Key Calculation Steps

The standard defines 50 µg mL⁻¹ DNA per 1.0 absorbance unit at 260 nm, following Beer’s Law where absorbance scales linearly with concentration.

For the diluted aliquot:

Concentration_diluted = (0.550 / 1.0) × 50 µg mL⁻¹ = 27.5 µg mL⁻¹

Dilution factor = V_aliquot/V_final = 20 µL / 1000 µL = 1/50

Thus, original concentration = 27.5 µg mL⁻¹ × 50 = 1375 µg mL⁻¹

Correct Volume Consideration

The aliquot is 20 µL from 50 µL original purified plasmid, diluted to 1000 µL. Since concentration is uniform in the original 50 µL solution:

• Total dilution factor = 1000 µL / 20 µL = 50-fold
• Original concentration = 27.5 µg mL⁻¹ × 50 = 1375 µg mL⁻¹
• Convert: 1375 µg mL⁻¹ = 1.375 µg µL⁻¹ × 50 µL aliquot portion

Detailed Solution

1. Determine diluted concentration: Since 1.0 A₂₆₀ = 50 µg mL⁻¹, 0.550 A₂₆₀ = 27.5 µg mL⁻¹
2. Account for dilution: V_aliquot=20 µL to V_total=1000 µL, fold dilution = 1000/20 = 50
3. Back-calculate original: 27.5 µg mL⁻¹ × 50 = 1375 µg mL⁻¹
4. Convert to required units: 1375 µg mL⁻¹ ÷ 1000 = 1.38 µg µL⁻¹ (rounded to two decimals)

Common Errors Explained

• Ignoring aliquot size: Treating 50 µL as diluted volume gives wrong DF=20, yielding 27.5 × 20 = 550 µg mL⁻¹ = 0.55 µg µL⁻¹
• Unit confusion: Forgetting µg mL⁻¹ to µg µL⁻¹ (/1000), or mL⁻¹ as µL⁻¹ directly
• Missing volume correction: Not accounting that 20 µL aliquot represents 40% of 50 µL total purified plasmid
• Purity oversight: A₂₆₀ assumes pure dsDNA; contaminants affect reading, but problem implies standard calibration

Final Corrected Answer