Sample Preparation for NMR by Solid Phase Extraction

SPE-NMR is a process that uses solid phase extraction (SPE) to prepare solutions for NMR. It is applied to complex aqueous samples like food matrices or body fluids. It simplifies the mixture by removal of disturbing compounds like inorganic salts and highly concentrated polar matrix components. Medium to non-polar compounds are retain and concentrated. And later eluted in a well-defined organic solvent (i.e. methanol). The SamplePro SPE is a robotic system which performs this process in a completely automatic way and supplies the solutions in NMR tubes ready for NMR measurement.

While NMR spectroscopy can analyze, classify mixtures and quantify for compounds in the mixtures, it often suffers from the fact that the spectrum is dominated by the intense signals of the matrix components of the mixture. Signals from characteristic minor compounds can be covered completely or at least may be difficult to detect. In addition matrix effects from salts and acids may lead to variations of signal positions and shape.

In contrast to SPE-NMR, LC-SPE-NMR is used for the identification or structure elucidation of single components the combination of chromatography and NMR with post-column-SPE. A single compound is isolated by a chromatographic separation. The SPE is here used to for enrichment of this isolated, pure component that elutes from the chromatography. The SPE collection enriches the sample, separates it from the complex chromatography solvent and provides it a concentrated solution in a deuterated NMR solvent.

For untargeted screening applications the isolation into individual pure components is not required and too time consuming. A coarse separation into fractions based on the polarity of the compounds is sufficient. SPE-NMR is the method of choice to achieve this goal.

The SamplePro SPE performs the SPE process with the original sample mixture and generates typically one or two fractions per incoming sample. The major components (typically polar like sugars in beverages or endogenous metabolites from bodyfluids) are not retained and are collected in the aqueous load fraction.
The compounds of interest (typically unpolar) are retained on the cartridge, enriched, dried and eluted as concentrated solution with an organic, NMR friendly solvent.
For ion exchange SPE applications, a third fraction is generated by flushing the cartridge with a different SPE solvent containing an acid or base according to the ions to be released from the sorbent.

The complete process of preparation, loading, drying, elution and transfer of the fractions into NMR tubes is fully automated and can process up to 48 samples yielding 96 fractions in an unattended process.
A standard method is provided that is optimized and validated by BRUKER for NMR screening. Quality and reproducibility is essential as small changes in water content, pH value or salt content may considerably affect position and shape of NMR signals and make the evaluation of the data difficult or impossible.