Assessment of the potable water mineral composition using LIBS technique
In procedures [1–3], such components of the mineral constituent as O, H, Cl and N are strongly shielded by water itself and used acids; that’s why they could not be detected (Table 1). During the dry residue test using LIBS technique, their measurement is carried out with relating weight fractions of many tested elements to 100%. Measurements are carried out in the argon medium.
In the simplified method version while operating LEA-S500 in atmospheric air (without use of the argon), precise identification of the contribution made by oxygen in dry residue is difficult while the nitrogen signal is shielded by air yet more than the oxygen signal. It is the only component in this case not subjected to measurement and accounting in the total sum of weight fractions, although, as a rule, its low concentration in the sample does not add significant uncertainty into measurement results for other components. As per oxygen, it is measured indirectly owing to the following analytical signals: carbon – as one associated with carbonates and bicarbonates in a sample (as well as organic compounds), sulfur – as one associated with sulfates, phosphorus – associated with phosphates, hydrogen – first of all, associated with hydration water of dry residue. Obviously, hydrogen concentration changes (increases) the most in dry residue as compared with mineral component in the liquid state. Significant change (decrease) by evaporation approximately two times according to ) is also typical for the carbon being the part of bicarbonates and carbonates.
Table 2 contains detection limits for some voluntary selected elements in potable water using LEA-S500 as compared with other analytical techniques. The recovery degree during measurement was checked for all specified elements using a spiking test. It was 90-105 %. It is obvious that the LIBS technique surpasses other ones considering detection limits while testing potable by dry residue.
The entire range of tested elements by ICP-OES [1, 2] and its extension according to Table 1 (totally 40 elements) should be referred to the LIBS technique as an atomic-emission one. The entire measurement range of ISCP-MS technique  (62 elements) may be considered as tested ones. This range includes REE, which are featured by the lowest ever detection limits (about 0.1 µg/l) using ISP-MS. The latest XLD values (≤1 ppm) obtained using LEA-S500 instrument for solid samples makes this technique competitive with ICP-MS measurement for REE content in potable water.
There is every reason to assume that the developed measurement procedure for element concentration in the dry residue using the LIBS technique will serve as one of the most significant information source concerning mineral composition of potable water during its application in practice.
1. ISO 11885:2007 Water quality. Determination of selected elements by inductively coupled plasma optical emission spectrometry (ICP-OES).
2. State Standard GOST 31870-2012 Drinking water. Determination of elements content by atomic spectrometry methods.
3. ISO 17294-2:2003 Water quality. Application of inductively coupled plasma mass spectrometry (ICP-MS). Part 2. Determination of 62 elements.
4. ISO 15587-1:2002 Water quality. Digestion for the determination of selected elements in water. Part 1. Aqua regia digestion.
5. ISO 15587-2:2002 Water quality. Digestion for the determination of selected elements in water. Part 2. Nitric acid digestion.
6. A, Muraviev. The guidelines for detection of water quality indicators using field techniques. – S.-Petersburg, 2009. - 220 p.