Measuring spoons and syringes measure volume.
Scales measure weight.
Two different things.
Just as a teaspoon of feathers would have a different weight than a teaspoon of lead, a teaspoon of trace element powder would have a different weight than a teaspoon of liquid trace element solution.
So there isn't a "1 teaspoon always weighs xx grams" conversion chart out there.
Nor have I seen a chart that lists volume/weight conversions for all the various aquarium ferts on the market, as Tom and Jason said, no one needs that level of accuracy.
Although, you could easily develop your own chart for the ferts you use in your aquarium, if you wished, simply by measuring out your ferts using your measuring spoons and weighing them on your scale, one by one.