Acute lung injury (ALI) is a series of pulmonary inflammatory reactions caused by various non-cardiogenic and internal and external pulmonary pathogenic factors. The main clinical manifestations are hypoxia, low lung compliance, and high physiological dead space. Severe cases of ALI can cause acute respiratory distress syndrome (ARDS), which has a high death rate in the late stages of multiple organ dysfunction syndrome [1,2]. Metabolomic profiling of patients with ALI has revealed a differential impact of various circulating amino acid levels, indicating worsening lung function. In the past several years, there has been an increased focus on the importance of amino acids and how they contribute to lung injury. The existing state of affairs may be resolved by developing treatment approaches that focus on amino acids.

As metabolic intermediates, amino acids and derivatives are vital to the production of proteins, the metabolism of energy, neurotransmission, and lipid transport [3,4]. In addition, amino acids and derivatives are involved in various types, such as the urea cycle, neurotransmitters, polyamines, etc., and play key roles as essential metabolites and regulators in many metabolic pathways. Therefore, accurate and rapid quantitative analysis of amino acids and derivatives is essential for assessing and diagnosing various diseases.

Metabolomics focuses on studying the dynamics of metabolite changes in biological systems following pathological or environmental disturbances, analyzing the metabolites' trajectory, and screening for biomarkers that can be used to determine the cause of the changes [5], [6], [7]. The research object of metabolomics is mainly small molecule metabolites (less than 1000 kb). The level of metabolites reflects a series of small changes occurring in the body, which is suitable for the diagnosis of diseases [8]. Currently, the scope of metabolomics has expanded to include the study of amino acids, with the aim to better understand how systems in biology respond metabolically to both internal and external perturbations [9]. The compounds from the methods established herein were subjected to the construction of metabolic networks, mainly involving the urea cycle and the TCA cycle. Different metabolic pathways are involved in the production and processing of amino acids, including arginine (Arg) biosynthesis, Arg-and proline (Pro) metabolism, aspartate (Asp), alanine (Ala), and glutamate (Glu) metabolism, tryptophan (Trp) metabolism, and so on (Fig. 1).

The identification of amino acids and derivatives has been the subject of several research initiatives and analytical techniques. One of the most widely used tools used for analyzing protein and metabolite levels is the liquid chromatography–mass spectrometry (LC-MS) [10], [11], [12], [13]. Most analytes show high polarity with low molecular weights, resulting in poor metabolite retention on traditional reversed-phase columns and weak analyte-stationary phase interaction [14], [15], [16]. Therefore, chemical derivatization pretreatments are commonly used in LC-MS analysis to enhance reversed-phase column retention and desired detection sensitivity. Unfortunately, this process has the disadvantage of being time-consuming and costly, and it frequently results in a number of problems, such as unstable sample preparation, poor reproducibility, side reactions, and reagent interference [17], [18], [19], [20], [21]. Thus, the creation of quantitative methods based on LC-MS without the use of derivatization procedures remains the superior advantage of flexibility and simplicity [22].

Without the need for ion pair reagents or derivatization, the validation and development of the LC-MS/MS method that can simultaneously determine 48 amino acids and derivatives in mouse plasma, urine, and lung tissue were reported in this study. To our knowledge, no studies have reported methods that can accurately quantify 48 amino acids and derivatives in three biological samples at the same time. The development of the method was demonstrated by detecting the presence of metabolites in the ALI mouse, which can provide ideas for the diagnosis and treatment of ALI. In addition, it is expected that the quantitative analysis produced by this approach will provide a thorough comprehension of the pathogenic mechanisms behind ALI. The procedure for analyzing mouse plasma, urine, and lung tissue samples is depicted in Fig. 2.