The Lucidity LC-UV Drinks Orange Juice
Last week, the GC-FID tried some sour candy to test for citric acid. This week the LC-UV gets to try some orange and lime juices to also test for citric acid.
Citric acid is what give citrus fruits their tart taste and was first found by chemist Carl Scheele, who crystalized it from lemon juice in 1784. From there full scale industrial production began in 1890, where Italian fruit growers began to mass produce the chemical. Currently, citric acid is produced from microbial molds, by feeding them sugar and isolating the citric acid.
The Lucidity LC-UV is able to observe and analyze up to 10 different wavelengths, so I decided to take advantage of this by looking at citric acid at three different wavelengths, 210, 230 and 243 nm. The analysis was done by the following method:
| Lucidity LC-UV Method | |
|---|---|
| Flow Rate | 1.0 mL/min |
| Column | Raptor C18 150 x 4.6 mm, 2.7 μm |
| Oven Temperature | ambient |
| Sample loop size | 10 μL |
| Wavelength | 210, 230, 243 nm |
| Mobile Phase A | 20 mM Potassium dihydrogen phosphate,
pH 2.5 with phosphoric acid |
| Mobile Phase B | 60:40 Methanol:Acetonitrile |
| Gradient | |
| Time (min) | %B |
| 0.0 | 5 |
| 2.0 | 5 |
| 2.1 | 25 |
| 13.0 | 25 |
| 13.1 | 90 |
| 18.0 | 90 |
| 18.1 | 5 |
| 25.0 | 5 |
The standard of 7 mg/mL was made by dissolving 99.9% pure citric acid in mobile phase A and sonicating for 10 minutes to ensure all of the citric acid was in solution.
Chromatogram of the citric acid standard in all 3 wavelengths
Chromatogram of citric acid at 210 nm
The citric acid was able to elute at 2 minute 1 seconds, the remainder of the method was to ensure the column was cleaned for each injection and for possible benzoic acid analysis in the future. The speed of the elution can allow for a much faster method time.
For the samples, about 45 mL of juice was put into a centrifuge tube and centrifuged at 5000 rpm for 10 minutes. Then, 5 mL of each sample was filtered through a 0.45 μm filter then 750 μL of sample and 750 μL of mobile phase A were added into a 1.5 mL sample vial for injection onto the LC-UV. The lime juice overloaded the detector and needed to be diluted another 50% with mobile phase A.
Chromatogram of orange juice with all three wavelengths
Chromatogram of citric acid in orange juice at 210nm
The concentration of both juices was calculated using a single point calibration. The citric acid in the orange juice sample was 7276 μg/mL and the lime juice was off the charts but calculated to contain 37,759 μg/mL citric acid.
The LC-UV did a great job analyzing the citric acid in different juices. This method was also able to show ascorbic acid, the peaks at about 1 minute 30 seconds, using the 243nm wavelength, but it was not measured in this experiment.