I am not sure what exactly the problem is. Do you analyse the bacillus or the supernatant? What method do you apply for eliminating the potential ArC+ interference?

It would be helpful to have some more information about the experimental set up. There are many different possibilities why ICPMS may not detect a particular isotope.They are mostly related to insufficient signal / noise (e.g. low concentration or spectral interference) or loss of the analyte before analysis.

Best regards

Bodo

Dear Bodo, thanks for your note. I have added Bacillus biomass (live/dead) to 50 ml (100 ppm Cr) for 1,2,3 hours. Then centrifuged and analysed the supernatant by ICP. But half of the samples shows the Cr (VI) and another half couldn't detect by ICP.

Hi, if you have checked the equipment itself and since you are doing adsorption, it could be due to a very high uptake of Cr after the adsorption process. in order to check if your equipment is working properly, you can make some standard solutions and test for the results. 

Cheers

A few more questions:

What Cr levels are you seeing in the positive samples and which dilution do you apply before ICPMS analysis? Do you correct for or eliminate the ArCr+? What is your instrumental LOD? Is Cr "lost" only in samples of a specific time or does that occur randomly?

Any ICP-MS instrument shows you information just about the total Cr concentration (plus - information about isotope ratios). But you says about Cr(III) and Cr (VI), so my question is if you use any on-line separation (e.g. Ion Chromatography) before introducing your sample to plasma.

P.S. As Bodo says, you should (1) check mass interfences, (2) check your instrumental detection limit and sensitivity.

Dear Bodo, I was assuming that it should be detected as a form of Cr (III), but those samples which detected shows Cr (VI), with dilution 1:20. And used Spectroquant Test Kit to digest and UV to run the samples. It was acquired randomly. Especially in those samples with alkaline pH.

Dear Arezoo as mentioned by Jerzy, the ICPMS cannot detect the oxidation state of Cr in solution. Again, we would need more information about the procedure and the individual results. You say you dilute the supernatant 20 times. Is that before analysis with ICPMS or UV? Which concentration of Cr do you detect by ICPMS when above LOD? What is the instrumental LOD? How do you calibrate? What is your internal standard? Cr should be easily detectable at sub-ppb concentrations when no spectral interferences occur.

When you say that samples with low pH showed no Cr more often, is that a general trend or still just a random effect?

As already indicated by Bodo Hattendorf and Jerzy Mierzwa I suppose a (strong) mass interference due to the mass-combination of Ar + C which is positioned on the 52 Cr-signal and makes the Cr signal not detectable. There are machine types by which you can deionize Ar+ to avoid mass combination; in my instrument this worked perfectly by rinsing a collision compartment (intercalated between cones and quadrupole) with nascent hydrogen, by which Ar+ is deionized to Ar0 and so it no longer contributes to form mass interfering compounds. Other collision techniques may be available. If a deionizing collision technique is not available or with too low efficiency you have to destroy the carbon in your sample. Adding strongly oxidizing acids will help - but only following an aqua regia (a.r.) treatment; otherwise there might be strongly exothermic reactions. After pre-reaction at ambient temperature evaporate a solution aliquot with a.r. until dryness (about 900 C). A following evaporation by HNO3 will help to destroy organics; whether an additional step with HClO4 or H2O2 (or both) is required must be found out. Do not forget blinds. Cr will be converted to Cr6+; but MS can not discriminate between Cr-valences anyway. Anyway, a device with deionization of Ar+ would be most helpful.

Dear Bodo, dilution was done before inject samples to ICP. Samples were checked by UV, if it was more than LOD (2 ppm) then diluted. Instrument was calibrated based on standard methods (0 to 8 ppm). And it happened randomly not a general trend. Is it possible that Cr has been fixed in well or cellular ingestion?  TQ.

Dear Dieter..thanks for your guidance..may I know how can I get the details of method for deionizing collision technique? Is that possible during the process of experiment Cr III converted to VI. 

Do you really measure 2 ppm Cr ? Not 2 ppb ??

Dear Arezoo,

unfortunately this fine and efficient tool is fixed as integrated part into the instruments: an external hydrogen generator provides the collision compartment of the MS-device and so the Ar+ is deionised  (ionisation potential of H < Ar) and does not combine to form interfering masses. Unfortunately also: the machine I used* is no longer on the market (Micromass Masslynx); To my information there are now other tools which promise similar efficiency in collision techniques without reducing sensitivity; I am not so sure. So possible my advise to destroy the organics might be a help. I forgot to indicate that you should avoid glass equipment and prefer PFA vessels (at least: PTFE); otherwise you take the risk to hamper your results.by adsorption/desorption processes.

Good luck

D. Zachmann

(* no longer active in the lab; retired)

You need to clarify whether you are trying to assay CrIII in the supernatant or in the bacteria pellet.  I suspect that all of the CrIII will be in the pellet and very little in the supernatant for two reasons:  CrIII should bind tightly to all proteins (and other biomolecules - it is used as a tanning agent!), and CrIII should be actively absorbed by the bacteria (Cr release assay depends on cells taking it up then releasing it again if killed).  If the bacteria are intact, they would not release CrIII into the supernatant.