Fresenius Z Anal Chem (1982) 3t3:34-37 Fresenius Zeitschrift flit

Determination of Strontium in Biological Samples by Atomic Emission Spectrometry with Electrothermal Atomization

Masami Suzuki and Kiyohisa Ohta

Department of Chemistry, Faculty of Engineering, Mie University, Kamihmna-cho, Tsu, Mie-ken 5t4, Japan.

�9 Springer-Vertag 1982

Strontiumbestimmung in biologischem Material dutch Atom- emissions-Spektrometrie mit elektrothermischer Atomisierung

Zusammenfassung. Eine hohe Atomemission wurde ffir Strontium bei Verwendung einer Molybd~in-Mikror6hre be- obachtet. Durch Wasserstoffzusatz zum Argon konnte sie noch weiter erh6ht werden. Weniger als 5 ng K, Ca und Mg verursachen keine St6rung. Andere Etemente st6ren nicht in den Konzentrationsbereichen, wie sie i~blicherweise in biolo- gischen Materialien vorkommen. Der Aufschluf3 der Proben wurde mit Satpeters~iure in Uni-SeaI-Gef~iBen vorgenommen. Die erhaltenen Werte stimmten gut mit zertifizierten Daten /iberein. D ie vorgeschlagene Methode erm6glicht eine ein- fache und empfindliche Strontiumbestimmung in biologi- schen Proben (Vm'iationskoeffizient 2,5 % bei 10 pg Sr).

Smnmary, Stromium showed a high atomic emission in a molybdenum microtube. The addition of hydrogen in the argon gas atmosphere served to increase the atomic emission. No interferences from less than 5 ng of potassium, calcium and magnesium were found. Other elements also had no interferences at the concentration levels usually found in biological samples. Samples were digested with nitric acid in a Uni-Seal decomposition vessel. The results obtained showed good agreement with the certified values. Atomic emission spectrometry with a molybdenum microtube atomizer per- mits a simple and sensitive determination of strontium in biological samples (coefficient of variation 2.5 % for i0 pg St').

1. Introduction

The relatively low excitation potentials of the resonance lines for alkaline earth elements allow sensitive measurements of these elements by flame emission spectrometry. However, this technique is accompanied by ionization interferences.

Suzuki et aL [I] have recently developed atorrfic emission spectrometry of barium with electrothermal atomization in a molybdenum lvdcrotube and recognized the potential of this technique as an analytical tool. The molybdenum microtube atomizer needs only a short heating period to provide an environment with a uniform temperature throughout the atomizer and reduces the possibility of formation of carbides for alkaline earth elements. Furthermore, the ionization of alkaline earth elements is much less in a molybdenum

Offprint requests to: M. Suzuki

microtube atomizer. Computer processing of the signals is also attractive for highly sensitive measurement of atomic emission.

This work has been done toexamine atomic emission spectrometry of strontium in a molybdenum microtube atomizer by a microcomputer processing of the signals and its application to the determination of strontium in biological samples.

2. Experimental

Apparatus"

The monochromator, photomultiplier, and amplifier were the same as used for atomic absorption measurement [2]. The output signal from the amplifier was fed to a microcomputer (SORD M 223) through an AD converter (DATEL ADC-HX 12BGC) and multiplexer (DATEL MX-808). The memory capacity of this microcomputer is 64,000 words and the basic cycle time is 500 gs. The BASIC program was operated by an interpretive routine. The memory capacity of the floppy disc used as sub-memory was 350 kbytes. The signals were also monitored on a Memoriscope (Iwatsu MS 5021).

The molybdem~m micrombe atomizer (20 mm long and 2 mm bore) and atomization chamber (300 ml) have been described [2]. A microtube was fabricated from 0.05 mm thick molybdenum sheet. The argon used as purge gas in the atomization chamber was mixed with hydrogen. Two light apertures (0.5 and 5 mm diameters) were positioned between the microtube and the lens in front of the monochromator entrance slit for reducing the amount of tube-wall radiation reaching the monochromator. Atomic and ionic emission measurements were made at 460.73 nm and 407.77 nm, respectively. The wavelengths of the monochromator were adjusted to the desired lines using a hollow cathode lamp which was then disconnected. The tube temperature was measured with a photodiode (Hamamatsu TV Co. S 641). The signal from the photodiode was calibrated with an optical pyrometer (Chino works) and is recorded with the emission signal simultaneously.

All sample solutions were injected into the microtube atomizer by use of a t gl glass micropipet.

The Uni-Seal decomposition vessel was used for decom- posing biological samples.

Reagents

Strontium Stock solution (1 mg Sr/mt) was prepared by dissolving high purity strontium carbonate (dried at 110 ~ C

0016-1 ~[ 52/82/0313/'0034/$01,00

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Fig. 2. Effect of hydrogen flow rate on atomic emission of strontium: (a) 500 ml/min H a ; (b) 200 ml/min H 2 and 300 ml/min Ar; (c) 100 ml/min H2 and 400ml/min Ar, (d) 50ml/min H2 and 450ml/min Ar; (e) 20 ml/min H 2 and 480 ml/min Ar, (/) 500 ml/min Ar, (g) temperature increase

for 2 h) in hydrochloric acid and diluting with demineralized water. This solution was 0.1 M in hydrochloric acid. Dilute working solutions were prepared immediately before use by diluting appropr ia te volumes of stock solution. All reagents were of analytical reagent grade and checked for s tront ium as an impurity.

General Procedure

Samples of 1 t~1 of s tront ium were injected into the microtube, dried at 100 ~ C for 2 s, and then atomized by heating to a final temperature of 2200 ~ C.

All of the atomizat ion signals were stored in a microcom- puter. Signals were subject to background (noise) sub- traction, compensat ion of base line, and smoothing as described for atomic absorpt ion [3]. Figure 1 shows the flow chart for sample analysis. First, the acquisitions were made with s tandard solutions for calibration curve. After process- ing the signals the peak heights were found and the cal ibrat ion curve was prepared by calculating with the least square method. Then, the sample signal was acquired and operated logically. The analytical value was displayed on a cathode ray tube.

Procedure .for the Determination of Strontium in Biological Samples

The sample (100mg) was transferred into the Uni-Seal decomposit ion vessel and 4 ml of nitric acid was added. The vessel was stoppered and placed into the preheated I20~ electric oven for 6 h to digest the sample. Then, the vessel was removed from the oven and allowed to cool to room temperature. The content of the vessel was transferred to the Teflon beaker and heated on a water bath to evaporate to dryness. The residue was moistened with 3 ml of hydrochloric acid and evapora ted to dryness. Then, the residue was taken with 1 ml of hydrochloric acid and demineralized water, and diluted to 1 0 - 5 0 0 ml according to the strontium content in sample. An aliquot (1 lal) of this solution was injected into the microtube atomizer and strontium was atomized after dehy-

drating. The calibration curve was prepared from standard solutions.

3. Results and Discussion

Atomic Emission Profile o f Strontium

The computer processing of the atomic emission signal facilitated the higher sensitive measurement of strontium. This resulted from the extreme reduction of background (noise). The dimension of the microtube was important for measuring atomic emission of strontium. A smaller diameter of the microtube was not favourable because of tube-wall radiat ion reaching the monochromator , while a larger diam- eter tube resulted in poor sensitivity of s trontium emission. A microtube of 2 mm diameter was recommended in order to minimize the impact of tube-wall radiat ion without the pronounced impairment of sensitivity. The microtube atom- izer was necessary to be set in such a way as to pass only the emission from the center of the tube diameter into the monochromator .

Effect o f Hydrogen on Atomic Emission o f Strontium

Hydrogen, added to argon as purge gas, had some contri- but ion to atomic emission profile of strontium. Figure 2 demonstrates the dependence of atomic emission profiles for s trontium on flow rates of argon and hydrogen as purge gases. The atomizat ion temperature of strontium was shifted to a lower temperature region in the presence of hydrogen. The highest peak emission of s trontium was observed at 20 ml/min of hydrogen flow. The appearance and peak temperatures were 1860 ~ and 2180 ~ C, respectively. Under this condit ion the ionization of s trontium was negligible. The peak emission of s tront ium was lowered with further increase in hydrogen flow. The atomizat ion of s trontium was slower in pure argon atmosphere and its profile was noisy. Hydrogen served to prolong the life time of the atomizer resulting in protect ion of the atomizer from oxidation by traces of oxygen in the atomizat ion chamber.

36

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Fig. 3. Effect of hydrochloric and phosphoric acids on atomic emission of strontium: (a) 50 pg Sr in 5 x 10 -5 M HC1; (b) 50 pg Sr in 0.1 M HC1; (c) 50 pg Sr in 0.1 M H3PO4; (d) temperature increase

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Fig. 6. Effect of potassium on atomic emission of strontium: (a) 50 pg Sr; (b) 50 pg Sr and 5 ng K; (c) temperature increase�9 Potassium was added as chloride

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Fig. 5. Effect of calcium on atomic emission of strontium: (a) 50 pg Sr; (b) 50 pg Sr and 5 ng Ca; (c) 50 pg Sr and 10 ng Ca; (d) 50 pg Sr and 20 ng Ca; (e) 50 pg Sr and 50 ng Ca; (f) temperature increase. Calcium was added as chloride

T a b l e 1. Determination of strontium in biological samples

Sample Strontium, ~tg/g

Found Certified

Bovine liver a 0.14, 0.18, 0.17 0.14 Oyster tissue" 11, 9.8, 8.8 10.36_+0.56 Pepperbush b 34, 34, 30 36+_4 Orchard leaves a 34, 34, 35 37

a NBS standard sample b NIES standard sample

Detection Limit and Reproducibility

The detect ion l imit was calculated f rom the peak height in the a tomiza t ion profi le and defined as that of the weight of analyte giving an emission signal equal to twice the s tandard devia t ion of the backg round signal�9 In the present a tomizer , the detect ion l imit fs restr icted by the small sample vo lume (1 gl). Therefore , the detect ion l imit is given in absolute terms (pg). As little as 0.3 pg of s t ron t ium could be detected under the opt imized conditions�9 The coefficient of var ia t ion was 2.5 ~o (10 determinat ions) for the a tomic emission of 10 pg of s t ront ium.

Interferences

The interferences were invest igated by analyzing compute r - processed a tomic emission profi les of s t ron t ium in absence and presence of concomitants . Figures 3 and 4 show the effects o f mineral acids on s t ron t ium emission at 0.1 M level. Both nitric and phosphor ic acids showed depressing effects�9 The interference f rom nitric acid was much more severe than that f rom phosphor ic acid. However , no interferences were shown by minera l acids at the 0.01 M level. A t a concen t ra t ion level higher than 0.1 M all the acids had depressive effects on s t ron t ium emission.

The test was carr ied out for elements o f biological samples likely to interfere in the measu remen t of strontium�9 Figure 5 shows the effect o f ca lc ium on a tomic emission o f s t ront ium.

Large amounts of calcium led to a depression of strontium emission. The effect of potassium on strontium emission is shown in Fig. 6. Some depressing effect on strontium emis- sion was observed in the presence of 5 ng of magnesium (by 7 ~o in peak height). No interferences from magnesium and sodium were observed at the 5 ng level of these elements together. Aluminium shifted the peak absorption tempera- ture of strontium showing no depressing effect [4]. A similar effect was also shown for strontium emission. The permissible amount was 5 ng for the individual elements tested irrespec- tive of the amount of strontium. Minor amounts of other concomitant elements in biological samples had no effect on the atomization of strontium.

Determination of Strontium in Biological Samples

Strontium in some biological samples was determined by the proposed method. The analytical results are shown in Table 1. A longer digestion time was necessitated for orchard leaves and pepperbush with nitric acid in the Uni-Seal decomposition vessel compared to bovine liver and oyster

37

tissue. A digestion of 6 h was recommended. The certified strontium content in the samples shows close agreement with the results obtained by the proposed method. These data provide further evidence for the absence of cation interference.

The results show that the atomic emission spectrometry in a metal microtube atomizer offers a simple and sensitive technique for the determination of strontium in biological samples. Computer processing of atomic emission signal allows the sensitive measurement of strontium.

This work was supported by the Ministry of Education, Science and Culture of Japan through a Grant-in-Aid for Special Project Research.

References

1. Suzuki M, Ohta K, Yamakita T (1981) Anal Chem 53:1796 2. Ohta K, Suzuki M (1979) Fresenius Z Anal Chem 298:140 3. Suzuki M, Ohta K, Yamakita T (1981) Anal Chim Acta 133:209 4. Suzuki M, Ohta K (1981) Talanta 28:177

Received January 30, 1982