Considerations for Mixing of HPLC Mobile Phase Solvents Off-line

It will be very rare that you will come across an HPLC method which uses a single solvent throughout the run. Pure solvents kept in mobile phase reservoirs need to be mixed in required proportions under isocratic conditions.

Mobile Phase Solvents

Mobile Phase Solvents

Mixing of solvents is a critical step as procedural deviations can affect the chromatograms and analysis.

Miscibility of solvents

Miscibility of solvents is directly related to their polarities. Remember ‘Like dissolves like ‘. The solvents to be mixed should be miscible with each other in all proportions as immiscible or partly miscible solvents cannot be used in HPLC analysis. Commonly used solvents such as water, methanol, acetonitrile and tetrahydrofuran are fully miscible with each other and are commonly used in reverse phase separations.

Temperature effects

The density of solvents varies with temperature which in turn affects the dispensed volume. For good reproducibility before mixing the solvent bottles should be kept for some time in the laboratory room so as to stabilize to laboratory temperature.

Solvents seldom mix stoichiometrically and on mixing occupy less volume than the sum total of their individual volumes prior to mixing. Additionally there can be temperature changes as a result of mixing which can lead to volumetric variations. Mixing of methanol with water results in heating whereas acetonitrile and water on mixing result in cooling. In order to compensate for the thermal changes on same mixing sequence should be followed every time. It is advisable to measure required volumes separately and mix them rather than taking a volume of one solvent in a graduated cylinder or volumetric flask and making up the volume to the mark with the other.

Purity

HPLC grade solvents should be used. This grade is commercially prepared from distillation of AR grade solvents followed by filtration. Water should be of high purity and can be obtained using reverse-osmosis based commercially available systems. See link on Water Requirements of HPLC.

UV cut-off wavelength

UV cut-off wavelength is important consideration for deciding which HPLC solvents can be mixed when using a UV detector. The UV cut-off specifies the lowest wavelength that UV detector can be used for a particular solvent.. The cut-off values of some common solvents are provided here :

 Solvent   UV Cut off (nm)
 Methanol 205
Water 190
 Acetonitrile 190
 Tetrahydrofuran 212
 Hexane 195

Refractive index

Refractive index of solvents should be considered before mixing of solvents when using RI detector. The refractive index of sample should be as different as possible from the refractive index of the mobile phase for significant response

Viscosity

Solvents should have low viscosity for efficient flow and mass transfer of analytes in and out of stationary phase pores. Viscous solvents also lead to high column back pressures.

Compressibility

Some solvents are more compressible than others. HPLC operates under high pressures and flow rate deviations can result on account of solvent compressibilities. Modern pumping systems correct for solvent compressibilities and ensure consistent flow rates.

Hazardous solvents

Safety of use should be your prime concern. Try to avoid as far as possible solvents which are poisonous, highly flammable or carcinogenic.

In this article considerations for manually mixing the mobile phase are covered. A subsequent article will cover mixing of mobile phases in system using low and high pressure mixing options.

We shall welcome your comments, suggestions and experience on the topic covered.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

Comments

  1. Hi,
    Thanks for the compliment.The retention time of the peak characterizes the peak qualitatively and its height or preferably area is used to decide the amount of the component present. For further details you may refer to the topic on suitability parameters that are related to the peak in our free course on HPLC or GC.
    regards

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