Purge and trap systems detect and measure volatile organic compounds (VOCs) in liquid and solid samples. This sophisticated technology combines two key processes: purging and trapping. Purging extracts VOCs from the sample using an inert gas, while trapping concentrates these compounds on an adsorbent material.

These systems are capable of analyzing trace-level contaminants in water and soil, making them indispensable in environmental monitoring, water quality testing, and various industrial applications. Purge and Trap systems are crucial in volatile organic compound analysis, complementing other separation techniques like flash chromatography columns in comprehensive chemical analysis.​

​Extracting VOCs with Precision​​

Purging a sample to extract analytes is a complex gas extraction process. Many factors affect the efficiency of this extraction. The amount of each compound purged is proportional to both its vapor pressure and its solubility in the sample. Both of these are, in turn, are affected by the sample temperature​.

In the case of a sample sealed in a closed vial, the sample matrix is a vapor space usually referred to as the “headspace.” If you allow the sample sufficient time, volatile organic compounds in the sample will migrate into the vapor space. After a certain period of time an equilibrium will be established; the concentration of the compounds in each phase will be stabilized.

At this point, a portion of the headspace can be removed and injected into the gas chromatographer (GC) for analysis. This technique is known as “equilibrium analysis” or “static headspace analysis.” The amount of material in the vapor phase will be proportional to the partial pressure of the component​.

In understanding the behavior of volatile compounds in a sample, it's important to consider the relationship between vapor pressures and compound concentrations. This relationship can be expressed through the following equation, which relates the total vapor pressure of a system to the partial pressures and mole fractions of its components:

P_T = P₁ + P₂ + P₃ + ... + P_n = X₁P₁^o + X₂P₂^o + X₃P₃^o + ... + X_nP_n^o

whe​re:

P_T = total vapor pressure of system
P₁, etc. = partial pressure of each compound
P₁^o, etc. = vapor pressures of the pure compounds
x₁, etc. = mole fractions of each compound

In purging a sample, the system is no longer at equilibrium because volatile compounds in the vapor phase are constantly being removed. There is no migration of components from the vapor to liquid phase. This means that the partial pressure of any individual component above the sample at any time is essentially zero.

This encourages even greater migration of the volatiles into the vapor phase more efficiently than equilibrium. This is true even if the volumes of headspace gas are the same. Purging a sample for 10 minutes with helium (at a flow rate of 50ml/min.) results in more efficient extraction of volatiles than equilibrium, using 500ml headspace.

This purging technique is called “dynamic headspace analysis." For aqueous matrices, the increase in efficiency can be upwards of 100-fold, using dynamic versus static headspace analysis.

Extraction efficiency increases with an increase in sweep volume. Sweep volume is the amount of purge gas used to purge and trap the analytes. Sweep volume is a function of sweep time and flow rate.

Since the analytes are being trapped on an adsorbent bed, there are limitations to the sweep times and flow rates that can be used. These limitations are determined by the compounds of interest in the sample and the packing material used in the trap.​

Con​​​centrating VOCs for Analysis

A trap is a short gas chromatograph column. Compounds entering the trap will slowly elute with a measurable retention volume. Retention volume is the amount of purge gas that passes through the trap before elution of the analytes begins to occur.

The requirements of a trap are as follows:

At the lower temperature used for trapping, retention times are long. At the higher temperatures used for desorption, retention times are much shorter, allowing rapid transfer to the GC. In this context, the correct parameter is retention volume.

When elution does occur, it is usually referred to as “breakthrough." The retention volume at which breakthrough occurs is often referred to as the “breakthrough volume." Adsorbents are usually chosen so that the breakthrough volume is high for analytes and low for water.

Care must be taken that the adsorbent chosen does not retain the analytes too strongly or it may not be possible to desorb it efficiently. Traps containing combinations of adsorbents are often used to enhance performance.

The trap is packed with the weaker adsorbent on top. The stronger sorbent is placed below the weaker sorbent. Less volatile analytes are not effectively desorbed by the stronger sorbent and are retained by the weaker sorbent. Therefore, the less volatile analytes fail to reach the stronger sorbent.

Only the more volatile analytes reach the stronger sorbent, and because of their volatility, these analytes can be efficiently desorbed. The desorption is carried out by backflushing the trap, ensuring that the heavier analytes never come in contact with the stronger sorbent. ​

​​Frequently Asked Questions​

What is purge and trap gas chromatography?

Purge and Trap gas chromatography is a method of removing volatile organic compounds from a solid or liquid sample. Once purged, trapped and desorbed, the sample is separated in a gas chromatography instrument. The EPA stipulates purge and trap guidelines for analysis of volatile organic compounds in drinking water, wastewater and soil samples. 

What is the purge and trap technique used to measure?

Purge an​​d Trap is a sample preparation technique for gas chromatography for the measurement of volatile organic compounds in a liquid or solid sample. 

What does it mean to purge a sample?

To purge a samp​le is to remove the volatile organic compounds from the sample.​​​​