Anal. Chem. 2001, 73, 1030-1036
Solid-Phase Microextraction-Mass Spectrometry: A New Approach to the Rapid Characterization of Cheeses Christophe Pe´re`s,* Christine Viallon, and Jean-Louis Berdague´
Laboratoire Flaveur, INRA de Theix, F-63122 Saint-Gene` s-Champanelle, France
This work describes a new method for the rapid characterization of cheeses by solid-phase microextraction coupled with mass spectrometry (SPME-MS). After four types of fiber were tested and the main extraction parameters studied, the volatile components were extracted using a Carboxen/PDMS 75-µm fiber placed for 10 min at 20 °C in the headspace of the cheese. The substances adsorbed were then transferred directly from the injector to the inlet of a mass spectrometer through a 1-m deactivated silica capillary column heated to 210 °C. The mass spectra thus obtained without prior chromatographic separation formed a “fingerprint” of the analyzed sample. For data analysis, the mass fragments of each spectrum (45 < m/z < 150 amu) were considered as potential descriptors of the composition of the headspace of the cheeses. Stepwise discriminant analysis was used to select a limited number of mass fragments that afforded an operational classification of the batches of cheeses studied. This new method offers the advantage of minimizing thermal, mechanical, and chemical modifications of the matrix, thereby reducing the risk of analytical artifacts. SPME-MS provides a simple and effective approach to rapid quality control by analysis of the volatile fraction of foods. The characterization of raw materials and foods has become an important strategic issue in the agrifood industry. Recent research has shown that the rapid analysis of the volatile fractions of foods by mass spectrometry offers effective analytical solutions. Direct coupling of mass spectrometry with methods such as static headspace (SHS-MS),1 dynamic headspace (DHS-MS),2-4 or solidphase microextraction (SPME-MS),5 affords “fingerprints” of analyzed products, with or without concentration of the volatile fraction. Owing to their rapidity, these nonseparative methods can be used for the classification or prediction of quality factors. * Corresponding author: (e-mail)
[email protected]. (1) Shiers, V.; Squibb, A. D. In 5th symposium on olfaction and electronic nose; Hunt Valley, Baltimore, MD, 1998. (2) Berdague´, J. L.; Vernat, G.; Rossi, V. Viandes Prod. Carne´ s 1993, 14 (5), 135-138. (3) Berdague´, J. L.; Viallon, C.; Kondjoyan, N.; Denoyer, C.; Thonat, C. Viandes Prod. Carne´ s 1998, 19 (1), 78-80. (4) Vernat, G.; Berdague´, J. L. In Bioflavoin Symposium, Dijon, France, 1995. (5) Marsili, R. T. J. Agric. Food Chem. 1999, 47, 648-654.
1030 Analytical Chemistry, Vol. 73, No. 5, March 1, 2001
The purpose of this work was to develop a solid-phase microextraction-mass spectrometry (SPME-MS) method for the characterization of cheeses by rapid analysis of their volatile fraction. This approach needs no chromatography step with prior concentration of volatile components in static mode. A series of separative analyses by solid-phase microextraction-gas chromatography/mass spectrometry (SPME-GC/MS) was first conducted to select the SPME fiber that would provide the richest chromatographic response. Inspection of the resulting chromatograms and consideration of the main parameters influencing the extraction of the volatile components then enabled selection of the conditions of SPME-MS analysis to characterize the cheeses. In practice, the discriminating power of the SPME-MS was evaluated using a range of five different cheeses of the “Camembert” type. EXPERIMENTAL SECTION Nature and Packaging of the Products Analyzed. The cheeses considered in this study were commercial products of the Camembert type at different stages of ripening (Table 1). These cheeses were cut up into eight equal portions, wrapped in aluminum foil, vacuum-packed in polyethylene bags, and stored at -25 °C. Sample Preparation. The packed portions were thawed at ambient temperature before analysis. Samples of 2 g of cheese were taken and placed in 10-mL flasks (Interchim, Montluc¸ on, France) sealed with butyl-Teflon septum caps (Interchim). SPME-GC/MS Measurements. The fibers tested for the extraction of the volatile components were as follows: poly(dimethylsiloxane) (PDMS) 100 µm, polyacrylate (PA) 85 µm, PDMS/divinylbenzene (PDMS/DVB) 65 µm, and Carboxen/DVB 75 µm (Supelco, Bellefonte, PA). The comparison of the performance of the four fibers was made using the “cru2” Camembert cheese, which produced a relatively complex volatile fraction. Each analysis was carried out three times. The volatile components were extracted by the static headspace method (SHS-SPME). During this step, each of the fibers was exposed for 10 min in the headspace of the cheese with the flask maintained at 20 °C (thermostatically controlled analysis room). The adsorbed molecules were desorbed by introducing the SPME fiber into the injector of a chromatograph (GC8060, Fisons Instruments, Milan, Italy). The injector temperature was set according to the nature of the fiber used: 260 °C for the PDMS, PDMS/DVB, and PA and 280 °C for the Carboxen/PDMS fiber. These temperatures, close to the maximum temperatures recom10.1021/ac001146j CCC: $20.00
© 2001 American Chemical Society Published on Web 01/27/2001
Table 1. Main Characteristics of Cheeses Analyzed code
type of cheese
cheese maker
milk treatment
ripening stage (days)
cru1 cru2 cru3 the cou
Camembert Camembert Camembert Camembert Coulommiers
A B B C D
raw raw raw heat-treated heat-treated
30 30 42 30 30
mended by the manufacturer (Supelco), allowed us to avoid the occurrence of significant carryover (
