[8bambi4]

I'm currently working on Silver analysis via ICP-OES.  The problem I have: I've tried about 4 different types of methods, and none is working.  I'm currently trying a method where I'm digesting my Concentrates with Nitric Acid, Perchloric Acid and Hydrochloric acid.  The Silver ppm value of my end result should be between 1170-1230.  I'm 50ppm too low. 

I've tried about everything now...increasing some acid volumes, decreasing them, digesting at different temperatures, digesting for shorter and longer periods.  I've googled and read about every topic I thought could help.

I'm considering to try a Nitric acid digestion without any other acids.  Hydrochloric acid causes AgCl, but only if the HCl concentration is low.  So I increased the HCl concentration to 50% (1:1 ratio).  But at this moment I want to use only the Nitric Acid, so no AgCl could form.

What I want to know is, am I going to waste my time with the Nitric-Acid digestion, or is there something I'm missing in general??  I've been working on this project for almost 7 months now.

The material I'm using, is Copper Concentrate.  The ICP-OES:  PerkinElmer Optima 7300DV.

I'll appreciate it if someone can push me in the right direction.  I'm not a Chemist, only a Laboratory technican in training, and will appreciate any kind of help.

Thanks 

[vex]

I just ran into a similar problem a few weeks ago. It's possible that, at that large of a concentration of silver, you're saturating your signal, or that the difference in absorption intensity for 50 ppm is insignificant compared to your total signal. (I don't know the intensity of the silver line, so I can't say for sure.)

I would dilute your sample solution 100 or 1000 times and redo the experiment, then back-calculate for the original concentration and see if you get the number you're looking for. The ICP-OES can give you very accurate results, but sometimes the concentration of your analyte works against you.

[8bambi4]

Thanks so much vex!!  Appreciate your reply.  You might be right, I've considered to dilute the samples once, but did not do so.  I'll try it and see what happens. 

Thanks a million!!!

[tlahren]

We used to use nitric only for graphite furnace and ICP-OES.  It works well.  We tried switching to a Nitric Only digestion for all elements so we could use the same digestion bottle for all instruments (furnace-AA, ICP-OES and ICP-MS) but some metals just require a different acids to be totally soluble.  We were unable to reliably use HNO3 only for all metals.  Works great for most though. I think the EPA method for Graphite Furnace-AA explicitly states HNO3 only but I would have to check.

[8bambi4]

Thanks a million tlahren!!  I figured that the nitric acid will only help with certain metals.  I'm going to take all the replies and work with it.  I've got nothing to loose.  I'm still collecting some info and net-surfing.  Our ICP-OES is currently out of order.  The quick release (where the argon tube connects to the ICP) is leaking and giving me inconsistent results.  So I have to wait till that's fixed before I can go on with my experiments.

Thanks you guys!!   

[osmium]

Thanks a million tlahren!!  I figured that the nitric acid will only help with certain metals.  I'm going to take all the replies and work with it.  I've got nothing to loose.  I'm still collecting some info and net-surfing.  Our ICP-OES is currently out of order.  The quick release (where the argon tube connects to the ICP) is leaking and giving me inconsistent results.  So I have to wait till that's fixed before I can go on with my experiments.

Thanks you guys!!   

Are you matrix matching your standards so that they contain the same acid concentrations that are in your sample?

If you don't, then the standards will nebulize more  efficiently.  The effect will be that your results can come back lower than expected.

Are you using the axial or radial view on your system?  A such a high concentration, the radial view would probably be better.

You can also try a less sensitive Ag line for the analysis.


-os

[8bambi4]

Hi Osmium,

Yes, the matrix of my standards are the same as the samples.

We used axial a few months ago, and figured out that Radial is less sensitive and showed better results than the axial.  I figured out that the Radial is best to use when your concentrations are high, and Axial for low concentrations.

I will look into the Ag line that's less sensitive.  We've tried only 3 so far, the one line dubbled each sample's value, where the other two were near to correct, but still about 50ppm to low.

What we also found is that on the ICP-OES, you get a small part called the "quick disconnect or release".  This part is connected on the machine where the spray chamber are located.  This small part had a argon gas leak, and caused the plasma to go brighter and dimmer.  So we figured this might also be a huge problem, for the sample's results aren't constant.  So today I've replaced that part (after 3 month's wait), and discovered that one part is too small.  But luckily the guy will arrive next week Monday to insert an argon filter.  So only after everything is been taken care of, I can go on with my experiment and see what the results will be.  We figured that when everything is fixed, and no leakages appears, and the results are still inconsistant, then there are something wrong with the ICP-OES itself.

The way we determine this, is to analyse the same sample, but instead of triplicate readings, we do 30 at once.  So it analyzes the same sample each second for 30 times.  And in that results, there should not be a 50ppm difference.  The sample is homogenous and in liquid form.

But thanks so much for your help Osmium.  I'll try that Ag line and see what happens.  Appreciate your reply!

Bambi