What Karen and Ed said about the pulse to analog mode could indeed be your problem and gave me an idea. I am also not familiar with the NexION (I run ELAN 9000) but I believe the software is similar to previous versions for the predecessor instruments. If 80 ppb of Pb is at the 2,000,000 cps range your detector could be switching to analog mode. First take a look at your method and make sure that on the "Processing" tab (if the name isn't changed) that the detector is set to "Dual." If it is set to pulse this could be part of the problem. Also, if you have it set to dual and you didn't do a "Dual Detector Calibration" for Pb then this could also be your problem. For the most accurate data when crossing from pulse to analog in your method it is best to perform a dual detector calibration every day (add to your "SmartTune Wizard") and include all elements in the dual detector method and solution for which you expect to go above the 2,000,000 cps threshold. This will give a smooth transition from pulse to analog and allows a nice linear dynamic range. Verify the accuracy of your LDR with a series of solutions beyond your calibration range.
If you are uneasy about changing methods, open the default dual detector method and save it as a separate file name then add all elements/isotopes you wish to quantitate. Important to add enough of each element to your solution for this so that the count rate exceeds 2,000,000 cps. Make sure you also change the method file in the "SmartTune Wizard" that is used for the dual detector to the new one you just created, save the work space (different name too if you don't want to mess with your defaults).
Randy J. Mercurio
Eastern Research Group, Inc.
601 Keystone Park Drive
Morrisville, NC 27560
Office: (919) 468 - 7922
Lab: (919) 468 - 7933
Fax: (919) 468 - 7803
Email: [log in to unmask]
>>> "Chisholm, William (CDC/NIOSH/HELD)" <[log in to unmask]> 9/21/2011 3:48 PM >>>
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
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