Minimizing non-volatile matter in the mobile phase

NOTICE

Long-term exposure of the detector to non-volatile mobile phases will damage the detector. Do not use non-volatile buffers.

Observe the following guidelines to minimize the concentration of non-volatile matter in the mobile phase:

  • Prepare all mobile phases with volatile solvents. As a general rule, use solvents that are at least as volatile as water, that is lower boiling point and higher vapor pressure.

  • Use ultra-pure water with high resistivity and low total organic carbon obtained fresh from a thoroughly flushed and well-maintained point-of-use purification system.

  • TIP

    Bottled water and stagnant water from a purification system typically contain a higher level of non-volatile impurities than freshly obtained ultra-pure water.

  • Choose solvents that are specified to have low residue after evaporation.

    For example, LC/MS-grade solvents typically have a lower residue after evaporation specification than HPLC-grade solvents and are often the better choice to achieve consistently low baseline noise.

  • TIP

    Many organic solvents have low viscosity, low surface tension and high volatility, which lead to a high mass transport efficiency within the detector. For such solvents, it is especially recommended to select a grade with lowest residue after evaporation.

  • If a particular solvent causes high background current or noise, try a different grade or vendor.

  • Most HPLC-grade or better solvents are pre-filtered and additional filtration may increase the concentration of impurities within the mobile phase. It is therefore recommended to only filter the aqueous buffer salt portion of a mobile phase.

  • TIP

    With solvent gradients, non-volatile impurities in the more polar solvent (for example, water in reversed-phase) can be concentrated on the column and elute later to produce large peaks or baseline artifacts.

  • With highly combustible mobile phases, such as tetrahydrofuran (THF) and other ethers or ketones, use only nitrogen as supply gas.

  • When changing the operating mobile phase: Make sure that the solvents are miscible and will not cause precipitation of buffer salts. If required, use an appropriate intermediate solvent.

  • TIP

    Keep this in mind particularly when switching between chromatographic methods.

  • Water or highly aqueous (< 5% organic modifier) mobile phases are highly susceptible to microbial growth. Prepare such mobile phases freshly on a daily basis.

  • Select an optimal evaporation temperature to selectively evaporate background components of the mobile phase. Observe the guidelines in Evaporation Temperature.

  • If the detector is inadvertently exposed to a non-volatile mobile phase, use water or appropriate solvent with no additives to remove the salts from the detector.

Typical compatible solvents for use with the detector

  • Water

  • Methanol

  • Isopropanol

  • Acetonitrile

  • Acetone

  • Methylene chloride

  • Hexane

  • Chloroform

  • Tetrahydrofuran

    NOTE: Use THF with nitrogen gas only.