Chaotropes in Supercritical Fluid Chromatography

Abstract

In this literature review, chaotropic additives in supercritical fluid chromatography (SFC)
were investigated. Due to the usage of CO2 as mobile phase in supercritical fluid
chromatography, the technique was assumed to only be appliable to non-polar analytes. Polar
modifiers were able extend the range to include polar analytes. Though it’s still insufficient to
separate some sets of polar compounds but can be improved by using additives. Chaotropes
are known to increase the solubility of proteins by reducing the hydrophobic effect and
inducing disorder in the water molecule network. In liquid chromatography (LC), chaotropes
are known additives and generally used to increase retention of basic analytes in reverse phase
conditions.

The aims were to investigate and compare which chaotropic additives, and in which
conditions, that are utilized in SFC and LC. Furthermore, to investigate if chaotropes can
increase the scope of SFC and how a chaotrope can be defined. For this, Scifinder was
primarily used for the literature search of chaotropic additives in the context of
chromatography. The keywords used at the start were general for the topic, and two relevant
sources was acquired with the search “Chaotrop* AND ‘supercritical fluid chromatography’
OR ‘SFC’”. This search was then extended to include LC. After collecting necessary
information, the searches were narrowed to specific chaotropes used as additives, with
acetate, trifluoroacetic acid and ammonium hydroxide being found in SFC conditions. The
anions hexafluorophosphate, perchlorate, thiocyanate, tetrafluoroborate and nitrate were
found in LC conditions. Resources were also found through the reference list of other articles.

In summary, common LC conditions included the mobile phase H2O:acetonitrile (ACN), the
stationary phase C18, and the three chaotropic anions ClO4 −, PF6 −, and trifluoroacetate
(TFA−). In SFC, stationary phases with differing polarity have been utilized, together with the
mobile phase CO2:methanol (MeOH), with 5 % water in MeOH. The articles reported the
chaotropic effects of perchlorate and bicarbonate/carbonate. Three examples of how to
classify a chaotrope includes the viscosity B coefficient, the Gibbs free energy of hydrogen
bonding (ΔGHB) and through temperature change in agar gel point.

In conclusion, the LC conditions are different in comparison to SFC conditions. Chaotropes
appears to be able to increase the scope of SFC. There is no standardization in how a
chaotrope is classified.

Popular Abstract

Structure breakers in analytical methods

Chaotropes, or ‘structure breakers’, are generally known to induce ‘chaos’ in water. The
structure of water becomes more disorderly, which in turn increases the solubility of water.
They may also affect compounds by directly interacting with them. Therefore, chaotropes
may be used to improve the effectivity of analytical methods to separate the compounds and
increase the number of compounds suitable to analyse with the method.

Through a literature study, the possibility of using chaotropes as additives in such analytical
methods has been investigated. The aim was to find out which chaotropes are used, with
which analysed compounds, and in which conditions. The conditions used will also be
compared between one method that uses liquids, and one method that uses supercritical fluids,
showing properties between those of a liquid and a gas. Furthermore, as it appears to be a
disagreement in how the word ‘chaotrope’ is used, the aim is also to find methods used to
classify chaotropes.

The results for the method using a liquid were that primarily water and acetonitrile were used
together with a carbon-based stationary phase. Three types of chaotropes were regularly used,
among 15 different types. For the method using supercritical liquids, mainly carbon dioxide
with methanol and a small amount of water was used. Different stationary phases were used,
though not one like the previous method. The analysed samples appeared to be either forms of
pharmaceutical compounds or biological substances. Only 3 chaotropes were investigated in
these conditions. There were different ways to classify an additive as a chaotrope by
investigating their interactions with the solvent, or how it affects the structure of the solvent.

The usage of the chaotropic effect, when used for something in between a liquid and a gas,
appears to still be rather limited, based on the literature, and further investigation is needed.
Despite this, chaotropic additives appear to improve the analytical method. There is no
standard or definite way of how to classify a compound to be a chaotrope.

• author: Bladh, Linnea
• supervisor: Mynta Norberg
• organization: Department of Chemistry
• course: KEMK10 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Chemistry
• keywords: Additives, Chaotropic, Hofmeister series, Liquid chromatography, Supercritical fluid extraction, Analytical chemistry
• language: English
• id: 9208097
• date added to LUP: 2025-08-25 10:32:42
• date last changed: 2025-08-25 10:32:42