A critical review of methods for the determination of dead time (volume)

Abstract

The critical review examines currently known methods for determining dead time, which are required for calculating retention factors and related patterns in chromatography. Particularly remarkable is the fact that the retention factor, determined using the theoretical plate method in relation to chromatography, initially on sorbents that did not have pores, could be converted into a retention factor on porous sorbents, characterised by the presence of mobile phase components in stagnant zones, specifically in the pores of the sorbent. However, the zone of unretained sorbent moved slower than the true speed of movement of the mobile phase. The most popular options from those previously proposed were considered, taking into account new experimental data. The possibility of the presence of gallery pores in sorbents called into question the gravimetric method. In addition, the gravimetric method, similar to many others, did not take into account the difference in the retention of solvents sorbed by adsorption or absorption mechanisms. The use of solvents that wet the stationary phase most likely resulted in the penetration of solvent molecules into the graft phase and should result in a dead volume applicable to the absorption mechanism. If the solvent did not wet the stationary phase well, the meaning of the obtained dead volume became uncertain. Calculation of the dead time by the retention of deuterated water was more acceptable for determining the dead time by the retention of substances using the adsorption mechanism, although the results may be overestimated due to the sorption of water on residual silanol groups. The use of deuterated organic solvents for the same purposes raises many questions. The use of inorganic salts as dead volume markers may present not entirely accurate results on exclusion effects, which should be considered in other options as well. The homological series method was also not unconditionally correct for a number of reasons mentioned in the text. The system peak method also could not be unambiguously confirmed, since it may correspond to retention with an indefinite proportion of exclusion effects. Therefore, the problem remains unsolved to date and the development of new special approaches are required.