Highly Detailed Analysis of cis/trans FAME Isomers Using 200 m SP-2560 and SLB®-IL111 Capillary GC Columns

Over the last half of the previous century, the use of partially hydrogenated vegetable oil (PHVO) replaced the use of animal fats for baking purposes in most western countries. During the process to make PHVO, cis fatty acids are converted into trans fatty acids, which increases the shelf life of food products. Scientific research over the last decade has shown that the increased intake of trans fatty acids coupled with our inability to properly metabolize them can increase the risk of coronary disease. In June 2015, the US FDA mandated that food manufacturers must stop using all artificial trans fats (i.e. they can no longer use PHVO) within three years.1

The qualitative and quantitative testing of cis/trans fatty acids is best accomplished using GC after conversion of the fatty acids to fatty acid methyl esters (FAMEs). The 200 m SP-2560 and SLB-IL111 are specifically designed and tested for the highly detailed analysis of cis/trans FAME isomers. Specifications for both columns are shown in Table 1.

 

Table 1. Column Specifications

SP™-2560  
USP Code This column meets USP G5 requirements.
Phase Non-bonded; poly (biscyanopropyl siloxane)
Temp. Limits Subambient to 250 °C (isothermal or programmed)
SLB®-IL111  
USP Code None
Phase Non-bonded; 1,5-di(2,3-dimethylimidazolium)pentane bis(trifluoromethanesulfonyl)imide
Temp. Limits 50 °C to 270 °C (isothermal or programmed)

 

C18 FAME Isomer Mix

Some of the most studied fatty acids are the C18 family. A custom mixture was injected in each column, and run conditions were adjusted to maximize resolution. The optimized chromatograms are shown in Figure 1. While neither column can separate all isomers, both provide a high degree of separation of trans FAME isomers from cis FAME isomers. Of interest is that SLB-IL111 provides resolution of C18:1Δ9c (a cis FAME) from all trans FAMEs. This is significant because C18:1Δ9c often results in a very large peak area while analyzing food extracts. Its entire peak area must be considered as being contributed by a trans FAME if there is a co-elution, resulting in trans fat values that are biased high.

 

38-Component FAME Isomer Mix

A custom standard comprised of FAMEs ranging from C4 to C24, including key monounsaturated and polyunsaturated fatty acids, was analyzed on each column under identical conditions. Figure 2 shows the chromatograms obtained from both columns. Of relevance is that SP-2560 (Product No. 29688-U) provides better resolution of saturated FAME isomers from unsaturated FAME isomers, and that SLB-IL111 (Product No. 29689-U) provides increased retention of unsaturated FAME isomers.

 

Rapeseed Oil FAMEs with CLA FAME Isomers

A custom mixture containing rapeseed oil FAMEs plus four CLA FAME isomers was injected in each column using oven temperature programs which optimized resolution. Rapeseed oil is a simple vegetable oil that contains a series of C14-C24 saturated and unsaturated fatty acids. CLAs are C18:2 fatty acids in which a single carbon-carbon bond separate the two double bonds. The resulting chromatograms are shown in Figure 3. Of note is that the co-elutions on one column are fully resolved on the other, providing complementary data.

 

Discussion

The SP-2560 / SLB-IL111 pairing allows the most comprehensive fatty acid composition information possible, able to provide accurate results (qualitative and quantitative) for both saturated and trans fatty acids. Observations include:

  • Analytes elute from SLB-IL111 at a lower oven temperature (Figures 1–3)
  • SLB-IL111 provides resolution of C18:1Δ9c from all trans FAMEs (Figure 1)
  • SP-2560 provides better resolution of saturated FAME isomers (Figure 2)
  • SLB-IL111 provides increased retention of unsaturated FAME isomers (Figures 2–3)

  

C18:0, C18:1, C18:2, and C18:3 FAME Isomers

Figure 1. C18:0, C18:1, C18:2, and C18:3 FAME Isomers

 

38-Component FAME Isomers

Figure 2. 38-Component FAME Isomers

 

Rapeseed Oil FAMEs with CLA FAME Isomers

Figure 3. Rapeseed Oil FAMEs with CLA FAME Isomers

 

Legal Information

SLB is a registered trademark of Sigma-Aldrich Co. LLC
SP is a trademark of Sigma-Aldrich Co. LLC

Materials

     

 References

  1. FDA Cuts Trans Fat in Processed Foods. http://www.fda.gov/ForConsumers/
    ConsumerUpdates/ucm372915.htm
    (accessed August 18, 2015)

 

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