Electrospray ion source (ESI) is the most used ion source type for connecting liquid chromatography (LC) with mass spectrometry (MS) due to its robustness and possibility to analyze a wide range of analytes. LC/MS in turn is currently the most used analytical technique for determining trace contaminants (pesticides, mycotoxins, drug residues, …) in the widest possible range of matrices (water, food, blood, …)

ESI is a very good LC/MS ion source but it still has two major shortcomings. Firstly, ESI is in general not efficient in generating gas-phase ions (meaning, the large majority of the analyte molecules pass the ion source without being detected). Secondly, ESI is susceptible to the so-called matrix effect – decrease of the analyte response in the presence of compounds co-eluting with the analyte. These drawbacks lead to lowering the sensitivity and increase of the risk of possible false-negative results.

Anneli Kruve from UT Chair of Analytical Chemistry has come up with an original idea how to improve the sensitivity of an ESI source and make it less susceptible to the matrix effects. The essence of the idea is to modify the nebulizer (the key component of an ESI source) by adding an additional capillary directing the nebulization gas right into the stream of solution (the thin innermost capillary on the image). The prototype of this ion source has been built (see the image) and experiments have shown that this design offers significantly enhanced ionization efficiency compared with the classic nebulizer design and leads to improved sensitivity (by three to 10 times) and decreases the detection limit, on an average 10 times. The patent application of this development has been filed and a communication has been published in J. Am. Soc. Mass Spectrom. (2012).

Recent studies have shown that the matrix effect in the ESI source mostly arises from the competition of ionized analytes and matrix compounds for the droplet surface. It is expected that by more effective nebulization the net surface area of the droplets increases and thus the competition decreases leading to decrease or elimination of the matrix effect. Testing of this hypothesis is currently in progress.

The gain in ESI sensitivity may open up new horizons in different fields of LC/ESI/MS application. It may become possible to detect different marker molecules at very low levels in complex biological matrixes in biochemical and medical analysis, possibly leading to e.g. early discovery of diseases; it may become possible to determine the background concentrations of organic pollutants in environment, which is very important for understanding and modeling their behavior in the environment; etc.

(Image by Ave Saluvee)



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