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Reprinted from tbe JOUR_ AL OF THE AMERICAN OIL CHEMISTS' SOCIETY, Vol. 49, No.2, Pages: 106--110 (February 1972) Purcbased by U.S. Dept. of Agriculture for Official Use. Headspace Gas Analysis of 3086 Salad Oils by Mass Spectrometry C.D. EVANS and E. SE LKE, Northern Regional Research Laboratory,' Peoria, Illinois 61604 ABSTRACT In this report we present a reliable sampling technique A mass spectrometer was used to analyze the and data on headspace gas analysis of several commercially content of hydrogen, nitrogen, oxygen, carbon diox­ processed vegetable salad oils packaged in glass bottles. ide and argon in headspace gas of commercially packaged soybean, cottonseed and corn salad oils. A EXPERIMENTAL PROCEDURE leakproof sampling system was designed to avoid air contamination and obtain a representative headspace Salad oils packaged in pint and 1.5 pint screw cap gas sample. Some edible oils are packaged under pure bottles came from retail stores, from warehouses, direct nitrogen, whereas other samples contained various from the processors, and in one plant directly off the amounts of oxygen in the headspace gas. The bottling line. Upon receipt of all bottles, some were held at presence or absence of argon in the headspace gas 34 F and others were frozen at 0 F until analyzed or placed indicates that some oils are packaged with pure under accelerated storage conditions. Part of the samples nitrogen and others with nitrogen obtained by con­ obtained directly from the bottling line were frozen trolled burning of hydrocarbons to remove all the immediately and transported in dry ice. The frozen samples oxygen in air. The presence of hydrogen in some were stored at 0 F and kept frozen until after the headspace samples where argon was also present suggested that gas analyses were completed. Accelerated storage samples ca talytic purifiers were used to remove the last traces were placed in 100 F forced draft air ovens. All samples of oxygen and to ensure pure nitrogen for packaging were stored in the dark. oils. The decrease in oxygen of oils bottled in air was A gas sampling tool of our own design was constructed followed during storage at room and at elevated from brass stock by the Laboratory's machine shops. Figure temperatures. 1 is an expanded view drawing showing details of construc­ tion. Several of these tools had to be made in order to fit each size of bottle cap. The tool is firmly fastened to the The composition of headspace gas is an important factor bottle cap by a 1/2 in. hose clamp. The threaded center in long term storage of packaged salad oils. Salad oils stored post of the tool has a small hole drilled in it to receive the under air for 6-10 weeks at 100 F receive lower flavor hypodermic needle and the bottom end of the post scores than those stored under nitrogen (1). Oxygen not contains a small cup that will hold and compress two silicon only dissolved in the oil, but also present in the headspace, rubber septums (gas chromatographic inlet septums) as the reacts with oil to give peroxides and oxidative cleavage post is tightened against the bottle cap. The septum next to products. Many different chemical and physical methods the bottle cap has a 3/32 in. hole at its center to allow a are followed throughout the food industry to determine the space where the hypodermic needle can be positioned to oxygen content of packaged foods. Older methods of evacuate the bottle gasket and the space around the bottle analysis based on a direct chemical determination of threads. It is advan tageous to use a small amount of silicon oxygen, such as the Orsat or Winkler techniques (14), have stopcock grease on each septum to ensure a vacuum tight been largely superseded by the physical methods of gas seal. chromatography (5-10) and paramagnetic measurement (4) The bottle cap is specially prepared before the sampling for the determination of oxygen content. In edible oil tool is attached. The bottom edge of the bottle cap is sealed processing, the polargraphic oxygen electrode has been to the glass with Apiezon wax W-2. The wax and bottle cap effective according to several pu blications ( 11-14). Ini tially are warmed sufficiently with heat lamps to soften (partially we explored its use, but the particular electrode reported liquefy) the wax and it is applied with a 1/8 in. diameter on by some of these investigators was no longer manu­ wooden dowel. After a ribbon of wax has been applied factured in the U.S. Its replacement, a new and consid­ completely around the bottle, the wax is worked by hand erably changed electrode, has not been satisfactory in our against the cap and bottle. By dipping the hands in cold hands for headspace analysis. Consequently we undertook water and not having the wax too soft, a good seal is studies using mass spectrometry. obtained and the wax does not stick to the fingers. A mass spectrometer resolves all components in a The metal at the exact center of the bottle cap, obtained headspace gas mixtu re with grea t accuracy and range of by marking with a centering square, is ground away with an concentration, if these elements are desired. Although electric hand grinder. When sufficient metal has been widely accepted for gas analysis, the application of the mass removed from an area about 1/8 in. in diameter, the thin spectrometer to storage headspace gas analysis of food layer remaining can be pierced with a steel pick and the packages is limited. Mass spectrometry is not a method gasket layer exposed. Grinding the metal stops before any suitable for control or routine analysis, and this disadvan­ gasket material is exposed. The sampling tool is now tage limits its use to research or basic investigation in fastened to the bottle cap, and the bottle positioned under well-defined areas. Detection of leaks or sampling errors is the sampling needle of the mass spectrometer. The bottle is probably more readily ascertained when a mass spectrom­ raised on a lab jack until the needle touches the sealing eter is the analyzing instrument. Drastic or unwarranted (top) septum. By careful feeling the needle is forced by changes in the ratio of normal components become obvious hand through the top septum. when all components of the mixture are determined. Error Vacuum is now applied to the entire system removing all detection is more difficult when changes in only a single the air from the bottle cap, gasket material, and the inlet component are measured. system of the mass spectrometer. The system is checked for leaks by a valve, which cu ts off the system from the I No. Market. and utT. Res. Div., ARS, USDA. vacuum pump. A system showing no pressure drop on a 106 FEBRUARY, 1972 EVANS AND SELKE: HEADSPACE GAS ANALYSIS OF SALAD OILS 107 Syringe Hole ~ Threaded Center Post Side View Exploded View FIG. 1. Expanded view and <hawing of headspace gas sampling tool for use on screw cap seals. mercury manometer within 2 mm is accepted as leak free. spectrometer for an air sample. From the standard devia­ Since practically all leaks involve the wax seal, many times tions, oxygen would have a relative error of about 7% of by working the wax with the thumb and fingers, the leak the value reported and levels of 0.1 % or less could be can be sealed and the sample saved. The cap liner prevents determined. Sensitivity and accuracy could be greater with leakage of the headspace sample into the evacuated system. instruments having less background and high sensitivity. When a leak free system has been established, the needle can now be forced through the gasket and through the bottle cap liner. Pint bottles have about 22 ml of free RESULTS AND DISCUSSION headspace and with our sampling system a pressure of 170-200 mm is obtained from the headspace gas. The Soybean Salad Oils pressure in the system is slowly lowered to 100 mm by Hydrogenated winterized soybean salad oils from four bleeding off the excess gas and another valve is closed different processors were evidently packaged under a trapping about 10 ml of gas at 100 mm pressure. This 10 ml variety of conditions. Table II contains headspace analyses sample is bled into the mass spectrometer through a of two lots of oil of the same brand name. The results differ gold-foil pin-hole system. After 2 min is allowed for surprisingly; one lot (A l-A3) contains an appreciable equilibrium to be established, readings from the mass amount of hydrogen and more argon than that found in air. spectrometer scanning system are taken. Figure 2 diagrams However both lots of oil have zero oxygen content in the the sampling technique and the inlet system to the mass headspace gas. These analyses have been interpreted as spectrometer. resulting from two types of nitrogen used in packaging the Mass spectra are corrected for background errors. Peak oil. In the first samples nitrogen gas was produced by heights are calculated on the basis of known sensitivities burning hydrocarbons in air to remove the oxygen; this and the appropriate amplification settings of the read-ou t treatment would increase nitrogen and argon contents system. These mass spectra are then normalized to 100% proportionately. The hydrogen has been purposely intro­ for the gases analyzed. Both background and sensitivity duced into the gas system to remove any oxygen com­ determinations are taken frequently between runs. Sensi­ pletely by passing the mixed gas system through a catalyst tivities can be determined from pure gas samples, air or a bed to turn the last traces of oxygen to water.

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