Validation of a measurement method for soluble aluminum compounds

Status:

completed 06/2024

Aims:

Aluminium is found in a wide range of industrial applications. A light metal, it is used, for example, for the construction of vehicles and machinery and in the packaging and construction materials industries. In powder form, its uses include as a pigment in metallic coatings. Aluminium compounds are used in the treatment of drinking water and wastewater. They are used particularly widely in the cosmetics industry, and as active ingredients and excipients in the pharmaceutical industry. Owing to their astringent and antiperspirant properties, aluminium chlorohydrate and other aluminium compounds are used in products such as deodorants.

Results of testing on animals suggest that workplace inhalation exposure to dust or fumes containing aluminium presents a health risk to employees. New maximum workplace concentrations (MAK values) for certain soluble aluminium compounds (e.g. aluminium chloride, aluminium sulfate, aluminium chlorohydrate) have been discussed and published by the German Research Foundation (DFG), not least owing to these substances’ irritant effect on the respiratory tract.

In 2014, a measurement method for the analysis of aluminium and its soluble inorganic compounds was published in the MAK Collection. This method is based on the use of graphite furnace atomic absorption spectrometry (GF-AAS) for quantitative analysis of these substances in the workplace air. The aim of the present project is to validate the analytical performance of an existing ICP-MS (mass spectrometry with inductively coupled plasma) method for the analysis of soluble aluminium compounds. The analysis results were to be evaluated in consideration of the MAK values published in 2024 (0.0002 mg Al/m³ for irritant aluminium compounds; 0.005 mg Al/m³ for non-irritant, soluble aluminium compounds in the inhalable fraction) and the performance characteristics set out in various standards, such as the minimum measuring range and the measurement uncertainty (EN 482; TRGS 402 technical rules for hazardous substances).

Activities/Methods:

Since the method to be validated is an established method, the sampling method is already defined. For analysis of soluble metal compounds, a consensus exists on the use of a conventional standard digestion method: an open digestion method employing 0.1 molar hydrochloric acid. For the GF-AAS method published in the MAK Collection, a microwave digestion method employing 0.1 molar hydrochloric acid was used. In addition to an open acid digestion method employing 0.1 molar hydrochloric acid, the project also studied a microwave pressure digestion method employing nitric acid (65%) and an open acid digestion method employing an acid mixture of nitric acid (65%) and hydrochloric acid (25%), 2:1, v/v. These two digestion methods are used routinely for sample preparation to enable a number of hazardous substances besides aluminium and its compounds (with the exception of poorly soluble aluminium compounds) to be analysed simultaneously in a multi-element method.

The project began with definition of the individual validation steps for the method in consideration of EN ISO 21832, DIN 32645, EN 482 and TRGS 402. The validation tests (in particular, determining of the limit of quantitation, recovery, analytical precision and storage life of aluminium on cellulose nitrate filters) were then performed for the analytical method described. The expanded measurement uncertainty for the measurement method was also calculated, taking into account the validation results. The results obtained were evaluated both against the performance requirements described in the applicable standards referred to above and with regard to the MAK values for soluble aluminium compounds.

Results:

The validated measurement method enables the concentration of aluminium in soluble aluminium compounds in the workplace air to be analysed quantitatively from the inhalable and respirable dust fractions by means of ICP-MS following acid digestion. The method is suitable, subject to constraints, for measurements in accordance with the TRGS 402 technical rules for hazardous substances for an assessment criterion (MAK value) of 0.005 mg Al/m³, but does not satisfy all requirements of EN 482 and EN ISO 21832.

Besides the limit of detection of the instruments, the limits of detection and quantitation of the measurement method were determined. The latter are in a range from 0.00017 to 0.00059 mg Al/m³, depending on the digestion method used. Aluminium compounds of highly purified material were used to determine mean recoveries for the measurement method in the range from 91 to 102%. The coefficients of variation of the aluminium analysis which were observed during the validation and serve as a measure of the analytical precision exceed the permissible tolerance of 5% in some cases. Storage tests demonstrated the storage stability of soluble aluminium compounds on cellulose nitrate filters over a period of four weeks. With consideration for a range of factors influencing sampling and analysis, the expanded measurement uncertainty of the measurement method is in the order of 21.4 to 27.6%, and therefore satisfies the requirements of TRGS 402.

Aluminium is ubiquitously present in the laboratory and therefore gives rise to blank values in all samples. The lack of consistency at times between the blank values, which has a direct influence on the analytical limits and the accuracy of aluminium analysis, is one reason why the requirements for the measurement method set out in the codes stated cannot always be met. Measures have already been taken in the IFA’s laboratory to reduce ubiquitous aluminium contamination. For aluminium analysis, microwave pressure digestion is preferred to open digestion, as it yields blank value concentrations for aluminium that are lower and exhibit lower variation.

The tests showed that the acid digestion method employing 0.1 molar hydrochloric acid (conventional standard digestion method for the analysis of soluble metal compounds) is not specific for the analysis of soluble aluminium compounds; aluminium present in metallic form can also be detected quantitatively when this digestion method has been used. At this point in time, the lack of selectivity considerably impairs the analysis results’ suitability for evaluation for soluble aluminium compounds.

Following on from this project, the focus is now to be placed on the analysis of aluminium and its "poorly soluble" and "insoluble" compounds (particularly aluminium oxide). Only once the associated validation has been completed will it be possible to specify future use of the measurement method in the MGU measurement system for exposure assessment (e.g. with regard to differentiation between groups of compounds differing in their assessment criteria). Besides the validation results, this will also entail consideration being given to aspects relevant to setting of the MAK values, such as toxicological evaluation of the aluminium compounds.

The results of this project are to be published in the form of a standard operating procedure on Q.Wiki and in the IFA folder, and in the DFG’s collection of analytical methods for testing harmful substances at the workplace. The corresponding manuscripts are currently in preparation.

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