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Discover Restek USLC™
DevelopMethods Quickly and Easily Using Ultra Selective Chromatography
LC Columns
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Ultra Selective Liquid Chromatography™
What is Ultra Selective Liquid Chromatography™? USLC™ is the directed application of selectiv-
ity—the most influential factor affecting resolution—to optimize separations and improve
method performance. Restek has extensively studied reversed phase selectivity to provide
practicing chromatographers with the most effective and widest range of USLC™ stationary
phase chemistries available.
Selectivity Drives Separations
By understanding and controlling selectivity through USLC™, chromatographers have the best opportunity for fast, effective analyte resolution.
One of the most significant challenges in method development is finding the proper stationary and mobile phase chemistry for a particular sep-
aration. As sample complexity increases, achieving adequate resolution between matrix components and target analytes becomes more diffi-
cult. Despite recent advancements in column format, such as sub-2 micron packings and pellicular particles, resolution can still be difficult to
obtain because, while these formats can increase chromatographic efficiency and analysis speed, they do not significantly influence resolution.
Selectivity, as shown in Equation 1, is the single most powerful factor affecting resolution, and it is largely dependent upon stationary phase
composition.
Real Diversity in Phase Chemistry
Restek columns offer the widest range of selectivities available on a single column line. More choic-
es mean optimized separations and more robust methods.
While numerous bonded phases are available for reversed phase chromatography, many are
similar and offer only moderate changes in retention (e.g. C8 and C18), rather than significant
differences in selectivity. Method development is less laborious and time-consuming when
using a full range of column selectivities, including orthogonal phase chemistries like polar
embedded, phenyl, and fluorophenyl columns. Restek has led the development of unique
USLC™ phases across these phase classes in order to provide chromatographers with a more
effective range of column selectivities and innovative column chemistries for method development. The phases shown in Figure 1 provide the
widest range of reversed phase selectivity available on any column line, and can be used to guide the least understood and most practically sig-
nificant part of method development—proper column selection.
Equation 1: Selectivity drives
resolution—USLC™ considers
column selectivity during method
development, resulting in fast,
effective separations.
R = 1/4 N x (k' /k'+1) x (α-1/α )
Efficiency Retention capacity Selectivity
RESTEK USLC™
Ultra Selective Liquid Chromatography™
Evaluating and Extending Selectivity
Restek leads the industry in USLC™ phase diversity because optimal differences in selectivity are built in during the research and development of our
bonded phases.
The diversity in selectivity provided by USLC™ columns can be demonstrated empirically using the hydrophobic-subtraction (HS) model [1]. This
model is a novel procedure for characterizing selectivity that uses test probes to define the solute and stationary phase interactions in reversed
phase separations. Restek is leading the commercial application of this model by implementing it in the research and development of USLC™
bonded phases. To evaluate phase selectivity using the hydrophobic-subtraction model, the retention characteristics of the solute probes are
compared across different phases on the same silica base. In this approach, the range of selectivity is indicated by the degree of scatter along
the regression line; high correlations indicate similarity and low correlations represent changes in selectivity across phases (Figure 2). The differ-
ence in selectivity across columns can then be quantified based on the correlation by calculating the selectivity (S) statistic for the comparison [2].
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