光路 length is the most important specification on a キュベット after the 材質 itself. This guide explains what it is, how it changes your readings, and how to pick 0.1 mm to 100 mm セルs for your samples.
光路 length is the distance the light beam travels through the sample inside the キュベット — the internal distance between the two optical 窓s. For a 標準 10 mm セル this is exactly 10 mm. It is the dimension that controls signal strength: absorbance scales linearly with 光路長, so the same solution produces a 10× stronger signal in a 10 mm セル than in a 1 mm セル.
Do not confuse 光路長 with 容積. A 微量キュベット can have a 10 mm 光路 but hold only 50 µL — the internal width is narrowed so the beam passes through less liquid over the same 光路. Both numbers matter; see section 7.
A = ε · c · l
Where A is absorbance, ε the molar absorptivity, c the concentration, and l the 光路長. Because ε is fixed for a given compound and 波長, you control the readout by choosing c (dilution) or l (光路長).
The practical target: keep absorbance between ~0.1 and ~1.5 AU. Below 0.1 AU, noise dominates; above ~1.5–2 AU, the detector becomes non-linear and readings flatten.
| Reading | What to do |
|---|---|
| A < 0.1 (too weak) | Increase 光路長 (e.g. 10 → 50 mm) or concentrate the sample |
| A = 0.1 – 1.5 | Perfect range — keep your current セル |
| A > 1.5 (too strong) | Shorten 光路長 (e.g. 10 → 1 mm) instead of diluting |
The 10 mm 光路 セル is what most UV-Vis spectrophotometers are 設計ed around. It fits every 標準 holder (12.5 × 12.5 mm external), works for typical dilute samples, and every instrument's factory validation is usually performed with it.
Start here unless you have a specific reason not to. Browse 10 mm 石英キュベット →
When your sample is concentrated, a 10 mm セル pushes the absorbance off-scale. Rather than diluting (which adds error, consumes time and changes the matrix), switch to a short-光路 セル — the same solution reads on-scale directly.
Typical users: protein concentration checks at 280 nm, concentrated API solutions, HPLC peak confirmation. Short 光路-length セルs →
For undiluted liquids with very high absorbance — neat organics, concentrated polymers, most IR transmission work — even a 1 mm セル reads off-scale. Ultra-short セルs of 0.1–0.5 mm bring the signal back on scale without any dilution.
Fill 容積 is tiny (a few microlitres to tens of µL), which is another advantage when sample is precious.
When concentrations are too low to read reliably at 10 mm, a longer 光路 multiplies the signal:
Also the right choice for turbid samples — longer 光路s average out particulate effects — and for color-grading (Lovibond/Hazen) where the measurement scale itself assumes a long セル. Long 光路-length キュベット →
A frequent confusion: "I need a 微量キュベット, so it must be short-光路." Not true. Micro and sub-micro セルs reduce 容積 by narrowing the internal width while keeping a 標準 10 mm 光路. A 50 µL sub-micro セル still gives you a full 10 mm of signal 光路.
Choose the 光路長 for concentration/sensitivity; choose the 容積 format for how much sample you have. 石英 微量キュベットs → / Sub low-容積 セルs →
| Application | Recommended 光路 | Why |
|---|---|---|
| Routine UV-Vis, dilute samples | 10 mm | Universal 標準, fits all holders |
| Concentrated protein / API | 1 – 2 mm | Keeps A on-scale without dilution |
| Neat liquids, IR, polymers | 0.1 – 0.5 mm | Very high absorbance by nature |
| Trace metals, environmental | 20 – 50 mm | Sensitivity boost for low concentrations |
| Lowest detection limits | 100 mm | 10× signal vs. 10 mm |
| Color grading (Lovibond/Hazen) | 10 – 50 mm | Scale defined on specific 光路長 |