Finding the optimal separation method/protocol entails a lot of work for chromatographers, and once the process is approved (especially for pharma), chemists depend on the sorbent’s quality and lot-to-lot reproducibility to deliver consistent results. Think about it, would you want to spend hours in the lab optimizing each time you want to run the same separation? When time and money are on the line, researchers want consistency in quality.

In industry, the most common way to demonstrate consistent quality from the product you are getting from the manufacturer is with the Certificate of Analysis (also known as COA or a CofA)

Have a look below at the template of one of SiliCycle’s certificates of analysis for the metal scavenger SiliaMetS Thiol. This is a common example that lists all the properties tested for functionalized silica gels as well as the properties tested on bare silica and the bare silica backbones of bonded gels.

The COA is divided into three sections for this bulk sorbents, each with its own properties, specification ranges and observed results. Let’s take a closer look at each section and what these properties mean.

We will see later how the packed sorbents' COAs are different.

Bare Silica Specification

This section is applicable to all silica-based sorbents produced at SiliCycle, whether functionalized or not, due to silica being the support backbone, and details the particle’s dimensions.

• Particle size is one of the key physical characteristics for silica as a stationary phase in chromatography. In fact, particle size, including its distribution, and the pore diameter on the particle each influence the performance of separations.

  When it comes to particle size distributions, tighter is better for more uniform packing. A larger particle size distribution might create preferential paths in the column since the compounds are more likely to elute following the path of least resistance. This means broader peaks and products exiting the column in a larger volume.

  Fine particles increase back-pressure. They can result in clogging, or they can pass through filters and contaminate the final product. Lack of fines gives a more regular, stable, and reproducible chromatographic bed with a faster and more even flow rate.
• The particle size distribution can be given as a percentage of particles smaller or bigger than a specified size or the median diameter.

Another way to represent particle size distribution is by the terms D10 and D90. These stand for specific particle sizes, namely the sizes where 10 % of particles are smaller, and 90 % of particles are smaller, respectively. The closer to 1 the ratio of these two is, the tighter the particle size distribution is.

 

In order to assess size distribution, SiliCycle’s QC lab uses both laser diffraction and sieving as explained below.


Laser diffraction
Malvern – Mastersizer analysis is a Dynamic Light Scattering (DLS) measurement. It measures the hydrodynamic diameter of particles in solution and provides a particle diameter distribution curve for the measured sample.
A laser beam passes through the sample that scatters light. Measuring the angular variation of the scattered light gives the particle size through a series of calculations. Small particles scatter light at large angles and large particles scatter light at small angles. We report the particle size as a volume equivalent to sphere diameter.

Sieving



In contrast, for sieving analysis, the sample passes through a series of sieves with mesh sizes that are progressively smaller. Each sieve retains a quantity of material that is then weighed.

We determine the particle size distribution this way but no distribution curve is available with this technique.

On silica with a median particle size of 25 microns or larger, both diffraction and sieving analyses can be conducted. For particles smaller than 25 microns, we use only laser diffraction because the particles are too small for the sieves to give accurate results.

What is the relation between the mesh size (mesh number) and the particle size?

Microns	Mesh
60 - 120	120 - 230
60 - 200	70 - 230
120 - 200	70 - 120
200 - 500	35 - 70
500 - 1,000	18 - 35

Microns	Mesh
5 - 20	625 - 2,500
15 - 25	325 - 625
15 - 40	400 - 1,250
20 - 45	325 - 625
40 - 63	230 - 400

The mesh size refers to the mesh number (which is a US measurement standard) and its relationship to the size of the opening in the mesh.

The mesh number is in fact the number of openings in one linear inch of screen. The higher this number, the smaller the particles are and vice versa.
• Specific surface area is a means through which a solid interacts with its surroundings. It affects how the silica gel interacts with the compounds in the sample, its reactivity, and adsorption capacity. Also, more pores or larger pores increase the surface area.
• Brunauer-Emmett-Teller (BET) analysis gives specific surface area, pore diameter, and pore volume. Degassing the sample is the first step. This removes water and other adsorbed vapors that may influence the results and thus leaves the surface exposed for the analysis. Degassing occurs at the highest temperature that does not denature or cause any structural change to the silica.

  Then under increasing pressure, an isotherm plots the relative pressure against the amount of adsorbed gas on the particle analyzed. Data points on the graph help determine the specific surface area, pore diameter, and pore volume. When relative pressure is close to 1, the gas fills the pores, and this gives the pore volume. When the relative pressure is low, it gives the specific surface area.

Tests Report

The measures that are not inherent to the silica particle’s specifications are listed in this section. Color and volatile content are tested on every gel, and elemental analysis is only tested on grafted silica.

• Volatile content is measured by humidity, at 160°C for non-functionalized silicas, and 110°C for the functionalized ones. It includes water and other solvents. Silica gel is hygroscopic and water content can change when exposed to ambient air like during transportation and storage. A consistent volatile content ensures that it does not affect the gel’s efficiency from one time to the other since there is a relationship between sorbent water content and retention characteristics.

  The Brockmann & Schodder Activity Test (Application Note SF002-0) can be used to evaluate activity grade of a silica gel based on the water content.

• Color is a visual test and is observed by one of our non-color-blind QC technicians. As trivial as this might seem, it allows us to notice quickly when something is wrong with the gel’s synthesis.
• Elemental Analysis (CNS) is only performed on functionalized gel and gives carbon, nitrogen, and sulfur content. In the case of SiliaMetS Thiol, sulfur content is naturally the specification used to control the product. The percentage represents how much of the molecule consists of carbon, nitrogen, and sulfur.

Physical Properties

These properties are calculated and not measured. They are mostly derived from the data gathered in the previous section and are only relevant on bonded silica.

• The carbon, nitrogen or sulfur content serves to calculate carbon, nitrogen or sulfur loading using the product’s molecular formula. Loading is thus a measure of the active part of the functionalized product and is useful in determining its activity. Higher loading equals more active functions, and it can be used to calculate how much metal scavenger is needed for your purification.
• Surface coverage and molecular loading are calculated from the carbon, nitrogen or sulfur loading observed depending on the bonded phase’s chemical structure.

  Surface coverage, very similar to molecular loading, represents the number of functions grafted on silica particles. The difference is that it is for the surface of the silica (including the surface within the pores, in m²) and not for a quantity of silica (in g).

• Endcapping is as simple as a yes or no question: has this product been endcapped? Endcapping is a process of reacting the residual silanol groups on the surface to make them unavailable. In some cases, a sorbent can come both in an endcapped and non endcapped (nec) version. To understand more on endcapping and why it is done, check our FAQ section on this.

Cartridge Visual Tests

First, the sorbents are tested in bulk and, when packed in a cartridge, the packed cartridge is tested too with a series of visual tests.

The QC technician (the color-blind one can do it now) checks if all the components are present, leak-free, and sturdy.

The cartridge’s COA will include all the sorbent’s observed properties and the cartridge’s visual tests. For flash cartridges, this might look like the image above. For SPE cartridges, there are slight differences.

Other Tests

Other tests that do not appear on the COA, namely purity assay and scavenging efficiency, are systematically performed on SiliCycle’s products before the quality control experts release them.

• Purity is observed by GC-MS with the aim to reduce metal content and maximize purity. As a result, SiliCycle’s typical metal content is lower than other manufacturers.

  Typical metal content comparison for 40 - 63 µm, 60 Â Silica Gels (mg/kg)
  Metals		SiliCycle	Manufacturer A	Manufacturer B
  Aluminum	Al	33	262	280
  Barium	Ba	9.4	59.7	32.5
  Calcium	Ca	336	1,150	502
  Chromium	Cr	0.5	0.6	0.4
  Copper	Cu	0.2	0.2	0.2
  Iron	Fe	32	75	41
  Lead	Pb	0.41	5.3	0.95
  Magnesium	Mg	61	149	104
  Nickel	Ni	0.4	0.5	0.5
  Silver	Ag	0.09	0.29	0.19
  Sodium	Na	466	945	585
  Tin	Sn	0.2	0.2	0.1
  Titanium	Ti	147	250	179
  Zirconium	Zr	32	75	56

  Aluminum, iron and lead are the most problematic because they cause peak tailing. Furthermore, when you use a metal scavenger to remove metals from your products, you do not want this scavenger to introduce more metal impurities in your products.

• Scavenging efficiency
  To control the quality of our products and make sure that they are effective in what they are supposed to do, we test SiliaMetS metal scavengers in scavenging efficiency for Pd removal, and measure the resulting Pd content in the mixture post scavenging using ICP-OES or ICP-MS.
• In a similar fashion, HPLC columns are tested for the separation of a known mixture. The separation test description and chromatogram appear on the HPLC column’s COA, as shown below, as a way for you to control if the column has lost any efficiency by running the same separation.

  COLUMN QUALITY CONTROL
  COLUMN VISUAL TEST	COLUMN PACKING EVALUATION
  Visual tests are made on SiliCycle® HPLC column to provide customers with default-free products to ensure 100% satisfaction.	To ensure the packing of the column, a standard separation is performed to validate the characteristic chromatographic experimental parameters.

  
  

  SEPARATION TEST DESCRIPTION Wavelength 254 nm Sample mixture in mobile phase Uracil(1) / Phenol(2) Nitrobenzene(3) / Naphtalene(4) Injection volume 1 µL Temperature 30°C Flow rate 1.0 mL/min Mobile phase 20% Water 80% Methanol OBSERVED COLUMN PARAMETERS (NAPHTALENE) Item Specification Test result Retention time (min.) 5.9 ± 0.50 5.640 Theoritical plates number ≥ 17000 19063 Tailing factor 0.98 ≤ TF ≤ 1.12 1.09 Observed backpressure (bar): 178

  CHROMATOGRAM

Quality Guaranteed

A product that does not pass any of these tests will go back to the production team to be corrected. When this is not possible, the product is destroyed. Non-conforming products are never shipped out, no matter how nicely you may ask. With ISO 9001:2015 certification, SiliCycle guarantees that the products out of the plant consistently meet customer and regulatory requirements.

If you are still unsure about the silica gel’s quality, the FDA classified silicon dioxide as Generally Recognized as Safe (GRAS), meaning it can be safely used for food and drug applications. (App note G001-0)

In addition, every shipment comes with the product’s corresponding COA. If by any chance you are missing yours (looking at you custom agents), just reach out to your representative for a new copy and they will be happy to get it for you.

Learn More

Application Notes:

• Brockmann & Schodder Activity Test (Application Note SF002-0)
• Generally Recognized as Safe (GRAS)

FAQs:

• Why are fines so important
• Silica analysis descriptions

Articles:

• Key physical characteristics of silica in chromatography

Videos:

• Understanding PSD

Tools:

• Mesh converter
• Calculating amount of scavengers

Other:

• SiliCycle’s quality control