ICP-MS manual tuning

I could use some resources on manual tuning for ICP-MS systems.


I'm a relatively new user, and I'm disappointed at the seeming lack of useful information available through Masshunter Help, Agilent user manuals, or online. 

 

Specifically, I'm looking for resources pertaining to how lenses, plasma settings, and various other tune parameters  affect sensitivities/peak shapes.

 

What are the purpose of the various lenses?  How do they affect the ion beam and sensitivities?

The same question for gas flows and all of the other various tune parameters.

 

 

We currently use a 7900 system for drinking water applications (EPA).  The autotune alone is often not sufficient to meet tune criteria, making manual tweaking necessary.

 

Thank you.

  • Hi,

     

    manual tuning can be a tricky thing and I don't have a manual describing every step, but also because the goals might differ between users.

    For typical liquid samples it is not even necessary. The auto tune function of MassHunter is terrific and will give you results near the optimum in a very short time.

    If you want to tune manually, the general approach is easy: Allow yourself the custom tune in the software, aspire a solution with your favorite elements (usually tune solution with Li, Y, Tl; but can be different for special applications) and change the lens voltage to achieve maximum sensitivity. Sometimes depending on the lens, you will get higher sensitivities for lower mass elements and lower sensitivities for high mass elements or vice versa... choose and weight according to your needs.

    The auto tune will give you a balanced result for the whole mass range but this is also adjustable with a right click in the tune window -> Tune -> Parameters for Auto Tune.

    The Plasma Mode pre settings can be trusted. Use General Purpose for most samples below 0.2% total dissolved solids. If you want to tune these, keep an eye on the oxide ratio (156/140). This should stay below 1.5% in no gas mode.

     

    Manual tuning is usually only done with fancy sample introduction systems like Laser Ablation, LC, GC or semiconductor applications. If you have some of this, tell me so I can give you more specific advice. Otherwise it is to trust the auto tune generally, but you can play around if you want without risk to damage your instrument as long as you stay away from the "override hardware settings" button.

     

    Best regards

    Matthias

  • Thanks for the response Matthais.  The issue actually is that autotune sometimes fails to match EPA compliance criteria, and so it becomes necessary to manually tweak settings.

     

    I get that Agilent doesn't want lab techs carelessly messing around with quadrupole settings.  That said, I find it surprising that every time I ask about manually tuning the response is typically "just play around with the lens settings!".  I figure there would at least be some information describing what mass ranges are affected by which lenses, etc.  Alas, c'est la vie.

     

    Really understanding how these instruments work has been an uphill battle, no doubt.  


      

  • Hi labrat,

     

    I understand your frustration, however this is how it works - the pros don't do differently. You expect the situation that you have a batch with settings that worked fine once. They don't do so anymore, probably because the lenses changed so you have to re-tune them. But it's impossible to know which lenses are affected and in which direction you must tweak them. You just try and if signal drops, you change them in the other direction. That's how things are, really. But I will give you some more info about the lenses at the bottom of my response.

     

    Anyway if you have constant issues with sensitivity, you might need to clean/replace the cones or clean the lenses. What you could do easily:

    In He mode, decrease the He flow to about 4.5 mL/min and energy discrimination to 4 V. This should give you better net sensitivity with still good interference control The energy discrimination is the "repelling lens" at the end of the cell which repels the now-slow molecular ions.

    If you like, you can try to reduce Nebulizer Gas flow by 0.1 L/min and increase Make-Up gas to 0.1 L/min. Sometimes this gives better sensitivity. But don't overdo, always check the oxide ratio when playing with the gases.

    Don't change anything else on the plasma settings.

     

    Now the lenses:

    Extract 1 and 2 extract ions from the plasma and makes them accelerate. Settings are specified so that sufficient sensitivity is exhibited for all mass numbers. But usually Extr. 1 is set around 0 Volts and Extr. 2 at -200 V. Changing them doesn't do much.

    Omega Lens separates ions, photons, and neutral particles, and efficiently guides only ions to the cell. Settings are specified so that sufficient sensitivity is exhibited for all mass numbers. Just play around. The Omega lens is a lens that has an interdependent relationship with the Omega bias.

    Omega Bias extracts the ions that leave the Omega lens and guides them efficiently to the cell.

     

    Cell Entrance Lens: Set to a lower voltage than the OctP Bias. When set to a higher voltage than the OctP bias, there may be an increase in ions at specific mass numbers.

     

    Cell Exit: Accelerates ions coming from the cell. The lower the voltage, the sharper the increase in off-mass- background.

    Deflector Lens: Lens that deflects the ions output from the cell (like Omega does). Optimal value depends on mass measured, gas flow of the cell, and the values of: energy discrimination, Cell Exit, Plate Bias, and OctP Bias. When tuning manually in gas mode, settings are specified so that the relationship between sensitivity and BEC is optimal, rather than so that the sensitivity becomes high. Not something you do in a minute.

     

    Plate Bias: The optimum voltage decreases when cell gas flow increases. There is an optimum voltage with respect to BEC. The Plate Bias has an interdependent relationship with the Cell Exit and Deflect.

  • Hi,

     

    for the Nebulizer gas, optimal settings depend on type and size and manufacturer of the nebulizer. Flows of 1.00-1.05 L/min are most typical for 400 µL/min glass concentric nebs. But of course different settings can be optimal, the manual is mostly suggestions. 0.9 L/min (+ 0.1 L/min Make-Up gas) are also not guaranteed to help, but I saw an improvement quite often.

     

    You can try to play with Extract 1 but just a few single volts up and down, it can be slightly beneficial but usually not much. You are right, the potentials between Extr. 1 & 2 are inversed when using cool plasma conditions or generally different from standard settings after moving the plasma out of the typical distance from the cones through manual tuning. This is only needed in special applications. I guess Extract 1 also does something when fixed at 0 V, but I'm not physicist enough to know how a grounded lens acts on the ion beam flying through the instrument... but I'm sure that if it wasn't there, it wouldn't work so well.

     

    BEC is the background equivalent concentration, meaning "if I translate the 1234 CPS of the blank into a concentration, what would it be"? And yes, it's the same as displayed in the calibration curves. The standard deviation of the blank(s) is a better approximation of the quantification limit but the BEC can also be seen as such. It's the background concentration of the instrument which can have various sources: Carryover of the element, contamination of this element, some interference which just looks like this element (e.g. ArO molecule on 56Fe) or adjacent mass "bleeding" on my analyte mass because the resolution of the instrument becomes too bad / the energy spread of the ions is too high.

     

    Best regards

    Matthias

  • Matthais,

     

    Thanks for the detailed response.  I think it will help me to make informed decisions on which tuning parameters to change during tuning and how to go about doing it.

     

    For nebulizer/carrier gas, the manual specifies a range of 1.01 to 1.11 L/min.  Given your suggestion of decreasing the flow by 0.1 L/min, do you think it is acceptable to deviate (a little) from the specified range?

     

    I find it interesting that the extract 1 lens is fixed at 0 V (per the manual recommended specification).  I'm interpreting this to mean that the extract 1 lens is not affecting the ion beam.  Is it used for other, more specialized applications?

     

    "When tuning manually in gas mode, settings are specified so that the relationship between sensitivity and BEC is optimal"  -  What is BEC?  Is it the same BEC displayed on calibration curves?  I was told this is an approximation of the quantification limit.

     

    Thank you.

  • Thank you for providing useful information.

  • Hi all, 

    Thank you for the useful information provided. 

    We've found some also useful resources at Agilent University portal. Recently a new module was released about system maintenance. There is a slide that talks about Auto RF matching and how it could be related to electronics not being tuned. There is also RF matching at the tune page. Could you please explain in more details the RF matching? 

  • Hi jujuba,

     

    I'm neither a physicist nor technician but I can try to explain as best as I understood  maybe a colleague can correct me if I'm wrong.

    The RF matching can be done automatically in Hardware -> Plasma -> Auto RF matching or in the manual batch tune under plasma parameters -> RF matching. The usual values are somewhat between 1.1-1.8 V.

    It needs to be adjusted when the reflected power is too high.

    What is does to my understanding is, it changes the capacitance of a variable shut capacitor parallel to the plasma RF current. This in turn changes the frequency of the RF generator. The plasma has sort of a natural or optimum frequency under particular conditions, and matching plasma and RF generator frequency helps to couple them better, i.e. let the plasma absorb more of the RF power.

  • Hi....I just have Mass Hunter on the 7700 and 7900 here. To fire them up toward the beginning of the day it normally inquires as to whether you need to go through the underlying startup program when you light the plasma which will heat up the instrument at that point move to the tuning arrangement and run an autotune and a tune check.

    You can avoid these and do the tune physically. You click on the Batch tab of the interface and you can get to the tune boundaries there and do it physically.

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