Multi-element Analysis of Biological Materials by ICP-MS using Alkali Dilution

Introduction

Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) is today the instrument of choice when it comes to elemental analysis of biological materials including urine, whole blood, plasma and serum. The fast, multi element capability of the technique and wide dynamic range are key benefits that allow the measurement of major, minor, essential and toxic elements, covering the concentration range from sub parts-per-trillion (ppt) to percent levels in a single analysis. This places greater pressure on the sample preparation procedure as the chemistry of elements will define the range of elements and level of accuracy that can be achieved. For example:

• Acidification of biological samples to improve elemental stability will cause proteins to coagulate and potentially retain certain elements, producing lower than expected results.
• The degree of ionization (and hence signal strength) of As and Se within the plasma is influenced by the carbon content in the samples and the effect must be minimized for accurate measurement.
• Volatile forms of iodine are formed at lower pH, greatly affecting elemental recovery during preparation and washout properties within the sample introduction system.
• The addition of gold has been reported to improve the washout properties of mercury, reducing rinse times for faster analysis especially after a high recording.

This application study evaluates an alkali sample preparation procedure that allows for the broadest range of elements to be measured accurately and precisely for different biological samples using a single, external calibration. While at the same time, maintaining method simplicity and keeping sample contamination to a minimum. To achieve this, the following steps were taken.

• Samples were simply diluted to reduce sample preparation times and minimize the potential for contamination by eliminating the need for complete microwave acid digestion
• Samples were prepared in an alkali mixture using ammonium hydroxide to break down the cell membrane (Lysis), preventing coagulation and allowing complete release of all elements
• Propanol was added to buffer the enhancement effects of variable carbon content in the matrix on the important elements of As and Se
• In the absence of acidification, EDTA was added to stabilize the elements
• 200ppb Au was added to improve the washout properties of Hg within the sample introduction system.