TV 9000
Introduction
A new apparatus for the measurement of volatile substances in food, or other material, is presented. The apparatus consists of a standard, "on column" or "packed column" injector of Gas Chromatography (GC), linked directly to a detector. The detectors we have tested are the Flame Ionisation Detector (FID) and the Nitrogen Phosphorous Detector (NPD). Other detectors can be adapted. The head space is produced in a vial and sampled by an Solid Phase Micro Extraction (SPME)-fiber. The SPME is desorbed directly into the detector and the signal is measured. The signal obtained consists of a single peak which is directly proportional to the concentration of the head space volatile. The signal can be quantified either by the integration, or by the measurement of height and width of the signal. The reproducibility of the measurement for standard mixtures, as measured by their variation coefficient, is better than 2%. The profile of the signal may be modulated to some extent by varying the injector temperature, as the signal is mainly influenced by the rate of desorption from the SPME-fiber. Two applications are presented : The measurement of the ageing of fish, the principal component of this odor being trimethylamine, and the hydrolysis of amygdalin into benzaldehyde, hydrocyanic acid and a sugar component.
The SPME in conjunction with both liquid and gas chromatography has been widely used in analysis of food [1], [2], [3], [4]. In a recent article we proposed a novel method for a rapid measurement of the volatile compounds of food [5]. In this method the volatiles of a head space are adsorbed on a SPME-fiber [6] and measured by desorption into a GC detector, without separation of the components as in classical SPME-GC. The method allows quantitative determination of the volatiles. Selectivity for different classes of analyte can be obtained by different SPME-fibers and by different detectors. This communication presents the apparatus we have developed for this purpose and some typical applications.
TV 9000 - Principle of operation.
The measuring device, TV9000, (Fig. 1) consists of a heated injection port and a detector of a GC linked by a heated capillary of 20 cm length. The temperature of the injection port is typically between 80°C and 280°C. FID (Carlo Erba EL 980 ) and NPD (Carlo Erba NPD 800) are used at their optimal operational temperatures, 250°C and 300°C respectively. The gas control unit is a Brechbühl (PCU/2 CH/BM-A). The signal is integrated using a PC-based chromatographic integration system Chrom-Card (Fisons Instrument, Ver. 1.21). The SPME-fiber is Polydimethylsiloxane Divinylbenzene (Supelco cat. no 57311). We use vials with screw caps (22 ml), with PTFE septum (Supelco cat no 2-6916).
Applications
Determination of Fish Freshness
Once harvested fish flesh begins to degrade, this lead to the evolution of odorous amines, of which trimethylamine (TMA) is the principal component. The lowering of the temperature to about 2°C in ice is the best way to store the fish and minimise unwanted odors. Even under such conditions, the degradation continues. In this application, we follow the ageing of different samples of salmon over a period of 24 days, which is greater than the recommended storage time for this fish. Unfortunately, due to geographical problems (!) we were not able to get certified fresh fish. The preparation method according to Miller et al. [7] is very simple : Cut a sample of about 1 g, put it into the vial, add 2 ml of NaOH 40%, close the vial, wait 15 min. at 70°C, plunge the SPME-fiber in the head-space of the vial for say 2 min. and desorb it in the measuring device. This method allows a high throughput of samples with few manipulations. All the manipulation can be performed by a non-specialist. The results (fig. 3) show a good correlation between the freshness of the fish (date after harvesting) and the measured signal using NPD (the correction of the area is made by measuring an standard solution of TMA).
Monitoring of an Enzymic Reaction
Amygdalin (the "laetril" or "vitamin B19" of some nutritionists) is an almost tastless component of bitter almonds. Once the bitter almond is ground, amygdalin undergoes enzymic hydrolysis in the presence of water to give benzaldehyde, sugar and hydrocyanic acid. Benzaldehyde is the typical flavor component of marsipain and is slightly volatile. The method we used to monitor the hydrolysis was the following : Place 1.0 g of ground bitter almonds in a 250 ml flask, add 1.0 ml water, close the flask with a septum and heat to a constant temperature. We choose 40°C. At regular intervals the head space of the flask was sampled by SPME with an absorption time of 2 min. and measured with the TV9000. The signal was measured with a FID, with 2 min. for the desorption process and integrated. The evolution of the signal with time (fig. 4) can be interpreted as representing the kinetic profile of the hydrolysis, as benzaldehyde and hydrocyanic acid are the only volatile species present. The signal can be easily standardised with a solution of benzaldehyde in water.
Ethanol in Apple Juice
A fruit juice should not contain ethanol, but in some cases a fermentation can occur. The maximum ethanol concentration allowed is 0.5% vol. We were asked to propose a rapid method for the measurement of this ethanol content. The method was as follows: Apple juice (8ml) was placed in a 22ml vial and ethanol was added in order to obtain a concentration of between 0.05 and 0.5% vol. The sample was thermostatised at 30°C for 30min. The head-space was sampled using a SPME-fiber, with an adsorption time of 2 min. The desorption took place in the TV9000 in 1 min. The signal was measured with a FID. The Fig. 5 shows the correlation between the amount of ethanol added and the area of the signal, for both standard ethanol solutions and spiked samples. The high area before any addition of ethanol (0 concentration) comes from the volatile compounds of the apple juice. Using this method, we can determine the concentration of ethanol in apple juice, and also monitor the evolution of ethanol during the fermentation of apple juice.
Conclusion
The TV 9000 is...
- Simple........
- Robust........
- Easy to use........