Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 253

analysis, but it should be realized that these modeling equations are not employed on
a daily basis.
A last, but not least, important parameter in separation is column temperature
control for GC and mobile phase selection in HPLC. The column temperature is a
compromise between speed, sensitivity, and resolution. At high column tempera-
tures, analytes spend most of their time in the gas mobile phase so they are eluted
from the column quickly (short retention time), but resolution is poor and sensitivity
is increased because of decreased spreading of the peaks. Temperature can be easily
programmed in operating software and, therefore, is considered less critical than the
selection of stationary phase. In HPLC, commonly employed in environmental
analysis, temperature has little effect on separation and hence room temperature is
well served. The equivalent operational parameter in HPLC is the types and
composition of mobile phases (solvent), or polarity of the mobile phase. This is
because polarity is the basis for the separation in HPLC. The selection of mobile
phase solvents in HPLC will be further discussed in Section 10.2.2.

Practical tips

Column internal diameter: GC capillary columns of 0.25 or 0.32 mm i.d.
represent the best compromise between resolution, speed, sample capacity, and
ease of operation. These are the reference columns against which all other
internal diameters are measured. The 0.10 mm fused silica columns have limit-
ed sample capacity, and are not well suited for trace analysis. The 0.53 mm,
calledmegabore or widebore columns, have lower resolution but offer increased
capacity and ease of operation (i.e., direct on-column syringe injection).
Column length: Increased column length will increase resolution, but will
also proportionally increase the retention time resulting in slow analysis.
Medium length GC capillary columns (25 or 30 meters) are recommended
for most environmental applications. If more than 50 components are to be
analyzed (such as for petroleum analysis), then 60 meters or longer column
length is needed.
Film thickness: For most analysis, a standard film thickness of 0.25

misa
reasonable starting point. It represents a compromise between the high
resolution attainable with thin film and the high capacity available with
thick films. High capacity means that larger sample quantities be accom-
modated while keeping usually the injection technique simple.
HPLC column parameters (i.d., column length, particle size, and pore size):
HPLC analytical methods usually are best developed with 0.46- or 0.3 cm
i.d. columns having particles in the range of 3-10

m

m
particles give the best compromise of efficiency, reproducibility, and relia-
bility. Columns of 0.3 cm i.d. reduce solvent consumption to one-half
of widely used 0.46-cm units. HPLC columns of these types should
allow mobile phase flow not to exceed a few mL/min. Smaller i.d. columns
are especially useful (and often necessary) when interfacing an HPLC

m m. Columns with 5- m