## 1 Introduction  
Human exhaled breath has been proposed as a potential specimen following efforts to search for reliable non-invasive samples for diagnostic purposes. Exhaled breath odors have been used since ancient Greek times as part of the physical examination to diagnose a disease [1; 2]. Eventually,studies have found that the distinctive odors from exhaled breath can provide clues in the diagnosis of disease [2; 3]. In the modern era, with the emergence of science and technology, the analysis of exhaled breath has raised clinical interest. In addition, exhaled breath sampling is rather easy to handle, cost effective, and can be obtained more easily than other commonly used specimens [3; 4; 5].  
The composition of exhaled breath is directly associated with the circulation of gases from the whole body to the capillary networks of the alveolus prior to exhalation [6; 7]. The chemical composition of exhaled breath samples varies between subjects due to genetic factors, infection, medication, and individual lifestyle [3; 4; 6; 8]. It has also been proposed that the chemical compounds in exhaled breath could originate from the inhalation of contaminants from the environment [4; 8], and be produced from the blood, peripheral tissue, and lungs as a result of biochemical reactions in the body [4; 6; 7]. As a disease progresses, its pathological processes are found to have a significant impact on the production of gases, either by producing new types of gases or by the metabolic consumption of the gases [7; 8].  
To date, exhaled breath samples have been widely studied in various types of diseases, such as lung-related diseases, infectious diseases, diabetes, breast cancer, prostate cancer, inflammatory disease, and others [9; 10; 11; 12; 13; 14]. Several studies have reported a correlation between the gases of exhaled breath and metabolic pathways [2; 3; 7; 8]. Hence, there is great potential for the use of exhaled breath as a non-invasive specimen to study neurodegenerative diseases, especially Alzheimer's disease (AD). The analysis of exhaled breath samples from patients with AD could be an alternative option, since there is no direct and definitive single diagnostic method. Previously, a study described the use of nanomaterial gas sensors to distinguish patients with AD from healthy individuals with an accuracy of 85% [15]. Thus, the further analysis of exhaled breath samples from patients with AD may also provide useful insights into the development of this disease.  
In this study, we analyzed exhaled breath samples from patients with AD, Parkinson's disease (PD), and from healthy persons using gas chromatography-mass spectrometry (GC-MS) and in-house fabricated exhaled breath sensor systems. We studied the chemical compounds in the exhaled samples to determine those distinctive to the AD group. We included PD patients in the study because PD is the second most common neurodegenerative disease after AD. It has a different mechanism, signs, and symptoms from those seen in patients with AD. As a result, patients with PD can be used as a group distinguishable from those in the AD group. With the limited data available on exhaled breath analysis for patients with AD, studies on the distribution pattern and chemical composition of exhaled breath from this group are of great interest and value. Exhaled breath compounds could provide fundamental information on AD progression. Additionally, an exhaled breath system could represent an alternative screening method for AD.