Alpha Resources smooth wall tin capsules for CHNS elemental analysis

Tin vs Silver Capsules for CHNS Elemental Analysis

At LabSupplies.com, the encapsulation decision comes down to one question first: are you determining oxygen? If you are, use silver. If you are not, tin is the default for CHNS and costs less. Everything after that — pressed or smooth-wall, light-weight or standard, ultra-clean or not — is about how low your blank needs to go and how often you are willing to stop and clean out the reaction tube.

This guide covers the material decision, the format decision, and how capsule mass quietly drives your ash interval and your minimum sample size.

Tin vs Silver: The Material Decision

Tin is the default for CHNS. Tin is not inert in the combustion step — it is doing work. Tin oxidises exothermically in the oxygen pulse, producing a local temperature spike that drives the sample to combust completely. That flash combustion is why tin is standard for carbon, hydrogen, nitrogen and sulfur determination.

Silver is for oxygen determination. That same helpful tin oxidation becomes a problem when oxygen is the analyte — forming tin oxide consumes oxygen from the sample and biases the result. Silver avoids the side reaction. The practical convention in many laboratories is to weigh into tin for C and N determinations and into silver for H and O determinations.

Silver is also the right call for samples that have been acid-treated to remove carbonates, because silver tolerates residual acid that would attack tin.

Analyte Capsule Why
C, H, N, S Tin Exothermic tin oxidation drives flash combustion
Oxygen Silver Avoids tin oxide formation consuming sample oxygen
Acid-treated samples Silver Tolerates residual acid that attacks tin
Total organic carbon after acidification Silver Same — residual acid

Pressed vs Smooth-Wall

Pressed capsules are formed with a crimped or folded wall and are designed to be closed by folding. They suit powders and most solids, and are the general-purpose choice.

Smooth-wall capsules have a clean cylindrical wall and a flat base. The flat base matters more than it sounds: it lets the capsule stand upright on the balance pan while you weigh into it, which is the difference between a manageable weighing session and a frustrating one when you are running dozens of samples. Smooth-wall is also the usual choice for liquids and viscous samples, where wall geometry affects how cleanly the capsule seals.

Light-Weight and Ultra-Clean Grades

Two variants are worth understanding because they solve different problems.

Light-weight capsules use less metal. Less metal means less residue after combustion, which means the reaction tube accumulates ash more slowly and you can run more samples between clean-outs. It also means a lower blank, which lowers the minimum sample size you can work with — useful when sample is scarce, as it often is in isotope and environmental work.

Ultra-clean grades are processed to reduce residual carbon and nitrogen on the metal surface. This is a blank reduction, not a mass reduction. Use them when you are quantifying at low levels and the capsule itself is a meaningful part of your signal.

The two are independent — you can have a standard-weight ultra-clean capsule, or a light-weight standard-cleanliness one. Pick based on whether your constraint is ash interval or blank level.

Sizing: Match the Capsule to Sample Volume, Not Sample Mass

Capsules are specified by height and diameter, and the useful figure is internal volume against your sample's bulk density. A low-density plant tissue and a dense soil at the same mass need very different capsules. Size so the sample fills the capsule without packing it — you need to be able to fold the capsule closed cleanly, and a capsule that will not close will leak sample into the autosampler.

Then check your autosampler. Carousel wells are dimensioned for a range of capsule sizes, and a capsule that is technically fine for your sample can still be wrong for your instrument.

Handling and Storage

Handle capsules with clean tweezers, never fingers — skin transfers carbon and nitrogen directly onto the surface you are about to combust, and it is one of the most common causes of an unexplained blank drift. Keep containers closed between weighing sessions, and if you are working at low levels, take capsules from a freshly opened container rather than one that has been open on the bench for weeks.

Selecting the Right Capsule for Your Application

If you are running… Material Format
Routine CHN on solids Tin Pressed, standard weight
Oxygen determination Silver Pressed or smooth-wall
Acidified / carbonate-removed samples Silver Pressed
Liquids and viscous samples Tin Smooth-wall, flat base
Scarce sample, low-level quantitation Tin or silver Light-weight, ultra-clean
High sample throughput Tin Light-weight (longer ash interval)

We stock tin, aluminum and silver capsules in pressed, smooth-wall, light-weight and ultra-clean grades, cross-referenced to the OEM part numbers your method already names. Browse tin and aluminum capsules, or start from your instrument: Elementar-compatible, Thermo Fisher-compatible, Costech-compatible and PerkinElmer-compatible consumables.

Related reading: crucible selection for carbon and sulfur analyzers, choosing a lab balance for the microweighing step, just-in-time inventory for consumables, and lab PPE requirements.

Frequently Asked Questions

Should I use tin or silver capsules for CHNS analysis?

Use tin for carbon, hydrogen, nitrogen and sulfur. Tin oxidises exothermically during the oxygen pulse and that temperature spike drives complete combustion, which is exactly what you want for CHNS. Switch to silver when you are determining oxygen, because tin oxide formation consumes oxygen from the sample and biases the result.

Why do some methods specify silver capsules for acidified samples?

Samples treated with acid to remove carbonates can retain residual acid, which attacks tin. Silver tolerates it. This is why silver is commonly specified for total organic carbon work where the sample has been acidified before analysis.

What is the difference between pressed and smooth-wall capsules?

Pressed capsules have a crimped or folded wall and are the general-purpose choice for powders and solids. Smooth-wall capsules have a clean cylindrical wall and a flat base that lets them stand upright on a balance pan, which makes weighing easier and suits liquids and viscous samples.

What do light-weight capsules actually do for me?

They use less metal, so less residue reaches the reaction tube. That means ash accumulates more slowly and you can run more samples between clean-outs, and it lowers the blank, which in turn lowers the minimum sample size you can quantify reliably.

Is an ultra-clean capsule the same as a light-weight one?

No, and they solve different problems. Ultra-clean refers to surface processing that reduces residual carbon and nitrogen on the metal — a blank reduction. Light-weight refers to using less metal — an ash and blank reduction by mass. The two are independent and can be combined.

How do I choose a capsule size?

Size to internal volume against your sample's bulk density, not to mass alone, so the sample fills the capsule without packing it and the capsule still folds closed cleanly. Then confirm the dimensions against your autosampler carousel, which is built for a specific range of capsule sizes.

Running a new lab? LaunchLab gives new laboratories 15% off consumables for their first 12 months.

Edited by Ray Dillman-Neu, Co-Founder, LabSupplies.com. LabSupplies.com supplies aftermarket consumables, certified reference materials and replacement parts for elemental analysis laboratories across the United States.

Specifications in this guide were verified against the published standards cited below. Always confirm consumable compatibility against your instrument manual and your validated method before substituting any part.

Sources and Technical References

Back to blog