Purity law for analytical laboratories
The requirements in respect of cleanliness of laboratory environments have increased dramatically, particularly in the areas of industrial production, packaging and in analytical laboratories and research. A particulate-free, sterile environment is often crucial in such fields. For this reason, considerable amounts of money are often spent and enormous technical and building effort are invested in order to erect large clean rooms even though the cleanliness and purity required for the given application is only actually needed in certain areas of the laboratory.
In particular, the requirements for cleanliness getting more important in analytical laboratories in particular, because the sensitivity of analytical instruments has undergone continuously significant improvement, so that it is no longer the analytical system alone that constrains the achievable detection limit, but also the purity of the chemicals and equipment used, such as pipettes and solvents. It is a fact frequently overlooked that humans and the laboratory environment itself can contaminate the analytical sample and thus affect the results of the analysis. Only if laboratory workflows are optimized adequately, chemicals of appropriate purity are used and equipment, containers and vessels are appropriately purified is it possible to fully exploit the capabilities of modern multi-element methods [such as total reflection fluorescence analysis (TRFA), continuum source atomic absorption spectrometry (CS-AAS), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS)]. Careful preparation and high-purity chemicals are essential, since not only the sensitivity of the instrument but also the standard deviation of the blank is included directly in the calculation of the analytical instrumental detection limit. A single metal particle that has been transferred to the analytical sample from the ambient air is already enough to degrade the detection limit for the corresponding elements by several orders of magnitude or to falsify the analytical result significantly. However, an incorrect analytical result can be more costly for a company rather than financing a preventive equipment to purify the ambient air in the laboratory. But not every analytical laboratory is always granted approval for a separate clean room when a new, more powerful analysis device is purchased. And construction of a new clean room is not absolutely necessary for many applications, since smaller, locally installed clean-room workbenches or laminar flow boxes guarantee the same cleanliness, namely, to protect the analytical sample from particles and dust. Therefore, this article is intended to highlight this aspect of clean-room workbenches and laminar flow boxes.
Relevant standard and the operating principles of a laminar flow box:
The quality of a clean room or similar clean room area, such as a flow box, is evaluated based on Din en iso 14644, which regulates the classification of clean rooms.
It is the intention of this article to investigate this laminar flow box in more detail concerning its applicability in an analytical laboratory to reduce the level of contaminations. The flow box was already tested and certified by the Fraunhofer Institute for Manufacturing Engineering and Automation and was finally designated as ISO class 5 (old US: class 100). This means that a maximum of 100 particles of min. 0.5 µm diameter per cubic foot may be detected inside the box (3.5 particles per litre or 3,520 particles per m3). The flow box thus has an isolation factor of 104, which results in a corresponding reduction in the number of particles and improves the air quality by at least 10,000 times compared to the ambient air in the laboratory. The way in which a flow box works is quite simple. The ambient air is sucked in by a fan and forced through a particle filter. The placement of the filter creates a laminar air flow in the working area behind the acrylic glass panels. This means that the air flows from top to bottom in parallel strata like a curtain and the sample is protected against the diffusion of particles into the box by overpressure. Particles that have nevertheless penetrated the box, for example when adding or changing samples, are captured by the air flow and removed through the perforated panels at the bottom of the flow box or discharged through the front aperture.
In this paper, the authors aim to show what influence the laboratory environment can have on the achievable analytical detection limit for analysis with modern, high-performance ICP-MS equipment. For this purpose, analytical samples were stored in open PFA containers for 12 hours in a clean room, in a "normal" analytical laboratory and in the same room but protected by a laminar flow box [10 mL, 1% v/v HNO3]. This is also where the standards for the calibration of the ICP-MS were prepared. A modern ICP-MS instrument (Plasma Quant MS Elite S; Analytik Jena, Jena, Germany) was used to analyse these samples using optimized operating conditions in particular by employing the BOOST and Nitrox technology to reduce spectral interferences and to significantly increase the instrumental sensitivity.
The measured and calculated limit of detection (LOD) is shown in ng L-1 in Figure 2 a,b for a total of 18 elements (the figure shows the isotopes used for each measurement), which cover the entire mass range of the ICP-MS instrument. The LOD for all elements can be significantly reduced compared with a “normal” laboratory if the samples are stored in a clean room or the flow box. This improvement is particularly markable for elements shown in Figure 2a, for which contaminants from ambient particles can lead to increased blank levels in most normal laboratories. Only by using the filtered laboratory air in the laminar flow box significant improvements have been achieved, thus allowing the capabilities of the ICP-MS instrument to be fully exploited for the alkali, alkaline earth and transition elements. In the best case, the improvement could be more than one order of magnitude. The elements in Figure 2b have been selected to demonstrate the capability of the ICP-MS instrument used, which allows detection limits in the lower pg/L range to be achieved for terbium (Tb) and holmium (Ho). On the other hand, the differences between storing the standards and blank solutions in a clean room and using a flow box in a “normal” laboratory tend to be marginal. Thus, we can conclude, that only in a clean laboratory environment it possible in routine analysis to fully achieve the best possible detection limits offered by the modern ICP-MS instrument technology as it was shown here. These LODs extend down to the lower pg L-1 range and thus meet the purity requirements for ultra-trace analysis.
Figure 2 a,b: LODs achieved for analysis of a blank solution stored under the same conditions in different laboratory environments. The following storage environments were compared: Clean room, ordinary laboratory with a laminar flow box (FB), ordinary laboratory without flow box.
In this article we have shown that the purity of a blank solution is a significant factor in the achievable detection limit for the analysis of ubiquitous elements using a modern ICP-MS instrumentation. It was possible to demonstrate that, in addition to the purity of the chemicals and the containers, the laboratory environment has a significant effect on the analytical result. In a normal laboratory environment, particles are unavoidable and can contribute to the contamination of calibration standards and samples. In our trial, filtration of the air in the laboratory, as can be done in clean rooms, led to a significantly higher purity of the blank solution and a considerable improvement in the limit of detection for many elements. It was possible to demonstrate that the same (comparable) improvements can be achieved by using a small, relatively inexpensive laminar flow box, which allows the purity conditions needed for ultra-trace analysis to be achieved with little outlay in any laboratory environment. The various module sizes of the flow boxes allow the system to be adapted to the given quantities of samples and to the floor space in the laboratory. An appropriate module for each laboratory or a complete clean room can also be purchased subsequently if the requirements for purity in everyday laboratory work needs to be improved. The purity of chemicals, equipment and containers, the laboratory environment and the use of modern, high-precision multi-element methods are the prerequisites for an efficient analytical laboratory fulfilling the purity law for ultra-trace analysis.