CALIBRATION CURVE ICP-OES

[Marianna]

Hello,
I'm new with ICP-OES and I need help about the calibration curve.
I have an iCap 7200 ThermoFisher coupled with an autosampler ASX 560. I want to anticipate that I will buy ultrapure water and ultrapure acids; I avoid to use glass in my preparation, but I use only a glass flask to prepare the washing solution (I use a 20% acid solution composed by 10 ml HNO3 and 10 ml HCl in 1L total) because I haven't yet plastic flasks.
I prepared five standard solutions from a 100mg/L multielemental stock: 20 ppm, 10 ppm, 1 ppm, 0.1 ppm, 0.01 ppm. I explain my operations.
In the 50 ml graduated falcons I pipetted:
1) 20 ppm : 10 ml stock + 40 ml from washing solution (I helped with the graduated scores on the falcon)
2) 10 ppm: 5 ml stock + 45 ml washing solution
3) 1 ppm: 0.5 ml stock + 49.5 ml washing solution
4) 0.1 ppm: 0.060 ml (i.e. 60 um) stock in 60 ml total volume
5) 0.01 ppm: 5 ml from 4) + 45 ml washing solution.

My problem is that all concentrations are double after acquisition. Why?
I add that I had the same operations on the last week and the concentrations were good.
Can someone help me to understand my mistakes? Where is the problem?

Thank you!

[MichaelVW]

Well, your standard concentrations look ok to me. I've never worked with that model of ICP. Some thoughts: Usually when our results double suddenly it's because the internal standard ran out, but I don't know if you're using one. Could it be something more fundamental, like gas pressure of pump tubing? Is the plasma stable? Sodium bullet test look ok? When you say concentrations doubled, what do you mean exactly? Were past calibrations prepared the exact same way? Does your calibration look linear, or is it all over the place?

PS: If anyone knows of an ICP forum as good as this one is for chromatography, I'd love to hear about it and I'm sure OP would too.

[Marianna]

Well, your standard concentrations look ok to me. I've never worked with that model of ICP. Some thoughts: Usually when our results double suddenly it's because the internal standard ran out, but I don't know if you're using one. Could it be something more fundamental, like gas pressure of pump tubing? Is the plasma stable? Sodium bullet test look ok? When you say concentrations doubled, what do you mean exactly? Were past calibrations prepared the exact same way? Does your calibration look linear, or is it all over the place?

PS: If anyone knows of an ICP forum as good as this one is for chromatography, I'd love to hear about it and I'm sure OP would too.

Hi Michael,
Thank you for your answer!
I will check gas pressure, but it was 5,6 bar during the work. How can I see if the plasma is stable?
I used Argon as internal standard but it seemed ok.
When I say that my concentrations are double, I mean that each one isn't my expected value, but for example my standard 0,01 ppm results 0, 02 ppm, so 1 ppm results 2 ppm, 10 ppm results 20 ppm and 20 ppm results 30 ppm.
My calibration spots aren't on the line but all over the place, infact my confidence limits are 86% and I know that it's a too low and bad value!
And, how can I do sodium bullet test?
The last time it was happened that a yellow sample colured yellow my tubes, could it be a problem?
I'm yet not expert with ICP, I used it only two times and this is my second calibration, but the first time it was ok.
If you have other information or tips to tell me, I'm very happy to learn them!
Thank you so much!

[LALman]

I think those acid concentrations are too high. Also, don't make a rinse mix and use that to make standards. When you make a calibration standard always use volumetrics and after adding a little water add concentrated acids directly from preferably a bottle top pipettor on the acid bottle. Letting your rinse acid stand around is just asking for contamination problems, expecially in your more dilute standards. Then make sure your dilution water is at least 17.6 MegOhm, preferably 18.2 MegOhm.

Its been a while since I was running ICP-OES. This is a recipe for ICP-MS you can modify for levels upward to use it for ICP-OES. Except, you cannot do Hg. I think probably a 10X increase in every concentration (except acid) levels should work our well.

The HP 4500 is the newest ICP-MS I've used. Generally, acids are the same as dissolved solids. Your matrix load on the plasma needs to be constant.

I digest my samples to a 20%HNO3 10% HCl final concentration. Then I do a 10X dilution of that for running. I run with 2% HNO3 1% HCL in samples, rinse, and standards. 1% HCl will let you keep 1000 ppb Tl, Ag, and Sb stable in solution without problem.

I make two stock standards. CAL4 at 1000 ppb for nearly every analyte. CAL3 at 200 ppb for nearly every analyte. Now you can make sucessive dilutions of these to have cal levels 5X apart. To each volumetric just at the moment you make the standard add a little DIWater and 2mL HNO3 and 1mL HCl. Then do as follows.
4mL CAL#4 /100mL = CAL#2
4mL Cal#3/100mL = CAL#1
4mL CAL#2/100mL =CAL#2A
4mL CAL#1/100mL = CAL#1A
Immediately transfer these into Teflon PFA bottles and you will have no problem with making Boron standards that remain stable.
This gives you 1000, 200, 40, 8, 1.6, and 0.32 ppb. CAL4 is your linear range check the 5 more dilute ones are your cal curve. The 40 ppb is my CCV that gets rerun every 10 samples.

If you use trace metal grade acids (you don't need optima grade) and PFA teflon bottles, CAL4 and CAL3 are stable for ~ 1 year. I make those two in liter amounts. All the others I make 500mL at a time. Just use 20mL in 500 mLs.

When you calibrate use the Log Linear cal type. These will be nicely spaced to let you easily evaluate your calibration curves in log linear graphs.

[LALman]

Once you see the concept, its easy to modify that a bit. I usually have Fe, Na, Ca, K, Mg at 20 ppm in CAL4. I make stock solutions for making CAL4 and CAL3. They are generally at 100-200 ppm prepared from 10,000 ppm primary single standards. I group them common stability. Make your rinse solution 1 ppm in gold. This will give you a good washout for mercury. Speaking of which CAL4 should be prepared with as 100ppb Hg and CAL3 at 20ppb. This lets you calibrate down to 0.16 ppb and your MDL will be around 0.05 ppb or better.

[Marianna]

Thank you very very much! You suggested me very important things. I will prepare my diluition with fresh acid solution. As regard to Hg, I'm waiting for the standard stock, in fact I ordered the 100 ml bottle and I will add also it in my future standardization.
If I will have other problems, could I write you again here?
Thank you!

[James_Ball]

I always made up 2L flasks of my working acid at 2%/1% HNO3/HCl and used that to dilute my stock standards to a working solution and also used that to rinse between samples. 20% does sound too high for the acid concentration, and if there is a difference in the standard and sample acid concentration there will be difference in results.

If you add mercury to the standards you will need to use gold to prevent the mercury from adsorbing on the plastic tubing, otherwise you will have varying results. When we do mercury we spike every standard, blank and sample with 50ppb gold solution to prevent that. It will also prevent loss of mercury when you do a heated digestion of the samples because it keep it from becoming volatile at the higher temperatures.

[Marianna]

I always made up 2L flasks of my working acid at 2%/1% HNO3/HCl and used that to dilute my stock standards to a working solution and also used that to rinse between samples. 20% does sound too high for the acid concentration, and if there is a difference in the standard and sample acid concentration there will be difference in results.

If you add mercury to the standards you will need to use gold to prevent the mercury from adsorbing on the plastic tubing, otherwise you will have varying results. When we do mercury we spike every standard, blank and sample with 50ppb gold solution to prevent that. It will also prevent loss of mercury when you do a heated digestion of the samples because it keep it from becoming volatile at the higher temperatures.

Ho James,
Ok, my first step will be to decrease the acid concentration in the diluition solution.
You gave me precious information.
Thank you very very much!

[MichaelVW]

Then make sure your dilution water is at least 17.6 MegOhm, preferably 18.2 MegOhm.

Really? I hadn't put much thought into our DI water. Is that amount of purity necessary for ICP-OES? What kind of behavior would you expect lower quality water to cause?

[Marianna]

Then make sure your dilution water is at least 17.6 MegOhm, preferably 18.2 MegOhm.

Really? I hadn't put much thought into our DI water. Is that amount of purity necessary for ICP-OES? What kind of behavior would you expect lower quality water to cause?

Hi Michael,
Yes, the purity of water is important, in fact a lower quality water has an higher metals concentration than 18.2 MegOhm. So, I ordered it yesterday.

[James_Ball]

Then make sure your dilution water is at least 17.6 MegOhm, preferably 18.2 MegOhm.

Really? I hadn't put much thought into our DI water. Is that amount of purity necessary for ICP-OES? What kind of behavior would you expect lower quality water to cause?

Depends which elements you are looking for. Iron, Sodium, Calcium, Potassium and Magnesium can be very high in low quality water. Even water with 18.2MOhm can have high ppb concentrations of the salts that are not completely removed from tap water by a DI water system. If you are working in high ppm levels then it would probably not be noticed, but when a regulation asked for detection limits at 5ppb, or lower then you need really good water.

If the conductivity is higher because of dissolved CO2, then you might be able to get away with it, which can happen even with the best water, so conductivity/resistivity are not the only specs to look for.