Hello, Amra!

May I ask you first, do you use an autosampler for injection and is it properly inject the sample on the wall or platform of the furnace. Also, why there are no RSD in your data. RSD of standard solution replicates' signal will allow you to understand is there any influences on the repeatability (for example autosampler issues).

I suggest you to try to inject the matrix modifier together with your sample/standard, because for Pb and Cd it may enhance the sensitivity (use Pd or mixture of Pd and Mg modifiers). Try to add 5 mkl of 500 ppm Pd or 500ppm Pd+250 ppm Mg. Also use the furnace with platform.

Use at least 0.1 mol/L HNO3 solution as blank and as dilution solution for your standards.

What temperature programm do you use? Have you tried to

plot the pyrolysis / atomization curves? It will help you to find the best pir/at. temperatures with higher sensitivity (but first add the modifier, it will reduce the atomization temperature for Pb and Cd).

Best regards, Nikita

Nikita Sobolev first of all, thanks for suggestions.

I use autosampler. Before each measurement I adjust the injecting needle position and I also watch the whole analysis and injection on the cam. So far I haven't noticed any significant deviations when it comes to injecting the sample in the furnace. In one of my earlier measurements, I obtained the RSD data. I had no problem with Cd. I didn't do analysis of some sample, but I made a spike with Cd standard and I got good result; I made concentration of 2ppb Cd and the reading was 2.131 ppb. But again I had problem with Pb; for a sample that had 10 ppb of Pb, I got reading around 4 ppb and 6 ppb in the second reading. For 25 ppb Pb, the reading was around 36 ppb. I prepared standards and samples just with water and on that occasion I had no problems with cal and sample blank. The pics are attached.

Tomorrow, I will have another try in measuring Pb and Cd concentrations in reference material and I am planning to do measurements with modifiers and I will also use 0,1 HNO3 as a blank and for dilution in one set of samples and just water in another set.

I have used 0,1 HNO3 for dilution and as a blank in the past, but the same problems have occurred, so I am not sure that is causing me a problem.

For Cd, atomization temperature is 1700° (without modifier) and 2100° (with modifier). For Pb, it is 1600°(without modifier) and 2100°(with modifier). This makes no sense as You wrote that modifier should reduce atomization temperature. But these are the temperatures written in user manual.

I don't know how to plot the atomization curve. As I said I just started doing this without anyone giving me proper introduction to the method and all the things that could go wrong. Is there any literature you could recommend me to get?

Dear Amra Salkic,

Yes the introduction of Pd modifier allow to increase (not decrease) the atomization and pyrolysis temperature (sorry for typo). And typical temperatures for Pb-Mg mixed modifier are about 2000° as written in your manual.

There are a lot of books and papers published about AAS. I don't know what type of matrix are you going to analyze using your instrument, but you may use the keywords like GFAAS, Zeeman, etc to find a lot of manuscripts on your specific topic and matrix as AAS is a pretty well known, reliable and frequently used analytical method for elemental analysis, therefore you can follow the recommendations of your manual in most cases). There is a very good book by Welz and Sperling: Atomic Absorption Spectrometry. Also you may find your specific matrix in this book: Applications of Zeeman Graphite Furnace Atomic Absorption Spectrometry in the Chemical Laboratory and in Toxicology.

But first of all I think you need to use platform furnace, Pd or Pd-Mg modif., temperatures recommended by manual, 0.1 M HNO3 as blank and for dilution of standards, 3 replicates for each sample and standard. And try to start with 10 ppb and also try to add 50 and 100 ppb in your calibration. And I recommend you to acidify samples for most of the elements especially for Pb (at least for pH 2) check this link: https://www.inorganicventures.com/trace-analysis-guide/stability-of-elements-at-ppb-concentration-levels

Best regards, Nikita

Dear Nikita Sobolev , thank you for the literature recommendation.

When it comes to analysis, I did as you suggested, though I didn't add higher concentrations in the calibration. I attached the results. What is the acceptable value of RSD? In my case, I guess it is too high. RSD implies that there is problem with the injection, right? Does it implies to anything else.

Although the RSD and the blank values are high, I am quite happy with the results, because the target value of Cd in the reference material was 670 ppb.

Thank you for all the help. All the best!

Dear Amra Salkic,

I glad that I could help.

RSD seems quite high. It depends on the methodology you are using, but typically for standard solutions it should be less than 5% (excluding blank of course as RSD will almost always be high). The first what you should check if you have high RSD is your injection system.

Best regards, Nikita