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Boron Determination-A Review of Analytical Methods

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Boron Determination—A Review of Analytical
Article in Microchemical Journal · July 1997
DOI: 10.1006/mchj.1997.1428
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Patrick H. Brown
University of California, Davis
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56, 285–304 (1997)
Boron Determination—A Review of Analytical Methods
R. N. Sah and P. H. Brown
Department of Pomology, University of California, Davis, California 95616
Received April 12, 1996; accepted September 1, 1996
This paper reviews published methods of sample preparation, determinand purification, and
the determination of boron concentration and isotopic composition in a sample. The most common
methods for the determination of B concentration are spectrophotometric and plasma-source
spectrometric methods. Although most spectrophotometric methods are based on colorimetric
reactions of B with azomethine-H, curcumin, or carmine, other colorimetric and fluorometric
methods have also been used to some extent. These methods, in general, suffer from numerous
interferences and have low sensitivity and precision. Application of nuclear reaction and atomic
emission/absorption spectrometric (AES/AAS) methods has remained limited because these methods have poor sensitivity and suffer from serious memory effects and interferences. Among a
large number of published nuclear reaction methods only prompt-g spectrometry has been of
practical use. The prompt-g method can determine B concentration in intact samples, which
makes this method especially useful for some medical applications, including boron neutron
capture therapy. However, this is a time-consuming method and not suitable for detection of low
levels of B. Inductively coupled plasma optical emission spectrometry (ICP-OES) created a new
dimension in B determination because of its simplicity, sensitivity, and multielement capability.
However, it suffers interferences and is not adequately sensitive for some nutritional and medical
applications involving animal tissues that are naturally low in B. All methods involving the
measurement of B isotopic composition require a mass spectrometer. Thermal ionization mass
spectrometry (TIMS) and secondary ion mass spectrometry (SIMS) have been used to measure
isotopic composition of B; however, these methods are time consuming and require extensive
sample preparation and purification. Development of inductively coupled plasma mass spectrometry (ICP-MS) not only overcame most of the drawbacks of earlier methods, but also its capabiltiy
of measuring B isotopes made possible (1) B concentration determination by isotope dilution,
(2) verification of B concentration by isotope fingerprinting in routine analysis, and (3) determination of total B concentration and B isotope ratio for biological tracer studies in the same run.
Therefore, plasma source MS appears to be the method of choice among present-day technologies.
q 1997 Academic Press
Boron is an essential element for plants. Boron is present in animal tissue in low
concentrations (about 1 mg B/L) and is probably an essential micronutrient for humans;
however, no essential biochemical function has yet been positively identified to establish its essentiality to animals and humans (1). Boron deficiency in plants may result
in reduced growth, yield loss, and even death, depending on the severity of deficiency.
Excess B is toxic to plants and animals. Boron toxicity symptoms may range from
necrosis of some plant organs to death of the whole plant depending on the extent
and severity of the toxicity. The tendency of B to accumulate in animal and vegetable
tissues constitutes a potential hazard to the health of those consuming food and water
with a high B content (2).
Boron occurs as a significant component of steel, glass, and the dielectric borophos285
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phosilicate glass films. Boron is widely used as a thermalizing agent in nuclear reaction
materials and as a dopant in silicon wafers in the semiconductor industries. Boron
carbide, used as control rods, is an important nuclear material (3). The flow and other
properties such as etch rates of borophosphosilicate glass films are directly dependent
on their B and P concentrations (4). A small change in B concentration/content can
influence the properties of semiconductor-grade silicon (5) and physical properties
such as hot workability, hardenability, and creep resistance of steel and alloys (6–8).
Boron is also used as a source of short range a particles in cancer treatment using
boron neutron capture therapy (BNCT) (9–10).
BNCT is a novel technique for the treatment of cancer that uses 10B-labeled compounds and neutron radiation to kill cancerous cells. The significance of B compounds
in BNCT stems from high neutron cross section or capture probability (3838 barns)
of the 10B atom compared to other biologically ubiquitous atoms such as carbon (0.003
barns), hydrogen (0.33 barns), nitrogen (1.8 barns) and oxygen (0.0002 barns) atoms.
The short range (10 mm) cytotoxic a radiation released in the neutron capture reaction
kills the targeted cancerous cells without affecting the neighboring healthy cells (11).
Precise determination of B and its isotopes is necessary for the evaluation of the tumor
specificity and pharmacokinetics of B compounds for BNCT. Often it is necessary to
be able to measure B in very small samples (e.g., biopsy-needle samples) to make
sure that the drugs are actually localized in target tissue before exposing the patient
to neutron sources (12). Therefore, accurate determination of the B concentration is
very critical for these applications.
Naturally occurring materials may vary enormously in B isotope proportions (13).
B isotope ratios (11B:10B) in naturally occurring rocks and minerals varied from 3.8
to 4.2 depending on the source and the nature of the materials (14). The 11B:10B ratios
of weathered rocks may show negative shifts while those of the marine sediments
show 11B enrichment relative to their natural abundance ratio (15). Aggrawal and
Palmer (16) have recently reviewed the methods of B isotope analysis. The National
Institute of Standard and Technology (NIST) certified Standard Reference Materials
(SRMs) such as NIST-boric acid standard for B isotope ratio and NIST-botanical
standards for total B concentrations are widely used for verifying the accuracy of a
determination. A small amount of siliceous or a calcareous mineral matter present in
these SRMs may contribute to the errors that may not be resolved by analytical
methods. Therefore, one must use caution to ensure that the heterogeneity found in
SRMs is appropriately considered and dealt with (17).
The purpose of this article is to review the significance, strengths, and weaknesses
of various techniques for B determination. The role of sample preparation methods
for B determination is also addressed.
Extraction from Soil and from Geological and Miscellaneous Materials
The hot water extraction method (18) has been widely used for establishing the
index of plant-available B in soil. However, the amount of B extracted by this
method is affected by the extraction time and temperature (19) and the potential
resorption of B during the cooling period (20, 21). The hot water extract of some
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soils may be colored which may affect spectrophotometric B determination. Treatment with activated charcoal to remove the color of the extract was suggested (22);
however, this treatment may lower B concentration in the extract due to B sorption
on charcoal (21). On the whole, the hot-water extraction procedure is difficult to
standardize (23), time consuming, and tedious for routine usage if reproducible
results are to be obtained (24).
A number of reports suggested the extraction of soils with dilute CaCl2 solutions
to alleviate the problems associated with hot-water extraction (25, 26). However, colddilute CaCl2 extracts less B than hot water or hot-dilute CaCl2 (27, 28). Studies
involving 31 US soils (29) and 100 Dutch soils (23) confirmed these findings but also
showed that the values of cold extraction (water or 0.01 M CaCl2) were highly correlated with those of hot extraction (water or 0.01 M CaCl2). A more convenient cold
0.05 M HCl extraction method was recommended for predicting plant-available B
status of acid soils (30, 31). However, Fe extracted with the acid extractants often
interferes in B determination by commonly available methods such as azomethine-H
spectrophotometry (32) and inductively coupled plasma optical emission spectrometry
(ICP-OES) (32–34). Alternative methods such as extraction with sorbitol as B-sorbitol
chelate (35) and with BaCl2 or water using microwave heating (24) were found to
give results in agreement with those obtained by the standard hot-water extraction
method for ICP-OES and spectrophotometric determinations. In light of clay dispersion
and filtration problems commonly encountered in the extraction with either cold or
hot water, alternative extraction methods using BaCl2 recently suggested by Vaughan
and Howe (35) and Deabreu et al. (24) appear appropriate as they would also allow
the determination of Ca in the same extract.
Decomposition of Biological Materials
Biological samples are commonly decomposed by dry ashing, wet ashing, and
microwave dissolution. However, several other methods have also been reported for
specific applications. Alkali fusion of biological materials caused negligible volatilization loss of B and resulted in 80–95% recovery of spiked 10B (36) but the high salt
environment of the fused materials may cause matrix interferences (14). Low temperature radio frequency (RF) current ashing followed by ion exchange separation of B
reduced matrix interferences in the hollow cathode emission method (37). Alwarthan
et al. (38) decomposed the seeds and pulp of date palms by treatment with saturated
Ca(OH)2, drying at 1057C, volatilization over a burner and then dry ashing in a muffle
furnace for colorimetric determination of B. For an interlaboratory B determination,
timber samples were extracted with HCl or H2SO4 (39).
Dry ashing of a sample is commonly performed in a muffle furnace using a suitable
container (e.g., quartz or platinum crucibles) by a method similar to those described
by Gaines and Mitchell (40) for the spectroscopic azomethine-H method and by Brown
et al. (41) for inductively coupled plasma mass spectrometry (ICP-MS). Dry-ashed
samples are usually dissolved in dilute HNO3 for determination.
Wet ashing is performed by placing samples and strong mineral acids such as
HNO3, H2SO4, HClO4, or their mixtures in digestion containers (ideally B-free) and
heating (usually with a reflux system) to decompose the sample. The use of Bcontaining digestion containers such as borosilicate glass could contaminate the sample
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and lead to higher blank values. HNO3 is preferred for wet ashing for plasma source
methods because it provides a simpler matrix than other mineral acids. If HF or HClO4
is used for wet ashing for ICP-MS determination, the digest is usually evaporated to
dryness and redissolved in HNO3 (42).
Several analytical methods for B determination have utilized high-pressure microwave dissolution of biological materials in closed Teflon (PTFE) vessels. Examples
of these methods include spectrophotometric (32, 43), ICP-OES (44, 45), and ICPMS methods (46 – 48). The microwave dissolution is also used for the decomposition
of nonbiological materials such as steel samples (6) using diluted aqua regia
(3HCl:1HNO3 v:v) and for coal ash samples (49) using HCl/HNO3/HF. Highpressure microwave digestion is faster than other methods, requires less acid, avoids
sample volatilization and cross contamination, and results in generally low blank
values. However, the digests of biological materials may contain a large amount
of dissolved carbon and organic compounds, which tend to interfere in some methods of B determination. For the microwave digestion of plant and animal materials,
the ICP-OES and MS methods were the best, the carminic acid method was unreliable, and the azomethine-H method was satisfactory only for materials with high B
content (32).
Published findings on sample decomposition methods for B determination are inconsistent and conflicting. Sample-B concentrations found by the wet ashing method were
reported equal to (50, 51), lower than (52), or higher than (45, 53) those found by
dry ashing and the microwave dissolution methods. Some examples of these contradictions are as follows: (a) decomposition of human hair samples by dry ashing gave
higher B values than by wet ashing with acid or base when B was determined by the
azomethine-H method (52); (b) wet-acid digestion of plant tissues gave substantially
higher B concentrations than dry ashing (53) and microwave digestion without predigestion (45); and (c) several reports have stated that there is no significant difference
among these digestion methods. Boron values of NIST SRM citrus leaves and other
materials for closed vessel microwave dissolution agreed well with those for dryashing (50) and open vessel wet ashing (43) determined by ICP-OES. Spiers et al.
(51) also found no difference between the decomposition of plant materials by wet
ashing (with HNO3) and dry ashing. Most recently, Bratter et al. (54) found good
agreement between microwave sample dissolution and high-pressure wet ashing of
diet samples for a number of elements determined by ICP-OES. We found quantitative
recovery of B and no difference in B values in the NIST SRM biological tissue
samples decomposed/dissolved by either (1) microwave acid dissolution with HNO3
and H2O2, (2) dry ashing, (3) wet ashing with HNO3 and H2O2 or (4) wet extraction
with hot 1 M HNO3 when digests/extracts were analyzed by ICP-MS(55).
Decomposition of Miscellaneous Materials
Complete decomposition of soil and of geological and silica-rich materials is generally accomplished either by alkali fusion (36, 56) or by wet digestion using HF or a
mixture of HF with other acids (42, 57, 58). Although Na2CO3 is the most extensively
used flux for alkali fusion, use of other fluxes such as NaOH, KOH, and Cs2CO3 has
also been reported (14, 36, 56). Beary and Xiao (3) noted significant advantages of
Cs2CO3 over Na2CO3 for TIMS determination of B. When the sample is decomposed
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with HF, excess HF is usually destroyed by evaporating the digest to dryness. Significant amounts of B may be lost during the decomposition and the evaporation processes
(33, 34, 59), owing to the high volatility of BF3, if proper precautions are not taken.
The volatilization loss of B during the evaporation process occurs more readily from
acidic solutions than from neutral or alkaline solutions (60). Addition of mannitol
during the HF digestion and the control of evaporation temperature below 807C (61)
or at 707C (62) were effective in preventing B volatilization loss (61) and isotopic
fractionation (62), probably due to the formation of B–mannitol complex. Hu (63)
used mannitol to control B volatilization during the evaporation of HF / HNO3 from
digests of high purity silica to near dryness (0.5 mL volume). The use of orthophosphoric acid instead of mannitol in the HNO3/HClO4/HF digest also gave 100% recovery
of B following the evaporation of the digests to dryness at 707C (64).
Some sample matrices may interfere with B determination. Matrix interference
reduction methods such as matrix matching, standard addition, and isotope dilution
methods are often adequate to overcome matrix effects. However, the presence of
substances such as high salt concentrations (e.g., in soil extracts, ocean waters, and
alkali fusion digests), organic substances, and the determinand species that directly
interfere with B signal make some sample matrices too difficult to analyze by the
above-mentioned methods. The isotope dilution technique, one of the most reliable
methods for many difficult matrices, cannot be used for the determination of isotopically altered samples commonly encountered in biological tracer studies. Similarly,
the determination of isotope ratios would be in error when species in the sample
matrix interfere with one of the two isotopes differentially. Under these conditions,
the determinand of interest needs to be separated from the sample matrix. If the
concentration of the determinand in the original sample is too low to be accurately
measured, then separation is combined with determinand preconcentration in the new
matrix. Several methods of separation and preconcentration of B from aqueous solutions have been reported in the literature. These include solvent extraction (65), ion
exchange separation (14, 66), chelation (14), B-specific resins (36), chromatographic
separation, and separation of B as gaseous methyl borate (67, 68) or boron fluoride.
Solvent Extraction
Boron may be extracted as one of its organic complexes such as (a) the boron-2ethyl-1,3-hexanediol complex of B into chloroform or benzene (69, 70), (b) the 2,4dinitro-1,8-naphthalenediol complex of B into toluene (71), or (c) the complex of B
with a 24% solution of 2,4-dimethy-2,4-octanediol into isopentanol (72). Novozamsky
et al. (73) suggested a solvent extraction system based on the formation of BF40 ions
that were extracted with liquid ion exchanger (Aliquat 336, tricaprylmethylammonium
chloride) in xylene for ICP-OES determination. The separation may be performed on
line and the separated phase is introduced directly into an ICP using pulse nebulization
(72). However, the use of solvents affects plasmas and often limits the applicability
of these methods for B determination (74).
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Exchange Separation
Boron may be converted to tetrafluoroborate (BF40) ions using 10% HF and separated
from the sample on an anion exchange resin (66). The HF digest of rocks often
contains high concentrations of several major cations. The digest was evaporated to
dryness, redissolved in an aqueous solution, cations were separated using a cationexchange resin and then B was separated from the digest matrix using an anionexchange resin (62). Hemming and Hanson (75) described a separation and preconcentration procedure using Amberlite IRA-743 B-specific anion-exchange resin that gave
100 { 1% recovery of B. Many authors have also used B-specific resins for sample
purification from complex matrices or for sample preconcentration (36).
Chromatographic Separation
High performance liquid chromatography (HPLC) may be used as a separation
device with several analytical methods. Sample B is converted into an ionic compound
such as BF40 ion using 10% HF (66) or a complex with organic compounds (76–78)
and separated from the sample matrix on an ion-exchange column prior to detection.
The ionic complexes of B with chromotropic acid (76) and H-resorcinol (78) were
separated by HPLC and detected spectrophotometrically by measuring absorbance at
350 and 510 nm respectively. When HPLC separation of inorganic ions is used in
conjunction with a conductivity detector, it is commonly called an ion chromatograph,
as several ions can be analyzed sequentially. Ion chromatography has been used for
B determination (58, 66); however, the sensitivity of this method is lower than that
of ICP-OES (79).
Gas Phase Separation
Conversion of sample B to volatile species such as boron fluoride or methyl
borate provides the basis of gas-phase separation of B from the sample matrix. This
technique is used to separate B from a number of matrices, such as wine (80),
waters (67), metal and metal alloys (70, 81), plants (68), and biological tissues
(12), for determination by atomic absorption spectrometry (AAS) as well as by
plasma-source spectrometric methods (80 – 82). Musashi et al. (83) described a
manual methyl borate distillation method for the separation of B from alkali fusion
digests of rocks. Novozamsky (84) and Johnson et al. (12) described continuousflow techniques for on-line generation and separation of methyl borate from the
sample matrix for determination by ICP-OES. The accuracy of the method, however,
depends on the quantitative conversion of the sample B to methyl borate. Some
workers have generated methyl borate in a concentrated sulfuric acid medium (67,
68) to utilize the heat generated by the hydration of sulfuric acid in the reaction
vessel for a rapid volatilization of methyl borate. This technique was used for the
sulfuric acid / hydrogen peroxide digest of wine for flame emission spectrometric
determination of B with 0.03 to 0.04 mg detection limits (82).
Pyrohydrolytic Technique
There are some reports of the separation of B from steel samples by the pyrohydrolytic technique (7). The pyrohydrolytic separation of B does not require sample decomposition. It resulted in 1.0 mg kg01 detection limits by ICP-OES (7).
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Boron tends to raise the base line in spectrometric and other procedures by adhering
to instrumental components, which affects subsequent readings of many determination
methods. This phenomenon is called ‘‘memory effect’’ and presents a major problem
in B determination (13, 85). Measures to minimize memory effects include running
only dilute concentrations of B, typically less than 0.2 mg/L, and increasing wash
time (86, 87); the use of a direct injection nebulizer (DIN) (36), and running an
acidified (0.02 mol/L HNO3) 2 mg/g sodium fluoride (NaF) wash solution for 60 s
between samples (48). Luguera et al. (85) suggested the use of 4% m/v solution of
NaF between the atomization cycles to transform residual B into a volatile species,
BF3 , which decreased the memory effect in AAS. In a comparative evaluation of
nebulizers for the memory effects of a 1.6 mg B/L solution during the washing cycle
with 2% HNO3 , a DIN significantly outperformed the concentric Meinhard nebulizer.
With the concentric Meinhard nebulizer, the B signal fell to within 2% above the
original baseline value after 8 min wash time while with the DIN it fell to about
0.03% above of the original value in one minute (36).
Spectrophotometric Methods
Colorimetric Methods
A number of spectrophotometric methods based on the use of specific reagents for
the color development are employed for B determination. Examples of these methods
are curcumin (88, 89), carmine (88, 89), methylene blue (89), azomethine-H (90),
and others such as quinalizarine, arsenazo, and crystal violet (65). Two B-curcumin
complexes, rubocurcumin in the presence of oxalic acid and rosocyanin in the presence
of strong sulfuric acid, are of practical significance for B determination (91). The
commonly used curcumin method employs the reddish-brown rosocyanin complex,
which has a 545 nm absorption maximum at about pH 1.0. The detection limit in
high purity silicon was 3.0 ppb B (92).
The Azomethine-H method is perhaps the most commonly used spectrophotometric
method of B determination. This method is fast, simple, and sensitive and does not
require concentrated acids, which make it desirable for automation (93). In a comparative evaluation of azomethine-H, carminic acid, and curcumin methods for B determination in water, the azomethine-H method suffered the least interferences and was
the most sensitive (94). Zenki et al. (95) reported that the use of azomethine-HR (1(2,4-dihydroxy-benzylidene-amino)-8-hydroxynaphthalene-3,6-disulfonic acid, a derivative of azomethine-H) increased the sensitivity 3.5-fold relative to the azomethineH method, but the azomethine-HR method suffered interferences from A1, Cu, Fe,
Ti, and Zr.
A number of flow injection (FI) spectrophotometric methods have been presented
in the recent years. Some of these methods have employed FI systems for sample
introduction in the existing spectrophotometric methods of B determination. Examples
under this category are the use of FI in (1) the chromotropic acid method (96), (2)
the azomethine-H method following an on-line partial dissolution of directly introduced solid soil samples (97), (3) the azomethine-H method following an on-line
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A Partial List of Published Spectrophotometric Methods for B Determination
Kaplan et al. (29)
McGeehan et al. (21)
Banuelos et al. (53)
Zarcinas (103)
Ciba and Chrusciel (52)
Lopez et al. (94)
Chen et al. (97)
Soil, plant, fertilizer
Sekerka and Lechner (99)
Zenki et al. (95)
Hulthe et al. (171)
Ostling (172)
Ishchenko et al. (173)
Higgs (174)
Hofstetter et al. (56)
Nose and Zenki (101)
Garcia et al. (65)
Sea water
Sea water
Steel / alloys
Azomethine-H, curcumin
Azomethine-FI and
carminic acid
Azomethine-FI / precon
Carminic acid
Carminic acid
Sorbitol/methyl orange
Crystal violet
Campana et al. (107)
Alwarthan et al. (38)
Lussier et al. (96)
Nogueira et al. (100)
Geological material
Water, eye solution
Plant and natural
Soils, waters, plants
Light water and
Al, Cu, Fe, Ti, and Zr
Mo, Fl
Al, Cu, Na, Mg, and
Flow injection analysis
Monosegmented FI with
relocating detectors
sequential injection and zone trapping for soil and plant tissue samples (98), and (4)
the azomethine-H method following an on-line ion-exchange preconcentration (99).
Other FI methods have used different approaches. Nogueira et al. (100) described a
monosegmented FI method with detector relocation. The method of Nose and Zenki
(101) determines B by measuring the change in the methyl orange indicator as a result
of the change in acidity of a sorbitol solution in presence of boric acid. A partial list
of published spectrophotometric methods of B determination is given in Table 1.
The spectrophotometric methods, in general, suffer interferences from several species including Al, Cu, Fe, Zn, and Mo (102). The sample pH, especially in the range
of 6.4 to 7.0, affects the color of the B-azomethine-H complex (98). Color of the
sample (especially in soil extracts) and high Fe levels may cause severe interference
and a wide variability in spectrophotometric B values in the azomethine-H and carminic acid methods (21, 32). The presence of Fe enhances azomethine-H values of
B. Fe interference may be suppressed by thioglycolic acid treatment (103); however,
thioglycolic acid reduced the sensitivity of the azomethine-H method. These interferences and lack of sensitivity limit the application of spectrophotometric methods for
the samples with low B concentrations and complex matrices.
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Fluorimetric Method
Boron forms fluorescent compounds with a number of reagents in concentrated acid
as well as under milder conditions. A list of these reagents is given by Chimpalee et
al. (104). The differences in the reported fluorimetric methods are mainly due to the
choice of reagents used for fluorimetric reaction. The FI fluorimetric method of Motomizu et al. (105) and the spectrofluorimetric method for plant tissues, fertilizers,
and natural waters employing B determination at the constant wavelength difference
of 24 nm (106) used chromotropic acid as a fluorimetric reagent. The fluorimetric
methods using Alizarin Red S (the sodium salt of 1,2-dihydroxyanthraquinone-3sulfonic acid) were described by Chimpalee et al. (104) with a 0.34 mg/mL detection
limit, by Campana et al. (107) with a 7.2 ng/mL detection limit, and by Blanco et al.
(108) for simultaneous determination of B and Mo using first and second derivatives
of the synchronous spectra. However, this method is sensitive to pH and temperature
and suffers from inferences from a number of chemical species. The fluorimetric
method using carminic acid also suffered from several problematic interferences (32).
Ionometric Methods
For potentiometric determination, B is generally separated from the sample matrix,
treated with HF and the resulting tetrafluoroborate ion (BF40) is measured potentiometrically with a suitable BF40 selective electrode. The BF40 conversion is complete within
a few minutes (109, 110). Ionometric methods not requiring B separation from the
sample have also been reported (2, 111, 112). These methods, however, are severely
affected by the sample matrix which may shift the potential. To achieve reliable
results, it is essential to either remove the matrix or match the calibration matrix with
that of the sample (2). A polarographic method based on the adsorptive characteristics
of the B complex with beryllium (III) (4-((4-diethylamino-hydroxyphenyl)-azo)-5hydroxy-2,7-naphthalenedisulfonic acid) at the dropping mercury electrode in a solution of potassium hydrogen phthalate (pH 3.7–4.6) was reported for the determination
of trace amounts of B (1 1 1009 g/mL detection limit) in foods (113). The amounts
of B obtained by this method reportedly agreed well with those by the ICP-OES
method. The ionometric method has not been very popular; however, some interest
in this technology remains for borophosphosilicate glass (114), rocks and ores (115),
and environmental samples (116).
Atomic Spectrometric Methods
Atomic Emission/Absorption Spectrometry Methods
Atomic emission spectrometry (AES) and atomic absorption spectrometry (AAS)
generally involve introduction of samples into a flame (usually of acetylene–air or
acetylene–N2O–air), where elements of the sample are atomized. The AAS measurement is based on the principle that free atoms of an element (e.g., B) in their ground
state absorb photons of discrete energy values (a characteristic wavelength) generate
by a hollow cathode lamp containing that element (e.g., B). The AES methods measure
emission from the atomized and excited species when they fall to ground state. The
AES/AAS determination of B often requires separation and preconcentration of B
from the sample matrix for acceptable results (117). After the separation of B from
06-12-97 21:28:22
the sample matrix as volatile methyl borate, atomic emission measurement of BO20
radical at 548 nm (67) or absorbance measurement at 149.7 nm (68) improved the
detection limit and sensitivity. Separation and preconcentration methods for B are
discussed in the earlier sections. Boron determination by the AAS/AES was recently
described by Usenko and Prorok (118) and Zakhariya et al. (119). This method
generally has poor sensitivity (120), serious memory effects of previous samples, and
numerous interferences.
The ET-AAS method can analyze solid or liquid samples without requiring sample digestion and atomization at high temperatures (117, 121, 122). The ET-AAS
method without the use of chemical modifiers has poor detection limit and sensitivity
due to (a) inefficient thermal dissociation of B-containing species (probably oxides
and carbides) produced by dissociative desorption of B2O3 and (b) severe memory
effects resulting from B atoms apparently undergoing a series of condensation –
vaporization steps, which causes a persistent plateau in the tail of AAS signals (85,
122). Use of chemical modifiers (a list has been compiled by Botelho et al., 117),
and the treatments of the pyrolytic graphite tube are often necessary for acceptable
results by ET-AAS. Coating of the graphite tube with tungsten carbide or lanthanum
carbide increased the optimum pyrolysis temperature of B from 8507C to ú22007C
and the addition of a Ca – Mg modifier to the B solutions increased the pyrolysis
temperature to 12007C (122). Higher pyrolysis temperature would be expected to
increase the efficiency of thermal dissociation of B-containing species. A chemical
modifier composed of nickel and zirconium salts and the treatment of the graphite
tube with zirconium solution mitigated the interference of iron for the determination
of B in iron and nickel-based alloys by ET-AAS (8). Barnett et al. (123) described
a method of matrix modification using totally pyrolytic graphite tubes. The use of
two chemical modifiers, (1) diammonium hydrogen phosphate to suppress matrix
interferences and (2) NaOH to retain determinand on the surface of the tungsten
boat furnace vaporizer, suppressed the loss of B during the ET vaporization process,
enhanced detection, and overcame matrix interferences (124). Ideal conditions for
ET-AAS determination in cell-suspension B were 4 s hold time at 25007C for
atomization and 249.7 nm wavelength for the detection (120).
Plasma-Source Methods
Introduction of plasmas as ionization sources and the development of plasma-source
analytical instruments (plasma-source-OES and MS) provided higher sensitivity and
lower detection capability for B determination than was possible by spectrophotometric, flame AES/AAS, and time consuming nuclear methods. There are several types
of plasma (125), namely, the direct current (DCP) (126, 127), the inductively coupled
plasma (ICP) (4, 6, 128), the microwave induced plasma (MIP) (129) and the glow
discharge plasma (GDP) (130). Plasmas have been generated from a number of gases
or their mixtures (125, 131); however, most commercial plasma-source instruments
use an argon ICP (i.e., ICP generated from argon) for ionization. The plasma source
instruments are of two kinds, based on the detection method they employ—(a) plasmasource optical emission spectrometry (OES) such as ICP-OES and DCP-OES and (b)
plasma-source mass spectrometry (MS) such as ICP-MS, DCP-MS, and microwave
induced plasma (MIP)-MS. OES has also been called atomic emission spectrometry
06-12-97 21:28:22
Examples of the Application of ICP-OES Methods for B Determination
Material analyzed
Plant, animal
Method used
Novozamsky et al. (73)
Spouncer et al. (19)
Evans and Krahenbuhl
(32, 48)
Hu et al. (133)
Ferrando et al. (43)
Fucsko et al. (4)
Hu (63)
Hunt and Shuler (175)
Plant leaves
Plant, animal
Borophosphosilicate glass
High-purity silica
Biological materials
Azomethine-H / other
Coedo et al. (6)
Fe for ICP-OES
Fluorinating ETV method
Low-temperature wet
(AES), mainly in the older literature. In order to maintain uniformity, we will use
OES consistently, even when AES was used in the original articles cited in this work.
Generally, samples are converted to liquid and introduced to the plasma of instruments, though, several alternate modes of sample introduction (e.g., slurry, powder,
gases, laser ablation, and electrothermal vaporization (ETV)) are used for specific
purposes, mainly to avoid sample preparation and to reduce interferences (125, 131).
When HF is used for the sample dissolution, its presence can be expected in the
digest. Presence of HF in the digest generally causes problems in B determination
and has damaging effects on the sample introduction and interface regions of the ICPMS instrument. For plasma source determinations, HF should be removed from the
sample by evaporation to dryness.
Plasma-source OES. Development of ICP-OES revolutionized the determination of
several so-called ‘‘problem elements’’ such as B, S, Mo, and all hard to detect trace
elements by virtue of its low detection limits, large linear range, and multielement
(several elements in the same run) detection capability. Reported detection limits for
B are 10 to 15 mg B/L in soil solutions and plant digests by ICP-OES based on a
linear self-scanning photodiode array (51), 0.1 ppm (100 mg B/L) in as little as 50
mg animal tissues (tumor, blood, liver, skin), or as few as 5 1 107 blood cells in cell
suspensions by DCP-OES (132), and 25 mg B/L in the microwave digest of mice
tissues using Ar-ICP-OES with a Babington nebulizer (44). Some examples of the
applications of the ICP-OES method for B determination in various sample types are
presented in Table 2. Boron determination in plant leaves by slurry introduction in a
fluorinating ETV ICP-OES system resulted in good sensitivity, avoided carbide formation, and decreased memory effects and interferences (133). The ICP-OES determination of small biological samples following an in situ conversion of sample-B to gaseous
methoxyborate improved the detection limit by more than tenfold over the conventional
ICP-OES procedures using liquid sample introduction (12). The FI-ICP-OES resulted
in a near unity dispersion ratio, high linearity of the concentration-peak height relationship and increased the sample throughput up to 320 per hour (128).
Interferences in ICP-OES: If the wavelength of the elements of interest is near the
06-12-97 21:28:22
wavelength of another element (in the sample) within the search window, then the
peak search routine becomes less reliable, even erroneous. Iron interferes with the
two most sensitive B lines at 249.773 (B1) and 249.678 (B2) in the ICP-OES method
(33, 34, 134). If the sample has high iron concentrations (as encountered in the digests
of soil, metals, geological and some biological materials) then the B 249.773 and B
249.678 lines cannot be used because of the overlap of Fe at 249.782 on at B 249.773
nm and Fe at 249.653 on B at 249.678 nm (64). Boron determination by ICP-OES is
also affected by other interfering species; for example, Si interferences may render
low levels of B determination unreliable (135, 136). The presence of Fe, Ni, Cr, Al,
and V depressed, while Mn, Ti, Mo, and high concentrations of Na enhanced B signals
(33, 34, 137).
Novozamsky et al. (84) found it essential to separate B (as methyl borate) from
the iron alloy digests for reliable B determination by ICP-OES. Din (136) suggested
successive fusion of geological materials with potassium dihydrogen phosphate and
potassium hydroxide to obtain iron-free solution. Alternatively, Kavipurapu et al.
(137) suggested a multiparametric linear regression model to mathematically correct
for Fe interference without separating B from digests of steel. Kato and Takashima
(138) suggested the use of Cu as internal standard to manage interferences for B
determination in sea water.
The presence of HF in a sample increases B transport into the plasma and B emission
signal (4). The presence of HF favors the formation of boron trifluoride, which has
higher transport efficiency as aerosol particles than boric acid. In fluoride-free methanolic solutions, most of B volatilizes in the spray chamber and reaches the flame
mainly as vapor (139, 140). The presence of HF in methanolic solutions deceased the
ICP emission signals by the replacing B–OCH3 bond with B–F bonds and the formation of a less volatile B-fluoro complex (139–141).
ICP-MS. The ICP-MS is often the method of choice over ICP-OES and spectrophotometric methods for B determination (32). The advantages of ICP-MS over other
methods are higher sensitivity, lower detection limits, and simultaneous measurement
of 10B to 11B isotopic ratio and total B concentration in a sample. The ability of ICP-MS
to measure B-isotope ratios renders this instrument especially suitable for biological B
tracer studies (142). The reported detection limits are at the ppb level, e.g., one ppb
(36) to 3 ppb (48) in biological materials, 0.15 ppb in saline waters (143), and 0.5
ppb in human serum (47). The uniqueness of ICP-MS is also due to its capability to
carry out B determination by the isotope dilution method which is considered the
most precise for quantitative determination.
The external calibration with an internal standard is most widely used method for
plasma-source OES and MS because of its simplicity and labor efficiency. Other
methods, such as standard addition, addition calibration, and isotope dilution methods
are less common and are generally employed to deal with difficult sample matrices
or to improve precision. For the external calibration method, an internal standard as
close as possible to the mass number of the determinand elements should be selected
(57). Beryllium is the closest in mass number to B and therefore it is the most
commonly used internal standard for B determination. Use of Be as an internal standard
was effective in mitigating matrix interferences (32, 48) and corrected the matrixinduced suppression of 10B signals (85.6 { 5.2% recovery relative to 0.14% HNO3),
06-12-97 21:28:22
yielding 99.6 { 2.5% relative signal intensities (47). For the microwave digests of
biological materials, the external calibration method using Be as internal standard was
as good as isotope dilution or standard addition methods for B determination (32, 48).
Boron determination is not affected by isobaric interferences or by spectroscopic
interferences from the elements originating from water, acid or plasma gas (144).
However, if samples vary in acidity, use of Be as an internal standard may result in
error in the determination of B/Be ratios by ICP-MS (32).
Mass discrimination in ICP-MS is the resultant effect of unequal transmission of
ions of differing masses through the interface, ion lenses, quadrupole mass filter, and
detector. Measurement of B isotope ratios by ICP-MS may be influenced by mass
discrimination effects arising from instrumental parameters as well as the sample
matrix (145). The apparent B isotope ratio (11B/10B) for NIST SRM 95 boric acid
(certified value Å 4.0436) varied from 3.2 to 4.7, depending on the ion lens voltage
setting alone (14). As a light element (atomic mass 10.8) with relatively low ionization
in the argon plasma (approximately 58% at 7500K) B is expected to have serious
nonspectroscopic interferences from species present in the sample solution (14). The
matrix related mass discrimination, in general, results in more severe suppression of
B relative to 11B, thus increasing the 11B/10B ratio (14, 143, 146).
Spectral overlap of the 12C peak on the 11B peak may interfere in 11B/10B ratio
determination if the sample solutions or the digest contains high levels of organic
carbon as commonly found in microwave digests of biological materials. Evans and
Krahenbuhl (48) noted a significant interference of 12C on B in the microwave digests
of biological materials when the measurement was made in the normal resolution
mode (0.8 m). In the high resolution mode (0.6 m) the effects depended on the sample
types: the 11B peak of the hay sample was resolved completely, but those for the
kidney and the flour sample were resolved only 75 and 55%, respectively. An analyst
should always watch for the possibility of error in B isotope ratio measurement due
to anticipated enhancement in 11B signals by 12C. Where the isotope ratio measurement
is not required, total B can be computed on the basis of 10B to avoid the error due to
the overlap of 12C-peak (47).
A number of approaches have been suggested to deal with the interferences in ICPMS determination. The readers are referred to two recent reviews in this area (125,
144). Nebulization of HF-containing solutions using a borosilicate glass sample introduction system results in the high B blank values due to the dissolution of glass by
HF. Boron determination in an HF digest of titanium using, a Galan-type nebulizer
made of an organic polymer and a PTFE sample introduction system to eliminate the
sample-glass contact, resulted in low background and detection limits (57). Sample
purification techniques discussed in the earlier section are also employed to overcome
Other Mass Spectrometry
Thermal ionization mass spectrometry (TIMS): TIMS provides the high degree of
accuracy and precision for the determination isotopic composition of an element in a
sample. The TIMS method for B isotopes determination is described in detail by Xiao
et al. (147) and Xiao (148). Bassett (15) reviewed published sample preparation
procedures and analytical methods for B11/B10 ratio measurements in minerals using
06-12-97 21:28:22
TIMS. In general, TIMS procedure involves the following steps: (1) sample decomposition, (2) separation of the determinand of interest from the sample or the digest
matrix, (3) ionization of the determinand, and (4) the determination of m/z ratios by
a mass spectrometer. The biological samples are ashed to destroy organic matter while
the nonbiological samples are wet digested. Vengosh et al. (149) found no difference
in isotopic ratios whether the sample were analyzed with or without chemical separation. The detection limit was 0.06 ppm with approximately {2% precision. Isotope
ratio (11B/10B) determination by TIMS may utilize thermal ionization of BO2/ or
BO20 salts of Na, Cs, or Rb (62, 150). The TIMS procedure for 10B/11B ratio measurement involving Cs2 BO2/ (mass peaks Cs210B16O16O/ at m/z Å 308 and Cs211B16O16O/
at m/z Å 309) resulted in better precision than that involving Na2BO2/ when measured
ratios were corrected for the interference from Cs210B16O17O/ at m/z Å 309 and its
oxygen isotope composition (3). Negative TIMS may have a significant advantage
over the positive TIMS methods for B isotope determination; however, this procedure
may be affected by organic materials which (1) interfere with the ionization of
BO20 and (2) increase the occurrence of isobaric interference at mass 42 (75). Drawbacks of TIMS are the long sample determination time, usually 0.5 to 3.0 h (151),
and laborious sample preparation steps (148).
Spark source mass spectrometry (SSMS) is mainly used to determine concentrations
of trace elements. Lukaszew et al. (152) noted a satisfactory performance of SSMS for
the determination of isotopic composition. Secondary ion mass spectrometry (SIMS)
employs sputtering of the surface atoms from small areas (typically, 250 1 25 mm)
of the sample by an energetic primary ion beam (153). The secondary ions generated
from the sample are detected in a mass analyzer. The applications of SIMS to BNCT
were reviewed by Moore (11). While SIMS can be used for quantitative B determinations in small samples also, it is particularly suitable for the determination of intracellular B concentrations in BNCT.
Nuclear Reaction Analytical Methods
A number of nuclear reaction analytical (NRA) methods have been reported for B
measurement. Some of these methods may be of mere academic significance, having
little practical value for routine B determination. All these methods involve the bombardment of B nuclei and the measurement of the reaction product(s). For convenience,
the reported NRA methods are divided into two classes—(a) neutron activation analysis (NAA) and (b) other NRA methods.
Neutron Activation Analysis
In general, the sample is bombarded with neutrons, the elements of interest are made
radioactive and quantity of the element is determined by measuring the radioactivity or
radioactive decay products. NAA is a nondestructive method capable of handling solid
samples with multi element detection capability and generally low detection limits.
However, it is not suitable for sample mass or liquid volumes that pose a threat of
radioactive leaks after activation (86). Ward et al. (86) found good agreement between
the value of 18 elements determined by NAA and ICP-MS. The nuclear methods for
B determination were recently reviewed by Pillay and Peisach (154).
06-12-97 21:28:22
The measurements by the NAA methods require access to a nuclear reactor for the
production of thermal neutrons for bombardment to convert isotope(s) of interest in
a sample to radioisotopes (151). Boron is an exception to this strategy. The activation
of 10B by an incident beam of thermal neutrons does not make it radioactive but
causes the following neutron-capture reaction (11):
B / neutron Å 7Li / a particles (2.31 MeV) / gamma-ray (478 KeV).
This reaction involves only the 10B isotope, which has approximately 20% abundance
in naturally occurring B. All NAA methods for B determination are based on the
measurement of one or more products (a-particles and g-photons) of this reaction.
Perhaps the most important method based on the measurement of a particles is neutron
activation (NA) MS while that based on the measurement of gama rays is prompt gray spectrometry. Reported methods of B determination based on the above nuclear
reaction are listed below.
Methods based on a particles. Neutron activation mass spectrometry (NA-MS):
Iyengar et al. (155) and Clarke et al. (156) described an NA-MS method for simultaneous determination of lithium and B in biological materials. The sample was placed
in an ultrapure polyethylene ‘‘liner’’ and freeze-dried. The liners containing the freezedried samples were placed in lead containers and evacuated to about 1005 Pa. The
lead containers were pinched-sealed following evacuation for neutron irradiation. A
static mass spectrometer was used to measure 4He (from 10B) and 3He (from 6Li)
generated by the NA reaction. The error rate at ¢1 ppm B concentration was 1–5%
but at 6 ppb B the error rate increased to 75%. This sensitivity is not adequate for
BNCT and some nutritional and environmental applications.
a-track etching. This technique, also called neutron capture radiography, is generally
used to determine microscopic distribution of the 10B isotope in tissues. This technique
was reviewed by Moore (11). The sample containing 10B is placed in contact with a
detector film and is irradiated with neutrons. Following irradiation, the film is stained,
reversed, and etched with KOH or NaOH. This technique has been used for mapping
the distribution of natural B in histological sections of mouse tissue (157) and in
parenchyma cells of clover leaves (158). The quantification of B is possible using an
image analyzer (159).
Neutron depth profiling (NDP). This method has been used for near surface analysis
of isotopes that undergo neutron-induced positive Q-value charged particle reactions
such as 10B(n, a)7Li for B determination where 10B is the target isotope, n is the
neutron as an irradiation source, and a particles and 7Li are the products of the
reaction. Lamaze et al. (160) used the NDP method to measure B in CVD diamond
surfaces. The samples were irradiated with cold neutrons and resulting particles escaping from the surface of the sample were detected with a silicon surface barrier detector.
Methods based on the measurement of g-rays. Prompt g spectroscopy is an extensively used method for the measurement of 10B (154, 161, 162). The g-ray emitted
from the disintegrating 10B nuclei due to the action of the neutron is detected. This
is also a nondestructive method; however, it is not sensitive for the detection of low
B levels (generally, õ5 mg g01) in the sample (163). As B concentration in the sample
06-12-97 21:28:22
Other Nuclear Methods
B(d, n)11C
B(a, a)
Particle detected
C, half life Å 20.4 min.
a with a surface barrier
(thin layers) silicon
B(p, g)
g-ray with a NaI(T1)
a activation
B(a, p)13C p by surface barrier
B(a, p)14C Same as above
Neutron transmission
Thermal neutrons
He activation
Neutron radiography
Electron energy loss
Strijckmans et al. (167)
Moncoffre et al. (165)
B in titanium Deuteron activation
B-implanted Prompt nuclear
McIntyre et al. (166)
B in thin
Szegedi et al. (168)
Pillay and Peisach (169)
Bayulken et al. (170)
Moore (11)
B tablets
decreased, counting time to achieve desired precision increases logarithmically (164).
For example, counting time necessary to achieve 1% precision was 10 h for a sample
containing one ppm B and 50 h for a sample containing 0.5 ppm B.
Other Nuclear Methods
A number other methods involving non-neutron irradiation sources such as a particles, protons, and deuterons (165–167) are reported in the literature and are listed in
Table 3. These methods have not been commercially adopted.
This paper compiles methods of determining total B concentration and its isotopic
composition. The evolution of B determination methods has generally progressed
with developments in analytical instrumentation. The application of nuclear reaction
methods (mainly prompt-g spectrometry) has remained limited to some specialized
fields. Atomic spectrometric methods such as AES and AAS revolutionized the determination of a large number of elements, but these methods were not very sensitive
for the elements such as B, P, Mo that occur mainly as their oxy-ions. As a result,
spectrophotometric methods remained the methods of choice for most routine applications until the development of ICP-OES. ICP-OES was also not adequately sensitive
for nutritional and medical research involving animal tissues that are naturally low in
B. Development of plasma-source MS (e.g., ICP-MS) not only has overcome most
of these drawbacks, but also its capability of measuring B isotopes has made possible
(1) B concentration determination by isotope dilution, (2) verification of B concentration by isotope fingerprinting in routine analysis, and (3) determination of total B
concentration and B isotope ratio in the same run for biological tracer studies. Therefore, plasma source MS appears to be the method of choice among present-day
06-12-97 21:28:22
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