Do you mean; " A blank is a substance that is made by combining two elements?" (I'm not sure if substances is the right term)
If so then the blank is a compound.
Answer
Manual samplers are prone to effects of temperature, speed of wind and air concentrations.
Explanation
Manual samplers face several challenges that can act as drawbacks to obtaining accurate results. They are subjected to effects of sampling duration where long sampling times are needed to obtain adequate mass for detection. Manual samplers face challenges when measuring non-volatile species because particles are observed into the adsorption medium at a slower rate of diffusion.
Answer: Water because if we say aqueous it means it resembles water or related to water.
