Methods to be Reviewed by OMB in 2016

time. Instrument conditions used for method valid

ation of acid-soluble/total P and K are listed in

Table

2015.18G

. ICP-OES differ in their design and options, so minor adjustment to the

conditions listed in Table

2015.18G

may be necessary; however, any adjustments to these

conditions should be performance based and validated. Special attention should be paid to the

recovery of phosphorus in fertilizer concentrates such as MESZ

(40% P

), DAP (46% P

)

and MAP (50% or 52% P

) since these materials pose the greatest need for optimal instrument

performance.

Calculations Alternative B

See Calculations for Alternative A.

H. Comments (Alternative B)

The

0.16 M HCl matrix used in Alternative B poses fewer analytical challenges for the ICP-OES

than does the citrate-EDTA solvent used in Alternative A. If minor method modifications are

necessary to accommodate different ICP-OES types or designs and/or to correct for variable or

low phosphorus recoveries, the following are likely watch areas. Increasing the plasma power

often benefits phosphorus. Decreasing the volume of the aliquot injected into the plasma can

also help improve recoveries of materials containing high concentrations of phosphorus. This

can be accomplished by using a smaller sample pump tube and/or larger internal

standard/ionization buffer pump tube, and/or by slightly decreasing the pump speed and/or

nebulizer pressure. The final matrix of the test solutions and standards should closely match.

Standards prepared from salts as provided in Table

2015.18E

provide the greatest match

andoffer the best Phosphorus recoveries. Stock standards preserved in acid solution are not

recommended. The comments provided for potassium in Alternative A section H also apply to

potassium in Alternative B.

Deviation from this method is not recommended, but if small revisions are

necessary to accommodate differences in ICP-OES types and design, then these

revisions should be validated.

References

J. AOAC Int

, 687(2014)

Candidates for 2016 Method of the Year

255