## 4 Discussion  
The combination of NMR and headspace SPME GC provides coverage of a selection of organic compounds of interest to brewers. The NMR technique has the ability to measure and easily quantify multiple compounds in the mg/L to percent range in a liquid sample, providing a good deal of information that allows the brewer to gain an understanding of the brewing process. In this study, 26 different compounds were measured in a single run including organic acids, carbohydrates, alcohols, and amino acids. Figure 3 provides an example of one utilization of the data. In this case, maltooligosaccharides and lactose concentrations are shown clearly, revealing that lactose has been added as a sweetener to the stout. It also demonstrates the'maltier' beer styles (those brewed with a higher amount of malt in the mash stage, and retain a residual amount in the final beer itself) such as the Scotch ale, Belgian ale, and stout, compared to Patersbier, Hefeweizen, and English brown ale styles. A comparison of %ABV, values obtained by NMR and FTIR were in good agreement, which provides validation of the NMR method. Both NMR and FTIR methods provided values generally the same or higher than the stated values. This may indicate that some of these unfiltered samples continued to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Stated \%ABV** & **FTIR \%ABV** & **NMR \%ABV** \\ \hline
1 & 6.5 & 6.5 & 6.6 \\
2 & 7.1 & 8.6 & 8.0 \\
3 & 8.2 & 8.8 & 8.6 \\
4 & 5.2 & 6.3 & 6.1 \\
5 & 5.4 & 6.1 & 6.2 \\
6 & 7.5 & 7.1 & 8.0 \\
7 & 9.0 & 9.2 & 8.9 \\
8 & 5.0 & 5.4 & 4.9 \\
9 & 5.4 & 5.9 & 5.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of methods for the determination of ethanol concentrations. Figure 2: Headspace SPME chromatogram of an IPA style beer (sample 2). ferent in the barrel or that the method used by the brewery to measure alcohol content is inaccurate. The number of identifiable compounds by NMR could be increased and the detection limit lowered by freeze-drying the sample. The headspace SPME GC method is a nice complement to NMR, as it provides information about volatile compounds which are present at concentrations that are too low for NMR to measure. In this work we focused on ester compounds, but any volatile compound in the sub mg/L to percent range could potentially be measured. We used a GCMS instrument after headspace SPME sample preparation to identify volatile compounds. Some of the compounds identified are listed in Table 5. A relatively simple and inexpensive conversion of a GC FID instrument provides a cheaper method for identifying and quantifying compounds. In this case, however, known standards need to be available and identification is only possible by comparison of retention times. Quantification is also much more complex and time-consuming than the NMR method. Initial attempts to quantify by external standards and internal standards calibration did not provide values in expected ranges. This highlights the existence of matrix effects and the challenge of selecting an appropriate, cost-effective, internal standard which closely mimics the behavior of analyte compounds. Standard addition calibration overcomes these problems, but has the disadvantages of being considerably slower and consuming larger volumes of samples. Some figures of merit of the headspace SPME GC method are shown in Table 7. Limits of detection were calculated as three times the standard deviation of the background signal divided by the calibration sensitivity. Values obtained for three ester compounds were within expected ranges [16; 17]. Isoamyl acetate is known to be produced in higher amounts by Hefeweizen and Belgian yeast, while other ales are characterized by higher ethyl caprylate and ethyl caprate concentrations [18]. In this study, the two Belgian style beers (samples 1 and 9) did show higher isoamyl acetate values. The Hefeweizen (sample 4) however, did not show a measurable isoamyl acetate concentration. We do not have an explanation for this discrepancy.