AOAC RI ERP EBOOK FOR FERTILIZERS

AOAC RESEARCH INSTITUTE AOAC OFFICIAL METHODS OF ANALYSIS (OMA)

OMAMAN-28/AOAC 2006.03 Study Director: Sharon Webb, University of Kentucky, Division of Regulatory Services, 103 Regulatory Services Bldg, Lexington , Kentucky 40546-0275 in radial ICP systems. These need to be addressed because they can have an adverse affect based on system hardware used. Perhaps more emphasis could be placed on Section F (e) regarding interelement interference in instruments that don't have software to correct for this issue.

ER 2

ER 3

Favors metals that are extractable in acid rather than alkali

Does not provide extractant pH levels

Reports total metals which is not indicative of plant availability

Could tend to mislead consumers if construed as a micronutrient label guarantee

Not an appropriate method for fertilizer materials in determining plant availability

ER 4 ER 5 ER 6 ER 7 ER 8

N/A

"cut" (page 112) could be replaced.

may not be applicable foe all fertilizer products

none

The following weaknesses of the proposed method may be mentioned:

Calibration and linearity Some deconvolution effects have to be considered (Fe, Co) when speaking of certain wavelengths for arsenic, cadmium, lead and selenium (calibration range: 0-10 μg/mL). Precision Many results show many HorRat values below 0.3 for a single validation study. However, RSD values for the proposed method are extremely reduced if compared with collaborative study 2006.03 results. In addition, RSD for Group B metals remain low enough, in general. LOD and LOQ It should be noted that lower limits for Group B elements can be achieved by altering the test portion size or choosing more sensitive wavelengths. Ruggedness trial It has to be noted that: 1) Se values in the sample are at or below the method LOQ. For this reason, the proposed method is not recommended for Se 2) Certain ‘Relative percent difference’ values show that the determination of iron has to be carried out carefully. This element is reported to be the most variable element and ruggedness test results seem to confirm the affirmation. All of the ruggedness work was done on the digestions portion of the study, but none was done on ICP variables. How software varies from instrument to instrument may yield different lines Page 8 of 14

ER 9