Ionization analyte

Electrospray and APCI spectra consist predominately of molecular species and the ways in which structural information may be generated from analytes ionized by these techniques have been considered in some detail. 

The use of TFA as a mobile-phase additive in LC-MS can be problematical when using electrospray ionization. In negative ion detection, the high concentration of TFA anion can suppress analyte ionization. In positive ion detection, TFA forms such strong ion pairs with peptides that ejection of peptide pseudo-molecular ions into the gas phase is suppressed. This problem can be alleviated by postcolumn addition of a weaker, less volatile acid such as propionic acid.14 This TFA fix allows TFA to be used with electrospray sources interfaced with quadrupole MS systems. A more convenient solution to the TFA problem in LC-MS is to simply replace TFA with acetic or formic acid. Several reversed-phase columns are commercially available that have sufficient phase coverage and reduced levels of active silanols such that they provide satisfactory peptide peak shapes using the weaker organic acid additives.15... 

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 ... 

Sample pretreatment is useful when a mass spectrometer with atmospheric pressure electrospray ionization (ESI/MS) is used as the detector for ion chromatography. High ionic strength matrices are known to suppress analyte ionization and cause poor reproducibility in ESI/MS [40]. We investigated the use of off-line sample pretreatment to remove chloride in order to improve sensitivity in... 

The primary amino group of the analyte ionizes in the presence of acid and forms ammonium ion (—NH3+). The ammonium ions form strong hydrogen-bondings with the oxygen atoms of CCE. Therefore, the presence of an acid in the mobile phase is essential to achieve the chiral resolution on these CSPs. However, recently, Aboul-Enein et al. [75] observed very interesting results for the chiral resolution of thyroxine and tocainide racemates on the (+)-(18-crown-6)-2,3,ll,12-tetracarboxylic acid CCE. The authors reported the chiral resolution of these molecules without using an acid in the mobile phase. Moreover, they... 

Is the sample matrix miscible primarily with water or organic solvents If the sample matrix is water soluble, is the analyte ionized or non-ionized ... 

Because the sensitivity of the detector decreases with decreasing analyte ionization, the pH of the mobile phase should be chosen to maximize solute dissociation. For example, anions with pKa values above 7 are not detectable by conductivity detection. However, conductivity detection is often the preferred method for organic acids with carboxylate, sulfonate, or phospho-nate functional groups, since the pKa values are below 5. For cations, most aliphatic amines have pKa values around 10 and are readily detected by conductivity detection. The pKa values of aromatic amines, however, are in the range 2 to 7, which is too low to be detected by suppressed conductivity. Sensitivity by nonsuppressed conductivity is also poor, so these amines are monitored by UV absorption or pulsed amperometric detection. 

The AP-MALDI source is illustrated in Figure 1.17. It works in a similar manner to the conventional MALDI source. The same sample preparation techniques and the same matrices used for conventional vacuum MALDI can be used successfully for AP-MALDI. The main difference is the pressure conditions where ions are produced. Conventional MALDI is a vacuum ionization source where analyte ionization takes place inside the vacuum of the mass spectrometer whereas AP-MALDI is an atmospheric ionization source where ionization occurs under atmospheric pressure conditions outside of the instrument vacuum. 

Liquid chromatography mass spectrometry (LC-MS) is now routinely used in analytical laboratories. Traditional IPRs are non-volatile salts that are not compatible with MS techniques because they play a major role in source pollution that is responsible for reduced signals. Moreover the final number of charged ions that reach the detector is impaired by ion-pair formation actually IPRs added to the mobile phase to improve analytes retention exert a profound effect on analyte ionization. Chromatographers who perform IPC-MS must optimize the eluent composition based on both chromatographic separation and compatibility with online detection requirements. 

It is a common practice in IPC to buffer the mobile phase at a suitable pH to control analyte ionization and avoid analyte acid-base equilibrium that would be deleterious for peak shape. For a long time, inorganic components of a buffer were considered... 

Equation (1-7) shows that in an ideal case the selectivity of the system is only dependent on the difference in the analytes interaction with the stationary phase. It is important to note that the energetic term responsible for the eluent interactions was canceled out, and this means that the eluent type and the eluent composition in an ideal case does not have any influence on the separation selectivity. In a real situation, eluent type and composition may influence the analyte ionization, solvation, and other secondary equilibria effects that will have effect on the selectivity, but this is only secondary effect. 

Most chromatographic systems with absence of the secondary equilibria effects (such as analyte ionization, specific interactions with active adsorption sites, etc.) show linear dependencies of the logarithm of the retention factors on the inverse temperature, as shown in Figure 2-11. 

Analyte ionization, tautomerization, or solvation equilibrium in the chromatographic column has a profound effect on the retention and efficiency. These effects are known as secondary equilibria effects [34,35]. The effect of the analyte ionization on the retention has been extensively studied [36, 37]. Fundamental work by C. Horvath and co-workers created a solid foundation in this held [30, 38] for ionic equilibria of... 

At proton concentration at least hundred times higher than the analyte ionization constant for a basic analyte (pH is two units lower than pKa), expression (2-80) reduces to Vr = Vq + SK +. It essentially represents the retention volume of only ionic form of the analyte. At a pH at least two units higher than the basic analyte >Ka (suppressed ionization conditions), expression (2-80) reduces to Vr = Vq + SK and it represents the retention of only nonionic form of the analyte at conditions where protonation is completely suppressed. Corresponding capacity factors for neutral and protonated forms of basic analyte could be written in the form... 

The analyte nature and its appearance (e.g., ionization state) in the mobile phase are also factors that affect the retention mechanism. Eluent pH influences the analyte ionization equilibrium. Eluent type, composition, and presence of counterions affect the analyte solvation. These equilibria are also secondary processes that influence the analyte retention and selectivity and are of primary concern in the development of the separation methods for most pharmaceutical compounds. 

The impact of the pH in hydro-organic mixtures on the analyte ionization and retention will be thoroughly discussed. The impact of pH on analyte UV absorbance will be discussed in the method development chapter. Chapter 8 (Section 8-6). 

A simple rule for retention in reversed-phase HPLC is that the more hydrophobic the component, the more it is retained. By simply following this rule, one can conclude that any organic ionizable component will have longer retention in its neutral form than in the ionized form. Analyte ionization is a pH-dependent process, so significant effect of the mobile-phase pH on the separation of complex organic mixtures containing basic or acidic components can be expected. 

Liquid chromatography has also been widely used for the determination of dissociation constants [88-92] since it only requires small quantity of compounds, compounds do not need to be pure, and solubility is not a serious concern. However, the effect of an organic eluent modifier on the analyte ionization needs to also be considered. It has been shown that increase of the organic content in hydro-organic mixture leads to suppression of the basic analyte pKa and leads to an increase in the acidic analyte pK compared to their potentiometric pKa values determined in pure water [74]. 

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