ICP-MS manual tuning

Question

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.

Agilent · Accepted Answer

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.

Agilent · Answer

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

Agilent · Answer

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

ICP-MS manual tuning

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