Chromatography Handbook Of Hplc - Katz Eksteen Schocnmakers Miller wiley Sons -117

Mechanisms and Importance of Zone-Spreading\n107\nfor diffusion without convection, whereas the exact solution to the mass balance equations does include convection.\nF. Broadening and Peak Shape from Nonideal and Extracolumn Processes\nIn the preceding discussion we assumed a priori that the concentration distribution of the migrating bands and the eluting zones from the separation column would possess the ideal gaussian character that will lead to symmetrical and bell-shaped peaks in the chromatograms. On careful inspection of the detection signals obtained in practice, often peak shapes are either symmetrical, but nongaussian or asymmetrical (often tailing or backward-skewing and sometimes fronting or forward-skewing) and peaks are wider than expected. Thus the gaussian shape is, at best, an approximation in greater or lesser degree and we envisage that these deviating peak shapes may arise from two sources:\n1. From the separative and transport mechanisms within the column itself\n2. From extracolumn contributions in the connected apparatus.\nFor the first source, we can think of neglected but potentially active phenomena such as nonidealities connected with the partitioning and broadening mechanisms, such as\n1. Interfacial resistance at the phase boundary, replacing the assumed equilibrium condition\n2. Concentration effects (nonlinear partitioning isotherms, thermal and sorption effects, viscosity of the injected sample, concentration-dependent diffusion)\n3. Compressibility of phases\n4. Mixed retention mechanisms (e.g., additional nonlinear [and slow in its kinetics] adsorption interactions of solute and column material, or even reactions occurring)\n5. A badly packed or deteriorated column creating voids in the packing structure.\nSome of these effects may be circumvented (for example, by choosing different operating conditions such as lower concentrations, lower pressures, elevated temperatures, or adding modifiers to suppress adsorption), or minimized (by producing more column efficiency), but others are not.\nFor the second source, extracolumn contributions in the connected apparatus to the column may easily interfere, to produce broader and asymmetrical peaks that elute from the column as such. Examples of such contributions are\n1. Nonideal connecting: too long connecting capillary lines, leakage of mobile phase, and remaining dead volumes or corners\n2. Broadening from injection (e.g., a too large injection volume, a nonsharp input distribution, a too viscous sample solution or dead volumes)\n3. Broadening from detection and data-handling systems (e.g., a too large detection cell volume, dead corners in the cell, or even nonlinearity of the detector response, and too large time constants of detectors and data systems)\nOften these sources of additional band-broadening are to be avoided (fast electronic data systems, check on dead volumes and short connections), but sometimes the required detection principle (with a large cell for instance) or the geometric conditions to be met (leading to long connecting lines) are imperative.\n1. Broadening from Nonidealities in the Partitioning\n(a) Nonequilibrium at the Phase Boundary: Interfacial Resistance. Since the early days of chromatography, workers in the field have objected to the common assumption that partition equilibrium is present at the exact position of the phase boundary. Especially liquid-liquid chromatography, with two potentially partly miscible liquid phases, and the advent of HPLC\n