Study of Dispersion in Packed Chromatographic Columns by Pulsed Field Gradient Nuclear Magnetic Resonance

Abstract

Pulsed field gradient nuclear magnetic resonance (PFGNMR) allows the direct determination of the average value of the axial and transverse apparent dispersion coefficient in a packed bed over a certain volume of this bed. From these data, the dependence of the local value of the height equivalent to a theoretical plate (HETP) of a chromatographic column on the fluid velocity can be measured accurately. The results are shown to be exactly accounted for by the Giddings equation. The PFGNMR measurements are made over a very short period of time. Hence they give the value of the HETP averaged over a limited volume of packing and a short time. This value is markedly smaller than the one afforded by the classical methods of chromatography and which is derived from the width of the distribution of the residence time of a small sample. The difference is explained by systematic variations of the local mobile-phase velocity across the column. PFGNMR techniques also allow the determination of the tortuosity of the column packing in the absence of flow. Their results confirm that the apparent dispersion experienced by a molecule is a function of the duration of the observation, until the tortuosity of the packing is completely felt. The values obtained agree very well with those derived from classical chromatography determinations.

Radial distribution of the contributions to band broadening of a silica‐based semi‐preparative monolithic column

“…Similar high A values have been reported earlier for monolith columns [12][13][14]. The structural heterogeneity of monolithic beds accounts for this high A contribution to band spreading [15,[20][21][22][23][24]. column or not.…”

Measurement of the parameters of the mass transfer kinetics in high performance liquid chromatography

“…In Eq. (19), the uncertainty in the estimate of D p is proportional to that of D m . As shown later (Figure 4), the contribution of surface diffusion to the intraparticle mass transfer in RPLC packing materials is remarkably high, usually as much as about 85 to 95% or more.…”

A Rigorous Model for Annular Chromatography

“…The correlation of Giddings [19] in connection with experimental results of other authors [20,21] generally describes the results quite well. Tallarek estimated the parameters of the Giddings equation based on NMR measurements [22,23] and the classical method of tracer injection [23]. The NMR method delivers much lower values than the method of tracer injection under the same conditions.…”

“…Therefore, these correlations are not recommended for the description of our data. The NMR based parameters by Tallarek [23] in connection with the simple Giddings model [19] shown in Eqs. (17) and (18) give the best results of all empirical correlations tested…”