# Solid-Phase Microextraction Arrow for the Sampling of Volatile Organic Compounds in Milk Samples
## 1 Introduction  
Volatile organic compounds (VOCs) count among the most important compounds in foods, since they determine the sensory characteristics of the food, which are closely related to the acceptance or preference of particular food varieties [1]. The determination of VOCs in foods is of high importance in order to understand their origins, to evaluate the aroma profile differences among similar foods and to ensure food safety and quality. However, due to the presence of a plethora of VOCs in largely varying concentrations, the determination of the volatile profile in complex food matrices is considered difficult. Additionally, individual VOCs may have very high odor activities and thus may contribute to the characteristic flavor of a given food even at very low concentrations [2]. Multiple solvent extraction or headspace (HS) analysis methods have been employed for the determination of VOCs in milk and dairy products. Typical examples of solvent extraction techniques include conventional solvent extraction, vacuum distillation, simultaneous distillation extraction (SDE/Likens-Nickerson extraction) and solvent-assisted flavor evaporation (SAFE), whilepurge-and-trap extraction and solid-phase microextraction (SPME) are the most commonly used headspace analysis methods [3]. Until now, various research methods regarding the identification of VOCs in milk samples have employed SPME for the sample preparation of the liquid sample in order to evaluate its volatile profile [4], to study the milk off-flavors [5] and to study the light-induced lipid oxidation products in milk [6]. Among the identified milk volatiles are low molecular weight aldehydes (e.g., pentanal, hexanal, heptanal), ketones (e.g., 2-heptanone, 2-nonanone), terpen (e.g., \(\alpha\)-pinene), carboxyl acids (e.g., butanoic acid, hexanoic acid, octanoic acid) and sulfur-containing compounds (e.g., dimethyl sulfide, dimethyl sulfone) [4]. SPME was initially introduced by the research group of [PERSON] in 1990 [7]. In this approach, the analytes are directly extracted and preconcentrated to the outer surface of a coated fused-silica fiber. Due to the appropriate choice of fiber coating material, the selectivity of the extraction can be tuned to reduce matrix interference. After the preconcentration step, the analytes can be desorbed by the addition of organic solvent or by direct thermal desorption into the injection port of a gas chromatograph [8]. SPME exhibits many advantages such as full process automation, avoidance of organic solvent consumption and application for in situ, in-field and in vivo sampling. However, SPME is characterized by low fiber mechanical robustness and easy fiber breakage, as well as small sorption phase volume which limits its sensitivity for analytes with low sample/fiber partitioning coefficient [9]. Stir bar sorptive extraction (SBSE) was introduced in 1999 from [PERSON] et al. [10] for the analysis of volatile and semivolatile micropollutants from aqueous samples. In this approach, a coated stir bar is inserted in the headspace or immersed directly into a liquid sample for the adsorption of the analytes. Subsequently, the bar is washed to remove nonvolatile components and the target analytes are finally desorbed either by the addition of a solvent or thermally. Compared to conventional SPME, SBSE provides a significantly larger extraction phase volume but loses the advantage of full automation and, moreover, requires a dedicated device for the thermal desorption of the stir bars [11]. Recently, a novel SPME-related extraction device, named Prep And Load (PAL solution) SPME Arrow, was proposed as alternative extraction method that combines the advantages of conventional SPME fiber and SBSE, while remediating the main inherent drawbacks of these approaches. SPME Arrow employs a robust stainless steel backbone, which carries a screw connection to the sampler, the sorption phase, as well as an arrow-shaped tip to provide conservative penetration of septa [9].