Analytico Chimico Acta, 113 (1980) 361-364 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

Short Communication

DISCRETE NEBULIZATION IN ATOMIC ABSORPTION SPECXROMETRY WITH A LONG ABSORPTION TUBE

TBTSUO UCHIDA, CHUZO IIDA and ISA0 KOJIMA

Laboratory of Analytical Chemistry. Department of Engineering Sciences, Nogoya Institute of Technology, Cokiso-cho, Showo-ku. Nagoya 466 (Japan)

(Received 9th July 1979)

Summary. The absolute detection limits obtained with SO ~1 of sample solutions for 10

elements in an air-hydrogen flnme are listed. The values for silver, cadmium, copper and manganese arc of the same order of magnitude as those obtained by graphite furnace atomization. The method was satisfactorily applied to the determination of copper and zinc in Bovine Liver.

In flame atomic absorption spectrometry, the sensitivity and detection limits for some clemcnts can be greatly improved by the use of a long absorption tube and an air-hydrogen flame [l-3]. Sample solutions are introduced into the flame by continuous nebulization and OS-2 ml of sample solution is needed for a single measurement. Recently, discrete nebulization techniques with conventional flame atomic absorption spectro- meters have been reported [4-7 ] and applied to various samples [ 4-10 1. Nebulization of about 100 ~1 of solution gives the same sensitivity as that obtained by continuous nebulization. The present communication deals with the combined use of the discrete nebulization and long tube techniques. The proposed method was successfully applied to the detxxmination of copper and zinc in NBS-SRM 1577 Bovine Liver.

Experimental The spectrometer used, operating conditions and preparation of standard

solutions were as reported previously [3]. To introduce small amounts of

sample solution to the nebulizer, a specially designed sampling cup was used (Fig. 1). This cup has four shallow holes (8 mm diam., 4 mm deep) of 200- ~1 volume, made by drilling the plane surface of a t&on rod (30 mm d&n.). The 509~1 droplet of sample solution placed in a hole with a micropipette was completely aspirated through a capillary tube, as shown in Fig. 1.

Results and discussion Spikelike signals similar to those observed in electrothermal atomization

with a graphite furnace were obtained, by using the system described. The signal increased with the aspirated volume of sample solution to a saturation

362

Hollow catho& la*p

Mcropipette t

Rtng burner

Fig. 1. Apparatus for nebulization of sample solution from cup.

value fixed by the uptake rate of the sample (Fig. 2). The peak height response became saturated for smaller volumes at lower flow rates. A sample flow rate of 2.7 ml min-’ was chosen for further study.

The calibration curves for copper (in 0.1 M nitric acid) obtained with various sample volumes are shown in Fig. 3. The curve obtained with 100 ~1 of solution was identical with that observed in continuous nebulization and was linear up to 120 ppb. With 50 ~1, the responses were also identical up to 40 ppb, but discrete nebulization gave less response than continuous nebulization at higher concentrations. With 25 ~1, the graph was slightly curved even at low concentrations and became pamlIe to the abscissa above 50 ppb, The differences were mainly due to the slow response of the in&u- ment. With 0.1 M nitric acid as solvent, the calibration graphs for copper had an intercept as shown in Fig. 3. This intercept was probably due to the procedure used to set the baseline without nebulization of 0.1 M nitric aoid. The relative standard deviations for peak height measurements were about 3% for 25911 samples and less than 1% for 50- and lOO+l samples.

Fig. 2. Effect of sample volume and flow rate on peak height for an 80 ppb CoPper xdution in 0.1 M HCI. The numbers on the cunws are the sample ilow rate in ml min-‘-

Fig. 3. Calibration curves for different volumes of copper solutions, 0.1 M in nitric acid. Sample volumes: (1) 100 ~1; (2) 50 ~1; (3) 25 ~1.

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Measurements were carried out with 50+1 samples to deterAmine the detection limits for 10 elements. These limits are listed in Table 1. The detection limits are given both as the concentration (ppb) and the absolute amount (pg) dissoived in 50-1.11 sample solution giving a signal twice the noise. For comparison, the detection limits obtained with a conventional flame apparatus and a graphite furnace are included. More than 10 times enhance- ment in detectability was obtained for silver, cadmium, copper, and zinc and l-6 times for the other elements by the present method compared to con- ventional flame atomic absorption spectrometry. The detection limits obtained for silver, cadmium, copper and manganese were of the same order of magnitude as those obtained with a graphite furnace. In practice, the present method was superior to electrothermal atomization in reproduci- bility, ease of use and rapidity of measurement.

Application to standard Bovine Liver. The technique was applied to the determination of copper and zinc in standard NBS-SRM 1577 Bovine Liver. The powdered sample (25 mg) was decomposed with 250 ~1 of nitric and 50 ~1 of perchloric acids in a sealed t&Ion vessel at 120°C for 3 h [ 121, and the digest was diluted to 250 ml with water. The results of six analyses were 184 * 4 ppm (certified value 193 + 10 ppm) for copper and 125 f 3 ppm (certified value 130 ? 13 ppm) for zinc. Agreement is obviously good.

The technique is simple, accurate, rapid and reproducible, and could be quite useful for determination of trace metals in numerous fields, especially when only minute amounts of the original sample are available for analysis.

TABLE 1

Comparison of detection limits in concentration (ppb) and absolute amount (pg)

AC Cd CO CU MC Mn Ni Ph %n

Resent method <pDb) 0.16 0.16 7.2 0.73 9.7 0.45 1.3 12 5.5 0.49 Hitachi 518= (wb) 3.6 11 20 8.1 13 0.52 3.7 20 31 14

Resent method (PC) 8 8 360 37 490 23 65 600 280 25 Hitachi 518D (Paz) 190 550 1000 410 GBO 26 190 1000 1600 700 E.ktrothcmmlb (PP) 0.02-20 0.2-2 2-30 5-10 2-20 0.1-I l-20 9-100 fi10 0.1-l

alO-cm air-acetylene flame. bRef. 11.

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