GB/T 7729-2021Chemical analysis of metallurgical products—General rule for spectrophotometric methods (English PDF)

6.2.1

UV spectrophotometer is usually tungsten lamp and deuterium lamp (wavelength range is about 180nm~1000nm). The light source shall have good stability.

Tungsten lamps shall be checked frequently to maintain the stability of the spectral composition.

6.2.2 Wavelength selector

A wavelength selector is a device that can separate a certain wavelength range from the optical radiation energy spectrum. It can select the wavelength band as needed.

There are two types of wavelength selectors: fixed wavelength selectors and continuously changing wavelength selectors.

Fixed wavelength selectors are often called light filters. In the optical path, placing an appropriate light filter can achieve wavelength selection. The most commonly used are glass filters, which have good stability to light and heat.

Continuously changing wavelength selectors: The light beam passes through the dispersive element and realizes the continuous change and selection of the wavelength by means of the adjusting mechanism. Commonly used dispersive elements are prisms and gratings. Gratings are linearly dispersive. The dispersion rate of a prism varies with wavelength. But it is easier to eliminate diffuse scattering of optical radiation. Quartz prisms are suitable for use in the ultraviolet, visible and near-infrared regions. In the visible light region, the dispersion rate of glass prisms is greater than that of quartz prisms, so glass prisms are often used in the visible light region.

The exit slit of the wavelength selector is generally adjustable. Usually adjust to the same width as the inlet slit. The width of the slit is one of the parameters of the spectrophotometer.

6.2.3 Absorption dish

Commonly used absorption dishes are glass absorption dishes and quartz absorption dishes. Glass absorption dishes are used in the visible and near-infrared regions. Quartz absorption dishes are suitable for near-ultraviolet, visible and near-infrared regions (wavelength range is 180nm~1000nm). The two walls of the absorption dish through which the light radiation passes shall be parallel to each other.

Commonly used spectrophotometers can use several sizes of absorption dishes. When using several absorption dishes for photometric determination, the length of the dishes shall be corrected. When calibrating, inject a solution with an absorbance of about 0.4 into a calibration dish and a standard dish with an accurate optical path length. Measure the absorbance. Take the absorbance of the standard dish as 1.0. Calculate the correction factor for the absorbance of the calibration dish relative to the standard dish. When the calibration dish is used for measurement, the actual absorbance of the measured solution is the product of the measured absorbance value and the calibration coefficient.

6.2.4 Detector

A detector is the component that detects the light intensity after passing through the solution and converts it into an electrical signal. It is often composed of photocells, photomultiplier tubes or photodiodes and their circuits.

6.2.5 Monitor

Commonly used spectrophotometers: The electrical signal output by the detector is indicated on the display head as absorbance (A) or transmittance (T), or both at the same time. Some spectrophotometer monitors give analytical results or other information directly in terms of concentration.

6.3 Main instrument types

6.3.1 Doable beam spectrophotometer

The photometric reading displayed by such instruments is the ratio of the intensity of light transmitted through the sample solution to the intensity of light transmitted through the reference solution at the same time. So, use two beams of light for the measurement at the same time. Usually, two beams of light split from the same light source are projected onto two symmetrical detectors, respectively; or both are projected onto one detector. When a detector is used, the detector receives two beams of light energy alternately. For this system, the frequency of alternation is quite high. It can be considered that the comparison of the two beams of light energy is carried out simultaneously.

6.3.2 Single beam spectrophotometer

The photometric reading displayed by the instrument is obtained by successively measuring the light intensity transmitted through the sample solution and the light intensity transmitted through the reference solution.

6.4 Instrument performance indicators

Instrument performance indicators include:

a) Wavelength accuracy: The deviation BETWEEN the wavelength at the maximum absorbance of the absorption curve of the optical filter or the wavelength at the maximum intensity of the emission line of a low-pressure mercury or deuterium lamp AND the given standard wavelength.

b) Repeatability of wavelength settings: The variance of measured wavelength values when repeated measurements are taken from the emission line of a low-pressure mercury or deuterium lamp, or the absorption curve of an optical filter.

7.2 Establishment of instrument operation conditions

7.2.1 Selection and setting of measurement mode

The measured values shall be energy-related quantities such as transmittance, reflectance, and absorbance, or set according to the instrument's operating manual.

7.2.2 Selection and setting of measurement wavelength

Wavelength can also be replaced by units such as wave number (cm-1), energy (eV), frequency (Hz).

Plot the molecular absorption spectra of the color developer and light absorbing substance, so as to determine the absorption wavelength and optimal contrast. Generally, the selected wavelength is within the maximum absorption peak range of the absorption curve. In some cases, for example, the maximum absorption peak is very sharp, or there is interference from other light-absorbing substances in the maximum absorption peak range, do not work at the maximum absorption peak of the absorption curve. Select additional absorption peaks or other flat portions of the absorption curve.

7.2.3 Selection of absorption dish

According to the optimum range of the spectrophotometer used, select the appropriate size of the absorption dish.

7.3 Measurement preparation

Measurement preparation shall be carried out as follows:

a) Confirm whether the device is running stably;

b) Place (or activate) a light shield in the sample beam path for the two-beam method and the beam path for the single beam method to cut off the beam path. Adjust the indication value of transmittance to 0;

c) Remove (or close) the light shield. Adjust the indication value of transmittance to 100;

d) Repeat the operations shown in b) and c). Confirm that there is no fluctuation in the indication value.

If the device has the function of automatically implementing the operations of a) ~ d), operate according to the device manual. In the case of devices using photodiode detectors, some devices require an output zero correction.

7.4 Measurement methods

7.4.1 Doable-beam measurement method

The measurement by doable-beam measurement method shall be performed as follows:

a) Place two reference samples in the sample light path and the reference light path, respectively. Adjust the transmittance to 100 or the absorbance to 0. This can also be done without a reference sample. Equivalent results can be obtained by data manipulation;

b) Put the testing sample and the reference sample into the sample optical path and the reference optical path, respectively. Read the transmittance or the absorbance.

When operating without a reference sample, the equivalent transmittance or absorbance can be obtained through data processing;

c) If necessary, calibrate the blank value of the absorption dish.

7.4.2 Single-beam measurement method

The measurement by single-beam measurement method shall be performed as follows:

a) Place the reference sample in the light path. Adjust the transmittance to 100 or the absorbance to 0;

b) Place the target sample in the light path. Measure the transmittance or the absorbance;

c) Repeat the operations shown in a) and b) above if necessary. Confirm that the readings are less scattered;

d) If necessary, calibrate the blank value of the absorption dish.

7.4.3 Measurement of the blank value of the absorption dish

In the measurement wavelength range, the sample absorption dish and the reference absorption dish preferably have the same optical characteristics. In the case where the optical eigenvalues are not exactly the same, but the difference is small, it needs to measure the blank absorption value of the absorption dish. Then calibrate it. If the device has a baseline calibration function, it is not necessary to calibrate the cuvette blank value.

Put the same solution in the sample absorption dish and the reference absorption dish (the absorption value at the measurement wavelength is smaller) as the testing sample and the reference sample, respectively. Operate as in 7.4.1 or 7.4.2. When the measurement result is transmittance, convert it to absorbance.

Take the obtained value as the blank absorbance value of the sample absorption dish to the reference absorption dish. When the blank value of the absorption dish is determined, it shall use the same absorption dish to combine in the same set of data.

9.1

c) The calibration curve within the straight line range is preferred;

d) When preparing the calibration curve, the concentration of the testing sample shall be between the concentrations of the solution drawn by the calibration curve;

e) When the sample is suspended, there is light attenuation due to scattering in addition to the sample absorbance. In this case, apply the absorption value at the wavelength where the sample absorption and scattering absorption coexist minus the absorption value at the wavelength where only the scattering absorption is present. Use the difference of the two absorption values to draw a calibration curve. This will use two wavelengths;

f) In the photometric determination, although the interference can be eliminated by certain analysis steps, the obtained photometric reading still includes the background absorption and the photometric reading of the reagent blank, which is usually called "reagent blank". When analyzing the specimen and drawing the calibration curve, a blank test shall be done along with the operation steps. If a solvent is used as the reference solution, the photometric reading of the blank test solution shall be deducted from the photometric reading of the test solution.

9.2 Calibration addition method

Pipette four or more samples of the same volume from the same testing sample solution.

Except for one sample, add a known concentration of the analyte solution to the remaining solutions. Make a concentration gradient in several solutions. The concentration of the analyte added shall be equivalent to or of the same order of magnitude as the concentration of the analyte in the sample solution. Colorize these solutions. Then set volume to the same volume. Measure its absorbance. Take the increase in the concentration of the analyte after adding the standard solution as the abscissa, and the absorbance as the ordinate to draw a correlation curve. The intersection of the correlation curve and the abscissa gives the concentration or amount of the analyte. The calibration addition method is only applicable if the relationship curve is linear. The values that draw the calibration curve correct the blank test values via the absorbance at origin. An example of the calibration addition method is shown in Figure 2. […]