Non-specific binding of analytes to metal surfaces inside HPLC columns and hardware can negatively affect quantitation of both analytes and impurities. Preventing these interactions requires “passivation,” a series of steps that treat metal surfaces with multiple rinses of water, solvent, and 6N nitric acid.

Waters recently introduced Arc™ Premier, reportedly the first MaxPeak™ high performance surface liquid chromatography (LC) system optimized for 2.5–3.5-micron columns. MaxPeak Premier columns virtually eliminate surface interactions occurring between analytes and instrument/column hardware, thereby reducing or eliminating the need for passivation, according to the company.

Analyte-surface interactions are common when running metal-sensitive analytes like phosphates, carboxylic acids, certain lipids, oligonucleotides, glycans, and many other compounds with metal-complexing capabilities, particularly when analytes or impurities are present at low concentrations. Such analytes are often under-reported or missed entirely. Analysts can waste many hours troubleshooting methods when the culprit is actually the instrument.

MaxPeak HPS is a proprietary, hybrid organic/inorganic surface treatment that reduces or eliminates Lewis acid-base-type interactions on the column, notes Patrick Flanagan, principal product marketing manager, Waters. It works by creating a barrier between the sample and the metal surfaces of both the system and column.

Waters claims additional benefits as well in analyte recovery, detector sensitivity, and peak shapes leading to improved reverse-phase and HILIC analysis of organic acids, phosphates, oligonucleotides, peptides, glycans, and phospholipids. Perhaps the main advantage is in system-to-system and user-to-user consistency and reproducibility.

According to Flanagan, non-specific interactions negatively affect the quantitation of both analytes and impurities, leading to under-reporting or missing these species entirely.

“Passivation involves multiple rinses with water, solvent, and six-normal nitric acid, and can take hours or days. Treatment time is variable and depends on the application, and how well the system was passivated,” he tells GEN. “Waters has developed a proprietary hybrid organic/inorganic surface technology that reduces or eliminates Lewis acid-base-type interactions by creating a barrier between the sample and the metal surfaces of both the system and column.”

Downside of nitric acid passivation

The big knock-on nitric acid passivation is the material is highly corrosive and requires care in its disposal. Companies who for whatever reason do not purchase pre-passivated columns might look into treatment with the much milder citric acid. That is the specialty of Stellar Solutions, a company that sells into food, pharmaceutical, and other markets that rely on stainless steel process equipment.

Ray Kremer, director of product management at Stellar, explains that citric acid works in much the same way as nitric acid, but is less hazardous and requires less risk mitigation.

“The key to passivation is removing as much iron as possible from the surface. Citric loves iron, whereas nitric acid goes after the iron first, but when it’s done with that it begins picking away at chromium and nickel, so it can only work so well,” he explains. “Citric acid provides significantly deeper iron removal, resulting in better corrosion resistance overall in addition to the safety benefits.”

Citric acid passivation using Stellar’s CitriSurf product, is normally performed once, before initial use. “But that depends on what kinds of fluids the surface normally comes into contact with,” Kremer says. “If it’s a corrosive liquid, or contains a lot of chloride ion, you may have to passivate every several years.”