Hello everybody,
 i was asked to determine high enriched 15N-fertilizer to check if the
content/specification is right. I have got enough matter (some grams) of 99
atom% 15N  fertilizer. I have guessed that I mix the high enriched
fertilizer with a natural fertilizer (it is both urea) to dilute the atom%
content to a low level ( max. 5 atom%) that I could analyse in my mass spec
(ANCA SL/20-20). 
But know I suppose all the errors by diluting by mixing both ferts,
homogenizing and weighing for analysing might be as big as the unknown
deviation from the specified value of 99atom%. I am right? 

Or has got anybody any idea how I could prepare the fertilizer? In this
case I have got another problem: one high enriched fertilizer has got 46%
total N, one high enriched  fertilizer has got 5 % total N; and the
"natural" fertilizer  has got 46% total N. Could anybody help me
calculating how much I have to weigh in to dilute the high enriched
fertilizers to max 5 atom% 15N by max.100 µg total N/sample? First
estimations by myself showed me that I have to weigh in 0,2mg (milligram)
of the 46% total N-fertilizer (mixed with high enriched fert. or not, is
disregarded now) to receive 100µg N; I suppose this is difficult to weigh in.
Would it be better to dilute the mixed fertilizers with water to a
specified concentartion (which?) and take something up (how much) to
receive 100µg total N?  Should the water be dehumidified, or should I use
chromosorb? What about the crystals that are remaining sometimes at the top
of the tin capsule...? These are the items, why I do not prefer liquid
analysis.

Hope for some helpful hints and many thanks in advance,
Deborah

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Rhein. Friedrich-Wilhelms-Universität Bonn
Institut für Nutzpflanzenwissenschaften und Ressourcenschutz (INRES)
Lehr- und Forschungsbereich Pflanzenernährung
Karlrobert-Kreiten-Str. 13
53115 Bonn, Germany
Phone (+049) 0228-73-1631, -3640, -1632
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