I am analyzing trace metal concentrations in algae pellets, which have been digested in HNO3. Some of the media used to grow the algae have very high concentrations of macro-level nutritional elements i.e. Na, Mg, P, S, and Ca. The range of concentrations is from 1000 ppm to 10,000 ppm. Can I run these samples directly on the ICP-MS without damaging the mass specs/multiplier?
I expect issues like signal suppression and deposits on the cone, but the main concern is not damaging the ICP-MS.
Dear Charles
I assume that you're not interested in the concentration of the nutrients, so you won't probably measure their isotopes. And it's not really clear if you have 10 g/l in your digest or only in the growth media before digestion.
But even if so, you could only damage the detector if you include the nutrients' isotopes in your method. You will certainly get deposits on the cones and ion lens stack, which will lead to signal drift. If you run a lot of samples undiluted you may even get some deposits on the rods of Q1, CRC and Q2 but I would not expect that to be substantial.
But considering the sensitivity of the instrument, I wonder why you need to run such samples without dilution.
Hope this helps,
Bodo
Thanks Bodo, I probably need to post more detail.
The 10-100 g/L is after digestion/dilution and is the concentration that will go into the nebulizer. We prefer not to dilute the samples too much, because then the trace elements we're interested in (Cu, Zn, Co, Fe, Ni) will become too low.
One thing I just found out from Agilent - HMI mode allows high matrix analysis by diluting with Ar in the nebulizer. That will reduce the signal suppression due to the high matrix. So this improves trace element sensitivity even though there is some dilution of the trace elements by the Ar. This may be the way to go.
Dear Charles
Well sure, HMI is also an option but the effect is pactically the same as dilution beforehand. Aerosol dilution effectively reduces the amount of sample transported to the ICP to a "tolerable" level to avoid/minimize matrix effects. In the end you will end up with approximately the same sensitivity for your trace elements. At least I haven't seen distinct improvements in analyte sensitivity with aerosol dilution when compared to diluting the solutions by a similar factor.
I generally prefer dilution because that also can reduce memory and matrix effects during sample transport and nebulization and possibly clogging of the nebulizer.
And I would never run a solution having 10s of g/L Ca, Mg even with HMI.
Best regards
Bodo
Hi Bodo,
thanks again for your reply. I spoke with an Agilent ICP-MS specialist today, and they were very confident in the ability of MHI to reduce the matrix effects while maintaining our trace metal sensitivities.
HMI mode is supposedly able to handle the analysis of undiluted seawater at 35 g/L or more.
It is possible Agilent are exaggerating these capabilities. I'll run tests of a range of standards in high and low Na matrices and post the results here.
Cheers!
Hello Charles, Just a couple of thoughts:
You state the media concentrations are quite high, but are you sure the algae have actually absorbed all of the available nutrients? Most organisms are reasonably good at regulating their intake, so luxury uptake is unlikely to see the algae absorb _all_ of the nutrients unless they are reproducing at a higher rate as well. Even then, you will still only be weighing out 100 mg of algae pellet per sample (or whatever mass you are using).
- Can you reduce the sample mass at all?
- Or run the digest through a resin of some sort to selectively remove the macro elements?
Following Bodo Hattendorf 's advice, is it worth diluting your samples down to a "reasonable" amount of macro elements (whatever you think reasonable is for your instrument/calibration/etc. is), and if that is too low to measure the trace elements, work your way back up until you can measure the trace elements at an acceptable degree (whatever criteria you have to measure them).
You won't damage the detector/cones etc. by measuring diluted samples, but you might do so measuring very high concentrations, even with the Argon dilution technology.
Hi Matthew, to measure the uptake of metals in an algal solution, we first pellet the solution in a centrifuge. Then both the pellet and supernatant are digested in HNO3 to produce two samples for ICP analysis. We calculate the uptake of trace metals by the algae by subtracting the concentration of trace metals in the supernatant from the concentration in the digested pellet.
The pellet and supernatant samples will have high matrix concentrations (Na, P, Ca, Mg) and low trace metals concentrations. We cannot dilute either appreciably without losing detection of the trace metals.
Hello Charles,
A follow-up thought; is it possible that the algal pellet you produce still has an appreciable volume of high nutrient liquid medium surrounding the cells? Could you try washing the cells with an isotonic solution (with known metal composition obviously), to resuspend the algae then centrifuging again? That will in essence dilute the few microlitres of solution around the cells but retain the same mass of algae. You can always measure the resuspension solution to see how much (if any) is leached out of the cells by this step.
Matthew Wheal The pellets are washed before digestion. Thankfully, previous tests showed that re-suspension and centrifugation did not measurably lower the macro nutrient concentrations found in the pellet samples.
On a separate note, the previous analyst in our lab prepared the standard solutions by weighing the water diluent, and then pipetting small amounts of HNO3 and trace metals standard into 50ml tubes.
In my previous ICPMS work I used PMP flasks and used serial dilution to make standards. Is there any benefit to weighing the water and constantly adjusting the pipet volumes?
Hi Charles,
I weigh all my pipetted stocks to ensure there are no gross errors (blunders is another term) in my preparations. Even with a printed spreadsheet, labelled tubes and a hour set aside for the task, it is quite surprising how often little mistakes sneak in and big mistakes just crash through.
As long as your solutions are all weighed and adjusted to a constant temperature and you know the density, you should be able to convert between mass and volume (mg/kg and mg/L) easily enough.
If the salt load proves too great you can resort to the well-tested, if labour intensive, technique of selective extraction
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