I have done pyrolysis of biomass and got pyrolysis oils, char and gas as products. To evaluate the distribution of inorganic matter in the biomass between the products, we did an ICP-MS analysis of the biomass and products. Assuming that inorganic metals don't vaporize, I expected to get mass balance like:
Minerals in biomass = Minerals in (aqueous phase oil + non-aqueous phase oil + char)
However, as you can see in the attached excel sheet, the balance is way off with mineral matter in char being much higher than in the source biomass.
I have listed the procedures I followed for the analysis:
I. Biomass:
- Add 0.5 g biomass to Teflon vessel and add 1 mL HF with 10 mL HNO3
- Digest the material according to the One Touch method for Straw
- Add 30 ML boric acid (4% w/v) to neutralize the HF and digest according to One Touch Method for HF Neutralization
- Filter the digested product and make the final volume up to 50 mL
- Dilute the sample to 1:10000 prior to analysis in ICP
- The result values given in the table account for the dilution factor
II. Biochar:
- Add 0.2 g biochar to Teflon vessel and add 1 mL HF, 5 mL H2SO4 with 10 mL HNO3
- Digest the material according to the One Touch method for char
- Add 30 ML boric acid (4% w/v) to neutralize the HF
- Filter the digested product and make the final volume up to 50 mL
- Dilute the sample to 1:10000 prior to analysis in ICP
- The result values given in the table account for the dilution factor
What am I missing here?
Is there a problem with the digestion procedure or is it a matter of instrumental error?
If the inorganic elements didn't vaporize, The elements' would be concentrated due to the mass-loss of the volatile matters. However, it will not be that high in your results. The elements you determined are mostly high-content elements. I suggest you determine these elements using ICP-OES except Cr. Too high dilution ratio tend to amplify the digest or instrument errors.
Dear Anubhuti Bhatnagar ,
I had a look at your data. I used Article An overview of the chemical composition of biomass
paper to get an idea of what element content to expect in biomass. When I compare the general elemental content with yours, all elements (except Cr and Ni) appear to be greater in your sample by a factor of 1.2 (Mg) to 14 (Al). This suggests that you have a systematic error.The mass of analysed elements in biochar alone (234 g) is already 3x greater than in original biomass (80.9 g). This is impossible.
Are you reporting the values [ug/g] on dry basis?
If the dilution factor is 10,000x, this suggests that you are measuring concentrations at very low levels (0.0013 ppb for Cr to 65 ppb for Si). If your HF acid and water are not suprapure, you will face issues with measuring such concentrations with good reproducibility. Instead of using 10,000x as dilution factor I would recommend 1,000x.
I am very surprised to see the nickel content to be 10x greater than Fe. Is your biomass really straw, or is it sewage sludge or some other contaminated biomass? If it is not contaminated then there is something wrong here.
How do you deal with isotope overlaps and other interferences? Have you done spike recovery for each analyte?
Do you use an internal standard (e.g. Rh or In) when running your samples?
How do you calibrate your ICP-MS (single-point multi-element standard)?
Robert Thomas Bachmann yes all the values are on dry basis.
I used suprapure HF, nitric acid and other chemicals.
The biomass is groundnut shell obtained from India. It was cleaned to remove the dust and other solid pieces that tend to come from farms during harvesting and storing, then pulverized to 0.9999).
I will repeat the measurements with lower dilution and update here if there is an improvement in the results.
Hi Anubhuti Bhatnagar
are you reporting the concentrations on dry or as received basis?
When you repeat the analysis try to include additional runs with spiked samples. Say you expect a concentration of 10 ppb Fe, and you spike with 10 ppb Fe, you should get 20 ppb Fe in your spiked sample. If you don't then you have an interference.
HI Anubhuti Bhatnagar ,
have you managed to repeat the analysis with lower dilutions and spike recovery? Did you get more realistic results?
All the best, Rob.
Hi Robert Thomas Bachmann I did repeat the measurements.
I have attached the updated results. The balances still seem way off. There are no issues with interference.
I don't know where is the problem.
Hi Anubhuti Bhatnagar ,
the mass balance is improving. In 1st trial the mineral amount in biochar was 2.9 x greater than theoretically possible. Now (2nd trial) it is 2.1 x greater than theoretically possible.
Two things to consider:
1. How effective is your digestion protocol in releasing all the minerals? Has the protocol been validated using a reference biomass and biochar for example ( https://www.researchgate.net/project/Standard-biochar )? For biochar digestion protocol you are using H2SO4, any reason why? Is the digestion time optimised?
2. Main contributors to mineral mass are Si, K and Ca. When we compare the ug/g values for biomass and biochar for 1st with 2nd trial there are big differences (decrease in concentration by factor 5 to 11) for Si and Ca. This suggests that the method is still not able to measure these elements correctly.
Another approach to consider is to ash the samples first then run them on SEM-EDX. This is one of the traditional methods (Article Methods for Characterization of Composition of Fly Ashes fro...
;Article An overview of the composition and application of biomass as...
).Hope that helps? Keep us posted on next move.
Hi Anubhuti Bhatnagar ,
how are you? Is your uni closed due to COVID19? My university is closed until 14.4.2020, at least.
Have you managed to squeeze in another run before any lab closure?
All the best, Rob.
Hi Robert Thomas Bachmann
Our university is shut till 31st May. i did try a new digestion method before the closure. This time I incinerated the char and biomass samples then performed the wet digestion using 10 mL HNO3 (65%) and 1mL HF (35 %).
I have attached the results here.
The concentrations seemed to have increased since everything is now converted to oxides.
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