Amphetamines in urine can be confirmed by mass spectrometry at a concentration of 500 ng/mL. Assuming a urine density of 1.025 g/mL, what is this concentration in parts per million (ppm)?

• A. 0.500 ppm
• B. 0.525 ppm
• C. 0.488 ppm
• D. 0.512 ppm

Answer: (C)

Verified step by step guidance

1. Understand that parts per million (ppm) is a way of expressing very dilute concentrations of substances. It is equivalent to milligrams of solute per liter of solution (mg/L) when dealing with aqueous solutions.
2. Start by converting the given concentration from nanograms per milliliter (ng/mL) to milligrams per liter (mg/L). Since 1 ng = 10^-6 mg, multiply the concentration in ng/mL by 10^-6 to convert to mg/mL, and then multiply by 1000 to convert to mg/L.
3. Recognize that the density of the urine is given as 1.025 g/mL. This means that 1 mL of urine has a mass of 1.025 grams. Since ppm is often used as mg of solute per kg of solution, you can use the density to convert the volume to mass.
4. Calculate the mass of 1 liter of urine using the density: 1 L = 1000 mL, so the mass is 1000 mL * 1.025 g/mL = 1025 g, which is equivalent to 1.025 kg.
5. Finally, use the formula for ppm: ppm = (mg of solute / kg of solution). Substitute the values you have calculated to find the concentration in ppm.