# Weekly and Longitudinal Element Variability in Hair Samples of Subjects Non-Occupationally Exposed
## 3 Results and Discussion  
### Quality Control and Quality Assurance (QC/QA)  
The environmental studies regarding the correlation between pollutants and biological tissues, such as heavy elements in human hair, need very sensitive and accurate analytical techniques in order to determine contaminants at trace and ultra-trace levels [51]. Among the different possibilities (e.g., spectroscopy, electrochemistry, etc.), the nuclear methods are still the main available techniques to address such stringent requirements. Their high accuracy and precision and the very low limits of detection (LODs) allow investigating a matrix deeply [52]. Furthermore, the possibility of avoiding chemical-physical treatments or performing radiochemical separations is of fundamental importance for achieving these results. Even if INAA is considered a primary analytical technique--i.e., it is possible to analyze the sample just knowing all the nuclear parameters (such as all the nuclear cross-sections of each radionuclide and the nuclear reactor data)--the easier way to analyze a sample is by comparing its activity with that of a standard irradiated in the same conditions. The comparison between the data analysis obtained by INAA and the certified values is the first step for assessing the quality assurance and the quality control (QA/QC). For this aim, standard reference materials (SRMs) such as BCR CRM 397 (Human Hair) and NIST 1515 (Apple Leaves) were chosen according to the matrix similarity and biological origin. Table 1 reports the differences (\(\Delta\) expressed as %) between our data and the certified values of both the SRMs for each element.  
It should be noted that the Human Hair SRM shows a \(\Delta\) below 6.5%: Se and Zn show high precision and accuracy as well as the indicative or informative element values (i.e., As, Co, Cr, Cu, Fe, Hg, and Mn). The comparison profile for the Apple Leaves is slightly different: among the certified elements, good \(\Delta\) (below 16%) are achieved for Ba, Ca, Fe, K, Mn, Na, Rb, Sr, and Zn, as also reported in previous paper [53], whereas Hg and Ni show quite relevant differences such as \(-\)48.4% and \(-\)21.9%, respectively. For Ni, the reason is due to the poor INAA sensitivity. Mercury shows two different results: for Human Hair SRM, the difference between measurements is 5.7%, whereas for Apple Leaves, the SRM is \(-\)48.4%. According to our evaluation, the cause has to do with the different Hg levels in  
Figure 2: Instrumental Neutron Activation Analysis (INAA) master scheme (\(\mathrm{t_{if}}\) irradiation time; \(\mathrm{t_{d}}\) decay time; \(\mathrm{t_{\frac{1}{2}}}\) half-life time). In brackets for each product, nuclides are reported as well as some nuclear analytical chemistry parameters, i.e., peak energy (keV), half-life time (m minute, h hour, d day, y year), and limit of detection (LOD).  
the two SRMs. In the first SRM, Hg is at 12.3 \(\upmu\)g g\({}^{-1}\), whereas the authors find 13.0 \(\upmu\)g g\({}^{-1}\) with a coefficient of variation (CV%, defined as the ratio between standard deviation and mean value \(\times\) 100) of 6.1, a good value according to the references [49; 54; 55]. In the second SRM, the certified Hg value is 0.0432 \(\upmu\)g g\({}^{-1}\) (or 43.2 ng g\({}^{-1}\)), whereas the authors find 0.0223 \(\upmu\)g g\({}^{-1}\) with a CV% of 45.7, which is an unsatisfactory value according to the ref [54]. The second SRM shows an Hg content almost 300 times lower than the first SRM: this occurrence justifies the high \(\Delta\) between ours and the certified data in the second SRM.