Surface Area Determination of Particle Size Using Nuclear Magnetic Resonance

Table of Content

Particle Size
Surface Area Determination
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Theoretically, it is possible to calculate a surface area value from particle size data, and there are different methods that can provide an approximate particle size distribution (PSD).

However, the PSD of various materials are broad and unequal. Three common descriptors include the mode, mean and median (d50). But as the distribution broadens, these values can vary significantly. It is therefore important to find the most suitable single value to determine the surface area. Figure 1 illustrates the PSD for a sample of a 2 vol% aqueous suspension of a micro fine titanium dioxide (TiO2).

Figure 1. The PSD for a sample of a 2 vol% aqueous suspension of a micro fine TiO2.

In the above figure, the particle size is evenly distributed about the modal value (70nm) where most of the particles exist; however, the suspension includes a "tail" of agglomerates that distorts the mean and median sizes. Table 1 shows the surface area values measured from the three sizes. They are markedly different.

Table 1. Surface area values calculated from the three sizes.

Particle Size (nm) Surface Area (m2/g)
91 (mean) 16
82 (median, d50) 18
70 (mode) 21

Particle Size

In effect, many real-world particles are not uniform or round. Light scattering instruments calculate an equivalent spherical diameter (ESD), which is the diameter of a sphere that would provide the same result as the true particle. Different methods can produce different ESD for the same particle; in other words, the more irregular the particle, the greater the difference in ESD.

Surface Area Determination

Any surface area value determined from the PSD data is just a rudimentary approximation. Contrary to PSD devices, when measuring wetted surface area values from nuclear magnetic resonance (NMR) relaxation data, no assumptions are made with respect to either particle shape or size.

Acorn Area, a revolutionary particle analyzer, can function with almost any particle in any liquid at any concentration. As measurements usually take less than 5 minutes, this makes it suitable for QC measurements.

It is important to measure the correct wetted surface area of active pharmaceutical ingredient (API), because API surface area directly affects bioavailability as well as the rate of dissolution (Noyes-Whitney equation).

Moreover, there is increasing evidence that for any kind of nanoparticle, it is the not the particle size, but surface area which is the defining metric that controls toxicological interaction.

Figure 2. Bimodal PSD

Figure 2 shows the complexity where the PSD is bimodal. Here, 10% of the API accounts for 90% of the wetted surface area. The wetted surface areas of six lots of the same API, which had passed QC by laser diffraction particle size measurements, are shown in Table 2. The reproducibility of the NMR measurements was ca 1%; Lot#1 is evidently different (smaller SA).

Table 2. Wetted surface areas of six lots of the same API.

Lot# 1 2 3 4 5 6
SA (NMR) 4.4 5.3 5.6 5.4 5.2 5.4


The surface area is the defining metric which controls toxicological interactions. NMR relaxation data is suitable for determining the correct wetted surface area of API.

About XiGo Nanotools

XiGo Nanotools was founded by Sean Race and Dr. David Fairhurst in 2005 with the mission to provide new innovate “tools” for the emerging nanomaterials industry. The Acorn Area is designed to measure the wetted surface area of concentrated dispersions with little or no sample preparation, providing a viable complementary technique to BET surface area, analyzing nanoparticles as they are made or used, dispersed in liquids.

Our goal is to provide scientists, researchers, and corporations with tools that are easy to use and serve as wide and diverse a customer base as possible. We have incorporated the latest technology available into an integrated, high quality package that provides precise measurements in a very small footprint.

This information has been sourced, reviewed and adapted from materials provided by XiGo Nanotools.

For more information on this source, please visit XiGo Nanotools.

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