The issue is that for many fresh water samples (not all) orthophosphate concentrations are slightly (for some samples more than slightly) higher than total P. For sample preservation I'm using HCl. Could that be the reason? Should I rather use H2SO4?
I have already tried eliminating arsenate, tried additional acidification and tried blowing the samples with nitrogen gas.
I do not see how this could be possible. Orthophosphates are the available forms of phosphorus in water and soils. Plants and all life forms have access to phosphorus in this form. Especially in unpolluted systems, the concentration of orthophosphates is very low by reason of ready consumption. This form of phosphorus is a component of total phosphorus in the system. How can a subset be higher than the set from which it was derived? I do not understand.
Do you use the same analytical method and same calibration curve in both cases? If not that might be the reason for the deviating results.
C.A. (Kees) Kan Because I use different reactives for orthophosphate and total P I have to use different standards (calibration curve). I checked the total P adsorbtions with the equation from orthophosphate calibration curve and it seems that this is not the problem, since they almost matched (with slight differences).
The method I use is mostly based on method 365.3 but also (Murphy & Riley,
1962 ; SFS 3025, 1986 ; SFS 3026, 1986)
Karin Sisask Dear Karin, Some general points which you have considered, but always good to keep in mind.
- Reproducibility of the analytical results; three aspects, firstly matrix related related; is the matrix sufficiently homogeneous? Secondly is the sample large enough to be representative? Thirdly, what is the inherent variability of the method (both cleanup/preparation of the sample as well as the detection method itself). Maybe the method is not well applicable to the matrix you are investigating.
- Possible evaporation of the solvent in sample during cleanup etc leading to a higher concentration of dissolved components.
Good luck in your search for an explanation of these unexpected/awkward results.
C.A. (Kees) Kan Thank you very much for your input, it is much appreciated.
First, switch to Sulfuric Acid (be sure the volume is correct for the method as it will severely influence the pH and thus the color of your samples if you add too much). Secondly, make sure your sample volume for the TP analysis is large enough to account for any evaporation that may occur in the autoclave process. I use 25 ml of sample/standard (at minimum) when analyzing for total phosphorus. Since you are having some issues, I would also do a method validation for both TP and Ortho and do all samples and standards with technical reps...to ensure the concentrations you are getting are consistent.
It can't. Therefore one of your two measurements is incorrect. We developed and have used the persulphate oxidation technique to digest freshwater samples to determine Total N and Total P (on an unfiltered sample) and Total Dissolved N and Total Dissolved P (on a filtered sample) in my lab for over 30 years. We conduct our digestions in a microwave digestion unit, which will digest around 40 samples in 1 hour. We then analyse them on a Skalar San ++ continuous flow autoanalyser, using the molybdenum blue method to determine all sample P as orthophosphate, and the copper hydrazine reduction method to determine all oxidised N (TON = nitrate + nitrite) and any residual ammonium in the samples. We use 10 ml of sample to 5 ml of oxidising reagent, introducing a dilution factor. We multiply all concentrations derived from the analysis of the digested samples by 1.5 to remove this dilution effect. If you then perform the following calculations you can derive the DON, PON, DOP and PP fraction concentrations: TN-TDN = PON; TDN-TON (on an undigested sample) - NH4-N (on an undigested sample) = DON; TP - TDP = PP; TDP - PO4-P = DOP. If you visit my researchgate page you can find multiple papers describing the method and the data we derive from it. Look for Yates et al 2019a, 2019b, 2016 and Lloyd et al 2019 for our latest publications on this technique and the quality assurance procedures we use to check the % recovery of these digested fractions.
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