6PPD-quinone: how a tire antioxidant became a salmon killer, and how scientists actually find it

Panyawut Tonanon

08 Jun 2026 • 4 min read

The salmon mystery

For scientists studying the life cycle of US Pacific Northwest coho salmon (Oncorhynchus kisutch), one problem has been bothering the scientists: they observed that coho salmon usually dies in streams near human activities hours after exposure to rainstorms with no explanation. This is a big issue considering salmon requires returning to freshwater river to begins spawning the new generation of salmons. Through Bioassay-driven fractionation, Zhenyu Tian at colleagues (2021, Science) identified a single molecule from tire leachates with median lethal concentration of 0.8 ± 0.16 ppb, which is less than a sugar cube in an Olympic-size pool. This molecule is called 6PPD-quinone, a transformation product of 6PPD.

The molecule — What 6PPD is and why tires need it?

6PPD, under IUPAC name N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, is an antioxidant / antiozonant additives added to tires. When exposed to natural elements, synthetic and natural rubber can be broken down causing weather cracking, hence, 6PPD are added to prevent such cracking. In a typical passenger tire formulation, 6PPD can accounts for ~1-2% of the tire mass by weight (it was listed as anti-ageing, Continental Tires).

6PPD works by creating a sacrificial protective shield (meaning 6PPD reacts with oxidant/ozone faster than the rest of the tire) over the tire surface. Once exposed, 6PPD can reacts with atmospheric O3, OH radical or other reactive oxygen species (ROS) in the presence of UV to form 6PPD-quinone. This reaction is intentional, it's literally how 6PPD protects the rubber; its sacrifice itself to ozone.

6PPD-quinone is classified as "very highly toxic" to aquatic organisms, with LC50 in coho salmon at less than 0.1 ug/L (Tian et al., 2021, and Brinkmann et al., 2022). The toxicity is also species-specific, as it is acutely toxic to rainbow trout, brook trout, and white-spotted char, but not toxic at environmentally relevant concentrations to Arctic char and white sturgeon. The mechanism of toxicity is not very well-understood, but was suspected of arising from toxicodynamic (different species handle the molecule differently) and toxicokinetic (different molecular targets) factor.

The analytical challenges — tiny amounts, messy matrices

Detecting and quantifying 6PPD-quinone in environmentally relevant concentrations is hard for a few reasons.

First, the concentrations are tiny — typically low ng/L (ppt) in surface water, with brief storm-event spikes that can be 40⨉ higher. Second, the matrix is messy: stormwater contains road dust, fuel residues, combustion byproducts, and thousands of dissolved organic matter features. Third, 6PPD-quinone is one of a family of p-phenylenediamine antioxidants and their transformation products, which share similar molecular features. The parent 6PPD itself, diphenylguanidine (DPG), and various N,N'-disubstituted PPDs all show up in the same chromatogram and overlap in the m/z dimension.

For the first wave of 6PPD-Q studies, the analytical method of choice was liquid chromatography–tandem mass spectrometry (LC-MS/MS) with multiple-reaction monitoring (MRM). MRM is sensitive and quantitative, but it answers the question "is 6PPD-Q exactly this molecule here?" rather than "what is everything in this sample that I might be missing?" And in 2024, when a group looking at disinfection byproducts asked the second question, they got a worrying answer.

The chlorination twist: disinfection is making it worse

When 6PPD-Q reaches a wastewater treatment plant, it encounters free chlorine — typically sodium hypochlorite, dosed for disinfection. A 2024 study in Water Research looked at what happens. The answer is uncomfortable.

With hypochlorite, 6PPD-Q reacts rapidly (pseudo-first order, pH-dependent) to form twelve transformation products, eleven of which are chlorinated. With chlorine dioxide, the picture is somewhat different: only four TPs, less chlorinated, more hydroxylated. In both cases, the proposed pathways involve Cl substitution, ring cleavage, ring closing, and hydroxylation — the alkyl branch of the molecule stays intact.

The bad news is that chlorination enhances toxicity. The more chlorine atoms substituted onto the 6PPD-Q scaffold, the more toxic the products become to aquatic organisms. The disinfection step that water utilities rely on to reduce toxicity is, in this specific case, creating more toxic byproducts.

This is what the technique I covered in the previous post (adding collision cross section to your identification) is built for. When you have twelve or more transformation products of a parent compound, sharing similar masses and chromatographic behavior, CCS separates them in a dimension that no amount of method optimization on the LC side can match. And when you have a chlorinated analog you have never seen before, a non-target screening workflow with HRMS + CCS + retention-time matching can flag it as a candidate even without a reference standard.

What it means for analytical chemistry

The 6PPD-Q story is a stress test for modern environmental analysis. It has all the elements that make a compound analytically difficult: trace concentrations, complex matrices, multiple isomers and transformation products, species-specific toxicology, and a fast-moving regulatory landscape. Commercial standards only became available in 2021. The chlorination TPs were published in 2024. We do not yet have a complete inventory of the transformation products, let alone their toxicological profiles.

Three questions are open right now:

How much 6PPD-Q and its transformation products are we missing in routine monitoring?
What is the complete transformation network under realistic disinfection conditions, including chloramination, UV, and ozone?
Of the dozens of transformation products, which ones are the actual toxicants?

These are not just scientific questions. They will shape the regulatory response, from water-quality guidelines to source-control measures on tire wear, over the next decade. The lab techniques that answer them are the same ones this blog is built around: high-resolution mass spectrometry, ion mobility separation, non-target screening, and increasingly, machine-learning-predicted CCS values for compounds that have never been measured.

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