Evaluation of high-pressure microwave digestion methods for hydride ...

Mikrochim. Acta 127, 77-83 (1997)

Mikrochimica Acta 9 Springer-Verlag1997 Printed in Austria

Evaluation of High-Pressure Microwave Digestion Methods for Hydride Generation Atomic Absorption Spectrometric Determination of Total Selenium and Arsenic in Sediment Chao Yan Zhou 1, Ming Keong Wong 1,*, Lip Lin Koh 1, and Yeow Chin Wee: Department of Chemistry,NationalUniversityof Singapore,LowerKentRidgeRoad, Singapore0511, Republicof Singapore e Departmentof Botany,NationalUniversityof Singapore,LowerKentRidge Road, Singapore0511, Republic of Singapore

Abstract. Five closed-vessel microwave digestion methods were compared for the accurate determination of arsenic and selenium in NIST SRM 1645 River Sediment by flow-injection hydride-generation atomic absorption spectrometric methods. The digestion methods using five different acid mixtures (HNO3/ H2SO4, HNO3/HC104, HNO3/HC1, HNO3/HC1/HF, HNO3/H2SOa/HC104) were all found to be reliable for the determination of the analytes. Taking into consideration the safety and suitability for the analysis of other metals, the methods based on the use of aqua regia are recommended for closed vessel microwave digestion with pressure control. Using the quick digestion program, the presence of up to 10% organic content in soil samples did not adversely affect the closed vessel digestion and did not cause the loss of volatile analytes. After digestion, opening the vessel under an inner pressure of below 345 kPa (50 psi) had no effect on the accuracy of the results. The recommended digestion methods (HNO3/HC1 and HNO3/ HC1/HF) for the reliable determination of arsenic and selenium in different sediment samples were demonstrated. The calculated detection limits (3crB) were less than 0.030 gg/g and 0.033 gg/g for arsenic and selenium, respectively. All analytical results for arsenic and selenium in SRM 1645 River sediment, NRCC BCSS-1 Marine Sediment and NIES CRM Pond Sediment were within or near the certified and reported ranges, with the exception of selenium in NIES CRM No. 2 Pond Sediment. * To whomcorrespondenceshouldbe addressed

Key words: microwavedigestion,arsenic,selenium,hydridegeneration atomic absorptionspectrometry,sediment.

Health, regulatory, and environmental concerns have made trace-level determinations of selenium and arsenic extremely important. Arsenic is a widely distributed element well known for its toxic effects [1]. Selenium is a trace element present in the environment at levels which may be anywhere between essential and toxic to humans and animals [2]. Hence there has been increasing interest in these elements in biological and environmental samples [3-5]. Hydride generation atomic absorption spectrometry (HG-AAS) is a preferred method for the determination of selenium and arsenic, particularly at low concentrations. The advantages of HG-AAS are that it is highly sensitive and free of matrix component related interferences [6]. By combining it with flow injection analysis (FIA), the FIA-HG-AAS provides excellent means to control the sample introduction, reagent delivery and reaction steps. Such control further improves speed, precision and sensitivity of the HG-AAS method [7]. Through FIA it is possible both to miniaturize batch scale processes and to automate the manual analytical procedure [8]. Decomposition of the sample is one of the most important steps in trace metal determination using voltammetric and atomic spectrometric methods [9, 10]. The measurement of hydride-forming elements by HG-AAS requires that the sample matrix be destroyed and the residue taken up in solution.

78

c.Y. Zhou et al.

H y d r i d e - f o r m i n g elements in the solution must be in s i m p l e forms

which

can be r e d u c e d

and form

and selenium measurement, respectively. Peak area measurement was used for accurate quantification.

hydrides. The closed-vessel m i c r o w a v e digestion system provides a reasonable alternative to the conventional wet digestion procedure for the d e t e r m i n a t i o n of trace metals [11, 12]. Saraswati et al. [5] compared four different acid mixtures based on H2SO4 to digest sludge samples for the determination of s e l e n i u m and arsenic. For trace e l e m e n t analysis, HNO3-based acid mixtures were r e c o m m e n d e d in closed vessel micro-

Reagents The concentrated acids (HNO3, HC1, H2SO4, HC104 and HF) and all other chemicals used for digestion were of analytical reagent grade (Merck, Germany). All glass and polypropylene apparatus was washed with 5% nitric acid and deionized distilled water (DDW). Stock standard selenium and arsenic solutions containing 1000 rag/1 of each element for atomic absorption spectrometry were obtained from BDH (UK). All working standard solutions were prepared by immediate serial dilution with 0.2% nitric acid solution.

wave digestion as the digestion procedure could be controlled by pressure [11-13]. N h a m and Brodie [14]

Sample Preparation

studied three digestion methods using three acid

Separate samples of three standard sediments were dried in an oven at ll0~ for 5 h to obtain mass loss data. Undried samples were analysed to avoid the effects of arsenic and selenium loss. The results for the undried samples were multiplied by the mean undried to dried sample mass ratio obtained from the separate drying experiment. All reported values are based on the dried mass. Five procedures using different combinations of acids were studied. Because of the low concentration of organic substances in the sediment samples, quick digestion under the following conditions was used: power, 100%; pressure, 1035 kPa (150 psi); temperature, 200 ~ digestion time, 30 rain. Approximately 0.3 g of sample and blank were digested with different acid mixtures in decomposition vessels. After digestion, the vessels were left to cool until the pressure of the inner vessel was reduced to below 345 kPa (50 psi). The caps of the vessels were then carefully removed and the contents filtered and diluted with DDW to 50 ml.

mixtures based on HNO3 for s e l e n i u m analysis by graphite furnace atomic absorption spectrometry and H G - A A S . Accurate results were obtained b y using HNO3/HC1/HF acid mixture. The

aims of this work were to evaluate the

microwave digestion m e t h o d using different acid mixtures based on HNO3 for the determination of s e l e n i u m a n d arsenic in s e d i m e n t samples.

An

accurate m e t h o d for the d e t e r m i n a t i o n of these elements in different sediment samples b y F I A - H G A A S was then developed. In order to demonstrate the accuracy and precision of the method, a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) River S e d i m e n t 1645 was used.

Experimental Apparatus Sediment samples were digested in the MDS-2000 Microwave Sample Preparation System (CEM Crop., USA). This system was designed to hold 12 low-pressure screw-top polytetrafluoroethylene (PTFE) digestion vessels. The digestion vessels had rupture membranes for safe operation under 1380 kPa (200 psi). The temperature in the digestion vessel was kept below 250 ~ Absorption measurements were made using a commercial FIA system (Perkin-Elmer FIAS-200, USA) interfaced to an atomic absorption spectrometer (Perkin-Elmer Model 4100ZL, USA). The FIA system was computer-controlled with two peristaltic pumps, four switching valves, a sample injection loop (500 gl), a mixing coil and a liquid-gas separator. Dilute (3% w/v) hydrochloric acid was used in the carrier stream to sweep the sample from the injection loop to the mixing coil where it reacted with a solution of 0.25% (w/v) sodium tetrahydroborate (NaBH4) stabilized in 0.05% (w/v) sodium hydroxide. The gaseous hydrides of selenium or arsenic produced were separated from the solution (in the liquidgas separator) and swept by a stream of argon to a resistively heated (900 ~ quartz absorption cell mounted in the light path of an eletrodeless discharge lamp in the atomic absorption spectrometer. The wavelengths 193.7 and 196.0 nm were used for arsenic

Reduction of Arsenic and Selenium Arsenic. An appropriate volume of the sample digest or calihration standard was transferred to the test tube used for auto-sampler AS90 of FIAS-200. Then 1 ml of the reducing agent, containing 10% w/v potassium iodide and 5% w/v ascorbic acid solution, was added and left for over 1 h. The solution in the test tube was then made up to 10 ml with DDW before analysis.

Selenium. A 5-ml volume of digest and calibration standard was introduced to the test tube used for auto sampler AS-90 of FIAS-200 and 5 ml of conc. HC1 (37%) was added. The contentswere heated in a water bath for about 25 min at 90 ~ and cooled to room temperature.

Analysis Sample solution concentrations were determined by establishing a linear response with a calibration graph prepared using four standard solutions ranging from 2 to 20 ng/ml of arsenic and 1 to 10 ng/ml of selenium that were matched in acid and reducing agent concentrations with the samples.

Results and Discussion Choice of Acid Mixture Traditional soil decomposition techniques for the determination of arsenic and s e l e n i u m were tedious

Evaluation of High-Pressure Microwave Digestion Methods

and time-consuming. Most of them involved a dangerous combination of acids (such as H2SO4 and HC104), accompanied by heating, and losses of volatile analytes [5, 15]. The digestion acid mixtures commonly contain some strong oxidants to break down the sample matrix. To avoid the loss of selenium, which is in the reduced form, oxidizing conditions must be maintained throughout the digestion procedure and some components must be added to enhance the partition coefficient of the elements between vapor and solution [2, 16]. The digestion methods commonly used involve different acid mixtures such as HNO3]H2SO4]HCIO4 [5] or H 3 P O 4 / H N O 3 ] H 2 0 2 [17], and so on. The risk of atmospheric contamination and loss of analytes by evaporation is a serious problem in the open system. Pressure feedback microwave digestion makes sample digestion easy and convenient [18]. The technique is safe, saves time and reduces the loss of volatile elements [18, 19]. As the sample matrix is decomposed in a closed vessel under high pressure and high temperature and the oxidizing power of acids is directly proportional to temperature, the addition of strong oxidizing acid or the addition of acids to increase the boiling points of the mixture becomes unnecessary. Simple acid mixtures such as HNO3/HF [19], HNO3/HC1 [11] and H 2 S O 4 ] H N O 3 [5] are frequently used in microwave digestion. Taking into consideration the factors discussed above, five different acid mixtures were selected for soil sample decomposition in closed vessel microwave digestion for arsenic and selenium determination. They were: M1, 7 ml HNO3 § 7 ml H 2 S O 4 ; M2, 7 ml HNO3 § 7 ml HC104; M3, 3.5 ml HNO3 § 10.5 ml HC1; M4, 3.5 ml HNO3 + 10.5 ml HC1 + 1 ml HF; M5, 8.4 rnl HNO3 + 4.2 ml HpSO4 + 1.4 ml HC104. Hydro-

79

fluoric acid was used in M4, as it is useful in destroying the silicate matrix in soil samples [11, 13].

Microwave Digestion Procedure In closed-vessel microwave digestion, the pressure and temperature must be controlled to avoid exceeding the maximum values allowed while still achieving successful decomposition of the sample matrix. The application of a suitable program ensures safety of the digestion procedure and saves time [11, 12, 8]. In trace metal analysis, a quick one-step digestion program is often recommended for soil samples. In pressure-controlled microwave digestion, the relation between pressure and temperature is very important. From previous studies with nitric acid based mixtures [5, 11, 12], the pressure in the vessel easily reached 1380 kPa (200 psi). However, the temperature was always below the safety level of 250~ In detecting trace metals in soil samples, quick digestion was suggested. The pressure setting at or above 1035 kPa was used [11]. Maximum microwave output was used to reduce digestion time and 30 rain was found to be sufficient for the complete digestion of the soil matrix [5, 11, 12]. The quick digestion program used in this study was: microwave output 100%, pressure setting, 1035 kPa (150 psi); temperature setting, 200 ~ digestion time, 30 min.

Comparison of Five Microwave Digestion Methods The SRM 1645 River Sediment from NIST was used to compare the various closed-vessel microwave digestion methods for the determination of arsenic and selenium. The results using the five different acid mixtures are given in Table 1. All five acid mixtures

Table 1. Determination of arsenic and selenium in SRM 1645 River Sediment (Reference value of arsenic, 66 pg/g; the reported value of selenium [30], 1.2 4- 0.3 gg/g) and the sediment spiked with NIST SRM 1515 Apple Leaves (Certified values: arsenic, 0.038 4- 0.007 gg/g; selenium, 0.050 :k 0.009 gg/g) digested by closed-vessel microwave digestion using different acid mixtures Digestion method

MI

M2

M3

M4

M5

No. of determinations

3

3

6

6

3

664-3

64-t-2

664-4

65-t-4

664-3

1.04-0.1

1.04-0.2

1.14-0.1

1.14-0.2

1.1•

674-2

65-4-4

65•

66•

65-t-4

1.04-0.2

1.04-0.1

1.1•

1.0 i 0.1

Mean As in SRM 1645 river sediment (gg/g) Mean Se in SRM 1645 fiver sediment (gg/g) Mean As in SRM 1645 fiver sediment + SRM 1515 apple leaves ([xg/g) Mean Se in SRM 1645 fiver sediment + SRM 1515 apple leaves (gg/g)

1.04-0.1

80 were found to be suitable. The soil matrix was destroyed by the selected acid mixtures under high pressure and temperature. Other forms of selenium and arsenic were completely converted to Se4+/Se6+ and As3+/As 5+. Losses of volatile analytes were limited with the use of the closed vessel. The pressure and temperature in the digestion vessel are affected by several parameters, such as the power output, the volume of digestion vessel, the digestion time and so on. However, the nature and mass of the sample and the volume and composition of acid mixtures used for the digestion were very important. When 0.3 g of NIST SRM 1645 River Sediment was digested by selected acid mixtures, the relationships of the pressure and temperature were different. When using volatile acid mixtures such as aqua regia in M3, the pressure setting was the critical parameter for controlling the digestion. The pressure and temperature quickly increased during the initial digestion. When the pressure reached its preset value of 1035 kPa, the temperature also reached its maximum (45~ and remained constant during digestion. Even when the pressure setting was at the maximum 1380 kPa (200 psi), the temperature was below 160~ When acids containing H2SO4 or HC104 were used, the temperature increased quickly during the initial period, and reached its maximum setting of 200~ in less than 5 rain. The pressure increased relatively slowly compared with that for M3. In M1, the pressure first reached the maximum, followed by the temperature. When the pressure reached the maximum setting, the microwave radiation power was controlled by the pressure, and the temperature slowly became lower during digestion. The relationships of pressure and temperature for M2 and M5 had the same trend as for M1. In the sediment digestion with acid mixtures containing H2SO4 or HC104, the temperature setting was considered first. When using pressure to control the digestion, a lowpressure setting must be used to prevent the temperature from rising above that of the vessel tolerance. When the pressure setting was 483 kPa (70 psi), the temperature was controlled under 200 ~ in M1. At this pressure setting for digestion, similar recoveries of arsenic and selenium were obtained for M1, M2 and M5. On the other hand, using temperature to control the digestion by M3, the relationships of the temperature and pressure are also studied. Different temperature settings such as 150 ~ 120 ~ 90 ~ and 60 ~ were used. All temperatures

C.Y. Zhou et al. Table 2. Recoveriesof arsenic and seleniumin NIST SRM 1645 River Sedimentdigestedby M3 with differenttemperaturecontrol settings (n=4) Element

Temperature(~ 150

Arsenic Selenium

120

90

69

96-102% 98-104% 87-96% 50-73% 9 1 - 9 9 % 9 0 - 9 7 % 75-89% 47-71%

in the four digestions reached the preset values within 5 min. When the temperatures reached the set values, the pressures in the vessel reached their maxima at the same time or a little later. The maximum pressure was up to 1380 kPa (200 psi), when the temperature setting at 150 ~ was used. The pressure within the digestion vessel was reduced when the low-temperature settings were selected. The recoveries of arsenic and selenium were significantly affected by the temperature setting, as shown in Table 2. The recoveries of these two elements were acceptable when the temperature settings were at 150 ~ and 120 ~ The recoveries were reduced when the low temperature setting was used. The temperature during the digestion could be controlled by the pressure setting when the volatile acid was used. When the pressure setting was higher than 900 kPa (130 psi), the temperature in the digestion vessel was higher than 120 ~ for most of the digestion time in M3. Good recoveries of arsenic and selenium were obtained and the temperature was always in the saftey range. Owing to the potentially hazardous nature of the digestion environment using perchloric acid and sulfuric acid [14] and the difficulty of pressure control, M1, M2 and M5 digestion methods are not recommended. Indeed, many laboratories avoid using perchloric and sulfuric acids. It was found that the other two procedures were simple to use and gave essentially the same results. Recent studies have indicated that the use of aqua regia and aqua regia § HF to digest soil samples for the determination of trace metals in closed vessel microwave digestion are preferred as they are easy to use [11, 12]. On the other hand, systematic errors may result from chemical reactions of the analytes with acid mixtures containing H2SO4 as a result of the formation of sparingly soluble sulfates (i.e., Ba, Pb and Sr) [20]. The acid mixtures M3 and M4 are thus preferred for pressure feed-back microwave digestion.

Evaluation of High-Pressure Microwave Digestion Methods

81 110

Effect of Organic Substances The decomposition of organic substances in a closed vessel can result in additional stresses on the vessel as compared with the decomposition of inorganic samples. Gaseous decomposition products are generated, which add to the total vessel pressure. Also, exothermic reactions often occur, which increase the temperature of the closed system [12, 21]. Arsenic and selen.ium are prone to volatile losses during common wet digestion, particularly in difficult matrices. The loss of selenium caused by volatilization occurs during charring in the digestion process [22]. With the exclusion of histosols, the majority of soils contain ~< 10% (w/w) of organic matter [23]. To evaluate the effect of organic contents on the digestion, a mixture of 0.300 g of SRM 1645 river sediment and 0.033 g of SRM 1515 Apple Leaves was digested with the five acid mixtures. The changes of pressure and temperature in the vessel using the different acid mixtures were similar to those for the SRM 1645 River Sediment digestion. The results indicated that increasing the organic content up to 10% in the soil sample had no effect on the digestion procedure. The volatile elements, arsenic and selenium, were not lost (Table 1).

100

~

9o

8O 0

I

I

I

I

200

400

600

800

'

I

1

1000

1200

Pressure (kPa) Fig. 1. Effect of opening the digestion vessel under different inner pressures on the arsenic recoveries (n = 4)

110 -

100,

~:~

90-

8o 0

I

I

I

I

200

400

600

800

I

10'00

1200

Pressure (kPa)

Effect of Opening the Vessel with Different Inner Pressures The actual pressure of the inner vessel during sample decomposition was dependent on the vessel itself, the type and quantity of acids used, the temperature of the acid and the size and composition of the sample digested [21]. The increase in pressure was contributed by gases generated during sample decomposition, vapor of the acid and the volatile components. Cooling was an important step before opening the digestion vessel, both for safety reasons and to avoid loss of volatile analytes [24]. After digestion, it took a long time to cool the contents to room temperature and for the internal pressure to reach the safety level. Various post-digestion cooling methods were developed using water, ice-water and even liquid nitrogen [24]. However, it is difficult to reduce the pressure to atmospheric level. Opening the vessel when it is under high pressure would result in loss of some volatile analytes. To study the loss of arsenic and selenium when opening the vessel under different pressures, we analysed five groups of SRM 1645 River Sediment samples using acid mixture M3. The mean recoveries of the elements

Fig. 2. Effect of opening the digestion vessel under different inner pressures on the selenium recoveries (n = 4)

are shown in Figs. 1 and 2. There was no loss of arsenic when the vessel was opened under the five different pressures. The recoveries of selenium when the vessels were opened at an inner pressure of over 518 kPa (75 psi) were lower than when opened at lower pressures. It was obvious that part of the selenium in the sample existed in the gaseous phase after digestion and some was lost during the opening of the vessels when the inner pressure was over 518 kPa.

Analytical Characteristics and Application Good linearity was obselwed in the concentration range 2 to 20 ng/ml for arsenic and 1 to 10 ng/ml for selenium. The calculated detection limits (30-8) for acid mixtures M3 and M4, were 0.15 ng/ml arsenic and 0.20 ng/ml selenium for M3, and 0.18 ng/ml As and 0.20 ng/ml Se in the case of M4. Hence, the detection limits for arsenic and selenium in sediment analysis are respectively less than 0.030 and 0.033 ~tg/g by both M3 and M4.

82

C.Y. Zhou et al.

Table 3. Arsenic and selenium determination in different sediment reference materials using digestion methods M3 and M4 (n=3) Sample

Arsenic

NRCC BCSS-1 NIES CRM No. 2

Selenium

M3 (lag/g)

M4 (lag/g)

Certified (lag/g)

M3 (lag/g)

M4 (lag/g)

Certified (lag/g)

11.04-0.5 11.84-0.6

11.8• 12.4•

11.1• 124-2

0.39• 0.664-0.07

0.41• 0.69•

0.434-0.06 -

Table 4. Results for arsenic and selenium concentrations in different marine sediments collected around Singapore and some samples spiked with different species of arsenic and selenium using digestion methods M3 and M4 (n=3) Sample

JWA JWB JWE TB 1 TB2 TB3 SSB M CP

Digested by M3 Se (gg/g)

As (gg/g)

0.64-0.2 0.5-t-0.3 0.7• 0.84-0.5 2.14-0.2 4.1=t=0.3 1.84-0.4 0.9• 0.7-t-0.1

1.4-t-0.2 0.54-0.3 0.64-0.2 0.8~_0.2 0.5• 0.3• 0.34-0.2 0.24-0.1 < 0.1

Digested by M4 Spiked*

NazSeO3 Na2SeO3 NazSeO4 Na2SeO4 As203 As203

Recovery %

96 95 99 104 101 94

Se (lag/g)

As (lag/g)

0.5• 0.6=k0.3 0.6+_0.1 0.74-0.3 1.9• 4.0:t:0.3 1.4• 1.14-0.3 0.8•

1.14-0.1 0.84-0.1 0.4• 0.6• 0.6• 0.54-0.2 0.4• 0.3• 0.2•

Spiked*

Recovery (lag/g)

Na2SeO3 Na2SeO3 Na2SeO4 Na2SeO4 As203 As203

94 99 97 96 104 98

* Approx. 2 mg of substance per 400 g of sediment.

The recommended M3 and M4 methods were used to digest other sediments of National Institute for Environmental Studies (NIES) Certified Reference Material (CRM) No. 2 Pond Sediment (from Japan) and National Research Council of Canada (NRCC) BCSS-1 Marine Sediment (from Canada) for the determination of arsenic and selenium. The results, shown in Table 3, are in good agreement with the certified values with the exception of the selenium result for NIES CRM No. 2 Pond Sediment. Two digestion methods were also applied to digest nine marine sediments. Some of these samples were spiked with different species of selenium and arsenic (about 0.0020 g spiked substances in 400 g sediments). The results obtained are given in Table 4. For these spiked samples, good recoveries of 94-104% were obtained, indicating that with our proposed methods there was no loss of either arsenic or selenium.

Conclusions Five selected acid mixtures were all suitable for digesting soil samples using closed vessel microwave system for the determination of arsenic and selenium by FIAHG-AAS. The loss of analytes by volatilization was limited when the digestion vessel was opened at low inner pressure (< 500 kPa). Although the five different

sample digestion procedures gave similar results, digestion by aqua regia or aqua regia with a small quantity of hydrofluoric acid is preferred because of safety reasons and their suitability for the determination of other trace elements. Pressure control is recommended for sediment sample digestion with aqua regia. Pressure setting is useful to control the temperature in the digestion vessel. The temperature must be maintained at above 120 ~ during most of the digestion time. Acknowledgement. C. Y. Zhou thanks the NationaI University of Singapore for a research scholarship.

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E Hoke, Scientist 1992, July 6, 19. T. Suzuki, M. Sensui, Anal. Chim. Acta 1991, 245, 43. E. J. M. Temminghoff, I. Novozamsky, Analyst 1992, 117, 23. H. M. Kingston, L. B. Jassie, Anal Chem. 1986, 58, 2534. M. Verlinden, Talanta 1982, 29, 875. E L Stevenson, Humus Chemistry, Wiley, New York, 1982, p. 1. [24] H. J. Reid, S. Greenfield, T. E. Edmonds, Analyst 1993, 118, 443. [25] E. S. Gladney, C. E. Burns, I. Roelants, T. E. Gills, Natl. Bur. Stand. (U.S.), Spec. Publ., 1984, p.1. Received May 29, 1996. Revision October 24, 1996.