Page 418 - Instrumentation Reference Book 3E

Process chromatography 401

are available with different degrees of protection necessary to automate a particular instrument
against fire or explosion hazard. For operation in are usually assembled into a single module,
particularly hazardous environments, for exam- known as the programmer or controller.
ple where there may be flammable gases: instru- At the most basic level the programmer may
ments are available where the operation, consist of mechanical or electromechanical timers,
including temperature control, valve switching, typically of the cam-timer variety, to operate
and detector operation is entirely pneumatic, with relays or switches at the appropriate time to select
the oven being heated by steam. the sample stream to be analyzed, operate the
injection valve, and start the data recording pro-
18.4.5 Detectors cess, combined with a facility to correct the output
of the chromatograph for baseline drift. Most
A gas-chromatographic detector should have a fast chromatographs now use built-in microprocessors
response, linear output over a wide range of concen- that incorporate the programmer as part of the
tration, be reproducible, and have high detection central control and data acquisition facility. The
sensitivity. In addition the output from the detector programmer itself normally contains a micropro-
must be zero when pure carrier gas from the chro- cessor and is capable of controlling and monitor-
matographic column is passing through the detector. ing many more of the instrumental parameters as
In process chromatography, the most com- well as acting as a data-logger to record the output
monly used detectors are the thermal conductiv- of the chromatograph. Computer-type micropro-
ity and flame ionization types. Both have all the cessor-based integrators are available for labora-
desirable characteristics listed above, and one or tory use, and in many cases these have facilities to
other is suitable for most commonly analyzed enable them to be used as programmers for the
compounds: the thermal conductivity detector is automation of laboratory gas chromatographs.
suitable for permanent gas analysis and also This equipment is then integrated into a Eabora-
responds universally to other compounds, while tory Information Management System (LIMS) by
the flame ionization detector responds to almost which analytical results are transmitted directly
all organic compounds. In addition these detec- from the analyzer to a central database for use
tors can be ruggedly constructed for process use by customers across the facility.
and can be used with a wide range of carrier When the process chromatograph is operated
gases. M'ost other detectors have disadvantages in the automatic mode, all the time-sequenced
in comparison with these two, for example, fragil- operations are under Programmer control. These
ity, non-linear response, or a requirement for will typically include operations to control the
ultra-pure carrier-gas supplies, and although gas chromatograph and sampling system, such
widely used in laboratory chromatographs, their as sample stream selection, sample injection, col-
application to process instruments is restricted. umn or detector switching, automatic zero and
The helium ionization detector may be used attenuation adjustment, and backflushing. The
for permanent gas analyses at trace levels where programmer will also carry out at least some
the katharometer is insufficiently sensitive, and initial processing of the output data, by: for
the ultrasonic detector may be a useful alternative example, peak selection. It is also necessary for
in applications where a flame cannot be used or a process instrument to incorporate safety devices
where a katharometer cannot be used for all to prevent damage to itself or to the surroundings
components in a mixture. The selective sensitivity in the event of a malfunction, and also to give an
of the electron capture detector to halogenated indication of faults, which may lead to unreliable
molecules may also find occasional application. results. Functions which may be assigned to the
A comprehensive list of gas-detecting devices, programmer include: fault detection and iden-
indicating which are suitable for use in gas chro- tification, alarm generation, and automatic shut-
matography, is given in Table 18.2 above. down of the equipment when a fault is detected.
In addition to the automatic mode of oper-
18.4.6 Programmers ation the programmer must allow the equipment
to be operated manually for start-up. main-
Analysis of a sample by gas chromatography tenance, and calibration.
requires the execution of a series of operations
on or by the instrument at predetermined times
after the analytical sequence is initiated by injec- 18.4.7 Data-processing systems
tion of the sample. Other instrumental para- The output from a gas chromatograph detector is
meters must also be continuously monitored and usually an electrical signal, and the simplest
controlled. Process gas chromatographs incorp- method of data presentation is the chromatogram
orate devices to enable the analytical sequence to of the sample, obtained by direct recording of the
be carried out automatically, and the devices detector output on a potentiometric recorder.