This Information Applies To: Agilent VGA 77 (Vapour Generation Accessory)
Issue: This article discusses the setup steps for the VGA 77.
|Warning: Noxious Fumes and Corrosive Liquids.
Vapor generation methods of analysis involve the generation of toxic hydrides and require the use of materials that are toxic, highly corrosive, or otherwise hazardous. Careless, improper, or unskilled use of such materials can cause serious personal injury. Always ensure that laboratory safety practices governing the use, handling and disposal of such materials are strictly observed. These safety practices should include the wearing of appropriate safety clothing. In the VGA 77 system, highly concentrated acids are being pumped under pressure. If a leak occurs, acid could be sprayed from the system and cause serious personal injury. Always wear approved safety glasses warranted to protect the eyes. Never pump concentrated sulfuric acid through the VGA 77 system.
Steps to Follow:
1. Check that the gas supply is set to the recommended pressure (350 kPa (50 psi)) and turn on the inert gas supply.
Tip: Ensure that the inert gas supply is turned on before switching on the VGA 77 power. If solutions are pumped through the system before the inert gas is turned on, the signal will be unreliable.
2. Place the three capillary tubes in a container of distilled water and switch on the VGA 77. The green ‘Power’ indicator light is illuminated when the power is on.
Tip: The peristaltic pump will run continuously once the power is switched on.
3. Slacken both pressure adjusting screws. Swing both tube beds against the pump tubing and close the pressure bar.
4. Slowly tighten the front (sample) pressure adjusting screw until water is being pumped through the tube. Tighten the pressure adjusting screw a further half turn. At this setting, the pumping rate will be close to optimum. Any further tightening of the screw will tend to reduce the pumping rate.
|Caution: Do not overtighten the pressure adjusting screws. Excessive tightening will shorten the life of the pump tubes and could cause permanent damage to the pump.
Tip: At this stage, inert gas may be seen bubbling out of both the other tubes. This is a normal consequence of pressurizing the system and this situation is automatically corrected when you adjust the pumping pressure in these tubes.
Tip: To see if the liquid is being pumped smoothly, you can deliberately create air bubbles by dipping the capillary in and out of the container
5. Repeat Step 4 with the rear pressure adjusting screw.
6. Measure the uptake rate through the sample pump tubes. The uptake rate should be within the range 6–8 mL/min.
7. Measure the uptake rate for each of the other pump tubes. The uptake rate for each should be within the range 0.8–1.2 mL/min.
1. Remove the cell in its holder from the burner and place it at the front of the reagent module.
2. Lower the burner so that it is clear of the optical path. It is recommended to use reduced slit height (0.5) in all methods.
3. Fit the required hollow cathode lamp and set up the Spectrometer as described in the instrument operation manual.
4. Select Optimize > Optimize Lamp from SpectrAA Software.
5. Position the cell in its holder on top of the burner.
6. Align the cell in the optical path as follows:
a. Hold a piece of white card between the right end of the absorption cell and the sample compartment window.
b. Using the burner positioning controls, adjust the position of the cell until the light from the hollow cathode lamp passes through the cell onto the card. Remove the card.
c. Use the burner positioning controls to adjust the position of the cell for maximum transmission.
Lighting the Flame
1. Carry out any safety checks described in the instrument operation manual. Follow all instructions and observe all warnings.
2. Set the acetylene flow higher than normal to obtain a fuel-rich air-acetylene flame. This is required for igniting the flame.
|Warning: Explosion Hazard. With the VGA 77 cell installed, the burner will be lower than usual, and a standard mixture or a fuel-lean mixture may not ignite readily. To avoid an explosive accumulation of fuel and oxidant in the sample compartment, ALWAYS ensure that the gas mixture is fuel-rich before operating the Ignite button.
3. Ignite the flame and adjust the fuel flow to give a lean flame. If necessary, lower the cell back to its normal position.
|Caution: If the absorption cell is left in a fuel-rich flame, carbon particles may be deposited on the outside wall of the cell. Under these conditions, the cell can be permanently damaged by local overheating.
The complete system requires “conditioning” to obtain a stable signal before calibrating the instrument and analyzing samples.
This procedure should be carried out:
1. Allow the pump to operate for three or four minutes to stabilize the flow rates.
2. Make sure that the instrument is in ‘Absorbance’ mode with a suitably short integration period.
3. Aspirate the highest standard, monitor the signal, and measure the time taken for the signal to rise from zero to its stable value. The measurement for this interval will be the pre-read delay that should be entered into the system software.
|Tip: When presenting solutions to the system, always wait until this period has elapsed before taking a reading.
4. Aspirate the analytical blank.
5. Continue to aspirate the standard and analytical blank in turn until a consistent response is obtained for the standard solution and then proceed to ‘Operation’ in VGA 77 User’s Guide (page 51).