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3 comments
Participant

Join Date: May 2008
Location: U.S.\ Springfield, Illinois
Posts: 2

Elimination of the Phosphate effect in Calcium Analysis

05/06/2008 3:36 PM

I am utilizing a flame photometer to analyze agricultural samples for calcium content. These samples also contain phosphorous. Addition of Lanthanum oxide is necessary for eliminating the phosphate effect in atomic absorption spectrometry. Is this necessary for flame photometry too? What percentage should I add to be in solution if this is true? I suppose I could experiment to find out the answer, but it would be less time consuming if someone could supply me the information.

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Power-User

Join Date: Dec 2006
Posts: 443
Good Answers: 31
#1

Re: Elimination of the Phosphate effect in Calcium Analysis

05/07/2008 11:23 AM

There only a few chemists here that post, so I thought I would give it a shot.

Of course, the most accurate (and maybe even the fastest) way would be to prepare spiked samples, and see if there is interference.

This paper seems to indicated that the interference levels off at 7 ppm P. This would indicate that spiking the samples and standards with P to be above this level solves the problem.

http://jds.fass.org/cgi/reprint/33/6/413.pdf

You may be able to Google around and find more.

AA utilizes absorption of a specific wavelength by ground state atoms, while FP utilizes emission as excited atoms as they fall to the ground state. Since they are different processes, it would seem that an ionization buffer such as lanthanum would have opposite effects in each technique. However, L is also called a "releasing agent" for refractory elements such as Ca. It allows the Ca compounds to decompose at lower flame temperatures, so it may increase sensitivity in FP as well.

Sorry if this does not help, but doing a few tests to determine interferences is surely the best way to go.

Tad

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Participant

Join Date: May 2008
Location: U.S.\ Springfield, Illinois
Posts: 2
#2

Re: Elimination of the Phosphate effect in Calcium Analysis

05/07/2008 1:57 PM

Thanks Tad. I was planning on doing this if noone had a concrete answer. I was just trying to reduce my workload.

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Power-User

Join Date: Jun 2006
Location: Birmingham, Alabama, USA
Posts: 316
Good Answers: 7
#3

Re: Elimination of the Phosphate effect in Calcium Analysis

05/08/2008 11:31 AM

Aside from using lanthanum oxide. another method for eliminating phosphorus interference in flame photometry is to increase the temperature of the flame. This is generally accomplished by using nitrous oxide (consult your photometer manufacturer). Inductively Coupled Plasma Spectrometry produces a flame of 10,000 degrees K, effectively eliminating nearly all interferences. An ICP, however, is too expensive for all but the largest of laboratories (about $250K for a good one).

An alternate method of eliminating the error produced by P interference is the use of calibration standards with the same matrix as your unknowns. If your sample matrix is consistent you can cancel out the error effects by calibrating using known samples with the same matrix. You can send material to a certified lab or even to NIST to certify the concentrations of the components.

Bill Morrow

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