Selective Chemometric Elimination of Co-Eluting Components in Chiral and Achiral Liquid Chromatographic Analyses

No stationary phase can address all
separation problems, as such
a capability is both statistically and thermodynamically implausible.
Overlapping peaks in complex mixtures are unavoidable for any given
eluent and stationary phase combination. Since UV spectra are broad,
it is not possible to eliminate contributing signals from interfering
analytes in HPLC by simply choosing different wavelengths. In the
derivative space, where the differentiated pure spectrum of one component
is zero, its peak profile can be selectively eliminated, yielding
improved quantitative and qualitative information that was not possible
from the original chromatogram. The transformed raw chromatogram is
called the derivative spectrum chromatogram (DeSC). This technique
is extensively useful in chiral analysis where two or more enantiomer
pairs overlap in the time domain but are distinguishable in their
UV–vis absorption spectra. Since differentiation degrades the
signal-to-noise ratio, the improved DeSC algorithm combines time-domain
Savitzky-Golay differentiation with a frequency-domain windowing technique
to suppress noise in the final DeSC. The protocol can extract information
from peaks with peak resolution as low as zero. Illustrative examples,
including amino-acid and nicotine enantiomers, fungicides, and illicit
street drugs such as tianeptine (which exhibits an overlapping impurity),
were analyzed in isocratic and gradient runs with nonlinear baselines.
Quantitation of extracted peaks from a real sample of street drug
with overlapping impurities showed excellent calibration linearity,
with an R2 value of 0.9997. For partially
resolved peaks recovered from DeSC, we suggest using a generalized
error-peak model for accurate area determination.