## 1 Introduction
However, it cannot be applied to mono-isotopic elements and requires wet-chemistry-based sample pre-treatments to dissolve the sample matrix, while it is often a difficult challenge to completely dissolve the environmental sample matrix including PMs [19; 20; 21; 22]. INAA is a nondestructive, multi-elemental analysis method based on nuclear properties regardless of the chemical state of elements [19; 23; 24; 25]. It is possible to quantify mono-isotopic elements that cannot be accessed with ID-ICP/MS. The chemical pre-treatment for dissolution of the sample and the potential risk of contamination during the process can be avoided [12; 19]. Therefore, the INAA method is the most appropriate and widely used method for elemental quantification of airborne particulates [8; 23; 24; 26; 27; 28]. In particular, the SC-INAA method is another primary ratio method, complementary to the ID-ICP/MS, which has been used mainly in NMIs, including KRISS, for complicated matrix analysis, such as environmental samples [12]. The SC-INAA method has the advantage of high accuracy and low uncertainty of analysis because it can offset the complicated parameters of the analysis by irradiating and measuring the standard with the sample under the same conditions [12]. However, in SC-INAA, all standards for the elements of interest must be prepared, and the reliability of measurement might be low if the differences in the elemental compositions, shapes, and concentration ranges of the standard and sample matrices are large [12]. In the case of the analysis of airborne PM, SI-traceable pure elemental standard solutions [29] for preparing working standards in the labs are sometimes not available for certain elements of interest. If the sample contains high amounts of aluminum (Al) and uranium (U), the SC-INAA method is likely to cause bias due to isotope interference and nuclear fission reaction [19; 30; 31]. Therefore, in measuring various elements in a complicated matrix, such as airborne PMs, more accurate results may be obtained when the \(k_{0}\)-based INAA (\(k_{0}\)-INAA) method is applied, than when the SC-INAA method is adopted [24]. As is well known, the \(k_{0}\)-standardization method of INAA uses gold (Au) as a standard, and composite nuclear constants (molar mass, isotopic abundance, cross-section, and absolute gamma-intensity) for nuclides are normalized to Au nuclear data (i.e., \(k_{0}\) factors), which were experimentally determined. The overall uncertainty of the \(k_{0}\)-INAA method is relatively large compared to the SC-INAA method because several individual uncertainty factors, such as neutron flux and detector efficiency, must be considered, which are negligible in SC-INAA [24; 32; 33; 34]. Typically, the combined standard uncertainty of the \(k_{0}\)-INAA is conservatively 3.5% (\(k=1\)). Nevertheless, the \(k_{0}\)-INAA method is suitable for analyzing a sample when it is not easy to make the standard of the same or similar matrix. The method is also capable of analyzing multiple elements simultaneously without the need to prepare specific standards. Therefore, it is the most suitable elemental analysis method for preliminary quantification of elements in the early stages of the development of CRM. This work is a preliminary study for the development and certification of KRISS urban PM CRM. As a part of the process of developing KRISS urban dust CRM, 46 elements were analyzed using the \(k_{0}\)-INAA method. The analysis was validated using NIST Standard Reference Material(r) (SRM) 1648a urban PM. Another sample with a similar matrix--that is, the KRISS ash CRM candidate--was also analyzed. Some of the elements that are difficult to analyze with the \(k_{0}\)-INAA method were analyzed using wavelength dispersive X-ray fluorescence (WD-XRF). The major source of the KRISS urban PMs was preliminarily estimated by comparing the concentration levels of the elements.