Environ. Sci. Technol. 1997, 31, 114-118
Application of Laser Ablation Inductively Coupled Plasma-Mass Spectrometry in Dendrochemical Analysis SHAUN A. WATMOUGH,* THOMAS C. HUTCHINSON, AND R. DOUGLAS EVANS Department of Environmental and Resource Studies, Trent University, Peterborough, Ontario, K9J 7B8 Canada
The potential of laser ablation sampling (LAS) in conjunction with inductively coupled plasma-mass spectrometry (ICP-MS) for multi-element analysis of tree rings in Acer pseudoplatanus L. (sycamore) growing in urban U.K. is evaluated. Annual growth increments in xylem tissue are readily observed. Individual rings formed between 1975 and 1992 were sampled and analyzed by ICP-MS after LAS or after conventional nitric acid digestion. To correct for differences in ablation, 13C was used as in internal standard. Significant linear relationships were found for Pb, Mg, and Mn concentrations recorded after acid digestion and values determined by LAS. Lead in xylem declined steadily after 1975 until the late 1980s, clearly reflecting a reduction in lead deposition in urban U.K. during this period. Using acid digestion, Cd was detected only in a tree close to a Cu-Cd refinery; concentrations generally remained steady in this tree through the study period. A similar response was found in this tree with LAS. Cadmium was also detected in one of the two other trees 6 km from the refinery using LAS, but at much lower concentrations. Distribution of Mg and Mn within tree rings differed between trees; more likely reflecting differences in soil chemistry rather than atmospheric deposition. The results indicate that LAS coupled with ICP-MS can be used directly to determine changes in xylem chemistry, which reflect changes in exposure to metal deposition or changes in soil chemistry. This provides a repeatable, nondestructive sensitive method for determining many elements in wood, with high spatial resolution.
Introduction The recent application of laser ablation sampling (LAS) in conjunction with inductively coupled plasma-mass spectrometry for analysis of trace elements in biological structures, including teeth, fin rays, mollusk shells, and otoliths, offers the potential for quick analysis with minimal sample preparation and high spatial resolution (1-3). Importantly, these structures grow incrementally throughout the lifespan of an organism, potentially maintaining a historical record of concentrations in various parts of the body (4). These increments are usually small in size, and LAS is currently the only available technique exhibiting high-resolution capabilities with sufficiently low detection limits (4). Limited sample preparation also reduces the risk of contamination (4). Most * Corresponding author fax:
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 1, 1997
previous applications using laser sampling have focused on the analysis of metal alloys, ceramics, and minerals (5, 6), and most commercially available laser samplers were developed accordingly. Results of pioneering studies on biological materials are encouraging, although several problems exist with the LAS technique, including non-uniform ablation, a lack of matrixmatched standards, and heterogeneity within an increment (4). If matrix-matched standards are available or internal standards are used, an accuracy of 20-50% is possible (7). Presently, however, LAS can only be considered a semiquantitative technique (1). Historical monitoring may also be achieved using dendrochemical data. Changes in trace element distribution within xylem of several tree species have been shown to accurately reflect changes in metal deposition, increased urbanization, and acidification of soils (8-14). Wood is composed of secondary xylem, formed by the vascular tissues of the cambium (15). Each growth increment may be divided into alternating layers of earlywood and latewood, the latter being more dark and dense because of the predominance of narrow cells with thickened walls. However, care must be taken when interpreting dendochemical data. Natural spatial trends in element composition from pith to cambium, or vice versa, may occur that differ between tree species and between elements (16). Peaks in element concentration at the heartwood-sapwood boundary have been shown to occur (17), and lateral migration of elements is another factor that must be considered (18-20). Changes in xylem chemistry of sycamore (Acer pseudoplatanus L.) have been found to reflect changes in metal deposition in the vicinity of a metal refinery with little lateral movement occurring (14). A change in soil chemistry during the 1980s was also reflected in tree rings (14). The aim of the present study was to evaluate the potential of LAS coupled with ICP-MS for analyzing trace element composition within xylem of A. pseudoplatanus. No matrix-matched standards are available, and the use of internal standards is complicated by natural trends in element composition. To compensate for these limitations, values obtained by LAS are compared to concentrations obtained from the same tree rings following conventional nitric acid digestion. Differences in ablation are corrected using 13C as an internal standard.
Experimental Section Work was carried out using an Elan 5000 in combination with a laser sampler 320 (Perkin-Elmer SCIEX, Thornhill, Ontario, Canada). The laser sampler consisted of a frequency quadrupled Nd:YAG laser operating at 266 µm, a sample stage with three controlled stepper motors, and a video camera system for the observation of the sample. The sampler was controlled by the ALAS (automated laser ablation sampling) software program (1). Changes were made to the commercial laser sampler Model 320 in order to make it user friendly and more suitable for biological materials, which have been described previously (1). The computer controlling the LS 320 was connected to the Elan via the flow injection analysis system (FIAS) control port to allow automatic triggering of the Elan. Samples. Three sycamore (Acer pseudoplatanus L.) trees growing in urban Liverpool in northwest England were sampled in December 1994. One of the three trees was situated in the immediate vicinity of a Cu-Cd refinery. The remaining two were growing 6 km distant from the metal refinery. Sycamore is the most dominant tree species at the sites and has been well studied in relation to metal resistance traits and more recently for dendrochemical analysis (14).
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1996 American Chemical Society
TABLE 1. Distance from Metal Refinery (km); Mean Pb, Mn, and Mg Concentrations; and Range (µg g-1 dry wt) in Sampled Acer pseudoplatanus L. (Sycamore) Xylem in Urban Liverpool, England
a
Pb
Mn
Mg
treea
distance from refinery (km)
mean
range
mean
range
mean
range
T1 T2 T3
0.5 6.0 6.0
2.04 3.57 2.47
0.9-3.6 1.2-6.7 0.9-5.2
42.8 24.6 6.4
35.7-50.3 16.7-33.4 3.7-9.1
300 170 187
154-641 148-204 132-247
Cadmium only detected in T1.
Two cores from each tree were taken at breast height (1.5 m) using an acid-washed (10% H2SO4) 5-mm stainless steel increment borer. After each coring, the borer was washed three times with Analar grade acetone and three times with distilled deionized water (dd-H2O). Samples were immediately sealed in dry, plastic straws for storage and transport to the laboratory and were stored at -10 °C prior to analysis. From each tree, one core was divided into annual rings formed after 1975. Annual increments were separated using a stainless steel knife, washed in 10% HNO3, and rinsed with dd-H2O between cuts. Samples were placed in prewashed (15% H2SO4) 50-mL borosilicate glass tubes, dried at 70 °C for 48 h, weighed, and then dry-ashed at 400 °C in a muffle furnace for 6 h. After being cooled, the ash was dissolved in 70% trace grade HNO3 overnight and then digested under reflux at 80 °C for 4 h. Samples were filtered through acid-washed (10% HNO3) Whatman No. 42 filter papers and diluted to 25 mL with dd-H2O. Solutions were analyzed for Pb, Mg, and Mn by ICP-MS (14). Precision and accuracy was tested through repeated analysis of ground hop-hornbeam reference wood samples (Canadian Forest Service) in which concentrations were determined by interlaboratory comparisons. These elements were chosen as they were known to have different radial trends within the tree bole at the study sites (14). Recovery of elements was 84-88% for Pb (2.1 mg kg-1), 103-118% for Mn (55.3 mg kg-1), 108-116% for Mg (200 mg kg-1), and 112-120% for Cd (0.4 mg kg-1). The duplicate core was sanded and then washed in 10% trace HNO3 followed by dd-H2O prior to LAS. Individual rings were not visible through the video camera system attached with a microscope, and so points were selected at regular intervals along the core. After analysis, ablation holes were matched to the year of tree ring formation under a microscope. As a result, not all years were sampled. Rings formed in 1993 were not used in the analysis due to unreliable acid digest data. Sampled tree rings contained between three and six ablation holes. Elan 5000 Operating Conditions. The laser was operated in Q-switch mode (8 ns pulse) at 50 J (15 mJ beam energy) with a Q-switch delay of 240 µs, using 40 pulses at 10.0 Hz. The beam was focused by eye using the video camera system to allow for unevenness of the sample. An attempt was made to use 12C as an internal standard but was not found to be suitable, so the less abundant isotope 13C was used. It is recognized that both 13C and 12CH will be formed, although the rate of hydration is assumed to be constant, allowing 13C to be used as in internal standard.
Results and Discussion Comparison of LAS and Acid Digest Data. Ring widths varied between 4 and 11 mm (0.038-0.128 g), providing enough material to obtain detectable levels of Pb, Mg, and Mn following digestion of individual rings (Table 1). Detectable levels of Cd were obtained only in the tree closest to the Cd alloying plant following acid digestion. Holes produced from laser ablation sampling (LAS) were approximately 50 µm in diameter, so potentially much greater resolution than from year to year could be achieved. However, the purpose of the study was to evaluate the potential of LAS for analysis of trace
TABLE 2. Comparison of Coefficients of Determination Using Values Obtained by Laser Ablation Sampling (LAS) Corrected and Uncorrected Using 13C as Internal Standard (n ) 49)
Pb Mg Mn
uncorrected with 13C
corrected with 13C
r2
p
r2
p
0.65 0.27 0.64
