A colorimeter is an instrument that quantitatively analyzes the concentration of a substance by measuring the absorption of specific wavelengths of light by a solution. It is widely used in fields such as chemistry, biology, and environmental monitoring. In practical applications, various factors may affect the accuracy and repeatability of measurements. The following systematically analyzes the influencing factors from the aspects of light source, wavelength selection, colorimetric dish, sample properties, environmental conditions, instrument calibration, operating specifications, and data processing.
1、 Light source characteristics
1. Light source stability
Colorimeters commonly use tungsten filament lamps (visible light region) or deuterium lamps (ultraviolet region) as light sources. The intensity of the light source is significantly affected by voltage fluctuations, bulb aging, or heat dissipation conditions. For example, unstable voltage can cause fluctuations in light intensity, leading to drift in absorbance readings; After long-term use, the emission spectrum of tungsten lamps may undergo a red shift, resulting in attenuation of specific wavelength light intensity and affecting the detection sensitivity of low concentration samples.
2. Spectral purity
The light source needs to provide narrow band monochromatic light. If the resolution of a monochromator (such as a prism or grating) is insufficient, stray light will increase background noise and lower the measurement limit. For example, when detecting nucleic acids in the ultraviolet region, if the light source contains visible light components, it may interfere with the linear range of high concentration samples.
3. Thermal effect
Continuous operation can cause the light source to heat up, which may result in wavelength shift or intensity attenuation. For example, after prolonged operation, the near-infrared intensity of tungsten halogen lamps may significantly decrease due to heat generation, affecting the measurement of near-infrared spectra.
2、 Wavelength selection and monochromaticity
1. Maximum absorption wavelength matching
The color reaction of the test substance needs to be measured at the characteristic absorption wavelength. If the wavelength deviates from the maximum absorption peak, the molar absorptivity decreases and the sensitivity decreases. For example, phenolic substances have strong absorption at 270 nm, and if the 400 nm wavelength is misused for detection, the absorbance may be lower than the detection limit.
2. Filter or monochromator error
Aging of filter or deformation of grating lines can lead to wavelength deviation. For example, when using a 500 nm filter to detect copper sulfate solution, if the actual wavelength shifts to 510 nm, errors may be introduced due to the deviation.
3. Bandwidth impact
The narrower the bandwidth of the emitted light from the monochromator, the higher the measurement specificity. Broadband light may contain interference signals from other absorbing substances. For example, in multi-component mixed systems, wideband absorption peaks of unrelated components may overlap, leading to quantitative distortion.
3、 Quality and use of colorimetric dishes
1. Material and Transparency
Quartz colorimetric dishes are suitable for the ultraviolet region, while ordinary glass colorimetric dishes are only suitable for the visible light region. If the material selection is incorrect (such as using a glass dish to measure UV spectra), the light will be absorbed by the material itself, resulting in a false high absorbance.
2. Optical uniformity
The thickness difference of the colorimetric dish should be controlled within ± 0.02 mm. If the two transparent surfaces are not parallel or have uneven thickness, it will violate the assumption of "uniform liquid layer thickness" in Lambert's law, resulting in a non-linear standard curve.
3. Pollution and scratches
Fingerprints, oil stains, or wear can scatter light and increase the absorbance baseline. For example, when measuring low concentrations of heavy metal ions, trace fingerprints on the colorimetric dish may cause an increase in absorbance of 0.01, which is equivalent to introducing a relative error of 1%.
4、 Sample properties and pretreatment
1. Color reaction conditions
The dosage of color developer, reaction time, pH value, and temperature need to be strictly controlled. For example, when using the ammonium molybdate method to measure phosphate, the pH needs to be controlled between 6.5-7.5, otherwise the stability of the generated phosphomolybdate blue complex is poor and the absorbance changes significantly over time.
2. Suspended particles and emulsions
Colloids or precipitates in a solution can scatter light (Tyndall effect), leading to an increase in absorbance. For example, unfiltered soil leachate may have high absorbance due to sediment suspension and require centrifugation or filtration pretreatment.
3. Color concentration range
The optimal range of absorbance is 0.2-0.8 (corresponding to T=80% -15%). When the concentration is too high, it needs to be diluted, otherwise it deviates from the linear range (such as solute aggregation at high concentrations causing a decrease in varepsilon). For example, when using the Coomassie Brilliant Blue method to measure proteins, if the absorbance exceeds 1.5, the sample needs to be diluted.
5、 Environmental interference
1. Temperature fluctuations
Color reactions are often exothermic or endothermic processes. For example, in the determination of COD by potassium dichromate oxidation, the reaction rate may accelerate by 10% for every 1 ℃ increase in temperature, resulting in incomplete color development or increased side reactions.
2. Humidity and dust
High humidity environments can easily cause condensation or dust adsorption on the surface of optical components (such as lenses and mirrors), reducing light transmittance. For example, a colorimeter that is not moisture-proof during the rainy season may experience baseline drift.
3. Electromagnetic interference
Detectors are mostly photodiodes or photomultiplier tubes, which are sensitive to electromagnetic fields. High frequency equipment near the laboratory, such as mobile phone base stations, may introduce noise and affect weak signal detection.
6、 Instrument calibration and maintenance
1. Blank comparison setting
The blank solution should be consistent with the sample matrix (such as solvent, pH regulator). If the blank solution is selected improperly (such as using pure water instead of a reference solution containing a color developer), it may introduce background absorption interference. For example, when using spectrophotometry to measure nitrate, the blank solution needs to contain the same volume of color reagent.
2. Regular calibration
Calibrate the absorbance accuracy using a standard solution (such as 0.01 mol/L K ₂ Cr ₂ O ₇). Instruments that have not been calibrated for a long time may have a nominal value deviation of>5% due to detector aging or light source attenuation.
3. Stray light correction
High end colorimeters are equipped with stray light correction function, but low-end models need to check the baseline by blocking the light path (should be close to 0 A). When stray light>2%, it may not be possible to accurately measure high transmittance samples.
7、 Operating standards and human error
1. Colorimetric dish operation
When holding a colorimetric dish, pinch the surface to avoid finger contact with the translucent surface; After pouring the liquid, it is necessary to wipe it in a single direction with mirror paper to prevent scratches. Incorrect operation may result in a difference in absorbance greater than 0.05.
2. Timing of Reading
The color reaction requires reading data during the stable period. For example, when using the DNS method to measure reducing sugars, the optimal reading window is 5-15 minutes after color development, and either too early or too late can lead to errors.
3. Repetitive measurement
The same sample needs to be measured at least 3 times and averaged to reduce random errors. For example, when measuring hexavalent chromium in water, the deviation of a single measurement may reach ± 3%, and the mean of three measurements can be reduced to ± 1%.
8、 Data Processing and Method Selection
1. Linear range of standard curve
The applicability of Lambert Beer law needs to be verified. If the high concentration area deviates from linearity (such as \ \ (R ^ 2<0.999 \ \)), a quadratic equation or segmented fitting should be used. For example, when using the ortho phenanthroline method to measure iron, if the concentration is greater than 5 mg/L, it needs to be diluted and refilled.
2. Blank deduction and interference correction
Complex samples require deduction of reagent blank and matrix interference. For example, when measuring heavy metals in soil extract, it is necessary to simultaneously prepare "reagent blank" and "matrix blank" to eliminate the interference of the reaction between the color reagent and soil components.