X-ray single crystal diffractometerBy detecting the elastic scattering (diffraction) signals of X-rays and crystal atoms, and analyzing the core structural information such as atomic arrangement, bond length, and bond angle of crystals (with an accuracy of 0.001 Å), it is a key equipment in the fields of materials, chemistry, and biology. High order diffraction interference (such as diffraction orders of n ≥ 2, such as 2nd order diffraction of Cu K α rays) will be superimposed on the low order diffraction signal of the target, resulting in diffraction peak overlap and intensity measurement deviation. It needs to be eliminated through "hardware filtering parameter regulation software correction" to ensure the accuracy of structural analysis.

1、 Hardware filtering: blocking high-order diffraction generation from the source

Screening X-ray wavelengths and diffraction orders through specialized optical components to reduce the generation of high-order signals:

Monochromator precise filtering: Install a graphite monochromator (commonly known as a bent crystal monochromator) between the X-ray source and the sample, utilizing the Bragg reflection characteristics of the crystal for specific wavelengths, allowing only the target wavelength (such as Cu K α ₁=1.5406 Å) to pass through, filtering out X-rays of other wavelengths (such as Cu K β rays, continuous radiation) - these miscellaneous wavelengths are prone to non target high-order diffraction (such as the 1st order diffraction of K β may overlap with the 2nd order diffraction of K α). The reflection efficiency of the monochromator is ≥ 80%, and the wavelength purity can reach 99.9%, reducing the high-order interference base from the source.

Slit and collimator control: A series of slits (such as diverging slits and anti scattering slits) are set between the sample and the detector to control the divergence angle of the X-ray beam (usually ≤ 0.1 °) and reduce stray signals from non Bragg diffraction; Cooperate with a collimator (such as a capillary collimator) to make the X-ray beam parallel to the sample, avoiding the diffusion of high-order diffraction signals caused by beam divergence, and ensuring that the detector only receives signals in the target diffraction direction.

2、 Parameter optimization: suppress high-order diffraction signal detection

By adjusting experimental parameters, the probability of high-order diffraction being mistakenly detected can be reduced

Diffraction angle range and step size control: Calculate the Bragg angle (2 θ) based on the lattice parameters of the target crystal, and scan only within the 2 θ range of low order diffraction of the target (such as when analyzing small molecule crystals with Cu K α rays, 2 θ is usually set to 5 ° -70 °, avoiding the high 2 θ region of high-order diffraction); At the same time, reduce the scanning step size (such as 0.01 °/step) to improve the resolution of diffraction peaks, so that low order and high-order diffraction peaks (if any) can be clearly separated, avoiding intensity misjudgment caused by overlap.

Detector energy resolution function: Select detectors with energy resolution capabilities (such as CCD detectors, pixel array detectors), use the energy difference of X-rays from different orders of diffraction (high-order diffraction energy=n x low order energy, n is the order), set an energy threshold during detection (such as only receiving signals that match low order energy), and automatically eliminate high-energy signals from high-order diffraction. The energy resolution accuracy can reach 5eV, and the high-order signal rejection rate is ≥ 95%.

3、 Software calibration: eliminate residual high-order diffraction effects

Correct a small amount of residual high-order diffraction interference through data processing algorithms:

Diffraction peak shape fitting and separation: perform peak shape fitting on the detected diffraction pattern (commonly using the pseudo Voigt function). If there is an overlap between low order and high-order diffraction peaks (manifested as asymmetric peak shapes or shoulder peaks), separate the intensity and position of the two peaks through fitting, and extract pure low order diffraction intensity data; At the same time, by combining the structural factor calculation of the crystal (based on theoretical models), the rationality of the fitting results is verified to ensure that high-order interference is effectively removed.

High order correction in structural refinement: During the crystal structure refinement stage (such as using SHELXL software), a "high-order diffraction correction factor" is introduced to calculate the theoretical intensity of high-order diffraction based on X-ray wavelength and lattice parameters. After comparing with experimental data, the disturbed low order diffraction intensity is corrected; At the same time, the correction effect is monitored through residual factors (R1, wR2), and usually R1 ≤ 0.05 after correction, indicating that high-order interference has been reduced to an acceptable range.

In addition, the sample preparation process also requires auxiliary cooperation: selecting single crystal samples of appropriate size (such as 0.1-0.5mm) to avoid multiple diffraction caused by excessively large samples (which can easily generate high-order interference); If there is orientation disorder in the sample, fix the crystal orientation by low-temperature cooling (such as -173 ℃) to reduce the fluctuation of high-order diffraction signals caused by orientation changes. Through the above methods,X-ray single crystal diffractometerThe intensity error caused by high-order diffraction interference can be controlled within ≤ 2% to ensure high accuracy in crystal structure analysis.