Under Construction

Reversed phase materials are manufactured by bonding alkyl chains to the silica gel base materials. The range covers anything from the very short length C1 to very large C 20 alkyl chain length: A list of material is enclosed (Click here) 

The performance of reversed-phase materials depends on many parameters. Two key properties, hydrophobicity and polarity, are of practical importance and dominate their selection. 

Hydrophobicity

The strength of hydrophobic interaction can be measured by the retention of neutral (non-polar) molecules. The percentage of carbon in the material is a simplistic but useful guide to the retention characteristics of the column.
In Figure 1 this loose correlation is demonstrated by the increase in retention observed when alkyl chain length (ie. carbon load) is increased. This results in an increase in hydrophobicity of the stationary phase.

Figures 2 and 3 compare the retention obtained for a selection of non-polar solutes with a range of Hichrom manufactured and other commercially available C8 and C18 columns.
 

C18 Hydrophobicity Comparison
Separation of dimethyl phthalate, toluene, biphenyl and phenanthrene on C18 bonded phases using a methanol-water
(90:10) eluent
 

Polarity (Silanol Activity)

The second key property of reversed-phase materials is their of a polar solute (involving hydrophobic and ionic interactions) silanol activity, often discussed in terms of polarity. This can be to that of a neutral solute (involving hydrophobic interaction measured on a relative basis by comparing the retention only).

High Purity Base Deactivated Phases

In recent years a number of new alkyl bonded silicas have been introduced. The cumulative metal ion impurity level within these base silicas has in some cases been reduced to <10ppm. As a result the number of isolated silanol groups and hence the polarity of the silica surface is also reduced.
When coupled with the use of more effective and reproducible bonding processes, a new generation of reversed-phase materials is produced, which gives significantly improved chromatography for the more basic polar solute molecules. Use of bonded alkyl groups containing hydrophilic substituent’s (ie. polar embedded) can either enhance the above effect and/or offer alternative selectivity.
Figure 4 demonstrates the reduced polarity of high purity base deactivated materials compared to lower purity products. Toluene is used as a hydrophobic reference. High purity (low polarity) materials generally give better peak shape with strongly basic compounds. However, low purity (high polarity) materials may offer a unique selectivity.

Traditional C18 (ODS) phases are hydrophobic and have a high polarity due to the lower purity silicas on which they are based. Use of the new high purity silicas reduces the resultant phases’ silanol activity and improves reproducibility. Employing a polar embedded functionality may also result in a reduced polarity material. Shorter alkyl chain phases are found at the lower hydrophobicity area of the graph. Alternative bonded phases (including phenyl and cyano) based on high purity silicas are best considered to effect changes in polarity.
For any separation it is possible to select the most suitable phase from Figure 6. For basic solutes which will interact strongly with surface silanols, lower polarity phases are recommended. If a column of significantly different polar selectivity is required, select phases from a different section of the graph. If only differences in hydrophobicity are desired, simply select phases that are well separated on the hydrophobicity axis.
 

Optimising selectivity

Figure 5 illustrates the change in polarity and hydrophobicity for Kromasil C18, C8 and C4 materials. As discussed previously, a decrease in hydrophobicity on reducing alkyl chain length is observed (see p.40). Greater ligand density and hence lower polarity is also seen as the length of the alkyl chain is reduced from C18 to C4.
Such variations offer the possibility of reduced analysis time and improvements in peak shape, but no major change in selectivity. Changing the chemistry of the bonded phase (eg. from C18 to cyano or phenyl) is a more powerful tool in altering the selectivity.