Sticky Gases: Why Some Substances "Stick" to Walls and How to Deal with It

Introduction

In gas analytical practice, there is a group of substances that require a special approach. These are the so-called "reactive" or "sticky" gases, which are capable of adsorbing onto the internal surfaces of sampling lines, fittings, and measurement cells. Unlike "light" and inert gases (N₂, O₂, CO), they do not pass through the sampling path freely — some molecules are "lost" along the way, settle on the walls, and then are gradually released, distorting subsequent measurements.

I have touched upon this topic before using the example of nitrogen oxide (NO), but now I want to analyze it systematically: which gases belong to this category, what is the physical nature of the phenomenon, how it manifests itself in practice, and, most importantly, what engineering solutions can minimize the associated errors.

Which Gases Are Considered "Sticky"

The category of reactive or "sticky" gases includes those that have polar molecules, tend to form hydrogen bonds, or dissolve easily in water. According to gas analyzer operating manuals, such gases include: nitrogen dioxide (NO₂), chlorine (Cl₂), hydrogen chloride (HCl), hydrogen fluoride (HF), and ammonia (NH₃). In the broader list encountered in industrial monitoring, they are joined by: nitrogen oxide (NO), sulfur dioxide (SO₂), hydrogen sulfide (H₂S), ozone (O₃), water vapor (H₂O), as well as a wide range of volatile organic compounds (VOCs) and mercaptans.

These substances can be roughly divided into two groups based on the nature of their impact on the sampling path:

• Acidic gases (SO₂, HCl, HF, NO₂, Cl₂): actively interact with metal surfaces, especially in the presence of moisture. The resulting acids are not only adsorbed but also cause corrosion.

• Alkaline and polar gases (NH₃, NO): tend to form hydrogen bonds with oxide films on metal surfaces and also dissolve readily in condensate. Studies show that ammonia exhibits a noticeable delay when passing through a stainless steel sampling system.

The Physical Nature of the Phenomenon

The "sticking" mechanism involves two main processes:

Adsorption is the accumulation of gas molecules on a solid surface due to intermolecular interaction forces. Unlike absorption (uptake into the bulk volume), this is an interaction specifically with the surface. Gas molecules form a monomolecular layer on the tube walls — and stay there.

Desorption is the reverse process, in which molecules leave the surface and return to the gas phase. It is triggered by a decrease in the gas concentration in the flow, an increase in temperature, or a change in pressure.

The key problem is that adsorption occurs quickly, while desorption is slow and uneven. This is due to the diffusion of molecules from the bulk gas to the surface: signal formation can take from several minutes to hours.

How the Phenomenon Manifests in Practice

In practice, the "stickiness" of gases leads to three main negative effects.

Understated readings. As long as the system is not saturated, some of the gas molecules settle on the walls and do not reach the analyzer. Studies of SO₂ losses have shown that on PTFE tubing, losses can range from 1.1 to 11.5% depending on the system design. For more active gases, this percentage is significantly higher.

Memory effect (overstated readings when purging with zero gas). After a tube has been saturated with gas molecules, when switching to zero gas (pure nitrogen or air), these molecules begin to desorb. As a result, the analyzer indicates the presence of gas where none should exist. It is this effect that requires particularly long purging of the path.

Increased response time. The gas does not appear at the analyzer immediately after being supplied to the sampling line inlet. The system needs time to first saturate the walls, and only then is an equilibrium established at which the concentration at the inlet and outlet equalizes. Delays are especially noticeable for organic compounds and can reach tens of minutes.

The Role of Sampling Line Material

The choice of tubing material is a critically important factor. Studies of delays for a wide range of gas-phase compounds on various materials have shown that the best results among the tested options are demonstrated by conductive PFA and Silonite tubing.

In industrial practice, the following approaches are applied.

PTFE (Teflon) and PFA. Chemically inert, minimally adsorb most gases. According to research, PTFE is the best choice for sampling sulfur-containing compounds; however, noticeable losses are still observed for SO₂ on this material. PTFE and glass are very resistant to all sample components.

Stainless steel. For active gases (NO₂, SO₂, HCl), untreated stainless steel is a poor choice. Chemical transformations occur on its surface: the reduction of NO₂ to NO in an oxygen-free environment and oxidation at temperatures above 400°C in a metal tube in the presence of oxygen have been recorded. For such gases, steel tubes with special inert coatings are required.

Coatings. The technology of applying an inert coating to the internal surface of a tube prevents chemical adsorption and unwanted reactions, which is particularly important when measuring concentrations at the ppb and ppm level.

Engineering Solutions

The problem of sticky gases is not insurmountable. There are proven technical solutions to minimize the associated errors.

Heated lines. This is the primary and most effective solution. Heating the sampling line to a temperature above the dew point (usually 180–200 °C) prevents the formation of condensate, which dramatically reduces the adsorption of polar molecules. For gases such as HCl, HF, NH₃, and SO₂, a heated line is not an option but a mandatory requirement.

Surface passivation. Special chemical treatment of the internal surface of tubes (including the application of an inert coating — Sulfinert, Silonite) creates a barrier between the active centers of the metal and the gas molecules, preventing adsorption. The passivation process is particularly important when working with gases such as NH₃ and HCl.

Minimizing the path length. The shorter the sampling line, the smaller the total surface area available for adsorption. This rule always applies, regardless of the material. The sampling path must be designed so that, while retaining the necessary functionality, its length is minimized.

Increased purge time during calibration. When calibrating analyzers working with reactive gases, the standard supply time for zero or calibration gas must be increased. How much exactly depends on the specific gas and the geometry of the path. The criterion is the stabilization of readings at an unchanging level for several minutes.

Dynamic calibration. For reactive gases, traditional static calibration methods using pressurized cylinders may be inapplicable due to adsorption on the cylinder walls and uneven distribution of components. In these cases, dynamic dilution methods are used, in which the gas mixture is prepared immediately before being supplied to the analyzer by evaporating a liquid standard into a flow of balance gas.

Conclusion

Sticky gases are not an anomaly or a rare exception. They are a large group of substances encountered in industrial environmental monitoring and process control. Ignoring their characteristics leads to systematic measurement errors that can amount to tens of percent, yet remain invisible to the operator. Adsorption on the surfaces of the sampling path is one of the most frequently overlooked sources of error when troubleshooting gas analytical systems.

Conversely, competent consideration of these characteristics at the system design stage — the correct choice of materials, the use of heated lines, surface passivation, and increased purge time — allows the problem to be completely neutralized and reliable, reproducible measurement results to be obtained.