Suppression of ionization

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... 

A detailed description of sources used in atmospheric pressure ionization by electrospray or chemical ionization has been compiled.2 Atmospheric pressure has been used in a wide array of applications with electron impact, chemical ionization, pressure spray ionization (ionization when the electrode is below the threshold for corona discharge), electrospray ionization, and sonic spray ionization.3 Interferences potentially include overlap of ions of about the same mass-charge ratio, mobile-phase components, formation of adducts such as alkali metal ions, and suppression of ionization by substances more easily ionized than the analyte.4 A number of applications of mass spectroscopy are given in subsequent chapters. However, this section will serve as a brief synopsis, focusing on key techniques. 

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. 

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 ... 

Although HPMC is not thought itself to be pH sensitive [6], the pH of a dissolution fluid is known to affect release rates of drugs from its matrices via the suppression of ionization [7]. The cloud points at 2% K100 gels (Table 1) were only affected by pH at low pHs. It was therefore considered unnecessary to modify the pH of electrolyte solutions used to determine cloud points. 

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. 

All TOF mass spectrometers measure accurate mass by reference to standard substances, otherwise known as lock mass. This is an internal reference which is introduced via the ion source by means of a second sprayer system (Fig. 4.11) or coinfused with the sample using a single sprayer (Williams et al., 2006). However, the latter method is not preferred due to dilution of analyte with reference compound and issues of suppression of ionization (Herniman et al., 2004). The reference compound... 

To find a compromise for a mobile phase with neither too large a chain length (because of slow equilibration) nor too high a modifier content (because of the suppression of ionization), but yet optimum capacity factors and stable operating conditions is an optimization problem on its own. 

Suppression of ionization of a weak electrolyte by the presence in the same solution of a strong electrolyte containing one of the same ions as the weak electrolyte. 

Certain types of traditional LC mobile phase additives should be avoided due to nonvolatility and ion suppression effects. Mobile-phase related ion suppression will not depend on the analyte proximity to the solvent front, or capacity factor. These additives include detergents surfactants ion pairing agents inorganic acids such as sulfuric, phosphoric, hydrochloric, and sulfonic acids nonvolatile salts such as phosphates, citrates, and carbonates strong bases and quaternary amines. Complete suppression of ionization as well as interferences in both positive and negative ion mode will occur when these agents are utilized. 

The Q-parameter for the sulfonate monomer, which is indicative of its general reactivity13, did not change in the range of pH 7-1.5, but the e-parameter, in which the polar effect of the substituent group on the reactivity of the monomer (as well as that of the radical derived from it) is reflected, increased threefold at the low pH value. This was attributed to suppression of ionization at pH 1.5, causing a shift in the e-parameter towards a more positive value. 

The interface between the two systems includes the 1 torr region where deposition of products can occur. Severe suppression of signal has been observed when high concentrations of dissolved solids are present. This may be caused by suppression of ionization by other more easily ionized elements present in the sample. The exact cause of this interference is not clear, but the fact remains that interference does take place. The problem can be overcome to some extent by limiting the concentration in the samples to less than 0.2% total solids. This can be a serious limitation, particularly when body fluids or fused minerals are being examined. 

C.E. Carroll, RT. Hioe, Coherent population transfer via the continuum, Phys. Rev. Lett. 68 (1992) 3523 Selective excitation via the continuum and suppression of ionization, Phys. Rev. A 47 (1993) 571. 

Suppression of ionization efficiency is important when the total ionizing capability of the ionization technique is limited, so that there is a competition for ionization among compounds that are present in the ion source simultaneously. In principle such a saturation effect must be operative for all ionization techniques, but in practice it is most important for electrospray ionization (Section 5.3.6), slightly less important for atmospheric pressure chemical ionization (Section 5.3.4), atmospheric pressure photoionization (Section 5.3.5) and matrix assisted laser desorption ionization (Section 5.2.2) it does not appear to be problematic under commonly used conditions for electron ionization and chemical ionization (Section 5.2.1) or thermospray (Section 5.3.2). Enhancement of ionization efficiency for an analyte by a co-eluting compound is less commonly observed and is, in general, not well understood. 

There has been a great deal of discussion of matrix effects on ionization efficiency in previous chapters, particularly Section 5.3.6a. (Not being discussed here is matrix interference , i.e. direct contributions from matrix components to the analytical signals in the SIM or MRM channels used to monitor the analyte and SIS these interferences are easily detected in analyses of extracts of control matrix or incurred sample). Suppression of ionization efficiency by co-eluting matrix components is the more common, but enhancement is also observed (Section 5.3.6a). While it is true that operating under conditions of total solvent consumption , e.g. by using very low flow rates for ESI combined with additional heating, can reduce matrix... 

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