Analysis of the Preferred Reasons for Methanol and Acetonitrile in Liquid Chromatography Mobile Phases
Basic Requirements for Solvent Selection in Mobile Phases
In high-performance liquid chromatography (HPLC) analysis, the choice of mobile phase is crucial as it directly affects separation efficiency, analysis speed, and detection sensitivity. As the 'blood' of liquid chromatography systems, mobile phases must meet several strict technical requirements to ensure a smooth analytical process. Among them, methanol and acetonitrile have become the most commonly used organic solvents due to their unique physicochemical properties.
An ideal solvent for liquid chromatography should possess key characteristics: first, it must have good solubility for target analytes; second, it should not be corrosive to chromatographic systems; third, it should exhibit low UV absorption within the detection wavelength range; fourth, its viscosity should be as low as possible to reduce system pressure; finally, from an operational safety perspective, solvents should be non-flammable and have low toxicity. These basic requirements form our foundational framework for evaluating and selecting mobile phase solvents.
Considerations on Solubility Performance
Solubility performance is the primary consideration when choosing a mobile phase. In liquid chromatography analysis, samples must remain stably dissolved in the mobile phase; any precipitation can lead to column blockage or abnormal detection signals. Both methanol and acetonitrile demonstrate excellent solubility capabilities for most organic compounds that satisfy nearly all analytical needs.
Notably, using a solvent that dissolves samples can avoid 'solvent effects.' When there is a significant difference in elution strength between sample solvent and mobile phase solvent, peak shape distortion such as tailing or splitting may occur—this phenomenon becomes particularly evident with larger injection volumes. Therefore, during method development processes we typically recommend using either a weaker eluting solvent or one similar to the mobile phase itself to dissolve samples. As versatile solvents capable of forming homogeneous mixtures with various buffer salt systems and aqueous phases,methanol and acetonitrile provide great flexibility during method development.
System Compatibility & Corrosiveness Issues
Liquid chromatography system flow path components are usually made from 316 stainless steel or PEEK materials which show good resistance against most solvents. However,some organic solvents like chloroform or dichloromethane can cause noticeable corrosion on stainless steel over time leading potentially to seal failure or metal ion leaching.Methanolandacetonitriledemonstrateexcellentperformanceinthisregard,andwon'tcorrodeeitherstainlesssteelorPEEKmaterials. PEEK material possesses excellent chemical inertnessandmechanicalstrength,yetlong-termusewithcertainstrongpolar solventslike dimethyl sulfoxide (DMSO)or tetrahydrofuran(THF)mightleadto material embrittlement.Comparatively,theimpactofmethanolandacetonitrileanPEEKtubingisnegligible,makingthemtimelyandsafeselectionsforvariousliquidchromatographysystems.Additionally,bothsolvents’volatilityismoderateenoughnottocausepumpingdifficultiesduetoexcessiveevaporation,norexcessivelyslowremovalfromthesystemdueinsufficient evaporation rates.
Analysis of UV Absorption Characteristics
UV detectors are amongthemostcommonlyusedmethodsindetectingliquidchromatography,hence,theUVtransmissibilityofthemobilephaseiscritical.Acetoni-trileshowsasignificantadvantageovermethanolinlowwavelengthdetec-tionbecauseitsUVcutoffwavelengthis190nmcomparedto210nmfor-methanol.In200nm-belowdetectableanalysisusingacetoni-tril-asamobilephasecanyieldmorestablebaselinesandhigherdetection sensitivities.Duringgradientelutionprocesses,varia-tionsinthecompositionofthemobilephasedoresultinbaseline driftespeciallyduringlow-wavelengthmeasurements.Thebaseline driftproducedbywater-acetoni-trilemixedsystemsissmallerthanthatofmeth-anol-watermixturesduringgradientelutionsmakingacetoni-trilethedominantchoicewhenhigher-sensitivitydetectionismost critical.Noteworthy evenhigh-puritychromatographic-grade-solventsmaycontaintracesimpuritiesthataffectUVabsorptionthereforewhenperformingsuper-highsensitivityanalysesconsiderationmustbe givenforthebatchdifferencesandpuritygradesofthesolventsused.