Firma:                   ECH

Modell:                Mobil GC

Kommentar:        Dokumente engl.

The inovative techniques applied by the fully automated MobilGC open up new application opportunities for various analytical tasks. lt has been specifically designed for on-site measurements. Operation with an-live techniques can be performed without reservations.

An important component of the gas chromatograph is the newly developed column oven. Its heating system consists of a Compact heating unit. The separation column is directly mounted in this unit. The complete system is insulated and protected by a metal cage. As a package, the separation case can be easily installed or taken out again thereby allowing a very flexible column choice.

The analog/digital convertor of the detector produces digitised measurement signals that are processed in the connected computer. A microcontroller contained in the MobilGC controls the process. The use of digitised signals allows a simple structure and miniaturisation. Due to a remarkably improved signal-noise-ratio the detection sensitivity is very high.

Communication of the MobilGC electronics of the with the connected computer is performed via serial Interface. Remote control of the MobilGC as well as recording of all important data and chromatograms is facilitated by the control program of the computer.

Technical Parameter

Dimensions:                           

width:                                       380       mm

height:                                      450       mm

depth:                                      235       mm

weight:                                     ca. 20 kg

Operating temperature:           5..40 °C

Storage temperature:              0..75 °C

Relative humidity:                    0..80

Mains voltage:                         230V/50Hz

The following components are connected in the basic device:

•            pneumatic module

•            detector oven (with detectors, injectors and valves)

•            column oven (with separation columns)

•            Power supply

•            Control electronics

Pneumatic module

The basic device contains a pneumatic modu le, consisting of two pressure regulators and the necessary manometers. All tubing in the MobilGC is made of 1,6 mm stainless steel tubes and aluminium or GRAPH-FITT-connections respectively. Gaschromatographic precision regulators are used as pressure reg ulators. They can be identified by appropriate labels.

carrier gas:      helium, argon or nitrogen

fuel gas:           hydrogen

synthetic air

In most cases the pre-pressure is adjusted within a range between approx. 0,5-1,6 bar. For smooth Operation, the pressure difference between inlet and outlet of the pneumatic module should be at least 1 bar.

The scheure of pneumatic part is shown in part 7.

Detector oven

The detector is placed in a heated, thermo-insulated chamber at the back of the MobilGC. Two FID detectors are included, they are working independently.

Power supply

The power supply transforms the mains voltage (220..230V/50 Hz) and supplies the device with the necessary low voltages.

Controt Electronics

In the MobilGC a micro Controller-card developed by the ECH is integrated, serving as separate measure card. It processes and interprets the signals coming from the detector and realises the control Gourmands of the control computer (PC). Communication between control electronics and the connected computer is performed via serial Interface employing the relevant connection line.

Control Computer

capacity or higher is employed on which the control software is installed. Only this combination makes it possible to use the MobilGC as a portable device for on-site analysis. Notebook-PCs are featured by an integrated accumulator that allows operation independent of mains power supply. For laboratory operation the notebook-PC is connected to the mains adaptor.

Further it is possible to use a converter for longer mobile operation, for example to convert the bord voltage of a car to 220 V/50 Hz AC.

Iniection svsterns

SplittlSolittiess-Iniector

The MobilGC can be equipped with an injector mounted in horizontal position on the left front panel. Gases or liquids can be introduced by the injector piercing through the septum. It is recommended to change the septum after 20 injections. By using a syringe with a rounded needle and a hole on the side of the needle , longer working period is possible.

The temperature of the injector is identical with the temperature of the FID oven. Note! The accuracy of the injection (always the saure procedure) has great influence on the reproducibility of the results.

In Order to reduce the volume to be injected into the injector, a needle valve is employed. It is found at the back of the Instrument (<split>). By turning the needle valve Counter-clockwise, the sample volume to be injected is reduced. The split decrease is measured at the tube <split out>. Normally the split outlet is closed. Note! After modifying the split ratio a completely new calibration is necessary.

Automated qas sample valve

This valve is used for sample application. It consists of a valve with sample loop and integrated membrane pump. By turning the valve to position 1 the sample loop fills up, in position two the sample is applied onto the column.

Separation columns and column oven

Column oven

The column oven is built as two compact Gases for the two columns, consisting of a heated element, the column, the fan (aerator) for efficient cooling, insulation material (ceramic wool) and the metal cage for protection. The oven can hegt the two columns independently up to 300°C isothermally, or with a temperature gradient.

Separation columns

For good qualitative analysis the column separates the components from the injected gas mix. The order of the components is always the saure, each having its own retention time. For best separation results a capillary column is used with an inner diameter from 0.32 up to 0.53 mm and a length of up to 30 meters.

Before the first measurement is run, the column has to be conditioned, until the base live reaches a stable value. The conditioning is performed at maximum oven temperature and with carrier gas current.

In the column oven the capillary column will be heated up to 250 °C isothermally, or with a temperature gradient of maximum 40 degree/min. For best reproducibility it is recommended work with a temperature gradient a little bit lower than maximum (f. e. 30 degree/min) in order to compensate differences in the local power supply. The oven module is built as a compact case, consisting of two heating elements, the column, fans for efficient cooling, insulation material (ceramic wool) and a metal cage for protection. For reasons of safety, the case is connected to the ground. The temperature is controlled by a micro Controller within the MobilGC. The temperature, or heating rate respectively, can be entered in the control program an the PC. After the heating is turned off (and before closing the main valve of the gas supply), the carrier gas current cools the column until it reaches room temperature. In case of breakage, the column can be pulled out to a limited extent. There are two possibilities to change the column:

•            Exchange of the whole inner part of the column oven, containing the mechanical part, the column and the heating elements or

•            dismantling the oven and just replace the column.

The first way is rauch more easier, but takes a longer time as the unit is send to and send back from the MobilGC manufacturer. To dismount the column or the complete

inner oven it is necessary to disconnect the both ends of the column from the system. Both ends are connected with a special stainless steel screw, sealed by graphite sealing. The left end comes from the injector/dosing valve and the right end goes to the thermal conductivity detector.

If any problems should occur, please consult your service team or our application laboratory.

Capillary as well as packed columns can be used for Separation. Capillary columns are sealed by the Graph-Fitt-system. The 1/8" packed columns are fixed with bite­type fitting connections. One column is connected with the FID detector 1, the other with the FID detector 2.

FID-Detectors(Flame lonization Detector)

The Mobi[GC contains two flame Ionisation detector (FID).

The FID responds to almost all organic components. Its sensitivity, stability and simple maintenance make it the most frequently used detector. The Sample components are measured destructively. After outflow from the detector, they cannot be analyzed by another detector.

Technical parameter

•       limit of detection:    ca. 5 x 10 ^-12 g carbon

•       linearity:                   > 10 ⁶

•       techn. data:             nozzle:                      Ceramics

                                      Constitution:              Gastfight housing

                                      Dead volume:           zero by Capillary columns,

                                                                       negligible by packed columns

                                      Make-up-Gas:           possible

                                      Polarisation voltage: -170 V

                                      Range:                      10 ^-9. .10 ^-12 A/mV

                                      Sensitivity:                 15 mC/g for hydrocarbons

                                      noise:                        10 bis 25 Counts

Operation

A voltage is put up at the flame nozzle of the detector. The carrier gas passing out of

the column is warmed at the nozzle top by a hydrogen flame. This produces ions which emit a weck current (signal) an the way to the collector: In case the carrier gas contains an organic substance its ionization increases the current (signal), identified as a peak in the chromatogram.

It is advisable to hegt the detector before ignition for 20..30 min until the desired temperature is reached. The FID has an own heating element and operates isothermally at 100..180°C. The temperature regulation of the detector oven is set by the control program of the Computer. The value raust not exceed 180 °C, but it is important to work with FID at a temperature above 100 °C, otherwise condensed water will be produced.

The hydrogen flow during ignition can be higher than the recommended flow during operation. A glow plug ignites the flame, controlled by a green electric button in front of the device. Successful ignition is marked by an immediate increase of the indicator value and a muffled bang. It is also indicated by the steaming of a mirror mounted above the detector outlet.

After the ignition, a remarkable variation of the indicator value occurs. It should be given time for re-stabilisation. Then the hydrogen flow can be reduced to normal until reaching a base line of some thousand pV.

For trouble reduction the hydrogen flow should not be changed - it is optimized for safe ignition and working.

The base line can be influenced by FID-temperature, H2-flow and others. Therefore adjustment to desired values is possible. A potentiometer, located at the back of the Instrument allows for adjusting the base line of the FID. Carefully change the potentiometer screw, a small turning causes a big difference of the base line value. The high gain of the amplifier often causes an overmodulation of the base line, or a setting in the negative range. This does not cause any damage to the amplifier, however, it makes it more difficult to find a new operation value and therefore the right adjustment of the base line.