# A Comprehensive Review of Milk Components: Recent Developments on Extraction and Analysis Methods
## 7 Micro-Constituents' Analysis
### Chromatographic Techniques  
Liquid-chromatography (LC) and gas-chromatography (GC) are the primary chromatographic techniques used for the determination of milk micro-constituents in milk.  
GC-MS is often applied for the fingerprinting of milk volatile compounds responsible for milk flavor, including aldehydes, ketones and esters, which are small molecules often responsible for the flavor and odor of milk [163]. Milk volatiles are extracted through solid phase microextraction from samples headspace, by using divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) 50/30 \(\upmu\)m fiber [163].  
In a recent study, [PERSON] et al. [164] found that pyridine, nonanal, dodecane, furfural, 1-decene, octanoic acid and 1,3,5,7-cyclooctatetraene could be used as markers and discriminant compounds of milk originating from different geographical regions of China [164]. This study was made by using a divinylbenzene/carboxen/poly-dimethylsiloxane fiber coupled to an E-nose and a E-tongue. The E-nose contained ten metal oxide sensors for characteristics VOCs and the E-tongue had sensors for human taste (umami, richness (aftertaste-umami), astringency, aftertaste-astringency, bitterness, aftertaste-bitterness, sourness and saltiness). With this approach, it was possible to build the entire organoleptic characteristics of milk, thus identifying milk and dairy product origins [165].  
Instead, non-volatile compounds are profiled though liquid-chromatography coupled to different detectors. Reverse phase HPLC (RP-HPLC) are used for the chromatographic separation of different kinds of constituents such as vitamins [166], BAs [167] and nucleosides [168]. For example, nucleoside AMP, GMP and IMP are typically analyzed with C18 column in fifteen minutes with a gradient of sodium acetate and methanol [168].  
Normal phase HPLC (NP-HPLC) separation is used for vitamins [169] and for oligosaccharidesalrides [170; 171]. As an efficient evolution of HPLC in terms of size of particles of stationary phase, UHPLC enables faster separations, significantly reducing analysis time (e.g., minutes instead of tens of minutes). Several applications of UHPLC are available for the analysis of vitamins [172], BAs [173], nucleosides [168], oligosaccharidesalrides [174], polyphenols [175] and milk metabolites [176].  
In addition, ion-exchange HPLC (IEX-HPLC) is particularly useful for carbohydrates separation [177; 178].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Element**} & \multicolumn{2}{c}{**LOD ICP-OES (mg/kg)**} & **LOQ ICP-OES (mg/kg)** & **Upper Limit of Range ICP-OES (mg/kg)** \\ \cline{2-4}  & **LOD ICP-MS (mg/kg)** & **LOQ ICP-MS (mg/kg)** & **Upper Limit of Range ICP-MS (mg/kg)** \\ \hline \multirow{2}{*}{Fe} & 0.14 & 0.46 & 125 \\ \cline{2-4}  & 0.093 & 0.279 & 500 \\ \hline \multirow{2}{*}{Mg} & 0.05 & 0.14 & 125 \\ \cline{2-4}  & 0.077 & 0.231 & 1000 \\ \hline \multirow{2}{*}{Mn} & 0.03 & 0.1 & 125 \\ \cline{2-4}  & 0.009 & 0.027 & 200 \\ \hline \multirow{2}{*}{Ni} & 0.67 & 2.01 & 125 \\ \cline{2-4}  & 0.008 & 0.024 & 50 \\ \hline \multirow{2}{*}{Pb} & 0.25 & 0.75 & 125 \\ \cline{2-4}  & 0.003 & 0.009 & 5 \\ \hline \multirow{2}{*}{Zn} & 0.24 & 0.72 & 125 \\ \cline{2-4}  & 0.072 & 0.216 & 1000 \\ \hline \hline \end{tabular}
\end{table}
Table 5: _Cont._