inductively coupled plasma mass spectrometry (ICP-MS). ICP-ES ... In most cases where comparison was possible the ICP-MS data compared favourably with ...
COMPARISON OF DIFFERENT MULTIELEMENT TECHNIQUES FOR ANALYSIS OF MOSSES USED AS BIOMONITORS EILIV STEINNES
Department of Chemistry, University of Trondheim, A VH, N-7055 Dragvoll, Norway ODDVAR JOHANSEN
Institute for Energy Technology, N-2007 Kjeller, Norway ODDVAR ROYSET
Norwegian lnstitute for Air Research, P.O. Box 64, N-2001 LillestrCm, Norway
and M A G N E 0DEG.~.RD
Geochemical Division, Geological Survey of Norway, P.O. Box 3006, Lade, N-7002 Trondheim, Norway (Received: November 1991) Abstract. Mosses are used as biomonitors on a regular basis to study trends in the atmospheric deposition of trace elements in Norway. In this paper the analytical scheme used so far, based on a combination of instrumental neutron activation analysis and atomic absorption spectrometry (INAA/AAS) is compared with inductively coupled plasma emission spectrometry (ICP-ES) and inductively coupled plasma mass spectrometry (ICP-MS). ICP-ES provided satisfactory data for nearly 20 elements, but detection limits were inadequate for some elements of importance. For ICP-MS quantitative data were obtained for 33 elements including the heavy metals of key interest in air pollution studies. In most cases where comparison was possible the ICP-MS data compared favourably with data obtained by the reference scheme or by ICP-ES. On the basis of this study ICP-MS is considered to be an equivalent alternative to INAA/AAS in multielement studies using mosses as biomonitors. In addition ICP-MS offers some promise for the study of elements such as Be, Ga, Mo, Te, T1, and Bi, for which very little information exists regarding their behaviour as air pollutants.
1. Introduction Mosses and lichens are being increasingly used as biomonitors in air pollution work. Since these organisms lack a root system, they are dependent on the environment to which they are exposed for the uptake of chemical substances. They have therefore been found useful to monitor many heavy metals and other trace elements contributed by atmospheric deposition, either by sampling of the organism in situ or by the use of transplants such as moss-bags. In Scandinavia epigeic mosses (mosses growing naturally on the ground) have been found to be preferable to epiphytic mosses and lichens in regional heavy metal deposition studies (Rtihling and Tyler, 1973; Pakarinen, 1976; Steinnes, 1977). Since 1980, inter-Nordic surveys have been carried out every five years (Gydesen et al., 1983; Rtihling etal., 1987) using the moss Hylocomium splendens (alternatively Pleurozium schreberi). In Norway, national deposition studies using Hylocomium splendens were carried out in 1977 and 1985, using a combination of instrumental Environmental Monitoring and Assessment 25: 87-97, 1993. (~) 1993 Kluwer Academic Publishers. Printed in the Netherlands.
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neutron activation analysis (INAA) and atomic absorption spectrometry (AAS) for the determination of 27 elements. In connection with a third upcoming survey in 1990 it was decided to see whether alternative multielement approaches could be used, and the techniques selected for an intercomparison study were inductively coupled plasma emission spectrometry (ICP-ES) and inductively coupled plasma mass spectrometry (ICP-MS). ICP-ES has been a well established technique for multielement studies for a number of years, and reported applications cover a wide variety of environmental matrices. Prior to this work the technique had been successfully applied by two of the authors to the determination of 14 mineral elements in different species of vascular plants (LCbersli et al., 1990). ICP-MS has been in routine use only for a few years, but appears to be an extremely versatile and sensitive analytical technique, facilitating the simultaneous determination of about 20 elements in plant material and over 30 elements in geological samples (NOltner et al., 1990; Longerich et al., 1990). To the knowledge of the authors ICP-MS has not been previously applied for the analysis of biomonitors used in air pollution research, but simultaneous work carried out on air filters (Berg et al., 1991) and preliminary results from analysis of precipitation samples [10] indicated that application on mosses might be successful. 2. Materials and Methods
2.1. SAMPLES This study was based on 11 air-dried and homogenized moss samples (Hylocomium splendens) representing different contaminant levels, prepared for an intercomparison study to be carried out among various laboratories potentially to be participating in the 1990 Nordic heavy metal deposition survey. Details on the preparation of samples and results of the intercomparison exercise will be reported elsewhere (Riihling et al., in preparation). No further drying of the samples was performed before analysis. 2.2. INAA, AAS The samples were subjected to the same procedure as previously used in the Norwegian national moss surveys (Steinnes, 1980) except that some elements with no apparent connection to airborne supply were not recorded. In the INAA 0.3 g portions of the samples were analyzed without any petreatment. For the AAS determination 2 g of sample was decomposed with 10 ml conc. HNO3 for 20 h at gentle heat. The solution was then evaporated to dryness and the residue dissolved in 25 ml 0.1 M HNO~. An air-acetylene flame was used for the determination of Cu, Zn, and Pb, while Cd determinations were performed with a graphite furnace. 2.3. ICP-ES Two different procedures were tested for the decomposition of samples:
ANALYSIS OF MOSSES USED AS BIOMONITORS
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I.
Dry ashing of 2 g sample at 430°C for 20 h. The ash was dissolved in 5 ml conc. HNO3 (4 h) + 1 ml HC1 (20 h) on a water bath, and the solution subsequently diluted to 50 ml with H20.
II.
1 g of sample was decomposed with 15 ml conc. HNO3 for 20 h on a water bath, and the resulting solution diluted to 50 ml with H20.
The sample solutions were analyzed as previously described (Odegfrd, 1981) using a Jarrell-Ash 975 Plasma AtomComp instrument. 2.4. ICP-MS An amount of 0.5 g of sample was digested in a closed PTFE bomb with 5 ml conc. HNO3 for 4 h at 150°C. The solution was then diluted to 50 ml with H20. Duplicates of each sample were prepared independently. The ICP-MS analyses were performed with a VG Plasmaquad I using the instrument conditions specified by the manufacturer. A Meinhardt nebulizer with a double-pass Schott spray chamber was used (16/0.3/0.71 min -1 respectively for cool/plasma/nebulizer gases). The ion lenses were focused on the 115In isotope used as internal standard. A multielement scan procedure with about 1 s total integration time per mass unit was employed. The isotopes of each element selected for the ICP-MS determination were those with the lowest frequency of overlaps from isobaric isotope of other elements. A multielement standard containing 35 elements was made from high-purity single element standards (Spectrascan, Teknolab A/S, Norway) and adjusted to the same HNO3 concentration as the samples.
3. Results and Discussion
3.1.
FEASIBILITY OF
ICP-MS
FOR TRACE ELEMENT DETERMINATION IN MOSSES
The reproducibility of the ICP-MS determinations based on duplicate analyses of the 11 Nordic moss reference samples is shown in Table I. The isotopes used in the analyses and the concentration ranges encountered are also shown. Of the 35 elements studied, all except Se and Te could be quantified. Of the remaining 33 elements, 24 elements showed a relative deviation of less than 10% between duplicates decomposed and analyzed separately, and for 14 of these elements, notably Na, Mg, A1, Ca, V, Fe, Ni, Cu, Zn, Ga, Rb, St, Ba, and Pb a mean deviation of less than 5% typically in the range 3-5%, was observed. This is considered very good for a multielement technique at these mostly low concentration levels, given that all deviations due to sample inhomogeneities, preparation of sample solutions, and instrumental variations are included in the precision figures. The applied ICP-MS technique obviously fulfills the major prerequisite for doing accurate trace element analysis of mosses, namely a high overall precision.
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TABLE I Reproducibility of ICP-MS in the analysis of duplicate portions of eleven Nordic moss reference samples. Element
Mass
Concentration range (ppm)
Mean deviation between duplicates (%)
Li
7
0.057-1.00
6.8
Be
9
0.008-0.096
22.3
B
11
0.92-5.02
9.4
Na
23
44-217
4.9
Mg
25
629-1330
4.0
A1
27
158-2279
4.2
Ca
44
1465-2925
4.0
Sc
45
0.21-0.40
20.1
V
51
1.86-30.3
3.6
Cr
52
0.85-6.03
6.3
Mn
55
83-815
5.2
Fe
57
183-2398
3.6
Co
59
0.144-1.18
6.6
Ni
60
1.51-10.9
4.6
Cu
63
5.7-87.8
4.6
Zn
64
26.7-70.8
4.0
Ga
69
0.42-2.42
4.0
As
75
0.09-0.81
17.1
Se
82
