Drop coating deposition Raman spectroscopy (DCDRS) as a tool for rapid determination and identification of contaminants and food additives
Abstract
Raman spectroscopy proved itself to be a powerful tool providing unique fingerprint molecular information. However, the conventional Raman spectroscopy from solutions is limited to highly concentrated samples.
To overcome this disadvantage and to study solutions in low concentrations (~μM) and small volumes (~2 μl), a special method - drop coating deposition Raman spectroscopy (DCDRS) - was introduced. The DCDRS is a simple and rapid method requiring minimal sample preparation.
It is based on the deposition of a small droplet of liquid sample on the hydrophobic substrate where subsequent evaporation of solvent leads to the preconcentration of the analyte into dried patterns from which Raman spectra can be acquired. Compared to conventional Raman spectra from the solution, the sensitivity of DCDRS can be significantly improved, often by several magnitudes.
DCDRS was already employed for the detection and identification of selected molecules and molecular mixtures. Here we measured DCDRS spectra on environmental contaminants - thiram, bentazon and picloram - in aqueous solutions in environmentally relevant concentrations.
The Raman fingerprint spectral bands of studied contaminants were distinguished and assigned to specific molecular vibrations. For milk infant formula, an illegal additive, melamine, was intentionally added to milk and subsequently detected and identified.
As for the powder infant formula itself, we found out that during the evaporation process, spatial distribution and separation of milk's main constituents (fats, mainly oleic acid and carbohydrates, such as lactose) in the dried deposit pattern occurred. The clear-cut detection of melamine in the dried deposit of contaminated milk was done because of no overlapping Raman bands in milk spectra with the main melamine spectral feature - the breathing vibration at 681 cm-(1).