Separation of Alcohols and Esters in Spirits using Serially Connected GC Columns
Introduction
In addition to containing ethyl alcohol, alcoholic spirits are complex mixtures of many compounds that may each influence the flavor of the product. It is the ratios of these other compounds that give each spirit its uniqueness of flavor. Therefore, it is critical for the alcoholic spirit industry (distilleries) to be able to identify and quantify these compounds to maintain a consistent tasting product for their consumers. Because these types of samples are complex, containing a multitude of similar compounds, it may be difficult to find the proper column to resolve all analytes. The following application, submitted by Dr. Maurizio Baccarini with Dister SPA in Faenza, Italy, provides a unique solution to this analytical problem.
Application Problem / Solution
An alcoholic mix injected at low concentration normally does not create any separation problem. However, problems become clearly visible when there is a need to inject raw products, such as when it is important to identify impurity compounds present at trace levels. It is a wellknown problem that methanol/ethyl acetate co-elute on polar columns and that methanol/acetaldehyde co-elute on non-polar columns. Therefore, the need to identify each of these compounds in spirits typically requires two analyses on separate columns, each of different phase polarity.
Through many years of experience working in distilleries performing tests using different column phases, Dr. Baccarini has determined that using a combination of two columns serially-connected can solve these well-known separation problems. Table 1 describes the column sets used for the work presented in this article.
Column set 1 allows the optimum separation between the acetaldehyde, methanol, and ethanol peaks that historically are found in alcoholic spirits. However, isopropanol co-elutes with ethanol. Therefore, this column set is a viable option only when the analysis of isopropanol is not required. Chromatography and analytical conditions are shown in Figure 1.
Figure 1.Analysis of an Alcoholic Spirit Using Column Set 1
As shown in Figure 2, column set 2 solves the ethanol/ isopropanol separation problem while maintaining acceptable separation between the acetaldehyde, methanol, and ethanol peaks, as well as all other compounds. Therefore, column set 2 presents a better choice for the separation of all compounds of interest in a single analytical test.
Figure 2.Analysis of an Alcoholic Spirit Using Column Set 2
Practical Considerations
When serially connecting columns, care must be taken to insure the ends are connected with no, or minimal, dead-space. This can best be accomplished if the column ends are butted together, end-to-end, using a device such as the Capillary Column Butt Connector. The use of these devices was described in detail in a previous issue of The Reporter (1).
Another popular choice for connecting columns is the use of a press fitting, such as a GlasSeal™ connector. These result in a perfect seal forming between the two columns. Using a small drop of a polyimide resin on each column makes the connection extremely durable. Care must be taken to ensure none of the resin enters either column or plugs the connector.
Whichever method is selected, it is recommended that the connection be evaluated with an electronic leak detector. Avoid liquid leak detector solutions with capillary columns due to the risk of pulling some of the solution into the system.
Conclusion
As shown, the use of serially connected columns of different phase polarity allows the analysis of the various components in spirits in a single run. This novel, timesaving approach is an improvement over the need for two analyses on separate columns, and should be considered for solving separation problems associated with alcohol beverage analysis.
References
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