Imagine that your initial absorbance reading for "low salt" fraction was too high, so you diluted your sample 4x. When you measure the absorbance using 100 ul of the diluted sample, the absorbance is 0.6. What is the concentration of the original (undiluted) sample? (mg/ml)

Answer 1

To find the concentration of the original (undiluted) sample, you need to account for the dilution factor and the absorbance measurement.

1. Understand the dilution: When you dilute a sample by a factor of 4 (4x), it means that the original sample has been diluted to one-quarter of its original concentration. Therefore, if you measure the absorbance of the diluted sample, the concentration of the original sample will be four times the concentration of the diluted sample.

2. Using the absorbance: The absorbance measured for the diluted sample is 0.6. Assuming that Beer-Lambert law holds (A ∝ C), the absorbance value from the diluted sample can be directly extrapolated to find the concentration.

3. Calculate the concentration of the diluted sample: Let’s denote the concentration of the diluted sample as \( C_d \). The original concentration \( C \) can be calculated with the following relationship:

   \[ C = 4 \times C_d \]

4. Calculate original concentration:

   • If we consider the absorbance directly correlates to concentration, we can say for the diluted sample that:

   \( C_d = A_d \)

   Where \( A_d \) is the absorbance of the diluted sample.

   Then,

   \[ C_d = \frac{0.6}{4} \quad (\text{because this is for the dilution factor}) \]

   But since we are asked for the undiluted concentration:

   \[ C = 4 \times 0.6 \text{ } (\text{using the factor directly}) \]

5. Final calculation: \[ C = 4 \times 0.6 = 2.4 \text{ mg/ml} \]

Thus, the concentration of the original (undiluted) sample is 2.4 mg/ml.