Page 67 - Subyek Encyclopedia - Encyclopedia of Separation Science

62 I / CHROMATOGRAPHY/ Derivatization

rates characteristic of narrow-bore open-tubular col- withdrawn at ambient conditions, equivalent to the
umns require more sophisticated sample-introduction volume of the injection loop, and then inserted into
techniques based on sample splitting or solvent elim- the fully pressurized mobile-phase Sow by a simple
ination and refocusing mechanisms. rotation of the valve to change the mobile-phase Sow
The principal methods of detection are varied, con- paths. Although most separations are performed at
veniently grouped under the headings of gas-phase room temperature, either the column alone or the
ionization devices, bulk physical property detectors, whole solvent-delivery system may be thermostated
optical detection and electrochemical devices. Further to a higher temperature when this is desirable or
classiRcation is possible based on the nature of the required for the separation. The separation is
detector response } universal, selective or speciRc. monitored continuously on the low pressure side of
The Same ionization detector and thermal conductiv- the column using several bulk physical property,
ity detector are examples of (near) universal de- photometric, or electrochemical detectors Rtted with
tectors; the Same photometric detector, thermionic microvolume Sow cells.
ionization detector and atomic emission detector Common detection principles are UV absorbance,
are element-selective detectors; and the photoioniz- Suorescence, refractive index and amperometry.
ation detector and electron capture detector are struc- Coupling to MS and IR spectroscopy is becoming
ture-selective detectors. GC coupling to mass more common, as is online coupling to nuclear mag-
spectrometry and IR spectroscopy is straightforward netic resonance (NMR) spectrometers. Detection is
and widely utilized for automated structure iden- a more difRcult aspect in the condensed phase
tiRcation as well as detection. Detection in the and neither the variety nor operating characteristics
gas phase is a favourable process and GC detectors of LC detectors compare favourably with GC de-
are among the most sensitive and versatile by tectors, although they allow a wide range of sample
virtue of the range of mechanisms that can be types to be analysed routinely. Special materials are
exploited. used in the fabrication of biocompatible and cor-
rosion-resistant instruments for the separation of bio-
polymers and for ion chromatography. Individual
Liquid Chromatography
pumps can handle solvent delivery requirements
Modern LC employs columns with small particle over a decade range or so of Sow rates. The diversity
sizes and high packing density requiring high pres- of column diameters used in modern LC for analysis
sures for operation at useful mobile-phase velocities. and preparative-scale applications demands Sow
Syringe-type or single- or multiple-head reciprocating rates that vary from a few L per minute to tens
piston pumps are commonly used to provide the oper- of litres per minute. Consequently, instruments
ating pressures needed in conRgurations that depend are designed for efRcient operation within a par-
on the design of the solvent-delivery system. A single ticular application range and are not universal
pump is sufRcient for isocratic operation. A single with respect to column selection. Furthermore,
pump and electronically operated proportioning analytical detectors tend to be designed with sensitiv-
valves can be used for continuous variation of the ity as the main concern and preparative-scale de-
mobile-phase composition (gradient elution) or, al- tectors for capacity, such that the two are generally
ternatively, independent pumps in parallel (com- not interchangeable even when the same detection
monly two) are used to pump different solvents principle is employed. For preparative-scale work
into a mixing chamber. Between the pump and some form of automated sample fraction collection is
sample inlet may be a series of devices (check valves, necessary and economy of operation may dictate in-
pulse dampers, mixing chambers, Sow controllers, corporation of an integrated mobile-phase recycle
pressure transducers, etc.) that correct or monitor feature.
pump output to ensure that a homogeneous, pulseless
liquid Sow is delivered to the column at a known
Supercritical Fluid Chromatography
pressure and volumetric Sow rate. These devices may
be operated independently of the pump or in a feed- Instrumentation for SFC is a hybrid of components
back network that continuously updates the pump used in GC and LC modiRed to meet the requirements
output. Mobile-phase components are stored in reser- of operation with a compressible Suid. The mobile
voir bottles with provision for solvent degassing, if phase is typically carbon dioxide (with or without
this is required for normal pump and detector opera- modiRer) contained in a pressurized cylinder and de-
tion. Loop-injection valves situated close to the head livered to the pump in liquid form. Syringe pumps or
of the column are universally used for sample intro- cooled reciprocating piston pumps modiRed for pres-
duction. This allows a known volume of sample to be sure control are commonly used. A high precision