Cryogenic concentration of trace gases allows great gains in detection sensitivity. If the trace molecules are chemically stable, they may be separated from the water and air before being placed in the long path absorption cell. This is especially applicable to hydrocarbons and halocarbons.
The separation is not feasible for gases that are reactive, such as ozone and peroxides, or for gases that are highly soluble in water, such as HCl and HNO3.
An application of the cryogenic method in the measurement of fluorocarbons, carbon tetrachloride, carbonyl sulfide and other atmospheric trace gases has been decribed by Hanst, et al., (1975a). A simple collection technique was used. Air was driven by a pump into a closed 4 liter vessel immersed in liquid nitrogen. Condensation took place until the vessel was full, after which the oxygen, nitrogen and argon were pumped off, leaving a residue of ice, solid CO2 and condensed trace gases. When the container with condensate was attached to an infrared absorption cell and warned to room temperature, the trace gases flowed into the cell in a highly concentrated state.
The method depends on the vapor-temperature relations, as shown in the figure here. The most system. In the lower spectrum the dominant bands obsc volatile trace gas is CO, as shown by the vapor pressure line at the top of the figure. The CO will condense and vaporize with the nitrogen, without separation.
Methane is next down the scale in volatility. At liquid nitrogen temperature its vapor pressure is approximately 0.01 atmosphere. This is low enough for separation. Toward the end of the distillation, when the nitrogen, oxygen and argon are nearly gone, a methane-rich fraction distills. Test spectra have shown that during the removal of the methane-rich fraction there is no appreciable removal of carbon dioxide, nitrous oxide, or other minor constituents of the residue. Even ethylene remains in the cryocondenser during methane removal.
Acetylene, ethylene, ethane, other non-methane hydrocarbons, carbon dioxide, nitrous oxide, halogenated compounds, and all other minor constituents of the air do not vaporize from the cryocondenser during the removal of nitrogen and oxygen.
Nitrous oxide gives an inherent calibration of the degree to which the concentrations of trace gases have been increased by the condensation and distillation process. Nitrous oxide occurs everywhere in the atmosphere at a mixing ratio of about 3 x 10-7. Since the vapor pressure of nitrous oxide at liquid nitrogen temperature is less than 10-9 atmospheres, all the N2O remains with the other trace gases until final removal from the cryocondenser. Because of its inertness, the N2O behaves in handling like the halocarbons, hydrocarbons and other inert pollutants. The strength of the N2O band in the infrared spectrum will then tell how much gain in pollutant concentration was obtained from the condensation. In other words, the N2O bands give the spectrum “amplification factor”.
CO2 bands will dominate the spectrum of the vaporized residue. While the CO2 bands can be subtracted by means of a reference spectrum of pure CO2, some regions of the spectrum are lost because the CO2 absorption is almost total These regions can be used, however, if the CO2 is removed by absorption in sodium hydroxide. Halocarbons, hydrocarbons, nitrous oxide and carbonyl sulfide are not taken up by the sodium hydroxide. Carbonyl sulfide was not discovered in the residue until the CO2 was removed. These experiments actually led to the original discovery of carbonyl sulfide as the principal sulfur-carrying constituent of the clean atmosphere.
A second limitation of the CO2 is that it holds down the spectrum amplification factor. If the collected residue with CO2 present is placed in an infrared absorption cell at a total pressure of one atmosphere, the gas will consist mainly of CO2 at about one atmosphere total pressure. This compares with the normal CO2 partial pressure of about 0.00035 atmospheres. The amplification factor of the trace constituent concentration would therefore be about 3000. If the CO2 were removed, however, the N2O would become the major gas in the mixture, allowing an amplification factor of about three million.
Usually, the collected residue will not fill a laboratory long path cell to one atmosphere. It will then be advisable to use a small long path cell with a large path- to-volume ratio.
The second figure shows an example of the detection power of the Fourier transform specrometer when it is used in conjunction with the cryogenic concentration technique and a miniature long path cell. The cell in this case had an optical path of 1100 centimeters folded in a volume of 200 cubic centimeters, giving a path-to-volume ratio of 5.5. The spectrum beween 700 and 1200 cm-1 shows identifiable bands of 11 species of trace gases, sampled on a clear day, October 22, 1976 in Research Triangle Park, North Carolina. The carbon dioxide has been removed from the gas mixture. The amplification factor indicated by the strength of the N2O bands was approximately 16,000. It is therefore calculated that approximately three cubic meters of air were condensed and distilled to yield the residue that produced the spectrum.
The figure also demonstrates the value of the spectrum subtraction routines that are available with a Fourier transform spectrometer ure the weaker bands. In the upper spectrum, however, the dominant bands of acetylene, trichlorofluoromethane, dichlorodifluoromethane, ethylene and nitrous oxide have been subtracted, revealing the weaker bands of other pollutants and allowing them to be measured. The table gives the calculated mixing ratios for the gases in the original air sample.
It is obvious from the table that an extremely high degree of analytical sensitivity is obtained when the long path FTIR method is combined with the concentration technique. A chromatographic separation is not required. The spectral subtraction technique will generally suffice for the unraveling of complex spectra.