Separation of Atmospheric Samples

While a number of specific applications utilize packed columns (notably in the analysis of methane, CO and N2O), current methods for the separation of atmospheric components are performed almost exclusively using capillary column gas chromatography.

The few applications remaining where packed columns are in use, generally employ molecular sieve packings for the separation of permanent gas species.

With the introduction of Al2O3 PLOT columns, fast high resolution analysis of even the highest volatility species has been made possible and many applications that previously used packed column GC are now being performed using capillary columns.

Where only one species is to be determined simple isothermal separations may be used, often in conjunction with a column backflush step. The analysis of PAN is an example of this where a short backflushing pre-column is used prior to a 10 m analytical column. A simple two-dimensional separation has also been proposed for PAN using heart-cutting.

The wide magnitude of analyte volatilities encountered imposes limits on the range of species that can be successfully separated on a given column. PLOT columns are used widely for the analysis of C1-C7 hydrocarbons and some high volatility halogenated compounds. The retention characteristics of this type of column are not favourable for the analysis of oxygenated compounds, and water often plays a significant role in degrading the quality of any PLOT column separation. Very strong retention of higher boiling species leads to extensive peak broadening coupled with lengthy analysis times.

Analysis of higher molecular weight species, including volatile organic compounds (VOCs), CFCs and hydrogen-containing chlorofluorocarbon replacements (hCFCs), aromatic and monoterpene compounds, has commonly been performed with nonpolar (methylpolysiloxane) or slightly polar (5% phenyl methylpolysiloxane) 1-5 |im-thick film capillary columns, 0.32 mm i.d. by 50 m long. Wide bore, 0.53 mm i.d. columns are often used where desorption is direct from a preconcentration trap to the analytical column. For the analysis of complex hCFC mixtures in the atmosphere, columns as long as 100 m have been reported.

To improve retention of volatile VOCs on thick film siloxane columns, without use of sub-ambient cooling, operation with columns containing films up to 15 |im thick have been reported. Band broadening effects through stationary phase diffusion become significant with films of this thickness and this approach has not been widely adopted. The highest molecular weight gas phase species such as naphthalene, fluorene and anthracene may be separated efficiently only on thinner film non-polar columns with film thicknesses of typically 0.25-0.5 |im.

Organic nitrate species in the atmosphere may also be determined using capillary GC with either charcoal adsorbent traps, extracted with aromatic organic solvent or via direct cryofocusing from a canister sample. Lengthy analysis times can result due to the necessity of using combinations of columns to achieve full separation of target analytes although commonly moderate polarity 50% phenyl/50% methyl poly-siloxane columns are used.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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