Preparation and testing of phosphate, oxalate and uric acid matrix-matched standards for accurate quantification of 2D elemental distribution in kidney stone sections using 213 nm nanosecond laser ablation inductively coupled plasma mass spectrometry

Abstract

Matrix-matched calibration for quantitative elemental mapping of kidney stones by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was developed with a 213 nm Nd:YAG laser ablation device and a quadrupole mass spectrometer. The method was applied to the imaging of P, Na, Sr, Zn, Ba and Pb distributions over a section of the kidney stone specimen containing phosphate and oxalate phases. Eighteen kidney stone specimens consisting of phosphate, oxalate and urate phases in various proportions were cut into halves for both preparation of calibration pellets and bulk analysis. Homogeneity of calibration pellets was examined by scanning electron microscopy (SEM) and LA-ICP-MS, concluding that areas of individual biominerals were thoroughly mixed and their size in units of micrometers was well below the size of the used laser spot. Calcium was employed as the internal reference element, being present in sufficient contents in the studied kidney stones. Mean values of calcium contents in oxalate and phosphate phases separately were determined using an electron microprobe (EMP) in the kidney stone section further subjected to the mapping. The actual (time- and space-resolved) Ca sensitivity was computed for each ⁴⁴Ca⁺ signal and used as the internal reference for LA-ICP-MS isotopic signals of P, Na, Sr, Zn, Ba and Pb when mapping. Dependences of particular isotopic signal intensity/Ca sensitivity ratios vs. average elemental contents by solution analysis were processed by ordinary least squares linear regression. Despite variable matrices the regression yielded calibration lines with insignificant intercepts, coefficients of determination R² > 0.9955, and relatively narrow prediction and confidence bands. However, in addition, the applicability of four-point calibration and four single-point calibrations as less time-consuming options was examined on the basis of the NIST SRM 1486 bone meal pellet analysis. Best fit was obtained for the four-point calibration and single-point calibration with the phosphate pellet. Quantitative elemental maps of the kidney stone section were recorded and computed for P, Na, Sr, Zn, Ba and Pb. The feasibility of quantification by matrix-matched single-point calibration was verified by determination of the median elemental contents in phosphate and oxalate phases by LA-ICP-MS and their arithmetic comparison mean values obtained over the same section area using EMP.