Ion suppression

Lastly, establishment of CID conditions for tandem MS analysis (i.e., product ion, precursor-ion, neutral loss, and SRM/MRM) are instrument dependent. It is always advisable to optimize these parameters for each instrument in a laboratory utilizing the parameters available in many review papers as a starting point [22,24,25]. 

Ion suppression is present in both shotgun lipidomics and LC-MS analysis although the mechanism(s) are likely different. In the case of shotgun lipidomics, low abundance and/or less ionizable species are always suppressed in the analysis by the full mass scan due to the coexistence of abundant and/or readily ionizable species. In the case of LC-MS, the varied matrix effect always affects its analysis, regardless of the sensitivity or selectivity of the mass analyzer used [26]. The origin and/or mechanism of ion suppression are still a mystery. 

In samples comprised of multiple components at high concentrations, competition for either space or charge in the process of ionization most likely occurs. Such competition leads to a lower signal of each component (i.e., they suppress each other). This appears to be the case for lipid analysis with direct infusion where there exist many components, and the ionization efficiency depends on the physical properties of each lipid class. Therefore, the current dogma is that ion suppression is present in all methods employing direct infusion. In turn, the presence of ion suppression leads to inaccurate quantification of lipid species. Thus, it is sometimes argued that lipids cannot be quantified by using a direct infusion approach, which can only provide a 

Actually, a similar phenomenon to this steady-state ion suppression in shotgun lipidomics is also present in any method developed with LC-MS for quantitative analysis of lipid mixtures. For example, if it is intended to quantify a species of a minor lipid class in the presence of other abundant species [24], the amount of total lipids that can be loaded onto a column are capped by the upper limit of the linear dynamic range of the most abundant species in the mixture under the experimental conditions. The loaded amount of total lipids to expand the linear dynamic range of the minor component in the method cannot be increased greatly if there is a need for quantification of major components as well. Of course, the minor species can be analyzed separately with a pre-isolated fraction or with a saturated concentration of the abundant species to increase the dynamic range for quantification of the minor components. 

In addition to the steady-state ion suppression, there exists another complication of ion suppression for any method based on LC-MS and LC-MS/MS if the species either within a class or between classes cannot be completely resolved. Since the concentration of each individual species constantly changes during elution from the column, competition for either space or charge during ionization most probably occurs, particularly for the other species at the same retention time. To distinguish 

The interference of co-eluting compounds with the ionization of the analytes of interest is the most frequent reason for insufficient response and reproducibility in LC/MS. The phrase ion suppression was introduced in the mid-1990s and describes the phenomenon only partially, as an enhancement of the signal may also occur. Methods with little sample preparation and/or little chromatographic separation are especially prone to this. Detection methods that monitor only single ions or MS/MS transitions (SIM, MRM) are quasi-blind to co-eluting compounds, while the detection of complete mass spectra allows the recognition of massive co-elution so that corrective action can be taken. 

After the initial euphoria about the high selectivity of LC/MS-based methods had subsided, some disillusionment followed. It was immediately recognized that compounds that are invisible to MS detection are not always inactive . The matrix effect is exerted solely in the ion source and is therefore independent of the 

The matrix effect can be produced by various processes - hindering of the analyte ion release in electrospray, e.g., by alkali or phosphate, micelle formation by tensides, formation of ion pairs or even precipitation of the analytes. It is important that efforts are made during method development to recognize and exclude ion suppression or matrix effects. 

Various experimental approaches are available for evaluating a matrix effect. The easiest way is to compare a solvent standard with a matrix spike . The relevant effects are shown schematically in Fig. 4 (B. K. Choi et al.,J. Chromate. A 907, 337-342 (2001)). 

Ideally, the sample matrix should have no influence on the analyte signal. It is recommended that tests are carried out with matrix blanks of different origins to exclude random results. The approach described in Fig. 4 can be extended with little effort for the additional determination of the method recovery and extraction yield. The required steps are depicted in Fig. 5. 

---

Chromatographic methods, notably hplc, are available for the simultaneous deterrnination of ascorbic acid as weU as dehydroascorbic acid. Some of these methods result in the separation of ascorbic acid from its isomers, eg, erythorbic acid and oxidation products such as diketogulonic acid. Detection has been by fluorescence, uv absorption, or electrochemical methods (83—85). Polarographic methods have been used because of their accuracy and their ease of operation. Ion exclusion (86) and ion suppression (87) chromatography methods have recently been reported. Other methods for ascorbic acid deterrnination include enzymatic, spectroscopic, paper, thin layer, and gas chromatographic methods. ExceUent reviews of these methods have been pubHshed (73,88,89). 

The presence of a sufficientiy strong chelating agent, ie, one where K in equation 26 is large, keeps the concentration of free metal ion suppressed so that pM is larger than the saturation pM given by the solubiUty product relation (eq. 29) and no soHd phase of MX can form even in the presence of relatively high anion concentrations. The metal is thus sequestered with respect to precipitation by the anion, such as in the prevention of the formation of insoluble soaps in hard water. 

A computer program was compiled to work out the ray-tracing of UV detector of high performance capillary electrophoresis at the investigation of 5 and 6 (98MI59). The capacity factor of 5 at different temperature and at different mobile phase compositions was experimentally determined in bonded-phase chromatography with ion suppression (98MI15). 

This explains the increase in the induction period which is apparent after exposure of the salt to ammonia, and the decrease in the induction period found for samples which contain traces of HCIO4, identified as the unstable species [59,925]. In the low temperature range, the presence of an outer layer of adsorbed NH3 and/or NH4 ions suppresses the formation of HC104 and, in consequence, the decomposition reaction. 

The other fragmentation pathways are typical for diaryl sulfoxides1-4-6,1. A corresponding ortho effect was found in chlorodiphenyl ethers and sulfides but not in sulfones12 (12) were the sulfinate ester rearrangements1-4,6,11 and the consequent formation of the m/z 125 and m/z 159 ions suppress the other possible fragmentations of the molecular ions (equation 4). It is also noteworthy that the ratio [m/z 125] [m/z 159] increases with increasing distance between the chlorine and the sulfur (equation 4). 

A potential problem encountered in these determinations is the ion suppression encountered in the presence of polar/ionic interfering materials which compete with the analyte(s) for ionization (see Section 4.7.2 earlier). Many of these analyses therefore involve some degree of off-line purification and/or an appropriate form of chromatography. Since it is not unusual to encounter closely related compounds that are not easily separated, it is also not unusual to employ both of these approaches, as in the following example. This illustrates the use of HPLC as a method of purification and demonstrates that highly efficient separations are not always required for valuable analytical information to be obtained. 

Both solubilities are low, as we would expect for a salt with a small value of. S sp. Notice that PbCl2 is about 350 times less soluble in the NaCl solution. This makes sense in terms of the common-ion effect. The excess chloride ion suppresses the solubility of Pb by Le Chatelier s principle. The actual concentration of lead in seawater is much less than 4.0 X 10 M. This is because other lead salts are much less soluble than lead(II) chloride. The ocean contains carbonate, for example, and. STsp for lead(II) carbonate is quite small, 7.4 X lO ". ... 

Similarly to the methods used to characterize natural chlorophylls, RP-HPLC has been chosen by several authors to identify the individual components in Cn chlorophyllin preparations and in foods. The same ODS columns, mobile phase and ion pairing or ion suppressing techniques coupled to online photodiode UV-Vis and/or fluorescence detectors have been used. ° ... 

Shioi, Y, Doi, M., and Sasa, T., Separation of non-esterified chlorophylls by ion-suppression high-performance liquid chromatography, J. Ghromatogr., 298, 141, 1984. 

As already mentioned, a challenge in the application of ESI, at least for quantification, is ion suppression. Another example of matrix effects is observed in the... 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

The sample drawn from the reactor consisted of an acid, several amines, and a neutral species. Two of the components were not resolved to baseline by reversed phase LC, so a dual column reversed phase ion-suppression/ ion-exchange technique was used. A chromatogram of the separation is shown in Figure 5. 

Silica-based columns are only stable in the range of pH 2 to 8, so ion suppression is particularly useful for weak acids and bases. The advent of... 

Many, if not most, pharmacologically active compounds are amines. For this reason, issues in the RPLC of substances of pharmaceutical interest tend to be similar to those encountered in the separation of amines. Incompletely endcapped silica-based phases may exhibit tailing. The use of ion pairing or ion suppression is common in the analysis of pharmacologically active substances. Also, derivatization of the amine functionality prior to analysis may be required. The RPLC retention indices of most common pharmaceutical compounds have been compiled.97... 

There is no need for perfect separation in the eluting peak MALDI can improve the resolution of chromatography. Semi on-line SEC-MALDI-ToFMS makes allowance for the separation of polymer, oligomers and additives. Ion suppression has been noticed for... 

---