The molybdenum spot test in pharma is a quick, on-the-spot positive material identification (PMI) screening test used mainly to confirm whether stainless-steel items in contact with product/utility are molybdenum-bearing grades (e.g., SS316/316L) rather than molybdenum-free grades (e.g., SS304). This matters because SS316 typically contains about 2–3% molybdenum (Mo), while 304 is essentially Mo-free.
Where it is used in pharma
- IQ/commissioning: verify MOC of tanks, pipelines, valves, fittings, tri-clamps, and critical spares before system release.
- Maintenance: confirm replacement parts (ferrules, bolts, small fittings) match the specified grade.
- Vendor receipt checks: fast segregation of mixed lots (304 vs 316) before fabrication/welding.
Principle
A proprietary acidic reagent (commonly based on thiocyanate chemistry) is placed on the metal surface. Mo-containing stainless steels cause a clear darkening/color change (often blue-gray to dark brown), while Mo-free steels show little or no change.
Typical procedure (field practice)
- Clean and dry a small, bare-metal area (remove oil/dirt; avoid coated surfaces).
- Place one drop of the test solution on the surface.
- Wait ~2–4 minutes and compare with known 304/316 references or the kit guide.
- Rinse/neutralize and wipe after reading to avoid prolonged acid contact and possible pitting.
- Record the result in the PMI log (item ID/location, date, kit lot/expiry, tester/signature).
Limitations / GMP expectations
A positive result confirms presence of Mo, not the exact grade; some duplex/high-alloy steels can also test positive. Use PPE (gloves, goggles) and dispose materials as chemical waste. For critical items, confirm by instrumental PMI (XRF/OES). Testing should be done on representative locations and not on finished critical surfaces without QA approval.