Screening for organic compounds in various sample matrices (drinking, surface and waste water, air) are performed by comparing a sample with a blank by three techniques which cover most of the organic compounds.
As such, it is possible to compare the results of processes, e.g. surface or waste water which has undergone a treatment. The result is a data file with compounds, which are present in the sample and not in the blank.
The number of organic compounds in water and air are essentially infinite. Most analyses are starting from a list of compounds, of which the concentration is determined (target analysis). Disadvantages are the use of expensive standards and the labor intensity. Quick results are difficult to obtain.
Finally, evaluating an analysis for other, structurally related, compounds is impossible, as these compounds were not included in the standard mix. Sometimes, results are to be obtained in a quick manner and is exact quantification of a compound not necessary. This a certainly the case for emerging contaminants. Untargeted or general unknown screening is perfectly fit for this purpose: one looks to which compounds are present in a sample. This poses high demands on the accuracy and the sensitivity of the instruments being used. GCxGC-TOF MS (Comprehensive Gas Chromatography-Time-of-Flight mass spectrometry), HS GC-MS (Headspace Gas Chromatography) and LC-HR MS (Liquid Chromatography-High Resolution Mass Spectrometry) are capable of detecting thousands of compounds in one run, giving full mass spectrometry details. GCXGC is a technique which consists of two GC-columns in series, resulting in a high peak capacity and, hence, in a high chromatographic resolution.
This technique is used for detecting semi-volatile compounds. More volatile compounds are detected by HS GC-MS. Finally, LC-HR MS can determine more polar and even ionic compounds. The mass spectrometer in this case has a resolution of over 20000 and a mass accuracy of less than 2 ppm. This makes it possible of reducing enormously the number of possible hits of molecular formulae. Structural information is obtained by fragmenting the molecule (tandem mass spectrometry).
To determine differences between groups of samples, statistical techniques, like multivariate statistics (e.g. principal component analysis), is used. With the aid of this technique, we see which compounds are responsible for the differences between samples.
When to use screening?
- When one wants to determine which new compounds are present in a sample.
- When quick results are necessary, e.g. in threats towards water sources.
- When no standards are available, and, hence, no target analysis is possible.
- When one wants to look afterwards if a compound was present in a sample of e.g. a year ago.