Stationary phases in liquid chromatography

Abstract

Although HPLC has only been available for fifteen years, the column part of the system has seen more changes than any other part of the system. Classical column chromatography was associated with large diameter, porous particle packings (>100 μm) in wide bore columns (1–2 cm i.d.) and a low head pressure (from gravity to 50 lbf in−2) which together lead to slow analyses. The large diameter porous particles are unsuitable for HPLC owing to slow diffusion into the deeper pores which causes a decrease in column efficiency and loss of resolution. The earlier column packing materials also had very irregular shapes which formed in-homogeneously packed beds producing great variation in the mobile phase velocity and band spreading so that column efficiencies and resolution were again low. Equation (2.42) shows the relationship between efficiency in terms of height equivalents to a theoretical plate and many other variables: $$H=C_{\text{D}}\prime\frac{D_{\text{M}}}{v}+C_{\text{S}}\prime\frac{d_{\text{f}}\,^{2}v}{D_{\text{S}}}+C_{\text{SM}}\prime\frac{d_{\text{p}}\,^{2}v}{D_{\text{M}}}+\frac{1}{1/C_{\text{F}}\prime d_{\text{p}}+D_{\text{M}}/C_{\text{M}}\prime v d_{\text{p}}\,^{2}}$$ (2.42) With LC regular adsorbents, d fthe film thickness of the stationary phase is equivalent to d p since the porosity of the adsorbent allows the solute to diffuse through the whole particle. Since C S, the resistance to mass transfer, is proportional to d f 2 , and since C S contributes linearly to the plate height increase in the flow velocity for HPLC the particle diameter must be as small as possible.