Agilent Technologies 1290 Infinity LC System System Manual And Quick Reference page 61

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Optimization of the Agilent 1290 Infinity LC System
How to Achieve Higher Sensitivity
For example, a signal at wavelength 250 nm with a bandwidth of 16 nm will be
an average of the absorbance data from 242 nm to 258 nm. Additionally, a
reference wavelength and reference bandwidth can be defined for each signal.
The average absorbance from the reference bandwidth centered on the
reference wavelength will be subtracted from its equivalent value at the signal
wavelength to produce the output chromatogram.
The signal wavelength and bandwidth can be chosen so that they are
optimized for:
• Broad band universal detection
• Narrow band selective detection
• Sensitivity for a specific analyte.
Broad band or universal detection works by having a wide bandwidth to
detect any species with absorbance in that range. For example, to detect all
absorbing molecules between 200 nm and 300 nm set a signal at 250 nm with a
bandwidth of 100 nm. The disadvantage is that sensitivity will not be optimal
for any one of those molecules. Narrow band or selective detection is used
most often. The UV spectrum for a particular molecule is examined and an
appropriate absorbance maximum is selected. If possible, the range where
solvents absorb strongly should be avoided (below 220 nm for methanol, below
210 nm for acetonitrile). For example, in Figure 23 on page 62, anisic acid has
a suitable absorbance maximum at 252 nm. A narrow bandwidth of 4 nm to
12 nm generally gives good sensitivity and is specific for absorbance in a
narrow range.
The narrow band can be optimized for sensitivity for a specific molecule. As
the bandwidth is increased the signal is reduced but so is the noise and there
will be an optimum for best S/N. As an approximate guide, this optimum is
often close to the natural bandwidth at half-height of the absorption band in
the UV spectrum. In the anisic acid example this is 30 nm.
The analytical wavelength is usually set at a wavelength maximum to increase
sensitivity to that molecule. The detector is linear up to 2 AU and beyond for
many applications. This offers a wide linear range for concentration. For high
concentration analysis the concentration linear range can be extended by
setting the wavelength to one with a lower absorbance such as a wavelength
minimum or by taking a wider bandwidth which usually includes lower
absorbance values. The use of wavelength maxima and minima for
quantitation dates back to conventional UV detectors which because of
mechanical tolerances in moving gratings needed to avoid steeply sloping
parts of the spectrum. Diode-array based detectors do not have this limitation
Agilent 1290 Infinity LC System Manual and Quick Reference 61