Problems in quantification by ICP-MS

Hi angelas20002000,

I audited a food testing laboratory not too long ago so I have this kind of situation fresh in my mind. I am going to assume that your instruments are working as required and were serviced according to a specified schedule, so the source of error is not there. 

I will also assume that your sample preparation procedures are well established and did not vary in this situation (as you pointed out, multiple technicians are getting the same results), so again the source of error is not in the procedures. 

My educated guess is that your problem is your reference materials. If your reference materials are "old" (namely Cd and Pb), then this could significantly affect your results. Even if they are not old but for some reason something happened to them, the same problem would occur. 

Again this is just a guess, but if you have any doubts about your reference materials, it may be a good idea to get a fresh batch and see if this solves your problem. 

Good luck

Angelas,

I know that this post is a little old, but I just wanted to throw this out there in the event that you are still having these issues.  I see where you stated that you verified the concentration of your reference material using your AA spec and that all of the concentrations checked out and were hitting their target values.  If that is the case then I seriously doubt this issue lies in your certified reference material as if the standard had issues such as stability or floc then it would show up in the AA analysis as well.  This leads me to think that the issue may be interferences.  Because of the sample types your are working with, I would imagine that you are putting some really harsh stuff through the instrument.  When I say harsh I am really just talking about in the sense of having a high concentration of interfering isotopes such as Na, Cl and C.  Since the istopes you are having issues with are all on the heavier mass side then I would look to interferences that exist with those masses and see if you can find a common thread between them.  If you do not already have your method set up to monitor the cps for interfering isotopes then I suggest you add that in.  This way you can look at your samples and see if they have an extremley high Na content or some other interfering isotope content.  If you see that the counts are really high or are saturated then go ahead and increase your rinse time and try a larger dilution factor for your sample prep (if possible).  You can also look to other digestion methodologies which help to get rid of those interfering isotopes within a sample while leaving the rest of the sample as is (I.e. reductive precipitation).

It would be very interesting to see what your data looks like.  For example to see what you cps for all of your internal standards are througout your calibration, blanks and samples.  Also to see what your cps for the isotopes you are calibrating for are.  I would also note your instrument to see if anything has changed within the time frame that you first started to notice the issue.  For example did anyone perform maintenance on the ICP-MS?  Even something as minor as changing/cleaning the cones, changing sample peri-pump tubing, changine the oil in a pump, changing the rinse water you use, changing lot number or manufacturer or acid you use, etc.  Look and see if you notice any fluctuations in your vacuum pressure (both plasma off and plasma on).  Things like this can really help narrow down a problem. 

If I can help you any further please let me know.  Hopefully you have gotten this issue corrected and if you have and would not mind, then it would be great if you would share in your troubleshooting efforts to let the community know what you did to fix the issue or what the root problem ended up being.

Thanks,
Tox

Thank you for your help. I solved the problem after the substitution of turbomulecular pump but I am not sure that the real problem was it . The technician did not give me an answer......

Well a turbo pump which is going bad will surely cause you issues that will only get worse until the pump is replaced or that instrument will no longer be able to operate, as the turbo pump is responsible for maintaining the extreme vacuum within the vacuum chamber.  If that vacuum pressure starts to go up it is going to start throwing everything off and eventually you will not even be able to sustain the plasma.  It does not surprise me that you would see the heavier masses go first.

This is one of the reasons that I was telling you to check your vacuum pressure both before lighting the plasma and after lighting the plasma, because if over several weeks of time you see those numbers bouncing around or steadily increasing then it can tell you right away that you are having an issue somewhere in the vacuum system.  At least when you are able to narrow it down to a particular system then you can know to focus your attention to those components.  In the case of the vacuum system I always start with the cones, then check out the vacuum backing pump, then the interface roughing pump, then the vacuum chamber itself and lastly the turbo pump.  I am not sure what instrument type you have, but if you have an Elan series then you can always at least see if your bag light is comming on.  Whenever the vacuum system is properly working that bag light is going to be on.  If you see that bag light getting dim or if you see that it is out completley then you know you have a big issue.  In your case with the problem being your turbo pump it is just as well that you had a service engineer come in and take care of it, because those things are a real pain to replace (At least in the Elans and the Agilents) and they are very expensive so the last thing you would want to do is get a new one and then not be sure exactly how to install it and end up breaking it.

Hopefully you will not see any more issues.  If you do then please feel free to post again.  I am sure that someone will be able to point you in the right direction or at the very least give you another angle on the problem.

Tox