New use for old spice: Curcumin as MALDI imaging matrix

Eureka!

Inspiration in the lab rarely comes from the kitchen but that’s what occurred recently with the spice turmeric, the main component of which is curcumin. Enticed by its golden colour, Indian scientists showed recently that powdered turmeric is an ideal substance for developing fingermarks on different types of surface, including various papers, plastic sheets, painted steel and compact discs, although it failed on skin due to a lack of contrast.

That work caught the attention of Simona Francese at Sheffield Hallam University in the UK. Her research group is working on ways to examine fingermarks at the scene of a crime, using matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI MSI) to squeeze out more information beyond the ridge pattern alone.

Co-funded by the Home Office Centre for Applied Science and Technology, they have shown that drugs of abuse, condom lubricants and natural lipids can be identified within the fingermarks. The ultimate goal is to roll out this technology for the crime fighters and forensic scientists to use at the crime scene.

Although Francese wasn’t on the lookout for a new matrix, she suddenly had an inkling that curcumin could be used to visualise fingermarks and act as a matrix. "My Eureka moment was let’s look at the molecular structure of curcumin to see whether or not it could serve as MALDI matrix thus allowing curcumin enhanced marks to be analysed by MALDI MSI to give additional physical and chemical intelligence," she told spectroscopyNOW.com.

When she realised that curcumin had a conjugated double bond system to make it a good UV light absorber, like other MALDI matrices, she suspected that they were onto a winner. After a series of tests on different types of samples in her research group, she was proven right.

Feasibility test

In the first instance, the team found that good mass spectral profiles were possible with curcumin from a set of fingermarks when it was applied as a solvent-free matrix, or as dry powder that was subsequently wetted. So, in forensic work on fingermarks, curcumin could double up as a dusting powder to reveal the ridge pattern and a matrix for subsequent MALDI MS imaging.

Next, they compared synthetic curcumin with commercial curcumin that was derived from powdered turmeric. The powders were sprinkled onto fingermarks and analysed by MALDI MS. The commercial sample gave a more complex background spectrum than the synthetic one, presumably because it is mixed with other natural compounds from the turmeric. The synthetic curcumin gave the clearer images of the ridges.

When the performance of the synthetic compound was compared with α-cyano-4-hydroxycinnamic acid (CHCA), a conventional MALDI matrix, some species were ionised better with one matrix and some with the other. For lipids in fingermarks, both matrices were efficient.

Versatile matrix

Having established that the matrix works well with fingermarks, its scope was expanded to test a number of different types of analyte in various specimens. Drugs of abuse, pharmaceuticals, peptides, proteins and lipids were all targeted.

A fingermark spiked with cocaine gave a strong signal corresponding to the protonated molecule of the drug which was far weaker when laser desorption/ionisation was attempted without a matrix. When a sweet spot was selected, at which the drug was more concentrated, the signal was sufficiently strong to allow tandem mass spectrometry. In a forensic setting, this is useful for confirmation of the drug.

Strong signals were also acquired for the prescription drugs paracetamol, acitretin and tiotropium bromide with curcumin as a matrix, as well as a number of standard peptides and proteins. These were analysed from solution but the versatility of curcumin was demonstrated when it was applied to the analysis of tissue samples.

Sections of lung tissue from a rat were fixed to a glass slide and treated with curcumin. The sample was scanned and the signals for phosphocholine clearly differentiated between different regions of the organ, like the trachea and the airways.

Similarly, a piece of living skin equivalent, which consist of layers of primary human fibroblasts and keratinocyte cells, was dosed with acitretin and left for a few hours while the drug penetrated the layers. Using curcumin as a matrix, different lipid types were located in the tissue and the distribution of the drug could be visualised by superimposing the images of acitretin and phosphocholine.

These examples show that curcumin is a versatile MALDI matrix that can be used to ionise a range of compounds from different substrates, including fingermarks and biological tissue. In fact, the fingermark application covered two applications, visualising the ridge pattern before analysing the chemicals present, which is more convenient and a time saver.

"I was not dissatisfied with CHCA from the MALDI point of view but curcumin is a better fingermark enhancer and optical and fluorescent images of fingerprints can be captured prior to the MALDI process," said Francese. "Sometimes, we use all these complicated products but quite often, the answer is so simple."