Agilent Single Quad 5975 GCMS Source Overview Video
Introduction
This video will present an overview of the source including function structure and common problems.
Source Operation
The function of the source is to create ions by means of E I or C I and transmit them into the quad. In E I, neutral compounds first enter the source from the GC transfer Line. Once the compounds are in the source they collide with 70 electron volt electrons from the filament forming ions. The repeller pushes the electron through the drawout plate. The beam of ions are focused by the ion focus and entrance lenses before they enter the quad. The magnet is helpful in optimizing the collision of the electrons with the compounds. Programing of the entrance lens voltage with the gain and offset help to facilitate the movement of the ions from the source into the quad. The entrance lens voltage is also critical in favoring biasing the transmission of light or heavy ions, depending on the application or user preference
Since the effluent from the column contains not just the compounds of interest but also unwanted sample matrix, the source can get dirty with time. When this happens, the build up of contamination on the repeller acts to "sheild" the compound from the repeller. To compensate, the autotune increases the repeller voltage to create the same number of ions
Helium present from the column effluent helps to facilitate the movement of compound ions away from the repeller and through the drawout lens. Because of this, less repeller voltage is needed at higher helium flows
Here is a detailed picture of the E I source. Several types of E I sources are available depending on the application. These include the stainless steel source, the Inert source and the extractor source. Inert parts are denoted with a special dot as shown in the picture.
The CI Source
The C I source is very similar to the E I source. The main differences include a tip seal composed of a spring and ceramic tip that is added to the end of the transfer line. This helps seal the source to increase the reagent ion pressure which is beneficial in the C I process. The C I source contains only one filament. Note the parts in yellow can be cleaned with abrasive aluminum oxide powder. These include the entrance lens, ion focus lens, drawout lens, ion source body and repeller.
Source Wiring and connections
Here we see the source connections. The wires connecting to the source go directly to the Ceramic source connector board. The wires shown are the blue wire to the entrance lens, the orange wire to the ion focus lens, the red wire to the repeller and white and black wires to the filaments. The source heater is also shown here. The Green wires are for the heater, the white wires are for the temperature sensor. This color coding scheme is standard on all Agilent GCMS quad equipment.
Voltages that are applied to the source lenses and heaters are generated on the Main board. For troubleshooting purposes, test points exist on the main board and side board to monitor the voltage with a Volt meter.
Source and electric fields
This picture will help you visualize how ions interact with the electrical fields generated by the different lens components. The red markings denote the path of the positive compound ions. The blue lines denote the applied electrical field potentials. The lens voltages are optimized by the auto tune process to facilitate the creation and movement of ions into the quad
The electrons are created from a high frequency AC current through the filament wire. The filament wire usually consists of a special metal alloy that emits electrons from it's surface when a current is passed over it. The filament current can be adjusted in the software method. The default for E I is 35. The default for CI is 75. The value of the filament current only changes the number of electrons produced. The energy of the electrons remain constant at seventy electron volts and is not dependent on filament current.
Common Problems
No emission current can be caused by several types of problems including a broken filament, a bad ceramic board, bad main or side boards or shorted filaments.
Low response of the PFTBA or tuning compound can be caused by several different problems. These include a dirty source, incorrect voltages or shorted or disconnected wires.
Filaments are coded to contain a manufacture time stamp. This can be useful for tracking purposes.
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