In a study undertaken by the UK Laboratory of the Government Chemist (LGC), GC column lifetimes using the BIP cylinder carrier gases from Air Products were predicted as more than 4 times longer compared with standard Industrial gas grades having several ppm levels of oxygen impurity. Such an increase in lifetime can result in up to 70% cost savings where column costs dominate overall analysis costs.

Problem

In the analysis of trans fatty acids which occur in many foods, and which have been linked to high levels of cholesterol and heart disease, they are normally esterified, and analysed as the fatty acid methyl ester (FAME). In this method it is very difficult to separate the trans from the less harmful cis isomers, and highly polar capillary gc columns are therefore often employed. In the LGC study Chrompack CP SIL 88 capillary columns costing some $1250 were used with a Perkin 8500 chromatograph. The cyano propyl polysiloxane stationary phase of the column relies on the separation capability of the very polar cyano group, but this makes the column very susceptible to degradation by reaction with oxygen and moisture impurities in the carrier gases. This effect is known as column 'bleed' by chromatographers, and eventually leads to failure of the column. The LGC experimental system is shown in Figure 1.

Figure 1: LGC Experimental System

Identical columns and detectors were used, and the chromatogram of standard FAME test mixtures using a cylinder of BIP cylinder helium carrier gas was taken through column 1, with a chromatogram using a standard industrial grade of gas through column 2. Analysis of the oxygen levels was confirmed through the analyser, and the effect of variable moisture removed via the dryer. Results therefore represented the difference solely due to the remaining difference in oxygen levels.

Results and solution

The effects on the number of theoretical plates and column efficiencies for the two systems are shown in Figures 2 and 3.

Figure 2: Effect on Theoretical Plates

Figure 3: Effect on Column Efficiencies

During an 8 day continuous test, the number of plates only reduced from 165000 to 145000 in the case of the BIP gas, whereas it fell from 135000 to 84000 with the standard gas. Similarly, the column efficiency only fell from 68% to 60% with the BIP gas, compared with a 20 point fall from 55% to 35% with the standard gas.

Both these effects are a qualitative indication of the superiority of the BIP gas. In order to quantify this in terms of actual column lifetime, the resolution of the two closest peaks in the FAME chromatogram was calculated for each gas and plotted over the test. Taking a linear regression analysis, and extrapolating to the end of column life, defined as when the resolution falls to below unity, gave the results of Figure 4.

Figure 4: Plot of Resolution versus Days of Continuous Use

Again the superiority of the BIP gas is clearly evident. Whereas the standard gas has a predicted life of 24.5 days, the BIP gas is 4.3 times longer at 104 days.

This means that with columns costing some $1250 each, the column costs over any analysis period totally dominate the overall costs, and in this example, a saving up to 70% in overall analysis cost would result as shown in Figure 5.

Figure 5: Cost Savings Using Built-In-Purifier Gas

Conclusions and applications

The new BIP gases from Air Products have demonstrated the potential for cost savings up to 70% in the FAME analysis system reported here. Similar cost savings would be expected in other systems, where the choice of individual columns, grade of gas used, and specific analysis would determine the actual cost savings achieved. Such savings would be in addition to the performance advantages of reduced baseline noise, limits of detection and improved resolution already reported for these gases.