Calibration troublles: Sodium

Hello everyone,

I am having the hardest time attaining a good calibration line for sodium on a 5800 ICP-OES. At the moment i am still developing a method to determine Al, Ca, Fe, K, Na, Ni, P, Si, V and Zn in a matrix akin to crude oil, using microwave assisted acid digestion. When taking into account the dilutions in the sample prep, my measuring range goes from 0.08 - 10 ppm (which corresponds to 4 - 500 mg/kg in the sample) using 5 standards (0.08, 0.2, 0.6, 2, 10 ug/mL). Standards are prepared using a multi-element standard containing the 10 elements above. For the 9 other elements, i have repeatable calibration lines for the selected wavelengths with axial viewing. Even when changing the plasma conditions most of the lines stay useable, only Ca and K have some trouble from time to time. IS correction is done with Y II 371 for all elements (using AVS 7).

Now sodium is a different story, no matter what conditions i tried, no valid calibration could be made for the 589 or 588 lines in axial mode with the multi-element standard. To further investigate this, i prepared the same calibration standards using a single element sodium standard. I used these standards to find more suitable conditions and wavelengths for sodium. I have tried different wavelengths with no avail, leaving only the 588 and 589 lines for further experimenting. Considering the low concentrations i initially thought radial viewing wouldn't be possible. But after some tests i found that either a viewing height of 2 mm (low intensity but also steadier background) or 16 (low background, high intensity but also more variance in the background signal) allowed for sodium to be calibrated succesfully using the 589 line. After trying the following conditions with the multi element standards, i got a calibration line with an RSE of 2.40%.

Read time: 5 sec (might want to make this longer but this was sufficient initially)
RF power: 1.4 kW
Neb flow: 0.75 L/min
Plasma flow: 12 L/min
Aux flow: 1.00 L/min
View height: 2 mm

The following day i raised the RF power to 1.5 kW and lowered the neb. flow to 0.6 L/min, this resulted in a valid calibration line twice RSEs around 3.20% (with the multi element standard). That day i forgot to loosen the sample pump tubing and after that day it had become impossible to attain a valid calibration for sodium. The tubing was relatively fresh so i stubbornly went on trying different parameters, thinking maybe i got lucky the first few times. But alas, nothing seemed to work. I have now replaced the tubing, gave the torch a little cleaning and will continue once it is dry.

If i still cannot get a good calibration, i am considering splitting the measuring range using multical. Using the radial conditions above for the first three standards (0.08, 0.2 and 0.5 ug/mL) and the axial viewing conditions used for the other elements for the last three standards (0.5, 2 and 10 ug/mL). To me this sounds counter intuitive, but with axial viewing the calibration error in the low region is just too high.

If anyone has some insights as to what i could be doing wrong or just some general tips for getting a better calibration for sodium. Both are more than welcome!

  • Hello

    Na and and all other first group elements only have one valence electron. Because of their low electronegativity, they are easily ionizable which is not what we want in OES, we want the electrons to jump an energy level, and emit light as they come back to ground state. Because of this phenomenon, you will see group 1 elements, the alkali metals referred to as “EIEs”, or easily ionized elements.

    I have attached an older applications note, which contains principles that still apply. The note was written for our older 700 series instrument, where customer had to choose between axial and radial. We get the best of both worlds on the 5800. On page 4 it has a good few paragraphs about EIEs, and how when looking at them axially, “atomic emission lines of alkali and alkaline earth elements tend to exhibit signal enhancement”. Because of this, customers typically can do 3 things :

    1. Decrease plasma power (lower power will decrease amount of ionization that occurs)
    2. Add an ionization buffer – commonly Cesium (further explained in the attached note)
    3. Third, and what I see most in the field, is to look at these EIE elements radially - since the ionization occurs higher in the plasma, the ionization signal enhancement can be avoided by looking directly at the analytical zone which we can’t do axially. 

    Hope that answers your question and have a great day!

  • Hi John,

    Thanks for the reply. I am familiar with EIEs and matrix effects. That's why i initially tried to approximate robust plasma conditions. After giving the ICP some TLC it is once again possible to calibrate for sodium using above conditions.

    Would adding an ionization buffer also limit the suppresion/enhancement effect of EIE on other analytical lines? If i'm correct an ionization buffer can just be added to the IS solution right? (when adding IS online using the AVS 7)

  • suppression/enhancement are physical interferences that can be improved by the addition of the internal standard. the ionization interference is improved by the addition of the buffer.

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