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FLEXO Magazine : February 2009
TECHNOLOGIES & TECHNIQUES ø z - The Importance of Gas Chromatography for the Quality Control of ! Residual Solvents By Burton S. Todd II. T he gas chromatograph (GC) is one of the most commonly used instruments in quality control laboratories around the world. For the flexible packaging printer/converter, its primary use is to test for residual solvents that remain in printed flexible packaging materials. This test is important for produc- tion start-up and optimization and is specified by many food manufacturers. Residual solvents are defined as the solvents that are retained in the ink on the surface and between the layers of printed flex- ible packaging. Excess solvents are normally removed during the drying steps of the printing process. However, if those sol- vents are not removed, they can migrate into the food and other products that are stored in the sealed package. Some of these solvents have an objectionable odor, and they may be hazardous to our health, even when present in very small amounts. Also, when solvents are not dried sufficiently on the printed film be- fore it is wound up again at the end of the printing process, the whole ream can stick together and become a costly waste. BASIC GC TEST FOR RESIDUAL SOLVENTS Your GC can tell you what solvents are retained in your flex- ible packaging material and how much of each solvent is pres- ent. This is accomplished in several steps. First, a printed film sample is sealed into a glass vial. This sam- ple vial is then heated to drive any residual solvents present in the film sample into the air or headspace of the vial. A small volume of this heated vapor is then removed from the vial and injected into a GC where the solvents are separated and detected. Those solvents found are automatically identified and their amount in the printed material is reported in mg/square meter or mg/ream. THE BASIC COMPONENTS The Carrier Gas. The carrier gas is an inert gas that is sup- plied in high-pressure cylinders. The purpose of the carrier gas, usually pure helium or nitrogen, is to move the injected solvent vapors through the Gc. Air and hydrogen are needed for the operation of the detector as well. Pressure or flow regulators are used to control precisely the various gas flows in the Gc. Gas purifier traps are usually installed on the gas supply lines to remove contaminants such as trace hydrocarbons, water, and oxygen. These contaminants are detrimental to the long-term performance of the GC column. - FIGURE 1. Printed flexible packaging film sealed in headspace vials. The GC Injector. The GC can only analyze gases or organic compounds that can be vaporized into a gas. Pure liquids and mixtures, such as ink solvents are injected into the heated injec- tor where they are vaporized. Once vaporized, they are carried onto and through the GC column, where they are separated into individual solvents. The injector also provides the connection for the front of the column. The GC Column. This is the heart of the Gc. Today's instru- ments use what are called capillary columns. This new genera- tion of columns is manufactured to perform reliably over many years and to provide very reproducible results. The capillary columns are open tubes that are usually 30m or 60m long with inner diameters of 0.25mm to 0.53mm. They are wrapped on a wire frame that holds the many coils in place. The capillary columns themselves are manufactured from chemically clean glass called fused silica, and are coated with a high-temperature polymer to prevent them from breaking eas- ily. As noted above, the column is an open tube, the inside walls of which are coated with a thin film of a very high-boiling liq- uid. This thin layer, from 0.1 to 5.0 microns thick, is chemically bonded to the inner wall and is called the liquid phase or the stationary phase. FEBRUARY 2009 www.flexography.org FLEXO
Sustainable Winter 2009