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): Learn in 5 minutes

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.

Height equivalent to a theoretical plate (HETP)

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.

Related Article: How to decide system suitability test (SST)?

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)

Related Article: Allowable GC Method Adjustment

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

Related Article: What is the difference between method development and validation?

Conclusion

HETP is widely used in chromatographic separation in the pharmaceuticalindustrys. Hopefull,y 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.

You may also want to check out other articles on my blog, such as:

• Difference between HPLC and GC
• How to develop a method using GC-MS?
• Need of Chromatographic Method in Drug Development
• How to develop the HPLC method for basic compounds?
• How to develop the HPLC method for acidic compounds?
• How to develop the HPLC method for non-polar compounds?
• What should be the Analytical method development approach?

FAQS

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

What is the difference between HETP and column efficiency or theoretical plate?

The difference between HETP and column efficiency or theoretical plate is that HETP (Height Equivalent to a Theoretical Plate) is a measure of column performance, while column efficiency or theoretical plate is a measure of the number of theoretical plates in the column. HETP and flow rate have an inverse relationship – as flow rate increases, HETP decreases. HETP and reduced height have a direct relationship – as reduced height increases, HETP also increases. HETP is used to evaluate the performance of packed columns in chromatography and distillation processes.

What is the relationship between HETP and flow rate?

H=A+B/v+Cv

Where:

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

What are the applications of HETP?

HETP helps to assess the efficiency of a chromatographic separation process. The following are the various applications of HETP:

• HPLC and GC
• Distillation Processes
• Process Engineering
• Food and Beverage Industry
• Pharmaceutical Industry
• Petroleum and Petrochemical Industries
• Chemical Analysis

HPLC and GC

• Performance Evaluation: HETP is used to assess the efficiency of chromatographic columns. A lower HETP indicates better separation performance and higher resolution in separating compounds.
• Optimization of Separation: By monitoring HETP, one can optimize the flow rate, column dimensions, and other parameters to improve the quality of separation in both analytical and preparative chromatography.
• Column Design and Selection: It helps in selecting or designing the most suitable columns for specific separations by considering the required resolution and efficiency.

Distillation Processes

• Designing Distillation Columns: In distillation, HETP is used to design and optimize distillation columns. The number of theoretical plates in a column correlates with its separation efficiency, and a lower HETP means fewer plates are needed for the same separation efficiency.
• Optimization of Operating Conditions: By adjusting factors like reflux ratio, feed composition, and column temperature, operators can reduce HETP, improving energy efficiency and product purity in industrial distillation processes.

Process Engineering

• Scale-Up: HETP is used when scaling up laboratory separations to industrial scales. It helps predict how separation efficiency changes when moving from small laboratory columns to large industrial equipment..

Food and Beverage Industry

• Purification of Food Ingredients: In the production of flavorings, additives, or essential oils, HETP helps optimize separation processes for extracting desired compounds with minimal loss and maximum purity.

Pharmaceutical Industry

• Purification of Drugs and Active Pharmaceutical Ingredients (APIs): HETP is important in optimizing the separation of active ingredients from impurities or solvents during the production of pharmaceuticals. It helps ensure high purity and consistency of the final product.

Petroleum and Petrochemical Industries

• Refining and Fractionation: In the refining of crude oil or gas, HETP is used to optimize the distillation processes involved in separating various hydrocarbon fractions such as gasoline, diesel, and kerosene.

Chemical Analysis

• Analytical Chemistry: In chemical analysis, HETP provides valuable insights into column efficiency and can be used for the validation of chromatographic methods, particularly in the analysis of complex mixtures.

References:

• Analytical Chemistry, Gary D. Christian
• HETP

Abbreviations:

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