Note 4: Direct Analysis of Spices and Coffee

By Santford V. Overton and John J. Manura


Large volumes of solvents including chloroform, ether, benzene, toluene, among many others are utilized in most laboratories for the extraction and concentration of various components in samples for subsequent analyses. New methods which reduce or eliminate the solvents required for sample purification provide many advantages to the laboratory. The exposure of laboratory staff to these solvents has become of major concern to both employees and health officials. The disposal of used solvents has become a serious problem and is rapidly becoming quite expensive. In addition, a sensitive and accurate analysis of residual solvents in the final product is necessary because of both toxicology and quality control. The new Short Path Thermal Desorption System permits the direct thermal analyses of samples without any prior solvent extraction. Samples to be analyzed are inserted directly in the Glass Lined Stainless Steel (GLT) Desorption Tubes and held in place with two quartz wool plugs. The Desorption Tubes can then be inserted in the Short Path Thermal Desorption System and heated to the desired temperature to vaporize the residues and solvents within the samples. A constant carrier gas flow through the sample will then flush the vaporized residues into the injection port of the gas chromatograph (GC) for subsequent analysis.


Approximately 1 milligram samples of several spices and dehydrated coffee from various manufacturers were placed inside a 3.0 mm I.D. GLT Desorption Tube. Quartz wool plugs (approximately 5 mm in length) were placed on both sides of the sample to hold it in position. The Desorption Tube containing the sample was attached to the Short Path Thermal Desorption System and flushed with helium for 2 minutes to remove all oxygen from the system. The sample was then injected into the GC and heated to 250 degrees C for the spices and 200 degrees C for the coffee with continuous gas flow of 2.0 ml per minute to drive the volatilized components into the GC injection port.

 The standard gas chromatograph conditions are given below:

Spices: Sample Heated For 5.0 Minutes at 250 Degrees C In the Desorption Tube

Splitless Injection On a 60 meter DB-1, 1.0 um Film Thickness, 0.32 mm I.D. on a Hewlett Packard 5971 GC/MS.

Initial Temperature -40 Degrees C For 5 Minutes During Sample Collection, Program To 40 Degrees C At 10 Deg Per Minute, Thereafter Program To 280 Degrees At 4 Deg/Min

Coffee: Sample Heated For 5.0 Minutes At 200 Degrees C

Splitless Injection On GC, 30 Meter DB-1, 1.5um Film Thickness, 0.53 mm I.D. on a Varian Model 3700 GC

Initial Temperature 0 Degrees C For 5 Minutes During Sample Collection, Program To 300 Degrees C At 10 Deg Per Minute

Subsequent Analysis Of All Peaks On a Hewlett Packard 5891 GC/MS For Identification. 60 meter DB1, 1.0 um Film Thickness, 0.32 mm I.D., Desorb At 250 Degrees C For 5 Minutes

Results and Discussion

Figures 1-4 are chromatograms of 4 spice samples including black pepper, basil, oregano and garlic which were analyzed by Direct Thermal Analysis. The technique of Direct Thermal Analysis utilizing the Short Path Thermal Desorption System permits the analysis of dry vegetative matter without the use of solvents or other sample preparation. Each sample of spice analyzed exhibits its own distinctive pattern of peaks which were subsequently analyzed by the mass spec for identification.

Figure 1

Figure 1 Black Pepper

Figure 2

Figure 2 Basil

Figure 3

Figure 3 Oregano

Fgiure 4

Figure 4 Minced garlic

Figures 5 and 6 show the comparisons of the Direct Thermal Analysis of two different brands of dehydrated coffee. Each chromatogram consists of approximately 50 peaks.

Figure 5

Figure 5 Brand A Coffee

Figure 6

Figure 6 Brand B Coffee

Figures 7 and 8 compare a regular and decaffeinated coffee product from the same manufacturer. The samples were analyzed on the GC/MS and the peak at 64 minutes was confirmed to be caffeine.

Figure 7

Figure 7 Caffeinated Coffee

Figure 8

Figure 8 Decaffeinated Coffee

This technique can be utilized for the analyses of similar type samples for quality control and also for the detection of contaminants in the finished product such as residual solvents and pesticides. Again, this technique verifies the usefulness of Direct Thermal Analysis for the detection of the volatiles and semi-volatiles present in samples without the need for solvent extraction or other sample preparation.