How to choose the appropriate optical prism for spectral analysis?

In the field of spectral analysis, such as atomic absorption spectroscopy, ultraviolet visible spectrophotometry, and infrared spectroscopy, an optical prism is the core component for achieving "dispersion and separation of light" - it can decompose composite light into monochromatic light according to wavelength, providing accurate wavelength signals for subsequent detection. Unlike prisms in camera lenses and telescopes that only need to meet basic refractive function, spectral analysis requires extremely high "wavelength adaptability, transmittance stability, and dispersion uniformity" of optical prisms. Improper selection can directly lead to analysis result deviation (such as inaccurate wavelength positioning, excessive absorbance error), and even result in detection experiment failure. This article will start from the core requirements of spectral analysis, break down the four selection dimensions of "material selection, structural matching, performance verification, and scene adaptation", and provide practical guidelines that can be implemented.

1、 Pre demand: Three core requirements for optical prisms in spectral analysis

Before selecting, it is necessary to clarify the "core goal" of spectral analysis - different analysis scenarios (such as ultraviolet detection, infrared detection, trace substance analysis) have significantly different requirements for prisms. It is necessary to lock in the requirements from the three dimensions of "wavelength range, resolution, and detection accuracy" to avoid blind selection:

（1） Wavelength range: Prism needs to cover the "effective spectral region" required for analysis

The core of spectral analysis is to detect the optical properties of substances at specific wavelengths (such as detecting organic functional groups in the ultraviolet range of 200-400nm, detecting solution concentration in the visible range of 400-760nm, and analyzing molecular structure in the infrared range of 760-10000nm). Prisms made of different materials only have good light transmission within a specific wavelength range. If the light transmission range of the prism does not match the analysis wavelength, it will result in "effective light signal attenuation" or even inability to detect. For example, using a glass prism that is only suitable for the visible region to detect 250nm ultraviolet light, the transmittance will be less than 10%, and the detection signal will be weak enough to be unrecognizable. Therefore, the first step is to clarify the "wavelength range required for analysis" and then select prism materials that can cover this range.

（2） Resolution: Prism needs to meet the "precision" of wavelength discrimination

Resolution refers to the ability of a spectral analysis instrument to distinguish between adjacent wavelengths (such as the ability to distinguish between sodium double lines at 589.0nm and 589.6nm), while the resolution of a prism depends on its "dispersion ability" (the difference in refractive angles of light at different wavelengths) and "aperture" (the area of collected light signals). For trace substance analysis (such as detecting heavy metal ions with a concentration below 0.1mg/L), higher resolution is required to avoid the influence of interfering wavelengths - if the prism resolution is insufficient, the signal of the interfering wavelength will be misjudged as the target substance signal, resulting in higher analysis results. Therefore, for trace analysis and simultaneous analysis of multiple components, it is necessary to choose prisms with high dispersion ability and large aperture; Conventional concentration analysis (such as 1-10mg/L) requires lower resolution, and a more cost-effective ordinary prism can be chosen.

（3） Detection accuracy: Prism needs to control the "system error" within the allowable range

The detection accuracy of spectral analysis (such as absorbance error ≤ 0.001, wavelength repeatability ≤ 0.1nm) is significantly affected by prism performance: fluctuations in prism transmittance can lead to unstable light intensity signals, which in turn affect absorbance calculation; The angle error of the prism can cause wavelength positioning deviation, resulting in a mismatch between the detected wavelength and the target wavelength. For example, in food additive testing, if the prism wavelength positioning deviation is 5nm, the concentration of "qualified" additives may be misjudged as "excessive". Therefore, based on the "error range" specified in the testing standards (such as GB/T 5009.33-2016 "Determination of Nitrite and Nitrate in Foods"), the performance indicators that the prism needs to meet (such as transmittance stability and angle accuracy) should be derived in reverse.

2、 Core dimension one: Material selection - matching the "wavelength requirements" of spectral analysis

Material is a key factor in selecting prisms for spectral analysis. Prisms made of different materials have significant differences in transmission range, dispersion ability, and chemical stability, and require precise matching based on the analyzed wavelength range. Common materials and applicable scenarios are as follows:

（1） Optical glass prism: suitable for conventional analysis in the visible range (400-760nm)

Optical glass is a commonly used prism material, which has the advantages of low cost, low processing difficulty, and high mechanical strength. However, its transmission range is limited to the visible and near ultraviolet regions (350-760nm), with a sharp decrease in transmission in the ultraviolet region (<350nm) (such as a transmission rate of<50% at 300nm), and almost no transmission in the infrared region (>2500nm).

1. Common types and characteristics of glass

K9 glass: a basic optical glass with a transmittance of up to 92% in the visible region (550nm) and moderate dispersion ability (Abbe number 64.1), suitable for conventional visible region analysis (such as solution concentration detection and turbidity analysis), with a cost only 1/5 of quartz prisms, and high cost-effectiveness;

ZF glass (heavy flint glass): Strong dispersion ability (Abbe number 30-40, such as ZF6 glass with an Abbe number of 36.3), suitable for visible region analysis that requires high resolution (such as wavelength differentiation of multi-component mixed solutions), but slightly lower transmittance than K9 glass (about 90% at 550nm), and the material is brittle and susceptible to impact damage.

2. Applicable scenarios

The routine testing in the food and beverage industry (such as sucrose concentration testing, pigment content analysis), water quality routine indicator testing (such as COD, ammonia nitrogen testing), and other visible area analysis scenarios are sensitive to cost and do not require high testing accuracy (such as an error tolerance of ± 5%).

（2） Quartz glass prism: suitable for wide spectrum analysis in the UV visible region (200-2500nm)

The major advantage of quartz glass (divided into natural quartz and synthetic quartz) is its "wide wavelength transparency" - the transmittance can reach over 80% at 200nm in the ultraviolet region, 94% at 550nm in the visible region, and still maintain over 85% transmittance at 2500nm in the near-infrared region. It also has extremely strong chemical stability (acid and alkali resistance, only corroded by hydrofluoric acid) and is the core prism material of UV visible spectrophotometers.

1. Natural quartz vs synthetic quartz: selection needs to distinguish purity

Natural quartz: contains a small amount of impurities (such as iron and aluminum ions), with slight absorption in the ultraviolet range of 200-250nm (transmittance decreases by 5% -10%), suitable for analysis in the ultraviolet visible range (250-760nm) (such as drug content determination, organic UV spectral scanning), and lower cost than synthetic quartz;

Synthetic quartz: extremely high purity (impurity content<1ppm), with a transmittance of over 85% in the ultraviolet region at 200nm, no obvious absorption peak, suitable for deep ultraviolet region (200-250nm) analysis (such as protein UV absorption detection, nucleic acid purity analysis), but the cost is 2-3 times that of natural quartz.

2. Applicable scenarios

The pharmaceutical industry's drug composition analysis (such as measuring vitamin C content using UV spectrophotometry), organic pollutant detection in environmental monitoring (such as detecting benzene derivatives in the UV region), and nucleic acid/protein analysis in life sciences require high detection accuracy (with an error tolerance of ± 1%).

（3） Infrared crystal prism: suitable for molecular structure analysis in the infrared region (760-10000nm)

Infrared spectroscopy analysis needs to be conducted in the infrared range of 760-10000nm, and glass and quartz have extremely low transmittance (<10%) in this wavelength range. Prisms made of infrared crystal materials need to be selected, and the transmittance range and dispersion ability of different infrared crystals should be adapted to different infrared sub regions:

1. Common types of infrared crystals and their adaptation scenarios

Sodium chloride (NaCl) crystal: with a light transmission range of 2000-15000nm (mid infrared to far infrared region), moderate dispersion ability, low cost, but easy to absorb moisture (the surface will deliquesce when the humidity in the air is greater than 60%, resulting in a decrease in light transmission), it needs to be used in a dry environment (such as an infrared spectrometer equipped with a drying oven), suitable for routine mid infrared analysis (such as polymer molecular structure detection, petroleum product component analysis);

Potassium bromide (KBr) crystal: with a transmittance range of 2500-20000nm (far infrared region), the transmittance is higher than that of sodium chloride (up to 90% at 1000nm in the mid infrared region), but it is more hygroscopic and has low mechanical strength (brittle), suitable for fine analysis in the far infrared region (such as lattice vibration detection of inorganic compounds);

Calcium fluoride (CaF ₂) crystal: with a light transmission range of 120-9000nm (deep ultraviolet to mid infrared region), strong chemical stability (non hygroscopic, acid and alkali resistant), high mechanical strength, suitable for infrared analysis that requires wide spectrum coverage (such as detecting special substances in the ultraviolet and infrared regions), but the cost is 5-8 times that of sodium chloride.

2. Applicable scenarios

In the chemical industry, molecular structure identification (such as infrared spectroscopy to determine organic functional groups), polymer composition analysis in materials science, mineral composition detection in geological exploration, and other infrared analysis scenarios require the selection of crystal types based on infrared sub regions and environmental humidity.

3、 Core Dimension 2: Structural Selection - "Spectral System" Adapted to Spectrometers

The structure of the optical prism used for spectral analysis (such as shape, angle, and aperture) needs to be matched with the spectral system of the spectrometer (such as prism monochromator, grating prism composite system), otherwise it will lead to problems such as "low optical signal transmission efficiency" and "difficulty in wavelength adjustment". Common structures and adaptation scenarios are as follows:

（1） Shape selection: Determine prism type based on spectral principle

The commonly used prism shapes in spectrometers are "right angled prism" and "equilateral prism", which have different spectral principles and applicable scenarios:

1. Right angle prism (45 ° -45 ° -90 °): suitable for "fixed tilt angle" spectroscopic systems

The working principle of a right angle prism is that "light enters from a right angle edge, undergoes oblique total reflection, and exits from another right angle edge". The deviation angle (the angle between the incident light and the outgoing light) is fixed at 90 °, and can achieve spectroscopy without adjusting the angle. It has a simple structure and high stability, making it suitable for miniaturized spectrometers (such as portable ultraviolet spectrophotometers). For example, in portable water quality detectors, right angle quartz prisms can quickly decompose composite light into monochromatic light, meeting the needs of rapid on-site detection.

2. Equilateral prism (60 ° -60 ° -60 °): suitable for "wavelength adjustable" spectroscopic systems

The deviation angle of an equilateral prism can be adjusted by rotating the prism (the deviation angle of light of different wavelengths is different, and the rotating prism can select specific wavelengths of light to be emitted). It has stronger dispersion ability than a right angle prism and is suitable for spectrometers that require continuous wavelength scanning (such as laboratory UV visible spectrophotometers). For example, in drug spectrum scanning experiments, a rotating equilateral quartz prism can continuously select different wavelengths in the range of 200-760nm to obtain the complete absorption spectrum curve of the drug.

（2） Angle accuracy: Control "wavelength positioning error"

The angular accuracy of a prism (such as a right angle deviation of ≤ 30 "for a right angle prism and a vertex deviation of ≤ 1 'for an equilateral prism) directly affects the wavelength positioning accuracy - angular deviation can cause the refractive angle of light to deviate from the design value, resulting in the emission wavelength not matching the target wavelength. For example, if the vertex angle deviation of an equilateral prism is 1 ', the wavelength positioning error can reach 0.5nm at a wavelength of 500nm, exceeding the allowable error of most spectral analyses (≤ 0.1nm). Therefore, angle accuracy should be selected based on the requirements of detection accuracy:

Conventional analysis (allowable error ± 0.5nm): Select prisms with an angle deviation of ≤ 1 ';

High precision analysis (error tolerance ± 0.1nm): Select a high-precision prism with an angle deviation of ≤ 30 "(angle accuracy needs to be detected by a laser interferometer).

（3） Optical aperture: matching the "collection efficiency" of optical signals

The aperture of a prism refers to the large area allowed for light to pass through (usually expressed as "edge length x edge length", such as 10mm x 10mm, 20mm x 20mm). The larger the aperture, the stronger the collected light signal and the higher the detection sensitivity (especially suitable for trace substance analysis). For example, when detecting trace mercury ions (concentration 0.01mg/L) in water, a 20mm × 20mm large aperture prism should be selected to collect sufficient light signals and avoid signal masking by noise; Conventional concentration analysis (such as 1mg/L) can reduce costs by selecting a diameter of 10mm × 10mm.

Please note that the aperture of the light passage should match the width of the incident slit of the spectrometer (usually 5-10 times the width of the slit). If the aperture is too large, it can cause stray light to enter the system, affecting the detection accuracy; If the aperture is too small, the optical signal will be insufficient and the sensitivity will decrease.

4、 Core dimension three: performance verification - locking in "key indicators" to avoid pitfalls

When selecting, it is necessary to focus on verifying the three key performance indicators of the prism, namely "transmittance, dispersion uniformity, and surface quality". These indicators directly determine the accuracy and stability of spectral analysis and need to be verified through professional testing data, rather than relying solely on the manufacturer's promotional judgment

（1） Transmittance: Ensure minimal attenuation of optical signals at the "effective wavelength"

Transmittance refers to the transmittance ratio of a prism to a specific wavelength of light (such as a quartz prism with a transmittance ≥ 85% at 250nm), which must meet the requirement of "transmittance fluctuation ≤ 5% within the analyzed wavelength range". If the transmittance fluctuation is too large (such as 85% at 250nm and 75% at 260nm), it will cause unstable light intensity signals at different wavelengths, leading to excessive absorbance detection errors.

Verification method:

Require the manufacturer to provide a "wavelength transmittance curve" (covering the wavelength range required for analysis), with a focus on the transmittance near the target wavelength (such as detecting proteins at 280nm, ensuring that the transmittance is ≥ 80% at 280nm and the transmittance fluctuation within the range of 270-290nm is ≤ 3%);

If conditions permit, a spectrometer can be used for actual measurement: the prism is installed in the spectrometer to measure the light intensity (I) at a specific wavelength and the light intensity without the prism (I ₀). The transmittance T=I/I ₀ × 100%, which needs to be consistent with the manufacturer's data.

（2） Dispersion uniformity: avoid insufficient "wavelength resolution"

Dispersion uniformity refers to whether the refractive angle of light at different wavelengths of a prism changes uniformly within the analyzed wavelength range. If the dispersion is uneven (such as a sharp change in refractive angle in one wavelength band and a gentle change in another wavelength band), it will result in insufficient resolution in some wavelength ranges, making it difficult to distinguish adjacent wavelengths. For example, in the analysis of multi-component mixed solutions, if the prism has uneven dispersion in the 500-550nm wavelength range, it will merge the two absorption peaks at 520nm and 521nm into one, resulting in inaccurate determination of the concentration of each component.