Particle Size Distribution (Microtrac) Test

Our standard turnaround time is 10 business days for most assays. There are some assays that require a longer turnaround time. We also offer a RUSH service that is half the time of the standard turnaround time of the assay at double the cost of the assay. A few assays that we provide cannot be rushed due to the nature of the test. Please check the specific assay you are interested in regarding the ability to RUSH the turnaround time.

The Microtrac system offers a broad analytical range that depends on the dispersion method used for the sample. The specific ranges are:

• Liquid Dispersion: 0.0117 to 2,000 micrometers (microns)
• Dry Powder (Air) Dispersion: 0.2650 to 2,000 micrometers (microns)

This versatility allows us to analyze a wide variety of materials, from very fine powders and emulsions to coarser grains and ingredients.

Samples that contain a component that exceeds 2 mm can be passed through a sieve before testing so that a gravimetric percent of ‘large’ particles can be provided along with the Microtrac distribution of the smaller particles that passed through the sieve.  This makes the test much more versatile and applicable to a wider range of samples. 

The fundamental measurement principle, laser diffraction, is the same for both powders and liquid dispersions. The instrument directs a laser beam through a stream of particles, and the particles diffract (scatter) the light at angles inversely proportional to their size. A detector measures this scatter pattern, and a software algorithm calculates the particle size distribution.

The primary difference lies in the sample handling and dispersion module.

• For liquid dispersions, the sample is suspended in a suitable carrier fluid (e.g., water, isopropyl alcohol or isopar) and circulated through a glass flow cell in the laser's path.
• For dry powders, a specialized feeder uses a controlled vacuum stream to disperse the powder and carry it through the measurement zone.

The choice between a wet or dry analysis depends on the nature of the sample and the analytical question being asked.

The Microtrac instrument itself is highly precise and provides very reproducible results. However, the overall reproducibility of a test is most often limited by the homogeneity of the sample material and the ability to obtain a representative aliquot for analysis.

• For homogenous materials, such as a well-mixed liquid emulsion or a very fine, uniform powder, the reproducibility between measurements is typically excellent.
• For heterogeneous materials, such as ground corn or gritty nut butter, achieving high reproducibility is more challenging. The small aliquot taken for analysis may not have the exact same proportion of coarse-to-fine particles each time, leading to variability in the results. 

Proper sampling technique is critical to ensuring the data accurately reflects the material as a whole.

Yes, there are several potential challenges when analyzing complex food systems. The primary limitation is that laser diffraction provides a single, volume-weighted distribution for all particles that pass through the laser. It does not chemically differentiate between them.

Specific interferences can include:

• Heterogeneity: As mentioned, obtaining a statistically representative sample from a mixture with widely different particle sizes and densities can be difficult.
• Particle Shape: The calculation assumes spherical particles. For highly irregular or elongated particles (e.g., fibers), the reported size is an "equivalent spherical diameter," which may not directly correlate with other methods like sieving.
• Solubility and Swelling: If a component of the sample swells or partially dissolves in the liquid carrier fluid during the analysis, the measured particle size will change over the course of the measurement.
• Color: dyes can cause the carrier fluid to absorb too much of the laser energy to allow a measurement (pigments do not!).

The amount of sample required is not a fixed weight or volume. Instead, it is determined by achieving an optimal particle concentration. The instrument software provides a real-time loading value, and the analyst adds sample until this target is reached. This ensures there are enough particles for a robust statistical measurement but not so many that they cause errors from multiple scattering events or a saturated detector.

To provide a practical idea:

• A highly concentrated liquid like milk may only require a single drop.
• A dry powder like flour or ground grain typically requires approximately 0.1 to 1 gram per measurement.

Sample homogeneity is critical for accuracy. If a sample is not uniform, the small aliquot analyzed may not be representative of the entire batch. For example, in nut butter, the analysis will be very good at characterizing the smooth, creamy base. However, if there are only a relative few large gritty fragments in the sample, they are unlikely to be properly represented in each aliquot tested. In such cases, laser diffraction is excellent for assessing the "background" texture (creaminess), while a different technique like the Hegman Gauge might be better for quantifying the presence of coarser, "gritty" particles

Particle size distribution is a critical predictor of these properties in food:

• Solubility and Dissolution Rate: For a given mass, smaller particles have a higher total surface area than larger particles. This increased surface area allows them to dissolve or hydrate more quickly, which is crucial for products like drink mixes or instant sauces or for running predictably on a production line. 
• Texture and Mouthfeel: The human tongue is highly sensitive to particle size.
  • Fine Particles (< ~30 microns): Generally contribute to a perception of smoothness, creaminess, and increased viscosity. Think of the texture of high-quality chocolate or yogurt.
  • Coarse Particles (> 50 microns): Can be perceived as "gritty," "sandy," or "chalky." The presence of even a small percentage of particles in this range can negatively impact the mouthfeel of a product intended to be smooth.
• Stability: In emulsions and suspensions, smaller particles are less prone to settling due to gravity, leading to a more stable product with a longer shelf life.

The turnaround time for standard analysis aligns with Medallion’s typical TAT. The analysis itself is quite rapid, often taking <20 minutes per sample once the instrument is set up. If expedited results are needed, this can often be accommodated with prior coordination with the lab.

Replicate measurements are built into the analysis. Typically, 3 replicates are run for each sample aliquot. This allows us to assess the reproducibility and stability of the measurement.  The three measurements are averaged for the final reported values.

True replicates are frequently done by the lab on difficult samples but are typically not reported.  If the customer requests replicates, we can do so and they allow the customer to verify the homogeneity of the test aliquots.

This method is laser diffraction, so it compares perfectly. This technique is fast, highly reproducible, and covers a very wide range of sizes. It provides a statistically robust, volume-weighted distribution of the entire sample. Its primary limitation is that it provides less information on particle shape, reporting an "equivalent spherical diameter”.  Here is how it compares to other common techniques:

• Sieving: Sieving physically separates particles based on their ability to pass through a mesh of a known size. It is excellent for coarse materials and is often used for quality control specifications.  It is influenced by particle shape (an elongated particle may pass through a sieve if oriented correctly).  Rotap is not practical for particles below ~50 microns.  The alpine goes smaller but the Microtrac measures the smallest particles.  The Microtrac is also influenced by particle shape but since it is not influenced in the same way (it measures ALL orientations at once) the results between the two techniques can diverge as the particle shapes shift away from a general spherical shape. 
• Microscopy: Image analysis provides direct visual information about particle size, shape, morphology and even identity. However, it is often a manual and time-consuming process that analyzes a relatively small number of particles. This makes it less statistically representative for characterizing an entire distribution compared to laser diffraction, which measures millions of particles in seconds.

Yes.  By measuring the particle size distribution of a product at various time points and under different storage conditions, we can quantitatively track physical changes such as:

• Agglomeration or Aggregation: Particles clumping together, which appear as an increase in the measured particle size.  This can be tricky to measure depending on the strength of the agglomeration.  But significant agglomeration is typically strong enough to measure.  
• Crystal Growth: In confections or frozen products, small crystals can grow larger over time (Ostwald ripening or crystal perfection), which can be tracked as a shift in the distribution.
• Settling and Phase Separation: In emulsions and suspensions, changes in the particle size distribution can indicate instability long before it is visible to the naked eye.