Search: mass spectrometry

Vinyl chloride is determined using gas chromatography with detection by means of mass spectrometry (GC-MS) through application of the static headspace technique (HS). This method detects selected volatile organic compounds including chloroethene (vinyl chloride).

A water sample is run through an ion exchanger in which all of the cations are replaced with hydrogen ions. The sulfate is determined through titration in the presence of a previously prepared barium chloride solution, of which a known amount in excess of that required is added in advance. The quantity is measured by complexometric titration. The difference between the initial concentration of barium chloride and the amount determined by back titration corresponds to the sulfate content.

In many cases, particularly at higher contents, an alternative and sufficiently accurate measurement is possible, called the “negative m value” or the “total mineral acid value” (without carbonic acid). This is achieved through titration. Subsequently, the mval values for the anions (Cl^-, NO₃^-, NO₂^-, PO₄^3-) are subtracted from the result.

Transformation of the chloride ions in the presence of silver ions produces silver chloride, which is not very soluble, until all the chloride ions are bound. The excess silver ions react with chromate ions to silver chromate, which exhibits a reddish brown color:

Cl ^-     +   Ag⁺            →     AgCI

2 Ag⁺ +   CrO₄^2-        →    Ag₂CrO₄

The hydride technique is used to determine the concentration of antimony ions in the water sample. In this process, antimony ions are reduced by sodium tetrahydroborate in an acidic medium. This compound is then transferred using an inert gas into a heated quartz cuvette, is pyrolyzed and measured using AAS.

The absorbance is determined at a wavelength of 217.6 nm, and the antimony concentration is calculated using a reference curve.

The hydride technique is used to determine the concentration of arsenic ions in the water sample. In this process, arsenic ions are reduced by sodium tetrahydroborate in an acidic medium to arsine. This compound is then transferred using an inert gas into a heated quartz cuvette, is pyrolyzed and measured using AAS.

The absorbance is determined at a wavelength of 193.7 nm, and the arsenic concentration is calculated using a reference curve.

The lead content is determined by employing a flameless method which utilizes graphite furnace atomic absorption spectrophotometry. This technique is suitable for determining the lead content of water with very little lead contamination. Any matrix effects can be eliminated by using the standard additions calibration technique.

An aliquot of the sample is dosed into a graphite tube and is subsequently subjected to a program comprising a three-step temperature regime through electrothermic resistance heating. As the temperature increases in each step, the consecutive steps include drying, matrix pyrolysis (incineration) and thermal dissociation into free atoms (atomization). These can be carried out separately. During the analysis, the graphite tube is under an inert gas atmosphere (argon).

Also important for graphite furnace AAS is background correction, which can be achieved using a continuum radiation source (deuterium) or through the Zeemann effect. Background correction with the Zeemann effect is used for particularly difficult sample matrices.

A hollow-cathode lamp or an electrodeless discharge lamp, which contains the relevant element in gaseous state, usually serves as the light source.