Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 275

1 (mho). In the International System
of Unit (SI), G has a unit of siemens (S). Note that 1 S ¼ 1

Since R has a unit of (ohm), G has a unit of

ðmhoÞ¼1ohm 1 .In

1

practice, microsiemens ( S) is used for relatively dilute solutions.

The second term conductivity (k) is commonly referred to as ''specific
conductance.'' While conductance (G) is dependent on the cell dimension, the
conductivity (k) allows the measure to be independent of the detection cell:

m

15Þ
where the upper case K (unit cm 1 ) is termed the cell constant , which is equal to
L / A . The K value cannot be obtained by direct measurement, but it is determined
using a standard solution for which the conductivity k is known. From
Eq. 10.15, the conductivity has a unit of

k ¼ GK ð10 :

1

cm 1 . The third term called

equivalent conductance ( ) is introduced to take into account the concentration

of the chemical. It refers to the conductivity of an ion with a valence z in a dilute
solution at 25 C, and is defined as:

1000 k
Cz

¼ ð10 : 16Þ

where C is the molar concentration of an ion in mol per liter (1000 cm 3 ), and

is the
equivalent conductance in a unit of S cm 2 mol 1 . By combining above two equations
to eliminate conductivity (k), we obtain:

G ¼

Cz
1000K

ð10 : 17Þ

Eq. 10.17 underlines the principles of conductivity detector, that is, for a given ion
(constant and z), the conductance (G) of the solution will be linearly proportional

to the concentration of the electrolyte (C). In a conductivity detector, conductance
(G in S) is measured, and through a readout device, a chromatogram is obtained as

a plot of conductance (

m

S) vs. time.
As a summary of HPLC and IC detectors described above, Table 10.7 illustrates
the major characteristics of selected HPLC and IC detectors. Mass spectrometry is
also listed for the purpose of comparison. For brevity, other less common LC
detectors are omitted, including electrochemical (amperometric) detector, solution
light scattering detector, evaporative light scattering detector, and FTIR.

m

10.4 APPLICATIONS OF CHROMATOGRAPHIC
METHODS IN ENVIRONMENTAL ANALYSIS

Since the first gas chromatography instrument became commercially available in
1955, chromatographic techniques have evolved to be the premier technique for
the separation and analysis of complicated organic mixtures. Now it is hard to
imagine an organic analytical lab without any chromatographic instrument. In many
labs performing petroleum and petrochemical analysis, clinical and pharmaceu-
tical analysis, forensic and crime lab testing, and environmental monitoring,