer in addition to the LC separation. No interferences were detected with any of these selected ions. The ions selected for quantitation were the most intense ions to obtain good sensitivity. Structure confirmation is inherent in an LC/MS method due to die structural information of the selected ion and their relative abundance. In addition LC/MS offers the LC chromatographic confirmation based on retention time relative to the reference standard. 

Selected ions should be structurally specific to the analyte and should be abundant in order to maximize the assay sensitivity. In the example of the assay for salicylic acid in tomato (Figure 14), the ion selected for MS2 was the structurally specific [M —CH3]+ ion for both the analyte and the IS. The ion at m/z 91, although intense, is a tropylium ion (C7H7+ ) and would be an inappropriate selection as it would be present in most analytes containing a benzyl moiety. The ions at m/z 223 and m/z 209 in the product spectra (Figure 16) were chosen for the quantification because they were the most intense. 

Note that three water molecules (ligands) from the coordination sphere of the metal ion are replaced by one nitrogen and two oxygen donor atoms in the imino-diacetate functionality. The arrows indicate the metal-ligand or LAB interaction and the high metal ion selectivity for this type of functional group is often attributed to the accompanying coordination... 

To characterize high metal ion selectivity for chelating ion exchangers from a thermodynamic perspective, the metal ion uptake can be divided into two consecutive steps— ion exchange (IX) followed by LAB interaction, i.e.. 

Le Blanc, Y.C.Y Bloomfield, N. Dynamic Background Subtraction to Improve Candidate Ion Selection for Information Dependant Acquisition LC-MS/MS Analysis, in Proceedings of the 52nd ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN, May 23-27, 2004. 

The hexapeptide, cycb( L-Pro-Gly)3, has been shown to form complexes with a number of metal ions175 The compound exhibits ion selectivity for Li+ and Na+ over K+ and larger alkali metal ions. It also forms a Ca++ complex which has a stability constant in acetonitrile of stab = 1.1 x 10s M 1. With Mg++ three different complexes with cyclopeptide-cation stoichiometries of 2 1,1 1, and 1 2 are formed. Hypothetical structures of these complexes (Fig. 45) have been proposed which are reminiscent of the enniatin sandwich complexes. 

Ions selected for measurements should be of high abundance... 

Analytical schemes concerned with the determination of blood ions and gases can be divided into two categories analyses done in vivo and those done in vitro. By far the most common method of determining blood ions in vitro involves atomic spectroscopy. Atomic absorption and flame emission have both been used although the latter is the most popular. In the clinical lab nearly all of the remaining determinations (both in vivo and in vitro) are performed with ion-selective (for ions, NH3 and CO2) or amperometric electrodes (O2 and H2). Two important characteristics of ion-selective electrodes, sensitivity and selectivity, should be mentioned. The applicability of a specific electrode in any particular situation can be determined by considering, on one hand, the ionic constituents of the solution to be measured and, on the other hand, the sensitivity and specificity of the electrode in question. Proper consideration of these points will allow an investigator to determine the accuracy and validity of the measurement. 

Here, Q is the initial solution concentration and Cf is the final solution concentration. Comparison of the k values for the imprinted and control blank gels can show the effect of imprinting on the metal-ion selectivity for a given material. A measure of the increase in selectivity due to molecular imprinting can be defined by the ratio of the selectivity coefficients of the imprinted (MIP) and nonimprinted (NP) materials ... 

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