Ionizable retention volume

The expression (dxldt) is the linear velocity of the analyte in the column and we get the final equation for the retention volume of ionizable analyte using the same conversions as in equations (2-33)-(2-39). 

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... 

An IGC method for the analysis of divided solids and fibers has been initiated by Gray et al. (2 ) It is illustrated here by the results obtained from the precipitated silica sample (PI). Injecting a series of n -alkanes at infinite dilution (at the limit of detection by the flame ionization detector) usually results in a linear variation of the logarithm of the net retention volumes (V ) with the number of carbon atoms in the n-alkanes. This is illustrated in Figure 1 for measurements performed between 71 and 130°C. Thermodynamic considerations show that and the standard free energy of adsorption of the alkanes are related by... 

Haddad et al. measured retention volumes for a variety of bases on a quaternary ammonium functionalized PS-DVB stationary phase using dilute aqueous sodium hydroxide as the eluent [11]. Values for the retention volumes and distribution coefficients of selected bases are given in Table 8.3. Strong bases, which are fully ionized a the eluent pH, elute at the column void volume and have a value of 1.0. Solutes intermediate between these two extremes are partly ionized and generally can be separated by an ion-exclusion mechanism. 

Less commonly used measurement techniques include the pH dependence of partition coefficients [74], fluorescence spectra [75], nuclear magnetic resonance chemical shifts or coupling constants, HPLC or CE retention volumes [76,77], and the dependence of reaction rates for ionizable substrates on pH (also called kinetic methods). Kinetic methods were amongst the earliest methods to be used for pKg determination. In some cases, they may be the only feasible method, for example, extremely weak acids (pKa > 12) without suitable absorption spectra. The difficulty with kinetic methods is that they may not actually measure the pKg value for the substrate, but that of the reaction transition state. If the electronic configuration of the transition state is similar to that of the reactant (early transition state), then the kinetic may be quite close to the equilibrium value. However, if the transition state more nearly approximates the reaction products (late transition state), then the kinetic value may bear little resemblance to that for the reactant. This explanation might account for the lack of agreement between the first apparent kinetic pK = 4.0) and equilibrium (pK = 8.6) pKg values for hydrochlorothiazide at 60 °C [78]. Similar restrictions may be placed on the use of pKa values from the pH dependence of fluorescence spectra, as these reflect the properties of the first excited state of the molecule rather than its ground state [75]. 

A mixture of 70% N2 and 30% He was employed as carrier gas. This mixture is recommended because use of helium alone at liquid nitrogen temperature causes a very strong retention of methane. With this mixture some nitrogen is adsorbed on the silica layer of the column. At values of the relative vapor pressures p/pjj between 0.7-0.9 a polymolecular layer of Ng covers all the surface, and pores with a radius below 80 A are filled. In this way the most active sites of the adsorbent, which are located in the small pores, are blocked, and as a consequence retention volumes are decreased and symmetrical peaks are obtained. Separation of all isotopic methanes obtained on the glass capillary column at liquid nitrogen temperature is shown in Fig. 6-21 [90]. In the upper part of the chromatogram there are the radioactive compounds detected by the ion chamber and in the lower part there are the stable isotopes monitored by the flame ionization detector. These chromatograms do not refer to the seime injection. 

Bortnikov et al measured the retention volume for triethyl and tetraethyl silanes on a stainless steel column (1 metre x 3mm) packed with silanized Chromosorb W supporting 20% of Apiezon L or 15% of Carbowax 20M and operated at 120°C and 90 0 respectively using helium as carrier gas and thermal conductivity or flame ionization detection. They also carried out measurement by gas-solid chromatography on a column of Carbochrom-1 (graphitized thermal carbon black with 0.01% of Apiezon L) operated at 190 0 or Silochrom C-80 (modified by chlorotrimethylsilane or dehydroxylated) operated at 100, 147 or 203°C, respectively. 

Pollard et al used a flame ionization gauge to determine retention data for germanium tetramethyl. The apparatus and conditions are described in Chapter 2 (Section b). Tetramethylgermanium had the following specific retention volumes (Vgml) on a 15% E 301 silicone oil on celite column ... 

Flow rate Injection volume Retention time Ionization mode Polarity Nebulizer gas Auxiliary gas Nebulizer temperature Collision gas Acquisition... 

Chromatography Interaction between phases Retention at given time or volume Thermal conductivity, flame-ionization, MS Chromatogram ... 

The van der Waals volume can be related to the hydrophobicity of the solutes, and retention of molecular compounds can be predicted from their van der Waals volumes, 7r-energy, and hydrogen-bonding energy effects [72-74], It should be noted that the isomeric effect of substituents cannot be predicted with good precision because this is not simply related to Hammett s a or Taft s other hand, the hydrophobicity is related to enthalpy [75], Retention times of non-ionizable compounds were measured in 70 and 80% acetonitrile/water mixtures on an octadecyl-bonded silica gel at 25-60°C and the enthalpy values obtained from these measurements. 

In acidic eluents (Figure 3.9C and D), the retention of acidic compounds becomes stronger and that of basic compounds becomes weaker. In this system, uric acid (h) is not suitable as a void volume marker due to its longer retention time. In neutral and basic eluents, an ionized acid can be used as the marker because no other compounds are eluted more rapidly. Fructose (c) is a very... 

Another problem is compounds that are too soluble to retain on the column and elute unresolved in the void volume. Modifications of either the sample ionization or of the surface nature can increase retention and, therefore, resolution. In this section, we will study modifications of the column or mobile phase that will allow us to improve our separations. 

Table 3-4 shows the relative standard deviation in the retention of the same analyte on all columns. For ionized analytes eluted very close to the void volume, the retention difference between all columns is significant, but for analytes retained on the column, the RSD of is much smaller than k and essentially within the experimental error range. 

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