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boron analysis by ICP-MS

In Vivo Percutaneous Absorption of Boric Acid, Borax, and Disodium Octaborate Tetrahydrate in Humans Compared to in Vitro Absorption in Human Skin from Infinite and Finite Doses

by Ronald C. Wester, Xiaoying Hui, Tracy Hartway, Howard I. Maibach, Kathleen Bell, Michael J. Schell, D. Jack Northington, Philip Strong, and Dwight Culver

published in Toxicological Sciences, 45, 42-51 (1998)

Abstract

Literature from the first half of this century report concern for toxicity from topical use of boric acid, but assessment of percutaneous absorption has been impaired by lack of analytical sensitivity.  Analytical methods in this study included inductively coupled plasma-mass spectrometry ( ICPMS ) which allows quantitation of percutaneous absorption of 10B in 10B-enriched boric acid, borax, and disodium octaborate tetrahydrate (DOT) in biological matrices.  This has made it possible, 

 in the presence of comparatively large natural dietary boron intakes for the in vivo segment of this study, to quantify the boron passing through skin. Human volunteers were dosed with 10B-enriched boric acid,  borax, or disodium octaborate tetrahydrate,   in aqueous solutions.  Urinalysis, for boron and changes in boron isotope ratios, was used to measure absorption.  These in vivo results show that percutaneous absorption of boron, as boric acid, borax, and DOT, through intact skin, is low and is significantly less than the average daily dietary intake.
Boric Acid Borax

Disodium Octaborate Tetrahydrate

H3BO3

Na2B4O7.10H2O

Na2B8O13.4H2O

Boron Analysis

Boron has two stable isotopes having atomic masses of 10 and 11.  The natural abundance of the isotopes is approximately 19% 10B and 81% 11B. A mass spectrometer is needed to measure the individual isotopes of boron. Hence ICPMS can quantify each isotope individually.  10B-enriched boric acid,  borax, or disodium octaborate tetrahydrate were used in this study.

In this paper, the flux and permeability of these boron compounds was measure in both in vitro and in vivo experiments conducted by scientists at UC San Francisco.  In the in vitro experiments, dose solutions of   

 10B-enriched boric acid,  borax, or disodium octaborate tetrahydrate were applied to the outside of human cadaver skin while flowing phosphate buffered saline was used as the receptor fluid on the under side of the skin.  Boron concentrations were then measured in the saline.  In the in vivo experiments doses were applied to 24 human volunteers. Urine levels of boron isotopes were measured over 17 days.  10B-enriched compounds were administered to the backs of the volunteers on days 5 and 12.   Detection of elevated 10B/11B ratios by ICPMS in the daily urine samples allowed the estimation of adsorption rates in the presence of approximately 1 ug/mL concentration of naturally occurring boron in the urine.

Boron Method Validation

We have validated ICPMS methods for boron at ppb levels in urine, saline, water, and tissue. We also measure boron isotopes with better than 2% RSD, which means very accurate data can be obtained using isotope dilution. These methods have also been applied to animal feed, blood, and many other matrices. While the sensitivity of ICPMS for boron is

  excellent, boron background, its tendency to adsorb onto glass, and its chemistry all have to be controlled for successful quantitative analysis in complex matrices.

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