**Acknowledgements**

*Mass Spectrometry in Life Sciences and Clinical Laboratory*

rent primary method used for quantification [97].

*Reprinted with permission from Ref. [94]. Copyright 2019 Elsevier Ltd.*

**Figure 6.**

better representation of the ionization process for analyte embedded within tissue. However, it is more time consuming, labor intensive, and requires more tissue for the calibration curve. Each method has been correlated with LC-MS data, the cur-

*Description of qMSI experiments where on-tissue spotting and homogenate spiking are two popular methods.* 

Specific examples with clinical relevancy are briefly described here. First, epertinib and lapatinib were quantified in a metastatic brain cancer mouse model using stable isotope labeling, and with liquid chromatography (LC)-MS validation [91]. The topical drugs roflumilast, tofacitinib, ruxolitinib, and LEO 29102 were examined in human skin explants to determine drug penetration and evaluate lipid markers [90]. qMSI data had a much lower quantitation range than LC-MS data of individual skin layers. Rifampicin in mouse liver tissue was quantified via a fragment ion of the intact molecule. The method used an in-house synthesized stable isotope and correlated the results with LC-MS/MS [98]. Lastly, there is one example that specifically focused on comprehensive mapping of NTs in Parkinson's disease lesioned mouse brain and demonstrated quantitation of dopamine using a stable isotope [87]. All of the drug molecules listed in this paragraph are above the general size range that NTs and metabolites fall within, ranging from 400 to 800 Da, but present possible future

avenues of research for the NT-focused methods discussed in this chapter.

The tissue mimetic model first gained popularity with examination of lapatinib

This chapter has introduced the utility of mass spectrometry imaging (MSI) for small molecules, with a specific focus on neurotransmitters (NTs). Methods that have resulted in enhanced signals of NTs were highlighted, with alternative matrix materials and chemical derivatization of analytes the two main points of discussion. Future research is needed in both of these areas to determine optimum conditions and applications, as well as establishing standard procedures so that broad application of MSI can continue. Finally, an area not discussed here that is relevant to these

and nevirapine in mouse liver by Groseclose and Castellino [99]. In addition to demonstrating high spatial resolution, they examined reproducibility and drug distribution within the homogenate. Fewer applications of the tissue mimetic model have been done, especially with small molecules as opposed to lipids [100]. A notable example includes the determination of the spatial distribution of gemcitabine, a chemotherapeutic agent, and its metabolites in mouse model pancreatic tumors using AuNPs and a traditional matrix as comparison [101]. Further experiments also work on the computational side of MSI and determining the best ways to

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normalize spectra [102, 103].

**6. Concluding remarks**

KS thanks the University of Scranton and the Chemistry Department for providing facilities, equipment, and financial support for any referenced experiments. KS thanks the Johns Hopkins Applied Imaging Mass Spectrometry (AIMS) Core Facility at the Johns Hopkins University School of Medicine for undertaking the imaging experiments referenced in this chapter. Lastly, KS thanks Nolan McLaughlin and Tyler Bielinski for tagging along for the science journey.
