Supply with carrier gas, combustion gas and auxiliary gases
A gas chromatograph must be supplied with carrier gas and, if applicable, combustion gas and other auxiliary gases. The carrier gas is used to transport the sample through the analytical system. Auxiliary gases are used to operate valves, as combustion gases for flame ionization detectors, and to purge the oven.
Injection system
The injection system is the link between the continuous process stream and the discrete analytical process. It is responsible for injecting an exactly defined portion of the sample in a reproducible and pulsed manner (as far as possible) into the carrier gas stream.
The injection can be carried out in the conventional manner using valves or by means of a live injection:
- Gaseous samples (0.1 to 5 ml)
- Vaporizable liquid samples (0.1 to 10 l)
Gas injection valves
Model 50 10-port valve:
- Combined gas injection and backflushing valve
- Activation by pressure on the membrane without moving parts
Model 11 6-port valve:
- Can be used as gas injection valve, liquid injection valve or for column switching
- Membrane controlled by tappet
- One million switching cycles without maintenance
Liquid injection valve FDV
A constant quantity of a liquid sample can be automatically injected using the liquid injection valve, and subsequently vaporized rapidly and completely. The valve can also be used to inject small volumes of gas.
The liquid injection valve consists of three sections:
- Thermostatically-controlled vaporization system
- Sample passage section with seal
- Pneumatic drive
Liquid injection valve FDV
Features:
- Vaporization temperature 60 to 400 °C
- Injection volume 0.3 to 9.5 l
- Ambient temperature -20 to +150 °C
- Material of parts wetted by sample: Stainless steel, mat. no. 1.4571, Hastelloy, Monel or special materials
- Control pressure 400 to 600 hPa
- Max. sample pressure 5 000 hPa, recommended 0.5 to 1 000 hPa
- Connections for pipe: 3 mm outer diameter
Live injection
Flexible selection of the injection volume which is exactly matched to the requirements of the columns is possible with a live injection.
Live injection
Oven
A further important factor for the separating performance is the temperature This has a very high influence on the vapor pressure of the individual components, and thus on the diffusion between the mobile and stationary phases of the column. This influences the retention times, and thus the identification of components. Therefore very high demands are placed on the temperature stability of the oven and also on the injection equipment and the detectors.
Two different types of oven are available:
Airless oven for extremely stable isothermal oven temperatures (0.02 °C control accuracy) up to 280 °C.
Airbath oven for
- isothermal (5 to 225 °C) or
- temperature-programmed mode
Both types of oven are available as
- single ovens or
- dual ovens.
With the dual ovens, two separate heating circuits provide independent oven temperatures. It is then possible to carry out two different applications in one chromatograph.
In order to measure sample components with highly different volatilities, a temperature program is frequently used for the chromatographic separation. In this case the column temperature is continuously increased according to a selectable heating-up rate. This method (PTGC) is available with the MAXUM edition II.
The internal oven consists of a chamber with low thermal capacity located within the standard oven. It contains the capillary column used for the separation.
The ovens have separate, independent temperature control. The temperature of the internal oven is freely-programmable. The temperature changes according to the time-dependent profile assigned to the respective analysis. Up to three linear ramps and four constant periods can be configured.
It is then possible to determine components with low and high boiling points in one analysis. Laboratory applications can be opened up by PTGC for use in the process industry.
"Simulated distillation" is an important application of PTGC in refineries. The distillation range - a quality criterion for fuels - is chromatographically traced "online".
Columns
The columns are the central component of the chromatograph. They resolve the gas mixture into its individual components. The following distinction is made:
- Packed/micropacked columns with inner diameter of 0.75 to 3 mm
- Capillary columns with inner diameter of 0.15 to 0.53 mm
Packed columns are mechanically stable and simple to handle. Capillary columns have a significantly higher separating performance, often with a shorter analysis period and lower analysis temperature.
Types of column
Column switching systems
Process chromatographs are almost always equipped with column switching functions. Column switching is understood to be the combination of several columns in the carrier gas path which are arranged in succession or parallel. These columns usually have different separating performances, and are interconnected by valves for switching over the gas path. A distinction is made between backflushing, cut and distribution.
A wide range of techniques is available for column switching.
The techniques comprise highly stable membrane gas valves, membrane piston valves, sliding vane rotary valves and also valveless switching techniques.
Valves
Model 50 10-port valve:
- Combined gas injection and backflushing valve
- Activation by pressure on the membrane without moving parts
- Switches gas samples at an overpressure of 0 to 5 000 hPa
Model 11 6-port valve:
- Can be used as gas injection valve, liquid injection valve or for column switching
- Membrane controlled by tappet
- One million switching cycles without maintenance
Valveless switching technique
The valveless live column switching is exactly controlled by electronic pressure regulators, and prevents falsification of results since the sample does not come into contact with valves. A special pressure-controlled coupling element connects the capillary columns.
This technique is optimally suitable for capillary columns, and offers the best long-term stability and reliability. Live column switching is a technique where backflushing, cut or distribution is carried out on two different columns without any switching of valves or other moving components in the separation path.
This is achieved using a unique coupling unit, the live T-piece. Its function is based on a pressure difference controlled by the electronic precision pressure controllers of the MAXUM edition II. Because there is no dead volume whatsoever, it is ideally suitable for the low flow rates used with capillary columns. Maintenance of the column switching configuration is then superfluous, the separating performance is improved, and complicated separating procedures are simplified.
Column switching systems (examples)
Live switching
Solenoid valve control module
- Contains all control elements in one module in order to reduce downtimes during repairs to a minimum
- Has 3-way and 4-way distributors for control of many different types of valve
- Uses separate, plug-on pipe connectors to permit implementation of variable gas supplies
Electronic pressure controller module (EPC)
- Permits exact control of pressure without mechanical pressure regulator. Shortens the setup time since the pressure is set by an operator input.
- Permits programmable pressure changes for fast chromatography and modern applications.
- Controls the supply of carrier gas and combustion gas Avoids drift and deviations which can occur with mechanical pressure control.
Detectors
Thermal conductivity detectors (TCD) and flame ionization detectors (FID) are mainly used in process chromatography. Specific detectors such as flame photometer detector (FPD), electron capture detector (ECD), photo-ionization detector (PID), or helium ionization detector (HID) are used to a lesser extent.
The detector modules described above can be combined together in many different manners in the MAXUM edition II.
- A maximum of three detector modules can be used in the airbath oven.
- Up to three modules (depending on the type) can be used in the airless oven, the dual airless oven and the ovens with temperature programming.
- Multiple modules can be operated in parallel at offset times in order to increase the number of analyses within a specific time.
- Parallel modules can each be used for one sample stream. This shortens the total cycle time with multi-stream applications.
- Parallel use of two identical modules provides redundant measurements which can be compared together, thus reducing the necessity for calibration.
|
Detector |
Measured value dependent on: |
Selectivity |
Application example |
|---|---|---|---|
 |
Concentration |
Universal |
Main and subsidiary components |
 |
Mass flow |
Hydrocarbons |
Hydrocarbons |
 |
Mass flow |
Substances containing S or P |
Traces of sulfur in HC matrices |
 |
Mass flow |
Universal (except He and Ne) |
Ultra-pure gas analysis |
 |
Mass flow |
Molecules with electronegative groups |
Traces of halogenated hydrocarbons |
 |
Mass flow |
Selective, dependent on ionization potential |
Suitable detectors for process gas chromatography
Thermal conductivity detectors (TCD)
The measuring principle of the TCD is based on the difference between the thermal conductivity of a pure carrier gas stream and that of a gas mixture containing carrier gas and a component eluted from the column. Therefore all components whose thermal conductivity differs from that of the pure carrier gas can be detected by a TCD.
TCDs mainly comprise two measuring cells and two reference cells which are electrically heated and contain wire resistors connected in the circuit of a Wheatstone bridge.
The bridge resistors are balanced as long as pure carrier gas flows through the measuring and reference cells. If a mixture of carrier gas and sample component flows through the sample chamber, the change in thermal conductivity of the gas mixture also changes the temperature and thus the resistance of the heating wires.
The resulting offset in the bridge circuit is directly proportional to the current concentration of the sample component in the carrier gas stream.
Versions of TCDs:
- Thermistor detector
- Filament detector
Both detectors are available for universal use, except that the filament detector is more sensitive than the thermistor detector and can also be used at higher temperatures. The thermistor detector is available as a block with 6 measuring detectors and two reference detectors. The filament detector is available as a 4-fold block.
Flame ionization detector (FID)
With the flame ionization detector (FID), the gas mixture leaving the column is burnt in a hydrogen flame. If this gas mixture contains flammable organic compounds, ions are generated during the combustion which are collected on an electrode.
An electrode voltage is applied between the nozzle from which the flame burns and the electron collector positioned above it.
The resulting current is amplified, and is the measured signal.
In contrast to the TCD (concentration-dependent signal), the signal with the FID is proportional to the mass flow of the components.
The FID features a linear range of 6 to 7 powers of ten, and permits detection limits of less than 0.1 ppm (referred to the concentration of the hydrocarbon in the sample). Non-flammable components (e.g. inert gases and water) cannot be measured with the FID.
In addition to the carrier gas, hydrogen and air are required as the flame gases to operate this detector.
Flame photometer detector (FPD)
Further detector principles are used for determination of trace concentrations of specific components. For example, the flame photometer detector is used to determine traces of compounds containing sulfur or phosphor. The emission of light of characteristic wavelengths is measured when burning the substances in a hydrogen flame.
Pulsed discharge detector (PDD)
The detector can be used in three different versions: HID (helium ionization detector), ECD (electron capture detector) and PID (photo ionization detector). Installation in the Maxum GC is possible without further modification, and the detector can only be used in non-hazardous areas. The PDD uses stable, pulsed DC discharges in helium as the ionization source. The detector's performance data is equal to or better than that of detectors which use radioactive ionization sources. Since a radioactive source is not used, the complex directives for radiation protection need not be observed by the customer.
- PDHID (helium ionization detector)
The PDHID works almost destruction-free with an ionization rate of 0.01 to 0.1%, and has a high sensitivity. The sensitivity for organic components is linear over five orders of magnitude, and the detection limit is in the low ppb range. The PDHID can be used universally for organic and inorganic components, with the exception of helium and neon. - PDECD (electron capture detector)
In electron capture mode, sample components with a high electron affinity can be selectively detected, such as halogenated hydrocarbons. The detector's properties and sensitivity are comparable with those of a 63Ni ECD. It is necessary to use a supplementary gas in this mode (recommended: 3% xenon in helium). - PDPID (photo ionization detector)
Addition of argon, krypton or xenon to the carrier gas results in ionization of the added gas. The detector is used in this configuration for selective detection of aliphatic compounds, aromatic compounds and amines. The selectivity can be determined through the choice of added gas. The sensitivity in this mode is limited to sample components whose ionization potential is below the emission energy of the added gas.
Accessories: Catalytic air purifier
Instrument air is usually contaminated by traces of hydrocarbons. If this air is used as the combustion gas for a flame ionization detector (FID), the impurities are evident as disturbing background noise.
The catalytic air purifier eliminates interfering impurities of hydrocarbons in the combustion air for the FID detector. The products of the catalytic oxidation (H2O, CO2) have no influence on the detector. Use of the catalytic air purifier significantly reduces the background noise. It has a flameproof enclosure and is therefore explosion-proof.
The air within the purifier is passed through a spiral lined with palladium. This metal spiral is heated up to approx. 600 °C. Palladium has a high activity at this temperature, and almost complete catalytic oxidation is achieved despite the short dwell time. The air subsequently passes through a cooling loop, and is output purified and cooled.
Parallel chromatography
This function divides a complex application into several simple sub-applications which are analyzed in parallel. This reduces the cycle times.
The hardware and software of the MAXUM edition II permit a complex chromatographic analysis to be divided into several simple analyses. Each of these simple analyses can then be simultaneously executed in parallel. This not only simplifies the complete analysis, it can also be carried out faster and with greater reliability.
State-of-the-art communication
TCP/IP communication and standard Ethernet hardware mean that MAXUM edition II is compatible with many networks.
Software
For simple operation and maintenance, MAXUM edition II offers an online software system with local operation over an HMI and a flexible GUI accessible using a computer workstation.
The online software system is installed in every MAXUM edition II or NAU and includes:
- Embedded EZChrom evaluation
- Embedded MaxBasic in the runtime version
- Communications software, network software, I/O driver in order to operate the gas chromatograph
The workstation software consists of:
- MAXUM edition II workstation tools:
- System manager for network overview
- EZChrom method builder
- MMI maintenance panel emulator
- Data logger
- MODBUS download utility
- Backup and restore utilities
- Online system download utilities
- Optichrom Advance APC version 8.1 (emulation of old Optichrom operation)
- Online help and documentation
and optional packages for individual ordering, e.g.:
- MaxBasic editor
- Simulated distillation method
- OPC communications server
Compatibility
MAXUM edition II is compatible with all older types of chromatograph from Siemens: PGC 302, RGC 202, Advance Maxum, Optichrom Advance.
Application
Certain parameters must be observed during application and subsequent operation of the MAXUM edition II. It can then be determined qualitatively whether the task is fulfilled. The basic prerequisite for this is that all components can be detected clearly isolated from the interfering components. Important parameters are: Analysis period, measuring ranges, detection limits and repeatability of the results.