SilcoNert Coatings for Trace Gas Analysis: Ensuring Sample Integrity at the Parts-Per-Billion Level

The Challenge of Trace-Level Sampling

When process engineers need to measure gas-phase contaminants at the parts-per-billion (ppb) or low parts-per-million (ppm) level, the sample pathway itself becomes a potential source of error. Bare stainless steel surfaces are reactive at the molecular level. Active sites on untreated metal surfaces adsorb and desorb trace compounds, causing the measured concentration to be lower than the actual process concentration. This phenomenon is particularly severe for polar and reactive species such as hydrogen sulfide (H2S), mercaptans, sulfur dioxide (SO2), mercury (Hg), and moisture (H2O).

For routine gas analysis where the compounds of interest are present at percent-level concentrations, surface adsorption is negligible relative to the bulk concentration. But at ppb-level measurements -- the domain of environmental monitoring, natural gas quality specifications, and emissions compliance -- adsorptive losses of even a few percent can make the difference between a passing and failing result. This is where inert coatings for sample probes and transport tubing become essential.

What Are SilcoNert Coatings?

SilcoNert is a family of proprietary surface treatments developed by SilcoTek Corporation that apply a thin, chemically bonded silicon layer to the interior surfaces of stainless steel components using chemical vapor deposition (CVD). The CVD process diffuses a silicon-based precursor gas into a vacuum chamber containing the parts to be treated. At elevated temperature, the precursor decomposes and bonds to the metal surface at the molecular level, creating an amorphous silicon coating that is typically less than 1 micron thick.

Unlike electroplating or paint, the CVD coating follows the exact contour of the substrate surface, coating even complex internal geometries such as the bore of a probe tube, the internal passages of valves, and the cavity of regulators. The result is a chemically inert barrier between the reactive metal surface and the sample gas.

SilcoNert 2000 (Sulfinert)

SilcoNert 2000, also marketed under the trade name Sulfinert, is the most widely specified inert coating for sulfur compound analysis. It is specifically engineered to minimize the adsorption of:

• Hydrogen sulfide (H2S) at ppb to low-ppm levels
• Sulfur dioxide (SO2)
• Carbonyl sulfide (COS)
• Mercaptans (methyl mercaptan, ethyl mercaptan)
• Elemental mercury (Hg)
• Low-level moisture (H2O)

SilcoNert 2000 reduces surface adsorption by more than an order of magnitude compared to untreated 316 stainless steel. For a typical 1/4-inch OD sample transport line running 50 feet from a probe to an analyzer, the difference between a coated and uncoated pathway can mean the difference between measuring the actual 50 ppb H2S concentration and reporting less than 10 ppb due to adsorptive losses on bare metal surfaces.

Maximum service temperature: Approximately 840 degrees F (450 degrees C)

SilcoNert 1000

SilcoNert 1000 is a hydrocarbon-inert coating designed for applications where the primary concern is adsorption of non-polar or weakly polar organic compounds. It is commonly specified for:

• Trace-level volatile organic compound (VOC) analysis
• Refinery gas analysis where heavy hydrocarbon carryover is a concern
• Applications requiring an inert surface for both aqueous and organic media

SilcoNert 1000 provides good inertness for hydrocarbons but is less effective than SilcoNert 2000 for sulfur-specific applications. The selection between the two depends on the target analytes.

Maximum service temperature: Approximately 840 degrees F (450 degrees C)

When Are SilcoNert Coatings Needed?

Not every sample probe requires an inert coating. The decision depends on the measurement requirements and the concentrations of the target analytes.

Coatings Are Strongly Recommended When:

• Sulfur species (H2S, SO2, COS, mercaptans) must be measured at less than 1 ppm, and especially below 100 ppb. At these concentrations, surface adsorption on untreated stainless steel can consume a significant fraction of the analyte.
• Elemental mercury must be measured at nanogram-per-cubic-meter levels, as required by EPA Method 30B and similar regulations. Mercury is extremely prone to adsorption on metal surfaces.
• Moisture must be measured at low-ppm or ppb levels in specialty gases, semiconductor process gases, or natural gas dehydration monitoring.
• The sample transport pathway is long (more than 10-15 feet from probe to analyzer), increasing the total surface area available for adsorption.
• Response time is critical, because adsorption and desorption create hysteresis that delays the analyzer's response to changing process concentrations.

Coatings Are Generally Not Needed When:

• The analytes of interest are at percent-level concentrations (e.g., methane, ethane, CO2 at greater than 1%)
• The sample transport path is very short (probe directly adjacent to analyzer)
• The process conditions exceed the temperature limits of the coating
• Cost constraints prohibit the coating and the measurement uncertainty is acceptable without it

Electropolish: A Lower-Cost Alternative

Electropolishing is a surface finishing process that removes a controlled layer of material from the inside of the tubing, smoothing the surface and reducing the total surface area at the microscopic level. While electropolish does not provide the chemical inertness of a CVD coating, it offers meaningful improvements over as-drawn or mechanically polished tubing.

How Electropolish Compares to SilcoNert

Electropolish is a reasonable choice when measuring sulfur compounds in the low-ppm range (1-10 ppm), where some adsorption can be tolerated without significantly biasing the result. For sub-ppm and ppb-level measurements, SilcoNert 2000 provides measurably superior performance.

Specifying Coated Sample Probes

When ordering a sample probe assembly with SilcoNert coating, the following considerations apply:

What Gets Coated

• Probe tube interior bore: The primary sample contact surface from the probe tip to the packing gland outlet
• Isolation valve internals: The valve body, ball or stem, and seats that contact the sample
• Transport tubing: All tubing from the probe outlet to the analyzer inlet
• Fittings and unions: Every wetted surface in the sample pathway

The entire wetted path from probe tip to analyzer must be coated for the coating to be effective. A single uncoated component in the pathway creates an adsorption site that undermines the investment in the coated components.

Procurement Notes

• SilcoNert coating is applied after final machining and before assembly. Components are shipped to SilcoTek (or a licensed applicator) for coating and then returned for assembly.
• Coating lead times typically add 2-4 weeks to the overall probe delivery schedule
• Coated components should be capped and sealed during shipping and storage to prevent contamination of the inert surfaces
• The coating is permanent and does not wear off under normal service conditions, but it can be damaged by abrasive cleaning or exposure to strong bases (pH above 10)

Benefits for Sample Integrity

The practical benefits of specifying SilcoNert-coated sample pathways extend beyond simple accuracy improvement:

• Faster analyzer response: Coated surfaces reach equilibrium almost immediately, eliminating the hours-long conditioning period required for bare metal surfaces when measuring trace sulfur
• Reduced calibration drift: Because the coated surface does not participate in adsorption/desorption equilibria, analyzer calibration is more stable over time
• Lower detection limits: The full analyte concentration reaches the detector, enabling reliable measurement at concentrations that would be undetectable through an uncoated sample system
• Compliance confidence: For regulatory measurements (EPA methods, custody transfer specifications), coated sample pathways provide documented assurance that sample integrity is maintained from extraction point to analyzer

The Bottom Line for Process Engineers

If your analytical requirements demand accuracy at the ppb or low-ppm level for sulfur compounds, mercury, or moisture, specifying SilcoNert 2000-coated sample probes and transport tubing is one of the most impactful investments you can make in measurement quality. The coating eliminates the largest single source of sample bias in trace-level gas analysis: the adsorptive loss of reactive analytes on untreated metal surfaces between the process pipe and the analyzer detector.