Determination of hydrocarbons and other compounds dissolved in water
Dissolved and suspended hydrocarbons in water can be transferred into gas phase and thus analysed with a gas analyser provided that the compounds to be measured meanwhile do not condense in the system. The condensation of high boiling compounds is mainly eliminated by keeping low sample concentrations in a matrix gas and do the analysis with a high sensitivity gas analyser. Obtaining low sample concentration in the gas phase is usually not a problem
The method of measurement
A vessel with the sample of water to be analysed is exposed to vacuum, deep enough to cause boiling. At the same time a flow of nitrogen is driven into the water through a number of small sparging filters at the bottom of the vessel. The vacuum pump keeps the pressure down in the vessel, compresses the mixture of dissolved gases, nitrogen and water vapour and leads it through a condensing cooler at +4 oC into a vacuum pumped gas cell of an infrared photometer or spectrophotometer. The type of gas cell may vary with the application but is typically a long path (10 meter) cell with a volume of about 2.5 litres. The cell with known volume now contains the various compounds in concentrations that can individually be determined by spectroscopy, and thus the mass of each individual compound can be calculated. The procedure is repeated for the same sample as necessary until the water is clean. The total mass of each of the analysed compounds will then be equal to its original presence in the water sample.
The time spent for a particular analysis is dependent on a compound's solubility in water as well as vapor pressure at the actual temperature. Low molecular compounds with low solubility are entirely transferred to gas phase already during the first sparging. If the water contains compounds with high solubility, then it is possible to increase the sparging temperature.
Detectability of the method depends mainly on two factors. In de-aerated/degassed process water it is at least theoretically possible to obtain very, very high sensitivity, possibly at the cost of a long time constant. Normal aerated water is saturated with air, i.e. nitrogen and oxygen where the oxygen content may have degraded due to chemical reactions. In this case the vacuum stripped gas at best will contain the compounds to be analysed diluted in the dissolved nitrogen and oxygen. An example worth mentioning is that the refrigerant HFC-134A in district heating water and wastewater in a 10 litre sample has been determined down to 0.05 ppb (w/w) which equals 50 nanograms per litre.
Any type of an infrared photometer equipped with a long path gas cell can be used for a particular analysis. We use a high resolution FTIR spectrophotometer. It is a powerful tool with which we can determine a number of compounds simultaneously and at the same time handle potential interference problems in a safe way. When spectra from the determinations are saved in some data base it is also possible to go back and investigate for instance whether recent finds of a particular compound also was present previously.
The method described can be used manually in the field as well as in the laboratory and is also very suitable for continuous automated process control. The illustration below shows an infrared spectrum from analysis of a water sample from a gasoline store.