There are countless types of coffee worldwide, ranging from caffeinated to decaffeinated coffee in different variations. Besides caffeine, coffee contains many other ingredients such as chlorogenic acids. However, due to the roasting process and other structural changes in the natural substances, it is difficult to accurately identify all the ingredients.
Several chromatographic techniques can be used to determine caffeine content. These techniques must ensure that even very small quantities, such as those found in decaffeinated coffee, must be detected with all the other ingredients. This is performed with a technique by incorporating a diverter valve and a guard column using a mixed-mode phase.
The amount of caffeine can be determined using a Primesep SB column which consists of a hydrophobic chain with basic functional groups as an anion exchanger. This method shows good selectivity and resolution with a caffeine retention time of 12.5 min. Using an isocratic method or gradient elution system may result in very long run times - the last components are only eluted after about an hour. In comparison, results when using RP columns without the use of a diverter valve, show long retention times and insignificant resolution and selectivity.
In order to avoid long throughput times, an optimal sample preparation is necessary, either with a solid phase extraction (SPE) or with the use of a diverter valve as described above. The guard column is incorporated in the diverter valve so that it is not placed directly in front of the main column. Once caffeine reaches the separation column after sample injection, the valve can be switched to reverse the flow of the remaining substances (backflush) to prevent them from sticking onto the separation column. To facilitate the switch, the guard column is placed behind the detection unit (UV detector), which requires a certain pressure stability. The detector cell of the Knauer Smartline UV detector used in this method has a pressure resistance of up to 400 bar.
It is important to determine the exact retention times to ensure that the caffeine is eluted on the separation column while the other components are flushed back to the guard column. The caffeine content can thus be determined quickly and the late elutes may be disregarded.