Agilent Single Quad 5975 GCMS Mass filter Overview Video
Introduction
In this video we will talk about the functions of the mass filter and how it works. We will identify quad related problems and adjustments from the tune report. We will also talk about removing and cleaning the quad.
Quad Hardware
In this picture you can see the quad through the the quad radiator. The quad radiator encloses the quad. Through the openings of the quad radiator are the electrical connections to the quad contacts. Positioned on the quad radiator is the quad heater. Typical values are 150 degrees Celsius.
The quad itself consists of quartz coated with gold.
In comparison to the metal machined quads, the quartz/gold design has several advantages. These include a scan range to 1050 A M U. Very repeatable performance and improved charging performance. In addition, the manufacture of the quartz/gold quad allows for better control and repeatable performance.
Quad Operation
The quadrupole is the mass filter that determines which ions will make it to the detector. This is done by applying both positive and negative DC potentials to each pair of rods. Superimposed on each pair is an RF voltage that is shifted 180 degrees out of phase on second set of rods. The mass to charge ratio of an ion is directly proportional to the amplitude of the RF voltage.
IOn motion in the quadrupole can be predicted by the Mathieu equation. This equation determines the combination of R.F. and D.C that allow which ions to pass through the mass filter. The R.F/D.C combinations determine the Scan line which is a line used to operate the GCMS. The slope of this line is referred to as the mass Gain. The y offset of this line is referred to as the AMU offset.
In this picture we can see both R.F and D.C voltages superimposed. The left side is U plus and U minus DC voltage. On the right is R.F voltage.
For each R.F voltage determines a stable trajectory for a particular mass to charge value. Only ions with this mass to charge can reach the detector.
In Chemstation or Mass hunter software, the quad parameters can be seen in the manual control screen. The autotune will determine the optimum AMU gain and offset and display the value in this screen
Quad Electronics
The voltages that drive the quad are located on the Main board and Sideboard. On the main board and sideboard are electronics associated with the R.F voltage. The D.C voltages, U plus and U minus are also generated on the main board. Test points are located on these two circuit boards for troubleshooting purposes.
In this picture we can see a circuit overview of the quad electronics. These will be covered in more depth in the electronic videos.
The sideboard contains two screws that need to be adjusted when the system is first setup. The adjustment is referred to as "dipping the coils". The adjustment optimizes the quad frequencies. In the software configuration the polarity of the quad is also configured. E.I and P.C.I use the same polarity. N.C.I uses the opposite polarity.
Quad Tuning
When the quad is properly tuned, the result is a report that looks like the one shown. Important items to note is the good peak shape,,, no precursors or ghost peaks,,, no split peaks... Also, the isotopes of each ion of the PFTBA should have the proper ratios and good resolution. The autotune by default will try to attain a peak width of point 6 atomic mass units. Mass assignment is also done by the autotune through the mass gain and mass offset parameters. The default target accuracy is point zero five atomic mass units.
Common Quad Problems
When quad related problems occur, this usually will result in poor tune results, unstable signal and R.F faults. To correct these issues all operating parameters of the M. S should be investigated. These include the vacuum, any electronic failures and power supply read backs. Also, in some cases the quad contacts can get dirty and require cleaning. Chips in the quads or burn marks on the quads can cause some of the symptoms shown on the left.
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