Thanks for all this excellent info. Whe do you think your papers with the tweaked protocol will come out? I would love to include this in the 2nd edition of my SIF book I am writing at present.
BTW, I have done the silver phosphate prep from bone many a time and every time I wondered there must be an easier way of doing this. Our H&S officer had an apoplectic fit when she found we working with HF. ;-)
We used to refurbish our reactors after every autosampler run since we had a nice stash of GC tubes and chips (sieved to "uniform" chip size). So, dealing with the barite problem you mention would not have been an issue.
I also noticed black spots of presumably reduced AG appearing in my nice yellowish-green crystals. I wrapped a desiccator in tin foil so I could dry the precipitate in the dark just in case the stuff was as photosensitive as silver halides. ;-)
Let me know about your paper.
Sent from my BlackBerry 10 smartphone.
From: gerard olack
Sent: Thursday, 12 May 2016 19:39
To: [log in to unmask]
Reply To: Stable Isotope Geochemistry
Subject: Re: [ISOGEOCHEM] BaSO4 analysis interferences in water samples?
Just to comment on the Ba3(PO4)2. If you try running Ag3PO4 in a TC/EA type reactor after a full barite run--you'll barely see any signal. If you clean and repack the reactor, you're fine. And barites after phosphates are not a problem--and benzoic acids always run fine. I haven't spent time trying to tease out all the issues--we just run them separately. If someone wants to try running barium phosphate--keep an eye on yields. Maybe use additives? (microfluorination???--Menicucci, Matthews, Spero, 2013)
Historically the phosphate would be precipitated out as a molybdenum complex, then re-ppt and re-ppt again as the bismuth salt for analysis. Variations still done today to clean up some samples. Then the isotopers came across an old-fashion gravimetric phosphate analysis that ppt out phosphate as silver phosphate (to measure phosphate concentrations in solution), and adapted that for 18O analysis. Other metals have been looked at too--sometimes in the re-ppt steps.
In the process, the analysis went from bomb fluorinations to sealed tube (with graphite) to TC/EA. The bismuth phosphate apparently picks up more water than silver phosphate, so less amenable to TC/EA runs.
Working with silver salts does have it's challenges. Organic matter may reduce the silver--so it could plate out. Typically not a problem with most samples. Silver oxide can also crash out--which is always a potential problem--as is hitting the right pH range. Oh, and silver nitrate plus ammonium cation--can lead to silver nitride formation. Contact explosives can always be fun. (ok, watch concentrations, don't store silver amine complex for long periods, and have NaCl in your waste collection jar to crash out the silver).
We should have a paper out soon--tweaking some of the current protocols so they'll be more robust when working on small samples.
p.s. yes, lots of papers--but you can start with (and work forwards and backwards, e.g. Longinelli):
Chemical Geology, 2002, 185, 321-326
Vennemann, Fricke, Blake, O'Neil, Colman
Firsching, F. Henry. 1961. “Precipitation of Silver Phosphate from Homogenous Solution.” Analytical Chemistry 33 (7): 873–74. http://pubs.acs.org/doi/abs/10.1021/ac60175a018.
Baxter, Gregory Paul, and Grinnell Jones. 1910. “A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. FIRST PAPER.—THE ANALYSIS OF SILVER PHOSPHATE.” Journal of the American Chemical Society 32 (3): 298–318. doi:10.1021/ja01921a003.
p.p.s. Ca++ typically removed by ppt with F- (fluorite).
On 5/12/2016 6:55 AM, Wolfram Meier-Augenstein (aps) wrote:
Precipitating sulphate as BaSO4 in the presence of phosphate and nitrate will most definitely cause Ba3(PO4)2 to precipitate too. Barium phosphate is even less soluble in water than barium sulphate. Depending on circumstances, barium nitrate may co-precipitate even though under “normal” conditions it is quite soluble (0.33 mol/L).
So, the short answer is yes, analysing your precipitate for 18O by HTC/EA-IRMS will yield artefact d18O values, that is measured d18O values will be a composite of sulphate and phosphate bound O. The extent of the “problem” will depend on how much or little phosphate is present in your water samples in relation to sulphate.
Just as an aside, barium phosphate (solubility: 9x10exp-9 mol/L) is considered insoluble in water which always made me wonder why we continue to bother precipitating and analysing phosphate 18O as silver phosphate (solubility: 1.6x10exp-5 mol/L)? Perhaps something for the younger generation to have a look at?
P.S.: The solubility products of Ba3(PO4)2 and BaSO4 are 6x10exp-38 molexp5/Lexp5 and 1.5x10exp-10 molexp2/Lexp2 respectively.
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Daniel Carrizo
Sent: 12 May 2016 11:53
To: [log in to unmask]<mailto:[log in to unmask]>
Subject: [ISOGEOCHEM] BaSO4 analysis interferences in water samples?
Dear ISOGEOCHEM colleagues,
I precipitate sulphate with BaCl2, for d34S measurments, I know that I have some other ionic species in the water sample, like phosphate, or nitrate, thus I have to expect interferences/problems in the BaSO4 IRMS analysis??
Many thanks for your help and thoughts!
Dr. Daniel Carrizo
Investigador Ramón y Cajal (RYC)
Centro de Astrobiología (INTA-CSIC)
Instituto Nacional de Técnica Aeroespacial
Ctra de Torrejón a Ajalvir, km 4
28850 Torrejón de Ardoz, Madrid, Spain.
Tel: (+34) 692521256
Email: [log in to unmask]<mailto:[log in to unmask]>
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