Ionisation suppression

Ionic strength adjuster buffer 565, 570 Ionisation constants of indicators, 262, (T) 265 of acids and bases, (T) 832, 833, 834 see also Dissociation constants Ionisation suppressant 793 Iron(II), D. of by cerium(IV) ion, (cm) 546 by cerium(IV) sulphate, (ti) 382 by potassium dichromate, (ti) 376 by potassium permanganate, (ti) 368 see also under Iron... 

Hsiao et al. [11] have studied the use of MALDI ionisation for the detection of antioxidants and hindered amine light stabilisers (HALS) in polyethylene extracts. Using 2,5-dihydroxybenzoic acid as matrix, diagnostic spectra were obtained on standards, but the presence of soluble low molecular weight polyethylene in extract solutions caused some problems with ionisation suppression. 

Ionisation suppression caused by co-elution of other analytes or natural matrix components or due to the presence of ion pairing (IP) agents or high salt concentrations in the mobile-phase buffer used [2-4],... 

Therefore, the way to ensure reproducible adduct formation is to use mobile-phase additives (e.g. ammonium acetate or formate, formic, acetic or trifluoroacetic acid (in APCI), ammonium hydroxide, etc.). Their application in the mobile phase can be an effective way to improve the intensity of the MS signal and LC-MS signal correlation between matrix and standard samples. However, it is observed that some additives like trifluoroacetic acid or some ion-pairing agents (triethyl-amine) may play a role in ionisation suppression [3]. In addition, high concentrations of involatile buffers will cause precipitation on, and eventually blocking of, the MS entrance cone, leading to a fast decrease of sensitivity. For the in volatile NaAc buffer, it is advisable to maintain... 

Finally, the use of extremely hot flames with certain elements can cause ionisation of the latter, which decreases the concentration of free atoms in the flame. This effect can be corrected by adding an ionisation suppresser in the form of a cation whose ionisation potential is less than that of the analyte. A potassium salt at the level of 2 g, 1 is often chosen as an ionisation suppresser. 

Typical of these samples are raw and treated waters, seawater, biological fluids, beer, wines, plating solutions, effluents, etc. With this type of sample very little preparation is usually required. If the solution is suitable for aspiration then its approximate concentration can be determined, to check whether dilution with water is necessary. Degassing may be required, and/or the addition of releasing agents, ionisation suppressants, complexing agents, etc., as required for interference compensation. Concentration methods will be described later. 

The alkali and alkaline earth elements are easily ionised in the flame. It is therefore necessary to add an ionisation suppressant when analysing for these elements. The potassium salt of naphthasulphonic acid or a commercially available potassium standard solution must be added to give a final potassium concentration of approximately 1000/igml-1 in both samples and standards. 

When determining the alkali and alkaline earth elements it is necessary to add an ionisation suppressant, usually potassium, at approximately 1000 /ig mr1 to both standards and samples. 

Using either commercially available metal in oil standards or organometallic standards, prepared as described in Section III, prepare calibration standards by dilution with white spirit. Standards for Ca, Ba and Mg must contain 1000/igKml-1 as ionisation suppressant. Choose a concentration range for each element which exhibits an approximately linear response. 

The standards are best prepared with a matching sodium chloride content (15 gl 1) to minimise errors from differing viscosity and ionisation suppression. 

One of the most undesirable processes that can occur during atmospheric pressure mass spectrometric analysis is a nonlinear decrease of ionization by sample or mobile phase. This ion suppression, or ionisation suppression, is an effect whereby the extent of ionization for an analyte is decreased due to competition between analyte and sample matrix components within the atmospheric pressure ion source. Studies have shown ion suppression to be a somewhat proportional effect [19]. That is, a quasilinear relationship is observed between the amount of salt present in a sample and the loss of analyte molecular ion signal until a limiting amount of salt is reached, whereby the response is constant with increasing... 

Chromatographic ionisation suppression is useful for increasing the retention of organic acids in reverse-phase systems. With this approach, the apparent pH of the mobile phase is decreased by the addition of acetic or formic acid. 

At some point, however, assay recovery for an LC/MS method will be estimated and when this is done several important points are worth keeping in mind. Over the instrument linear range, mass spectral response is proportional to the number of ions which have entered the high vacuum region of the instrument. This number of ions is, in turn, dependent on how efficiently an analyte is ionized in the source. For a given analyte, the extent of ionization dictates its ability to compete with the matrix components present. As the amount of ionizable matrix components increases, the ability of an analyte to ionize, hence its apparent concentration, is diminished. This so-called ionisation suppression, or ion suppression, can lead to nonquantitative behavior in LC/MS [51,52],... 

Direct injection (infusion) MS of unpurified samples has also been used as a metabolomics technique since it uses a single unpurified extract to examine all ions in a sample to produce a single spectrum. Although it has some drawbacks, such as ionisation suppression, it is considered useful for reducing data complexity especially when used with accurate mass, as long as the resolution of the instmment is sufficient to distinguish ions with the same nominal, but different monoisotopic mass (Dunn et al, 2005). 

At the start the cathode is invariably a metal different from that to be deposited. Frequently, the aim is to coat a base metal with a more noble one, but it may not be possible to do this in one step. When a metal is immersed in a plating bath it will corrode unless its potential is sufficiently low to suppress its ionisation. Fortunately, a low rate of corrosion is tolerable for a brief initial period. There are cases where even when a cathode is being plated at a high cathodic (nett) current density, the substrate continues to corrode rapidly because the potential (determined by the metal deposited) is too high. No satisfactory coating forms if the substrate dissolves at a high rate concurrently with electrodeposition. This problem can be overcome by one or more of the following procedures ... 

Thus, for example a solution containing potassium ions at a concentration of 2000 mg L "1 added to a solution containing calcium, barium, or strontium ions creates an excess of electrons when the resulting solution is nebulised into the flame, and this has the result that the ionisation of the metal to be determined is virtually completely suppressed. 

Desorption/ionisation techniques such as LSIMS are quite practical, as they give abundant molecular ion signals and fragmentation for structural information. In the conditions of Jackson et al. [96], all the molecular ion and/or protonated molecule ion species were observed in the LSIMS spectrum when only 1 pmol of each additive was placed on the probe tip. However, as mentioned above, in LSIMS/MS experiments the choice of the matrix (e.g. NBA, m-nitrobenzylalcohol) is very important. Matrix effects can lead to suppression of the generation of molecular ions for some additives. LSIMS is not ideal for the quantitative detection of polymer additives, as matrix effects are very important [96]. 

Table 6.18 lists the main characteristics of FD-MS. FD is a superior ionisation technique for quantitative analysis, as there are no matrix effects as in LSIMS or MALDI which might suppress the generation of ions from certain additives. However, the technique has some serious drawbacks. The primary difficulty is that FD produces only short-lived, highly variable currents of analyte ions. These analyte ion currents are also very... 

We want the acids to be present only as ions, as the two forms of each acid will have different retentions on the ion-exchange stationary phase. As a rough guide, to suppress ionisation completely, we want to buffer at pH = (pKa - 1.5) and to cause complete ionisation we need pH = (p/Ca + 1.5). 

To recognise ion suppression reactions, the AE blend was mixed together either (Fig. 2.5.13(a) and (b)) with the cationic quaternary ammonium surfactant, (c, d) the alkylamido betaine compound, or (e, f) the non-ionic FADA, respectively. Then the homologues of the pure blends and the constituents of the mixtures were quantified as presented in Fig. 2.5.13. Ionisation of their methanolic solutions was performed by APCI(+) in FIA-MS mode. The concentrations of the surfactants in the mixtures were identical with the surfactant concentrations of the blends in the methanolic solutions. Repeated injections of the pure AE blend (A 0-4.0 min), the selected compounds in the form of pure blends (B 4.0—8.8 min) and their mixtures (C 8.8— 14.0 min) were ionised and compounds were recorded in MID mode. For recognition and documentation of interferences, the results obtained were plotted as selected mass traces of AE blend (A b, d, f) and as selected mass traces of surfactant blends (B a, c, e). The comparison of signal heights (B vs. C and A vs. C) provides the information if a suppression or promotion has taken place and the areas under the signals allow semi-quantitative estimations of these effects. In this way the ionisation efficiencies for the pure blends and for the mixture of blends that had been determined by selected ion mass trace analysis as reproduced in Fig. 2.5.13, could be compared and estimated quite easily. 

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