Use of piezoelectric microdispensing to produce a reference material of sulfuric acid and phosphoric acid on quartz fibre filters

Status:

completed 08/2018

Aims:

Quality control is becoming increasingly important in analysis. Reference materials are equally essential for calibration, quality-control processes, and verification of the accuracy and reliability of the results obtained. EN ISO/IEC 17025 specifies the use of reference materials in order to assure the quality of results. Reference materials are used above all for instrument calibration and calibration tracking; this is performed in the course of routine quality control in laboratories, by which the correctness of the analysis results can be checked and measurement uncertainty estimated. Reference materials are also used in the development and validation of measurement methods and in laboratory proficiency tests. To enable observance of the workplace limit values for hazardous substances to be monitored, the methods used must be validated in some cases in very low concentration ranges. It follows that reference materials must also cover these ranges. Very few reference materials suitable for workplace measurements are available.

Activities/Methods:

One method that is already known but has not yet been used for the production of reference materials is piezoelectric microdispensing. In this method, the generation of ultrafine liquid droplets permits high-precision, non-contact dispensing of liquids. A microdispensing system of this kind was to be used for the production of a reference material for the inorganic acids sulfuric acid and phosphoric acid on quartz fibre filters. The system for the dispensing of dilute acids had to be created first. For constant dispensing to be attained, the relationship between the droplet volume and the adjustable parameters of voltage, pulse width and frequency had to be examined in particular. Test series (> 100) were then loaded and studied in order to demonstrate the reproducibility of loading. 100 is however not an adequate sample number for a reference material. In order for the method to be accelerated, loading of the filters was therefore automated as far as possible. This was achieved by the use of stepper motors that move the filter such that coverage is automated and uniform over the entire filter. In the next step, larger series of samples (> 500) are to be produced. This requires the sample holders to be loaded on different days in order to show that the coverage is then still reproducible. The sample holders are analysed by means of ion chromatography in accordance with the standard method used for inorganic acids in the MGU measurement system. An important quality criterion for reference materials is the sample stability. Following their production, this is determined on the samples in storage over a duration of at least two years. A certification process is required in order for series of samples to be used as reference material. For this purpose, the samples were sent for analysis to as many suitable laboratories as possible, where they are analysed by means of the laboratories' methods. The results are interpreted statistically and documented comprehensively for certification purposes.

Results:

The project concerned the development and production of a reference material for sulfuric acid and phosphoric acid on quartz fibre filters. A microdispenser employing ink-jet technology was first created. The relationships between the droplet volume and the adjustable parameters of voltage, pulse width and frequency were studied for this purpose. A working range was then determined for the dispensing head and dispensing solution used in which changes in these parameters have virtually no influence upon droplet formation. Initial series of tests (100 samples) were dispensed with dilute acids. This showed the reproducibility over one day to be below a relative standard deviation of 1%; the reproducibility over several days was however substantially poorer. Fitting of a higher-resolution camera enabled the droplet size to be determined more precisely, in turn enabling a change in droplet volume to be compensated for by adjustment of the dispensed number of droplets. This led to considerably greater consistency of dispensed volumes, even between different days. In order for larger series of samples to be loaded more quickly, a semi-automatic loading unit was developed comprising two stepper motors and a filter holder. This was coupled to the dispenser in such a way that loading unit and dispenser can be programmed interdependently. The filter loading is applied swiftly and evenly over the entire filter. The dispensing unit is programmed such that the dispensing volume can be changed by adjustment of the frequency, and reprogramming of the dispensing process is not required. This combination of dispensing and loading units was used for the loading of large series of samples (≥ 500). 50 filters in each series underwent internal quality control; the samples were analysed and the results interpreted statistically. PROLab Plus, a software application designed specifically for the organization, performance and evaluation of inter-laboratory tests, was used for this purpose. All tests performed showed the samples produced to be adequately homogeneous. An inter-laboratory test was also performed in which eleven laboratories from six different countries took part. For each of the two concentrations, each laboratory received ten filters, a reference standard solution and three blank value filters. Interpretation of all results yielded relative standard deviations of between 1.5 and 2.6% depending upon the acid and concentration; results from nine laboratories were considered. Earlier studies of the stability in storage also show the samples to be stable; they are therefore suitable for use as reference materials. They have been available for purchase in sets from the IFA website since June 2018.

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