Suppression effect

Detection limits in ICPMS depend on several factors. Dilution of the sample has a lai e effect. The amount of sample that may be in solution is governed by suppression effects and tolerable levels of dissolved solids. The response curve of the mass spectrometer has a large effect. A typical response curve for an ICPMS instrument shows much greater sensitivity for elements in the middle of the mass range (around 120 amu). Isotopic distribution is an important factor. Elements with more abundant isotopes at useful masses for analysis show lower detection limits. Other factors that affect detection limits include interference (i.e., ambiguity in identification that arises because an elemental isotope has the same mass as a compound molecules that may be present in the system) and ionization potentials. Elements that are not efficiently ionized, such as arsenic, suffer from poorer detection limits. 

EINSTein s combat adjudication engine also includes weapon and ammunition selection, suppression effects, lethality contours, and individual- and squad-based targeting strategics. 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

Cromones suppress the release of mediators from mast cells by a mechanism that is not known (Fig. 4). In order to achieve the complete suppressive effect, cromones have to be given prophytactically several days to weeks before exposure to seasonal allergens can be expected,... 

The appetite-suppressing effect of (3-phenylethylamine drugs is either related to their sympathomimetic effect (metamphetamine, phentermine, diethylpropion), to... 

A major difference between MALDI and FAB is that a solid rather than a liquid matrix is used and a mixture of the analyte and matrix is dried on the laser target. For this reason, the effective combination of HPLC with MALDI is not as readily achieved although, since MALDI is largely free of the suppression effects experienced with FAB, it is able to provide useful analytical data directly from mixtures. 

Ions at m/z 55, 60, 214 and 236 are observed but do some or all of these arise from the background and are present throughout the analysis, or are they present in only a few scans, i.e. are they from a component with insufficient overall intensity to appear as a discrete peak in the TIC trace An examination of reconstructed ion chromatograms (RICs) from these ions generated by the data system may enable the analyst to resolve this dilemma. The TIC shows the variation, with time, of the total number of ions being detected by the mass spectrometer, while an RIC shows the variation, with time, of a single ion with a chosen m/z value. The RICs for the four ions noted above are shown in Figure 3.15. These ions have similar profiles and show a reduction in intensity as analytes elute from the column. The reduction in intensity is a suppression effect. 

Suppression effects are experienced in static FAB, with signals from more hydrophilic materials being reduced compared to those from hydrophobic components. There are fewer suppression effects in dynamic FAB and this is of benefit when it is not possible to achieve complete chromatographic resolution. 

The pressure difference between the source of the mass spectrometer and the laboratory environment may be used to draw a solution, containing analyte and matrix material, through the probe via a piece of capillary tubing. When an adequate spectrum of the first analyte has been obtained, the capillary is simply placed in a reservoir containing another analyte (and matrix material) and the process repeated. This may therefore be used as a more convenient alternative to the conventional static FAB probe and this mode of operation may also benefit from the reduction in suppression effects if the analyte is one component of a mixture. 

The electrospray process is susceptible to competition/suppression effects. All polar/ionic species in the solution being sprayed, whether derived from the analyte or not, e.g. buffer, additives, etc., are potentially capable of being ionized. The best analytical sensitivity will therefore be obtained from a solution containing a single analyte, when competition is not possible, at the lowest flow rate (see Section 4.7.1 above) and with the narrowest diameter electrospray capillary. 

The suppression effects associated with electrospray ionization have been discussed earlier although if the compounds present are similar in behaviour these may be minimal. The intention, when using chromatography as an introduction device, is to allow individual components to enter the mass spectrometer for analysis. The separation capability of HPLC has been discussed previously and it is not unusual, particularly when complex mixtures are being studied, to encounter electrospray spectra from more than one component. 

Suppression effects may be observed and the direct analysis of mixtnres is not always possible. This has potential implications for co-eluting analytes in LC-MS. 

MALDI-ToF is a technique that allows the molecular weights of proteins and peptides to be determined. It is less susceptible to suppression effects than electrospray ionization and thus is able to be used for the direct analysis of mixtures. In the case of a crude tryptic digest, MALDI-ToF will provide a molecular weight profile of the polypeptides present without the analysis time being extended by the need to use some form of chromatographic separation. 

It is well known that electrospray ionization (El) suffers from suppression effects when polar/ionic compounds other than the analyte(s) of interest, such as those originating from the sample matrix, are present, with this phenomenon being attributed to competitive ionization of all of the appropriate species present [33]. Matrix effects can, therefore, be considerable and these have two distinct implications for quantitative procedures, as follows ... 

Table 5.16 LC-MS-MS signal responses" obtained from wheat forage matrix samples using various mobile-phase additives (injection volumes of 50 p,l). From Choi, B. K., Hercules, D. M. and Gusev, A. I., LC-MS/MS signal suppression effects in the analysis of pesticides in complex environmental matrices , Fresenius J. Anal. Chem., 369, 370-377, Table 2, 2001. Springer-Verlag GmbH Co. KG. Reproduced with permission...

Four methods of overcoming suppression effects were discussed in this paper, as follows ... 

Suppression effects The decrease in analyte signal intensity brought about by the presence of extraneous materials in the sample. 

The suppressing effect of the acetic acid can be interpreted by reactions (101) and ... 

GAO Y H and YAMAGUCHI M (2000) Suppressive effect of genistein on rat bone osteoclasts Involvement of protein kinase inhibition and protein tyrosine phosphatase activation. Int J Mol Med 5, 261-7. 

Reeve, V.E. et al.. Suppressive effect of 2-acetyl-4-tetrahydroxybutylimidazole on contact hypersensitivity in the Skh HR hairless mouse, Int. Arch. Allergy Immunol, 102, 101, 1993. 

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