To evaluate 4-NC for detecting LMW compounds in animal tissues, we applied the optimal 4-NC matrix solution onto sections of rat brain tissue and used MALDI-MSI to investigate the distribution of LMW compounds. According to previous studies [
2,
39], LMW compounds were categorized into small molecule metabolites (< 500 Da) and lipids to facilitate our further analysis. Fig. S11a (Supporting information) showed that small molecule metabolites were predominantly detected in the
m/
z 100–300, while lipids were detected in the
m/
z 500–900. Additionally, a total of 218 LMW compound ion signals could be detected from a rat brain tissue section (Table S4 in Supporting information). Upon further analysis using MALDI-MS/MS and LC-MS/MS, we identified 212 LMW compounds, including 160 lipids (
i.e., 41 phosphatidylcholines (PCs), 22 phosphatidylserines (PSs), 12 phosphatidic acids (PAs), 12 phosphatidylglycerols (PGs), 12 phosphatidylethanolamines (PEs), 10 diacylglycerols (DGs), 9 lysoPCs (LPCs), 9 sphingomyelins (SMs), 7 cyclic PAs (CPAs), 6 ceramides (Cers), 5 phosphatidylinositols (PIs), 3 monoacylglycerols (MGs), 2 triacylglycerols (TGs), 2 fatty acids (FAs), 2 lysoPEs (LPEs), 1 lysoPA (LPA), 1 lysoPI (LPI), 1 glucosylceramide (Glc-Cer), 1 ceramide PE (PE-Cer), and 1 ceramide PI (PI-Cer)), 1 PI monophosphate (PIP)), and 52 small molecule metabolites (Fig. S11b and Table S4 in Supporting information). Our results showed the strong ability of 4-NC for detecting LMW compounds in biological tissues. The optical and H & E staining images further showed distinct compartments within the structure of rat brain, including cerebellum (Cer), hippocampus (Hi), thalamus (Th), fornix (Fo), caudate putamen (Cp), corpus callosum (Cc), superior colliculi (Sc), and frontal cortex (Fr) (
Fig. 1). Meanwhile, several lipid ions (
i.e.,
m/
z 562.328 [PC(18:0) + K]
+,
m/
z 725.512 [PA(38:4) + H]
+,
m/
z 734.576 [PC(32:0) + H]
+, and
m/
z 770.510 [PC(32:1) + K]
+, were observed in Cc, Cer, Cp, Fo, Fr, Hi, and Sc. Moreover,
m/
z 826.540 [PC(40:10) + H]
+ and 848.580 [PS(P-40:3) + Na]
+) were mainly detected in Cc, Cer, and Th. These distributions of lipids in rat brain tissue sections are related to the structures and biological functions [
13]. Notably, these lipids are essential for transmembrane transport, membrane localization, and signal transmission [
40]. Additionally, specific small molecules (
i.e.,
m/
z 137.045 [hypoxanthine + H]
+ and
m/
z 166.058 [cyclocreatine + Na]
+) were mainly distributed in Cc, Cer, Cp, and Th. Also,
m/
z 198.129 [citrulline + Na]
+ and
m/
z 204.099 [glucose + Na]
+ were mainly detected in Cc, Cer, Cp Fr, Hi, and Th. Meanwhile,
m/
z 233.027 [galactaric acid + Na]
+ and
m/
z 295.023 [stearic acid + H]
+, were mainly detected in Cc, Cer, Sc, and Fr. Generally, the localization of small molecule metabolites and lipids is crucial for discovering biomarkers, understanding signaling pathways, and uncovering disease mechanisms [
2,
41]. These findings suggest that 4-NC has great potential for detecting and imaging LMW compounds in animal tissue samples.