Agilent Single Quad 5975 GCMS Detector Overview Video
Introduction
In this video we will present an overview of the detector on the Single QUad. This includes the High energy dynode, or H.E.D , the Electron multiplier or E.M. and the log amplifier. We will also cover the circuitry and test points for trouble shooting purposes.
In addition we will cover the detector voltage ranges, the main electronic components as well as faults. Lastly we will cover the removal and installation of the detector components.
Decector Components
The detector assembly consists of a DIffused ion focus, or D.I.F which allows for focusing of the ions entering the detector. The second component of the detector is the H.E.D., or high energy dynode, which creates secondary ions, and the third component of the detector is the E M or electron multiplier. The voltage rage of the E M is zero to 3000 volts. The H.E.D voltage range is 10000 to negative 10000 volts. The D.I.F range is 650 volts. A unipolar H.E.D voltage supply is used when the system is E I only. If the C I option is installed, the Bipolar supply must be used.
In this picture you can see the different components of the Detector which is the older design used on the 5973 and earlier 5975 models.
Detector Operation - Old detector
When the positive ions exit the quad, they enter the 3 lenses of the D.I.F. This accelerates the ions so that when they strike the High energy dynode they produce secondary charged particles. These charge particles then strike the electron multiplier which is -1000 to -3000 volts. The E.M. produces a signal which is detected by the electronics in the log amp board. The signal is then sent over to the smart card for processing.
It should be noted that if negative ions are being analyzed, such as in Negative Chemical ionization, the voltages of the D.I.F, H.E.D and E.M are reversed.
The single quad uses a standard Electron multiplier. The multiplier is a glass tube coated with a semiconductor material that has a voltage of 1000 to 3000 volts applied to it. Ions striking the walls of the E M produce electrons. These electrons then collide with the side of the walls again producing secondary electrons. Because of the positive bias, the electrons move from one end of to the other producing a current which is detected by the log amp board.
Detector Operation - Triple axis
The newer design of the detector is called a triple axis detector. It has a different part number
For the triple axis detector, The E M also has a new part number.
The main difference of the triple axis detector is that the ions exiting the quad are moved around a guide rod. The beam is again shifted in the zee direction where they collide with the High energy dynode. The ions then move in the opposite direction back to the E.M. Since the ions-charge particles shift axis three times before finally reaching the E M, the detector is denoted as "triple axis". This technique helps to shelter the Electron multiplier from extraneous unwanted secondary particles and photons, lowering the noise and improving the signal to noise ratio.
Installation
At Installation, the E.M. must be installed by the service engineer. The Electron multiplier is located in a bag near the turbo pump. It is sealed in a moisture proof bag to minimize the exposure to air which can lead to long pump down times and premature aging of the multiplier.
EM Saver
ANother feature of the detector is the E M Saver. This is only used in Single ion monitoring or S.I.M. The E M Saver limits the accumulated ion current. This spares the E M from excessive exposure and minimizes "burn" due to high signal.
The E M voltage is driven from the high voltage drive. This is located on the side of the analyzer are shown. A test point to monitor the voltage is located on the side board next to the signal cable. The test point voltage is 0 to 10 Volts. The D C to D C converter produces 0 to 3000 volts which are directly proportional to the test point values.
HED Power Supply and Test Points
The voltage for the High energy dynode comes from the H.E.D. power supply. The supply has only two output voltages which is minus 10000 volts or positive 10000 volts. The D.I.F voltage also is produced on the H E D power supply. The H E D input voltage is 24 volts. A test point can be used to monitor the H E D supply with a volt meter. This test point is labeled H E D MONITOR. In normal operation, 5 volts indicate the H E D is turned off. Two volts indicate the H E D is producing negative 10000 volts used for E I or P C I operation. In negative CI where the H E D produces positive 10000 volts, the test point will read 8 volts in normal operation. If the read back shows 0 volts, the H E D is defective.
Log Amp
The Log amplifier converts the Signal from the Electron multiplier to a voltage. An input current of 20 micro amps would produce an output voltage of 10 volts. This would correspond to an abundance of 400 to 600 thousand counts.
Diagnostics of the log amp board can be run through the chemstation or mass hunter acquisition software. To do this, first edit the M S D chem dot I N I file to enable the diagnostic menus. Find the entry "P C S" in brackets. Below it, insert a line with the string UNIVERSAL equals UNIVERSAL. After restarting chemstation or mass hunter, under the "tune and vacuum control" page, a menu item will show called C E diagnostics. Choose the "log Amp" test. When completed, you should see a series of triangles as show in this page.
Signal Limits
It should be noted that the maximum detector signal that the log amp can process is roughly 4.2 billion counts. The maximum Extracted ion count is 8.3 million. No message will occur if the signal is overloaded. If the signal is greater then these values, the peaks will appear as "flat-tops".
Detector Troubleshooting
If an H E D fault occurs on the M S , there is most likely a malfunction in the H E D power supply. If this is the case, first check all general operating parameters on the M S including electronics and vacuum. ALso, a good troubleshooting technique is to remove the H E D power supply cable from the E module and sideboard. If the error persists, most likely the H E D power supply or cable is defective. If the error goes away after removing the cable, most likely the H E D is dirty or defective.
In the case where no signal is observed, first check the vacuum. If the vacuum is OK, confirm the E.M and H.E.D voltages are correct on their corresponding test point locations with a voltmeter. If the voltages are correct, continue troubleshooting by removing the signal cable from the side plate. When doing so, you should see some response. If not, it is possible the Log amp or main board is defective. To check the Log amp board, run the log amp test mentioned in the previous slide. If no signal continues to be observed, try replacing the E M or detector assembly and or main board.
For Isolating chemical noise caused by a dirty H.E.D and/or multiplier, try setting the E.M to 3000 volts, the filament and P F T B A to off, and threshold to zero. You can then turn the H.E.D on and off by entering a command through the chemstation or mass hunter acquistion command line. To set the H.E.D on,, type M S param comma comma 1. To turn the H.E.D off, type M S param comma comma zero. If turning the H.E.D on results in large abundances relative to when it was off, more then likely the detector is dirty or needs to be blown out with inert gas to remove particulates.
To determine if the noise is cause by the multiplier, turn off the H.E.D and set the threshold to zero. Set the multiplier to zero and observe the response. Then turn the multiplier to 3000 volts. If you observe a large increase in abundance, the E M might be defective.
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