ICP test question

[dough]

I sent in ICP test to two separate testing labs (ATI and ReefLabs). Both samples were drawn back to back, so with in minutes of each other. There are significant differences in values such as
Br (83.27 vs 68.3 mg/l)
Ca (417.9 vs. 384.6 mg/l)
K (356.2 vs 320.3 mg/l)
S (798.2 vs 839.7 mg/l)

The iodide values differ by >50% (.02461 mg/l to 0.05 mg/l).

Several additional significant discrepancies exist, among them the Ba, Al, Cu, Mn, Ni, Se, V, and Co values. One test would show no detection and another a value.

So I'm wondering is anybody has experienced these type of discrepancies between two labs and if so what did you do with the numbers?

 

[Randy Holmes-Farley Verified Community Expert]

I sent in ICP test to two separate testing labs (ATI and ReefLabs). Both samples were drawn back to back, so with in minutes of each other. There are significant differences in values such as
Br (83.27 vs 68.3 mg/l)
Ca (417.9 vs. 384.6 mg/l)
K (356.2 vs 320.3 mg/l)
S (798.2 vs 839.7 mg/l)

The iodide values differ by >50% (.02461 mg/l to 0.05 mg/l).

Several additional significant discrepancies exist, among them the Ba, Al, Cu, Mn, Ni, Se, V, and Co values. One test would show no detection and another a value.

So I'm wondering is anybody has experienced these type of discrepancies between two labs and if so what did you do with the numbers?

Some values can change during transportation, such as phosphate.

The ones you note seem within my expectation of the variability expected. It pays to not have excessively high expectations of such an inexpensive analytical test. lol

In my professional work, we spend $50 or more for a single chemical to be analyzed, and that is only if we have a whole bunch of similar samples to meet a minimum batch size. :)

 

[KStatefan]

All you can go on is feelings none of these test come with any NIST certification or would most want to pay for that level of confidence.

 

[taricha]

So I'm wondering is anybody has experienced these type of discrepancies between two labs and if so what did you do with the numbers?

Check out a recent write-up on this topic in reefbuilders by Sanjay and Rich Ross.
Forum post discussion here :
https://www.reef2reef.com/threads/e...ty-and-differences-between-icp-tests.1040318/

In these posts (post 37, and post 42) I tried to give a snapshot version of their data for what variation might be expected within and across ICP vendors for the same sample.

basically, your results:
Br +-10%
Ca +-5%
K +-5%
S +-3%

and much greater variations for Iodine and trace metals as you saw are all, frankly typical of what people see when they send the same samples around to different ICP vendors.
We should think of these as typical variations in the testing data we get and not conclude those size variations are actually in the water.

 

[Fishingandreefing]

Another reason why I don’t even bother with ICP testings. I have done it twice then it got me confused and just didn’t trust it.

 

[dough]

Thanks everyone for their replies and inputs.