Well, one possibility would be ultrasonic bath, which would disperse your sample in water much better I presume, and also raising the water temperature should help, as the solubility increases with temperature. As it is powder sample, the particles might be to big to remain suspended in water and so they sink down, maybe ballmilling or some other mechanic method would help you out reducing the size of the particles, before dissolving them in water.

One thing to think of is that creating a homogenous aqueous solution might not be so simple considering that your sample might be of heterogeneous nature, as some components, might be less or more soluble in water, therefore solution might not be a good representation of your sample.

One more thing that you might have to consider is that your sample might not be soluble at all in water, or just partially. And also you may have reached the saturation point already, but can’t see it with naked eye and the rest of the sample just sinks to the bottom, but the sample should be detectable by spectroscopic method. If your sample has low degree of solubility in water, adjusting pH should help out, above or beyond pH 7.0, by adding some acid or base, depending on your sample. But then this wouldn’t be aqueous solution any more.

These are some thoughts. I hope it helps out…

Fantastic, thanks Marijan! I had thought of the sonic bath and might give it a go if my current plan doesn't work out. I'm going to attempt to increase the viscosity of the water using fumed silica to see if that helps the dispersion. I'm only looking to disperse the sample, not dissolve any of it so if the media was a little thicker, perhaps the powder (more like granules) will be easier to disperse.

Thanks for the suggestions!!!