One of the predominant trends of trace metal analysis is elemental speciation. The greatest interest to the metal speciation in natural water is probably explained by their influence on the bioavailability and toxicity of metals. Estimation of the degree of the toxic effect of metals on hydrobionts and water quality may be carried on the data basis about their distribution among coexisting forms in aquatic environment.

METAL SPECIATION: Terms and definitions (IUPAC Recommendations)

The International Union for Pure and Applied Chemistry (IUPAC) has presented guidelines, which contain concepts related to speciation of elements, speciation analysis and chemical species:

Chemical species are specific form of an element defined as to isotopic composition, electronic or oxidation state, and/or complex or molecular structure.

Speciation analysis is the analytical activities of identifying and/or measuring the quantities of one or more individual chemical species in a sample.

Speciation of an element, speciation is the distribution of an element amongst defined chemical species in a system.

D.M. Templeton, F. Ariese, R. Cornelis, L.-G. Danielsson, H. Muntau, H.P. van Leeuwen, R. Lobinski. IUPAC Guidelines for
Terms Related to Speciation of Trace Elements // Pure Appl. Chem. - 2000. - Vol.72, №8. - P.1453-1470 (full-text, pdf)

METAL SPECIATION: What for it is necessary?

   • The toxicity, bioavailability and mobility of the metals in aquatic ecosystems varies with its physico-      chemical form;

   • Individual metal species have a different chemical activity and different ability to transformation;

   • For forecasting of metal fate in aquatic ecosystems;

   • For development of effective methods for water quality monitoring.

METAL SPECIATION: Structural aspects

The dissolved forms of trace metals are mainly present as free (hydrated) ions and complexes with a large variety of dissolved inorganic and organic matters. Free metal ions are regarded as the most toxic form. The complexing of HM with dissolved organic matter (DOM) known to be decreases their toxicity.

METAL SPECIATION: Tools for investigation

The speciation technique should make possible to quantify the amount of free and bound metal. Only a small part of total dissolved metal exists as free ions. Hence, only the most sensitive techniques are suitable for speciation analysis. ICPMS, ICPAES, ETAAS may be considered as the most sensitive and selective techniques, but their main drawback is the impossibility of individual metal species determination without preliminary division. In most cases various forms of metals need to be separated into several fractions related to their specific activity such as charge, molecular weight, chemical and physical properties etc. Such separation inevitably results in the disturbance on the chemical equilibrium between the forms of the element in a sample. Therefore, sample pretreatment should be kept to a minimum to decrease of the changes in the native species distribution. It is achieved by using the so called direct speciation methods, which provide complete information about the species in a sample without the need of additional separation procedures.

Chemiluminescent (CL) method is the most suitable for these purposes. CL methods are characterized by a low detection limit and wide calibration ranges. It is reasonably cheap, and it does not require preliminary sample concentrating (extraction, evaporation, freezing out, etc.). The short time of analytical signal registration is a very important factor, as the equilibrium between the particular HM forms in the water sample is not disturbed during the detection. CL methods are species-specific technique for the direct determination of the concentration of free metal ions and some labile complexes in natural waters.

Chemiluminescent method: advantages and drawbacks

In our research we used CL method based on the catalytic effect of some metals in the reactions between luminol or 4-diethylaminophthalhydrazide (4-DEAPH) and hydrogen peroxide. The selectivity of metal detection can be achieved by changing pH of a reaction mixture.

Chemiluminescent method in speciation analysis: application area

Determination of toxic species (free metal ions + labile complexes)

Concentration of Fe(III), Co(II) and Cu(II) dissolved forms in water from the Dnieper reservoirs and lakes of Kyiv
Water bodies	Sampling date	Free metal ions and labile complexes, μg/l	Total content, μg/l
Fe(III)
Kaniv reservoir, upper part	08.10.02	18±2	253±28
Lake Telbin, Kyiv	03.08.01	5,1±0,7	99±6
Lake Verbne, Kyiv	10.10.01	17±2	331±7
Co(II)
Kyiv reservoir, upper part	25.04.02	0,54±0,08	5,6±0,4
Kaniv reservoir, upper part	08.10.02	0,51±0,08	3,6±0,3
Cu(II)
Kaniv reservoir, upper part	22.10.01	1,8±0,2	16±2
Ros' River	17.11.02	3,0±0,4	20±2

The chemiluminescent method enables
the direct determination
of free metal ions and labile complexes
without their preliminary separation.

Determination of metals in different oxidation states

The chemiluminescent method allows to determine one of the valent forms of these metals
(Cr(III) and V(IV)) without their preliminary separation

Investigation of kinetic of metal species transformation

Kinetic of reduction of V(V) by humic (HA) and fulvic acids (FA) as a function of pH
C_V(V)=50 μg/l; C_HA=20 mg/l; C_FA=30 mg/l

Investigation of the chemical nature of metal complexes with dissolved organic matters (DOM)

Distribution of Co, Fe(III), Cu and V(IV, V) among their complexes
with DOM of different chemical nature in the water of Kaniv reservoir

Determination of the molecular weight distribution (MWD) of metal complexes with DOM

Distribution of Fe, Co and Cu among complexes with DOM of different molecular weight in some water bodies of Ukraine
Molecular weight, kDa	Percentage
	Fe (III)	Co (II)	Cu (II)
<1	13-59	22-51	29-66
1-5	11-19	13-22	8-26
5-15	6-28	7-18	6-17
15-30	4-14	4-10	3-7
30-50	5-13	3-10	3-10
50-80	3-20	5-25	4-10
>80	6-24	2-15	3-18

CONCLUSIONS:

   • due to such characteristics as a low detection limits, short time of of analytical signal registration, species-selectivity, the chemiluminescent method is suitable for use in the speciation analysis;

   • unlike the other methods of detection, i.g. spectroscopic, the chemiluminescent method allows to determine directly the most toxic forms of metals (free ions and labile complexes) without their preliminary separation;

   • combined with ion-exchange and gel-permeation chromatography the chemiluminescent method is a powerful tool for the investigation of the coexisting forms of metals in natural waters;

   • to apply the described approach in speciation analysis more effectively it is necessary to study thoroughly such a problem as on-line coupling of separation technique with chemiluminescent detection.

Key publications:

Linnik R.P., Zaporozhets O.A. Solid-phase reagent for molecular spectroscopic determination of heavy metal speciation in natural water // Anal. Bioanal. Chem. - 2003. - 375. - P. 1083-1088 (full-text, pdf, 0,47 Mb)

Линник Р.П., Васильчук Т.А., Запорожец О.А. Сосуществующие формы ванадия в природных водах // Химия и технология воды. - 2003. - Т.25, №6. - С.549-563.

Линник Р.П., Запорожец О.А. Сравнительная оценка расчетных и экспериментальных данных о сосуществующих формах железа, кобальта, и никеля в пресных поверхностных водах // Экологическая химия. - 2003. - 12, №2. - С.79-92 (full-text, pdf, 0,69 Mb).

Линник Р.П., Линник П.Н., Запорожец О.А. Методы исследования сосуществующих форм металлов в природных водах (Обзор) // Методы и объекты химического анализа. - 2006. - 1, № 1. - С. 4-26. (full-text, pdf, 0,77 Mb)

Линник Р.П. Комбіновані спектроскопічні методи визначення співіснуючих форм Ванадію, Феруму, Кобальту та Купруму в природних водах. Дисертація на здобуття наукового ступеня кандидата хімічних наук за спеціальністю 02.00.02. – аналітична хімія. – Київський національний університет імені Тараса Шевченка, Київ, 2004.