Note 32: Selection and Use of Adsorbent Resins for Purge and Trap Thermal Desorption Applications

Introduction

Adsorbent resins such as Tenax® TA, Tenax® GR and a wide variety of activated charcoals are widely used to trap volatile and semi-volatile organics from gas or liquid samples when utilizing purge and trap techniques for the trapping and subsequent analysis of volatiles from gas, liquid or solid matrix samples. Adsorbent resins are specified in several EPA methods for air and water testing. Manufacturers of these resins report the breakthrough volumes for organics on these resins at room temperature. This data is required in order to determine the gas volume which can be sampled in order to efficiently trap the organics on the resin bed without any loss due to elution off the resin bed. However, little data is available on the breakthrough volumes at elevated temperatures. This data is required in order to determine the desorption temperature, gas volumes and desorption times required to thoroughly elute the analytes off the resin bed for quantitative analysis of these analytes.

This paper describes the parameters which must be considered when selecting an adsorbent resin for a particular application. A GC equipped with a FID detector was used to analyze adsorbent traps packed with seven different adsorbent resins (Figure # 1) to determine the breakthrough volumes for a large number of hydrocarbons and alcohols as a function of desorption temperature. A series of Breakthrough Volume charts were developed to permit the proper selection of the adsorbent resin which will permit the accurate and quantitative collection and desorption of analytes off the various resins. Finally, all the data was condensed into histogram bar charts to permit the quick comparison of different adsorbent resins for classes of organic compounds. These charts will aid the analyst in the selection of appropriate resins for a particular application.

Figure 1 - Determination of Breakthrough Volumes Of Adsorbent Resins

Experimental

The term Breakthrough Volume has also been referred to as retention volume and also the specific retention volume. The units of breakthrough volume are usually expressed as liters/gram. The breakthrough volume is defined as the calculated volume of carrier gas per gram of adsorbent resin which causes the analyte molecules to migrate from the front of the adsorbent bed to the back of the adsorbent bed.

GLT desorption tubes, 1/4 in.O.D. x 4.0 mm I.D. x 100 mm long were packed with 250 mg of each of the adsorbent resins between two glass wool plugs. The packed tube was attached between the injection port and the detector of a gas chromatograph (Figure # 1) using 1/16Ó O.D. stainless steel tubing and the appropriate fittings to mate to the GC. In essence, we have made a GC column using the adsorbent resin as the column packing in the short desorption tube. Helium was used as the carrier gas, and a flame ionization detector was used for the detection of the eluted organics at GC temperatures between 0 degrees C and 360 degrees C in 20 degree increments. Approximately one milligram of each of the analytes was injected in the GC injection port. Carrier gas flow rates were accurately adjusted and measured using a primary flow calibrator (Gilibrator TM, Gilian Instruments) between 1.0 ml/min and 500 ml/min to obtain retention times between 0.1 and 5.0 minutes. The GC oven temperature was accurately controlled to within 5 degrees C using the GC oven temperature controller. From this data, the breakthrough volumes were determined by multiplying the retention time by the gas flow rate through the adsorbent resin and dividing this value by the weight of the adsorbent resin (Figure # 2).

Figure 2 - Calculation Of Breakthrough Volume (Bv)

A correction was made for the dead volume of the packed tube and connecting plumbing by injecting a non- retained volatile at high temperature. A minimum of 7 temperature data points were determined experimentally in triplicate for each of the analytes studied on each resin. This data was used to construct a plot of the log of the breakthrough volume (Bv) versus the analysis temperature (degrees C) This straight line plot was then extrapolated via linear regression analysis to obtain the breakthrough volume at the remaining temperatures (Figure # 3).

Figure 3 - Breakthrough Volumes Of Alcohols On Tenax TA As a Function Of Temperature (log-log scale)

For this study, the breakthrough volumes for the straight chain hydrocarbons and alcohols were determined on seven different adsorbent resins. The calculated and extrapolated breakthrough volumes for the hydrocarbons and alcohols at the various temperatures were placed into the Breakthrough Volume Data Charts (Figures # 4 and # 5). The breakthrough volume data for each of the analytes, expressed in Liters of Gas per Gram of Adsorbent resin at the various temperatures, can readily be obtained from this chart. The blue and green blocks indicate breakthrough volumes greater than 10 liters per gram of adsorbent resin. These are considered to be the generally usable ranges for the trapping of the analytes on the adsorbent resins. Breakthrough volumes of less than 10 liters/gram of resin (white and red blocks), would generally not be acceptable temperatures for the efficient trapping of organics on the resins, unless the gas volume samples were kept very small. The red blocks indicate the breakthrough volumes less than 10 ml/gram of resin. These are considered to be acceptable values for the efficient desorption or release of the analytes from the adsorbent resins. Breakthrough volumes greater than 10 ml/gram (white, blue or green blocks) would require excessive volumes of gas to efficiently desorb (or purge) the analyte off the adsorbent resin. Using these charts, one can readily determine the usefulness of the particular resin to both adsorb (trap) and desorb (purge) the various organics. In addition, these charts can be used to determine the temperature and gas volumes for gas sampling and collecting as well as for the desorption of the analytes into the GC injection port for subsequent analysis.

Figure 4 - Hydrocarbon Breakthrough Volumes Data Chart

Figure 5 - Alcohol Breakthrough Volumes Data Chart

The Breakthrough Volume charts are helpful for determining the usefulness of a particular resin for a particular analyte or a range of analytes in a group of organics such as the hydrocarbons or alcohols. However, in comparing different adsorbent resins, the amount of data can be somewhat overwhelming. Therefore, the bar chart histograms comparing the different adsorbent resins for a range of analytes of a class of organics were developed (Figures #6 and # 7). A lot of information is condensed into these charts. The numbers at the top of the chart indicate the number of carbons in the analyte carbon chain. The numbers at the bottom of the chart indicate the boiling point of the analyte. The chart is divided into 5 vertical bar columns indicating the broad classification of the analytes as a gas, volatile, low boiler, medium boiler or semivolatile. The four horizontal bars on the body of the chart indicate the range of analytes that can be effectively analyzed with each of the seven adsorbent resins. In addition, each of the horizontal bars is divided into smaller sections which indicate the temperature which will be required to desorb the analytes off the adsorbent resin.

These charts can be readily utilized to compare the adsorbent resins and to develop mixed bed resins. For example, it can be observed that Carbosieve SIII can be used to analyze hydrocarbons between ethane (C2) and hexane (C6). Hydrocarbons greater than hexane can not be desorbed off this resin below 400 degrees C. Tenax TA can be used to analyze hydrocarbons between pentane (C5) and tetracosane (C24). Hydrocarbons smaller than pentane would not be effectively trapped on Tenax TA at room temperature and compounds larger than C24 would not be effectively desorbed off this resin at 300 degrees C. A mixed bed resin of Carbosieve SIII and Tenax TA could be used to analyze hydrocarbons from ethane (C2) through tetracosane (C24).

Figure 6 - Selection of Adsorbent Resins For Desorption System

Figure 7 - Selection of Adsorbent Resins For Desorption System

Conclusion

The Breakthrough Volumes for a large number of hydrocarbons and alcohols have been determined on seven different adsorbent resins at various temperatures. This data has been presented in Breakthrough Volume Data charts for easy interpretation and calculation of both the adsorption and desorption characteristics of the organic analytes for the various adsorbent resins studied. This data can be used to determine the usefulness of the adsorbent resin for a particular application. In addition, the optimum conditions for the adsorption of the organics on the resin trap and the subsequent desorption off the resin and into the GC can be determined. The system can be further refined to develop methodology to remove solvent fronts from samples for increased performance, elimination of interfering or tailing peaks and increased resolution of early eluting peaks.

The bar chart histograms compare the various adsorbent resins for use with various classes of volatile organics. From these charts, the analyst can determine the optimum resin to use for a particular application or to develop a mixed bed resin to expand the range of analytes that can be trapped and analyzed via the purge and trap thermal desorption methods.