Two-dimensional mapping of copper and zinc in liver sections by laser ablation-inductively coupled plasma mass spectrometry.

Abstract

Metals are not homogeneously distributed in organ tissues. Although most mapping techniques, such as histologic staining methods, have been developed for element imaging on a subcellular level, many suffer from either low precision or poor detection limits. Therefore, small variations in elemental distribution cannot be identified. We developed a method for two-dimensional mapping of trace elements to identify the influence of metabolic zonation by the liver on trace element distribution. A prepared homogeneous Certified Reference Material (CRM; LGC 7112, pig liver) was used to characterize the laser ablation-inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-MS) in terms of precision. Different isotopes for copper and zinc were monitored, and the use of carbon as an internal standard was investigated to correct for differences in ablation efficiency to identify the most precise mapping technique for liver samples. For the homogeneous CRM, the reproducibility of the copper and zinc signals was approximately 3-24% depending on spot size and number of pulses. When carbon was used as an internal standard, the reproducibility was improved significantly. Line scan signals over a length of 1.5 mm were more precise [relative SD (RSD), 1.6-6.1% for copper ((63)Cu, (65)Cu) and zinc ((64)Zn, (66)Zn) depending on the spot size, the scanning speed, and the element]. Thin section of sheep liver achieved precisions of 27-59% (raster scan) and 9-47% (line scan) RSD for copper, whereas the precision for zinc was significantly better: 8-18% (raster scan) and 4-21% (line scan) RSD. Long line scans and two-dimensional element maps of the thin sections revealed the zonation of copper in sheep liver containing extremely low copper concentrations. Elemental mapping of trace elements generated by LA-ICP-MS can be very precise so that small changes in the elemental concentration in the tissue can be detected and nonuniform spatial distribution of the elements in tissues can be established.