Learn the complete Calibration of the UV spectrophotometer including control of absorbance using potassium dichromate solution, resolution power using toluene in hexane, limit of stray light and wavelength accuracy with step-by-step procedures and acceptance criteria.
Accurate analytical results are critical in pharmaceutical quality control, and proper calibration of instruments ensures data integrity. This guide covers the Calibration of the UV spectrophotometer including control of absorbance using potassium dichromate solution, resolution power using toluene in hexane, limit of stray light and wavelength accuracy in a clear, practical manner.
🎯 Objective
To establish a validated procedure for calibration of a UV spectrophotometer covering:
- Control of absorbance
- Resolution power
- Limit of stray light
- Wavelength accuracy and baseline flatness
⚙️ Pre-Calibration Checks
- Ensure all spectrophotometer connections are properly fitted.
- Switch ON the instrument and allow sufficient warm-up time.
🧪 1. Control of Absorbance (Potassium Dichromate Solution)
🔬 Preparation
- Dry potassium dichromate at 130°C to constant weight.
- Accurately weigh 57.0 mg to 63.0 mg.
- Transfer to a 1000 ml volumetric flask.
- Dissolve and dilute with 0.005M H₂SO₄ up to 1000 ml.
📊 Measurement
Measure absorbance at the following wavelengths:
- 235 nm
- 257 nm
- 313 nm
- 350 nm
📈 Calculation
A(1%,1cm)=Weight (mg)Absorbance×10000
✅ Acceptance Criteria
| Wavelength (nm) | A (1% 1cm) | Limits |
|---|---|---|
| 235.0 | 124.5 | 122.9 – 126.2 |
| 257.0 | 144.0 | 142.8 – 145.7 |
| 313.0 | 48.6 | 47.0 – 50.3 |
| 350.0 | 106.6 | 105.6 – 108.2 |
✔ Limits are as per IP & BP (stringent criteria)
🔍 2. Resolution Power (Toluene in Hexane)
🔬 Preparation
- Prepare 0.02% v/v toluene in hexane.
- Check hexane transmittance (≥ 97%) using water as blank.
📊 Measurement
- Scan spectrum from 260 nm to 420 nm.
📈 Acceptance Criteria
Absorbance at 266 nmAbsorbance at 269 nm≥1.5
🌫️ 3. Limit of Stray Light
🔬 Preparation
- Prepare 1.2% w/v potassium chloride solution.
📊 Measurement
Measure absorbance at:
- 198 nm
- 199 nm
- 200 nm
- 201 nm
- 202 nm
✅ Acceptance Criteria
- Absorbance should be greater than 2
📏 4. Wavelength Accuracy, Resolution & Baseline Flatness (Inbuilt Test)
⚙️ Procedure
- Connect printer directly to the instrument.
- Navigate: MODE → F3 (Maintenance) → Press 1
- Press Start/Stop twice (ensure no sample in optical path).
- Print results after test completion.
✅ Acceptance Criteria
Wavelength Accuracy
- 656.1 ± 0.3 nm
- 486.0 ± 0.3 nm
Resolution
- Not more than 1.0 nm
Baseline Flatness
- Within ± 0.002 Absorbance
🔄 Calibration Frequency
- Perform calibration once a month
🔧 Maintenance & Repairs
- If results fall outside limits:
- Label instrument as “OUT OF CALIBRATION”
- Perform servicing/repair
- Recalibrate before use
📄 Calibration Documentation
Maintain calibration records including:
- Absorbance values
- Calculations
- Observations
- Remarks (Satisfactory/Not satisfactory)
- Signatures of analyst and reviewer
📚 Reference
- Indian Pharmacopoeia (IP)
- British Pharmacopoeia (BP)
- UV Spectrophotometer Manual (e.g., UV-1700)
❓ Frequently Asked Questions (FAQs)
1. Why is UV spectrophotometer calibration important?
It ensures accurate absorbance readings and reliable analytical results.
2. Why is potassium dichromate used in calibration?
It provides well-defined absorbance peaks for validating instrument accuracy.
3. What is A (1%, 1cm)?
It is the specific absorbance value used to standardize measurements.
4. What is the purpose of resolution power testing?
It verifies the instrument’s ability to distinguish closely spaced wavelengths.
5. Why is toluene in hexane used for resolution testing?
It provides characteristic peaks required to evaluate resolution performance.
6. What is stray light in a spectrophotometer?
Stray light is unwanted light reaching the detector, affecting accuracy.
7. How is stray light tested?
Using potassium chloride solution with high absorbance at low wavelengths.
8. What is acceptable wavelength accuracy?
± 0.3 nm at specified wavelengths.
9. What should be done if calibration fails?
Label the instrument “OUT OF CALIBRATION,” repair it, and recalibrate.
10. How often should UV spectrophotometer calibration be performed?
Typically once a month or as per SOP requirements.