The present work aims at accessing the stability of biological material stored for diagnostic and scientific purposes. The influence of the temperature, storage time, and cyclic thawing on concentration stability of selected oxidative stress parameters in human serum was investigated. The study group consisted of 20 serum samples collected from healthy volunteers aged 18–52. The parameters whose reference ranges were not determined and to which validated determination methods did not correspond were examined by manual methods (FRAP and AOPP). Automatic methods were used to determine routine laboratory tests (albumin, total protein, bilirubin, uric acid) using the Konelab 20i^® analyzer. The samples were stored at various temperatures (room temperature, 4 °C, −20 °C, −80 °C) for max 6 months and were subjected to cyclic thawing at 1 month intervals. In order to check whether any differences between the concentrations of the studied parameters existed when the samples were stored in various conditions, the paired Student t-test or Wilcoxon test and comparison to desirable bias were applied. Based on the obtained results, it was found that the temperature and time of serum sample storage significantly affected the stability of the analyzed parameters and determined different shelf lives of serum samples for oxidative stress examination. Therefore, continuing the investigation concerning the impact of storage conditions on various serum parameters seems justified due to the discrepancy between the individual results obtained by different researchers and the inconsistencies between the results of scientific research and the applicable recommendations.

Background/Objectives: Body water, including its distribution between the intracellular (ICW) and extracellular (ECW) compartments, plays an important role in skeletal muscle physiology. The aim of this study was to assess whether body water distribution indices, namely ECW/ICW and ECW/TBW, differ according to age and sex, and whether they are associated with skeletal muscle index (SMI) after adjustment for BMI, age, and sex. Methods: The study included 188 healthy individuals: 71 older women (71.6 ± 6.4 years), 37 older men (72.7 ± 4.7 years), 40 female students (22.5 ± 0.9 years), and 40 male students (22.8 ± 1.8 years). Body composition was assessed using a Tanita MC-780 bioimpedance analyser. The ECW/ICW and ECW/TBW ratios were calculated. Nonparametric analyses, Kendall’s rank correlations, and linear regression models adjusted for BMI, age, and sex were used. Results: Older adults had significantly higher ECW/ICW and ECW/TBW values than younger participants, and women had higher values than men within the same age groups. Both indices correlated positively with age and BMI, and negatively with SMI. In multivariate models, ECW/TBW remained negatively associated with SMI after adjustment. BMI and male sex correlated positively with SMI, whereas age was not a significant predictor. Conclusions: Body water distribution indices (ECW/ICW, ECW/TBW) reflect age- and sex-related differences in water distribution and correlate with SMI independently of BMI, age, and sex. They may serve as complementary markers of skeletal muscle ageing.

It is shown that in presence of certain external fields a well defined self-adjoint time operator exists, satisfying the standard canonical commutation relations with the Hamiltonian. Examples include uniform electric and gravitational fields with nonrelativistic and relativistic Hamiltonians. The physical intepretation of these operators is proposed in terms of time of arrival in the momentum space.

The revised manuscript contains several presentational improvements, but the principal mathematical and physical objections remain unresolved. In particular, the manuscript does not adequately distinguish the auxiliary space from the constrained phys...