## 1 Introduction  
3-alkyl-2-methoxypyrazines (MPs) are kinds of nitrogen-containing heterocyclic compounds, which are considered to be one of the main sources of herbaceous and vegetal sensory characteristics of grapes and wine [1; 2; 3]. At present, seven MPs have been identified in grapes and wines [4], and 3-isobutyl-2-methoxypyrazine (IBMP), which imparts a green pepper aroma, is considered to be the most important one, due to the fact that its concentration is always significantly higher than the sensory threshold [5; 6; 7; 8]. In fact, the concentrations of MPs in grapes and wines are typically low (2-30 ng\(\cdot\)L\({}^{-1}\)) [9; 10]. The reason why such low concentrations still have an important influence on wine aroma and flavor [11; 12] is their sensory thresholds are also extremely low. Previous studies have shown that the sensory threshold of IBMP was 2 ng\(\cdot\)L\({}^{-1}\) in water, 1-6 ng\(\cdot\)L\({}^{-1}\) in white wine, and 10-16 ng\(\cdot\)L\({}^{-1}\) in red wine [6; 7; 8]. In view of the extremely low concentration of MPs in grapes and wines, one of the key points in this research field is the detection and quantitative methods developing. In recent years, many studies were involved in this field and great achievements were made, and many accurate and efficient techniques with higher sensitivity were developed [13]. Among them, isolation methods such as liquid-liquid extraction (LLE), solid-phase microextraction (SPME), headspace solid-phase microextraction (HS-SPME), and so forth, and clean-up procedures such as solid-phase extraction (SPE) provide powerful tools to detect MPs in concentrations on the ng\(\cdot\)L\({}^{-1}\) level [14; 15]. And the quantitative analyses of MPs have used gas chromatography-mass spectroscopy (GC-MS), GC combined with time-of-flight mass spectrometry (TOF-MS), or GC combined with nitrogen phosphorous detectors (NPDs) and so on [13; 16; 17; 18], which provide a methodological guarantee for different researchers to carry out relevant studies. Generally speaking, excessive levels of MPs are unacceptable for both the wine maker and consumer due to the fact that it will make the wine be regarded as poor quality, which is made from the unripe grape [19; 20]. The MPs' concentration in wine mainly depends on the final concentration of MPs in the grape berries it is made from, because these compounds mainly exist in the grape berry and show almost no change during fermentation [21]. Therefore, the key to controlling the concentration of MPs in wine is to reduce their content in the grape berries. Unfortunately, the biosynthesis pathway of MPs remains largely unclear [4]. To date, there are two hypotheses for MPs' biosynthesis. One hypothesis proposed by [PERSON] et al. [22] suggests that the initial step of MPs' biosynthesis involves amidation of an amino acid, followed by condensation with glyoxal or glyoxylic acid to form 3-alkyl-2-hydroxypyrazines (HPs), and finally the HPs are methylated to MPs (hereinafter referred to as \"Amino acid amidation pathway\"). According to the \"Amino acid amidation pathway\", the IBMP biosynthesis begins in the amidation of L-Leucine (L-Leu), that is, the L-Leu amides to 2-amino-4-methylpentanamide (AMPA), and the AMPA condensates with the glyoxal or glyoxylic acid to form 3-isobutyl-2-hydroxypyrazine (IBHP). Finally, the IBHP is methylated to IBMP. Whereas the other hypothesis proposed by [PERSON] et al. [23] believes that the MPs' biosynthesis begins in a condensation of two amino acids, which forms a cyclic dipeptide, followed by several steps to convert into the HPs, and finally the HPs are methylated to MPs (hereinafter referred to as \"Amino acid condensation pathway\"). Take the 3-isopropyl-2-methoxypyrazine (IPMP) biosynthesis that had been studied in _Pseudomonas perolens_, for instance; the IPMP biosynthesis begins in the condensation of valine with glycine to form the 2,5-dicarbonyl-3-isopropyl-piperazine (DCPP), and then the DCPP was converted into the 3-isopropyl-2-hydroxypyrazine (IPHP).