Calibration Points - Recommendations?

Discussions about IC and related topics

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I'm still pretty new to IC, and I'm trying to develop an internal test method for my lab (which had no documentation prior). I'm following EPA Method 300.0, but my lab does not do any kind of environmental testing - we are using the IC to detect ions as part of failure analysis.

I am looking for recommendations on what concentrations my anion calibration standards should be. I purchase premade stock solutions from AccuStandard. I noticed that many premade multielement standards have different concentrations for each species - is there a reason for this, or does it solely depend on the method/what you're looking for? I did find one standard that contains 100 ppm across the board of all species of interest, so I was planning on using this.

For my purposes, I want to be able to analyze F, Cl, Br, NO3, PO4, SO4 at concentrations of 0.1-20 ppm. I need to use linear curves, preferably unweighted to make uncertainty calculations simpler. How many calibration points should I use, and at what concentrations?

Just looking for some opinions to make sure my thinking is reasonable. All suggestions and advice are appreciated!
This may be a deeper dive than you were interested in, but your situation is almost exactly covered in the Coleman & Vanatta series on statistics in American Laboratory (especially the first dozen or so articles in the series -- they're short): ... lpi_4346=4
-- Tom Jupille
LC Resources / Separation Science Associates
+ 1 (925) 297-5374
With my Dionex the linearity problem limits F and NO2 and I calibrate those from 0.1 - 4 ppm. I weight the curves though, because the lab has to get the low standard to pass within 15% as the reporting limit standard. I use 6 points 0.1/0.2/0.5/1.0/2.0/4.0. Cl SO4 and NO3 are more linear, but I don't care about levels as low as 0.1 ppm for those. For NO3 I use 0.5/1.0/2.5/ Cl 5.0/10.0/25.0/50.0/100.0/200.0. We see lots of high Cl. For SO4 I only go to 120 ppm. The problem with going higher n that is that it time shifts at levels around 100 ppm and gets identified as Br as the column ages.
Thank you! Tom, those articles were super helpful. I can't believe I hadn't come across them before, with all the searching I've done.

Do you have any recommendations on where to purchase stock solutions? Most ASTM and EPA standards say that performance check solutions need to be from a different source than what the calibration standards are made from. This is leading me to believe that I need to find stocks from two different manufacturers? I haven't been able to find many options where the species in a mixed element standard are all the same concentration - usually fluoride is low, while phosphate/sulfate are high.
Many companies will make a custom stock for you. Inorganic Ventures and SPEX make them. I have gotten stocks that have too low Cl and just spiked it with some Cl to get it up to a good level. Right now I am having trouble getting an NaCl stock - seriously. I could make it in 5 minutes.
Most standard companies offer different combinations of concentrations. We use multi level standards because some anions respond better than others to the detector. Chloride may do well from 0.1ppm to 100ppm while sulfate is good from 0.4ppm to 400ppm, giving almost the same peak heights for each. That is why many offer the multi concentration mixtures.

Linearity will depend on your injection volume and column size. I would buy individual standards and test the range over which each is linear, then either make a mix from those that you can dilute into your working calibration or see if someone like SPEX will make a mixture to match that for you. We normally buy the 10,000ppm standards and mix them into a working stock at the levels we need then use that to dilute and make the calibration curve.
The past is there to guide us into the future, not to dwell in.
I like making my standards from dry chemicals because those never expire and a small bottle will last forever. But NaF is considered too toxic for us.
So as long as it's above my instrument's limit of detection (which I've calculated as between 0.01 and 0.04, depending on the anion), I should be fine to have the calibration curves for all species cover the same concentration range? It doesn't need to look pretty, it just needs to give a detectable peak from which the area can be integrated. All anions I'm looking for are equally important to my application.

How often should a calibration curve be run? It seems excessive to calibrate daily, but that's what I'm seeing a lot of. I run an intermediate check (mid-range standard, usually 2 ppm) with every batch of samples, but was only planning on generating a new calibration curve every few months, or when the intermediate check no longer passes within 10%.
Peak areas are pretty stable. What changes is retention time. Method 300.0 says you must recailbrate if the retention times are off by more than 10%:

10.4 If the response or retention time for any analyte varies from the expected values by more than ±10%, the test must be repeated, using fresh calibration standards. If the results are still more than ±10%, a new calibration curve must be prepared for that analyte.

I'm having to calibrate my ICS-2100 every 3-4 weeks because RTs are shortening pretty quickly. I think there is something wrong with it, but I've had 2 people from Thermo look at it and they can't find out what. My Integrion has been good since mid-December.

How are you calculating detection limit? The old rules for calculating it made no sense. Now they say to analyze 50 blanks or 6 months of blanks:
If some (but not all) of the method blanks for an individual analyte give numerical results, set the MDLb equal to the highest method blank result. If more than 100 method blanks are available, set MDLb to the level that is no less than the 99th percentile of the method blank results. ... 3-2016.pdf

In my experience chloride and sulfate calibrated well up to 200 ppm. Nitrate is good up to 20 ppm. Fluoride and nitrite don't works so well above 5 ppm because linearity breaks down. The peaks for later anions also tail pretty badly at high concentrations.
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