FAME Standard for Optimizing GC System Performance

Vicki Yearick, Katherine Stenerson

Reporter US Volume 28.1

vicki.yearick@sial.com

AOCS and AOAC methods for determining the trans fatty acid composition of foods by gas chromatography require the use of a highly polar cyanosilicone capillary column to provide the best peak resolution attainable of the numerous geometric (cis & trans) and positional isomers. Due to the highly polar nature of these phases, subtle changes in the GC system can alter the chromatographic separation of the positional cis & trans isomers. Therefore, it is important to continually monitor column performance and correct for unexpected changes in the column flow, oven temperature, column degradation and/ or sample concentration as they occur.

AOCS Method Ce 1h-05 recommends using a well-characterized mixture of fatty acid methyl esters (FAMEs) covering the range of fatty acids under investigation to monitor changes in column performance over time. Sigma-Aldrich’s new cis/trans FAME Column Performance Mix is an excellent choice for monitoring these changes. This qualitative mix is specifically designed to optimize and to monitor performance changes per AOCS Method Ce 1h-05 and AOAC Method AOAC 996.06. The mix contains C18:1 cis/trans positional isomers of critical importance to the food industry.

Because some cis/trans positional FAMEs are difficult to separate and baseline resolution may not be possible, regular use of the Sigma-Aldrich FAME Column Performance Mix is of utmost importance to achieve the required column performance. Small changes in the sample size, sample concentration, or oven temperature may be required to achieve the best resolution between the C18:1 cis/trans isomers. Once optimal resolution for the column is established, weekly analysis of the mix allows monitoring of the column’s performance.

The analyst injects the mix and identifies the peaks labeled as “1” and “2” in Figure 1. The resolution between these peaks is then calculated using the equation shown in Table 1. It is important to run the mix upon initial installation of a new capillary column to ensure that conditions are established to optimize resolution. Optimum resolution between two peaks is defined as “baseline” resolution. Full baseline resolution is achieved when the R-value calculated using the equation in Table 1 is approximately 1.5 or greater. The example chromatogram shown in Figure 1 illustrates the measurements required when calculating the resolution value - specifically peak width and retention time. As is evident in Figure 1, baseline resolution of peaks 1 and 2 will not be attained under normal testing conditions. The minimum acceptable resolution value will be described in a future revision of AOCS Method Ce 1h-05. For now, laboratories must establish their own criteria.

Figure 1. cis/trans FAME Column Performance Mix (24056), (40495-U)

Table 1. Resolution Equation

R=1.18 (tR2-tR1)/ (w(1/2)1 + w(1/2)2)

tR1= the retention time of peak 1

tR2= the retention time of peak 2

w(1/2)1= peak width at half height of peak 1 in mm

w(1/2)2= peak width at half height of peak 2 in mm

For example:

R=l.18(23.053-22.922)/(0.0846 + 0.0912)

R=0.88

At present, no column will fully resolve all C18:1 cis/trans positional isomers. By utilizing the cis/trans FAME Column Performance Mix, analysts can optimize column performance before assaying complex mixtures. Regular use also makes it possible to monitor column degradation and to recognize when to replace the capillary column.

References

1. 2005. AOCS Official Method Ce 1h-05. Determination of cis-, trans-, Saturated Monosaturated and Polysaturated Fatty Acids in vegetable or Non-ruminant Animal Oils and fats by Capillary GLC..

2. 2001. AOAC Official Method Ce 996.06. Fat (Total, Saturated, and Unsaturated) in Food, Hydrolytic Extraction Gas Chromatographic Method..