ChromFAQ:Volume

From ChromFAQ

What are the differences among dead volume, dwell volume, void volume, extra-column volume, etc.?

Dead volume (also often called "void volume", and symbolized by either V_m or V₀) refers specifically to the volume of liquid phase contained inside the column. The same terms are sometimes also used informally to refer to the volume of a cavity in the column, tubing or fittings. The distinction is usually evident from context, but this can cause some confusion.

• Relation to flow rate and dead time. The dead time of a column (symbolized by either t_m or t₀) is the elution time of an unretained compound (i.e., a compound whose distribution is 100% in the mobile phase). Because there is 1 dead volume worth of mobile phase already in the column when the sample is injected, the dead time is determined by the time required to pump that much liquid: the volume divided by the flow rate (F):
  t₀ = V_m/F
• Measuring dead volume is trickier than it might appear, because even a molecule which spends 100% of its time in the mobile phase may still not have access to the entire volume of liquid in the column (large molecules, for example, may be excluded from very small pores which are still accessible to the mobile phase solvent). Some of the issues are discussed in a thread in the Chromatography Forum Archives (http://www.lcresources.com/discus/messages/5133/3134.html?SaturdayJuly320040107am).
• Dead Volume can be estimated (for reversed phase columns) from the column dimensions:
  V_m ≈ 0.5 L d_c²
  where V_m is the dead volume (mL), L is the column length (cm) and d_c is the column diameter (also cm). This estimate is based on the assumption that about 60% of the volume of the column cylinder consists of liquid. This accounts for the inter-particle volume (the volume between particles) plus the intra-particle volume (the liquid inside the pores of the stationary phase packing). Remember that this is only an estimate, good to about ±15%.
  As it happens, the most commonly used analytical HPLC columns have an internal diameter of 4.6 mm. for that diameter, the dead volume can be estimated from L/10, where L is the column length in cm (if you specify length in mm, then divide by 100!)
  
  
  

Extra-column volume is the volume of liquid accessible to the analyte but outside the column. It includes part of the sample loop, all of the swept tubing between the injection valve and the column inlet, all of the swept tubing between the column outlet and the detector cell, and part of the detector cell volume.

• Importance of extra-column volume
• Effect of extra-column volume with small columns
• Measuring extra-column volume.

Dwell volume (sometimes called "gradient delay volume) applies only to gradient separations. It is the volume of liquid in the system from the point where the gradient is formed to the inlet of the column

• Dwell volume and instrument characteristics (HP vs LP mixing systems)
• Effect of dwell volume differences
• Measuring dwell volume
• Correcting for dwell volume differences

Void volume is a loose term that is sometimes (incorrectly) applied to the column Dead Volume, but more generally used to describe "pockets" or "cavities" in the column packing, the tubing, or the fittings which can result in excessive spreading of sample zones.

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