Supplementary information files for "Direct analysis of volatile organic compounds in foods by headspace extraction atmospheric pressure chemical ionisation mass spectrometry"

Supplementary information files for "Direct analysis of volatile organic compounds in foods by headspace extraction atmospheric pressure chemical ionisation mass spectrometry" Figure S1. APCI-MS analysis of a 16.5 μg/ml indole standard in 50:50 MeOH/H2O + 0.1% Formic acidFIGURE S2. Calibration curve of indole standard that demonstrates the quantitative capabilities of the systemTable S1. Variables list used in the cheese PCA-X model.Figure S1. APCI-MS analysis of a 16.5 μg/ml indole standard in 50:50 MeOH/H2O + 0.1% Formic acidFigure S2. S:N ratio measurements from 3 replicate APCI-MS analyses of a 1 μg/ml indole standard in 50:50 MeOH/H2O + 0.1% Formic acid AbstractRationaleThe rapid screening of volatile organic compounds (VOCs) by direct analysis has potential applications in the areas of food and flavour science. Currently, the technique of choice for VOC analysis is gas chromatography/mass spectrometry (GC/MS). However, the long chromatographic run times and elaborate sample preparation associated with this technique have led a movement towards direct analysis techniques, such as selected ion flow tube mass spectrometry (SIFT-MS), proton transfer reaction mass spectrometry (PTR-MS) and electronic noses. The work presented here describes the design and construction of a Venturi jet-pump-based modification for a compact mass spectrometer which enables the direct introduction of volatiles for qualitative and quantitative analysis.MethodsVolatile organic compounds were extracted from the headspace of heated vials into the atmospheric pressure chemical ionization source of a quadrupole mass spectrometer using a Venturi pump. Samples were analysed directly with no prior sample preparation. Principal component analysis (PCA) was used to differentiate between different classes of samples.ResultsThe interface is shown to be able to routinely detect problem analytes such as fatty acids and biogenic amines without the requirement of a derivatisation step, and is shown to be able to discriminate between four different varieties of cheese with good intra and inter-day reproducibility using an unsupervised PCA model. Quantitative analysis is demonstrated using indole standards with limits of detection and quantification of 0.395 μg/mL and 1.316 μg/mL, respectively.ConclusionsThe described methodology can routinely detect highly reactive analytes such as volatile fatty acids and diamines without the need for a derivatisation step or lengthy chromatographic separations. The capability of the system was demonstrated by discriminating between different varieties of cheese and monitoring the spoilage of meats.

本补充信息文件配套于论文《通过顶空萃取大气压化学电离质谱法直接分析食品中的挥发性有机化合物》，具体包含以下材料：
1. 图S1：对50:50甲醇/水溶液（含0.1%甲酸）中16.5 μg/mL吲哚标准品的大气压化学电离质谱（atmospheric pressure chemical ionization mass spectrometry, APCI-MS）分析结果
2. 图S2：吲哚标准品的校准曲线，用于展示本系统的定量分析性能
3. 表S1：奶酪PCA-X模型所用变量列表
4. 图S1：对50:50甲醇/水溶液（含0.1%甲酸）中16.5 μg/mL吲哚标准品的APCI-MS分析结果
5. 图S2：对50:50甲醇/水溶液（含0.1%甲酸）中1 μg/mL吲哚标准品开展3次重复APCI-MS分析得到的信噪比（S:N）测量结果

### 摘要
#### 研究依据与背景
挥发性有机化合物（volatile organic compounds, VOCs）的直接快速筛查在食品与风味科学领域具有潜在应用价值。当前VOC分析的首选技术为气相色谱/质谱法（gas chromatography/mass spectrometry, GC/MS），但该技术存在色谱运行时长较长、样品前处理流程繁琐等缺陷，推动学界转向选择离子流管质谱法（selected ion flow tube mass spectrometry, SIFT-MS）、质子转移反应质谱法（proton transfer reaction mass spectrometry, PTR-MS）与电子鼻等直接分析技术。本研究设计并搭建了一款基于文丘里射流泵的紧凑型质谱仪改装接口，可直接引入挥发物用于定性与定量分析。

#### 实验方法
通过文丘里泵将加热顶空瓶顶空中的挥发性有机化合物萃取至四极杆质谱仪的大气压化学电离源中，无需预先进行样品前处理即可直接分析样品。采用主成分分析（principal component analysis, PCA）对不同类别的样品进行区分。

#### 实验结果
本接口可常规检测脂肪酸、生物胺等疑难分析物，无需衍生化步骤即可完成检测；采用无监督PCA模型可有效区分4种不同品种的奶酪，且具备良好的日内与日间重现性。以吲哚标准品开展定量分析，得到的检出限与定量限分别为0.395 μg/mL与1.316 μg/mL。

#### 研究结论
本研究所描述的分析方法可常规检测高反应性分析物，如挥发性脂肪酸与二胺，无需衍生化步骤或冗长的色谱分离。本系统的分析能力通过区分不同品种的奶酪以及监测肉类变质情况得到了验证。