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Gas Chromatography - General Theory
Type of Sample: Vapor or liquid
Sample Pressure: 0 to 100 psig (0 to 7 bars)
Ambient Temperature: -4 to 122 º F (-20 to 50ºC); however, a sheltered environment is recommended
Analysis Zone: 140 to 356ºF (60 to 180ºC) provided desired temperature is at least 18ºF (10ºC) above ambient
±0.1º F (± .05ºC) at steady ambient, ±0.1º F (± .05º C) FOR –4 TO 122ºF (-20 TO 50º C) ambient change
Contacting Material: Stainless steel, Teflon
Auxiliary Utilities: Instrument air, dry oil-free, at 50 psig (3.5 bars) minimum and 2 to 3 scfm (1 to 1.5 sl/s).
Carrier gas, zero air and hydrogen for flame ionization detector (FID). Possibly steam tracing for sample lines
Cycle Time: 2 to 20 minutes depending on application and packed or capillary column operation
Special Features: Accepts other inputs (e.g., flow rate and density) to include in calculation of heat rate in units of
kal/hr or BUT/hr; Multicomponent readout
Location: Class 1, Groups B, C, and D, Division 1 hazardous areas
Analyzer Cost: Basic analyzer $25,000 to $35,000 (depending on number)
Installed Cost: $60,000 to $125,000 (depending on number and application)
Suppliers: ABB Process Analytics; Applied Automation/Hartman & Brown; Carlo Erba; Daniels; Delsi; Fluid
Data/Amscor; The Foxboro Co.; Servomex and Beckman/Rosemount; Siemens; YEW Yokagowa Electric
The word "chromatography" has been coined to describe a physical method of separation that is based on the
differences in solubility (or adsorption) of substances between a mobile and a stationary phase. It has become one
of the most often used procedures in analytical chemistry for separation and analysis. The reasons for its popularity
can be traced to its ease of use for the separation of complex mixtures, its high sensitivity, and the small sample
required for the analysis.
There are a number of different versions of chromatography depending on the physical characteristic of the mobile
and stationary phases and on the mode of sample introduction. We shall be concerned in this section with the
elution method of chromatography using a gaseous mobile phase and a liquid (gas liquid chromatography-GLC or
GC) or solid (gas solid chromatography-GSC) stationary phase.
In the elution method of GLC, the sample is vaporized as a narrow plug, injected into a carrier gas (the mobile
phase), and forced to flow through a small diameter tube containing the stationary liquid phase. This liquid phase
may take the form of a thin film suspended on the walls of the tubing (wall-coated open tubular [WCOT] column, or
simply a capillary column) or it may be distributed over a solid support that is then packed into the tubing (packed
column).
In either case, separation takes place primarily because of differences in the solubility of the sample components in
the stationary phase. Thus, the column retains some of the components longer than others, and consequently, they
pass through the column at varying speeds. Provided that a stationary phase has been selected so as to maximize
these solubility differences, complete separation occurs, with the least soluble components coming through first,
followed by the next soluble one, and so on. Obviously the degree of separation (resolution) is a function of the
above-mentioned solubility differences and also the efficiency of the column, that is, its ability to produce very
narrow peaks.
Similar results are obtained with GSC. However, the separations are the result of differences in sample component
adsorption on active solid surfaces such as charcoal, silica gel, or alumina.
Chromatograph Operation
The basic instrumentation necessary to accomplish the above-described chromatography is extremely simple. It
consists of a supply of carrier gas (commonly helium, hydrogen, or nitrogen) with appropriate pressure regulation, a
sample valve (described further on) to inject a fixed volume of sample into the carrier gas stream, and a column
containing the stationary phase. Since the degree of solubility (and hence the retention time) is temperaturedependent, a precisely regulated temperature environment must be provided for the column. Finally, a detector that
responds to some property difference between the carrier gas and the sample components must be available, so
that it can be placed at the end of the column and indicate when the components are emerging. This detector
signal, when recorded as a function of time, produces the familiar chromatogram.
The time required for each of the components to emerge is called the retention time, and the magnitude of the
detector signal (peak height or peak area) is related to the amount of component present in the injected sample. If
these individual retention times and detector responses are made to remain constant (by adequate control of carrier
gas flow rate, amount of stationary phase, column temperature, and detector variables), the process can be
automated so that the instrument can provide this information quite precisely and cyclically on a repetitive basis.
The process gas chromatograph (PGC) is quite different from its laboratory counterpart. These differences appear as
the result of a number of factors, not the least of which are:
The need for continuous, reliable operation of the unit
The need for analysis within the time required for the process control system to achieve the proper control action
The nature of the environment in which the PGC is placed
The diverse user attitudes regarding maintenance, and user capability regarding the proper operation of the unit
The need to interface with modern computer-controlled process control system
Thus, a premium is placed on simple, reliable design to ensure a low mean failure rate but straightforward, rapid
repair when required; important, too, is the strategic placement of appropriate sensors in the system to aid in
diagnosing problems and provide alarms with regard to the operation of the unit.
To summarize, the PGC has the following distinguishing attributes: Located in the plant as near as possible to the
sample point
Dedicated to monitor one or more components in one or more process streams
Designed for continuous, unattended operation
Designed for operation in hazardous environments
Designed for exposure to weather, humidity, dust, and corrosive atmospheres
Contains integral hardware and software to allow communication with the process control system as well as remote
maintenance stations.
Contains alarms and diagnostic aids to continuously monitor the health of the instrument and aid in diagnosing
problems
Components of a Process Gas Chromatograph
What follows is a general discussion of these various parts and their specific function.
Analyzer
The analyzer section contains all of the basic elements of the chromatograph, namely, columns, sample valves,
column switching valves, and detectors, all enclosed in a precisely thermostatted oven. In some designs the
associated pneumatic components are also placed in the oven, and in others they are not. Which option is elected
depends on the quality of the components and/or robustness of the software to tolerate retention time changes that
may occur when pressure and flow controllers experience a varying temperature environment.
Oven
For economy and design simplicity, a single temperature zone is most frequently used. The basic designs that have
evolved are the air bath type and the explosion-proof bell enclosure. In the air bath design each of the appropriate
elements of the analyzer (such as the oven heater and the detector) is itself designed to be explosion-proof for
mounting in basically a non-explosion-proofed air bath enclosure, while the alternative design places all the elements
into an explosion-proof enclosure. Both designs satisfy Class 1, Group D, Division 1 classification.
Of the two designs, the most popular is the circulating air bath, in which air (at 3 to4 scfm, or l.5 to 2 sl/s), is passed
over an electrical heating element.
The temperature range is usually 122 to 338º F (50 to 170) (up to 437º F, or 225º C on special jobs) with a stability
of 0.2º F (0.1º C) or better. The operation of a typical air-purged oven and heater is illustrated in Figure 11d.
Although only isothermal oven operation has been offered in the past, recent market demands for rapid simulated
distillation, Reid vapor pressure, and other volatility measurements has encouraged some vendors to offer a
temperature-programmed version. At this writing none of these units has proven to be as reliable as the isothermal
versions, nor have any been certified as yet for use in Division 1 areas.
Temperature programming is a procedure for decreasing the analysis time for a sample consisting of compounds of
widely divergent boiling points and polarity. The temperature of the column is usually maintained at some initial
value for a fixed period of time while the low boilers elute from the column, and the temperature is then raised in a
linear fashion (and at a suitable rate) during the remainder of the analysis period so as to ensure the separation of
the remaining sample components within a much shorter period of time than if the analysis has been run
isothermally. The oven must then be cooled rapidly to exactly the same starting temperature and programmed again
at exactly the same rate in order to obtain reproducible retention times for the various constituents of the sampleclearly, not a trivial design task.
Valves
Valves are used for column switching and for the injection of the sample. The most popular of these are the rotary,
sliding plate, and diaphragm mechanical valves and the so-called valveless, or Deans switch . The use of the older
linear, or spool-type, valves has been all but abandoned because of their inability to produce the sharp input sample
profiles required for modern, efficient chromatography.
The sample valve is designed to deliver a sample of constant volume. For very small samples (less than 1 microliter),
the volume is defined internally by a hole or slot machined into the movable member (sliding plate or rotor); for
larger volumes, it is defined by an external loop made from a length of tubing of known dimensions attached to the
valve.
In the past, the smaller volumes were used for liquids, but with the increasing popularity of capillary columns
(requiring much smaller samples than packed columns these valves are being used for the injection of gases. In any
case, liquid samples pose particular problems, from the point of view of sample valve reliability, sample handling,
and safety. Thus operators should try as much as possible to avoid the necessity for liquid sample injection by
vaporizing the sample before presentation to the sample valve.
E-mail: [email protected]
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Copyright © 2003 Analytical Specialties Incorporated
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THEORY
Gas Chromatography - Detector Theory
Thermal Conductivity Detector (TCD)
Theory of Operation
The Thermal Conductivity Detector uses four spiral wound filament wires supported inside cavities in a metal block.
The filaments are manufactured from a material whose electrical resistance varies significantly with variations in
temperature.
A constant DC current of up to 100 mA is applied to the filaments in an electronic bridge circuit. With pure carrier
and reference gas flowing across the filaments, the heat loss is constant leading to a constant filament temperature.
This consistent filament temperature produces a constant filament resistance. The currents in the electronic bridge
can be balanced to produce a zero signal level as a reference.
When component peaks enter the TCD with the carrier, the heat dissipated from the filaments on the measure side
changes. This leads to an imbalance in the electronic bridge. The resulting electrical signal is then used to measure
the quantity of the component.
8-Port TCD
An 8-port TCD is typically used to measure hydrogen and nitrogen on the same analyzer. This detector functions by
connecting a second injection valve and column set to the pair of detector filaments used as the reference in a
standard TCD. One side (train) uses a nitrogen carrier to measure hydrogen. The other side uses hydrogen or
helium carrier to measure nitrogen, carbon dioxide, oxygen, etc. Each side serves as a reference for the other side.
This configuration provides the advantage of measuring hydrogen, nitrogen, carbon dioxide, oxygen, etc., with one
analysis method that prints all the results on the same report. The injection time for each train (side) is set to ensure
that no peaks elute from both trains at the same time.
Detector sensitivity
Three factors affect TCD sensitivity. The first factor is bridge current. Increasing the bridge current causes an
increase in the voltage across the bridge circuit. This results in an increased signal amplitude and detector
sensitivity. However, if the bridge current is increased too much, the base line signal can become unstable and the
filament life may be shortened.
The second factor affecting TCD sensitivity is the relationship between the thermal conductivity of the carrier gas
and that of the sample. Greater sensitivity results from greater differences between these. Helium or hydrogen have
high thermal conductivity compared to nitrogen and air. Using a helium or hydrogen carrier permits much higher
bridge current than possible with nitrogen. This results in increased sensitivity.
The third factor affecting TCD sensitivity is the filament temperature. Increasing the bridge current will increase the
filament temperature and detector sensitivity. However, increased filament temperature can lead to shortened
filament life.
Flame Ionization Detector (FID)
Description
The FID is a horizontal flame detector incorporating a collector fashioned from stainless steel. The polarizing voltage
is applied directly to the flame tip assembly which is electrically isolated from the FID body. Typical polarizing
voltage is +15 volts DC. Higher polarizing voltages may be used although tests show that this has minimal effect on
detector sensitivity.
Flame On and Flame Off conditions are sensed by a thermocouple junction just outside the flame area. The Excel V
monitors the temperature near the flame to sense the presence or absence of the flame. Automatic flame ignition
can be enabled or disabled for each detector. The ignitor can also be fired manually. Typically, burner air flow is set
at 200 to 245 cc/min and burner hydrogen is set at 40 to 45 cc/min.
Theory of Operation
When sample material enters the FID, the compounds are burned by the flame. The ions produced result in a
proportional current flow inside the detector. This current is amplified and converted to a voltage signal. The
amplitude of the signal is proportional to the concentration of the component ionized. For more detailed theory on
FID operation, please consult appropriate literature on detectors used with chromatography.
The FID requires a hydrogen flow rate of 30 to 50 cc's per minute and an air flow of 200 to 300 cc's per minute for
best overall performance. Optimum sensitivity may be achieved by varying the flow rates somewhat; however, this
model FID is relatively unaffected by this technique.
Column connection to the FID is made by teeing the column effluent with the hydrogen at the FID. Experience
indicates that a standard tubing tee can be used for this connection. Some applications may require a zero volume
tee for reduced peak tailing.
Flame Photometric Detector (FPD)
Description
The Flame Photometric Detector (FPD) is a flame FPD used to detect volatile sulfur compounds. It is primarily used
to measure trace levels (< 500 ppm) of sulfur compounds (e.g., H2S, COS , SO2, CS2, and Mercaptans) in various
hydrocarbon backgrounds. This FPD provides about 10,000 to 1 discrimination of hydrocarbon to sulfur compounds.
There are two methods for output linearization. Square root extraction uses an electronic circuit to linearize the FPD
output. Sulfur addition uses the addition of a small amount of methyl mercaptan to the FPD air to provide a linear
output and increased sensitivity.
Theory of Operation
The FPD provides a hydrogen rich flame where sulfur compounds are reduced to the elemental S2 species which
produce a blue chemiluminescent emission in the visible light spectrum. The blue light emission is passed through a
narrow band (395 nanometer) optical filter to the photomultiplier tube. The signal from the photomultiplier tube is
further amplified by an electrometer and interface board.
To minimize condensate buildup in the detector, the FPD block is typically maintained above 100° C. The
photomultiplier tube housing is maintained at a cooler temperature to extend its service life.
The intensity of the blue light emission is approximately proportional to the square of the sulfur concentration in the
flame. In some applications, a small amount of methyl mercaptan (2 to 5 ppm) is continuously added to the FPD air
to bias the output level upward to the linear portion of the output curve. Generally, a permeation device is used to
provide a consistent addition of methyl mercaptan to the FPD air. A small sample size is also used to ensure
operation in the linear portion of the curve. The FPD is available with a square root extraction option that eliminates
the need for sulfur addition.
Hydrocarbons entering the detector have a quenching affect causing false or real flame-outs. If a component of
interest and a hydrocarbon peak elute at the same time, erroneous readings can result. Large hydrocarbon peaks
can cause a gradual buildup of carbon deposits on the detector wall and the quartz window. As these deposits
reduce the amount of light entering the photomultiplier tube, detector sensitivity decreases. Valve and column
arrangements should be used to minimize the amount of hydrocarbon materials entering the FPD detector.
Sulfur Chemiluminescence Detector (SCD)
The Sulfur Chemiluminescence Detector (SCD) is used for detection of sulfur. In this detection process, sulfur
monoxide is reacted with ozone to form an electronically excited species of sulfur dioxide which emits light when it
relaxes to the ground energy state. The light emitted by this reaction is detected and amplified by a photomultiplier.
The response from this reaction is linear over at least three to five orders of magnitude and demonstrates a
selectivity to sulfur over carbon of 106 – 108 to one.
A chromatograph injection valve injects the sample into a flow of air carrier gas which passes to a dual chamber,
flameless combustion chamber. Additional air is introduced just prior to entry into the first combustion chamber.
Hydrogen is introduced into the second combustion chamber.
In the combustion chambers, the hydrocarbon matrix of the sample is oxidized primarily to carbon dioxide and
water, while the sulfur components are oxidized to sulfur dioxide (SO2), water, carbon dioxide (CO2), and some
sulfur monoxide (SO). Sample from the second combustion chamber is drawn to the reaction cell by a vacuum
source. Ozone from an ozone generator is also routed to the reaction cell.
For the chemiluminescence reaction to take place in sufficient quantity for the required sensitivity, the reaction cell
must be maintained at a predetermined vacuum. As reaction cell pressure is increased, relaxation of the excited
sulfur dioxide to its ground state increasingly takes place through mechanisms of intermolecular collision rather than
the chemiluminescence reaction. In addition, the SO formed in the combustion chamber is a short-lived free radical
and must be transferred to the reaction cell quickly before competing reactions can occur. The low pressure of the
reaction cell causes both a fast transfer time and a quenching of possible competing reactions. The primary
reactions taking place in the system can be represented as follows:
RH + RS + O2 <-> SO + SO2 + CO2 + H2O
SO + O3 <-> SO2 <-> SO2 + hv
At the low pressures of the reaction cell, carbon dioxide and water do not cause appreciable interference or
intermolecular quenching of the signal, and water vapor condensation does not occur.
Although many sulfur compounds can chemiluminesce directly by reaction with ozone, the combustion steps in this
process are necessary for several reasons. The direct chemiluminescence reactions exhibit a wide variation in molar
response, probably due to different mechanisms of the attack of the ozone oxidant. In each case, the SO radical is
probably the reactive intermediate that ultimately causes the chemiluminescence.
However, differing reaction kinetics and mechanisms of oxidation determine the various sensitivities observed. The
chemiluminescence reaction is not affected by hydrocarbon or carbon dioxide quenching. Combustion of the sample
in the flame serves to break down the large hydrocarbon molecules in order to maximize the sensitivity and
selectivity of the detector. Combustion also serves to provide an equimolar response by breaking down the complex
sulfur containing molecules into a common sulfur species prior to reaction.
Since the SCD pre-combusts the sulfur containing compounds in the sample to SO and SO2 prior to the reaction with
ozone, the chemiluminescence response is equimolar to all different types of sulfur compounds found in the sample.
The burner used for combustion is a two-stage, or dual-chamber unit. Use of the dual-chamber combustion chamber
has two advantages. Since combustion is more complete, hydrocarbon quenching and non-equimolar response are
minimized. Use of combustion chamber with the SCD results in a detector that is equimolar and virtually free from
quenching.
Thermionic Ionization Detector (TID)
The Thermionic Ionization Detector is a highly selective detector. For example, when the appropriate thermionic
source is used, the TID is very sensitive to chlorinated compounds and relatively insensitive to other compounds.
This characteristic of the TID can be used to improve measurement of certain types of components, especially when
these components are obscured by other components when using an FID.
The type of components detected by a TID are determined by the type of mineral source chosen for the detector.
The mineral source is heated to generate negative ions which produce a signal current that is measured with an
electrometer. Measurement components react with the generated ions producing a change in signal flow resulting in
production of a negative peak on the chromatogram. (The Excel V can reverse the negative peaks.) Other
components have little effect on the ions and produce very little change in signal current.
The TID uses a modified Flame Ionization Detector body. The FID ignitor is removed and a modified Swagelock
fitting is mounted on the top of the FID cap. The TID source is mounted in this fitting. The thermionic source is
heated by passing a precisely controlled current through a wire inside the mineral source. The current heats the
mineral source to its operating temperature causing it to generate a flow of ions between the source and the
collector in the detector. The heat of the source also burns the hydrogen and air mixture in the detector.
In typical applications, the wire providing current to the TID source is routed a short distance inside the analyzer
oven. This wire typically has a relatively low operating temperature limit of 80° C. The oven temperature must not
be set higher than the rated operating temperature of the TID source wire portion that is located inside the oven.
E-mail: [email protected]
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Copyright © 2003 Analytical Specialties Incorporated
All Rights Reserved