## 2 Fecal Volatile Organic Compound Analytical Methods  
Roughly, available VOC detection techniques can be divided into two different categories. Chemical analytical techniques, residing at one end of the spectrum, allow for the quantitative and qualitative detection of individual VOCs, whereas, at the other end of the spectrum, electronic devices containing an array of different VOC sensors allow for discrimination between gaseous mixtures based on pattern recognition algorithms. Gas-chromatography mass spectrometry (GC-MS) is currently considered the gold standard in VOC detection and can be placed under the chemical analytical techniques. In GC-MS, compounds are volatized and transported by a carrier gas over a coated capillary column, thereby promoting maximum separation of VOCs based on the ability of individual compounds to interact with the column surface. Subsequently, after separation of individual analyte molecules by the GC, the specimen moleculesare ionized using electron impact ionization and pushed through an electrical field, manipulating the velocity of the charged molecules by varying the voltage value. Eventually, separated by the electrical field, these charged molecules reach a detector plate, resulting in a spectrum of peaks. In this \"mass spectrum\" each peak represents a particular molecule mass, with increasing peak height demonstrating a higher concentration present [10]. GC-MS provides very good specificity and reliable quantitation, however is time-consuming, expensive, labor intensive, requires highly trained operating personnel and has high maintenance requirements. Another chemical analytical technique increasingly used in clinical research setting is selected ion flow tube mass spectrometry (SIFT-MS), allowing for real-time analysis of complex gaseous mixtures. SIFT-MS first chemically ionizes VOCs with precursor ions, resulting in less fragmentation compared to GC-MS. Subsequently, these product ions are separated and quantified by a downstream (analytical) quadrupole mass spectrometer [11]. Although fast results are ensured, SIFT-MS devices provide less detailed information compared to GC-MS.  
A third chemical analytical technique is field asymmetric ion mobility spectrometry (FAIMS). With this technique, VOCs are ionized before being pushed through two metal plates. Over such metal plates, an asymmetric high voltage waveform is applied, allowing for separation of individual VOCs based on their ion mobility. Depending on the applied electrical field, only ions with a specific mobility value reach the end of the drifting tube onto the detector plate. Using a gradient of electrical fields, a spectrum of individual peaks is generated [12]. Electronic nose devices, residing the other end of the spectrum, come in many different shapes and sizes. They enable real-time and high-throughput analysis, entail inexpensive purchase and measurement costs and produce relatively easy to interpret outcomes. A complex gaseous VOC mixture is presented to an array of different sensors, influencing the measurable attribute (for example, electrical resistance or oscillation frequency) of each individual sensor. Based on pattern recognition algorithms, obtained VOC-profiles could be compared and potentially discriminated from each other. Therefore, different studies assessed usability of eNose devices in medical diagnostics of various diseases [13; 14; 15; 16; 17]. However, the main limitation of such eNose devices is their inability to identify individual VOCs and their concentration, consequently hampering inter-device outcome comparison. Furthermore, reproducibility of obtained results is hampered by sensor drift, preventing from reliable comparison of study results and application as a bed-side tool in clinical practice. Table 1 provides an overview of the advantages and disadvantages of previous described analytical techniques.