Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 262

present. These components are critical in, respectively, monitoring system pressure,
removing particulates generated by mobile phase or piston seals, and protecting
expensive analytical columns from clogging. The guard column is a very short
replicate (same packing material) of the analytical column and is used to protect the
sample-born materials (Note that the inline filter only removes the particulate matter
from the mobile phases).
When compounds enter into the detector, their signals are detected and the data
are analyzed by the data processing unit. HPLC detectors include ultraviolet detectors
(most popular), the differential refractometers, fluorescence detectors, and conductiv-
ity detectors. Details on these HPLC detectors will be described in Section 10.3.2.
Since solvent mobile phase is the most often changed component in the HPLC
once the separation conditions are established, we will further examine more details
on solvent (mobile phase) selection in this section. The question here is, given a set
of compounds and the selected column, what solvent or mixtures of solvents should
be chosen to maximize the separation efficiency while keeping other problems at a
minimum. Solvent regime can be isocratic (solvent or mixed solvents at constant
composition over time), or gradient (solvent or mixed solvents composition changes
over time). Note that the gradient elution is used to improve separation, just like the
temperature programming is used in GC to improve the column separation of the
analytes.
In selecting solvents as the mobile phase, several properties must be examined,
including viscosity, UV cutoff, refractive index, boiling point, and polarity
(Table 10.3). Solvents with a higher viscosity will cause a larger pressure drop
and, therefore, should be cautioned. If a UV detector is used, the solvents used in

Table 10.3 Properties of selected solvents used as HPLC mobile phase

UV cutoff Refractive Viscosity Boiling Polarity

index (20 C) point ( C) Solvent (nm) (cP) (P)

Pentane 190 1.3575 0.23 36.07 0.0

Hexane 195 1.3749 0.31 68.7 0.1

Cyclohexane 200 1.4242 1.0 80.72 0.2

Toluene 284 1.4969 0.59 110.62 2.4

Methyl tert-butyl ether 210 1.3689 0.27 55.2 2.5

Methylene chloride 233 1.4241 0.44 39.75 3.1

Isopropyl alcohol 205 1.3772 2.40 82.26 3.9

Tetrahydrofuran 212 1.4072 0.55 66.0 4.0

Chloroform 245 1.4458 0.57 61.15 4.1

Ethyl acetate 256 1.3724 0.45 77.11 4.4

Acetone 330 1.3587 0.36 56.29 5.1

Methanol 205 1.3284 0.55 64.7 5.1

Acetonitrile 190 1.3441 0.38 81.60 5.8

Water 190 1.3330 1.00 100.0 10.2

Adapted from Snyder et al. (1997). Solvents in the table are arranged in an increasing order of polarity.
cP: Centipoise.