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(1) Why is baseline correction necessary? As is well known, the optical system of a photometer basically consists of three components: a light source (deuterium lamp, tungsten lamp), a monochromator (grating, slit), and a detector (photosensitive diode, photomultiplier tube). In the wavelength range we use (typically, UV-Vis instruments operate in the range of 190 nm to 1100 nm), the response values of the aforementioned components at different wavelengths (the emission intensity of the light source, the dispersion capability of the monochromator, and the amplification factor of the detector) vary ； Put simply, even in the absence of a sample, if the instrument does not perform baseline correction, then within the range of 190nm to 1100nm, the absorbance or transmittance value will not form a straight line; this is an objective physical phenomenon, as shown in the figure below ； Although the graph above shows the energy profile of a single beam, in the absence of baseline correction, even if a dual-beam measurement method is used to scan a sample, the resulting spectrum or absorbance values are not reliable. (II) Types and uses of the corrected baseline (1) System baseline: The so-called system baseline refers to the overall baseline of the wavelength range inherent to the instrument ； For example, if an instrument is designed to operate over a wavelength range of 190 nm to 1100 nm from the factory, then its system baseline is this range. Generally speaking, it is quite rare for an analyst to conduct a full-scale scanning test on an instrument ； The general purpose of establishing a system baseline is to adjust the response values of the instrument’s optical system to be essentially consistent ； It’s similar to a marathon race; as long as everyone starts from the same starting point (note: not the starting line), it doesn’t matter if they start a few meters apart. (2) User baseline: The so-called user baseline refers to a baseline segment of the wavelength range defined by the analyst themselves for measurement ； Since this is the area required for analysis, and to ensure the accuracy of the tests, it is very important and necessary to correct the user baseline ； It’s similar to a 100-meter race: athletes must compete from the same starting line and cannot start ahead of others, otherwise it’s impossible to calculate the results accurately. (III) Methods of baseline correction (1) System baseline: The correction of the system baseline is relatively simple; generally, no sample is placed in the sample chamber, and only the optical system is calibrated ； If full-band measurement must be used, that’s another matter. It should also be noted that the system baseline does not need to be adjusted frequently; generally, an adjustment is sufficient every half month or month. For some instruments, excessive system baseline correction can instead lead to severe baseline drift. (2) User baseline correction: The correct method of correction is to place two cuvettes containing blank solutions in the sample and reference light paths respectively, with the correction wavelength range being wider than the analysis wavelength range ； For example, if the analysis range is set to 220nm–500nm, then the calibration wavelength should be set to 210nm–510nm ； Wait until the calibration is complete before setting the wavelength back to the original range of 220nm–500nm. The advantage of this correction method is that if the correction wavelength matches the analysis wavelength exactly, large noise may appear at both ends of the baseline after correction ； If the correction range is larger than the actual analysis range and the extremes are removed, the analysis accuracy can be improved. I named this correction method the “Mung Bean Sprout Principle” to make it easier to remember ； (Because when eating bean sprouts we always pinch off the ends and roots, eating only the middle part, old restaurants referred to the dish of stir-fried bean sprouts as “stir-fried picked vegetables”)” ； Sorry, I got off track). Regarding this correction method, many users are often unaware of it or ignore it; I would like to introduce it to fellow printmakers here. It is worth noting that some instrument operators, when performing baseline calibration, do not use a reference solution on the reference side; instead, they use air as the reference. This method may have little effect on some samples in the visible range, but the effect becomes significant in the ultraviolet range. Strictly speaking, the results obtained using air as a reference are not true corrected spectra. (IV) Precautions for baseline calibration: (1) Ensure that the instrument has sufficient time to warm up during baseline calibration. (2) Re-perform baseline calibration after changing each reference solution. (3) If the absorbance of the reference solution is higher than that of the sample, the test results will be negative; in such cases, it is necessary to consider which solution should be used for baseline calibration. (4) When performing baseline correction, it is necessary to take into account the wavelength range in which the reagents are used, as the absorbance value of some reagents becomes infinite below a certain wavelength; performing correction in such cases would exceed the instrument’s effective measurement range, preventing the acquisition of accurate results. Regarding the wavelength range for using the reagent, it is generally indicated on the label of the reagent bottle. Wavelength range for common solvents: Cycloethane (above 200 nm) ； Ethanol (above 220nm) ； Methanol (above 220nm) ； Ether (above 220nm) ； Ethane (above 220nm) ； Trichloromethane (above 250nm) ； Isopropyl alcohol (above 250nm) ； Acetic acid (above 250nm) ； Ethyl acetate (above 270nm) ； Carbon tetrachloride (above 275nm) ； Benzene (above 280nm) ； B-type (above 340nm) ； Carbon disulfide (above 380nm)