## 1 Introduction  
There has been a long history for human beings using a variety of pesticides for agricultural purposes. Other than herbicides and fungicides, insecticides are usually primarily used. Carbamate, organophosphate, and pyrethroids are the most common ingredients of commercial insecticide products used in the residential environments, and have recently become the most popular target insecticides in the domain of risk assessment because of the adverse effects [1]. Carbamate insecticides (CIs) serve as enzyme inhibitors of insects, but could directly bind to the melatonin receptors of mammals to cause an imbalance in circadian rhythm. It has been found that CIs could enhance the risk of diabetes to humans. Organophosphates (OPs) are inhibitors of the neuromuscular enzyme receptor (e.g., acetylcholinesterase) of insects, and also function similarly in humans and many other animals [2]. Recent studies have shown the neurotoxic effects of OPs on children, including behavior change, learning rates, normal physical coordination, and attention deficit hyperactivity disorder, which can be triggered even by routine exposure to low levels of OPs [2; 3; 4]. Some OPs, showing a relation with cancer, are regarded as possible carcinogens, as suggested by the International Agency for Research on Cancer (IARC) [5; 6]. Pyrethroids (PYRs) are artificial synthetic products of pyrethrins, which have been one of the oldest and most widely used botanical insecticides applied in the control of lice on humans and animals for more than 150 years [1; 7]. Artificial pyrethroids, instead of natural pyrethrins, became the major commercial household insecticides worldwide because of their stability to photolysis. The reversible health effects of PYR exposure include allergies of skin and mucosa, headache, and dizziness [8]. In 1999, the US Environmental Protection Agency (EPA) classified PYRs as \"likely to be a human carcinogen by the oral route\" [9]. Once entering the human body, these insecticides are metabolized and excreted in urine, which can be used for biomonitoring of insecticide exposure [10]. There are six major urinary metabolites of OPs that are the most commonly measured, including dimethyl phosphate (DMP), dimethyl thiophosphate (DMTP), dimethyl dithiophosphate (DMDTP), diethyl phosphate (DEP), diethyl thiophosphate (DETP), and diethyl dithiphosphate (DEDTP) (Figure 1a) [11; 12]. The major urinary metabolite of CIs is 7-phenol 2,3-dihydro-2,2-dimethyl-7-benzofuranol, also known as 7-phenol [13]. The metabolites of common commercial PYRs are _trans_-chrysanthemumdicarboxylic acid (_trans_-CDCA), _cis_- and _trans_-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylic acid (_cis_/_trans_-DCCA), _cis_-3-(2,2-dibromovinyl)-2,2-dimethylcyclopropanecarboxylic acid (_cis_-DBCA), 3-phenoxyozoic acid (3-PBA), and 4-fluoro-3-phenoxybenzoic acid (F-BPA) via ester cleavage of original compounds (Figure 1b) [14; 15]. A summary of the main metabolites of commerical carbamate, organophosphate, and pyrethroid insecticides is shown in Table 1.  
The main assay methods for urinary metabolites of these three insecticides include immunoassay, high performance liquid chromatography-mass spectrometry (LCMS), and gas chromatography-mass spectrometry (GCMS). The immunoassay method is easier to apply and less costly than LCMS or GCMS, but the lower sensitivity and specificity mean it is not favored for precise and accurate analyses [16]. The main LCMS assay method was established in 2004, and became popular recently because of the relatively easy pretreatment of sample extraction [17]; the disadvantage, however, is probably the cost of sample analysis and routine maintenance of LCMS, which is the highest among the assay methods. The early GCMS assay methods with relatively low cost were favored but limited to applications analyzing OP or PYR metabolites individually, and also had relatively high limits of detection and quantification (LODs and LOQs) [18; 19].