Fundamentals of Purge and Trap Operation
A measured amount of sample is placed in a sealed vessel. The sample is purged with inert gas, causing volatile compounds to be swept out of the sample. The volatile compounds are retained in an adsorbent trap, which allows the purge gas to pass through to vent. The volatiles are desorbed by heating the trap: they are injected into the GC by backflushing the trap using the GC carrier gas. Then, separation and detection is performed by normal GC operation.
In the section above it states, “The sample is purged with an inert gas, causing volatile compounds to be swept out of the sample.” This is a very simple-sounding way of describing what is in reality a rather complex process. Purging a sample to extract analytes is a gas extraction. There are many factors that effect 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, effected by the sample temperature.
Consider the case of a sample sealed in a closed vial. Above the sample is a vapor space, which is usually referred to as the “headspace”. If you allow the sample sufficient time, volatile 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 volatile compounds in each phase will be stabilized.
At this point a portion of the headspace can be removed and injected into the GC for analysis. The 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.
PT=P1+P2+P3+...+Pn = X1P1o+X2P2o+X3P3o+...+XnPno
PT=total vapor pressure of system
P1, etc.=partial pressure of each compound
P1o, 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. This is because the volatile compounds that move into the vapor phase are constantly being removed by the purge gas. Under these circumstances, 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 encourage 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 a 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 extract 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 limitation are determined by the compounds of interest in the sample and the packing material used in the trap.
Trapping and Adsorption
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 tapping, 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 use of retention time is not correct. The correct parameter is retention volume.
When elution does occur, it is usually referred to as “breakthrough”, and 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 efficiently desorb it. 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 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.