determination of magnesium in botanical reference materials by instrumental neutron activation...

Journal o f Radioanalyttcal and Nuclear Chemistry, Articles, Vol. 148, No. 2 (1991) 257-263

DETERMINATION OF MAGNESIUM IN BOTANICAL REFERENCE MATERIALS

BY INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS §

APPLICATION TO BEECH LEAVES AND SPRUCE NEEDLES

N. LAVI,* K. HEYDORN**

*Soreq Nuclear Research Center, Yavne (Israel) **Riso National Laboratory, Isotope Divison, DK~IO00 Roskilde (Denmark)

(Received April 3, 1990)

Instrumental neutron activation analysis (INAA) was applied to the rapid determination of magnesium in the botanical reference matelials Beech Leaves-100 and Spruce Needles- 101. The magnesium content was quantitatively determined by measuring the gamma-ray photopeak at 1014 keV of the short-lived radionuclide 21Mg (9.46 m). The magnesium concentrations in the two materials were found to be 834.6-+50.2 ~g �9 g - t dry weight and 618.6+36. 2 vg . g-t, respectively. When assaying a 0.1 g sample under the same experimental conditions the limit of detection is 30 vg of Mg.

Introduction

The influence of the aerosols deposited on trees and plants as well as the soil

acidification on metal uptake in plants is well known and discussed in the literature.1,2

Soil acidification could affect the supply of essential as well as non-essential ele-

ments absorbed from the soil by plants, which in turn provide food for animals,

thereby entering the food chain.

During recent years considerable attention has been given to understanding of the

physiological roles of both essential and non-essential elements in human and animal

nutrition as well as their toxicity.

Magnesium is one of the most plentiful elements on earth, found in rocks, soils,

water and all living matter.

The magnesium deficiency and its toxicity in humans 3 as well as its roles in

photosynthesi~ and in oxidative phosphorylation 4 have already been discussed in the

literature.

+Work carried out at Ris~b National Laboratory, Isotopes Division, DK-4000 Roskilde, Den- mark.

Elsevier Sequoia S. A., Lausanne Akad~miai Kind6, Budapest

N. LAVI, K. HEYDORN: DETERMINATION OF MAGNESIUM

In recent years neutron activation analysis (NAA) has become one of the most promising and attractive analytical methods for simultaneous multielemental analysis of biological and geological materials. Usually, the amount of radionuclide produced has been measured by gamma-ray spectrometry either without any chemical treatment, i.e., instrumentally, or after radiochemical separation.

Non-destructive reactor neutron activation analysis of biological as well as geological materials is generally not feasible because of the considerable activity produced by the major components (sodium, potassium, chlorine, bromine and phosphorus) in these matrices. Only very short irradiations may be carried out, and only short-lived isotopes can be determined by this method.

Magnesium can be determined through its short-lived radionuclide 27Mg (9.46 min), by measuring its gamma-ray photopeaks at 843.6 and 1014.3 keV. The intensity of 843.6 keV peak in 27Mg is 3 times higher than that of the 1014.3 keV peak, but the 843.6 keV peak suffers from interference by the 847 keV peak of s 6 Mn (2.58 h). Consequently, it is very important to verify the half-life of the 843.6 keV peak (9.46 m) compared with 2.58 hours for the 847 keV peak or else to use the 1014 keV peak despite its lower abundance.

In the present work INAA has been applied to the determination of Mg (via 27Mg ' 1014 keV peak) in two botanical reference materials CRM-100 (Beech Leaves) and CRM-101 (Spruce Needles) in the framework of an intercomparison of analytical results organized by the BCR, Reference Materials.

Experimental

Preparation of Mg standards

Five kinds of magnesium standard solutions (1-10 mg ml- 1 of magnesium) were prepared by dissolving MgSO4 �9 7H2 O, analytical reagent grade, Merck as well as Johnson Matthey (specpure), MgCI2 �9 6H20 analytical reagent grade (Riedel-De Haen) and Mg(NO3)2 �9 6H20 analytical reagent grade Merck, in redistilled water. The fifth standard was prepared by dissolving MgO, analytical reagent grade Merck in HNO3 ("Suprapur") and diluted with redistilled water.

0.5 ml of these dilute solutions were transferred directly by micropipette into precleaned and dried half-dram polyethylene vials used for irradiation." In each sample, the weights of the magnesium standard solution were determined by precise weighing. All the polyethylene vials holding the standard solution were heat-sealed to prevent any loss of magnesium during the irradiation. Blanks of 0.2-0.5 ml redistilled water were used in all runs.

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Preparation of botanical samples

Samples of 70-100 mg of Beech Leaves and Spruce Needles were weighed in a half-dram polyethylene vial used for irradiation. By increasing the weight of sample (> 100 mg), it was found that the detection of 27Mg, via the 1014 keV peak, was influenced by the high level of dead time, achieved during measurements of the irradiated samples under the same experimental conditions of irradiation and counting. All the standards, blanks and botanical samples were placed in heat-sealed polyethylene bags in order to avoid contamination of the external surface of the polyethylene vials.

The dry mass correction of these two botanical reference materials was determined by drying separate portions of the sample in a dessiccator containing P20s at room temperature for 48 hours. After this period samples were weighed and dried again in a dessiccator loaded with fresh P20s for 8 hours. No change of mass was observed. The dry mass correction factors for Beech Leaves and Spruce Needles were found to be 0.945 and 0.955, respectively. It should be mentioned that both samples for magnesium determination as well as for dry mass correction were prepared at the same time.

Irradiations

Samples were irradiated in the pneumatic transfer facility of the Danish reactor DR 3 at a thermal neutron flux of 2.5 �9 10 x3 n �9 cm -2 ' s -1 for 30 seconds.

Counting

The gamma-rays of the activated samples were measured using a calibrated Ge(Li) detector at a gain of 0.4 keV/channel. All samples were counted for 600 s clock time after a decay time of 90 seconds, which was sufficient for removing the samples from the polyethylene bags and to transfer them to the counting room.

A dead time of 5-+1% was achieved during measurements of magnesium nitrate and sulfate comparators. Dead times of 25-+2% and 20-+2%, respectively, were obtained during measurements of the irradiated Beech Leaves and Spruce Needles samples.

Both standards and samples were measured at a distance of 98 mm from the surface of the detector.

A 50 Hz pulse generator peak was used to correct for dead time losses.

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Dead time corrections

Since the dead time achieved during measurements of the irradiated botanical

reference materials is higher than that obtained from irradiated magnesium com-

parators, a -dead t ime correction has to be made. Therefore, we calculated the correc-

tion factors s due to dead time losses for magnesium standards as well as for

botanical reference materials.

Results and discussion

Determination of magnesium concentration

Values achieved after performing all calculation and corrections (for dry mass as

well as for dead time losses) were used to determine the magnesium content in an

unknown specimen, using comparator standards of magnesium nitrate.

Table 1 Specific activities obtained from irradiated standards of magnesium by instrumental neutron

activation analysis, using 2 ~ Mg (1014 keV) gamma-ray spectrometry

Standard N* Live time, Mg, Mean, s mg eounts/mg �9 s

MgSO 4 �9 7H20 32 977 541 5.50 11.08 analytical reagent grade (Merck) 32 635 547 5.40 11.05

32 780 543 5.45 11.08

MgSO 4 �9 7H20 18 533 565 2.94 11.16 (Specpure) 18 801 568 2.96 11.18

Mg(NO3) 2 �9 6H20 17 703 570 2.80 11.09 analytical reagent grade (Merck) 17 796 576 2.82 11.07

MgCI2 �9 6HaO 2 900 519 0.50 11.17 analytical reagent grade 2 880 521 0.50 11.05

*Calculated true net counts corrected for dead time losses.

The results obtained for a set of four kinds of standards containing 0 .5-5 .5 mg

of Mg, irradiated and counted under the same experimental conditions are given in

Table 1. It is seen that the mean activity (counts/mg �9 s) obtained from irradiated

magnesium sulfate, analytical grade, specpure, magnesium chloride and magnesium

nitrate comparators ranged from 11.04 to 11.18 counts/mg �9 s. A fifth MgO standard

260


was omitted from the present calibration, because it gave a count rate lower by 11%

compared to the other four kinds of standards, which agree within -+0.4%.

The concentration of magnesium in an unknown specimen was calculated by the

following equation:

Mg(/ag �9 g - ' ) = N s . Tst �9 mst

Ns t " Ts" ms

where Ns, Nst - calculated net counts, corrected for dead time losses of the sample

and standard, respectively,

Ts, Tst - counting times (live time in s) of the sample and standard, respec-

tively,

mst - amount of magnesium comparator in ~tg,

ms - weight of the sample in g (dry mass).

Table 2 gives the concentration of magnesium detected in 10 samples of each

botanical reference material determined by gamma-ray' spectrometry via 2 7Mg" The

Table 2 Determination of magsiesium in botanical reference materials Beech Leaves-100 and Spruce

Needles-101 by neutron activation analysis using 27Mg (1014 keV) gamma-ray spectrometry

No. Range of Mg, Mean, Intercomparison,* Sample of samples gg. g-~ #g. g-~ tzg. g-2

Beech Leaves 10 765.4-912.9 834.6 • 50.2 875.4 • 29.3 Spruce Needles 10 558.8-661.7 618.6 +--36.2 ~ 619.2 • 12.8

*Organized by the BCR; Reference Mzte.rizls.

magnesium concentration in Bee:h Leaves and Spruce Needles ranged from 765.4 to

912.9/ag �9 g-~ (mean, 834.6-+50.2/ag �9 g - l ) and from 558.8 to 661.7/ag �9 g-~

(mean, 618.6-+36.2 ~g �9 g-~) , respectively.

The values obtained in this work were found to agree well with data obtained by

other methods. 6

Detection limits of magnesium

The detection limit of a radionuclide is usually defined as the smallest photopeak that can be detected with a certain level of confidence above the background con-

tinulam. CURRIE 7 showed that the detection limit (LD) for the radioactivity is

261


given by the equation:

LD(Counts) = 2.71 + 4.65 X/~a

where/a B is the true mean of the blank. The minimum detected mass, M D is given

by M o = LD/K, where K is the sensitivity factor (counts �9 mg-1) . For a 0.1 g

sample irradiated for 30 seconds and counted for 600 seconds after a decay o f

90 seconds the detection limit for the determination of magnesium by gamma-ray

spectrometry (using the 1014 keV peak o f the short-lived radionuclide 2 TMg ) is

30/ag of magnesium.

Correction for the amount of 2 7Mg from Si and K

The contributions to the amounts of 2 7Mg by reaction 3 o Si(n, a) 2 7Mg, and

from the gamma-ray photopeak of 1013.7 keV of 42 K (12.36 h) were checked.

It was found that the contr ibution from fast neutrons is negligible due to the low

fast neutron flux ( ~ 5 �9 101~ n �9 cm -2 �9 s -1 ) as well as the lower cross section 8

for this reaction.

The contribution of the 1013.7 keV peak of 42K to the 1014 keV peak of

27Mg was checked by irradiating K standards as well as by determining the K con-

centration in botanical reference materials. Under the same experimental conditions,

the activity of the 1524 keV peak in 42K is 70-fold higher than that o f the 1013.7 keV

peak. It was concluded that the contr ibut ion from 4: K to the activity of 2 7 Mg is

negligible due to the K concentration in these two botanical reference materials,

which was found to be 5 - 1 0 mg �9 g- 1, together with lower sensitivity of 42 K acti-

vity.

Table 3 Determination of A1, C1, K, Mn and Na in botanical reference materials Beech Leaves-100

and Spruce Needles-101 by instrumental neutron activation analysis

Sample No. of

A1, CI, K, Mn, sam-

t~g" g-1 m g �9 g-I mg �9 g - i ~ g . g-1 pies

Na, mg- g-~

Beech Leaves Range Mean

Spruce Needles Range Mean

5 368.47-388.60 1.51-1.53 9.17-9.56 381.50• 8.77 1.52-+0.01 9.29-+0.31

5 149.56-166.45 0.67- 0.69 5.97- 6.26 156.39_+ 6.81 0.68-+0.01 6.06-+0.15

1298.50-1352.42 0.315-0.361 1331.20,+ 21.50 0.336_+0.017

896.82-920.12 0.308-0.316 911.20,+ 9.88 0.312.+0.017

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Determination orAl, CI, K, Mn and Na concentrations

The concentra t ions o f A1, C1, K, Mn and Na de tec ted in 5 samples o f each

botanical reference material de termined by gamma-ray spec t romet ry are summarized

in Table 3. The results obta ined were found to be in good agreement wi th publ ished

data. 6

References

1. A. WYTTENBACH, S. BAJO, L. TOBLER, Modem Trends in Activation Analysis, Copenhagen, Vol. 2, 1986, p. 1097; J. Radioanal. Nucl. Chem., 114 (1987) 137.

2. J. CAPLAN, E. LOBERSLI, R. NAEUMANN, E. STEINNES, Modern Trends in Activation Ana- lysis, Copenhagen, Vol. 2, 1986, p. 965; J. Radioanal. Nucl. Chem., 114 (1987) 13.

3. E. B. FLINK, Trace Elements in Human Health and Disease, Vol. 2, 1976, p. 1. 4. J. K. AIKAVA, Trace Elements in Human Health and Disease, Vol. 2, 1976, p. 47. 5. K. HEYDORN, Neutron Activation Analysis for Clinical Trace Element Research, Vol. 1, 1984,

p. 135. 6. Intercomparison of ahalytical results organized by the BCR, Reference Materials. 7. L. A. CURRIE, Anal. Chem., 40 (1968) 586. 8. Handbook of Nuclear Activation Cross-Sections, Technical Reports Series, No. 156, IAEA,

Vienna 1976.

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