Agilent Single Quad 5975 GCMS Tuning Overview Video
Introduction
In this video we will cover the fundamentals of MS tuning. This will including the various tuning methods available on the chemstation or mass hunter software. We will also cover Manual tuning and gain tuning.
Tuning determines the optimum voltages for each of the source elements. It sets the A M U Gain and offset for correct spectral width. The E M voltage is also determined. The mass gain and offset are set for proper mass assignment.
Tuning Calibrant
For E I tuning, the compound P F T B A is used. The properties of this compound make it ideal for E I tuning. It is stable, volatile, and fragments over a wide mass range. It also contains C-13 and N-15 isotopes only. On the right side is the spectra generated using S tune dot U.The autotune dot u is the tuning used to setup and checkout the 5975 Mass spectrometer.
Tuning Operation
The tune is selected from the "tune and vacuum control" section of chemstation and mass hunter software acquistion program. From the tune menu select the Autotune dot U option. When the autotune completes, a standard tune report is sent to the printer or screen.
Here is a diagram of the inert source showing the various components set by the autotune. The filament creates the electrons and is usually fixed at 35 micro amps for Electron Impact ionization. However some tunes such as B F B auto tune will optimize the filament emission relative to the repeller and entrance lens. Normally the electron energy of the filament is set to 70 electron volts to generate N I S T searchable spectra. The repeller helps to "push" the ions in the direction of the quad. It's range is 0 to 42 volts. Typical values are 20 to 30 Volts. The draw out lens in the inert source is held at ground potential. This lens helps to "pull" the ions in the direction of the quad. The ion focus lens helps to focus the ion beam. tune values are typically around 90 Volts. The entrance lens helps to assist the movement of the ion beam into the quad. Typical A tune values are 20 volts. In the case of S tune and BFB tune, the entrance lens voltage is programmed during the scan to meet target ion criteria. The AMU Gain and offset will be discussed in later slides. We will also go into the E M setting later in this video.o.
During the autotune process, the PFTBA is infused in the source by opening and closing of the two way solenoid. The temperatures of the source and quad are also set in the tuning method. The typical temperature of the source is 230 degrees Celsius. The typical temperature of the quad is 150 degrees Celsius.
Tune Files
There are several different choices when selecting a tune. The S tune is also called a target tune, this type of tune allows for entry of a target response for each of the PFTBA ions. This is useful when trying to meet BFB or DFTPP criteria. Other types of tunes are the low mass autotune and the PCI and NCI tune. The CI tunes will be discussed in more detail later.
A-Tune is the tune used in instrument checkout, optimizes the system for maximum sensitivity without criteria for ion ratios. This is a good starting tune selection if the requirements of the application are not known. In the report shown,, the system will try to maximize the abundance of 69, 219 and 502.
Auto tuning the instrument provides a way to keep data generated from instrument to instrument reproducible. It also allows us to see small changes of the system with time. Monitoring operating parameters in the tune or Tune failures can be used for diagnostic purposes. The tuning parameters are logged and recorded into a file where the results can be graphed and compared to previous values over a period of time. The Autotune is a good starting point for users wishing to adjust the operating parameters to get the optimum performance from their application.
Sequence of events during tuning
The following slide shows the flowchart used by the Auto tune process. First, the system uses it's default values to find the peaks associated with each ion. It then does a coarse adjustment of the E M and peak widths. It then does profiles of each of the lens components for each tuning ion, to determine optimum voltages. Using these values, it "fine tunes" the width of the response and width of each of the selected ions. Finally, the mass axis is adjusted based on the position of each of the tuning ions. A gain profile is gathered by setting the E M to different values and building a gain curve. AFter the autotune completes, it saves and prints the report.
The following are screen shots of what the autotune process looks like when optimizing each of the source parameters. The repeller lens is ramped and the response versus voltage of each of the selected ions is graphed. In the case shown, the system chooses the maximum response by drawing a vertical line near the optimum voltage.
This screen shows the same process for the Ion focus lens.
This screen shows the same process for the entrance lens
This screen shows the same process for the entrance lens offset.
AMU Gain and Offset
The peak width of each of the selected tune ions are a function of the A M U Gain and AMU offset. The Slope of the scan line is determined by the response of each of the tuning ions based on the relation of DC to RF voltage. The slope of the scan line is equal to the AMU gain value. The scan line is chosen so that the line intersects the tip of the stability regions for each mass. The lower the slope the larger the intensity of each ion but larger the peak width. The A M U offset is the Y axis intercept of the scan line.
Increasing the A M U Gain or slope of the scan line will result in decreased peak widths. This has a larger effect at the higher mass and less at the lower masses. When the A M U offset or Y intercept of the scan line is increased, the peak width and amplitude are both decreased. It has the same effect at both high and low masses.
Since the PFTBA is a reference compound, it has known mass to charge values. The autotune adjusts both the mass axis gain and offset to match these values. A calibration curve is generated using these calibration points.
The Autotune Report
Lets look at the autotune report. First is the width values. The target width of each of the tuning ions is zero point 6 AMU. The peak shape of each ion should be both smooth and symmetrical. Jagged peaks can arise when the system has excess noise or other problems. The report also shows the abundance of each of the tuning ions. Typical values are 400,000 to 600,000 counts for mass 69. Electron Multiplier values are reported in the tune report , we will discuss this later but typical values are 1000 to 2400 volts. The mass assignment should be very close to the expected mass for each of the ions. The system also measures both Air and Water levels. Values are shown here. The nitrogen or 28 mass to 69 mass ratio should be less than 10 percent but ideally less than 5. The water should be less than 20 percent but ideally less than 5.
Difference between Tune types
Another type of auto tune is the standard tune or S tune. In a standard tune, each of the PFTBA ion ratios must meet a designated criteria. This provides reproducibility from instrument to instrument. As in A-tune the S tune has diagnostic capability and system parameter tracking as well as a starting point for manual tuning.
The main difference between a A tune and S tune is that the entrance lens gain and offset is dynamically programmed to assure the spectral ratio criteria is met. In the case shown here, the custom values for each mass can be seen in the standard autotune report
As WIth A-tune, the same parameters are displayed in the report and S tune generally has the same attributes as the A-tune.
Another tuning option is the quick tune. In this tune only the mass axis, peak width and E M voltage are determined. The relative abundances of the selected tuning masses are not adjusted.
Target tuning, as with the standard tune, allows for custom tuning of the Mass spectrometer to meet different application requirements. Examples of Target tunes are the DFTPP tune, the BFB tune and the custom Target Tune. In all of these tunes, the entrance lens gain offset is dynamically changed with each mass to meet the required spectral ratios.
HEre are the input windows for the target tune. Most importantly, the user can input the desired ratio of each of the tuning masses relative to mass 69. Also the maximum values for each of the source lenses can be input.
The following slide shows the difference in these values when compared to the A Tune.
Here is the input parameters for both the BFB tune and the DFTPP tune. Note the differences.
Manual Tune
In some cases, the user may wish too override the values determined from auto tunes to meet custom requirements. This can be done through manual tune. Manual tune can also be used to trouble shoot instrument problems and or gather diagnostic information.
Here is a screen shot of the manual control page. Note all the lens parameters as well as the other operating values can be manually entered. The user can view profile or spectral Plots in real time to see the effects of changing one of the parameters in the tune.
Here we can see both the AMU Gain and Offset values determined from the autotune. If desired, the user can change the values and see how the changes effect the results real-time.
Also is a special correction for the 219 ion.
Mass assignment is primarily determined by mass Gain and Mass offset values in this page.
All lenses can be directly changed in manual tune. The values shown are determined from the Auto tune
Changing the A M U gain or offset will change the peak widths.
Changing the Mass gain or mass offset will change the mass assignment of the selected ions.
In the acquisition and display parameters, the profile masses can be changed from the default of 69, 219 and 502.
For spectral display, the range can also be changed here.
Under the ramp parameters configuration, you can change the starting and stopping voltage used when performing
a lens ramp.
In the tune file and manual control you can change both the source and quad operating temperatures.
Manual control allows you to view the mass spectrometer output as either profile, spectra or ramp.
For a profile plot the profile masses of the 3 tune ions can be plotted. The abundance, width and mass assignment are shown
in the profile plot.
IN the scan plot, the centroided scan can be shown in a real time plot. Also shown in this page is the
abundances of the selected ion values.
All tune parameters are recorded in files. For each autotune, the values are recoreded and a history of tune parameters
can be accumulated. This can be useful for plotting and seeing how the parameters change with time.
Here we can see the history of ion abundances over time.
Here we can see all the lens values determined on the day the autotune was run
Electron Multiplier and Gain Curve
WHen the autotune is run, the electron multiplier voltage is optimized. As part of the Electron multiplier optimization process, the autotune first determines the voltage required so that one ion striking the multiplier generates one hundred thousand electrons. This is known as ten to the fifth gain. As the multiplier ages, the output capability of the E M decreases since the surface of the E M erodes with time. To compensate the autotune increases the E M voltage to make the response consistent. Ten to the fifth gain is also referred as a gain of one. Added response can be added to the system by increasing the E M value above the gain one voltage. Here we can see the additional response attained by increasing the voltage above the gain one setting.
In the chemstation or mass hunter software, the user can select a E M gain as part of the method. The default gain is one. If a gain of 2 is entered in the method, the system will determine the appropriate voltage necessary to increase the signal by two from the gain curve.
Since the Electron multiplier ages with time, the same E M value will not result in the same signal over a period of time. To correct this, a "gain curve" is collected. From this plot, a reproducible response can be attained regardless of degradation of the electron multiplier.
If the user wishes to update just the gain profile without running an autotune, they can choose the update E M V coefficients from the parameter screen.
In the acquistion method, the user can now select a gain value. THe limits are point 3 to 25. this results in greater reproducibility and better consistency between instruments.
More....