520 Gases
Contents
1. methylene chloride or other halogenated solvent to clean tubing that will be used with an electron capture detector They will cause elevated baselines and detector noise until they are completely flushed out of the system Gases should be supplied to the instrument only through preconditioned copper tubing part no 5180 4196 Do not use ordinary copper tubing it contains oils and contaminants Do not use plastic tubing to supply detector and inlet gases to the GC It is permeable to oxygen and other contaminants that can damage columns and detectors and can melt if near hot exhaust or components The tubing diameter depends upon the distance between the supply gas and the GC and the total flow rate for the particular gas One eighth inch tubing is adequate when the supply line is less than 15 feet 4 6 m long Use larger diameter tubing 1 4 inch for distances greater then 15 feet 4 6 m or when multiple instruments are connected to the same source You should also use larger diameter tubing if high demand is anticipated for example air for an FID 6 of 14 Site Preparation Jun 2001 Agilent 6890 Gas Chromatograph Service Manual Gases 520 The gas plumbing Be generous when cutting tubing for local supply lines a coil of flexible tubing between the supply and the instrument lets you move the GC without moving the gas supply Take this extra length into account when choosing the tubing diameter Two stage pressure re2. 520 Gases Gas requirements Gases for packed columns The carrier gas you use depends upon the type of detector and the performance requirements Table 520 1 lists gas recommendations for packed column use In general makeup gases are not required with packed columns Table 520 1 Gas Recommendations for Packed Columns Detector Carrier Gas Comments Detector anode purge or reference gas Electron Capture Nitrogen Maximum sensitivity Nitrogen Argon Maximum dynamic range Argon Methane Methane Flame lonization Nitrogen Maximum sensitivity Hydrogen and air for detector Helium Acceptable alternative Flame Hydrogen Hydrogen and air for Photometric detector Helium Nitrogen Argon Nitrogen Helium Optimum performance Hydrogen and Phosphorus air for detector Nitrogen Acceptable alternative Thermal Helium General use Reference must be same as Conductivity carrier Hydrogen Maximum sensitivity Note A Nitrogen Hydrogen detection Note B Argon Maximum hydrogen sensitivity Note B Note A Slightly greater sensitivity than helium Incompatible with some compounds Note B For analysis of hydrogen or helium Greatly reduces sensitivity for other compounds Jun 2001 Site Preparation 1 of 14 Agilent 6890 Gas Chromatograph Service Manual 520 Gases Gas requirements Gases for capillary columns When used with capillary columns GC detectors require a separate makeup gas for optimum sensitivity For each dete
3. ctor and carrier gas there is a preferred choice for makeup gas Table 520 2 lists gas recommendations for capillary columns Table 520 2 Gas Recommendations for Capillary Columns Preferred Second choice Detector anode Detector Carrier makeup gas purge or reference gas gas Electron Capture Hydrogen Argon Methane Nitrogen Anode purge must N be same as makeup Helium Argon Methane Nitrogen Nitrogen Nitrogen Argon Methane Argon Argon Methane Nitrogen Methane Flame lonization Hydrogen Nitrogen Helium Hydrogen and air for detector Helium Nitrogen Helium Nitrogen Nitrogen Helium Flame Hydrogen Nitrogen Hydrogen and air for Photometric detector Helium Nitrogen Nitrogen Nitrogen Argon Nitrogen Nitrogen Helium Nitrogen Helium Hydrogen and Phosphorus air for detector Nitrogen Nitrogen Helium Thermal Hydrogen Must be same Must be same Reference must be Conductivity as Carrier and as Carrier and same as Carrier and reference gas reference gas makeup Helium Nitrogen 2 of 14 Site Preparation Agilent 6890 Gas Chromatograph Service Manual Jun 2001 Gases 520 Gas requirements When using hydrogen with a thermal conductivity detector vent the detector exhaust to a fume hood or a dedicated exhaust to avoid buildup of hydrogen gas Helium is not recommended as a makeup gas at flow rate gt 5 mL min Flow rates above 5 mL min shorten detector life Jun 2001 Site Preparation 3 of 14 Agil
4. e the entire valve assembly if you want to change coolants The liquid CO valve kit is part no G1565 65510 and the liquid No kit is part no G1566 65517 Choosing a coolant When selecting a coolant consider these points e The lowest temperature you need to reach e How frequently you will use cryogenic cooling e The availability and price of coolant e The size of the tanks in relation to the size of the laboratory e Liquid Ns cools reliably to 80 C e Liquid CO cools reliably to 40 C CO is the choice for infrequent cryogenic cooling because it does not evaporate and is less expensive than Ns However a tank of COs contains much less coolant than a tank of Ny and more CO is used for the same amount of cooling Although liquid N evaporates from the tank regardless of frequency of use Ns tanks contain more coolant than do COs tanks and therefore may be better for frequent use Jun 2001 Site Preparation 9 of 14 Agilent 6890 Gas Chromatograph Service Manual 520 Gases Cryogenic cooling requirements WARNING Caution Using carbon dioxide Pressurized liquid CO is a hazardous material If CO escapes its container it exits at high pressure and low temperatures that can be dangerous to personnel CO in high concentrations is toxic to humans Consult your local supplier for recommended safety precautions and delivery system design Liquid CO should not be used as a coolant for temperatures below 40 C b
5. ecause the expanding liquid may form solid CO dry ice in the GC oven If dry ice builds up in the oven it can seriously damage the GC Liquid CO is available in high pressure tanks containing 50 pounds of liquid The CO should be free of particulate material oil and other contaminants These contaminants could clog the expansion orifice or affect the proper operation of the GC Additional requirements for the liquid COs system include e The tank must have an internal dip tube or eductor tube to deliver liquid CO instead of gas see Figure 520 4 e The liquid CO must be provided to the GC at a pressure of 700 to 1 000 psi at a temperature of 25 C e Use 1 8 inch diameter heavy wall stainless steel tubing for supply tubing The tubing should be between 5 to 50 feet long e Coil and fasten the ends of the tubing to prevent it from whipping if it breaks e Do not install a pressure regulator on the CO tank as vaporization and cooling would occur in the regulator instead of the oven e Do not use a padded tank one to which another gas is added to increase the pressure 10 of 14 Site Preparation Jun 2001 Agilent 6890 Gas Chromatograph Service Manual Gases 520 Cryogenic cooling requirements Dip tube Correct configuration Incorrect configuration Figure 520 4 Correct and incorrect liquid CO tank configuration WARNING Do not use copper tubing or thin wall stainless steel tubing wit
6. ent 6890 Gas Chromatograph Service Manual 520 Gases Gas requirements Gas purity Some gas suppliers furnish instrument or chromatographic purity grades of gas that are specifically intended for chromatographic use We recommend these grades for use with the GC Generally all gas supplies used should be in the 99 995 to 99 9995 purity range Only very low levels lt 0 5 ppm of oxygen and total hydrocarbons should be present Oil pumped air supplies are not recommended because they may contain large amounts of hydrocarbons The addition of high quality moisture and hydrocarbon traps immediately after the main tank pressure regulator is highly recommended Refer to the next section The gas plumbing for more information on using traps Table 520 3 Gas Purity Recommendations Carrier gases and capillary makeup gases Helium 99 9995 Nitrogen 99 9995 Hydrogen 99 9995 Argon Methane 99 9995 Detector support gases Hydrogen 99 9995 Air dry Zero grade or better 4 of 14 Site Preparation Jun 2001 Agilent 6890 Gas Chromatograph Service Manual Gases 520 The gas plumbing The gas plumbing WARNING All compressed gas cylinders should be securely fastened to an immovable structure or permanent wall Compressed gases should be stored and handled in accordance with the relevant safety codes Gas cylinders should not be located in the path of heated oven exhaust To avoid possible eye injur
7. gulators To eliminate pressure surges use a two stage regulator with each gas tank Stainless steel diaphragm type regulators are recommended Figure 520 2 Two stage pressure regulator The type of regulator you use depends upon gas type and supplier The Chemical Analysis Consumables and Accessories catalog contains information to help you identify the correct regulator as determined by the Compressed Gas Association CGA Agilent Technologies offers pressure regulator kits that contain all the materials needed to install regulators properly Pressure regulator gas supply tubing connections The pipe thread connection between the pressure regulator outlet and the fitting to which you connect the gas tubing must be sealed with Teflon tape Instrument grade Teflon tape part no 0460 1266 from which volatiles have been removed is recommended for all fittings Do not use pipe dope to seal the threads it contains volatile materials that will contaminate the tubing Jun 2001 Site Preparation Jof 14 Agilent 6890 Gas Chromatograph Service Manual 520 Gases The gas plumbing Traps Using chromatographic grade gases insures that the gas in your system is pure However for optimum sensitivity it is highly recommended that you install high quality traps to remove traces of water or other contaminants After installing a trap check the gas supply lines for leaks Table 520 4 Recommended Traps Description Pa
8. h liquid CO Both harden at stress points and may explode Jun 2001 Site Preparation 11 of 14 Agilent 6890 Gas Chromatograph Service Manual 520 Gases Cryogenic cooling requirements Using liquid nitrogen WARNING Liquid nitrogen is a hazard because of the extremely low temperatures and high pressures that may occur in improperly designed supply systems Liquid nitrogen can present an asphyxiant hazard if vaporizing nitrogen displaces oxygen in the air Consult local suppliers for safety precautions and design information Liquid nitrogen is supplied in insulated Dewar tanks The correct type for cooling purposes is a low pressure Dewar equipped with a dip tube to deliver liquid rather than gas and a safety relief valve to prevent pressure build up The relief valve is set by the supplier at 20 to 25 psi WARNING fliquid nitrogen is trapped between a closed tank valve and the cryo valve on the GC tremendous pressure will develop and may cause an explosion For this reason keep the delivery valve on the tank open so that the entire system is protected by the pressure relief valve To move or replace a tank close the delivery valve and carefully disconnect the line at either end to let residual nitrogen escape Additional requirements for the liquid Ny system include e Nitrogen must be provided to the GC as a liquid at 20 to 30 psi e The supply tubing for liquid No must be insulated Foam tubing used for refrigerati
9. on and air conditioning lines is suitable for insulation Since pressures are low insulated copper tubing is adequate e The liquid nitrogen tank should be close only 5 to 10 feet to the GC to insure that liquid not gas is supplied to the inlet 12 of 14 Site Preparation Jun 2001 Agilent 6890 Gas Chromatograph Service Manual Gases 520 Supplying valve actuator air Supplying valve actuator air Some valves use pressurized air for actuation others are electrically or manually driven Actuator air must be free of oil moisture and particulates It can be supplied from a dried regulated cylinder although house air supplies or air from a compressor are acceptable Most valves require 20 to 40 psi of pressure to operate High pressure valves may require 65 to 70 psi Valves require a dedicated air supply Do not share valve air supplies with detectors See Valve Control in the Agilent 6890 GC Operating Manual CD ROM for more valve requirements Jun 2001 Site Preparation 13 of 14 Agilent 6890 Gas Chromatograph Service Manual 520 Gases Supplying valve actuator air 14 of 14 Site Preparation Jun 2001 Agilent 6890 Gas Chromatograph Service Manual
10. rt No Preconditioned moisture trap metal casing s shaped trap for carrier gas cleanup 5060 9084 Contains Molecular Sieve 5A 45 60 mesh and 1 8 inch fittings Hydrocarbon trap metal casing s shaped trap filled with 40 60 mesh activated 5060 9096 charcoal and 1 8 inch fittings Oxygen trap for carrier and ECD gases metal casing and 1 8 inch brass fittings 3150 0414 Oxygen trap cannot be reconditioned Moisture in carrier gas damages columns We recommend a type 5A Molecular Sieve trap after the source regulator and before any other traps A hydrocarbon trap removes organics from gases It should be placed after a molecular sieve trap and before an oxygen trap if they are present An oxygen trap removes 99 of the oxygen from a gas plus traces of water It should be last in a series of traps Because trace amounts of oxygen can damage columns and degrade ECD performance use an oxygen trap with carrier and ECD gases Do not use it with FID FPD or NPD fuel gases Oxygen trap Figure 520 3 Traps 8 of 14 Site Preparation Jun 2001 Agilent 6890 Gas Chromatograph Service Manual Gases 520 Cryogenic cooling requirements Cryogenic cooling requirements Cryogenic cooling allows you to cool the oven below ambient temperature A solenoid valve introduces liquid coolant either carbon dioxide CO3 or nitrogen No to cool the oven to the desired temperature CO and Ns require different hardware You must replac
11. y wear eye protection when using compressed gas Follow the general plumbing diagram when preparing gas supply plumbing Two stage regulation On off valve Main supply on off valve 7 Moisture trap Hydrocarbon Oxygen trap trap Main gas supply Figure 520 1 General plumbing diagram e Two stage regulators are strongly recommended to eliminate pressure surges High quality stainless steel diaphragm type regulators are especially recommended Jun 2001 Site Preparation 5 of 14 Agilent 6890 Gas Chromatograph Service Manual 520 Gases The gas plumbing Caution Caution e On off valves mounted on the outlet fitting of the two stage regulator are not essential but are very useful Be sure the valves have stainless steel packless diaphragms e FID FPD and NPD detectors require a dedicated air supply Operation may be affected by pressure pulses in air lines shared with other devices e Flow and pressure controlling devices require at least 10 psi 138 kPa pressure differential across them to operate properly Source pressures and capacities must be high enough to ensure this e Auxiliary pressure regulators should be located close to the GC inlet fittings This insures that the supply pressure is measured at the instrument rather than at the source pressure at the source may be different if the gas supply lines are long or narrow Supply tubing for carrier and detector gases Do not use
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