Gas Chromatography - Applications > Free Fatty Acids from Milk > Page 66

 

Courtesy of Supelco Inc.

1/ n-Valeric Acid 2/ n-Caproic Acid 3/ n-Caprylic Acid
4/ n-Capric Acid 5/ n-Lauric Acid 6/ n-Myristic Acid
7/ n-Palmitic Acid 8/ n-Stearic Acid 9/ n-C16-1 ene
10/ n-Oleic Acid 11 /n-Linoleic Acid 12/ n-Linolenic Acid.

Figure 42. The Separation of the Free Fatty Acids from Milk

If an external standard is used, then the extract is merely diluted to a known volume (e.g., 10 ml) and an aliquot placed on the column. This method could be considered as typical of the preparation procedures used in GC. It is clear that there can be considerably more time spent on the sample preparation than on the actual separation itself. This type of separation, however, lends itself to automation either appropriately designed hard-wired equipment or by the use of a laboratory robot. The hard wired device is generally inflexible, the laboratory robot, on the other hand, can be programmed to carry out many different types of analysis.

The separation itself has some interesting properties. Free acids are very readily adsorbed onto active sites on the support which can result in very asymmetric peaks and, as a result of the strong adsorption, significant quantitative losses can occur. In the above example, the effect of the adsorptive sites on the support is reduced by blocking them with phosphoric acid. Phosphoric acid is very involatile and thus can tolerate the high temperature and although it is active enough to block the adsorption sites it is not active enough to cause sample decomposition. It is seen that the peaks exhibit excellent symmetry for free acids. Teraphthalic acid has also been used for this purpose to deactivate the support. The column was glass, 3 m long and 2 mm in diameter and packed with a silicone polymer, SP-216-PS on 100/120 mesh Supelcoport which is a proprietary support that has already been deactivated and treated with phosphoric acid. The column was temperature programmed from 130oC to 200oC at 15oC/min. Nitrogen was used as the carrier gas at a flow rate of 20 ml/min.. The separation is effective, relatively rapid, and accurate quantitative results should be easily obtainable from the system. This analysis also demonstrates the need for rapid sample preparation techniques as well as rapid separations. Fast chromatography is of little use if the chromatograph is idle for long periods between samples while they are being prepared.