核酸は260 nmの深紫外域で定量します。使い捨てプラスチックも光学ガラスもこの波長域では不透明です。吸光度でDNAやRNAを測定する場合、石英キュベットは選択肢ではなく必須です。本ガイドでは測定法、2つの純度比の解釈、光路長と容量の選び方を解説します。

Nucleic acids are quantified at 260 nm — deep UV, where disposable plastic and optical glass are both opaque. If you are measuring DNA or RNA by absorbance, a quartz cuvette is not optional. This guide covers the measurement, how to read the two purity ratios that tell you whether your sample is actually clean, and how to pick path length and volume.

1. 260 nmに石英が必要な理由

The absorbance maximum of the nucleotide bases sits at 260 nm. Measuring there gives concentration directly from Beer-Lambert, with no reagents and no consumption of the sample beyond the measurement itself.

Material cut-offs make the choice for you: polystyrene and optical glass both stop transmitting around 340 nm. At 260 nm they absorb essentially everything, so a plastic cuvette returns noise, not data. JGS1 fused silica transmits from 200 nm, placing 260 nm well inside its usable window.

2. A260の読み方：換算係数

At a 10 mm path length, an absorbance of 1.0 corresponds to:

Nucleic acid	Concentration at A260 = 1.0 (10 mm)
dsDNA	50 µg/mL
ssDNA	33 µg/mL
RNA	40 µg/mL
Oligonucleotides	~33 µg/mL (varies with base composition)

Values assume a 10 mm path. For other path lengths, divide by the path in cm. The path length calculator handles the arithmetic.

3. 純度比が実際に示すこと

Ratio	Clean value	Low value indicates
A260 / A280	≈1.8 (DNA), ≈2.0 (RNA)	Protein or phenol contamination
A260 / A230	2.0 – 2.2	Salts, carbohydrates, phenol, guanidine, TRIzol carry-over

Both ratios only mean something if the absorbance values sit in the linear range (roughly 0.1–1.0 AU). A ratio calculated from a 0.02 AU reading is noise dressed up as data — dilute less or use a longer path.

4. 光路長と容量

Extraction yields are small and getting smaller. The standard 10 mm macro cell wants ~3.5 mL, which no modern extraction protocol produces.

• Standard extracts — micro or semi-micro quartz cells, 50–500 µL, keeping the 10 mm path so the conversion factors above still apply.
• Very concentrated samples — short path cells avoid dilution error.
• Very dilute samples — long path cells increase sensitivity without concentrating.

5. キュベットか NanoDrop か

Pedestal instruments measure 1–2 µL and are convenient, but they cap the measurable concentration at the top end and are sensitive to how the droplet is formed and whether the pedestal is perfectly clean. A quartz cuvette is more reproducible, is easy to validate, and costs far less per sample over its life. We compared both in depth: Cuvette vs NanoDrop.

6. 推奨キュベット

• Everyday DNA/RNA quantification — JGS1 quartz micro cell, 10 mm, 100–500 µL.
• Very small yields — sub-micro cells down to 50 µL.
• Teaching labs — standard 10 mm macro quartz cells, robust and reusable across many cohorts.

Related: Cleaning Protocol · UV-Vis Troubleshooting · Solvent Compatibility · Path Length Guide · Path Length Calculator · Cuvette Care Guide