7 Replies Latest reply on Mar 3, 2019 8:35 PM by koos

    No change after cleaning MS

    koos

      Good afternoon everyone,

       

      Over the last year, the EM volts of the MS have been slightly creeping up. Where it was still ~EMV 900 now we are roughly around 1800.

      Since we had a bit of downtime, there was a chance of cleaning the MS source to increase the sensitivity.

      The cleaning progress of the Agilent 5977B Operating Manual in combination with the How to clean an Agilent GCMS Ion Source  was followed.

      The overall process went smooth and the source was looking (visually) clean after the process.

       

      However after installing the source I did not notice any difference in the tune reports. Same EMV, same abundance of the ions.

      And my question is how this is possible? I would expect an increase in performance after cleaning the source, but did not see any.

      The tune reports are added

       

      This all happened on a

      5977B MSD

      EI SS single quad

      Fil 1 was replaced with a new one, Fil 2 was kept.

      There is no difference between the old filament 2 and the new filament 1 in the tune reports.

       

      Thanks for your time and a happy Friday,

      Koos

        • Re: No change after cleaning MS
          nickbharden

          Hi Koos,

           

          This occasionally happens. Depending on your use of the MS you may not notice any visible effect on the PFTBA ions used for tuning.

           

          I have taken a look at the tune reports and noted a few things.

           

          • Column flow rate is quite low: 0.6mL/min before clean and 0.55mL/min after clean. This is slightly outside an optimal range for Helium carrier gas: 0.7-1.5mL/min (range is approximate and depends on your needs).
            • What is our carrier gas?
            • Tuning the system under the conditions that you typically run is recommended (ie if you typically run your system with 0.55mL/min carrier gas, then you should tune under those conditions). I am not saying you have done anything wrong, I am just stating the facts about your tune reports.
            • Are you running RTL? Very low flow rates can happen when a column is trimmed a lot. RTL compensates for the shorter column with lowering the flow rate.
          • Your observation that the EMV changed from 900 to 1800 over time can indicate a dirty source however this would only be the case if the analytical conditions did not change and the "tune gain factor" increased as well.
            • The gain factor did not change significantly between pre and post clean. In fact the "gain factor" is quite low pre-clean (0.15) and post clean(0.2) meaning the ions being transmitted to the EM are of significant abundance that the EM is not working hard to produce the tune report.
            • A SS source in a diffusion pump system and He flow rate around 1.2mL/min commonly has a "gain factor" of around 0.5 (gain 50,000), meaning that the EM is required to multiply the signal 50,000 x times to get an abundance of ~500K for either 69 or 219. Your system only requires 15,000x electrons from the multiplier to give an acceptable tune abundance. This may be a factor of the carrier gas flow rate being so low or some other factor.
            • If your source gets dirty you should expect the tune "gain factor" to go up. Meaning more electron multiplication is required to bring the tune abundances up to the expected number of counts (~500K for 69 or 219). When the source can not produce enough ions (or transmit the ions) gain factor goes up.
            • The change you observed in your EMV here is an indication that the EM is aging. ie it requires more volts now to produce the same number of "counts" in the tune report (or your analytical conditions have changed over time (ie carrier gas flow rate, etc)). Many things effect the "age" of an electron multiplier. Think of it like a battery, they run low on electrons the more you use them, and the more current you draw from them the quicker they wear out.
          • The Entrance Lens voltage did change by approx. 5V change between pre and post tune
          • The abundance of 69 increased between pre and post tune (350K to 500K) but the 219 and 502 did not see the same increases. This is simply saying your clean helped with the low mass ions but did little for the mid to high mass ions.

           

          In summary, I think your system is performing as expected. If you want to test it further, you can set He carrier gas flow to 1.2mL/min and tune from the default tune file (reconfigure instrument to get back to default). You may see the EMV drop and the tune gain factor increase to around 0.5, to me, this would be considered performing as expected.

           

          Best regards,

           

          Nick

            • Re: No change after cleaning MS
              nickbharden

              One more quick note:

              Your source may have been visibly dirty (ie you saw discolouration) but that does not always mean that the voltages are being shielded from your ions by that discolouration, generally this is cause by helium ions. Helium ion burn does not effect source performance a lot.

              • Re: No change after cleaning MS
                koos

                Good morning Nick,

                 

                First of all thanks for your detailed reply, the information is much appreciated!

                 

                As I understand your remarks, my source was pretty clean before I started, and that is why I did not see much difference after cleaning. The EMV have been ramping in the last year due to 'battery-like-degradation', and the proper way to identify a dirty source is the rise of EMV in combination with the rise in 'gain factor'.

                 

                To answer some of your questions:

                • The column flow rate of 0.55 ~ 0.6 mL/min is in the "standby mode" of the GC, when the GC is idle between runs. Normally we run 1 to 1.2 ml/min using helium as a carrier gas. And no, I'm not running RTL.
                • It was a mistake to not tune the system under normal conditions. It was my belief that tuning only needed to happen ~once a week (dependant on usage etc).
                  • Does that mean I need to tune my system prior to every method I have?
                  • Or are we speaking generally - i.e. oven temperature needs to be low, but whether is 40 or 60 should not matter
                • 'normal' gain factors, measured with a flow of 1.00 mL/min in our tune reports are historically between 0.25 and 0.42, with EMV of ~1700-1900
                • You are clear on the EM-battery part. But going forward, when would the EM horn require replacement?  Is there some rule of thumb when evaluating the 'EMV * gain factor = <....." to evaluating when to change? I.e. i've always been thought to clean the source if it reached >2000 counts.
                • What does the Entrance Lens voltage change of 5V tells me?
                • I've added a tune report with the chromatographic settings set to standard, including a 1.2 mL/min flow. I dont see any difference between the low-column flow speed and the normal one.
                  • Re: No change after cleaning MS
                    nbohni

                    We do have a different System (7890 GC and 7010B TripleQuad), but we had also observed that there was no improvement in the tune file after cleaning the source. However, Peak Areas and s/n had improved substantially when injecting a sample of known analytes after cleaning the source.

                      • Re: No change after cleaning MS
                        nickbharden

                        Hi Nbohni,

                         

                        TQ systems tune the electron multiplier (EM) differently and use a different logic when producing the tune report to the SQ systems.

                         

                        On TQ instruments, Agilent fixes the gain factor to produce the tune report (7000x gain factor =1, 7010x gain factor = 0.1). This means your EMV will only change voltage if the EM is aging. The voltage will generally drift up and they generally require replacement when the EMV gets to between 2000 and 2500V (but like the SQ instruments, the EM can be ran to 3000V). There is an additional field on the TQ tune reports the gives you the max gain factor. This is an extrapolated number, and represents the gain factor if the EM has 3000V applied to it... Generally a new EM has a very very high max gain factor. The number drops off exponentially (ie very fast at the start but slows after that and gradually drops as the EM ages). It is a measure of the "age" of the electron multiplier. (age does not mean "years", it means more how spent the available electrons for multiplication are...).

                         

                        On a TQ instrument, generally, you will notice the abundance of 69, 264 and 502 dropping as the source gets dirty. Eg, if on a clean source you are getting 10,000,000 counts for m/z 264, then a dirty source may only give you 5,000,000 counts. If the "dirt" on the source effects your compounds more then the tune solution (PFTBA) then you may not see much change in the tune report, but your application/compounds will see the change. So you are correct, you will see a rise in the number of counts and s/n when you clean the source (if it was dirty in the first place).

                         

                        Regards,

                         

                        Nick

                      • Re: No change after cleaning MS
                        nickbharden

                        Hi Koos,

                         

                        Answers to your questions below:

                         

                        It was a mistake to not tune the system under normal conditions. It was my belief that tuning only needed to happen ~once a week (dependant on usage etc).

                        • Does that mean I need to tune my system prior to every method I have?
                        • Or are we speaking generally - i.e. oven temperature needs to be low, but whether is 40 or 60 should not matter

                        If column flow rate, source, quad temps (or collision gas flow rates on TQ) change between methods then it is probably a good idea to re-tune the MS. A change of 0.1 or 0.2mL/min won't make much of a difference but 0.5mL/min could. Oven temperature doesn't really matter when a system is being used in constant flow mode. If you have constant pressure methods (or ramp pressures/flows) then you should set the oven temp (and/or pressures/flow) to a mid range (often 200C was traditionally used on 5890 systems) or near the main analytes of interest elution temp/conditions.

                         

                        Frequency of tuning is open to debate but if you don't see much change between tunes then you could probably tune less frequently. Some tune every day, before each sequence, once a week, once a month, it really depends on your use case/application, requirements, SOPs and regulations.

                         

                        You are clear on the EM-battery part. But going forward, when would the EM horn require replacement?  Is there some rule of thumb when evaluating the 'EMV * gain factor = <....." to evaluating when to change? I.e. i've always been thought to clean the source if it reached >2000 counts.

                         

                         

                         

                         

                        Knowing when to clean a source really comes down to your application but as a general rule, I have found that an increase in gain factor on the tune report indicates your source is getting dirty. Example: If a clean source tunes with a gain factor of 0.2 (in the tune report), but now the gain factor has increased to 0.8 (in the tune report), your system is transmitting 4x less ions to the EM than it did when the source was clean... Therefore I would say it is over due for a clean... but if your application is still performing well, you may wish to put it off to another day when the gain factor reaches 1.5 in the tune report... generally I wouldn't let a system run with < 1/2 it's peak performance, let alone a 1/4, so pushing the gain factor to 1.5 on such a system would be concerning to me. Comparing tune reports like this requires that they are run under the same conditions. Keeping a good history of your tune reports is the best way to know your system. Late versions of MassHunter Acquisition auto saves a PDF of your tune reports to \MassHunter\GCMS\x\597x\PDF. (Note: Agilent has a variety of sources and configurations. Difference sources tune with different gain factors in the tune reports. A HES source makes ~10x more ions so you expect the gain factor to be ~10x lower in the tune reports eg gain factor = 0.05).

                         

                        Generally, when the EM voltage in the tune report approaches 2500V then it would be time to order a new EM. The maximum EM voltage is 3000V but since most applications run with a gain factor of between 1 and 5 (~delta EMV +100V to +400V), the EM is going to hit the 3000V limit when running your method before the tuning process does. The software warns at the end of the tune with a message telling you that a gain factor of 25 or less is not able to be achieved on your EM, at which point it would be a good idea to consider changing the EM.

                         

                        What does the Entrance Lens voltage change of 5V tells me?

                        Not a lot.

                         

                        I've added a tune report with the chromatographic settings set to standard, including a 1.2 mL/min flow. I dont see any difference between the low-column flow speed and the normal one.

                        It is always good to have a history in some known standard conditions. (ie column flow rate 1.2mL/min, source temp 280C, quad temp 150C). This helps for comparison when troubleshooting. You may not see any differences currently but bringing it back to known optimal conditions ensures the changes are not the root cause of an issue should one arise. Agilent default source temp is 230C but most applications will perform better with higher temps (~250C-300C).

                         

                        Regards,

                         

                        Nick

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