Page 253 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

EilUlPMENT FOR HPLC 8.3

(b) A linear response. The linear range of a detector is the concentration range
over which its response is directly proportional to the concentration of
solute. Quantitative analysis is more difficult outside the linear range of
concentration.
(c) Type of response, i.e. whether the detector is universal or selective. A universal
detector will sense al1 the constituents of the sample, whereas a selective
one will only respond to certain components. Although the response of the
detector will not be independent of the operating conditions, e.g. column
temperature or flow rate, it is advantageous if the response does not change
too much when there are small changes of these conditions.
A summary of these characteristics for different types of detectors is given
in Table 8.2.
Table 8.2 Typical detector characteristics in HPLC
-
Response Ca (g mL - ' ) Linear range*
Amperometric Selective 10 - l0 104-IO5
Conductimetric Selective 10 - ' IO3-104
Fluorescence Selective 10- l2 103-104
UVIvisible absorption Selective 104-105
Refractive index Universal 103-104
*The range over which the response is essentially linear is expressed
as the factor by which the lowest concentration (C,) must be
multiplied to obtain the highest concentration.
A detailed description of the various detectors available for use in HPLC is
beyond the scope of the present text and the reader is recommended to consult
the monograph by A brief account of the principal types of detectors
is given below.
Refractive index detectors; These bulk property detectors are based on the
change of refractive index of the eluant from the column with respect to pure
mobile phase. Although they are widely used, the refractive index detectors
suffer from several disadvantages - lack of high sensitivity, lack of suitability
for gradient elution, and the need for strict temperature control ( I0.001 OC)
to operate at their highest sensitivity. A pulseless pump, or a reciprocating pump
equipped with a pulse dampener, must also be employed. The effect of these
limitations may to some extent be overcome by the use of differential systems
in which the column eluant is compared with a reference flow of pure mobile
phase. The two chief types of RI detector are as follows.
1. The deflection refractometer (Fig. 8.4), which measures the deflection of a
beam of monochromatic light by a double prism in which the reference and
sample cells are separated by a diagonal glass divide. When both cells contain
solvent of the same composition, no deflection of the light beam occurs; if,
however, the composition of the column mobile phase is changed because
of the presence of a solute, then the altered refractive index causes the beam
to be deflected. The magnitude of this deflection is dependent on the
concentration of the solute in the mobile phase.
2. The Fresnel refractometer which measures the change in the fractions of
reflected and transmitted light at a glass-liquid interface as the refractive