Secondary retention

The fraction of the atoms produced in a nuclear reaction and found in the foim of the original chemical compound is called retention. Retention can be due to nonbreaking of the chemical bonds (primary retention) or to breaking of the bonds followed by recombination or substitution reactions (secondary retention). 

After the recoil atoms have come to rest, subsequent reactions are stopped. Further reactions including recombination of reactive atoms or molecular fragments are possible after diffusion of the reactive species or after dissolution of the solid. Diffusion can be enhanced by increasing the temperature (thermal annealing) or by iiTadiation with y rays or electrons (radiation annealing). Dissolution may lead to recombination, reaction with other species or reaction with the solvent. Due to these processes secondary retention may increase or decrease. As an example, the retention of in the form of sulfate after neutron irradiation of ammonium sulfate and dissolution is plotted in Fig. 9.13 as a function of the time of thermal annealing at... 

TFA is frequently applied as an additive in LC-MS, for instance in the RPLC separation of peptides (Ch. 16.3.2). TFA is applied as an ion-pairing agent and to mask secondary retention effects of RPLC stationary phases. Without TFA in the mobile phase, the peptides would be almost irreversibly adsorbed. TFA restrlts in significant signal suppression due to both ion-pairing and sirrface-tension effects. The TFA anion more-or-less masks the positive charge on an analyte molecttle at the droplet surface and thereby prohibits lEV of that ion. In the TFA-fix , a post-column addition of propionic acid in 2-propanol (75 25, v/v) is used to counteract the suppression [103-104]. This approach has been found to be successful in some cases, but not in all. 

Secondary retention effects residual silanol interactions. —> Use ion pair reagent, or competing base or acid modifier. Trie-thylamine for basic compounds, acetate for acidic compounds. 

If only some of the peaks tail, secondary retention effects, such as residual silanol interactions, may take place. Another possibility is that a small peak is eluting on the tail of a larger peak. If all peaks tail, this may be due to a bad column or build up of contamination on the column inlet frit. 

Chapter 12 of Ref. 79)]. Next, secondary>retention effects 88) that can affiwt separation are considered, and ropriate changes in mol -phase composition are made ... 

In the case of SEC separations, knowing the sample molecular weight allows us to choose the right pore size column prior to the first HPLC run. An initial separation is then followed by adjustments in mobile-phase composition or additives, in order to minimize any secondary-retention effects (see preceding section). 

Secondary retention effects can idhd to excels band broadening ahd/or distorted peaks as noted above. These effects can also result in incomplete recovery of the sample firom the column. Often such effects can be diagnosed (and cumd) by adding 10 mM of triethyl amine or morpholine to the mobile phase. In most cases of secondary retention, this treatment will block the silanols responsible for poor romatography and markedly improve the separation. If an amine modifier is found to improve the separation, but interferes with the subsequent workup of sample fifactions, tten other col ... 

When samples other than neutral polar compounds are analyzed, one has to take into account ion exchange as a secondary retention mechanism. An example of this is the separation of sialylated oligosaccharides. In this case, a buffeitKl mobile phase is used to control the retention. Another example is the analysis of polar adds such as glycolic add. Careful consideration should be given to the concentration of the buffer. A high concentration can lead to a reduction in retention, but a low concentration (e.g., 10 mM) leads to excessive equilibration times. See the previous paragraph for additional examples. 

This chapter first discusses the characteristics of silica as it pertains to size exclusion chromatography. Next, several methods for molecular weight calibration in SEC are examined and the effects of secondary retention discussed. The chapter concludes with an overview of practical aspects associated... 

The importance of hydrophobic interactions as another secondary retention mechanism is also illustrated in Table 8, in which the distribution coefficient for phenylethanol is listed as a function of ionic strength for the same set of commercial GFC columns (34). Indicative of hydrophobic interaction, Kd values increase with increasing ionic strength for this uncharged solute. Thus, a balance must be stuck between the need to increase ionic strength to reduce ionic interactions and to decrease ionic... 

It is also important to understand, that retention in RP-LC is never solely based on a pure hydrophobic or dispersion interaction, but always influenced by secondary retention mechanisms. These secondary mechanisms can be both of wanted or of an unwanted nature. A good understanding of these relationships is extremely helpful for selectivity optimization and the fundamentals are discussed in more detail in Chapter 4 on modern HPLC columns. 

Secondary retention effects residual silanol interactions, for basic compounds, acetate for acidic compounds. 

He verified that there are two kinds of retention one is primary retention at the original site and the other is billiard ball secondary retention. For the system K2ReBr6-K2ReCl6, Br yield was 12%, primary retention 8%, billiard ball secondary retention 74%, and some products of larger disorder [Re BrBr Cl5 ] (n = 0... 4) were about 6% altogether. 

Some pitch components that are soluble in sebum can stimulate epithelial cell production. Hair follicles and sweat-gland openings can be mechanically plugged by the CTP particles and by the resulting cell proliferation. Secondary retention of sebaceous material, pustule formation, and inflammation often result (Schwartz et al. 1957). Acneiform lesions were much more common in pitch workers (93%) than in controls (31%) with a peak incidence of these lesions in the 40-to 49-year-old age group (Hodgson and Whitely 1970). 

Commercially available silicas differ in their physical (specific surface area, mean pore diameter, pore volume, etc.) and chemical (contamination of different metals such as Al, Mg, Fe) characteristics. These metal contaminations can cause secondary retention effects (e.g., strong tailing of basic solutes). Additionally, commercial silicas differ widely in their Na content. The higher the Na content of the silica, the more alkaline its behavior in contact with water. This often acts as a catalyst for the hydrolysis of the Si -C bonds of chemically modified silica phases (see Section 12.4.3). Good silicas have an Na content of < 30 ppm. Excessively high contents of metal contaminants could easily be removed by the manufacturer by means of a thorough acid-washing... 

Reduce the likelihood of a drop through the use of secondary retention devices. 

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