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neutral and acidic aqueous solutions was intensively studied [14,15]. To avoid such
losses the complexation of boron with mannitol was proposed [16-20]. Volatilization
procedures at moderate temperatures stopped before drying showed identical
recoveries of boron regardless mannitol was added or not [19]. Gaillardet et al. were
not able to confirm a complexation of boric acid with mannitol at a pH lower than 7
[16]. In contradiction to the volatile BOx species, an evaporation of volatile BF3 from
HBF4 during the removal of the silicon and acid matrix components according to Eq.
7 had not been detected yet [10].
( ) ( )
Δ
⎯⎯⎯⎯⎯⎯⎯→7
J J >+) FRQWDLQLQJ VROXWLRQ@ +%) +) %) (7)
In summary, the digestion of Si and the evaporation of the silicon and acid matrix
are the most sensitive steps in the analysis of MG-Si. Both can cause serious losses of
phosphorus and boron. The aim of this work is the development of a new procedure,
which minimizes the losses of boron and phosphorus during the digestion and which
completely renounces the critical step of matrix evaporation. Because of the high-
silicon and acid matrix ICP OES was the method of choice to quantify the impurity
concentrations. The new method was applied for the analysis of the SRM’s NIST57b,
IPT134, IPT135, as well as in three interlaboratory comparisons. A detailed statistical
assessment of the method including Gaussian error propagation over the whole
method is applied to demonstrate their accuracy and precision.
Experimental
Materials and reagents
Solid silicon samples were dissolved in mixtures of deionized water (18 Mȍ cm-1,
Milli-Q), 65% (w/w) HNO3 (suprapur, Merck Millipore) and 40% (w/w) HF
(suprapur, Merck Millipure). The following silicon samples were used: Electronic-
grade p-type silicon (Silchem GmbH Freiberg, Germany: 10.5 ȍ cm) with a particle
size • 2 mm, the standard reference materials SRM 57b (National Institute of
Standards and Technology), IPT 134 and IPT 135 (Instituto de Pesquisas
Tecnológicas, Sáo Paulo, Brazil), as well as a total of 9 MG-Si samples from
interlaboratory comparisons (FerróPem) [25-27] with a particle size < 250 μm.
For preparation of calibration standards and sample dilutions a mixture of
1% (v/v) HNO3 / 1% (v/v) HF (each suprapur, Merck Millipore) is used. Multi-
element calibration standards are prepared from stock solutions with a concentration
of 1 g L-1 (Merck, Roth).
Microwave assisted digestion procedure
500 ± 5 mg of silicon samples were directly weighed into the microwave vessel.
For silicon samples with an average particle size < 2 mm, 28 mL of an acid mixture of
5.65 mol L-1 HF / 0.85 mol L-1 HNO 3 were added. Before sealing the vessel the total
mass of the sample mixture was noted. For samples having an average grain size > 2
mm, 18 mL of an acid mixture of 8.6 mol L-1 HF/ 1.35 mol L-1 HNO3 were added.
After a successful digestion 10 mL of water (18 Mȍ cm-1, Milli-Q) were added to
obtain a comparable acid matrix for any digestion solutions.
Adapted from about 50 variations in acid mixture compositions and temperature-
time programs for the high-pressure microwave digestion system (MLS Ethos Start,
100 mL PTFE vessels, ϑmax =120°C), the following procedure was found to be
optimum. The temperature was first raised from room temperature to 50°C within 3
min, and then held at that temperature for 20 min. Next, the temperature was
97