Introduction to Basic Gas Chromatography Utilization of Theoretical Efficiency (UTE%)
Coating Efficiency (CE%) is a historical term that compares the measured column efficiency and its theoretical maximum efficiency. It is calculated using Equation 6.
UTE% = Equation 6 ( ) Hactual Htheoretical x 100
Historically, Htheoretical was usually so heavily impacted by heterogeneities in the stationary phase film that extra-column contributions to Hactual could be ignored (such as injection anomalies, insufficient or misdirected makeup gas, mechanical and electronic lag times).
Because of improvements to coating efficiency, this is no longer the case and Hactual is usually more heavily impacted by extra-column contributions than the column itself. Column
contributions to Hactual become more meaningful with increasing film thickness or polarity, both of which affect stationary phase diffusion. Many authorities prefer the term "utilization
of theoretical efficiency," UTE, which take the above factors into account. Typically, UTEs are 85 to 100% for non-polar stationary phases and 60 to 80% for polar phases.
Resolution (RS)
It is not surprising that the higher the resolution, the less the overlap between two peaks. Separation is only the distance or time between two peak maxima (alpha, a). Resolution takes into consideration both alpha (a) and the width of the peaks. It is calculated using either form of Equation 7. Baseline resolution usually occurs at resolution number 1.50; however, there is no visible baseline between the two peaks. Numbers greater than 1.50 indicate there is baseline between the peaks and numbers less than 1.50 indicate there is some degree of co-elution.
R = 1.18 R = 2 Equation 7 ( ) tR2 - tR1 wh1 +wh2 ( ) tR2 - tR1 wb1 +wb2
tR1 = retention time of first peak tR2 = retention time of second peak
wh1 = peak width at half height (in units of time) of the first peak
wh2 = peak width at half height (in units of time) of the second peak
wb1 = peak width at base (in units of time) of the first peak
wb2 = peak width at base (in units of time) of the second peak
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