Short answer: You’re most likely resolving cis/trans geometric isomers of quaternium‑15 rather than seeing a split peak or an LC–MS artifact. Quaternium‑15 (a quaternary ammonium preservative also known as Dowicil‑75™/100™) is supplied as a mixture of cis and trans isomers around the chloro‑allyl double bond.

These isomers have identical exact mass but different 3‑D geometry, leading to distinct chromatographic interactions and two separate retention times—often both detected at the same precursor m/z in LC–MS. 

What’s happening chemically?

• Isomerism: Quaternium‑15 comprises cis (Z) and trans (E) configurational isomers at the allylic double bond. Commercial products (e.g., “Dowicil‑75™/100™”) typically contain a mixture of these isomers, whereas some grades (e.g., “Dowicil‑200”) may emphasize a specific isomer. The two isomers share the same elemental composition and thus the same m/z in MS, but their steric profiles differ—enough to elute at different times under common LC conditions. 

• Confirmed identity resources: Regulatory/cheminformatics records explicitly list cis‑quaternium‑15 as a validated substance entry; mass spectral libraries also include LC–MS/MS reference spectra for quaternium‑15 (free base), confirming [M+H]+ ≈ 216.11 and typical fragments.

How to confirm that the two peaks are isomers (not artifacts)

1. Exact mass & MS/MS: Acquire HRAM or QqQ‑MS/MS on both peaks. If precursor m/z and key fragments match (within tolerance) and relative ion ratios are similar, you’re likely observing geometric isomers rather than contaminants. 

2. Retention robustness: Slightly adjust organic strength, buffer, or temperature. Geometric isomers typically maintain distinct retention order, while split peaks from poor mixing/injection often collapse into one with method clean‑up. 

3. Spike with authentic material: Spiking a known quaternium‑15 standard should increase the area of both peaks proportionally if your sample already contains the isomer mixture. (Vendor references describe mixed cis/trans composition). 

4. Orthogonal selectivity check: Run a short selectivity screen (e.g., slightly altered pH/ionic strength or a different stationary phase) to see if the ΔtR persists. Cis/trans pairs remain separable across multiple conditions due to geometry‑driven interactions. 

Why LC mode matters (and why Diamond Hydride helps)

• ANP/normal‑phase selectivity: With polar/ionizable analytes, Aqueous Normal Phase (ANP) or related modes often enhance differential surface interactions that accentuate isomeric selectivity, producing clear two‑peak profiles. That’s why your Cogent Diamond Hydride™ (TYPE‑C) method can separate cis vs. trans quaternium‑15 while MS reports the same m/z for both. 

• General principle: Cis/trans isomers interact differently with stationary phases because of shape/steric and dipole/accessibility differences; many LC modes (RP, NP/ANP, occasionally chiral) can exploit these.

Practical method guidance (LC–MS)

Below is a pragmatic checklist you can adapt to your Diamond Hydride or other LC methods when quaternium‑15 is expected to show as two peaks:

1) Mobile phase & additives

• Volatile acid (e.g., 0.1% formic) or low‑level ammonium salt (e.g., 5–10 mM ammonium formate) in water/organic helps robust ESI while maintaining isomer resolution. Avoid non‑volatile buffers that suppress ionization. 

• For ANP‑like methods, keep organic content relatively high at start, then adjust aqueous fraction to tune retention—monitor both peaks’ resolution (Rs) as you iterate. 

2) Column & temperature

• Cogent Diamond Hydride™ (TYPE‑C) is appropriate for polar cationic analytes and can differentiate geometry. Start with 30–40 °C; modest temperature increases can sharpen peaks and tweak ΔtR without collapsing isomer separation.

3) Injection & diluent

• Use a diluent similar to initial mobile phase to avoid distortion/splitting unrelated to isomerism (e.g., methanol‑rich diluents can broaden or split peaks in ANP if mismatched). 

4) MS detection

• Positive ESI typically yields [M+H]+ ≈ 216.11 (free base form), with characteristic fragments; confirm both peaks share the same precursor and diagnostic fragments. If using HRAM, lock mass to keep ppm error tight for both peaks.

• For quantitative assays, monitor both peaks or sum their areas if the specification is total quaternium‑15; if you must report isomers separately, document which peak (earlier/later) corresponds to cis vs. trans under your exact conditions. (Many suppliers state mixtures are common.) 

Diagnostic decision tree

1. Two peaks, same m/z
   → Acquire MS/MS for each → fragments match → likely cis/trans. 

2. Peaks move together when you adjust %B/pH but remain distinct → isomers. If one disappears with improved injection solvent matching → injection mismatch artifact; fix diluent and re‑test. 

3. Only one peak in a reference grade sample (known single isomer) versus two in your sample → your sample contains the isomeric mixture; consider summing areas for total content. 

Reporting recommendations for regulated work

• System suitability: Define Rs ≥ 1.0 between the two peaks (or method‑specific), %RSD ≤ 2% for area/RT across replicate injections for each isomer. (Typical LC–MS performance practices; adjust to your SOP.) 

• Identification criteria: Same precursor m/z (within HRAM or QqQ tolerance), consistent MS/MS fragments, and RT window vs. standard. Use a bracketing standard to verify identity across sequences. 

• Quantitation: If compendial/label claim is total quaternium‑15, sum both peaks. If isomer‑specific limits apply, validate linearity, accuracy, precision per isomer. Reference libraries (e.g., MassBank) can assist in confirmatory fragment checks.

FAQ (quick answers for support)

• Q: Could one peak be a degradation product?
  A: Possibly, but quaternium‑15 is widely documented as a cis/trans mixture; if MS/MS matches on both peaks and there’s no new mass or unique fragment set, isomerism is the parsimonious explanation. 

• Q: Can I force them to co‑elute?
  A: You can reduce resolution with stronger organic, different buffer strength, or temperature change—but document the change. Many labs prefer to keep the resolution to avoid quantitation bias if isomer ratios vary by source/lot. Q: Which peak is cis vs. trans?

• A: Assignment can be method‑dependent (stationary phase and mobile phase determine relative order). If assignment is required, analyze enriched/certified isomer or use orthogonal conditions and MS/MS comparison. 

References & further reading

• MTC CRC article (original): “Why am I seeing two peaks for quaternium‑15 or Dowicil‑75™ in my LCMS data – FAQ.” (Explains cis/trans isomer separation on Cogent Diamond Hydride.) 

• Quaternium‑15 background and isomer info (cis/trans; product names including Dowicil‑75/100/200). 

• FDA/UNII record for cis‑quaternium‑15 (stereochemical designation).

• MassBank LC–ESI–QTOF reference spectra for quaternium‑15 (free base): precursor [M+H]+ ≈ 216.11, MS2 metadata. 

• LC–MS method and platform guidance (HRAM/QqQ considerations for complex matrices).