TROUBLESHOOTING — Suppressed Conductivity Nitrate (NO₃⁻) Peak Below is a compact, practical troubleshooting guide you can work through step-by-step. Start at quick checks, then follow the diagnostic tests and the symptom → likely cause → fix table. I’ll keep it hands-on and prioritized for the issues labs actually see.
Common symptoms, likely causes and fixes
Symptom: No or very small nitrate peak (low sensitivity)
- Likely causes
- Suppressor exhausted or current off/misaligned
- Poor eluent strength (wrong concentration) or contaminated eluent
- Conductivity cell shorted, dirty, or zero drift
- Sample matrix with high ionic strength / sample dilution too high
- Column voided or damaged
Run a system blank (matrix blank, mobile phase only). Look at baseline noise/drift and any ghost peaks.
Inject a fresh nitrate standard (known concentration). Compare retention time (RT), area, and shape to expected.
Check suppressor status: current setpoint, regeneration water feed (if required), and any alarm messages.
Inspect baseline: is it high, noisy, drifting, or with CO₂ spikes?
Check column and guard: installed correctly, not bypassed, correct flow direction.
Verify degassing and eluent prep: fresh, correctly mixed, and degassed eluent.
Confirm instrument temperatures (column oven) and pump pressures/backpressure
Column contamination or overloaded column
Sample contains organic material or surfactants
Dead volume (loose fittings, long tubing, large injector loop)
Suppressor inefficient / partial suppression
Flush or backflush column (if manufacturer allows), change guard/column.
Pre-treat sample (filtration, SPE cleanup) or reduce injection volume.
Shorten or replace transfer tubing, tighten fittings.
Check suppressor condition and regeneration.
Coelution with another anion (e.g., nitrite or organic anions)
Injection port problems (air bubble, partial loop fill)
Column phase change (damage/contamination)
Run high-resolution column or adjust eluent strength/gradient.
Check autosampler aspiration/dispense, purge lines, ensure loop fills fully.
Replace column or perform cleaning protocol.
Symptom: Retention time shift (RT earlier or later)
Eluent composition/strength changed (wrong prep, concentration drift)
Flow rate incorrect or pump pulsation
Column aging or temperature change
- Re-prepare eluent to correct recipe; degas.
- Verify pump flow and check for leaks or air in pump.
- Stabilize column at correct oven temp; replace if RT drift persists.
Symptom: Ghost peaks / unexpected nitrates at blank
- Likely causes
- Carryover from autosampler or previous injections
- Contaminated vials, pipettes, or eluent
- Sample loop or lines not flushed
CO₂ ingress into eluent, degassing failure
Dirty conductivity cell or electronic noise
Suppressor malfunction (incomplete suppression)
Mobile phase contamination or particulate matter
Blank → Standard → Sample (same injection volume). If standard looks fine, issue is sample prep or matrix.
Swap conductivity cell (or connect to a second cell) to check detector.
Run alternate standard (chloride or sulfate) to check suppression across different anions.
Vary injection volume (reduce by 50%) to rule out overloading.
Change eluent concentration slightly to see effect on RT/resolution.
Bypass column test: inject standard directly after suppressor (if safe) to isolate column vs detector issues — only if instrument/protocol allows.
Replace guard column and re-inject standard.
Check peristaltic/regeneration water flow and tubing for electrolytic suppressors.
Acceptance criteria & quick verification
- Standard %RSD (area) ≤ 2–5% for replicate injections (lab dependent).
- Baseline noise less than specified in method (e.g., < 5 nS).
- Retention time RSD ≤ 0.2–1%.
- Peak shape: Gaussian with tailing factor < 1.5 (use vendor/method limits)
Suppressor current: manufacturer recommended value (confirm in manual).
Regeneration water flow: steady and as specified.
Eluent: correct salt(s), concentration, pH (if applicable), degassed, made with high-purity reagents.
Autosampler wash: strong wash solvent between injections for samples with organics.
Filter samples: 0.2–0.45 µm syringe filter to remove particulates.
Column care: use guard; flush column after runs; store per vendor instructions.
Conductivity cell check: measure cell constant with standard solution; inspect electrodes for deposits.
