Removal of Water

The inevitable presence of water in atmospheric samples and its removal prior to sample analysis is a complex area. In certain circumstances its presence may be both beneficial, e.g. with canister samples where it occupies the active sites on the canister walls, or detrimental, e.g. where it affects analysis in some way. Alumina porous layer open tubular (PLOT) columns are particularly affected by moisture in the sample, and large changes in stationary phase affinity occur when water is introduced. Detectors such as mass spectrometers are also extremely sensitive to water introduced with the sample and high background noise may result. Even the flame

Figure 2 Aromatic hydrocarbon species determined using an online Tenax TA adsorbent trap in a programmed temperature vaporization injector. Column 60 m, 0.53 mm i.d., poly(dimethylsiloxane) 3 ^m film thickness. (Restek RTX-1). Desorption temperature was at 16°Cs~1 from 0°C to 220°C, and column temperature programmed from 35°C to 240°C.

Figure 2 Aromatic hydrocarbon species determined using an online Tenax TA adsorbent trap in a programmed temperature vaporization injector. Column 60 m, 0.53 mm i.d., poly(dimethylsiloxane) 3 ^m film thickness. (Restek RTX-1). Desorption temperature was at 16°Cs~1 from 0°C to 220°C, and column temperature programmed from 35°C to 240°C.

1.

unresolved volatile material

13.

toluene

25.

p-ethyltoluene

2.

hexane

14.

2- and 4-methylheptane

26.

1,3,5-trimethylbenzene

3.

methyl cyclopentane

15.

3-methylheptane

27.

o-ethyltoluene

4.

2,4-dimethylpentane

16.

octane

28.

1,2,4-trimethylbenzene

5.

benzene

17.

ethylbenzene

29.

decane

6.

2-methylhexane

18.

m- and p-xylene

30.

1,2,3-trimethylbenzene

7.

3-methylhexane

19.

styrene

31.

indane

8.

2,2,4-trimethylpentane

20.

o-xylene

32.

1,4-dimethyl 2-ethylbenzene

9.

heptane

21.

nonane

33.

dimethylethylbenzenes and undecane

10.

methyl cyclohexane

22.

isopropylbenzene

34.

1,2,3,5-tetramethylbenzene

11.

2,4- and 2,5-dimethylhexane

23.

propylbenzene

35.

naphthalene

12.

2,2,3-trimethylpentane

24.

m-ethyltoluene

36.

dodecane

(Reproduced with permission from Lewis AC et al. (1996) Atmospheric monitoring of volatile organic compounds using programmed temperature vaporisation injection. Journal of High Resolution Chromatography 19: 686-690.)

ionization detector can be affected by injection of water, and the flame may be extinguished when water elutes from the column.

Many forms of selective water removal exist, the simplest of which is the use of condensation traps or stripping coils. Losses of low boiling molecular species are insignificant although condensation of higher boiling organic material may arise at low condensa tion temperatures. Inorganic adsorbents are also commonly used, notably potassium carbonate and magnesium perchlorate. Adsorbents such as these, however, have limited capacity and often require frequent regeneration or replacement. A combination of initial condensation and second stage adsorbent scrubber often provides sufficient capacity to dry a sample stream of air for many hours or days. Con-

Figure 3 CFCs and halon species in clean marine air determined using an online Carbosieve micro adsorbent trap and direct injection to capillary GC. Detection by dual ECD/oxygen-doped ECD. Column 60 m, 0.33 mm i.d., 1 ^m film DB-1 (J&W). (A) Stage 1: standard ECD; (B) Stage 2: oxygen-doped ECD. (Reproduced with permission from Bassford M, and Simmonds PG, University of Bristol, Cantocks Place, Bristol.)

often then performed to remove the remaining components from the analytical column. A further example of direct loop injection is in the analysis of PAN where, due to thermal decomposition of the sample, analysis must be performed in situ. It is performed using direct loop injection to a cooled isothermal pre-column followed by a short analytical capillary column coupled to an electron capture detector. An ancillary measurement of carbon tetrachloride is often gained from using this approach.

Sample concentration methods Whether an atmospheric sample is collected using adsorbent or canister techniques, several stages of sample preparation are required prior to injection into the analytical column. The sheer size of sample - often many litres - precludes any form of direct injection except in the case of the high concentration species described earlier, making sample concentration a vital step prior to injection.

Canister samples require complete concentration prior to introduction to the capillary column.

Figure 3 CFCs and halon species in clean marine air determined using an online Carbosieve micro adsorbent trap and direct injection to capillary GC. Detection by dual ECD/oxygen-doped ECD. Column 60 m, 0.33 mm i.d., 1 ^m film DB-1 (J&W). (A) Stage 1: standard ECD; (B) Stage 2: oxygen-doped ECD. (Reproduced with permission from Bassford M, and Simmonds PG, University of Bristol, Cantocks Place, Bristol.)

tinuous drying may be achieved using permeation membranes such as Nafion. These forms of driers operate by means of a concentration gradient across a membrane that is permeable only to highly polar material such as water. A counter-current of dry gas is passed around the outside of the membrane as a sheath gas and carries moisture away to waste. However, this type of drier is unsuitable for samples where the quantitation of polar materials is required.

Direct loop injection High concentration atmospheric species require the least amount of sample preparation, often only the removal of excess water from the sample. Methane, carbon monoxide and N2O analyses are performed simply by filling a known volume injection loop, followed by direct injection into the analytical column. Backflushing is

Figure 4 GC-SIM/MS of natural halocarbons (CH3I, CH2ClI, CH2BrI and CH2I2) in marine air. PLOT column trap cooled with liquid nitrogen. Flash heating to capillary GC. Column 60 m, 0.33 mm i.d., 1.8 ^m film DB-VRX (J&W). Detection by single ion monitoring HP mass selective detector. (Reproduced with permission from Carpenter, L and Sturges, WT, School of Environmental Sciences, University of East Anglia, Norwich, UK.)

Figure 4 GC-SIM/MS of natural halocarbons (CH3I, CH2ClI, CH2BrI and CH2I2) in marine air. PLOT column trap cooled with liquid nitrogen. Flash heating to capillary GC. Column 60 m, 0.33 mm i.d., 1.8 ^m film DB-VRX (J&W). Detection by single ion monitoring HP mass selective detector. (Reproduced with permission from Carpenter, L and Sturges, WT, School of Environmental Sciences, University of East Anglia, Norwich, UK.)

Analytes are removed from the canister via either internal canister pressure (for pumped-in samples) or vacuum pump (for atmospheric pressure samples) over a cryogenic trap. Liquid nitrogen is most commonly used but liquid argon has been used to reduce the amount of oxygen retained in the focusing stage. The concentration zone may consist of a packed tube containing either an absorbent such as Tenax TA or glass beads or it may simply be empty stainless steel tubing. Since the concentration stage is at such low temperatures, the majority of water vapour in the sample must be removed prior to focusing in order to stop blockage of lines with ice. Once a sufficient volume has been collected on the focusing trap, the trap is generally flash heated either electrically or by using hot water. This results in a very sharp band of compounds being introduced to the head of the analytical column.

With adsorbent tube analysis the collected analytes are generally thermally desorbed either directly on to the analytical column (in the case of programmed temperature vaporization injection), or on to a focusing cold trap. The desorption temperature is generally defined by the maximum temperature that the adsorbing material can support. For polymeric adsorbents this may be relatively low (<250°C) while carbon-based materials may support desorption at temperatures of 500°C or higher.

If a programmed temperature vaporization (PTV) injector is used, the desorption may be sufficiently rapid (> 15°C s"1) so that initial column focusing is sufficient to obtain well-resolved peaks. If the desorption from an adsorbent tube is relatively slow then a focusing step is used, with a similar concentration mechanism to that used with canister samples. Once again, water must be removed from the sample since it may affect the column or the detection system.

While the majority of species are thermally desor-bed from the trap to the analytical column, a few types of compounds require solvent extraction prior to syringe injection. Organic nitrates fall into this class along with higher molecular weight polycyclic aromatic compounds that may suffer from incomplete or slow thermal release.

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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