动态顶空采样和TD-GC-TOF MS快速灵敏分析草莓香气

Components of different concentrations are often found in food aroma profiles, but compounds that are odour- active are usually present in small amounts. Hence it is important to ensure that all aspects of the analysis and sampling processes are optimized to allow detection and identification of a wide variety of analytes.

The aroma of strawberries is among the most complex fruit aromas and has attracted significant attention, especially in terms of identifying the olfactory components that distinguish different varieties of strawberries.

This article describes the application of a microchamber-based analytical setup that provides low detection limits required for identifying important volatiles without affecting the speed, flexibility, and ease-of-operation in food analysis.

Background to the Sampling and Analysis Methodology

抽样和分析方法的背景

Sampling and analysis of the strawberry samples are performed using the following techniques:

草莓样品的取样和分析使用以下技术进行:

• Dynamic headspace sampling removes the organic vapours from the strawberry and transfers them onto a tube filled with sorbent

• 动态顶空采样从草莓中去除有机蒸汽，并将其转移到充满吸附剂的管中
  

• Thermal desorption concentrates the organic vapours and transports them into the GC of carrier gas for improved sensitivity

• 热解吸将有机蒸汽浓缩，并将其输送到载气气相色谱中，以提高灵敏度
  

• Analysis utilizes the time-of-flight mass spectrometry method, which is highly sensitive

• 分析采用飞行时间质谱法，这是高度灵敏的
  

• Compound-identification ramps up data processing by detecting and validating the characteristics of headspace components

• 化合物识别通过检测和验证顶空成分的特征来提高数据处理速度
  

Dynamic Headspace Sampling 动态顶空采样

The Micro-Chamber/Thermal Extractor from Markes is an advanced sampling accessory specifically designed to allow headspace sampling of organic vapours from an extensive range of materials. The accessory is easy to use and offers short sampling times of less than 60 minutes. Depending on the selected model, it can analyze four to six samples simultaneously.

For the experiment, the strawberry sample is placed in one of the microchambers that are inert-coated and can be heated, as shown in Figure 1. Inert gas or air is then passed into the multi-hyphenated microchamber at a steady flow rate.

This process ensures that the headspace vapours are transferred onto a tube packed with sorbent for the thermal desorption stage. During this constant flow, large amounts of headspace vapours are collected from the volatility range. This improves the level of sensitivity and also ensures that the sample represents the whole aroma or odour profile under the specified temperature conditions.

Figure 1. Schematic showing operation of an individual microchamber for sampling organic vapours from samples such as fresh foodstuffs.

图1    示意图显示从新鲜食品等样品中取样有机蒸气的单个微室的操作

Thermal Desorption 热脱附

Thermal desorption (TD) is a ‘front-end’ technique developed for GC and GC–MS instruments and can be used for studying volatile and semi-volatile organic compounds (VOCs and SVOCs) in a range of sample matrices such as solids, liquids, and gases.

This method integrates processes like pre-concentration, desorption and extraction, and GC injection into a fully automated and sensitive operation, which eliminates the necessity for solvent extraction with its related limitations of interferences, dilution and manual preparation of samples. When TD is used along with dynamic headspace, it provides a high level of sensitivity for studying VOC profiles from fresh foodstuffs.

In this experiment, the sorbent tubes that were utilized to obtain strawberry headspace vapours were examined using TD in tandem with GC–TOF MS method. The sampled tubes were then heated in an inert carrier gas during the two-stage thermal desorption method. The components thus released from this process were delivered to a smaller ‘focusing’ trap that was electrically cooled and integral to the TD solution.

Once the primary desorption stage was over, the focusing trap was desorbed rapidly by heating it in a reverse flow of carrier gas to inject or transfer the organic compounds within the capillary GC analytical column. The two- stage desorption process not only helps in optimizing concentration enhancement, but also creates narrow chromatographic peaks, which in turn improves the level of sensitivity.

Markes’ TD systems also provide other benefits, which make them ideal for complex odour profiling applications. These benefits include:

• Electrical cooling of the secondary focusing trap removes the necessity for liquid cryogen, saving significant amount of time and money. It also allows for accurate water management and temperature control.

• 二次聚焦阱的电冷却消除了液态致冷剂的必要性 ，节省了大量的时间和金钱。它还允许精确的水管理和温度控制。
  

• The backflush or reverse operation of the focusing trap and sorbent tube enables sequential use of various sorbents. This helps in extending the volatility range of compounds.

• 聚焦冷阱和吸附剂管的反冲洗(反吹)或反向操作使各种吸附剂的顺序使用 成为可能。这有助于扩大化合物的挥发范围。
  

• Gentle heating of the tube along with the inert flowpath makes the system suitable for reactive compounds such as sulfur compounds and amines.

• 管沿惰性流道的温和加热使该系统适用于活性化合物 ，如硫化合物和胺。
  

• Samples can be divided during desorption process and the remainder samples can be again collected and examined under same or different conditions.

• 在解吸过程中可对样品进行分离 ，剩余样品可在相同或不同条件下再次收集和检测。
  

Markes’ fully automated thermal desorber TD-100 was used in this study. This system can carry 100 sorbent tubes and enables complete automation of sample desorption and re-collection.

Time-of-Flight Mass Spectrometry 飞行时间质谱

In this experiment, ALMSCO’s BenchTOF instrument was used that is specifically designed for gas chromatography. The BenchTOF systems provide complete spectral data at high sensitivity which makes them suitable for detecting analytes even at trace levels. Reference-quality ‘classical’ EI spectra obtained from these instruments help in comparing against NIST library.

Compound Identification Software

化合物鉴定软件

TargetView is a data-processing software package that further improves the sampling and analysis setup. The software enables automated and precise identification of trace compounds in challenging GC–MS profiles, and utilizes advanced algorithms to process data related to total ion chromatography (TIC). With the help of TargetView, unknown as well as target compounds can be easily identified at trace levels.

Experimental Framework 试验流程

Figure 2 shows the extraction and analysis process, where whole strawberries have been accommodated by the microchamber system, which is otherwise not possible with a conventional 20mL headspace vial.

Figure 2. Flow chart showing the process used to sample and identify the volatile compounds released from strawberries.

图2    流程图显示了从草莓中提取和鉴定挥发性化合物的过程。

For sampling, Markes’ Micro-Chamber/Thermal Extractor, inert-coated chambers, and sorbent- packed tubes were used; for thermal desorption process, TD-100 thermal desorber and Material Emissions trap were used; and for TOF MS method BenchTOF system was employed.

采样时，使用了Markes的微室/热萃取器、惰性涂层室和吸附剂填充管;热脱附过程采用TD-100型热脱附器和物质排放捕集器;TOF质谱法采用BenchTOF体系。

Results and Discussion 结果和讨论

Emission Profile of a Whole Strawberry

全草莓的风味释放谱

Figure 3 illustrates an example of emission profile for a whole strawberry. In order to create a list of the major components in this sample, the TargetView software includes options for the ‘all-component’ search that were adjusted to identify the 100 most abundant peaks against the NIST 11 commercial database. Ultimately, a list of 76 known compounds was obtained, with most of them being esters as major olfactory components.

Figure 3. Analysis of the headspace profile of a whole strawberry, sampled onto a Tenax TA–SulfiCarb sorbent tube using the Micro-Chamber/Thermal Extractor, and analyzed by GC–TOF MS. The 40 most abundant compounds are labeled.