Single-cell analysis using inductively coupled plasma mass spectrometry

Abstract

The technique of single-cell analysis using time-resolved inductively coupled plasma-mass spectrometry has been characterized and optimized. Determination of the metal contents of individual cells provides data on the natural metal contents of the cells and the corresponding distributions in the population. The distribution is a useful indicator of the health and the state of development of the cells. The contents of sorbed metals of individual cells over a duration of time are required to understand the dynamics of metal-cell interactions.

A green alga, Chlorella vulgaris, was used as a model biological cell in this study. The criteria and procedures for proper sampling of the cells into the ICP will be discussed. Ideally, each ICP-MS spike corresponds to one cell, but cell overlapping occurs because the cells enter the ICP randomly. Selection of cell number density and sample uptake rate to minimize spike overlapping will be discussed. A cell counting method based on the frequency of the spikes has been developed.

The distribution of the metal contents of cells was determined by measuring large number of spikes. The minimum number of spikes required was determined by statistical analysis. The spike intensity distribution was correlated with the size distribution of the cells. The peak maximum of the spike intensity distribution was used for the determination of the average metal content of the cells. The use of the peak maximum reduces errors due to spike overlapping in the measurement. Quantitative determination of the metal contents was achieved using standard particles for calibration. Errors in calibration using standard solution nebulization were discussed.

The technique was applied in the study of metal-cell interactions. Sorption of heavy metal ions (as environmental pollutants) by Chlorella vulgaris, and uptake of biometal (as nutrient) and metallodrug (as toxin) by Helicobacter pylori were studied. The technique requires simple sample preparation of removing the culture medium by filtration or centrifugation. The health state of the cells in the presence of toxic metals was related to the change in cell number density. The ratio of the FWHM of the spike intensity distributions of the sorbed metals to the natural metal contents of the cells is identified as a possible indicator of the location of the sorbed metals. The kinetics of metal sorption by the cells can be studied using a single cell culture. The method reduces errors due to uncertainties in cell number density and metal concentration in multiple samples that are required in conventional methods.

The optimal ICP-MS sampling depth of 17 elements, introduced into the ICP by conventional solution nebulization of aqueous standard solutions, has been determined. The elements were selected to represent a wide range of boiling points and ionization potentials. Boiling point of the dried residues and ionization potential of the analyte element were identified as the major factors that determine the optimal sampling position. Since dried sample solution aerosols are effectively nanoparticles, the study provides useful insight on the optimization of the operation conditions and calibration strategies for single-particle analysis using ICP-MS.