Chapter 19 Boron

Chapter 19

Boron

Boron (B, atomic weight 10.81, d = 2.34 g cm -3 ) is a metalloid that exists as a brown amorphous powder or, in higher purity, as dark grey crystals. Boron occurs in the earth's crust with an average abundance of 0.001%, principally in colemanites. Boron is widely used in electronics as dopant of amorphous silicon. Boron (10B) is an effective neutron absorber in the nuclear industry and is used as a hardener for steel and high purity metals. Boron is an essential micronutrient for plants but is toxic at higher concentrations. In medicine boron compounds are used in cancer therapy. Boron is a valuable tracer of various geochemical processes and of industrial pollution. Boron is unattacked by non-oxidizing acids and hot concentrated alkalis but readily dissolves in concentrated HNO3 and oxidizing mixtures. On fusion with alkali metal hydroxides, carbonates or peroxides, boron is oxidized to borate. Most metal borates, apart from those of alkali metals, NH' and TI(I), are sparingly soluble in water but dissolve in acidic solutions. In analytical chemistry only B(III) compounds are of importance. In water a number of oxy-anions are present: orthoborate (BO3-), metaborate (BO-) and tetraborate (B 402-) which are related to each other by hydration-dehydration reactions. In aqueous solution, boric acid reacts with HCl and HF to produce volatile boron fluoride (BF 3) and chloride (BCI3 ), respectively. In excess of HF involatile tetrafluoroboric acid (HBF 4) is formed. In anhydrous medium, boric acid reacts with methanol to form volatile trimethyl borate. Several boron hydrides (boranes) are also known. Boric acid and metal borates are toxic in large amounts whereas the boron hydrides are very poisonous.

347

19.1 SEPARATION AND PRECONCENTRATION Volatilization Volatilization of low boiling boron compounds: borofluoride (boiling point -101°C), trimethyl borate (boiling point 69°C) or diborane, B2H 6 (boiling point -92.5°C) is widely used for the separation of B from the sample matrix [1-9]. Boron fluoride, BF3, is usually distilled from an HF medium in the presence of H 2SO4 using a N2 carrier stream in a Teflon apparatus. The generation of trimethyl borate is an esterification reaction which requires an addition of concentrated H2SO4 and methanol. The heat to volatilize the trimethyl borate can be produced either by the addition of a small volume of water to the H 2SO4 solution or by heating the aqueous phase in a water bath prior to methanol injection. The inhibiting effect of water which is also a reaction product of the esterification can be eliminated by the addition of saturated Ca(OH) 2 to the sample and its evaporation prior to distillation of the trimethyl borate [5]. The interference of fluoride which masks the boron can be removed by adding A13+ which leads to the formation of A1F 4 instead of BF 4 [5]. The distilled trimethyl borate can be sorbed in a 4% NaOH solution [4,7] or water [2,5], or conveyed in a carrier stream directly to the atomizer [1,3,6]. An intermediate condensation step is often incorporated to increase the sensitivity. Diborane generation by heating a solid mixture of borate and LiAlH4 at 160°C has been reported [9]. Extraction Boron can be separated by extraction of H 3BO 3 with 1,3-diols, typically with 2-methylpentane-2,4-diol or 2-ethylhexane-1,3-diol [10,11]. Extraction of ion associates of BF 4 with onium cations or basic dyes (e.g. Methylene Blue) has been used [121. Ion exchange Cation exchangers [7,13-16] or chelating resins, e.g. Chelex-100 [14], do not retain B while removing dissolved cations. They are recommended to be used prior to sensitive determination techniques with low tolerance to the dissolved salts, e.g. ICP-MS. Conversely, anion exchangers which retain boron selectively are preferred in the analysis of samples with low boron concentration or when poorly sensitive techniques are used. Synthetic boron-binding resins (Amberlite XE-243, Amberlite IRA 743) contain a hydrophobic styrene backbone and Nmethylglucamine groups. The latter complex borate ions while matrix 348

components are eluted with NH 3(aq). Boron can be eluted with HCI [17-20] or 1% HNO 3 [21]. A more complex scheme is based on the conversion of the borate into BF 4 (with 10% HF), which is quantitatively retained on the tertiary amine groups and subsequently eluted with NaOH [14]. Another possibility is anion exchange of the boron-mannitol complex [22]. 19.2 DETERMINATION TECHNIQUES The position of spectrophotometry, although considerably recently eroded by ICP-AES because of often insufficient sensitivity and interferences, is remarkably strong. This is due to many difficulties occurring in AAS (formation of a thermally stable carbide), XRF (low atomic weight) and NAA (the lack of a suitable nuclear reaction). Spectrophotometry

In acid non-aqueous media, curcumin and boron form a violet-red complex called rosocyanin. The curcumin method offers an exceptionally high sensitivity (e = 1.8x10 5 at 550 nm) but results depend critically on the quality of the reagent. Numerous elements, oxidants and anions (fluoride) interfere so a separation step is mandatory. Other common reagents include Carmine (Carmine Red, a glycoside derivative of the a-hydroxyanthraquinone) [10,11,19] and Azomethine-H [5,19,23-27] both requiring separation of B prior to determination. An alternative is the extraction of an ion pair of BF 4 with Methylene Blue [12,28]. Flame atomic absorption spectrometry

Flame AAS offers a sensitivity of about 10 fig m-1 at the most sensitive 249.7 nm line in the recommended N 20-C2 H2 , reducing (rich, very red) flame. Atomization of trimethyl borate gas with a D2 background correction is recommended. Fluoride interference can be overcome by adding A13+ [29]. Electrothermalatomic absorptionspectrometry

The determination of B by GF AAS is difficult because of the involatile nature of this element (boiling point 2550°C) and the formation of a refractory carbide (boiling point 3500°C). The latter reduces the tube lifetime and is responsible for the memory effect. The loss of sensitivity can be prevented by adding matrix modifiers usually alkaline earth 349

metals (as salts) which form with B more volatile species (e.g. calcium boride). The memory effects were removed by injecting NaF and a CH 3OH-H 2SO4 mixture between measurements to destroy the residual boron carbide [30]. An alternative is the pretreatment of the graphite tube with a carbide-forming element (e.g. Zr). When combined with the use of a Ni modifier it was reported to give high sensitivity (80 pg), a negligible memory effect and an increased tube lifetime. The use of a Zr-treated L'vov platform gave lower sensitivity than that obtained with pyrolytically coated graphite tubes [30]. The latter, however, showed a considerably longer lifetime and allowed for the determination of B at high atomization temperatures on the routine basis. The memory effect was also eliminated [31]. Another difficulty is associated with loss of molecular boron forms below the appearance of atomic boron which cannot be prevented with the Ni(NO 3) 2 modifier [321. Electrothermal AAS was applied to the indirect determination of boron after extraction of the ion pair of the Cd-1,10-phenanthroline cation with BF4 and measurement of the Cd signal [33]. Flame atomic emission spectrometry

Molecular emission of boron oxide at 518 nm or the BO2 radical at 547.8 nm [34,35] is measured. Introduction of trimethyl borate or diborane vapour into an H 2 diffusion flame [91 is recommended to avoid interferences from alkali and alkaline earth elements. Fluoride greatly diminishes the flame [36] and plasma [37] emission of boron in methanolic solutions. This effect is caused by the replacement of a B-O bond by a B-F bond leading to the formation of the stable and involatile BF4 [34]. The presence of elements that form stable complexes with fluoride reduces the interference. Inductively coupled plasma atomic emission spectrometry

Inductively coupled plasma AES offers high sensitivity (a detection limit of 2-3 ng ml-) at the most sensitive lines: B I 249.773 nm and 249.678 nm [38,39]. Both lines are interfered with iron, because of the spectral overlap with Fe II at 249.772 nm and Fe I at 249.65 and 249.70 nm. The separation of B from the matrix or instrumental correction for the Fe content is required. The side line indexing method in scanning ICP AES was used to overcome the Fe interference in the determination of B in soils [40]. The iron interference free lines (B I at 208.959 nm and B I 208.893 nm) are less sensitive and are often not suitable to be used for the analysis at lower levels. The choice of wavelength is of particular 350

importance in the determination of B in refractory metals because of a high density of their emission lines in the spectral range used for B measurements. The 249.678-nm line was chosen for B determination in tantalum and the 249.773-nm line in W, Mo, Co, Nb and Ti metals [39]. To avoid interferences volatilization of trimethyl borate is the usual approach [3,6]. Care must be taken to remove excess of methanol to prevent quenching of the plasma. A sensitivity enhancement (DL 0.1 ng ml-l) and freedom from interferences were observed by ETV ICP AES using NaOH-(NH4 ) 2HPO 4 as a matrix modifier [411. With GF furnace volatilization addition of polytetrafluoroethylene to the sample prevents the formation of refractory carbide and eliminates the memory effects [42]. The filament vaporization has been proposed for the analysis of amorphous silicon [43,44]. Thermal ionization mass spectrometry

Boron has two naturally occurring isotopes 0°B and lB with natural abundances of 19.78% and 80.22%, respectively. Thermal ionization MS of Na2BO' has been widely applied to the determination of B [17,18,45] and its isotope ratios [13,17]. The ion emission intensity is strongly dependent on the ratio of the NaOH and H3BO 3 used to form the borax. The chemical stoichiometry problems are alleviated by thermal ionization of Cs 2BO after direct fusion of the sample with Cs 2CO 3 on a tantalum filament [2,22,46-48]. High concentrations of dissolved organic carbon suppress formation of Cs 2BO2 ions [2]. Chemical ionization MS has been proposed to quantitate the ' 0B isotope from the 11B/l°B ratio in enriched boron by using the (CH 3 0) 3BH+ ion. Direct introduction of trimethyl borate into the ion source has been employed [1]. Inductively coupled plasma mass spectrometry

Inductively coupled plasma MS offers high sensitivity (ca. 0.4 ng ml-l) and selectivity [7,14,49]. A serious concern is the matrix-induced mass discrimination which can be significant owing to the proportionally large mass difference between 0°B and 11B [14]. Consequently an apparent increase in the measured 0°B/"UB ratio in the presence of an abundant matrix component such as Na, is observed [14]. Therefore, a separation of B from the matrix is advised. The intense peak from 12C+ in biological samples can complicate the measurement of 11B+ so it is advisable to use 10B instead [50,51]. The 11B+ can be resolved from carbon in HR mode [24,50]. Use of ETV ICP MS with a Ni matrix modifier has been reported [32]. 351

19.3 ANALYSIS OF REAL SAMPLES General Boron standard solution is prepared on the basis of H 3BO3 and must be stored in a polyethylene bottle. If standard glassware is used, HF should be avoided since it attacks borosilicate glass leading to elevated blanks. The excess fluoride can be complexed with A13+ to protect the borosilicate and quartzware and to release boron from the stable BF 4 [52]. The use of a PTFE ware and sample introduction system for ICP is recommended [53]. Boron is often lost during decomposition or evaporation of HF containing solutions because of the high volatility of BF 3 [54]. Many workers added mannitol to prevent losses of B [22,33,52,55], but sometimes low recoveries were still observed. In some works H3 PO4 was added to HF-HNO 3 [44] and HF-HNO3 -HCl04 [40] mixtures to prevent losses. Contamination from mineral acids and fluxes is common [21]. It is necessary to remove B from the reagents before use, e.g. by a boron-selective anion-exchange resin. Closed systems, made of an inert material and having a minimum surface area are recommended to reduce the risk of contamination and to minimize blanks. Waters Boron is found in natural waters, brines and pore fluids at concentration levels ranging from less than 1 pg 1-1 to in excess of 200 mg 1-1 depending on the nature and the source of the fluid. River and waste water samples were collected in polyethylene bottles, filtered and acidified with HNO3 to pH <2 [30]. Spectrophotometric methods for environmental waters were compared; the most sensitive Azomethine-H gave a DL down to 20 jg 1-1 [23]. Flow-injection sample introduction systems for water samples has been proposed [56]. Combined methods for the determination of B in waters are summarized in Table 19.1. Geological materials Colemanites are dissolved by refluxing with dilute HCl; this is best done in a quartz apparatus. Aqua regia, HNO 3 , HC104 are used to oxidize carbonaceous matter and dissolution of the silicate matrix is accomplished with HF [54]. Losses of boron occur on evaporating to dryness. Boron is often present in resistant minerals such as tourmaline for which fusion is favoured for sample decomposition. Fusion with Na 2CO 3 followed by the dissolution of the melt in dilute HCl or warm water is preferred. During this procedure the borate anion passes into 352

TABLE 19.1 Methods for the determination of boron in water Water sample (amount) Fresh,saline (15 ml) Fresh,saline (40 g) River, tap Irrigation (5 ml)

Separation/ preconcentration

cation exchange anion exchange anion exchange volatn. as trimethyl borate Nuclear reactor (0.1 ml) volatn. as trimethyl borate Heavy water volatn. with Cs 2CO 3 Seawater ion exchange

Detection

Detection limit Ref. (range)

ID ICP MS VIS ID TIMS VIS

0.15 a (0.3-2)C (5-250)a n.g.

16 19 17 5

FAES

0 . 15 d

34

TIMS TIMS

(0 .04 - 17 )b n.g.

46 2

a In ppb in the sample; b in ppm in the sample; c in ppm in the soln. analyzed; d absolute detection limit, gg.

an aqueous extract of the melt. Many of the rock constituents (Al, Fe, Ca) do not dissolve in the alkaline solution and must be removed by filtration or centrifugation. NaOH is less efficient and gives higher blanks [7]. The bulk of the flux can be easily removed by subsequent addition of HC104 [38]. Interference from Fe was eliminated by using Na 2CO 3 -NaNO 3 fusion followed by an aqueous leach [57]. Boron can be separated from a carbonate melt by distillation of its methyl ester after acidification. Pyrohydrolytic leaching of B from rocks has been developed [2]. Boron is usually leached from soils with hot water [58] or CaC12 [26] solutions. The methods for the determination of B in geological materials are summarized in Table 19.2. Operational speciation of B in terms of total, leachable and mobile boron in sedimentary rocks has been discussed [20]. Biological samples Serum can be analyzed on dilution with HCl by ICP AES with a DL of 0.7 ng ml-1 [59]. Ashing or digestion of the sample is required to destroy the complex boron compounds (such as e.g. B1oH2o) prior to any combined procedure. Fusion with alkalis or sintering with Ba(OH) 2 or CaCO3 [18] have been reported but losses of organically bound boron in the tissue are likely. The most common is tissue digestion with a H 2 SO4 -H 2 0 2 either open vessel [60] or with microwave-assisted high 353

TABLE 19.2 Methods for the determination of boron in geological samples Sample (amount)

Decomposition

Separation

Detection

DL (range) Ref. (Rlg/g)

Rocks (0.2 g) Rocks (0.1-0.4 g) Rocks Rocks (0.5-1 g)

fusion with K2CO 3

none

ICP AES

2

38

fusion with Na2CO3

cation exchange

DCP AES

8a

15

fusion with K2 CO3 fusion with NaOH or Na 2C03

anion exchange volatn. as trimethyl borate; removal of Na + by cation exchange extrn. with 2ethylenehexane1,3-diol,stripping with NaOH anion exchange preconctn., elimination of interferences by cation exchange cation exchange on-line volatn. as trimethyl borate

VIS ICP MIS

n.g.

19 7

VIS

1

11

Rocks (0.15 g)

fusion with Na2C03

Rocks

leaching with HCI or fusion with Na 2CO 3

Sediments Soils (3 g)

fusion with Na2CO3 HNO 3 ,HF-HCl

Soils (0.2 g)

HNO3-HF-HC10 4-

ICP MS

14

TIMS ICP AlES

n.g. 50 a

13 6

ICP A]ES

n.g.

40

ICP AlES

5a

54

TIMS

(1-35)

17

ICP AES

1

57

VIS

n.g.

10

TIMS

n.g.

2

H3 P0 4 Soils, rock (1 g) CRMs CRMs (0.2 g) CRMs (1 g)

CRMs

HF-aquaregia fusion with Na 2CO3-KN03 fusion with Na2CO3-NaNO3 fusion with Na2CO3

pyrohydrolysis

a In ng/ml in the solution analyzed.

354

anion exchange

extrn. with 2ethylenehexane1,3-diol, stripping with NaOH volatn. as trimethyl borate, conversion to Cs 2B4 07

pressure [24]. Care must be taken as this mixture reacts explosively with acetone [60]. Tumour samples were decomposed with concentrated HNO3 in closed PTFE vessels [61]. Several determination methods of B in biotissues have been compared [24]. Measurement of "B by ICP MS is interfered with by the immense 12 C; oxidation of the organic material with HNO3 or HC10 4 is required often at the expense of higher blanks [50]. Procedures for the determination of B in biological materials are summarized in Table 19.3. TABLE 19.3 Methods for the determination of boron in biological materials Sample (amount)

Decomposition

Separation/ Detection preconcentration

DL (range) (tg/g)

Ref.

BioCRMs (0.5 g)

fusion with

anion exchange

ICP MS*

0.001

21

BioCRMs

Na 2CO 3 Ba(OH)2

anion exchange

TIMS

(1-35)

17

Animal tissue (0.1-0.2 g)

HC1O4 ,H2 02

none

ICP AES

(0.05-100)

28

Animal tissue (0.05 g)

H2SO4-H 2 0 2

none

DCPAES

0.1

60

Animal tissue (3-5 g)

HNO3 ,H2 02

none

ICP AES

n.g.

62

Plant CRM (0.25-1.25 g)

Ba(OH)2 or

anion exchange

ID MS

Plant tissue (0.5 g)

HNO 3-HCIO4 (bomb)

volatn. as ICP AES trimethyl borate

40 a

18

Plant tissue (0.4-0.5 g)

dry ashing

FAAS volatn. as trimethyl borate

1 .5 b

29

Plant tissue (5 g)

dry ashing, dissoln. in HCI

FI-VIS

n.g.

28

Plant CRM (0.2 g)

HNO 3 -HF-

ICP AES

n.g.

40

Wine (125 ml)

H2 SO4 -H2 0 2

6a

35

18

CaCO3

HC104-H 3 PO4

volatn. as FAES trimethyl borate

a In ng/ml in the solution analyzed; b absolute detection limit, g; * 11B/1OB isotope ratio measured.

355

Industrialsamples

Metals, quartz and synthetic glasses are decomposed at a mildly elevated temperature in closed Teflon vessels. The oxidizing properties of the solvent are maintained by adding H 2 0 2 to H2SO 4 or HNO3 . Methods for the determination of boron in industrial materials are summarized in Table 19.4. TABLE 19.4 Methods for the determination of boron in industrial materials Sample (amount)

Decomposition

Separation/preconcentration

Detection technique

Silicon

HNO3-HF

none

ETV ICP AES

Silicon (0.4 g)

HNO3-HF

matrix volatn. as SiF 4

DCPAES

Silicon (0.1 g)

HNO 3 -HF

extrn. as Cdphen 3 .2BF 4 (nitrobenzene)

indirect GF AAS

Silicon (0.1 g)

HF-HNO3, H2SO4

none

Silica (0.4 g)

HF-HNO 3 mannitol

Trichlorosilane (10 ml)

DL (range) (lLg/g)

Ref.

44

0.8

44

5b

33

VIS

n.g.

63

matrix volatn. as SiF 4

DCP AES

0.8

55

hydrolysis, dissoln. in HF

extrn. as Cd(Ph)3.2BF 4 (nitrobenzene)

indirect GF AAS

5b

33

Chlorosilane (10 ml)

hydrolysis, dissoln. in HF

none

VIS

n.g.

63

Steel (2 g)

n.g.

volatn. as trimethyl borate

ICP AES

20b

8

Steels (1 g)

aqua regia,H2SO4 -

volatn. as trimethyl borate

ICP AES

40 a

3

volatn. as trimethyl borate

ICP AES

0.070

4

H3 PO4 Iron,steel CRMs (0.5 g)

356

H2 SO4 -H3 PO4

Sample (amount)

Decomposition

Separation/pre- Detection concentration technique

DL (range) Ref. (Plg/g)

Titanium CRM

HF

none

ICP MS

1

64

Refractory metals

HF-HNO 3 or H2 02 (W)

none

ICP AES

from 0.4 (Ti) to 2.5 (Ta)

39

Coal (0.5 g)

HNO3 -HF

none

ICP AES

n.g.

65

Coal (1 g)

oxygen bomb, HF-HNO3

none

ICP AES

2

52

Fertilizers (CRMs)

HCI

anion exchange

ID TIMS

(1-35)

17

Chemicals (100 ml)

none

extrn. as MBBF-4 into 1,2-dichloroethane

VIS

125 b

12

MB = Methylene Blue. a In ppb in the solution analyzed;

b absolute

detection limit, ng.

REFERENCES 1 2 3 4 5 6 7 8 9 10 11

C.J. Cook, S.V. Dubiel and W.A. Hareland, Anal. Chem., 57 (1985) 337. A.J. Spivack and J.E. Edmond, Anal. Chem., 58 (1986) 31. T. Kumamaru, H. Matsuo, Y. Okamoto, M. Yamamoto and Y. Yamamoto, Anal. Chim. Acta, 186 (1986) 267. M. Hosoya, K. Tozawa and K. Takada, Talanta, 33 (1986) 691. J.M. Monz6, F. Pomares and M. de la Guardia, Analyst, 113 (1988) 1069. J. Novozamsky, R. van Eck, J.J. van der Lee, V.J.G. Houba and G.O. Ayaga, At. Spectrosc., 9 (1988) 97. M. Musashi, T. Oi, T. Ossaka and H. Kakihana, Anal. Chim. Acta, 231 (1990) 147. A. Lopez Molinero, A. Ferrer and J.R. Castillo, Talanta, 40 (1993) 1397. K. Fujiwara, H. Tsubota, S. Tsumura, S. Iwata and T. Kumamaru, Anal. Chim. Acta, 246 (1991) 413. G. Troll and A. Sauerer, Analyst, 110 (1985) 283. A. Hofstetter, G. Troll and D. Matties, Analyst, 116 (1991) 65.

357

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 358

J. Kdthe and G. Ackermann, Fresenius'Z. Anal. Chem., 320 (1985) 545. G.H. Swihart, P.B. Moore and E.L. Callis, Geochim. Cosmochim. Acta, 50 (1986) 1297. D.C. Gr6goire,Anal. Chem., 59 (1987) 2479. L.H.J. Lajunen, A. Kurikka and E. Ojaniemi, At. Spectrosc., 8 (1987) 142. D.C. Gregoire, J. Anal. At. Spectrom., 5 (1990) 623. N.L. Duchateau, A. Verbruggen, F. Hendrickx and P. De Bi6vre, Anal. Chim. Acta, 196 (1987) 41. A. Lamberry, V. Holland, F. Hendrickx and P. De Bievre, Fresenius'Z. Anal. Chem., 332 (1988) 645. E. Kiss,Anal. Chim. Acta, 211 (1988) 243. G. Zeibig, Mikrochim. Acta, III (1989) 389. F.G. Smith, D.R. Wiederin, R.S. Houk, C.B. Egan and R.E. Serfass, Anal. Chim. Acta, 248 (1991) 229. T. Ishikawa and E. Nakamura, Anal. Chem., 62 (1990) 2612. F.J. L6pez, E. Gim6nez and F. Hernandez, Fresenius'J. Anal. Chem., 346 (1993) 894. S. Evans and U. Krihenbfihl, Fresenius'J. Anal. Chem., 349 (1994) 454. M.A.Z. Arruda and E.A.G. Zagatto, Anal. Chim. Acta, 199 (1987) 137. P. Carrero, J.L. Burguera, M. Burguera and C. Rivas, Talanta, 40 (1993) 1967. A.R.A. Nogueira, S.M.B. Brienza, E.A.G. Zagatto, J.L.F. Costa Lima and A.N. Araijo, Anal. Chim. Acta, 276 (1993) 121. S.T. Tamat, D.E. Moore and B.J. Allen, Anal. Chem., 59 (1987) 2161. J.R. Castillo, J.M. Mir, C. Bendicho and C. Martinez, At. Spectrosc., 6 (1985) 152. M. Luguera, Y. Madrid and C. Camara, J. Anal. At. Spectrom., 6 (1991) 669. E. Norval, Anal. Chim. Acta, 181 (1986) 169. J.P. Byrne, D.C. Gregoire, D.M. Goltz and C.L. Chakrabarti, Spectrochim. Acta, 49B (1994) 433. C.Y. Liu, P.Y. Chen, H.M. Lin and M.H. Yang, Fresenius'Z. Anal. Chem., 320 (1985) 22. J.R. Castillo, J.M. Mir, C. Martinez and C. Bendicho, Analyst, 110 (1985) 1435. J. Sanz, R. Lopez Martin, J. Galban and J.R. Castillo, Analusis, 18 (1990) 279. A. Canals, V. Hernandis and J.V. Sala, J. Anal. At. Spectrom., 1 (1986) 277. A. Canals and V. Hernandis, J. Anal. At. Spectrom., 2 (1987) 379. J.N. Walsh, Analyst, 110 (1985) 959. G. Bauer, I. Rehana, W. Wegscheider and H.M. Ortner, Spectrochim. Acta, 43B (1988) 971. L. Xu and Rao Zhu, Fresenius'Z.Anal. Chem., 325 (1986) 534.

41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

Y. Okamoto, K. Sugawa and T. Kumamaru, J. Anal. At. Spectrom., 9 (1994) 89. Hu Bin, Z. Jiang and Zeng Yun'e, Fresenius'J. Anal. Chem., 340 (1991) 435. E. Kitazume, Anal. Chim. Acta, 187 (1986) 313. E. Kitazume, S. Ishioka and E. Mitani, Anal. Chim. Acta, 199 (1987) 245. K.G. Heumann, W. Schindlmeier, W. Zeininger and M. Schmidt, Fresenius' Z. Anal. Chem., 320 (1985) 457. K.L. Ramakumar, O.S. Khodade, A.R. Parab, S.A. Chitambar and H.C. Jain, J. Radioanal.Nucl. Chem., 107 (1986) 215. Y. Xiao, E.S. Beary and J.D. Fassett, Int. J. Mass Spectrom. Ion Process., 85 (1988) 203. E.S. Beary and Y. Xiao, Analyst, 116 (1990) 911. A.R. Date and A.L. Gray, Spectrochim. Acta, 40B (1985) 115. H. Vanhoe, R. Dams, C. Vandecasteele and J. Versieck, Anal. Chim. Acta, 281 (1993) 401. C. Vandecasteele, H. Vanhoe and R. Dams, J. Anal. At. Spectrom., 8 (1993) 781. J.C. Mills, Anal. Chim. Acta, 183 (1986) 231. F. Vanhaecke, H. Vanhoe, C. Vandecasteele and R. Dams, in: G. Holland and A.N. Eaton (Eds.), Applications of PlasmaSource Mass Spectrometry, RSC, Cambridge, 1991, pp. 149-161. B.A. Zarcinas and B. Cartwright, Analyst, 112 (1987) 1107. W.D. Hu, Anal. Chim. Acta, 245 (1991) 207. P.L. Kempster, H.R. van Vlirt and J.F. van Staden, Anal. Chim. Acta, 218 (1989) 69. G.E.M. Hall and J.C. Pelchat, Analyst, 111 (1986) 1255. W. Haubold, E. Koenig and R. Schmid, Fresenius'Z. Anal. Chem., 331 (1988) 713. Y. Mauras, K.S. Ang, P. Simon, B. Tessier, F. Cartier and P. Allain, Clin. Chim. Acta, 156 (1986) 315. R.F. Barth, D.M. Adams, A.L. Soloway, E.B. Mechetner, F. Alam and A.K.M. Anisuzzaman, Anal. Chem., 63 (1991) 890. D. Pollmann, J.A.C. Broekaert, F. Leis, P. Tsch6pel and G. T61g, Fresenius' J. Anal. Chem., 346 (1993) 441. D.L. Mann, Spectroscopy, 3 (1988) 37. J.S. Chen, H.M. Lin and M.H. Yang, Fresenius'J.Anal. Chem., 340 (1991) 357. F. Vanhaecke, H. Vanhoe, C. Vandecasteele and R. Dams, Anal. Chim. Acta, 244 (1991) 115. M.A.B. Pougnet, J.M. Wyrley-Birch and M.J. Orren, Int. J. Environ. Anal. Chem., 38 (1990) 539.

359