Judge DNA/RNA purity from A260/A280 (pure DNA ~1.8, pure RNA ~2.0; low = protein/phenol contamination, DNA > 2.0 = RNA contamination) plus the optional A260/A230 secondary check. Derived from Manchester 1995, Sambrook & Russell, Thermo Fisher T042. Educational use only.
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Key facts
Category
Education
Input types
select, number
Output type
json
Sample coverage
4
API ready
Yes
Overview
The OD260/280 Nucleic Acid Purity Ratio Calculator evaluates DNA or RNA purity from A260 and A280 absorbance readings, with an optional A260/A230 secondary check for organic or salt carryover. It compares the calculated ratios with expected ranges for DNA and RNA and is intended for educational use.
When to use
Check whether a DNA preparation is near the expected A260/A280 ratio of 1.8.
Assess whether an RNA preparation is near the expected A260/A280 ratio of 2.0.
Use A260/A230 as a secondary check for guanidine, phenol, organic, or salt carryover.
How it works
1Select DNA or RNA as the nucleic acid type.
2Enter A260 and A280 readings from the same diluted sample.
3The calculator divides A260 by A280 and compares the result with the selected nucleic acid range.
4Optionally enter A230 to calculate and interpret the A260/A230 ratio, then choose the desired decimal places.
Use cases
Screen plasmid DNA preparations before restriction digestion or sequencing.
Review RNA extraction quality before RT-qPCR.
Teach spectrophotometric purity assessment using realistic absorbance readings.
Examples
1. Clean DNA preparation with an A260/A280 ratio of 1.80
Molecular biology student
Background
A plasmid preparation produced A260 = 0.45 and A280 = 0.25 readings.
Problem
The student needs to determine whether the DNA purity is consistent with the expected ratio.
How to use
Select DNA, enter 0.45 for A260 and 0.25 for A280, enter 0.20 for optional A230, and set decimal places to 2.
The calculator returns A260/A280 = 1.80, consistent with clean DNA in the 1.7–2.0 window. A260/A230 = 2.25 does not indicate organic or salt carryover.
2. RNA preparation with low A260/A280 and A260/A230 ratios
RNA extraction researcher
Background
A TRIzol RNA preparation produced A260 = 0.60, A280 = 0.36, and A230 = 0.90.
Problem
The researcher needs to check whether contamination may affect downstream RT-qPCR.
How to use
Select RNA, enter the three absorbance readings, and set decimal places to 4.
FAQ
What does the A260/A280 ratio measure?
It compares absorbance at 260 nm and 280 nm to help assess nucleic acid purity and identify possible protein or phenol contamination.
What is the expected A260/A280 ratio for pure DNA?
Pure DNA is expected to be approximately 1.8, with an acceptable window of about 1.7–2.0.
What is the expected A260/A280 ratio for pure RNA?
Pure RNA is expected to be approximately 2.0, with an acceptable window of about 1.8–2.2.
What does a low A260/A280 ratio indicate?
A low ratio can indicate protein or phenol contamination; interpretation depends on whether the sample is DNA or RNA.
What is the optional A260/A230 check used for?
A low A260/A230 ratio can indicate carryover of guanidine, phenol, organic compounds, or salts. A pure sample is commonly near 2.0–2.2.
The calculator returns A260/A280 = 1.6667, below the 1.8–2.2 RNA window, indicating likely protein or phenol contamination. A260/A230 = 0.6667 also indicates guanidine, phenol, or salt carryover.