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Cocaine metabolite imaging in fingerprints with Water Cluster SIMS

Detection of drug compounds and their metabolites in natural environments is a critical topic for both forensic and pharmaceutical applications, and requires overcoming some of the limitations in existing microscopic and analytical techniques.

Time of Flight Secondary Ion Mass Spectrometry (ToF SIMS) is a powerful analytical technique capable of providing detailed chemical and spatial information about a surface, and as such has recently been employed in a number of forensic studies for drug and metabolite detection. However, ToF SIMS can suffer from low sensitivity due to insufficient ionisation efficiency, and this is particularly true for complex biomaterials, i.e. those of most interest to forensic and medical analysts.

Recently, we have led the development of a powerful unique gas cluster ion beam (GCIB) using water clusters. The Water Cluster Source is capable of enhancing ion yields by many orders of magnitude compared to other conventional ion beams (C60+, Bi3+ etc.), and is particularly effective for biomolecular imaging and 3D analysis of organics such as tissue, cells, fingerprints, etc.

Plot displaying increase in signal intensity using water clusters
Water clusters enhance sensitivity to intact biomolecules such as lipids, even compared to current state-of-the-art GCIB technology.

In this application note, an experimental fingerprint detection approach using the Water Cluster Source identifies traces of ingested cocaine on human skin. The use of the J105 SIMS equipped with the Water Cluster Source (Water Cluster SIMS) provides both visualisation of the latent fingerprint as well as discrimination between contact-only and ingested cocaine by looking for metabolites of the drug excreted through the skin.

Detectable levels of metabolite in a fingerprint are extremely low, for instance 25 mg of ingested cocaine excretes less than 2.5 ng/mL in sweat,1 and previous attempts using other mass spectrometry imaging (MSI) techniques such as MALDI and DESI were unsuccessful. Using Water Cluster SIMS, it was possible not only to detect the metabolite, but also to generate a high-contrast chemical map of the entire fingerprint.

The fingerprint specimen, provided by University of Surrey, was collected on a piece of silicon wafer from a donor who had previously ingested cocaine,2 then a ToF-SIMS analysis was acquired on an 18×6 mm2 area with a 70 kV (H2O)29k+ primary ion beam in the J105 SIMS.

Figure 1(a) shows the chemical image of the 290.14 m/z signal, demonstrating the characteristic fingerprint features with ridges, valleys, as well as sweat pores. Due to the high mass accuracy of the J105 SIMS, this signal is confidently annotated as the cocaine metabolite benzoylecgonine (BZE, C16H20NO4+). Figure 1(b) shows a colour overlay of BZE (magenta) and the cocaine molecular ion (C17H22NO4+, 304.15 m/z – yellow). As expected, cocaine was observed in particulate form (see arrow) due to direct contact of the donor with the powder, and is not co-localised with BZE.

ToF SIMS image of cocaine metabolite BZE in a fingerprint.
Figure 1(a) Positive ion image of BZE (C16H20NO4+, 290.14 m/z) in a fingerprint. (b) Overlay positive ion image with BZE (magenta) and cocaine (C17H22NO4+, 304.15 m/z – yellow). (c) BZE peak, with high mass accuracy and high mass resolution.

These images, with the small amounts of BZE and cocaine present, demonstrate the benefits of Water Cluster SIMS for enhancing sensitivity, particularly for trace detection of organic compounds in complex sample matrixes.

The J105 SIMS is a powerful tool for 2D and 3D molecular imaging, providing high sensitivity analysis with a range of powerful features. Now featuring the new Water Cluster Source, the J105 takes another leap forward to offer even greater sensitivity and to intact molecular ions. This exciting new technology has been shown to dramatically improve the imaging of drug metabolites ingested by the body, and is a powerful tool for visualising molecular information in a wide range of applications.

To find out more about how the J105 SIMS can benefit your research, get in touch via our Contact Page.

References

  1. Kacinko, S. L., Barnes, A.J. et al. , Disposition of Cocaine and Its Metabolites in Human Sweat after Controlled Cocaine Administration, Clinical Chemistry, 51, 2085 (2005). https://doi.org/10.1373/clinchem.2005.054338
  2. Jang, M., Costa, C., Bunch, J. et al. On the relevance of cocaine detection in a fingerprint. Sci Rep 10, 1974 (2020). https://doi.org/10.1038/s41598-020-58856-0

Drug detection with high-sensitivity using ToF SIMS

The high attrition rate of pharmaceutical drug compounds adds enormously to the cost of those that make it to market, so there is an urgent and growing need to identify failure at earlier stages of drug development.

In order to do so, researchers require as much information as possible. Specifically, there is a need to measure the concentration of a drug at the target in order to accurately predict its pharmacological effect. This then requires a means of generating label-free sub-cellular imaging, as fluorescent labels may affect drug chemistry, altering results.

Time of flight secondary ion mass spectrometry (ToF SIMS) is a powerful tool for label-free chemical imaging, having typically very high lateral resolution capable of resolving sub-cellular features with 3D analysis capabilities.

ToF SIMS is thus a potentially powerful analysis tool for the screening of new drug compounds. However, the use of high energy projectiles for ToF SIMS analysis can cause molecules to fragment, preventing the molecular ion from being detected. This can lead to a lot of ambiguity, for example distinguishing between a drug compound and its metabolites.

Another possible stumbling block is the issue of sensitivity, particularly for those compounds of most interest. In a recent study by the National Physical Laboratory (NPL), Vorng et al. demonstrate that the sensitivity in ToF SIMS is proportional to the Log P of that compound, such that compounds with low or negative Log P values are extremely difficult to detect.  

Log P, or partition coefficient, is a measure of hydrophobicity, and is a major factor used in pre-clinical assessment of a compound’s druglikeness.  It is advisable that a drug candidate be as hydrophilic as possible while still retaining adequate binding affinity to the therapeutic protein target, i.e. that the Log P be as low (or negative) as practicable. This presents an obvious problem for the use of ToF SIMS as an analytical tool in this context.

Cluster beam colliding with a surface.

We have recently led the development of a new type of ion source for ToF SIMS that provides unparalleled sensitivity particularly for organic species. Available exclusively on the J105 SIMS, the Water Cluster Source simultaneously reduces fragmentation while increasing ionization, for truly unparalleled sensitivity of drugs, metabolites, biomarkers, lipids, peptides and more.

Combining this new ion source with the already impressive sensitivity of the J105 SIMS, even low Log P compounds can be detected in tissue and cells, with direct, label-free imaging of the molecular compounds at sub-cellular resolutions.

To demonstrate this, we doped tissue homogenate with 4 different pharmaceutical compounds that span the range of Log P from -0.8 to 7.6. The relationship between sensitivity and Log P reported by NPL is observed in this data, however the slope of the line is greatly reduced, with only a factor of 40 between the highest and lowest values.

ToF SIMS sensitivity to drugs as a function of Log P
ToF SIMS sensitivity of four different drugs using the Water Source. Sensitivity shows a linear relationship to the partition coefficient, Log P, though the slope is not steep.

As a comparison, we performed the same experiments with a state-of-the-art Ar gas cluster ion beam and plotted the yield against that of the new Water Source. The Water Source increased sensitivity by an order of magnitude in most cases, with the largest increase being for those compounds with the lowest Log P values. This indicates that the improvement in sensitivity is greatest for those compounds that need it the most.

Comparing sensitivity of argon and water cluster beams for four different drugs
Comparing sensitivity of a state-of-the-art Ar cluster source with the Water Source. Sensitivity improves by roughly an order of magnitude when using water, with the largest increase for those compounds with lower Log P values.

As a final demonstration of the capabilities of the J105 with the Water Source, we performed tandem MS analysis on the homogenate samples. Tandem MS is an important step for confirming any assignment in mass spectrometry, however the inefficiency of the process often means it can only be performed on high intensity peaks. With the boost in sensitivity provided by the Water Source, tandem MS analysis is possible even on compounds with relatively low Log P values, such as ciprofloxacin.

Tandem MS analysis of the drug ciprofloxacin
Tandem MS performed on the J105 SIMS with a Water Source. Greater sensitivity allows definitive confirmation of many more peaks.

ToF SIMS is a potentially powerful analysis tool for the screening of new drug compounds, however research is hampered by the inherently low sensitivity to many drug candidates. The J105 SIMS in combination with the Water Cluster Source provides unparalleled sensitivity to drug compounds, particularly in complex matrices such as tissue and cells, even for low Log P compounds. This unprecedented sensitivity combined with sub-cellular imaging and high-resolution 3D imaging mean the J105 SIMS is a powerful tool for drug analysis.

To learn more about how the J105 SIMS can benefit your research or to set up a demonstration, get in touch via our Contact Page.