External Reference

Keeping the standard outside the sample prevents it from mixing with the analyte solution. This separation reduces the possibility that the reference will contaminate a limited sample, react with sample components, or introduce a signal that overlaps with the analyte signals. The measurement can therefore retain a known comparison point without changing the sample’s chemical composition.

A separate tube or coaxial insert provides a physical location for the reference while the sample remains in its own container. The instrument can compare sample signals with the reference’s known resonance without adding the standard directly to the sample. This arrangement is especially useful when the sample is sensitive, reactive, or available only in a small amount.

A known resonance provides a basis for assigning or calibrating chemical shifts, while a known concentration supports quantitative comparison. The sample’s observed signals are evaluated against the corresponding known property of the external standard. Consequently, the same reference approach can support either identification-related measurements, quantitative measurements, or both, depending on the analytical objective.

An internal standard is introduced directly into the sample, whereas an External Reference remains physically separated in another tube or coaxial insert. The external arrangement avoids adding material that could contaminate the sample, react with it, or overlap with its signals. The tradeoff is that the analytical comparison depends on the separately positioned reference rather than a substance mixed into the sample.

First, place the sample in its measurement tube and position the known reference in a separate tube or coaxial insert. Then acquire the measurement so the sample signals and reference signal can be compared. Finally, use the reference’s known resonance for chemical-shift calibration or its known concentration for quantitative analysis, according to the experimental goal.

This approach is useful when adding a standard to the sample could create contamination, chemical reaction, or signal overlap. It also suits sensitive samples and samples available in limited quantities, because the reference remains outside the sample solution. In NMR-based chemistry, researchers can use the resulting comparison for chemical-shift calibration and quantitative measurements.