Chemical analyses
Introduction.
The analysis of siloxanes in biogas can be carried out with the same techniques that are used for the analysis of air contaminants.
The simplest procedure could consist of introducing a sample of air inside an instrument for study that in a short period of time will indicate the pollutants present and their relative concentrations in the analyzed sample. Instruments that allow the analysis of inorganic gases (NOx, SO2, CO, HCl...) and volatile organic compounds or particles in suspension exist. Some of these instruments require the presence of an analyst (manual instruments) whereas others work automatically (analyzers), and can even send its results to a data processing center away from the sampling point. In all the cases the analysis takes place "in situ" and the concentration of the contaminant of interest is obtained at the same moment at which the sample is taken from the air (instantaneous concentration).
Nevertheless, not always is there an instrument that allows "in situ" analysis in such a simple form available. An alternative strategy consists of introducing the sample into a container and taking the sample to a laboratory for analysis. In this case, the result is not obtained at the same place and time in which the sample was taken, although the result indicates the concentration of the contaminants in the air at the moment, normally a short period of time, during which the sample was taken.
Sometimes the desired compound concentration for analysis is too low therefore it is not possible to measure with the currently available instruments. A solution to this problem consists of circulating a sufficient volume of air through a "trap" which retains the substance of interest and lets the air pass through. The trap can be analyzed "in situ" or more frequently it is transported and analyzed in a laboratory. In this case the amount of contaminants retained in the trap must be related to the total volume of air that has been made circulate. The normal process is to circulate great volumes of air through traps of a much smaller volume, with which the concentration of the contaminants in the trap is much higher than their concentration in the air. That is to say, with this process one is able to concentrate the contaminants.
The air can have varying levels of contaminants throughout the time in which the sample is taken and, therefore, the information that is obtained will be an average of the contaminant concentration throughout the sampling time (concentration average).
In the sampling techniques that we have previously presented, the air with the help of a pump is drawn in towards the interior of the measurement instrument, a storage container, or through a trap. In short, an element is used that moves a certain volume of air and, for that reason, these sampling schemes are said to be active.
As opposed to the active systems, the possibility exists of taking advantage of the entropic property of gases that allows them to move randomly in all the directions, to spread between molecules of another gas and to arrive at the surface of a liquid or a solid that, if they have been selected properly, can act like as a trap for the contaminant of interest. These sampling schemes are said to be passive.
"In situ" Analysis of Contaminants
Manual analysis with colorimetric tubes. The air is circulated through a glass tube that contains an inert solid (silica, aluminum, or another adsorbent solid) on which has been deposited a substance that instantaneously reacts with the compound of study and generates a certain coloration (Fig. 1). The length of the colored tube is proportional to the amount of pollutant present in the aspirated air volume and, therefore, it is proportional to the concentration in the air. The sample taking is made in a few second and the result is obtained instantaneously.
Analyzers. In general, analyzers consist of a pump that draws in the air that is desired to be analyzed and leads the air through a detection chamber in which a physical-chemistry phenomenon takes place based on the characteristic of the substance or set of substances that is desired to be determined. The magnitude of the physical-chemistry phenomenon is related to the concentration of the contaminant in the air, in many cases the phenomenon is directly proportional to the concentration. |
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The physical-chemistry process can consist of: the absorption of electromagnetic radiation, that is to say of light (absorption spectrophotometry), electromagnetic radioactivity (fluorescence or chemical luminescence), change of t electrical conductivity (conductimetry) or pH of a solution (potentiometry), oxidation of the contaminant, and electron liberation (electrochemical detector), combustion of the contaminant in a hydrogen flame and electron liberation (flame ionization detector), retention of the contaminant in a filter and its interaction with ß radiation (attenuation ß radiation), ...
Thus, for example, when burning the organic substances in a hydrogen flame and in air the CH, CH2 groups of these substances ionize releasing electrons. The amount of electrons generated is proportional to the concentration of organic contaminants present in the air (flame ionization detector-FID).
Portable Gas Chromatograph. With the FID detector the total volatile organic compound content can be determined in the analyzed air, however it is not possible to say specifically which compounds are present and in which concentration. A gas chromatograph allows for the separation of different volatile organic compounds presents in the air sample by means of a chromatographic column with an adapted stationary phase and to separately detect the different volatile compounds with a FID or other types of detectors. The compounds are separated by their degree of interaction with the stationary phase, eluting first those that which interact less. With these instruments it is possible to identify the contaminants present in the air and to quantify their concentrations.
Normally, gas chromatographs are installed in conventional laboratories, since they are heavy instruments (about 100 kg), consume large quantities of energy (can work to 350ºC), and work connected to one or several compressed gas cylinders (of about 50 Ls of capacity). Nevertheless, a portable version of these instruments exists allowing "in situ" analysis and results obtained immediately.
Taking and conservation of samples ¨in situ¨ and laboratory shipment.
- Recipients for the air sample.
The airs is introduced inside a plastic bag, in a glass container, or in a metal canister with the help of an aspiration pump and is subsequently taken to the laboratory as rapidly as possible for analysis.
Bags of TedlarR, a plastic material are usually used. The bags are inert, re-usable, have excellent mechanical and thermal properties, and generate minimum losses during the period of storage (Fig. 2). The plastic bags allow the sample to have the same pressure as that of the original air. They have a perforated septum that allows extraction of an aliquot of the sample with a special syringe made to manipulate gases.
- Physical-chemical Traps
Cold trap. The air circulates through a container that stays at a sufficiently low temperature that the desired substances condense for subsequent analysis.
Absorbent liquids. A known volume air is made to bubble through an absorbent liquid that can simply dissolve the contaminant of interest or, in many cases, to react with the absorbent liquid. The air is directed to the liquid cavity by means of a tube ending in a small hole or by means of a tube ending in a porous plate (depending if the air contains or not particles in suspension) (Fig. 3).
Solid adsorbent tubes. With the help of an aspiration pump, the air to analyze is made to drip (circulate) through a glass tube whose interior contains an adsorbent solid. If the adsorbent has been chosen correctly, the contaminants present in the air of interest interact with the adsorbent and are retained in their surface (Fig. 4).
Activated charcoals (of coconut or petroleum) are the adsorbents most frequently used. Nevertheless, the range of adsorbents used is very ample and includes among others: carbon graphites, silica gel, aluminum, magnesium silicate (FlorisilR), porous polymers (polyurethane divinylbenzene-styrene, foam, TenaxR...), molecular sieves... Also, it is possible to use adsorbents impregnated with a suitable reagent. In this case the contaminant reacts with the reagent generating a new substance that is more stable or easier to analyze.
In fact, the contaminants advance through the adsorbent, dragged by the air, and they do so much more slowly with greater adsorbent interaction. If the interaction is very intense, the contaminant practically does not advance, whereas if the interaction is very weak and the time during which the contaminant is made to circulate is sufficiently long, a part of the contaminants can cross the adsorbent tube without being retained. One can say that the used air volume has surpassed the rupture volume of the tube used.
On the other hand, if the concentration of the contaminant of interest is too high all the active points of the adsorbent can get to be blocked and, in this case, one will not be able to retain the contaminant of interest quantitatively. The adsorbent has been saturated.
For that reason, tubes in which the adsorbent has been divided in two parts (sectors A and B) is frequently used. If the capture works correctly, the amount of contaminant retained in the first sector will be remarkably superior to the amount captured in the second sector.
Filters. The air is forced to cross a filter of a certain type and size pore. The contaminants that are present in the air in particle form are retained. Normally, mixed acetate membranes are used of nitrocellulose cellulose, fiber glass, polyvinyl chloride (PVC), teflon (PTFE), and silver, or the membranes, can be impregnated with a reagent that reacts with the contaminant that is desired to be studied, facilitating its retention.
In some cases it is possible to connect in filters series and a tube with adsorbent solid. In the first trap particles in suspension are retained, whereas in the second trap the dissolved gases are retained.
Sample analysis in the laboratory.
The sample will be in a gaseous state if it was taken in a container of plastic, glass or metal. In this case, one will take aliquot from the sample having squeezing the walls of the bag, if the bag is made of plastic, the gas can be taken with a gas syringe through the perforated septum or the gas can be taken by use of an aspiration pump. In general, the aliquot will be analyzed by gas chromatography.
The sample will be in a liquid state if it has been retained in an absorbent liquid. In some cases it is possible to analyze this liquid directly. In other cases it may be necessary to carry out a more or less complex preparation process: dilution of the sample, pH fit, or transform the contaminant into another substance which is easier to measure, to extract compounds of interest with an organic dissolvent, to concentrate the organic extract, and or to purify the sample...
The sample will be in solid state if the contaminants have been retained in a filter or a tube with adsorbent.
In the filters the particles are retained (solid or liquid) that were suspended in the air. The particles can contain metals, metallic compounds, inorganic salts, and organic compounds of high molar mass. The filter can be weighed and the increase of weight attributed to the retained particles (gravimetry), or, can be treated with water to extract anions corresponding to soluble salts (extraction with water) if it is desired. The filter can be treated with acid to determine heavy metals (acid digestion) if it is desired or with an organic dissolvent to analyze organic compounds (extraction with solvent) if it is desired.
In the tubes with adsorbent organic gaseous compounds are retained that have interacted with the adsorbent. In order to extract the compounds the forces of interaction between the contaminants and the adsorbent must be weakened. The simplest option is to put the adsorbent in contact with a suitable solvent and to simply leave the adsorbent in contact with the solvent for a suitable amount of time (5min - 1 hour). Sometimes it may be necessary to shake the sample in order to favor the contact between the adsorbent solvent and to provide energy in the form of heat.
Another option is to weaken the interactions providing heat while the adsorbent sample circulates in an inert gas; the gas drags the contaminant to the interior of the analysis system (gas chromatograph). This process is denominated as thermal desorption. Thermal desorption can be used if the studied contaminants are thermally stable. The ideal condition is that the desorption is very fast and does not to dilute the contaminants in the gas too much and the introduction of the gas is instantaneous into the chromatograph. The desorption gas used is the same as the gas that feeds the chromatograph.
The analysis of siloxanes in biogas.
In the consulted bibliography procedures that allow the generated ("in situ") analysis of the siloxanes in biogas, in the garbage dump are not described.
Some authors, transfer the samples of biogas to the laboratory using steel canisters, glass collection containers or Tedlar bags.
In other cases, the siloxanes in the garbage dump are concentrated in an absorbent liquid or in an adsorbent solid and, later, the liquid or the solid is transferred to the laboratory.
The liquid absorbents that are used include methanol, acetone or n-hexane. Later, these liquids are analyzed without carrying out previous treatments.
The solid adsorbents utilized include activated charcoal, polyurethane foam, XAD-2 and XAD-4 resins. The siloxanes are desorped using carbon sulfide, methylene chloride or methylisobutylcetone (MIBK)
Finally, the liquid or gaseous solutions of siloxanes are analyzed by gas chromatography/flame ionization (HRGC/FID), gas chromatography/mass spectrometry (HRGC/MS) or gas chromatography/atomicx emission spectroscopy (HRGC/AES).
