Analytica Chirnica _-Icfa, 116 (1980) 205-210 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Nether!nnds

Short Communication

DETER~IIXA’I’ION OF METALS IN SMALL SAhlPLES BY A’I’OXIIC ABSORPTION AND EMISSION SPECTROMETRY WITII DISCRETE NEBULIZATION

TETSUO UCHIDA, ISA0 KOJIitlA and CHCIZO IID:\*

Laboratory of Analyiical Chemistry, Department of Engineering Sciolccs. .Vtrgoya Instilcc te of Technology, Gobiso-ciao, Showa-ftr, :\:afoya -166 (Japan)

(Received 13th August 1979)

Summary. Eight elements in NBS-SRhI 15ii bovine liver and other biological standards are determined by flame atomic absorption (Ca, Cu, Fe, hlg, bin and Zn) and emission (Ii

and Na) spectrometry. Samples (3 mg) weighed into a 32%ml teflon vessel are decom- pcsed with 40 ~1 of (-5 + 1) nitric-perchloric acids in a sealed teflon vessel. A 75-pl or loo-pi aliquot of the diluted sample solution is injected into a small teflon funnel

coupled directly to the nebulizer needle of the spectrometer. The results obrnintd agree

well with certified values: the standard deviation is about 2%.

In flame atomic absorption spectromctry (a_a.s.), 1-J ml of sample solution is generally used for the detemlination of a single clement. For multi-element determinations, the volume of sample solution or the (in7ounts of original sample must be large. Thus for small amounts of sample, the concentration of elements to be determined may be less than their detection limit as a result of the high dilution after sample decomposition, necessary to allow the determination of a series of elements. To overcome this difficulty, it is possible to determine an element by flame a.a.s. in a micro- liter sample volume (Al00 ~1). Various workers [l-d] have studied this technique, proving that nebulization of a 100-111 sample gives the same sensitivity as that obtained by conventional nebulization. Two nebulization techniques have been proposed for small volumes, i.e. nebulization from a small teflon funnel coupled directly to the nebulizer needle [l--3, ?k-71 or from a small hole cut in the surface of a teflon rod [4]_ These methods have been applied to the determination of elements in NBS-SRM 1571 orchard leaves [ 23, serum [Z] , fish [S] , NBS-SRAI 1577 bovine liver [4] , steel 191 and blood [ 9]_

Many workers [lo-121 have reported that the decomposition of organic materials, silicates, etc. with an acid mixture is easily done in a sealed teflon vessel at room or elevated temperatures. However, it was not known whether the decomposition of milligram amounts of organic materials was possible with small volumes of acid in such vessels.

The present communication deals with the decomposition of milligram amounts of biological materials with small volumes of nitric and perchloric

acids in a teflon vessel, followed by atomic absorption (aas.) or emission (a_e.s_) determination of the elements with a discrete nebulization technique_ The proposed method has been successfully applied to the determination of eight elements in standard biological samples_

Rx-perimental Reagents. The acids used were of SSG quality (Wake Pure Chemicals Co.).

Twice-distilled water was used for all dilutions. Metal stock solutions (2000 ppm, 0.5 M HCl or HNO,) were prepcared by

dissolving metals (Cu and Zn, 99.999%; Mitsuwa Pure Chemicals Co-j or metal compounds (hIgO ignited at ca. lOOO”C, C&O,, Fe203, MnO,, all Johnson Matthey Specpure; KC1 and NaCl, Merck p-a_) in HCl or HNO, and diluting to 200 g.

Lanthanum stock solution (1% w/v in O-1 Al HK02) was prepared from La,O, (99.99%; Nakarai Pure Chemicals Co_)_

\Yorking mised standard solutions were prepared gravimetrically by diluting the stock solutions to the appropriate concentration in lo-ml polypropylene bottles. These solutions contained the eight elements and the same concentrations of nitric and perchloric acids as in the sample solutions_

Apparatus. _A Hitachi Model 51s atomic absorption spectrometer equipped with a lOO-mm burner head (acetylene-air flame) was used in conjunction with a fast-response strip-chart recorder (Rikadenki B-%1 _ Mark II)_ The flow rate of ausiliary air could be freely varied at a constant sample aspiration rate. The optimum operating conditions were: acetylene, 3.3 1 niin-’ (0.5 kg cm-‘); air, 4.8 1 min-’ (1.5 kg cm-‘) for the nebulizer and 11.0 1 min-’ (1.5 ligcm-’ ) auxiliary; burner height, position 2.5; sample flow rate, 3.2 ml niin-‘. The wavelengths used were: Cu. 324-S nm; Zn, 213.9 nm; Fe, 248.3 nm; Mn, 279.5 nm; Ca, 422-7 nm; Mg, 265-z nm; Na, .SS9_0 nm; and K, 766.5 nm. Solutions (5100 ~1) were nebulized by injection into a small teflon funnel coupled directly to the nebulizer needle (Fig. 1).

Powdered samples (l-2 mg) were weighed 31: a Chyo Jupitor Al,-20-A microbalance. An agate mortar and pestIe are used to repulverize standards. Gilson (P5000, PfOOO, P200) and Nichiryo (Juster V50) micropipettes were used for dilution_

Samples were decomposed in the sealed digestion bomb shown in Fig. 2. This bomb contains two teflon vessels, an inner 3.S-ml (ca. 5.8 g) mini- vessel with a cap and an outer 23-ml vessel.

Recommended procedure. The sample (2 mg ) was weighed into the mini- vessel and 40 ~1 of (5 + 1) nitric acid-perchloric acid was added. The small vessel was placed in the large vessel which was then sealed. After decompo- sition for 3 h at 13O”C, and cooling to room temperature, the contents of the mini-vessel were diluted to 1.5 g (analytical balance) with twice-distilled water. Aiiquots of this solution (100 or 75 ~1 for a.a.s. and 100 ~1 for a-es.) were injected for measurement. Calibration curves were prepared with the same volumes of solution_ For calcium, 2000 ppm lanthanum was tolerated.

Micropipette

Small Funnel

Nvasizing of eyriipmen t. Teflon vessels mci polyethylene and polyprop~;- lcnc bottlr~s were clennerl 1,~ soaking first in .1 .\I I-ISO,3 for a clay at room temperaturt~, G 11 I-ICI for two chgs at WCC, and 0.1 .\I I-ICI for three clays tit 9022. in :i teflon tank. and finally t)!: \k-ashing with clistillecl water. Clean bottles and vessels were dried under reduced pressure at ambient. temperature.

~Yebrtlizatio?z tcchniqrte. Lp to lOOr.ll of sample solution is injected as sho\vn in Fig. 1. Figure 3 shoe-s the spike-like signals ol,tainccl with various volumes

(10-1000 ~1) of 0.5 ppm copper solutions at the sample flo~v rate of 3.2 nil

min- I_ ‘I’hc signal incwascs with injected volume up to about 60 p!: tlwre- il ter, f tlw kgnnl intensity remains the smie as that. ot,tainecl by continuous nehlizntion. How-ever, the injected volume necessary to obtain the sxnt’ Gpn31 as that of continuous nebulization is about 100 ~1 for the flamct emission of 10 111x-n sodium. l~wause the olectricnl circuitry for cmissiol? nwasurements is more highly clanil~td. In the prewnt study. ‘73 ~1 (Cu. Fta. CL. LIg? AIn) and 100 ~1 (Zn) \vek-c used for zzs. and 100 pt (Xu. K) for B.c.s.

The reproclucibilitirt~ of the signals obtained for various volunir~ 9f 0.3 ppm copper solution and 10 ppm sodium solution arc given in Table 1. The relative standard deviation for 75~1 samples in atomic absorption anti loo-/11 samples in emission is similar to that obtained by cont.inuous nt4,ulization.

Snr~lpic ciccomposition. ‘he effects of time and temperature on tIit% clecomposition of 0 mg of bovine liver were studiecl at 100--l 50% and for l.--5 11, in a simihr way to previous stutlks [ 12 ] _ \Vitli cleconiposition at 13O’Y_

Fig. 3. Effect of injected sample volume on absorption signal for copper.

TABLE 1

Reproducibility of peak height (n = 10)

hlethod

r1.a.s. (0.5 ppm Cu)

&es. (10 ppm X!a)

Volume Peak height (PII (mm)

20 66.6 10 95s ‘75 101.0

30 56.9 .50 59.3 100 121.S

S.d. R.s.d. (mm) WI

2.3 3.3 1.4 1.4 0.9 0.9

1.0 1.8 1.2 1.3 0.7 0.6

no significant differences were observed in the results for Cu, Fe, Zn, Na, K, Ca or Big with digestion times of l-5 h_ However, sample solutions obtained by digestion below 110°C for less than 2 h were a little viscous, and could be ejected from the micropipette tip only with difficulty, making it difficult to obtain reproducible results. Solutions from samples decomposed above 120°C for longer than 3 h did not adhere to the tip wall and reproducible results were easily obtained_ Therefore, it is recommended that samples are decomposed at 130% for 3 h_

It was shown previously [ 121 that 500 mg of sample was completely decomposed by a misture of concentrated nitric (5 ml) and perchloric (1 ml) acids in a sealed teflon vessel for 2 h at 120°C_ For the decompo- sition of 5-mg amounts of sample, the volume of acid mixture used was decreased proportionately, i.e. a mixture of nitric (50 ~1) and perchloric (10 ~1) acids was used. Satisfactory results were obtained, but for the decomposition of 2-mg (or 1-mg) samples, about twice the proportional volume of acid mixture was used (40 ~1 or 20 ~_rl), to ensure adequate

209

TABLE 2

Results for different weights of NBS-SRM 157i bovine liver

Sample weight

(mg)

Certified values

Final ZUTlOUIlt

Element (ppm) -.--.-_

cu Zn Fe Na K Ca Mg

____ ____ --._-__-__ -_-

25 ml 189 131 3i6 - 119 600 25 ml 1Sl 1% 237 - - 1’7-1 572

3.s e 189 133, 266 2390 9s10 12; 593 1.5 ; 195 13i 2i-1 a420 9690 127 602

0.5 g 193 - - 9.500 - - 602

193 130 “66 2.130 9700 13-1 6Od :lO -13 28 : 130 I 600 -6 -9

TABLE 3

Results for NBS-SRM 1 Si’i bovine liver in 2-mg and 1 -mg samples

Element (ppm)

cu Zn Fe __.- -_

x3 K Ca Ilg

Bovine liver 2 mg ._ final lveight i .5 g; -10 pl (5 - 1) IiNO ,-HCIO, Ilean” 195 -1 137 * -1 P7.l f 5 2430 . 50 9690 _ “20 1s; . 7 602. 11 Certified vdue 193 . 10 130 : 13 “GS - s a-130 . 130 9ioo : 600 I”-1 . 6 60-f - 9

Bovine liver 1 -- final weight 0.5 30 .ul mp g; (.5 - 1) I-fSO,--HClO~

.\Ieana 193. -1 - - ‘2500~ I”0 - GO2 . 9 --_---._

.‘.\Ican 0T .?I complete analyses.

moistening of the sample. The results obtained (Tables 2-4) sho~v that the sample was completely decomposed by such small amounts of the ac~ri mixture. The solutions obtained after dilution as descrilwcl in the Procedure were clear and not viscous.

Spectrometric measweme)z ts. The absorbance of calcium and the emission intensity of potassium decrease with increasing perchloric acid concen- tration, whereas t.he absorbance of Cu, Zn, LIg, Fe and hln and the emission intensity of Na are not affected by varyin g amounts of perchloric acid less than 0.5 &I. For a11 eight e1cment.s studied, the presence of nitric acid beloxv 0.5 ;\I does not alter the signals. Thus the same concentration of perchloric acid was added to the standard solution of elements used for preparing calibration graphs.

As NBS-SRM 15’7’7 bovine liver contains 1.1% phosphorus, the final solution contains less than 25 ppm of pl~ospl~orus. It was found that the addition of lanthanum to give a concentration of 2000 ppm in the final solution allowed calcium to be determined without interference from

210

TABLE 4

Results for S elements in biological samples (ppm) 01 = 5) ---

CU (0.8) - - (3.5) 3.3” - 63 63

Zn (I’,) 10 15 (Iii) 13.5 - 8tiO 859

Fe (12) 2i 12 5.3 5; - “09 195

Mn (0.6) - - (0.9) 0.; - 19 -

C;r “-II? - 100 110-310 - 287 X&IO-3370 1500

MR 9x-t - 1080 1150 -_ 1180 1280 1210

_s 2 2990 3510 4060 1020 1030 1130 4920 -

li 15500 15700 li200 12i00 1”HOO 1 “SOO Y77Q 9SlO ~~- _-.. -.-- _.. -.-._-

(Sample 5 mg -- final weight 1.5 g (10 my! -- 0.5 g).)

GO0 ppm of phosphorus. This was achieved by adding 50 ,ul of the lantha- num solution (I.%. w/v) to 200 ~1 of sample solution and then injecting 75 ,ul of the mixture. Lanthanum was also added to the standard calcium solutions.

Analysis of bovine liver_ Results for seven elements in NBS-SRM 15’77’ bovine liver obtained with va-ious sample sizes are summarized in Table 2. The 250-mg and 2%mg samples were decomposed in two steps, at 90°C for 3 h and then at 120°C for 3 h; the small samples were decomposed as recom- mended above. Reproducibility and accuracy are satisfactory. The results obtained for Zmg samples and for 1-mg samples (Table 3) also show reason- able precision and accuracy_

Analysis of other biological statzdards. The recommended method was

applied to the analysis of shark powder (Tokyo University) and NBS albacore tuna and oyster tissue (Table 4 j_

REFEREXCES

1 E. Sebxtinni, K. Ohls and G. Riemer, Fresenius 2. Anal. Chem., 26-I (19i3) 10.5. ‘7 D. C. Alnnning, At. Abs. Newsl., 14 (19i3) 99.

3 H. Berndt and E. Jackwerth, Spectrochim. Acta, Part B, 30 (1975) 169. -1 T. Uchida, I. Kojima and C. Iida, Bunseki Kagaku, 3’7 (19554) T4-i.

5 E. Jackwerth and H. Berndt, Anal. Chim. Acta, 74 (19;s) 299.

6 Ii. Berndt zmd E. Jackwerth, At. Absorpt. Newsl., 15 (19i6) 109. i P. D. Goulden, At. Absorpt. New&, 16 (19ii) 121. S G. J. Kamelow and T. I. Balks, Anal. Lett., 10 (19’7’7) ‘733. 9 K. C. Thompson and R. G. Godden, Analyst, :Ol (19i6) 96, 17-I.

IO B. Bernas, At. Absorpt. Newsl.. 9 (1970) 52. 11 T. Uchida, 31. Nagase, I. Kojima and C. Iida. Anal. Chim. Acta. 94 (19TT) g’i.5. 12 C. Iida, T. Uchida and I. Kojima, Anal.

therein. Chim. Acta, 113 (19’SO) b61, and references

12 K. Kudo, private communication. 14 Y. Dokiya, S. Kurosawa, S. Toda and

36-19. 15 ‘i*. Dokiya, private communication. 16 T. L. Barnes, private communication.

Ii. Fuwa, Bull Chcm. Sot. Jpn., 51 (1975)