Boron - Units and Measurement Methods

Units & conversion factors - Measurement Methods

Boron - Units & conversion factors

Boron which is measured in environmental samples may either be reported in:

• units of mass per volume, such as µg/l or mg/m, or
• parts per million by weight, or ppm

When describing concentrations of boron in air as a volume fraction:

1ppm. = 0.4421 mg/m³ (or 1 mg/m³ = 2.262 ppm.).

Sometimes, however, the amount of boron used in toxicity testing can be reported as

• the total weight of the boron-containing compound - such as boric acid - used in the test, rather than as the weight of only the boron present in the sample.

It is important that the same type of unit is used when amounts of boron in different samples are compared.

Table of conversion factors:
Boron-containing compounds	weight of Boron in 1 gram of compound
boric acid	0.175 g
borax	0.113 g
anhydrous borax	0.215 g
sodium perborate tetrahydrate	0.070 g
sodium perborate monohydrate	0.108 g
metaboric acid	0.247 g

Finally, the amounts of boron measured in different samples may refer to different sample preparation techniques.

Dried samples: Since the amount of water in a biological tissue can vary, many samples are dried thoroughly before measurements are made. These samples will have units such as µg/g dry weight of sample.

Wet samples: In some work it is not appropriate that the samples be dried, and in these cases the wet weight of the sample will be used, with units such as µg/l wet weight.

It is important that the same units are used if comparisons are to be made between different samples.

Source & © based on the full IPCS document Environmental Health Criteria (EHC) 204, Section 2.3: Conversion factors  (1998)

Units & conversion factors - Measurement Methods

Boron - Measurement Methods

Extraction: In order to measure the amount of boron present in water, soil, or in biota such as plants or other environmental species, the boron is usually removed from the sample by a process called extraction. Hot water is used to extract boron from soil. Acid and chloroform are used to extract boron from water, while acid can be used to extract boron from biological samples. Error can be introduced into the measurement process if extraction is not complete.

Analysis: The solution containing the extracted boron is then analysed, to determine the amount of boron originally present in the sample. For the low levels of boron present in environmental samples, sensitive measurement techniques are needed. The best techniques use ICP, or Inductively Coupled Plasma, methods, with either Atomic emission spectroscopy (AES) or Mass spectrometry (MS) used to detect the boron. ICP-MS is the most sensitive, as shown in the table below. In addition, it requires only a small volume of sample for the measurement.

Overview of Analytical Techniques
Analytical Technique	Samples for which the technique is useful	Detection limits
ICP-AES	Bone, plasma, food, tumour, blood, liver, skin, cell suspensions, wastewater, and fish tissues	5 to 50 µg boron/litre of solution analysed
ICP-MS (with direct nebulization)	Plant, rat, and human tissue	1 ng/g (or 1 µg boron/kg material)
ICP-MS	fresh and saline water, sewage, wastewater, soils, plant samples and all biological materials	0.15 µg boron/litre sample.
Azomethine-H*	Environmental water samples	20 µg boron/litre of solution analysed
Curcumin*	Environmental water samples - nitrate, chloride, and fluoride will interfere with method	100 - 1000 µg boron/litre of solution analysed
Neutron Activation with MS analysis	Blood and other biological samples. Also freeze dried leaves, human erythrocytes, and food items	10-8 g boron per g of sample analysed (or 10 µg boron/kg of sample analysed)
* Method must be used with caution, as interference is common in biological and environmental samples.

IPC and neutron activation methods shown in the table above require expensive analytical equipment. For this reason, the older (and less expensive) colorimetric techniques based on changes in the colour and intensity of certain dyes when boron is present are sometimes still used. Some information on these techniques is also given in the table above. However, many substances, present in biological and environmental samples, can interfere with these colorimetric measurements. For this reason, the World Health Organization (WHO) ECH report states that the IPC methods are preferred for the analysis of the low levels of boron found in biological and environmental samples, and that colorimetric methods must be used with caution.

Source & © based on the full IPCS documentEnvironmental Health Criteria (EHC) 204, Section 2.4: Analytical methods  (1998)

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