Dosing leaves with Zinc Nitrate for pollution experiment - Need specific concentration

Question

I am trying to assess the pollution tolerance of an insect using Zn as a proxy. I have a supply of Zinc Nitrate Hexahydrate that I want to use to contaminate dried leaves (the insect's food). The LD50 for my insect is around 2000 micrograms g^-1.

I was thinking I could achieve this by using 0.1 g squares of dried leaf that I would contaminate by topically applying 1 ml of Zn solution. To get my desired concentration, I am thinking that I would dissolve 200 micrograms of Zn into 1 ml of distilled water, so that when I apply 1 ml of this solution to a 0.1 g square of leaf I would have a final concentration of 2000 micrograms/gram of food. But if I wanted to apply only 0.5 ml of Zn solution to 0.1 g of leaf but still have the same final concentration of 2000 microg/g food, I would create a solution of 400 micrograms/ml. And if I wanted to use 1.0 gram of leaf and have a concentration of 2000 micrograms/g, I would make a solution of 2000 micrograms/ml. Is all of this correct?

I am also open to suggestions of how to better contaminate the food in a more consistent way. I thought about soaking leaves in a defined concentration, but then I am worried that some leaves will absorb more or less liquid than others, making the final Zn/g food concentrations unequal across different leaves.

Thank you for any solutions (pun intended) anyone might provide.

Answer 1

It seems to me that it will be difficult to get a uniform concentration of Zn in a non-uniform material like dried leaves, unless you measure only in bulk (this pile of dried leaves weighs X grams and contains 2000X micrograms of Zn). Things get more confusing if the insects don't eat all the dried-leaf structure (leaving stems, for example).

I'm also not sure how you'll get all the zinc from 1 ml of liquid onto a 0.1g leaf sample. (And, of course, the leaf won't be dry for a while after that!) I think a more concentrated solution would make your task easier. Your math for adjusting the concentration to match the amount of leaf matter appears correct.

As an aside, is your question confusing LD50 (micrograms per gram of insect) with leaf concentration (micrograms per gram of leaf)? I'm not sure why the concentration in the leaves would need to correspond to the LD50, unless there's something significant about the insects eating their own body weight in dried leaves...?