1. Normal-phase separation techniques (NP) with non-polar mobile and stationary
phases based on unmodified silica gel.
2. Reversed-phase separation techniques (RP phases) with polar, usually aqueous
mobile phases and modified silica gel and other polar stationary phases.
HPLC was developed in the late 1960s based on normal-phase separation systems. Normal-phase (NP) separation is mainly based on hydrophilic, polar and π-π
interactions. Thus, NP is suited for effective separation of a large variety of
non-polar compound mixtures (Cutillas and Timms 2010). However, the restricted
retention properties of NP column-mobile-phase systems limit the applicability to
liquid chromatographic separation systems considerably. Therefore, reversed-phase
HPLC (RP) was developed enabling the analyst to attempt new challenges in
chromatographic separations by increasing the mobile-phase strength with decreasing polarity and, thus, extending the polarity range for effective chromatographic
separation (Kalász et al. 1993; Lipton and Paša-Tolić 2009). Dependent on the
choice of the stationary-mobile-phase system, a variety of interactions can be chosen
for the effective separation of target compounds mainly H–H interactions and van
der Waal’s strength are governing the retention properties of the RP chromatographic separation system. Already in the 1980s, these two branches of HPLC
were the most used analytical separation methods in the research and chemical
industry, but now also made their way into relevant environmental applications.
Further improvements in HPLC/UHPLC and highly selective interactive stationary
phases such as monolithic stationary phases, molecular imprinting and hydrophilic
interaction liquid chromatography (HILIC) were introduced during the last decade
(Zambonin 2003; Clonis 2006; Kumar et al. 2012; David et al. 2013) as an integrated
method for new, emerging research fields like proteomics, genomics, metabonomics,
toxicogenomics and lipidomics (McGuire and Casado 2004; Gomase and Tagore
2008; Lu et al. 2008; Andjelkovic et al. 2017; Novotny 2017). These techniques, for
which the column size was continuously reduced, and micro-columns containing
stationary phases with <3 μm particle size were introduced to minimise solvent
usage and increase resolution (Novakova and Vlckova 2009; Howard et al. 2012;
Cielecka-Piontek et al. 2013; Nishi and Nagamatsu 2014; Fekete et al. 2015; Patel
et al. 2016). These methods and technologies primarily developed for technical
applications in industry and product development are now also widely used in
environmental trace analysis and pollution research (Varma et al. 2010; Ghosh
2012; Nunez et al. 2012).
Nowadays, a large variety of detectors including high- and ultrahigh-resolution
mass-selective detectors are available allowing the trace-level determination of all
types of compounds including chiral pollutant residues in the environment. HPLC
and UHPLC separation methods reached or even surpass detection limits earlier only
achieved for gas chromatographic methods. Thus, HPLC/UHPLC in combination
with sensitive detection systems, preferably mass-selective detectors together with
gas chromatography combined with suitable detection systems, are powerful and
complementary scientific tools for today’s multi-compound or even non-target
analysis of environmental micro-pollutants including chiral substances.
30
3 Enantiomer-Selective High- and Ultra- High-Performance Liquid Chromatography
phases based on unmodified silica gel.
2. Reversed-phase separation techniques (RP phases) with polar, usually aqueous
mobile phases and modified silica gel and other polar stationary phases.
HPLC was developed in the late 1960s based on normal-phase separation systems. Normal-phase (NP) separation is mainly based on hydrophilic, polar and π-π
interactions. Thus, NP is suited for effective separation of a large variety of
non-polar compound mixtures (Cutillas and Timms 2010). However, the restricted
retention properties of NP column-mobile-phase systems limit the applicability to
liquid chromatographic separation systems considerably. Therefore, reversed-phase
HPLC (RP) was developed enabling the analyst to attempt new challenges in
chromatographic separations by increasing the mobile-phase strength with decreasing polarity and, thus, extending the polarity range for effective chromatographic
separation (Kalász et al. 1993; Lipton and Paša-Tolić 2009). Dependent on the
choice of the stationary-mobile-phase system, a variety of interactions can be chosen
for the effective separation of target compounds mainly H–H interactions and van
der Waal’s strength are governing the retention properties of the RP chromatographic separation system. Already in the 1980s, these two branches of HPLC
were the most used analytical separation methods in the research and chemical
industry, but now also made their way into relevant environmental applications.
Further improvements in HPLC/UHPLC and highly selective interactive stationary
phases such as monolithic stationary phases, molecular imprinting and hydrophilic
interaction liquid chromatography (HILIC) were introduced during the last decade
(Zambonin 2003; Clonis 2006; Kumar et al. 2012; David et al. 2013) as an integrated
method for new, emerging research fields like proteomics, genomics, metabonomics,
toxicogenomics and lipidomics (McGuire and Casado 2004; Gomase and Tagore
2008; Lu et al. 2008; Andjelkovic et al. 2017; Novotny 2017). These techniques, for
which the column size was continuously reduced, and micro-columns containing
stationary phases with <3 μm particle size were introduced to minimise solvent
usage and increase resolution (Novakova and Vlckova 2009; Howard et al. 2012;
Cielecka-Piontek et al. 2013; Nishi and Nagamatsu 2014; Fekete et al. 2015; Patel
et al. 2016). These methods and technologies primarily developed for technical
applications in industry and product development are now also widely used in
environmental trace analysis and pollution research (Varma et al. 2010; Ghosh
2012; Nunez et al. 2012).
Nowadays, a large variety of detectors including high- and ultrahigh-resolution
mass-selective detectors are available allowing the trace-level determination of all
types of compounds including chiral pollutant residues in the environment. HPLC
and UHPLC separation methods reached or even surpass detection limits earlier only
achieved for gas chromatographic methods. Thus, HPLC/UHPLC in combination
with sensitive detection systems, preferably mass-selective detectors together with
gas chromatography combined with suitable detection systems, are powerful and
complementary scientific tools for today’s multi-compound or even non-target
analysis of environmental micro-pollutants including chiral substances.
30
3 Enantiomer-Selective High- and Ultra- High-Performance Liquid Chromatography