tessenderlo kerley, inc.technical information background and history there are two existing old aoac...

Name Tessenderlo Kerley, Inc.

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Michael Hojjatie Ph.D. Tessenderlo Kerley, Inc.

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Technical Information

Background and History

There are two existing old AOAC Official Method, 959.03 Urease Method, 1959-1960), and AOAC Official Method 988.01 (HPLC, 1988). These two methods have been introduced for the determination of urea in fertilizers on those years. At the time of their introductions, the methods were valid and worked for rather simple urea containing fertilizers (such as urea solutions). These methods are not working for more complex fertilizers that have been introduced since then, such as ureaforms and Triazone containing fertilizers. A method (2003.14) was made official, First action 2003, second action 2008. AOAC 2003.14 works for all class of fertilizers containing urea. Comparative evaluations showed that the older AOAC methods (959.03, and 988.01) do not work for the complex urea containing fertilizers.

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Aqueous Urea Containing Fertilizer Products Methods of

Analyses for Free Urea contents

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Existing Method of Analyses

• AOAC Methods 959.03 (Urease, 1960)1

• AOAC Method 983.01 (Liquid Chromatography, 1984)1

• AOAC Method 2003-14 (Liquid Chromatography 2008)2

1 Official Methods of Analysis (2000) 17th Ed., AOAC INTER NATIONAL, Gaithersburg, MD,

Methods 959.03 and 983.01

2 Official Methods of Analysis (2016) 20th Ed., AOAC INTER NATIONAL, Gaithersburg, MD, Method

2003.14

SRN Determination

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• The composition of fertilizers are complex with often several

components.

• SRN is coming from urea reaction compound (MMU, DMU, etc.).

• Free Urea does not provide SRN and it is readily available N.

• To accurately report the SRN, one has to determine the Free

Urea accurately.

• SRN is determined by subtracting the total Nitrogen present from

the amount of free Urea

Free Urea Determination

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HPLC (AOAC 2003-14) Analyses of Twelve Fertilizer Samples for Free Urea

Total -N Fertilizer

Sample Lab ID

% Urea-N Mean Std. dev.

Lab 1 Lab 2

29.89 1 26-068-03 12.04 12.45 12.25 0.21

28.54 2 26-051-01 8.63 8.51 8.57 0.06

28.44 3 26-068-02 8.68 8.57 8.63 0.05

28.33 4 26-076-07 8.45 8.45 8.45 0.00

28.47 5 26-076-06 8.58 8.60 8.59 0.01

24.95 6 26-076-08 16.08 15.70 15.89 0.19

30.78 7 26-079-01 12.55 12.70 12.63 0.07

30.06 8 26-086-04 12.56 12.07 12.32 0.25

28.01 9 26-103-02 12.09 11.36 11.73 0.37

28.13 10 26-103-03 6.85 6.83 6.84 0.01

26.36 11 26-100-03 15.55 15.28 15.42 0.14

29.87 12 26-010-02 9.37 9.51 9.44 0.07

Comparison of the Results by HPLC (2003-14) and by Urease

Method (Powdered Sample)

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SRN Determination

Product %Total N

%SRN Claimed Method

HPLC (2003-14) Urease

28-0-0 72 72 88

30-0-0 60 60 77

30-0-0 50 50 68

28-0-0 70 50 70

30-0-0 85 46 85

Urease Method Results

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• Often not reproducible

• Type of Urease is important (powder, Tablet,

Liquid form)

• Urease activities varies with different sources

• Interference from ureaform components

AOAC HPLC Method 983.01

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Method 983.01 is not reliable for determination of urea in the

cited class of fertilizers because in most, if not all, cases, there

is co-elution of a non-urea peak with the urea peak.

Mobile Phase: Water

Column: ODS

Detector: Refractive Index

Ring tests analyses of 4 different samples for biuret contents using

the two HPLC Methods showed the 983.01 official Method is not

suitable for the Triazone products.

Comparisons of the two HPLC Methods

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Sample ID

Samples

Analyzed by

TKI

Grade

%

Biuret

Test 1

%

Biuret

Test 2

%

Biuret

Test 3

%

Biuret

Test 4

Average

% Biuret STDEV

By 2003.14 Method

25-012-05 Sulfur coated urea 39-0-0 0.911 0.943 0.979 0.933 0.942 0.025

25-068-06 Triazone Fertilizer 26-0-0 0.118 0.124 0.131 0.139 0.128 0.008

25-070-01 Fertilizer “A” 15-15-15 0.668 0.689 0.762 0.708 0.707 0.035

25-070-02 Fertilizer “B” 25-11-10 0.296 0.356 0.440 0.416 0.377 0.056

By 983.01 Method



25-070-01 Fertilizer “A” 15-15-15 0.560 0.560 0.557 0.555 0.558 0.002

25-070-02 Fertilizer “B” 25-11-10 0.299 0.302 0.303 0.300 0.301 0.002

By SRIC Method (same

as 2003.14 Method)



25-070-01 Fertilizer “A” 15-15-15 0.663 0.711 0.734 0.727 0.709 0.028

25-070-02 Fertilizer “B” 25-11-10 0.286 0.367 0.388 0.382 0.356 0.041

Separation of Biuret and Urea by AOAC 983.01 and 2003.14

Conclusions

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AOAC official Method 959.03 (Urease Method) is not suitable

for the determination of FREE Urea in the Water soluble slow

release Triazone products

Results are inconsistence (high or low biased)

AOAC Official Method 983.01(HPLC method) also not suitable

for these determinations due to interference of some Non-Urea

peaks

AOAC Official Method 2003-14 accurately determines the FREE

Urea contents in these Fertilizers

Supporting Documents

Publications

Peer review

AOAC Amendment

IFA Amendment

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IFA Decision

IFA Method Harmonization Committee has accepted the petition to revise the 2014 Monogram

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