Karen, Thank you for the note. We've been using the auto-tune and default settings, and until we get more familiar with the system we're reluctant to change them, especially since the system works well at lower concentrations for Pb and other elements.
I'm thinking that the next step is to determine if the issue is chemical or instrumental. We'll look at some other elements, maybe as chloride salts rather than as nitrates, in the 80 - 100 ppb range and see if they mimic the Pb behavior. If they do, I would suspect it's a detector problem. If not, I would suspect a chemical interaction in the sample introduction system.
Anyway, I do appreciate everyone's comments and suggestions.
From: PLASMACHEM-L: Analytical Chem.(ICP's, DCP's, MIP's). [mailto:[log in to unmask]] On Behalf Of Karen Spaleta
Sent: Wednesday, September 21, 2011 4:38 PM
To: [log in to unmask]
Subject: Re: Drift in Pb signal
could that be a P/A tuning issue with the detector?
Karen J. Spaleta
ICP-MS Research Professional
Advanced Instrumentation Laboratory
University of Alaska Fairbanks
PO Box 755780
Fairbanks, AK 99775-5780
e: [log in to unmask]
On Wed, Sep 21, 2011 at 11:48 AM, Chisholm, William (CDC/NIOSH/HELD)
<[log in to unmask]> wrote:
> We have a new Nexion 300D ICP-MS that we use for analyzing lead and other elements. Whenever we try to run a sample or standard with more than about 80 ppb lead we get a large drift in the lead signal. We use the Nexion software to set up a routine that flushes the system and then takes 6 replicate measurements, one right after another. All six replicates are finished in a few seconds. For Pb concentrations above 80 ppb, the Pb signal (raw counts) falls with each replicate till the sixth (last) replicate, which is typically half or two-thirds the value of the first.
> We see this behavior for all lead isotopes. The drift is always towards lower counts, and we see it no matter what sample we ran previously, pure water, acid blank, or 100 ppb Pb standard, 5 ppb Pb standard. But for concentrations much below 80 ppb the signal from replicate to replicate is constant within a percent or less. We see the drift with ultra-pure Pb standards diluted with ultra-pure HNO3 in 18.2 megohm-cm water, and we see it in complex geological samples with multiple elements in HNO3.
> I assume this behavior is not normal; Does anyone have any idea what's causing this?
> Many thanks, - Bill
> William P. Chisholm
> Research Physical Scientist
> NIOSH m/s L-3030
> 1095 Willowdale Road
> Morgantown WV 26505
> [log in to unmask]<mailto:[log in to unmask]>