Adsorption on the tube wall, not pressure rating, most often spoils a result. Polar products such as methanol and formic acid stick to the oxide surface of stainless steel, so a long line can strip product before it reaches the detector. Use PTFE or glass for trace organics, stainless steel where pressure and heat dominate, and prove the choice with a blank run at your own line volume.
A dilute product stream loses a disproportionate share of its analyte to the first surface it touches. At low concentration the wall is far from saturated, so the fraction retained is largest exactly where the measurement is hardest — which is why the effect is routinely read as chemistry rather than as plumbing.
Three symptoms follow, and each one is usually blamed on the catalyst:
The magnitude scales with wall area per unit of gas volume. A narrow bore of the same length exposes more wall per unit of charge, so a long run of small-bore tubing between reactor and valve can hold more product than the sample loop that is supposed to represent it.
No material wins on every axis; the table shows which axis each one gives up, so choose by the axis your experiment cannot afford to lose.
| Material | Adsorption of polar products | Pressure and temperature | Wall permeation | Typical duty |
|---|---|---|---|---|
| Stainless steel, passivated | High for alcohols, acids and water; electropolishing reduces it | Highest of the three; suits reactors rated to 1.6 MPa and above | Negligible | High-pressure and high-temperature lines, permanent gases |
| PTFE | Low; the usual choice for trace polar organics | Low pressure only; creeps under sustained compression | Significant for H₂, CO₂ and O₂ over long dwell times | Low-pressure sampling lines, corrosive media, flexible routing |
| Borosilicate glass | Low and cleanable; blockage is visible | Atmospheric or slight vacuum; fragile under thermal shock | Negligible | Ambient liquid-phase and gas-solid rigs, short runs |
Read the table by elimination. If the line has to hold 1.6 MPa, as in the PLR-PTSR II reactor, stainless steel is the only candidate, and adsorption is managed by shortening the path and heating it rather than by changing material. If the line carries a few micromoles of methanol in a carrier gas, PTFE or glass wins, and the pressure question is answered by the reactor rather than by the tube.
The phase of the reacting mixture decides which failure mode you can afford, so settle it before comparing materials. In a gas-solid rig the whole loop belongs to the mass balance: a closed circuit holding a 65 mL line volume, as in the Labsolar-6A, circulates the same charge past every fitting, and each surface in that circuit both adsorbs and desorbs.
In a liquid-solid rig the liquid inventory dominates, and a small adsorption loss on a sampling line matters less — but only until the product becomes dilute. At micromolar concentrations the sampling line, not the reactor, is the limiting surface. Photocatalytic CO₂ reduction sits in the worst corner of both cases: low rates, polar products, and often a gas phase with a small inventory.
All three fail in ways that look like instrument drift rather than like a material problem. Stainless steel corrodes in chloride-containing electrolytes, and the resulting surface roughness increases adsorption exactly where it is least wanted; a hot wall can also catalyze reactions that the reactor was not meant to run. PTFE cold-flows at compression fittings, so a joint that was tight at assembly loosens over thermal cycles and produces a slow leak. Glass fractures at the joint, and ground joints seize after long exposure to water and heat.
A leak-rate acceptance test catches the PTFE case before it reaches the data. The system specification of ≤0.1 μmol/h (O₂) on a water-splitting rig sets the budget that every added fitting has to fit inside, so a line that cannot hold that figure is spending error budget that the experiment needs.
Run the four steps in order and stop at the first one that eliminates a candidate; that sequence prevents optimizing adsorption in a line that cannot hold pressure anyway.
Record tubing length, bore and material in the method section. When a result later fails to reproduce, the line is one of the first things to check, and it can only be checked if it was written down.
PTFE adsorbs little, but it is not a barrier. Small molecules such as H₂, CO₂ and O₂ permeate the wall, so a long PTFE run in a closed system loses pressure and composition over hours. Keep PTFE runs short where the mass balance matters, and use it where adsorption is the bigger risk.
Yes on the high-pressure side, where nothing else will hold the pressure. Keep the run from reactor to valve as short as the layout allows, heat it above the dew point of the products so they do not condense on the wall, and confirm with a blank that recovery is acceptable.
Compare two configurations: the installed line, and the shortest line you can connect. Run the same charge through both and compare peak areas. A consistent shortfall in the long configuration is line loss; a shortfall in both points upstream. Then repeat with a known addition to separate adsorption from leak.
