Isotopes: A brief introduction

An isotope is a variant of a chemical element. Lead, for example, exists in nature in four forms: lead-204, lead-206, lead-207, and lead-208. These four forms are chemically identical; they react the same way, form the same bonds, and share the same toxicological properties. They differ only by their mass: lead-208 is slightly heavier than lead-204.

This tiny mass difference is measurable using specialized instruments called mass spectrometers. It is this measurement that opens the door to source identification.

How an isotopic signature is formed

The relative proportion of each isotope in an ore deposit or a rock depends on its geological history. Lead-206, lead-207, and lead-208 are produced by the radioactive decay of uranium and thorium over billions of years. A deposit formed in uranium-rich rock will accumulate more lead-206 and -207. A deposit formed in thorium-rich rock will accumulate more lead-208.

As a result: every lead deposit in the world has a unique combination of isotopic ratios, reflecting its geological age and the radioactive element composition of the source rock. This unique combination is what we call the isotopic signature.

Source A
Ancient deposit
²⁰⁶Pb/²⁰⁴Pb: 18.2
²⁰⁷Pb/²⁰⁴Pb: 15.6
²⁰⁸Pb/²⁰⁴Pb: 38.1
Source B
Recent deposit
²⁰⁶Pb/²⁰⁴Pb: 17.1
²⁰⁷Pb/²⁰⁴Pb: 15.4
²⁰⁸Pb/²⁰⁴Pb: 37.5
Sample
Contaminated soil
²⁰⁶Pb/²⁰⁴Pb: 17.8
²⁰⁷Pb/²⁰⁴Pb: 15.5
²⁰⁸Pb/²⁰⁴Pb: 37.9
Isotopic ratios of two different lead sources and a contaminated sample. The sample falls between the two sources, a sign of a mixture whose proportions can be calculated.

Why signatures resist time and transformations

This is one of the fundamental properties that makes this approach so robust. When lead is smelted in a blast furnace, ground into fine powder, dissolved in water, or precipitated as salt, the isotopic ratios do not change. Ordinary chemical and physical processes do not alter the isotopic composition of a heavy element like lead.

This is not entirely true for light elements; isotopes of hydrogen, carbon, nitrogen, or sulfur are slightly fractionated by certain chemical and biological reactions. For heavy metals, however, the signature is practically preserved from the moment the deposit was formed.

The measurement: High-resolution mass spectrometry

Isotopic ratios are measured using Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS). The instrument ionizes the sample, accelerates the ions, and separates them by mass within a magnetic field. The precision achieved is on the order of 0.01 to 0.1 per mil, which is sufficient to distinguish geological sources that differ by only a few percent.

These instruments cost several hundred thousand euros and require specialized laboratories. Analyzing a single sample involves several hours of preparation and measurement.

Key Takeaways
  • An isotopic signature is the relative proportion of the different mass variants of an element in a sample.
  • It is formed during the crystallization of the ore deposit, millions or billions of years ago.
  • It is preserved during transport, smelting, dissolution, and precipitation of heavy metals.
  • Its measurement requires high-resolution mass spectrometry.
  • It constitutes a virtually unfalsifiable "fingerprint" of a source.