Selection of laser particle size analyzer

Selection of laser particle size analyzer

The previous particle size analysis methods usually used sieving or settling methods. The commonly used settlement method has disadvantages such as slow detection speed (especially for small particles), poor repeatability, large errors for non spherical particles, unsuitability for mixed materials (i.e., the particle ratio must be consistent to be better prepared), and narrow dynamic range. With the invention of laser diffraction method, particle size measurement has overcome the drawbacks of sedimentation method, greatly reducing labor intensity and accelerating sample detection speed (from half an hour to 1 minute).

The laser diffraction method for measuring particle size is based on the fact that small particles have a larger scattering angle for activation, while large particles have a smaller scattering angle for laser. The particle size can be converted by measuring the scattering angle. The optical concepts it is based on are the Mie theory and the Fraunhofer theory. The Fraunhofer theory is an approximation of the large particle Mie theory, which omits the imaginary subset of Mie theory and assumes that the particles are opaque; And ignore the light scattering system and absorption system, that is, set all dispersant and dispersion parameters to 1, so the mathematical processing is much simpler, and the error for colored substances and small particles is also much larger. Similarly, the approximate Michaelis concept is not applicable to emulsions.

In addition, according to Rayleigh scattering law, the intensity of scattered light is proportional to the sixth power of the particle diameter and inversely proportional to the fourth power of the wavelength of the scattered light source. This means that if the particle diameter decreases by 10 times, the scattered light intensity will decrease by 1 million times! The shorter the wavelength of the light source, the higher the intensity of scattered light.

Furthermore, due to the large scattering angle of small particles and the limited area of the main detector, it can generally only receive scattered light at angles of up to 45 degrees (i.e. particles larger than 0.5 microns). So, how to detect small particles and overcome the problem of low light scattering energy of small particles beyond the range of the main detector has become the key to evaluating laser analysis technology.

So, to judge the advantages and disadvantages of a laser particle size analyzer, we mainly look at the following aspects:

1. Particle size measurement range: The particle size range is wide and suitable for a wide range of applications. Not only should we look at the range reported by the instrument, but also at how small particle scattering (<0.5 μ m) beyond the main detection area is detected.

The approach is to directly detect the entire range, in order to ensure the consistency of background deduction. Mixing different methods of testing and then using a computer to create a graph will definitely bring errors.

2. Laser light source: Generally, a 2mW laser is used. If the power is too low, the scattered light energy will be low, resulting in low sensitivity; In addition, the wavelength and stability of gas light sources are affected by solid-state light sources.

Detector: Because the larger the radius of the laser diffraction ring, the weaker the light intensity, which can easily cause a decrease in the signal-to-noise ratio of small particles and lead to leakage. Therefore, detecting the distribution of small particles can reflect the quality of the instrument. The development of detectors has gone through several stages: circular, semi-circular, and fan-shaped.

3.1 The first generation detector of Marvin Company in the UK

Adopting a non-uniform cross arranged 3D fan-shaped detection system, the actual resolution is the highest, with no signal blind spots, equivalent to 175 circular or cross star arrangements and 93 semi-circular arrangements, making the direct detection angle reach 135 degrees.

3.2 Number of Channels

In a laser particle size analyzer, unlike in a counter, there is no concept of a channel. It is actually used to detect the number of light receiving areas, and it has a concept and actual value: insufficient received scattered light, poor accuracy; excessive sensitivity, resulting in poor reproducibility.

To compensate for the limitation of sampling speed, some manufacturers use more channels to achieve sensitivity requirements at the expense of attention, so such measurement times are over 20 seconds or 1 minute.

The first generation Mastercizer 2000 laser particle size analyzer from Malvern Instruments in the UK samples 1000 times per second with a measurement time of only 2 seconds (average of 2000 results), which optimizes accuracy and repeatability.

4. Whether to use the Mie scattering concept of *: Because the Mie scattering concept is very complex and requires a large amount of data processing, some manufacturers ignore the optical properties of particles such as refraction and absorption, and adopt approximate Mie theory, which limits its applicability and increases the probability of missed detections.

5. Accuracy and repeatability indicators: The higher, the better. Adopting NIST standard particle detection.

6. Stability: Instrument stability includes the stability of the optical path, the stability of the dispersion system, and the influence of the surrounding environment. Generally speaking, choosing a gas laser and using an optical platform can help stabilize the optical path. Internal heating components (such as 50 watt tungsten lamps) will affect the surrounding environment of the light path.

The stability index is not included in the manufacturer's instrument instructions, and users can only judge based on their judgment of the instrument's marriage and visiting or inquiring with other long-term users.

7. Scanning speed: Touching the speed can improve data accuracy, repeatability, and stability.

The scanning speed of instruments from different manufacturers varies, ranging from 1 to 1000 times per second. Generally speaking, the more times the cyclic scanning test is conducted, the better the accuracy of the average results, so the higher the speed, the better; Spray dry method and spray require higher speed; Although the free fall dry method is not fast, it is still faster because the particles only pass through the sample once.

The amount of samples that users need to process every day is also a factor that considers speed.

8. It can automatically align without the need to change lenses and can automatically calibrate

9. Simplicity of use and maintenance: This is often overlooked before purchase, and in fact directly determines the efficiency and lifespan of the instrument. The method of understanding is based on my understanding of the instrument's marriage and feedback from other existing users.

Is disassembly and cleaning convenient? The particle size analyzer is divided into two parts: the main unit and the disperser. The sample flow cell always needs to be cleaned regularly, and the cleaning interval depends on the nature of the sample. The instrument that combines the host and disperser often places the sample cell deep inside the instrument, making it cumbersome to remove and disassemble, and easily damaging the optical system.

10. Compliance with standards: ISO 13320 standard is a basic requirement for laser particle size analyzers. But not all manufacturers follow this standard. The use of non laser diffraction methods in measuring the distribution of submicron particles does not comply with the ISO 13320 standard.

11. Dispersant

11.1 Wet process

Whether it has physical dispersion functions such as ultrasound and stirring, and whether the ultrasound power and stirring speed are continuously adjustable, are the key to ensuring the repeatability of analysis results.

11.2 Dry process

Is it a closed measurement and is the sample easily dispersed? If not, have you chosen a jet disperser? A shock airflow of 300 meters per second is a prerequisite for ensuring that the sample can be fully dispersed and obtain accurate analysis results.