## 1 Introduction
The measurement might take many hours, which, in the case of fluorescence excitation with UV radiation, often means photochemical decomposition of the substance or significant changes in its lifetime. Therefore, other fast and sensitive methods are being sought. These include, among others, the stroboscopic method. In the article [13], we showed an example of its use in the analysis of suspensions of several bacteria. The stroboscopic method was introduced to the market in 1990 and allows the measurement of the fluorescence lifetime of bacteria and its biological interferents [14]. Its main advantage is high sensitivity and the short time of a single measurement at 10-60 s. The name characterizes the measurement method, which examines the fluorescence intensity in very short (relative to the fluorescence decay time) moments in time synchronized with the moment of excitation. The great advantage of the stroboscopic technique is its relatively low cost and high sensitivity, allowing measurement of low fluorescence signals. The classic method, Time-Correlated Single Photon Counting (TCSPC), is burdened by very long measurement times, up to several hours, and during the irradiation, the biological sample can be degraded. Rapid progress in semiconductor technologies allowed the development and miniaturization of bio-analytical systems based on UV LED. For that reason, it is important to test the new light sources for their potential capabilities for the future development of new generations of detectors [30; 31; 32]. While the literature research concerning the fluorescence lifetimes of bacteria, spores, fungi, and pollens is broad, the data are strongly fragmented, and there is a lack of reliable comparison between different species. Moreover, experiments are conducted in different conditions, which makes them inconsistent. In our research, we examined 27 substances of biological origin under controlled and identical parameters. We believe that this approach will improve the comparability among the studies. Bacteria, bacterial spores, fungi, pollens, and tissue are a combination of thousands of compounds. A number of them are fluorescent, which makes them suitable for investigation using UV sources. The main source of endogenous fluorescence in cells and biological tissues are aromatic amino acids used to build proteins. Of the 20 amino acids that are basic units of proteins, only tryptophan (Trp), tyrosine (Tyr), and phenylalanine (Phe) fluoresce under UV excitation [33]. Among these amino acids, tryptophan exhibits the strongest fluorescence with excitation and emission maxima of about 280 and 340 nm, respectively. In contrast, phenylalanine fluorescence is very short, and tyrosine fluorescence is usually quenched due to interactions with other protein chains [34]. Both the spectral shift and the fluorescence lifetime of tryptophan are highly sensitive to the ligand they are bound to, as well as to the polarity of the microenvironment of their occurrence [35; 36]. While the fluorescence of proteins and amino acids is an indicator of the presence of biological material, the fluorescence of metabolic mediators such as reduced forms of nicotinamide adenine dinucleotide (NADH) and its phosphorylated form, NAD(P)H, flavin dinucleotide adenine (FAD) is considered an indicator of the activity of a microorganism. The maxima of excitation and emission of both NADH and NAD(P)H are about 350 and 450 nm, respectively. The free, unbounded NADH has about 0.4 ns lifetime, while bounded proteins present a lifetime of about 2-3 ns. The oxidized form NAD\({}^{+}\) is non-fluorescent. The fluorescence lifetime of free and protein-bound FAD is less than 0.1 and 2.3-2.9 ns, respectively [33; 37; 38; 39; 40]. Table 1 contains the absorption and emission parameters of selected compounds contributing to the fluorescence of biological systems. The current work concerns the analysis of dry samples of bacteria and their interferents (fungi, pollens), i.e., real situations with air pollution and contamination with biological materials.