Multielement determination in estuarine suspended particulate matter


Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J1, Canada. An instrumental neutron activati...
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A N A L Y T I C A L CHEMISTRY, VOL. 51, NO. 7, JUNE 1979

Multielement Determination in Estuarine Suspended Particulate Matter by Instrumental Neutron Activation Analysis Katherine M. Ellis’ and Amares Chattopadhyay” Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J1,Canada

An instrumental neutron activation analysis (INAA) method has been developed for the simultaneous multielement determinations In samples of estuarine suspended particulate matter (SPM) weighing between 0.75 and 7.5 mg. The INAA method consists of four sequential irradiations at two different fluxes and counting after four decay periods using a highresolution Ge(Li) detector. Concentrations of up to 28 elements have been measured in estuarine SPM collected from water with salinity varying between 2.23 and 29.9%. The average precision of measurement has been observed to vary between f10 and 16% for most elements. The accuracy of determination has been evaluated by analyzing standard reference materials, and agreements within * l o % of the certified values have been obtained. The detection limits for most trace elements have ranged between 0.37 ng and 1.0 Pg.

Large quantities of suspended particulate matter (SPM) are generally discharged from rivers to oceans via estuaries. Intense industrial and agricultural activities close to rivers and estuaries have increased not only the S P M loads but also the amounts of toxic and potentially toxic elements in them. In view of the increasing awareness of heavy metal pollution of the aquatic ecosystem, it is important that suitable analytical methods be available for determining concentrations of several elements in aquatic samples. Relative elemental content of particulate and soluble fractions of water and of sediment are necessary for studying sources, pathways, and sinks of these elements. Trace elemental content can also be used t o determine the nature of suspended matter ( I , 2). T h e determination of a number of trace elements in estuarine SPM is difficult for several reasons such as very low concentrations of the elements of interest in presence of large amounts of interfering elements, microgram quantities of total SPM generally available for analysis, and large reagent and filter blanks. In the past, atomic absorption spectrometry (AAS), colorimetry, X-ray fluorescence, and emission spectrometry methods have been applied to trace element analysis of SPM (1-11). Both flame and flameless AAS methods have been used t o analyze SPM collected from river, coastal, and ocean waters ( I , 3-7). In these studies, large amounts of reagents have been added to either dissolve the membrane filters containing SPM or extract the particulates from filters. Use of ultrapure reagents for wet ashing of S P M in closed Teflon bombs substantially reduces the reagent blanks ( 5 ) ; however, it does not eliminate the problem. Moreover, AAS methods are generally applied for the determination of single elements, or sequentially for a small number of them. Nondestructive multielement techniques are best suited fur such purposes. Among these techniques, neutron activation, X-ray fluorescence, and emission spectroscopic methods are noteworthy. ‘Present address: Atlantic Environmental Radiation Unit, Chemical Oceanography,Atlantic Oceanographic Laboratory, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada. 0003-2700/79/0351-0942$01 O O / O

Instrumental neutron activation analysis (INAA) has been successfully applied in the past to a number of complex sample matrices including atmospheric particulate materials (12-14). It should be noted here that t h e weight of atmospheric particulate materials normally analyzed by INAA is considerably higher than t h a t of the estuarine SPM. A few researchers have used INAA for the determination of selected elements in SPM collected from river, coastal, and ocean waters (15-19). Almost no studies have involved the analysis of elements by assaying their short-lived nuclides. T h e full potential of INAA for measuring trace element content of estuarine SPM has not previously been investigated in detail. T h e objective of the present work is to develop an INAA method for the simultaneous determination of trace multielement concentrations in microgram to milligram quantities of estuarine SPM. Precision and accuracy of measurements have been evaluated by replicate analysis of samples and standard reference materials. Limits of detection and determination for the elements measured have been calculated.

EXPERIMENTAL Sample Collection a n d Preparation. Samples of estuarine SPM were collected from the Saguenay Fjord, Quebec, Canada, during the C.S.S. Hudson Cruise z76-006 organized by the Atlantic Oceanographic Laboratory, Bedford Institute of Oceanography, Dartmouth, Nova Scotia. The SPM load of water a t a depth of 5 m in the Saguenay Fjord ranged from 0.59 t o 16 mg L-’. The salinity varied between 2.23 and 29.9 parts per thousand (ppt). Thirteen samples of water from a depth of 5 m and one each from depths of 125 m and 150 m were collected using 20-L Niskin water samplers in which the internal rubber spring was replaced by a Teflon-coated stainless steel spring. Samples of SPM were obtained by passing water through prewashed and preweighed Nuclepore membrane filters (47-mm diameter, 0.4-pm pore size) under a pressure of 5 psi of gaseous nitrogen according to the method prescribed by Spencer and Sachs ( 1 ) . Nuclepore membranes were mounted on disposable Millipore filter holders. Samples of SPM weighing between 0.75 and 7.5 mg were collected by filtering 0.23 to 2 5 L of water immediately after sampling. The membrane filters containing SPM were then washed with 50 mL of distilled-deionized water, dried in an oven at 60 “C for 2 h on board ship, and were placed in small precleaned plastic Petri dishes. The filters were reweighed after transferring them to the Analytical Laboratory. Since a small size would be desirable for both irradiation and counting, the 47-mm membrane filters containing SPM were compressed to pellets of 5 mm in diameter using a clean die. The pellets were placed in prepared precleaned high-quality polyethylene bags (6 X 6 mm in size) which were then closed by heat-sealing. Standards. For multielement analysis of SPM, a multielement comparator standard is preferred to a number of monoelement comparator standards. A t the initial stages of the present work, several monoelement standards were prepared by dissolving pure elements in “ULTREX” acids. Commercially available atomic absorption standards were also used whenever they were obtained in high purity. These monoelement standards were used to determine the concentrations of several elements of interest in the Orchard Leaves standard reference material (NBS SRM-1571) supplied by the US.National Bureau of Standards (NBS). Dried Orchard Leaves SRM was dissolved using “ULTREX” concentrated hydrochloric and nitric acids with the dropwise addition of 30% hydrogen peroxide. and the solution was kept near boiling. c 1979 American Chemical Soctetv

ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE

1979

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Table I. Nuclear Data for Elements Detected in Estuarine SPM by INAA element

target isotope

A1 Ba Ca

~1 Ba "Ca

c1

I Mg Ti V

Eu K Mn Na

27

l3

37c1 1271

j5Mg "Ti V 151EU K j5Mn "Na

41

As Br Sb

"As 8'Br I2lSb

Ba Ce

130Ba I4OCe

co

Cr Eu Fe Gd Hf Hg La Lu Nd Rb sc Sm Th Yb Zn

j9C0

j0Cr 151Eu

jbFe l 5 2Gd "OHf Hg "'La Ie6Lu '"Nd "Rb 45sc 2'j

l52Srn 23

2Th

I6'Yb

h"Zn

isotopic abund., %

product cross section, b nuclide ( A ) Length of irradiation = 3 min 100 0.23 '*A1 71.7 0.35 L39Ba 0.185 1.1 49Ca 0.43 38 c 1 24.5 I281 100 6.2 0.03 *'Mg 11.2 5.34 Ti 0.14 4.8 52 v 99.8 (B) Length of irradiation = 1 5 m i n 3100 L52rnEu 47.8 6.88 1.3 42 K 100 13 56Mn 100 24 Na 0.53 (C) Length of irradiation = 5h 100 4.3 7 5 As 49.5 3.3 82 Br 57.2 6.2 Ij2Sb ( D ) Length of irradiation = 34h 0.1

88.5 100 4.31 47.8 0.33 0.20 35.4 29.8 99.9 2.6 17.2 72.1 100 26.7 100

0.14 48.9

8.8

0.54 37 16 5700 1.2 125 12.6 4.9 9.5 2100 1.3 0.91 23 210 7.4 3200 0.82

After cooling to room temperature, the solution was transferred to a 50-mL volumetric flask and the volume was made up using ultrapure water. Two milliliters of this diluted Orchard Leaves solution were added to a blank Nuclepore membrane, dried under an infrared lamp, pelletized using a clean die, and placed in a polyethylene bag which was closed by heat-sealing the edges. These Nuclepore membranes spiked with Orchard Leaves solutions were used as comparator standards for the multielement determinations of estuarine SPM. Concentrations of certain elements, particularly the rare earth elements, in diluted Orchard Leaves solution were too low to be used as standards in their accurate measurements. In such cases, monoelement standards were used. Geological standard reference materials, namely USGS G-2 and PCC-1, supplied by the US.Geological Survey were also used in this work to determine concentrations of a few elements in estuarine SPM as well as to evaluate the precision and accuracy of the analytical methods developed here. Irradiations. Samples of estuarine SPM, blank Nuclepore membranes, monoelement standards and standard reference materials were irradiated first a t the Dalhousie University SLOWPOKE-2 Reactor (DUSR) a t a flux of 1 x 10" n cm-'s-' for the determination of short- and medium-lived nuclides. The details of composition, homogeneity, and reproducibility of DUSR neutron flux have been reported elsewhere (20). The actual variation in the neutron flux was measured to be less than f 2 % for a given operating power level and pneumatic site of the DUSR over several months of operation. This excellent stability of the neutron flux was advantageously used in determining multielement content of estuarine SPM. Samples, blanks, comparator standards, and standard reference materials of identical geometry were irradiated separately in the same pneumatic site, and the specific activities induced in the selected y rays of the individual

I3'Ba I4lCe

50c~

"Cr 1S2EU Fe '"Gd I s I Hf 2"3Hg "'La I5'Lu l4?Nd "Rb jg

4h

sc

'j3Sm 23 3Th-233Pa Ib9Yb b5Zn

half -life

photopeak used, keV

2.3 rnin 8 3 min 8.8 min 37.3 min 25 min 9.5 min 5.8 rnin 3.8 min

1779 166 3083 1642 443 1014 320 1434

9.3 h 12.5 h 2.58 h 15 h

122 1525 84 7 1369

26.3 h 35.5 h

67.2 h 12d 33 d 5.26 a 27.8 d 12.4 a 45 d 242 d 42.5 d 46.6 d 40.2 h 6.7 d 11.0 d 18.7 d 84 d 47 h 22 min-27 d 32 d 244 d

559 554 564 196 145 1332 320 1208 1099 97 $82 '279 lC596 208 li31 1077 889 103 312 177 1115

nuclides were calculated after appropriate decay periods. These specific activities, in turn, were compared to determine the elemental content of SPM samples. Nuclear data for elements detected in estuarine SPM are shown in Table I. Samples were irradiated for 3 min at the DUSR facility, allowed to decay for 1 2 or 19 min. and counted for 300 s for determining Al, Ba, Ca, C1, I, hlg, Ti, and V. Concentrations of Eu, K, Mn, and Na were measured following 15-min irradiation, 4-h decay, and 1000-s counting periods. Samples were further irradiated for 5 h, "cooled" for 90 h, and counted for 1000 s for measuring levels of As, Br, and Sb. For the sensitive determination of elements which produce long-lived nuclides on thermal neutron activation, the estuarine S P M samples, blank Nuclepore membranes, comparator standards, and standard reference materials were simultaneously irradiated in the McMaster Nuclear Reactor for :34 h at a flux of 1.5 X 1013n cm-2 s-l. Following decays of 15 d and counting for 1000 s, y rays from nuclides (Table I) of Ba, Ce, Co, Cr, Eu, Fe, Gd, Hf, Hg, La, Lu, Nd, Rb, Sc, Sm, Th, Yb, and Zn were detected. Counting System. y-ray spectra of all irradiated materials were recorded using a 60-cm3 Canberra Ge(Li) detector with a full width a t half-maximum of 1.88 keV at the 1.332-MeV photopeak of 6oCo,peak-to-Compton ratio of 271, and an efficiency of 9.5%. This detector was used in conjunction with a Tracor Northern model TN-114096-channel pulse height analyzer assisted with a 16-k memory PDP-11/05 minicomputer.

RESULTS AND DISCUSSION y rays emitted by thermal neutron activation products 33 isotopes (31 elements) have been detected in irradiated estuarine SPM deposited on Nuclepore membrane filters. T h e

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nuclear d a t a (21) of these isotopes are shown in Table I. Possible interferences from epithermal and fast neutron reactions, viz. (n,p), ( n p ) and (n,2n), of the neighboring isotopes to the thermal neutron activation products of interest have been studied. In almost all cases, contributions by interfering isotopes have been estimated to vary between 0.5 and 170and considered negligible with the exceptions of 2'A1 and 2sSi contributions to "Mg and 28A1concentrations via the 27Al(n,p)27Mga n d 28Si(n,p)28A1reactions, respectively, for which appropriate correction factors have been applied. y rays of the individual nuclides listed in Table I are generally free from interference after appropriate decay periods selected in this study. Half-lives of the nuclides have been measured using these y rays and have been found to agree within h107~ of t h a t reported in t h e literature. However, the 103-keV y ray of 153Smhas been observed t o be interferred with by the 103- and 105-keV y rays of '"Gd and ''limLu, respectively. Concentrations of both Gd and Lu have been independently measured using their interference-free nuclides and y rays (Table I); their contributions to the 103-keV photopeak area of 153Smhave been calculated and subtracted. In the y-ray spectra of short-lived nuclides (half-lives 5 8 3 min), "A1 has been identified as the major activity-producing nuclide. Although it has been possible to measure concentrations of eight elements, namely Al, Ba, Ca, C1. I, Mg, Ti, and V, after 12-min decay periods with dead times of about 10% in nearly 6770 of the estuarine S P M samples analyzed in this work; the remaining 33% of the irradiated samples produced dead times greater than 20% a t the end of the same decay period. These remaining samples, with A1 content greater than 47 mg g-', have been allowed to decay for 19 min when dead times reduced to about 10R before recording their y-ray spectra. T h e 1811-keV photopeak of j6Mn has been detected in the y-ray spectra of samples recorded after 3-min irradiations and 12/19-min decays. However, Mn content of all the estuarine S P M samples could not be determined with desired sensitivity, precision, and accuracy using t h e above experimental conditions. Levels of Mn could also be measured through the 847-keV 7 ray of 56Mn after a 3-min irradiation and approximately 100-min "cooling" to allow the interfering highly-abundant 844-keV y ray of "Mg to decay. This is possible if M n is present in large enough quantities to be activated t o a sufficient degree during the short 3-min irradiations and to be detected with good precision after the 100-min or so decay time required. Alternatively, estuarine S P M samples have been irradiated for 15 min and allowed t o decay for u p to 4 h for the determination of Mn through t h e 847-keV ray of 56Mn. Additionally, concentrations of Eu, K, and Na have been measured using the same irradiation and decay conditions. T h e 122-keV y ray of 1 5 2 m could E ~ be interfered with by the 121- and 124-keV y rays of j5Se and 131Ba,respectively. However, concentrations of Eu determined in the same samples through the 122-keV peak of 1 5 2 m Eand ~ the interference-free 1408-keV peak of "*Eu have been found to agree with each other within i 7 7 0 . T h e possibility of any large error due t o interference from y rays of and 13'Ba in E u measurements through the 122-keV peak of 1 5 2 m Ehas ~ therefore been discarded. Levels of As, Br, and S b have been conveniently measured using 5-h irradiation a t 1 x 10l2 n cm-2 s-l and 90-h decay periods. Photopeaks of nuclides of Ba, Ce, Co, Cr, E u , Fe, Hg, La, Sc, T h , and Zn have also been detected in these samples following a decay of 12 d and counting for 6000 s. However, concentrations of the above elements could not be ;timated in all samples of estuarine S P M using these experimental conditions because of lack of required sensitivity. In order t o determine these elements as well as Gd, Hf, Lu,

-,

Nd, Rb, S m , and Yb, samples have been irradiated for 34 h a t a flux of 1.5 X 1013n cm-' s-l and counted for 1000 s after a decay of about 15 d. Attempts have been made t o incorporate these two separate irradiations, viz., 5 h and 34 h, into one for determining all the elements of interest. However, swamping activities of 24Na and 82Br have been found t o interfere with the measurement of several nuclides, particularly 76As,when counted a few days from t h e end of irradiation a t the high flux. I t has already been pointed out that the S P M of estuarine water were collected on Nuclepore membrane filters. Trace elements present in blank Nuclepore filters may increase the background activity and contribute toward the total elemental content of S P M measured. I t is, therefore, necessary t o determine concentrations of elements of interest in these blank filters. Random specimens of the blank Nuclepore membrane filters have been analyzed from t h e same package as those used for collecting the estuarine SPM. Since the amounts of most elements are expected t o be low, the filters have been analyzed in two groups of three using the INAA method developed here. T h e average concentration for several elements as measured from these six 47-mm, 0.4-pm Nuclepore membrane filters are shown in Table 11. The ranges found and values recommended by the Nuclepore Corporation (22, 23) and those reported by other researchers ( 5 , 6) are also included in the table for comparison purposes. Levels of Al, Co, Fe, Mn, Na, Ti, V, and Zn recommended by the Nuclepore Corporation (22, 23) are significantly higher t h a n t h a t measured in this work. T o the contrary, our estimates of Ca, Ce, Cr, Hg, La, Mg, and S b content are higher t h a n t h a t of the Nuclepore's recommended values. Comparable values have been obtained for Ba, Br, and K. T h e concentration ranges for individual elements shown in Table I1 (22) have been observed to vary by more than an order of magnitude in several cases. However, these ranges are expected to narrow down as more reliable determinations are reported and provided there is no large variability between batches. Recently, many researchers have reported trace element content of various types of filter material (22, 17, 24-26) which did not include Nuclepore polycarbonate membranes. In general, Nuclepore membrane filters have very small blanks for most of the elements analyzed in this study. Although these blanks are negligible in comparison to the trace element content of estuarine S P M (Tables I11 and VI), appropriate corrections for the blanks have been made. Large correction factors have been needed for determining Co content because of high Co blanks. Concentrations of Cr, Hg and S b in blank Nuclepore filters (Table 11) have been observed to be far too large for their reliable determinations in samples of estuarine SPM. Polyethylene bags, in which Nuclepore membranes containing estuarine S P M were irradiated, have also been analyzed for trace elements by the INAA method. These bags have been found t o contain very small amounts of t h e elements detected. Nevertheless, appropriate correction factors have been applied. Concentrations of 28 elements have been determined in the estuarine S P M samples using t h e INAA method developed in this study. Concentration ranges and averages of these elements are shown in Table 111. Values of Na a n d C1 are included only for interest and should not be considered as actual content of S P M since large amounts of these elements are present in estuarine water and it is possible t h a t not all of the salt were completely removed by washing with distilled-deionized water-a process carried out immediately after sample collectior.. Levels of major, minor, and trace elements in S P M shown in Table I11 compare favorably with those reported in literature (27-29) for the same or adjacent area of sampling. I t has been stated earlier that the samples of

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Table 11. Elemental Content of Nuclepore Membrane Filters by INAA

element A1 As

Ba Br Ca Ce

c1 co Cr Eu Fe Gd Hf Hg I

K La Lu Mg Mn Na Nd Rb Sb sc Sm Th Ti \i

a

this work whole filter, pgifilter 0.30 i 0.15