Suppression effects ionization

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. 

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. 

The effect of the buffer on the efficiency of electrospray ionization was mentioned earlier in Section 4.7.1. This is a good example of the dramatic effect that this may have - good chromatographic separation and ionization efficiency with formic, acetic and propionic acids, and good separation, although with complete suppression of ionization, with trifluoroacetic acid (TFA), the additive used for the protein application described previously. Post-column addition of propionic acid to the mobile phase containing TFA has been shown to reduce, or even... 

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 ... 

Remedy The resulting effects of shifts in ionization equilibrium may be eliminated effectively by the addition of an ionization suppressor, that promptly gives a comparatively high concentration of electrons to the flame. This ultimately results in the suppression of ionization by the respective analyte. 

Atomic absorption spectroscopy is highly specific and there are very few cases of interference due to the similar emission lines from different elements. General interference effects, such as anionic and matrix effects, are very similar to those described under flame emission photometry and generally result in reduced absorbance values being recorded. Similarly, the use of high temperature flames may result in reduced absorbance values due to ionization effects. However, ionization of a test element can often be minimized by incorporating an excess of an ionizable metal, e.g. potassium or caesium, in both the standards and samples. This will suppress the ionization of the test element and in effect increase the number of test atoms in the flame. 

Any ions that need to be introduced into a sample either to prevent ionization or suppression effects (for example, a lanthanum and caesium mixture) or as an internal reference (for example, lithium) are usually incorporated in the diluting fluid in which the samples are prepared. 

While the technique of ionic suppression (or ionization control) is only effective with weakly ionic species, ion-pair chromatography has been developed for strongly ionic species and again utilizes reverse-phase chromatography. If the pH of the solvent is such that the solute molecules are in the ionized state and if an ion (the counter-ion) with an opposite charge to the test ion is incorporated in the solvent, the two ions will associate on the basis of their opposite charges. If the counter-ion has a non-polar chain or tail, the ion-pair so produced will show significant affinity for the non-polar stationary phase. 

Hence an ion-exchange resin cannot be used at a pH that suppresses the ionization of the group, and weak anion exchange resins, for instance, are only effective over the pH range 2-8. The stronger acids and bases, however, are capable of being used over almost the whole pH range. 

Muller, C., Schafer, P., Stortzel, M., Vogt, S., and Weinmann, W. (2002). Ion suppression effects in liquid chromatography-electrospray-ionization transport-region collision induced dissociation mass spectrometry with different serum extraction methods for systematic toxicological analysis with mass spectra libraries. /. Chromatogr. B 773, 47— 52. 

One has to emphasize that MS also is associated with several drawbacks when it comes to bioactivity screening. First of all, the optimum, native conditions for bioactivity screening (pH 7.2, addition of 150 mM sodium chloride) are entirely incompatible with optimum conditions for MS detection which, for ESI-MS, typically require acidic pH values and the presence of organic modifiers to enhance ionization properties of the analytes. Assay development for MS-based assays therefore mainly requires the testing of different assay conditions, particularly the replacement of nonvolatile buffers with MS-compatible volatile buffers. Furthermore, it is essential to monitor ion suppression effects, for example, by the... 

For many elements, the atomization efficiency (the ratio of the number of atoms to the total number of analyte species, atoms, ions and molecules in the flame) is 1, but for others it is less than 1, even for the nitrous oxide-acetylene flame (for example, it is very low for the lanthanides). Even when atoms have been formed they may be lost by compound formation and ionization. The latter is a particular problem for elements on the left of the Periodic Table (e.g. Na Na + e the ion has a noble gas configuration, is difficult to excite and so is lost analytically). Ionization increases exponentially with increase in temperature, such that it must be considered a problem for the alkali, alkaline earth, and rare earth elements and also some others (e g. Al, Ga, In, Sc, Ti, Tl) in the nitrous oxide-acetylene flame. Thus, we observe some self-suppression of ionization at higher concentrations. For trace analysis, an ionization suppressor or buffer consisting of a large excess of an easily ionizable element (e g. caesium or potassium) is added. The excess caesium ionizes in the flame, suppressing ionization (e g. of sodium) by a simple, mass action effect ... 

Finally, we want to emphasize an interesting result of the numerical calculation that has been proven experimentally. As shown in Fig. 6, there exist two pulse sequences (b, c) leading to a small population of level 2. In case (c) most of the population is transferred to level 3 while in case (b) nearly all of the population remains in the initial level. In coherent ion dip experiments case (b) is used as it provides deeper dips due to the more effective suppression of ionization. Using higher laser intensities would allow us to achieve nearly 100% ion dips also in case (c) however, for off-resonant conditions the ion current would be smaller by an order of magnitude than in the pulse sequence of case (b) and dips are more difficult to detect. 

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