Fresenius Z Anal Chem (1988) 332:447-452 Fresenius Zeitschrift fiir

�9 Springer-Verlag 1988

Arsenic determination in the ultratrace range by atomic absorption spectrometry after preconcentration of the hydride Victor Arenas 1,,, M arkus Stoeppler 1, and Giinther Bergerhoff 2

1 Institut fiir Angewandte Physikalische Chemie der Kernforschungsanlage Jfilich, Postfach 1913, D-5170 Jfilich, Federal Republic of Germany 2 Anorganisch-Chemisches Institut der Universit/it Bonn, D-5300 Bonn, Federal Republic of Germany

Arsenbestimmung im Ultraspurenbereich dutch AAS nach Anreicherung des Hydrids

Summary. A method for the determination of extremely low levels of arsenic by hydride AAS is presented. It is based on hydride preconcentration in a quartz tube at liquid nitrogen temperatures under carefully evaluated and optimized ex- perimental conditions, followed by rapid heating up to 100~ and measurement in a heated quartz cell. Under the given experimental conditions an absolute detection limit of 0.05 ng and a linear range up to 6 ng could be achieved, representing a 6.5-fold improvement of detection power in comparison to commercially available hydride systems.

1 Introduction

The determination of arsenic and its compounds in environ- mental samples has increasingly gained importance during recent years due to the large differences in toxicity between its various organic and inorganic compounds .[1, 2]. Their separate determination ("speciation") in many different bio- logical and environmental materials is therefore urgently required.

Since these various arsenic compounds are often present in very low concentrations, it has been tried for about 20 years to improve the detection limits of the methods available in order to reliably determine even small quantities.

Many experiments were carried out to analytically utilize the capability of the elements of the main groups IV, V and VI of the periodic table to form gaseous covalent hydrides. The introduction of these hydrides into the flame of an AAS instrument had the advantage that most interfering compounds were left in the reaction vessel. Holak [3] pro- posed this method for the first time in 1969, although hydride formation had already been known for more than

* On leave from Universidad de Antofagasta, Departamento de Quimica Analitica, Antofagasta, Chile. Taken in part from the Ph. D. thesis, University of Bonn Offprint requests to . V. Arenas

100 years as a test for arsenic (Marsch test and Gutzeit test). In the presence of arsenic, he evolved hydrogen in hydrochloric acid solution with the aid of zinc and collected the arsines produced by this reaction in a trap cooled with liquid nitrogen. He then heated the trap in order to determine the arsines by means of an atomic absorption spectrometer, after transporting them into an argon- hydrogen diffusion flame by means of a nitrogen flow.

Modifications concerning the reductant [4-6] (sodium tetrahydroborate instead of zinc for hydrogen generation), the atomization system [7, 8] (electrically or flame-heated quartz cells instead of the sole flame), and the lamp [9-12] (electrodetess discharge lamp instead of a hollow-cathode lamp) then permitted the development of a significantly more sensitive method for the determination of arsenic at low concentrations. Although quite a number of other meth- ods are available today for analysis [13], hydride evolution using either AAS or electron capture detection [14] is prob- ably the most sensitive method, especially in conjunction with an arsine trap.

The arsine trap technique was further improved by Braman [15] and Andreae [14]. They collected different arsenic compounds in a cold trap, passed them sequentially into a flame due to the different boiling points and deter- mined the arsenic by emission or absorption spectrometry.

Attempts have been made for many years to achieve lower detection limits by modifications of the hydride method, and absolute values between 0.1 and 5 ng were reached [16 - 19]. Alone with the aid of a rather sophisticated miniaturization of the sample introduction using a continu- ous flow hydride generator system it was possible for Ikeda [20] to attain an absolute detection limit of 24 pg.

Welz [21-23] performed various studies to lower the detection limit of the hydride method. He achieved a 20-fold increase of the peak height by means of an arsine trap [23].

Our study aimed at a quick procedure with exactly inves- tigated parameters in an electrically heated quartz cell for the routine analysis of arsenic concentrations down to the absolute picogram range. The method is also based on the preconcentration of arsines formed prior to the arsenic de- termination by atomic absorption spectrometry. Thus it was possible to achieve an absolute detection limit of 50 pg and a 30-fold enhancement of the peak height.

Water-tra

I MHS- 20 I Transform. I

I I Regutator I

Helium

A

Hydrogen Arsine-trap

AAS.-I?E,-4000

illJ

i Lr " Recorder

Integrator

448

Fig. 1 Scheme of the complete atomic absorption spectrometric instrument with hydride generation device for the determination of arsenic at picogram levels

2 Experimental

2.1 Equipment

Electrically heated aluminium block, Coreci Palcor type RNS 2R4, from Liebisch-Bielefeld in West Germany, for the decomposition of biological samples in quartz vessels. The system allows a temperature program in arbitrary steps and has a capacity for 28 15 ml quartz flasks.

The determination apparatus (Fig. I) consists of: a) Perkin-Elmer MHS-20 hydride system including

arsine generator with electronical control of gas-flow, tem- perature of the quartz-cell and reductant feed.

b) Water vapor separator consisting of a quartz U-tube with arms 14 cm in length and different inside diameters (inlet 15 mm ~ i ; outlet 6 mm ~i) . This tube is cooled by an ethanol/liquid nitrogen mixture to approximately - 60 ~

c) The arsine trap is a quartz U-tube 280 mm in length with 8 mm inside diameter, the lower part of which is filled with 0.11 g of silanized quartz wool. A NiCr heating wire (4.6 O/m; 0.55 mm ~ ) is wound around the tube. The tem- perature of the U-tube can be regulated by a transformer (Platthaus type 108-121 Pr. 220 V _+5% 15 V 10 A) and a regulator (MFN Schiele) in such a way that heating from - J80~ to 200~ is possible in less than 10 s.

d) Chrompack gas flowmeter to control the hydrogen flow.

e) Atomic absorption instrument Perkin Elmer model 4000 with an EDL excitation unit, also from Perkin Elmer.

f) Recorder model BD-4104 from Kipp & Zonen, The Netherlands.

g) Integrator model 3390A from Hewlett Packard, USA.

2.2 Chemicals and standards

All acids used for decomposition and sample treatment (nitric acid 65%, sulphuric acid 96%, perchloric acid 70% and hydrochloric acid 30%) are of "suprapure" quality supplied by Merck, Darmstadt.

The sodium tetrahydroborate solution is prepared as follows from NaBH4 powder supplied by Riedel de H~ien AG, Seelze: 16.5 g of Merck Suprapur sodium hydroxide solution (30%) and 15 g of sodium tetrahydroborate are added to 468.5 g of distilled water. This solution can be used

after filtration up to a maximum of 3 days, if stored in a refrigerator.

The measuring solution is a mixture of 50 m130% hydro- chloric acid, 4 ml 96% sulphuric acid and 946 ml distilled water. The I mg/1 arsenic standards are prepared from sodium arsenate (Titrisol, Merck) and acidified to pH 3 with hydrochloric acid prior to filling up. They are diluted to l0 ng/ml prior to each series of analyses.

The carrier gases used were 99.9996% helium and 99.999% hydrogen, both from Messer-Griesheim AG, Dfisseldorf, details see below. Liquid nitrogen from Linde AG, FRG, and ethanol from Merck were used for the cooling mixture of the water vapor separator.

2.3 Decomposition procedure

For the determination of total arsenic in biological material an automated decomposition procedure, described else- where in detail [24, 25] is used: Subsamples up to 300 mg are first treated in quartz tubes, positioned in the heating block of a programmable digestion system with 3 ml nitric acid at about 90~ for about 15 min with occasional shaking, then are cooled down to room temperature. Sub- sequently 0.5 ml sulfuric acid and 0.2 ml perchloric acid are added and the block is stepwise heated up to 310~ within about 2.5 h in order to achieve complete decomposition.

Each sample solution is then made to volume in 25 ml borosilicate glass flasks with distilled water.

For the determination step, an aliquot containing approximately 1 ng of arsenic is pipetted from this solution into the MHS-20 reaction vessel which already contains 10 ml of 1.5% hydrochloric acid measuring solution. The sample is then analyzed by AAS under the conditions given in Table i after addition of the reductant.

2.3.1 Water separator. The quartz U-tube used as a water separator was tested without filling as well as filled in part with glass spheres of 3 mm diameter and in part with silanized quartz wool. This was done by repeated measure- ment of an aliquot with 2 ng As 5 +

2.3.2 Investigated parameters. Five parameters were principally investigated: Preconcentration time, addition time for sodium borohydride, carrier gas flow rate, influence

449

Table 1. Working conditions of the preconcentration method for arsenic determination

PE-4000 Wavelength 193.7 nm Slit width 2.0 nm Lamp EDL-As Mode Cont. 0.5 s Helium current 538 ml/min Hydrogen current 20 ml/min No background compensation

Water trap - 60~ Arsine trap - 185 ~ Heating temperature 100 ~

PE-MHS-20 Purge I 25 s Reaction 13 s Purge II 50 s Cell temperature 900 ~

of the additional hydrogen flow and temperature of the arsine trap.

In all parametric investigations at least five points de- pendent on the parameter and the associated blank values were measured several times and plotted against the absorp- tion obtained.

To examine the temperature of the arsine trap the sample is only transferred into the reaction vessel when the trap has reached -190~ The analysis is then started. During the entire measuring process the temperature of the arsine trap is registered by the recorder. At the beginning of addition, the temperature (TR) is noted in order to check whether the trap is sufficiently cold for a quantitative preconcentration. During the heating process, the temperature is also recorded when the peak appears (Tp). Finally the highest temperature reached by the arsine trap (TE) is also noted.

3 Results and discussion

3.1 Memory effect

During multiple determination of a sample, a non-reproduc- ible peak height increase was observed from measurement to measurement. This memory effect can be avoided by analyzing a blank solution after each analyte solution.

3.2 Water separator

During the rigorous reaction of sodium tetrahydroborate with the acidified sample aliquot much water vapor is formed in addition to the arsines. However, water interferes with the measurement since it is collected together with the arsines in the arsine trap where it freezes to ice. During trap heating, the rapidly vaporizing arsines are retained by the ice. The constant gas flow produces a pressure increase in the space in front of the ice.

When the ice evaporates, different gas flow velocities arise from measurement to measurement, i. e. the arsines are passed into the cell under non-reproducible conditions and the peaks show always a different height. A cooled quartz U-tube is best suited to separate the water from the arsines. The use of chemical drying agents is not advisable since the

large volumes often occupied by these chemicals and the need for frequent exchange due to rapid saturation make measurements more difficult.

A partial filling of the U-tube with glass spheres or quartz wool is neither advisable since a layer of fine ice crystals is formed after about ten measurements, preventing a determi- nation with adequate detection power.

An unfilled U-tube has the best separation properties for this trap and has only to be cleaned and dried after approximately 40 measurements.

Cleaning of the empty U-tube is much simpler since the free ice crystals formed can be blown out with the argon flow. The filled tubes must be completely emptied and cleaned when the ice crystals are molten.

3.3 Preconcentration time

When using the MHS-20, the determination can be per- formed in three steps.

1. Purge I phase: the air passed into the system by the addition of a new sample is flushed out with the carrier gas. This purge phase prevents a hydrogen-oxygen reaction during measurement.

2. Addition of the reduction solution. 3. Purge II phase: the residues of previously measured

samples are removed from the system with the carrier gas. When determining arsenic as arsines, a certain amount

of hydrogen [Eqs. (1) and (2)] and diborane [Eq. (2)] is formed in the reaction vessel in addition to the desired arsine [Eq. (1)1

As 3+ + B H 4 + 2 H / O ~ AsH3 + BO2 + 3 H + -l-H2 (1)

2 BH4 + 2 H + -~ B2H6 + 2 H2. (2)

This hydrogen plays a decisive role forming H radicals in the hot quartz cell because these radicals lead to a better atomization of the arsines after they are expelled from the arsine trap [21 - 2 3 , 26]. During preconcentration, however, the hydrogen formed in the reaction flows through the trap and reaches the cuvette earlier than the arsenic. In the case of excessively long concentration times it is either possible that the H radicals concentration in the cell is too low or that no radicals are present any more. This leads to thermal decomposition due to the cell temperature and to a decrease of the detection limit since this process probably only generates As2 and/or As4 [22].

It can be seen from the investigation results for these parameters (Fig. 2 a) that the highest useful net absorption for arsenic is reached when the trap is heated 20 s after termination of the sodium tetrahydroborate addition. How- ever, this time does not only imply that hydrogen is still present after 20 s of preconcentration, but also that the arsine has only formed quantitatively some time after the addition of sodium tetrahydroborate.

3.4 Time for adding sodium tetrahydroborate

The investigation of this parameter has shown (Fig. 2b) that an addition time of 13 s is long enough to reduce a maximum of 6 ng arsenic under the experimental conditions. This time is equivalent to an addition of 3 ml of 3% NaBH4.

A surplus of sodium tetrahydroborate is not to be recom- mended since the blank value will then be too high.

1

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8 . 7

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a 8] 8

1 ,

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8 . 8

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n o 7 ~

I l I I

18 28 38 48

Blank , , c,-

I I 1 58 68 ?8

Preconr time [s]

5ngAs

f I I I

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28 25 Time of addition of Ha.H4 [s]

5n As

I

195 I I I I I

288 3 ~ 488 588 ~8B

Blank

1 I [ t

780 BB8 988 I888 Gas flow [ml He/mini

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Fig. 2 a Influence of the preconcentrat ion time of the arsine in the cool trap on the absorbance, b Influence of the time of addition of reductant (NaBH4) on the absorbance, c Influence of the helium current during the arsenic determination on the absorbance. d Influence of supplementary hydrogen added to the helium current on the absorbance, e Influence of the heating temperature of the arsine trap on the absorbance

3.5 Influence of the carrier gas flow rate

Argon is normally used as the carrier gas in arsenic determinations with hydride generation and atomic absorp- tion spectrometry. Since in this case, however, argon is condensed, freezes and clogs the pipe due to the low temper- ature in the arsine trap (about - 1 8 0 ~ a more volatile carrier gas such as helium (melting point - 269 .7~ or hydrogen (melting point - 2 5 9 . 2 ~ has to be used.

As can be seen from Fig. 2c, the gas flow rate influences the sensitivity of measurements. It rises until the helium flow has reached a flow rate of approx. 540 ml/min and then decreases again. The opt imum flow rate is therefore in the range of about 540 ml He/rain.

3.6 Influence of an additional hydrogen flow

As already mentioned, hydrogen participates in the atomiza- tion of the arsines in the heated cell. The influence of an additional hydrogen flow leads to various effects. As can be seen from Fig. 2d, the absorption initially decreases until 3 ml Hdmin . The measured value then increases again and exceeds the predefined value without hydrogen by approx.

Table 2. Evaluation of detection limit under different conditions

Peak height (in arbitrary units)

No trapping Trapping

Blank 5 ng As 5 + Blank 1 ng As s +

1.7 11.6 1.6 10.5 2.2 10.5 1.8 11.4 1.9 11.5 1.8 11.0 1.9 10.8 2,0 11.3 2.2 11.1 2.0 11.3 2.0 X4- S 1.9 4- 0.2 11.8 RSD (%) 9.8 3.6 Detection limit

326 pg

4- 0.4

11.0 35.4 11.2 35.4 10.7 36.5 11.5 34.9 11.1 34.3 12.1 34.4 11.1 33.8 11.3 36.7 11.2 35.3 11.6 35.0

11.3 ___ 0.4 35.2

3.4 2.6

50 pg

4-0.9

Recorder: 10 mV

451

2% at 20 ml/min. Hydrogen should therefore be added to the helium flow at a rate of 20 ml/min to achieve maximum sensitivity.

3.7 Temperature of the arsine trap during the process

In order to identify the relationship between the heating temperature in the arsine trap and the sensitivity of measure- ments, the trap temperatures were measured by means of a temperature sensor placed into the tube through a bypass.

Since the temperature sensor interferes with the measure- ment, Fig. 2e only presents relative measuring values in order to demonstrate how the sensitivity of determination behaves in connection with changes of the temperature in the arsine trap.

The arsine trap must reach at least - 1 7 0 ~ for a quantitative preconcentration, since otherwise a part of the arsines do not remain in the arsine trap, but are immediately passed on into the cuvette. At the same time, the arsenic concentrations in the cell are so low that no absorption can be identified. The loss is only observed during the subsequent heating process because the peak is then markedly lower.

The arsines are highly volatile due to their low boiling point (boiling point = - 116.3 ~ There is no difference in the sensitivity of measurements between a trap end tempera- ture of - 10 ~ and 100 ~ Temperatures below - 10 ~ are very difficult to measure. It was not possible to hold the exact end temperature, since the room temperature (25 ~ caused a slow, but steady increase. A difference can be clearly seen, however, in the time within which the peak appears after heating; the higher the temperature, the sooner the peak appears.

3.8 Detection limit and linearity

The detection limit of the method described here is 50 pg of arsenic. It was calculated from the threefold standard deviation of the blank value (Table 2). In order to determine the standard deviation, ten blank values were alternately measured in each case with arsenic aliquots. Depending on the level of the blank, the linearity of the calibration curve extends up to 6 ng of arsenic. The detection limit is 326 pg under the same conditions without preconcentration.

3.9 Arsenic determination in certified biological reference materials

Seven biological reference materials were analyzed for total arsenic to confirm the accuracy and reproducibility of the method (cf. Table 3).

Table 3. Arsenic values found for various reference materials

NBS Standard NBS No. Sample weight Concentration found RSD n Certified Reference materials (mg) (lag/g) (%) (lag/g)

Citrus leaves ] 572 100-- 150 3.2 ___ 0.3 9.4 11 3.1 4- 0.3 Rice flour 1568 290-310 0.41 4- 0.02 4.9 6 0.41 4- 0.05 Oyster tissue 1566 100-200 13.0 4- 0.8 6.2 14 13.4 4- 1.9 Tomato leaves 1573 300--400 0.29 + 0.02 6.9 13 0.27 4- 0.05 Orchard leaves 1571 100-250 10.2 4- 0.4 3.9 4 10 4- 2 Pine needles 1575 300-400 0.20 4- 0.01 5 8 0.21 4- 0.04 Bovine liver 1577 390--400 0.049 4- 0.006 12.2 4 0.047 4- 0.006

452

TRAPPIN6

He/H2"current I Cooling ~

Reduction Trapping ~_

Volat i l izat ion | I

8 l i t

i i i i i I i i 28 38 48 Sll 68 7"8 88 98

Time (e) NO TP.~PP I NG

A r - c u r r o n t ~ ..... Reduction T , , ,

II 18 28 38 48 i i i i i

58 68 7B Bg 9B Time ($)

Fig. 3. Comparison of the required times in both trapping and no trapping methods for the determination of arsenic at picogram levels

oJ u

e o

.JD <~

1.0

0.8

0.6

No trczpping Trapping Volatilized in He*H=

150 ng As 5ng As

OA SOngAs

5ng As

o o I 1 Fig. 4. Comparison of the peak heights of both trapping and no trapping methods for the determination of arsenic at picogram levels

Conclusions

The method presented here permits a highly sensitive deter- mination of traces of arsenic in organic and inorganic mate- rials.

In spite of the concentration time, there is only a slight difference in time (14 s) as compared to the common method because the concentration coincides with the Purge II phase (Fig. 3). By shortening the concentra t ion time or the Purge I or Purge II phases it is possible to carry out an even more rapid determinat ion without influencing the precision of the

result, since the change of the different parameters only influences the sensitivity.

The gain in sensitivity achieved by this method can be seen from Fig. 4. The high net absorpt ion for small arsenic aliquots permits a determinat ion of arsenic even for very small sample quantities.

References

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Received June 15, 1988