A common technique for the sampling of Volatile Organic Compounds (VOCs) is purge and trap concentration. During purge and trap concentration, the sample to be extracted is either placed in an airtight vial with no headspace or introduced into an airtight chamber and mixed with water. An inert gas such as helium or nitrogen is bubbled through the water; this is known as purging or sparging. The volatile compounds move into the headspace above the water and are drawn along a pressure gradient (caused by the introduction of the purge gas) out of the chamber. The volatile compounds are then swept through a heated transfer line onto an adsorbent trap. The trap is a column of adsorbent material at ambient temperature that holds the volatile compounds. Next, the trap is heated and the sample compounds are introduced to the GC column via a second heated transfer line, which is in line with the split inlet system on the GC. The GC is interfaced with a detector for the analysis of the volatile analytes in the sample. Purge and Trap in conjunction with a GC is well suited for volatile organic compounds (VOCs) and BTEX compounds (aromatic compounds associated with petroleum) in environmental samples and can also be used to sample volatile aromatic analytes in food and flavor samples.
Food – Aroma profiling
Environmental Drinking Waters – USEPA 524
Environmental Waste Waters, Soil, and Sludge – USEPA 5030, 5035, 624, 8260
Purge and Trap Sampling has six steps:
Standby/Purge Ready – the concentrator is at the required temperature, flows, etc. and is ready for the sample to be transferred to the sparge vessel for a water matrix or purged in the sample vial for a soil, sludge matrix
Purge – the sample is purged with inert gas and the analytes are swept onto the adsorbent trap
Dry Purge – purges the adsorbent trap with inert gas while bypassing the sparge vessel, thus driving some of the moisture out of the adsorbent trap
Desorb Ready – the system is done purging the sample and the adsorbent trap is ready to be heated
Desorb – the adsorbent trap is heated to the required temperature and the analytes are transferred to the GC
Bake – the adsorbent trap is heated and swept with inert gas in order to ready the trap for the next sample
The headspace is defined as the vapor space above the sample. VOCs, if given time, will migrate out of the matrix and into the headspace until an equilibrium is established. Purge and trap is based on headspace equilibrium.
Purge and trap sampling is actually gas extraction. Gas extraction is the process of sweeping the headspace of a sample with an inert gas thus reducing the vapor pressure of the sample. Reduction of the vapor pressure, in turn, encourages the VOCs out of the sample and into the vapor phase. By purging the sample, the headspace is constantly disrupted with the VOCs collected on the sorbent trap.
VOCs are defined as compounds with a boiling point of less than 180°C. Typical purge and trap compounds are not only volatile, but are also insoluble or slightly soluble in water. Purge and trap concentrates the analytes onto the adsorbent trap for transfer to the GC, thus the technique is more sensitive than static headspace. Since many GC columns and detectors are adversely affected by the presence of water, purge and trap is ideal as the analytes are transferred over in vapor form.
The VOC response is determined by the purge efficiency of the compounds. Purge efficiency can be affected by several factors. First is the vapor pressure. The greater the vapor pressure the greater the purge efficiency. High vapor pressure causes the analytes to migrate into the vapor phase more rapidly due to their preference to exist in the vapor state. Thus, when increasing the temperature of the sample the vapor pressure also enhances therefore producing better purge efficiency. It is important to note that too much water can cause issues with the GC column and detector, so when increasing the temperature, the amount of water transferred has to be considered.
Solubility of the compound also plays a role in purge efficiency. The more soluble the compound is in water, the lower the purge efficiency. Consequently, the more polar volatile compounds have lower compound response. Sensitivity can be raised by increasing sample size, but purge efficiency is decreased. Furthermore, the chosen purge method can also affect purge efficiency with purge through a frit in the sparge vessel being the best option.
Most purge and trap methods recommend a 40mL/min purge flow for 11 minutes for purge and trap sampling. This leads to a 440mL purge volume. The purge volume affects the sensitivity of the sample analysis. This purge flow and time is recommended due to the fact that it gives the optimum results and almost complete trapping of the sample VOCs. It has been found that lesser purge volumes will decrease the sensitivity while increasing the purge volume does not aid in compound response.
The United States Environmental Protection Agency (USEPA) has promulgated a number of methods for the determination of VOCs. VOCs can be found in air, water and soil. The USEPA determinative methods include USEPA Methods 8260, 624, 8021 and 524 while the purge and trap preparative methods are USEPA Methods 5030 and 5035.
USEPA Method 5030 describes the purge and trap sampling of water and water miscible samples while Method 5035 describes the sampling of high concentration soil and waste sample extracts. Both of these sampling techniques can be done by the Evolution Purge and Trap concentrator and are outlined as proper sample preparation techniques for Method 8260D which involves employing a GC/MS for the VOC determinative technique. Other determinative methods that utilize the Evolution purge and trap are Method 8015 for the determination of gasoline compounds using GC/FID and Method 8021 which examines gasoline fractions by GC/PID. In order to determine VOCs in drinking water, the USEPA promulgated Method 524.
Since VOCs evaporate rapidly in the air, VOC samples have to be sampled for purge and trap carefully. If an air bubble is found in the sample vial, the results of some of the volatile analytes can be biased low. If a sample vial is opened before sampling, results can be also be biased low. Thus, the Centurion W/S purge and trap XYZ autosampler was designed to ensure sample integrity. Samples are transferred directly from the vial or purged in the vial, this ensures the sampling system is closed the entire time and that there is no loss of analytes.
For food and flavor analysis, the samples need to be sealed in a sample vial as most food and flavor samples are not water, but are liquid matrices or solids. As food and flavors are composed of an assortment of VOCs, purge and trap can optimize the analysis due to it being an exhaustive technique.
There are many automated systems on the market and they each have their advantages and disadvantages. The Evolution purge and trap concentrator has the advantage of having an 8-port valve in order to better control the water contamination and a heated sparge vessel for lowering sample carryover. When coupling the Evolution to the Centurion W/S XYZ autosampler, the system provides both efficiency and reliability. As the Centurion can transfer water to both the sparge vessel or another vial and has a needle that goes directly to the sample vial for water transfer thus elimination cross contamination between water and soil samples.
If you are just getting started with Purge and Trap sampling and analysis, contact us to reach one of our application specialists who would be glad to offer some tips and tricks. Feel free to look at some of our application notes which will help you on your way.