# Ensuring the Quality of the Analytical Process in a Research Laboratory
## 3 Results
### Standard Addition Approach as a Solution to Lack of Suitable CRM for Trueness Estimation  
The second case presented a problem with available CRM and a standard addition as an approach to solve it. The aim was to estimate the trueness of a method for the determination of plant available K and P in arable soils. The method applied followed a specific procedure for extraction followed by well-established flame atomic emission spectrophotometry for K quantification and spectrophotometry for P determination in extracts. Due to the lack of soil CRM for extractable P and K by lactate/acetate reagent, the standard addition approach was applied to estimate the method bias. The results are presented in Table 3. The addition of an aliquot of a standard solution of K was performed on a dry, solid soil sample. The sample was homogenized and left to equilibrate for 6 days. Thus, prepared samples were analyzed following the lactate/acetate procedure. As can be seen from Table 3, the determination of extractable K in soil was performed with 87% recovery. Due to the questionable chemical equilibrium of K added to the solid dry sample, the obtained bias appeared to be an approximate estimate of the bias of the method for determination of the available potassium in soil samples. However, in the case of available P determination, it was found that standard addition before extraction could not be applied due to the very strong fixation of P in the soil matrix. The recovery obtained was 17%. Thus, the standard addition was made after extraction to the obtained solution, and the recovery was 96%. It allowed us to estimate the bias of the measurement step and, thus, only partially the method bias. The uncertainty of the bias of measurement was typically lower, and it could result in underestimation of the expanded method uncertainty calculated following the method validation and quality control approach [21]. The efficiency of extracting solution depended not only on its composition and procedure used but on the soil type and its physical and chemical properties. According to [21; 23], the CRM should be chosen with characteristics as close as possible to the studied material. However, often, an appropriate CRM is not commercially available. We encountered two problems: (1) analyzing CRM of soils with various characteristics was a well-spread procedure for estimating the analytical behavior of the applied method for testing mine tailings; however, the discrepancy of the matrix composition or analytes content, as well as their state and speciation of the sample, did not allow for the method behavior to be fully studied; (2) although a large variety of soil CRM was available, the extractable forms of plant nutrients depended on a specific procedure and often certified concentration following the same procedure were not available. A standard addition approach could help for at least an approximate estimation of the uncertainty of measurement. However, it could be noted that it was difficult to control the chemical equilibrium of the added analyte with the matrix. Thus, the state of the added analyte could differ from its state in the CRM sample or testing samples, especially in solid samples. An advantage of the standard addition approach was that performing additions at different stages of the whole procedure allowed us to estimate the bias contribution of a specific step to the overall bias. Standard addition before the extraction allowed for the estimation of bias of the whole procedure. The efficiency of extracting solution depended not only on the composition of the reagents and procedure used but also on the soil type and its physical and chemical properties. Thus, the trueness could be evaluated in sample types usually encountered in the laboratory. A significant limitation of the standard addition