Table of Contents

• Height equivalent to a theoretical plate
• Formulae for calculating HETP
• Factors affecting HETP
• Applications of HETP
• Conclusion

Height equivalent to a theoretical plate (HETP)

Introduction and Outcome

Height equivalent to a theoretical plate (HETP) is one of the widely chromatographic parameters in HPLC method development. Chromatographers are not using it effectively due to a lack of clarity on it. That is why I decided to share my skill-based knowledge on it and clarify the doubts. Having read this article you will be able to answer questions like:

• What is a Height equivalent to a theoretical plate (HETP)?
• How a Height equivalent to theoretical plate (HETP) is calculated?
• What is the difference between HETP and column efficiency or theoretical plate?
• What is the relationship between HETP and flow rate?
• What is relationship between HETP and reduced height?
• What are the factors affecting the HETP?
• What are the applications of HETP?

Height equivalent to a theoretical plate

Height equivalent to a theoretical plate is the properties of the column which is defined as the height of the column required by the analyte to achieve equilibrations between the stationary phase and the mobile phase. It is denoted by HETP or H.

Formulae for calculating HETP

Following formulae is used to calculate the HETP:

HETP = L/N

Where L is the length of the column (in millimetres/centimetre/meter) and N is the theoretical plate or column efficiency.

When N will be 1 then HETP = L

Hence HETP can be defined as “HETP is the length of the column which represents 1 (one) theoretical plate. It means lower the HETP and better the column.

Difference between HETP and Column efficiency

Relationship between HETP and flow rate or Van-Deemter equation

H=A+B/v+Cv

Where:

• A : Eddy diffusion
• B: Longitudinal diffusion
• C: Equilibration mass transfer &
• v: Flow rate

Factors affecting HETP

Following are the resolution controlling factors in HPLC:

1. Column efficiency: HETP is inversely proportional to the column efficiency
2. Column length: L is directly proportional to the column length
3. Particle size: HETP is directly proportional to the particle size.
4. Column temperature: HETP is inversely proportional to the column temperature
5. Injection volume: HETP is directly proportional to the buffer concentration
6. Sample concentration: HETP is directly proportional to the sample concentration
7. Eddy diffusion: HETP is directly proportional to the Eddy diffusion.
8. Longitudinal diffusion:HETP is directly proportional to the Longitudinal diffusion.
9. Equilibration mass transfer: HETP is directly proportional to the Equilibration mass transfer
10. Flow rate:HETP is directly proportional to the flow rate

Relationship between HETP and Reduced plate height

h=H/d_p

Where:

• H:HETP (in micrometers)
• h: Reduced plate height
• d_p: Particle diameter (in micrometers)

Applications of HETP

• Very helpful in selecting the column during chromatographic method development
• Lower the value of HETP better is the method/column
• Lower the value of HETP sharper the peak

Conclusion

Hopefully this article has cleared all your doubts related to HETP and you can now use it effectively during HPLC separation method development. Write your opinion/ question related to this article in the comment section and I will answer it on a priority basis.

FAQS

What is HETP in HPLC?

Height equivalent to a theoretical plate is the properties of the column which is defined as the height of the column required by the analyte to achieve equilibrations between the stationary phase and the mobile phase. It is denoted by HETP or H.

How do you calculate the HETP of a chromatography column?

HETP = L/N; where L is the length of the column (in millimetre/centimeter/meter) and N is the theoretical plate or column efficiency

What is the significance of HETP?

Lower the HETP and better the column. HETP = L/N; where L is the length of the column (in millimetres/centimetre/meter) and N is the theoretical plate or column efficiency.

What is the relation between HETP and theoretical plate?

HETP is inversely proportional to theoretical plate

References:

• Analytical Chemistry, Gary D. Christian

Abbreviations:

• HETP: Height equivalent to theoretical plate
• N: column efficiency
• h: Reduced height.
• HPLC: High performance liquid chromatography