Food Analysis: Chocolate

More resolution, more selectivity

Both, the PTR-TOF 10k and the Next-Gen FUSION PTR-TOF 10k offer a mass resolution between 10 000 and 15 000 m/Δm.

The level of analytical detail which is revealed by stepping up the example of nominal m/z 147 in the mass spectrum of exhaled breath after the consumption of salt-caramel flavored chocolate. Where previously only two slightly "deformed" peaks were visible, with very high mass resolution no less than nine peaks can be separated and easily identified. The resolved isobars comprise important aroma constituents like benzalacetone (C₁₀H₁₀O) and butyl lactate (C₇H₁₄O₃) as well as metabolism and lab-air compounds.

However, the most crucial benefit becomes immediately obvious during the analysis of chocolate flavor in the complex matrix of nosespace air. Trimethylpyrazine (C₇H₁₀N₂; protonated m/z 123.092) is of utmost interest, as it belongs to the key aroma compounds of cocoa and chocolate. The isobar C₉H₁₄ on the other hand is an omnipresent VOC which can be detected at high concentrations in indoor air. With 5 000 m/Δm the two molecules can hardly be distinguished, while at 10 000 m/Δm two clearly separated peaks are visible in the mass spectrum.

This has a considerable effect on compound quantification capabilities: With an instrument tuned to 5 000 m/Δm (left figure) only the sum concentration of trimethylpyrazine and C₉H₁₄ can be measured. During the blank exhalations through the nose (cycle 550 – 650; exhalations are indicated by the isoprene signal, which is originating from human metabolism) this sum concentration oscillates around several hundred pptv. After ingestion and starting to chew a piece of chocolate at about cycle 750 the signal behavior does not change significantly. It is not clear if trimethylpyrazine is released into the nosespace at all.

With the PTR-TOF 10k, where the two isobars can be quantified independently, the concentration of trimethylpyrazine in blank nosespace (up to cycle ~150) is only at about 20 pptv, i.e. the signal is not masked by C₉H₁₄ from room air. Therefore, the release of trimethylpyrazine into the nosespace can be beautifully monitored with sub-second time resolution. Moreover, when the test subject removes the disposable nose-pieces from the utilized NASE interface at cycle 350, the tentative source of C₉H₁₄ release can be identified – it is most probably an ingredient of the subject's hand lotion, as only the orange line corresponds to the proximity of the hand to the sample inlet.

More sensitivity, more speed

FUSION PTR-TOF 10k outperforms common PTR-MS devices with a stunning sensitivity of up to 80 000 cps/ppbv. In food and flavor research such an extreme sensitivity can be crucial when the available time per measurement is strongly limited but still high quality data need to be produced.

In order to provide a simple proof-of-concept for this statement, our science team set up a challenge: "Can a FUSION PTR-TOF 10k unambiguously identify the flavor of a piece of chocolate by measuring its headspace at room temperature for only 1 s ?".

As marker compounds for the different chocolate flavors, we chose:

Grappa (grape based brandy)

• Ethanol: C₂H₆O, alcohol.
• Ethyl octanoate: C₁₀H₂₀O₂, found in wine.
• Ethyl decanoate: C₁₂H₂₄O₂, found in grapes.

Raspberry

• Acetic acid: C₂H₄O₂, sour taste.
• Methyl butyrate: C₅H₁₀O₂, berry aroma.

 Caramel

• Maltol: C₆H₆O₃, caramel aroma.

The results were even clearer than we expected. With ion count-rates in the 10⁴ range and above after only 1 s of integration time, the relative errors are already below 1%. And yes, there are error bars plotted in the graphs, but hardly visible because the errors are negligible. Thus, for great data quality measurement times in the ms region would be more than sufficient.

FUSION PTR-TOF 10k passed the admittedly slightly tongue-in-cheek challenge with flying colors. However, a serious application of this result could be high-throughput analysis of large amounts of food headspace with an autosampler with excellent data quality.