Information produced by Dr. Wall  for the GC lab.

Measuring Ethylene with Gas Chromatography

Gas chromatographs use the flow of an inert carrier gas (the mobile phase) to carry the injected gas sample through a metal or glass column (the stationary phase). The column is packed with a substrate that differentially retards the passage of gas components, and therefore separates them. The column used for ethylene analysis is packed with activated alumina, and the carrier gas is nitrogen.

There must be a pressure drop across the column for the carrier gas to flow through the column. The gas pressure at the start of the column must be greater than the pressure at the end of the column near the detector. Injection of the sample at the start of the column is therefore injected against a positive pressure.

Ethylene is detected with a flame ionization detector. A hydrogen flame burns the samples within the carrier gas, forming ions and electrons. The electrical conductivity is then proportional to the concentration of charged particles within the gas. A flame ionization detector does not work for He, O2, N2, H2O, NH3, CO and CO2.

Calculations: C2H4 production rate under static conditions

The gas chromatograph must be calibrated with a known standard of ethylene gas. The GC will determine ethylene in parts per million (ppm).

V1 = volume of container
V2 = volume of fruit
W = fruit weight (grams)
T = time in jar (hours)

Rate of C2H4 evolution = ppm C2H4 x (V1- V2) x 1/W x 1/T
(nl C2H4/gfw/h)
The units are nanoliters per gram fresh weight per hour, which is the same as microliters per kilogram fresh weight per hour.
nl/g/h = ul/kg/h


DETERMINATION OF ETHYLENE PRODUCTION RATES IN HARVESTED HORTICULTURAL COMMODITIES.

Measrue the weight of fruit in the jar  and the volume of the jars after the experiment.  Each group will determine the concentration (ppm) of ethylene in 1 replication of the fruit by withdrawing 1 ml of gas and injecting the sample into the GC. Be sure to record the time, weight, and necessary volumes. Check with members of the other groups to get the gas concentrations for the other replications, and use the formula on page 1 to determine the ethylene production rates for each of the  jars. Fill in the following table, calculate mean C2H4 production rates and write (typed) a brief paragraph discussing your results along with the other components of the report describing the CG.

 

Commodity Rep Fruit Weight
grams
Fruit Volume
liters
Jar Volume
liters
Hours in Jar PPM C2H4 C2H4 Rate
  1            
  2            
  3            
  4