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Example of the Use of Breakthrough Volume Data


Removal of Ethanol from a Wine Sample

In the analysis of wines, the ethanol concentration is greater than 10 percent for most samples with the combined flavor components comprising less than 1 percent of the sample. As a result, if the entire wine sample were analyzed via direct injection or purge and trap of the entire sample, the ethanol peak will be so large that it would overload the capillary column and make the analysis of the trace flavor compounds very difficult. In order to analyze the wine samples for the flavor ingredients, the ethanol must be removed from the sample which can be collected and analyzed via the purge and trap thermal desorption technique.

In the studies below, Tenax® TA was selected as the adsorbent resin. Tenax was selected due to its low affinity for water and for its low breakthrough volumes for methanol and ethanol. Desorption tubes were packed with 100 milligram of Tenax TA and the samples were collected by placing the wine sample (5.0 mL) into a purge and trap tube and purging with pure nitrogen. A complete description of the analysis of wines via this technique is presented in Application Note No. 17. After the sample was collected it was necessary to determine the volumes of gas that would be needed to purge the ethanol from the sample. Breakthrough volumes are typically listed in the units of Liters per gram of resin. Since our collection tubes contain 100 milligram (0.10 grams) of Tenax TA the Breakthrough Volumes for the analytes need to be converted to milliliters per 100 milligram for our application. The breakthrough volumes for methanol, ethanol and several other organic analytes were obtained from the Breakthrough Volume tables and are listed in the table below:

In order to remove the methanol and ethanol from the samples but not remove any of the other flavor components, the purge gas volume must be carefully selected using the data in the table as a guide. The chart below demonstrates what volume of gas is required to remove the methanol and the ethanol.

Breakthrough Volume of Alcohols

Keep in mind that the Breakthrough Volume Numbers in the table above correspond to the top of a GC peak. The compounds eluting off the adsorbent resin bed have some width and in order to assure the complete removal of the alcohol, the values must be doubled or tripled. Typically the analyte begins to be eluted off the resin at one half the Breakthrough Volume value and is totally eluted off the resin bed at 2 to 3 times the Breakthrough Volume value. Therefore in order to remove the methanol from the adsorbent resin bed, about 80 milliliters of nitrogen purge gas is required. To remove the ethanol, about 400 to 500 milliliters of nitrogen is required. The "Rule-of-Thumb" of 2 to 3 times the Breakthrough Volume is a good safe bet to remove all of the analyte as will be demonstrated below.

Purging of Tenax to Remove Methanol

This can be demonstrated quite nicely in the example above of a mixture of alcohols dissolved in methanol at a concentration of 100 ng/ul. One ul of this methanol solution was injected onto the front of a desorption tube and then purged with various volumes of nitrogen in an effort to remove the methanol solvent. Afterwards the desorption tubes were analyzed using the SIS Short Path Thermal Desorption System to desorb the alcohols off the resin and into the GC for analysis. As noted above the Breakthrough volume for methanol is about 36 mL per 100 milligram of Tenax TA at 20 degrees C. Purging with 20 milliliters removes very little of the methanol. Purging with 40 milliliters removes a good portion of the methanol, enough such that the ethanol peak is observable and the propanol is completely resolved from the methanol tailing peak. Increasing the purge flow to 80 milliliter removes most of the methanol and the ethanol and propanol are well resolved. Increasing the purge flow to greater than 100 milliliter would begin to elute the ethanol off the resin, which would not be preferable in this case. Without this purging of the sample after collection, the ethanol and propanol would be impossible to isolate and identify.

Purging Acetone to Remove Methanol

In the example above, a mixture of acetone in methanol solvent was prepared as above and purged with 120 milliliters of nitrogen to remove the methanol. By increasing the purge gas flow to 120 milliliters, the methanol is totally removed from the sample, and 100 percent of the acetone is retained since the breakthrough volume for acetone is 600 mL/100 milligram. Quite often in our analysis of samples we use deuterated internal standards dissolved in methanol solvent in order to quantitate the levels of analytes in our samples. After the sample is collected we inject 1.0 ul of the internal standard in methanol onto the top of the Tenax resin bed and purge the desorption tube with 120 milliliters of nitrogen to totally remove the methanol solvent but retain the internal standard along with the analytes of interest. The samples are subsequently analyzed via the S.I.S. Short Path Thermal Desorption System.

Wine Sample

For the analysis of wine samples, we want to purge the ethanol off the collected samples. Since the Breakthrough Volume for Ethanol is 180 mL/100 milligram Tenax TA, we have selected a purge volume of 400 milliliters as our post purge gas volume ( 2 times the Breakthrough Volume). This volume will assure complete removal of the ethanol but most of the flavor component analytes from the wine will be retained for analysis as shown above. This is the method that was used to analyze the wine samples described in Application Note No. 17 and in other wine samples we have analyzed for customers in the wine industry.

The above description demonstrates the use of the breakthrough volume data to purge off solvent fronts or major components from a sample collected on adsorbent resins via the purge and trap technique. After selecting the analytes that you wish to elute from the adsorbent resin bed, looking up its Breakthrough Volume in the tables at the sample collection temperature and then after correcting for the adsorbent resin bed volume and multiplying by 2 you will have determined the amount of nitrogen gas needed to purge the analyte off the adsorbent resin bed.

A more detailed description of Breakthrough Volumes is in the article entitled: "Calculation and Use of Breakthrough Volume Data." and in Application Note 32.

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