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Technical Articles

trans Fat: Resolving cis and trans FAME Isomers by GC

trans Fat: Resolving cis and trans FAME Isomers by GC
By Julie Kowalski, Innovations Chemist
  • Highly polar Rt-2560 column resolves individual cis and trans FAME isomers.
  • Analytical reference mixes for quantifying FAMEs in foods and dietary supplements.
  • Use column and reference mixes to meet new trans fat labeling regulations.

Concern over the detrimental effects of diets high in trans fats has prompted the US Food and Drug Administration (FDA) to require trans fat content to be reported separately on food labels after January 2006. The FDA estimates that by 2009 this rule will save $900 million to $1.8 billion per year in medical costs and lost productivity. The monetary savings will far more than offset the FDA-estimated $140-250 million in one-time costs of determining amounts of trans fats, revising Nutrition Facts panels, and voluntarily reducing amounts of trans fats(1) that the food industry will incur to comply with the rule.

The highly polar Rt-2560 biscyanopropyl stationary phase has the selectivity needed for resolving cis and trans FAME isomers to comply with the FDA guidelines. Individual cis and trans isomers are resolved on a 100-meter Rt-2560 GC column (cat. RE13199), making this the column of choice for analyzing partially hydrogenated fats. The trans isomers elute before the cis isomers (Figure 1), a reverse of the elution order on Carbowax®-based phases such as FAMEWAX™ or Rtx®-Wax. AOAC method 996.06 (2) specifies the determination of total fat content based on the fatty acid content after conversion of the fatty acids to the methyl esters, and is the accepted analytical method for determining total fat content for nutritional labeling. A 100-meter Rt-2560 column meets the requirements of this procedure, and also allows quantification of the total trans fat content.

To calibrate the GC system for these assays, we recommend a carefully formulated FAME mixture, such as our 37-component Food Industry FAME Mix (cat. RE35077 - Figure 1, below) or our 28-component NLEA FAME Mix (cat.RE35078). Each of these mixes includes a gravimetric certificate of analysis to help ensure accurate quantification. To ensure correct identifications of individual C18:1 cis or trans isomers, use our cis/trans FAME Mix (cat. RE35079), as shown in Figure 2.

An Rt-2560 column is the column of choice when determining trans fat content and total fat content in food products. Whatever your fatty acid analysis requirements, Restek can provide the consistent-performance analytical columns and reference materials that will help you to accurately characterize your materials.

 

Figure 1 Food Industry FAME Mix (cat. RE35077) resolved on an Rt-2560 column.

  1. C4:0
  2. C6:0
  3. C8:0
  4. C10:0
  5. C11:0
  6. C12:0
  7. C13:0
  8. C14:0
  9. C14:1 (cis-9)
  10. C15:0
  11. C15:1 (cis-10)
  12. C16:0
  13. C16:1 (cis-9)
  1. C17:0
  2. C17:1 (cis-10)
  3. C18:0
  4. C18:1 (trans-9)
  5. C18:1 (cis-9)
  6. C18:2 (trans-9,12)
  7. C18:2 (cis-9,12)
  8. C20:0
  9. C18:3 (cis-6,9,12)
  10. C20:1 (cis-11)
  11. C18:3 (cis-9,12,15)
  12. C21:0
  1. C20:2 (cis-11,14)
  2. C22:0
  3. C20:3 (cis-8,11,14)
  4. C22:1 (cis-13)
  5. C20:3 (cis-11,14,17)
  6. C20:4 (cis-5,8,11,14)
  7. C23:0
  8. C22:2 (cis-13,16)
  9. C24:0
  10. C20:5 (cis-5,8,11,14,17)
  11. C24:1 (cis-15)
  12. C22:6 (cis-4,7,10,13,16,19)

Rt-2560, 100m, 0.25mm ID, 0.2µm (cat. RE13199)

Sample:

Food Industry FAME Mix (cat. RE35077),
30mg/mL total FAMEs in methylene chloride

Inj.:

2.0µL split (split ratio 200:1), 4mm inlet liner (cat. RE20814)

Inj. temp.:

225°C

Carrier gas:

hydrogen, constant flow

Flow rate:

1.2mL/min.

Oven temp.:

100°C (4 min. hold) to 240°C @ 3°C/min. (10 min. hold)

Det.:

FID @ 250°C



Figure 2 An Rt-2560 column resolves cis C18 FAME isomers from trans C18 isomers.

  1. C18:0
  2. C18:1 (trans-6)
  3. C18:1 (trans-9)
  4. C18:1 (trans-11)
  1. C18:1 (cis-6)
  2. C18:1 (cis-9)
  3. C18:1 (cis-11)
  4. C18:2 (cis-9,12)

Rt-2560, 100m, 0.25mm ID, 0.2µm (cat. RE13199)

Sample:

cis/trans FAME Mix (cat. RE35079), 10mg/mL total FAMEs in methylene chloride

Inj.:

1.0µL split (split ratio 20:1), 4mm inlet liner (cat. RE20814)

Inj. temp.:

225°C

Carrier gas:

hydrogen, constant flow

Flow rate:

1.2mL/min.

Oven temp.:

100°C (4 min. hold) to 240°C @ 3°C/min. (10 min. hold)

Det.:

FID @ 250°C

References

  1. http://www.fda.gov/food/ingredientspackaginglabeling/labelingnutrition/ucm274590.htm
  2. Official Methods of Analysis, 17th ed., AOAC International, 2000.

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