Chemistry Reference
In-Depth Information
chromatographic resolution. Spectra will also be changed if the organic concentration
or pH is altered, for example, during method development. The changes in spectra
resulting from mobile phase differences often result in a shifting of the spectra, affect-
ing the quality or the “fit” of the match, but not necessarily the information obtained.
Recent improvements in the ability to efficiently nebulize an HPLC column efflu-
ent has led to the increased utility and popularity of the evaporative light scattering
detector (ELSD). The ELSD works on the principle of evaporation (nebulization) of
the mobile phase, followed by measurement of the light scattered by the resulting
particles. The column effluent is nebulized in a stream of nitrogen or air carrier gas in
a heated drift tube, and any nonvolatile particles are left suspended in the gas stream.
Light scattered by the particles is detected by a photocell mounted at an angle to the
incident light beam. Carrier gas flow rate and drift tube temperature must be adjusted
for whatever mobile phase is used. Detector response is related to the absolute quan-
tity of analyte present; and while decreased sensitivity will be obtained for volatile
analytes, unlike the UV detector, no chromophores are required and it has orders of
magnitude more response than the refractive index (RI) detector, another common
detector in situations where analytes do not have strong chromophores. The ELSD
also has the advantage over RI detection in that the response is independent of the
solvent, so it can be used with gradients, and is not sensitive to temperature or flow
rate fluctuations. Mobile phases, of course, must be volatile, similar to those used for
MS detection, as listed in Table 3.3 [6]. Linearity can be limited in some applica-
tions, but is certainly quantitative over a wide enough range if properly calibrated.
Recent applications of the ELSD have also been extended to UHPLC.
Corona charged aerosol detection (CAD), sometimes referred to as corona dis-
charge detection (CDD), is a unique technology gaining in popularity in which the
tAble 3.3
properties of common organic solvents used in liquid
chromatography
solvent
uv cutoff (nm)
a
viscosity (cp)
boiling point (°c)
Acetonitrile
190
0.38
82
1-Butanol
215
2.98
118
Dimethylformamide
268
0.92
153
Dimethylsulfoxide
268
2.24
189
Heptane
200
0.40
98
Hexane
195
0.31
69
Methanol
205
0.55
65
n-Propanol
210
2.30
97
Tetrahydrofuran
212
0.55
66
Water
190
1.00
100
a
Wavelength at which solvent absorbs 1.0 AU in a 10-mm cell. (
Source:
Adapted
from Snyder, L. R. et al.,
Introduction to Modern Liquid Chromatography
, 3rd
edition, John Wiley & Sons, Hoboken, NJ, 2010, p. 882.)
