Go Direct Polarimeter provides a visual representation of chirality by measuring the optical rotation of optical isomers, such as sugars, amino acids, and proteins.
Note: Vernier products are designed for educational use. Our products are not designed nor are they recommended for any industrial, medical, or commercial process such as life support, patient diagnosis, control of a manufacturing process, or industrial testing of any kind.
Using the Product
Follow these steps when using the Go Direct Polarimeter when performing optical rotation experiments.
- Connect the Go Direct Polarimeter according to the Getting Started instructions.
- Calibrate Go Direct Polarimeter.
- Pour distilled water or the appropriate solvent in the Go Direct Polarimeter cell to a height of 10 cm. It is important to read the height to the nearest 0.1 cm. Read to the bottom of the meniscus.
- Place the cell in Go Direct Polarimeter and select Finish Calibration.
- When the polarimeter is ready, select Done.
- You are now ready to add your optically active sample into the polarimeter cell.
- Rinse the polarimeter cell with a few milliliters of your optically active sample. Pour your sample into the polarimeter cell to a height of 10.0 cm (1.00 dm). It is important to read the height to the nearest 0.1 cm. Read to the bottom of the meniscus.
- Place the sample cell in the polarimeter.
- Start data collection. Data collection will stop automatically. Data are stored automatically in Instrumental Analysis. In LabQuest App, you can store a run by tapping the file cabinet icon on the graph screen.
- Record the first angle closest to 0° where the illumination is at a maximum. This is the observed angle of rotation for the optically active sample (α). There are several ways to locate this angle.
- Statistics: To simply get the angle with the highest illumination, highlight the peak of interest in Instrumental Analysis or LabQuest App. In Instrumental Analysis, click or tap Graph Tools and select Statistics. In LabQuest App, choose Statistics from the Analyze menu. Record the angle value where the illumination is at a maximum, as presented in the box. This method is the fastest and will result in reproducibility of the angle of rotation measurement of ±2.0°.
- Cosine Squared: To incorporate all your data into the fit, you can fit the data to their true waveform, a cosine squared, in either Instrumental Analysis or LabQuest App. In Instrumental Analysis, click or tap Graph Tools and select Curve Fit. Then select cosine squared from the drop-down list. In LabQuest App, choose Curve Fit from the Analyze menu. From the list of available General Equations, select Cosine Squared. The fit will run automatically. In this fit, the x-value corresponding to the maximum y-value is obtained from the negative of the phase shift parameter, –C. This is a nonlinear fit that undergoes numerous iterations and may not converge, which may result in an unreasonable answer. Make sure the resulting value is reasonable based on the data. This method is the most time consuming; however, it will result in reproducibility of the angle of rotation measurement of ±0.1°.
- A compound will consistently have the same specific rotation under identical experimental conditions. To determine the specific rotation of the sample, use Biot’s law:
α = [α] ℓ c
where α is the observed optical rotation in units of degrees, [α] is the specific rotation in units of degrees (the formal unit for specific rotation is degrees dm-1 mL g-1, but scientific literature uses just degrees), ℓ is the length of the cell in units of dm, and c is the sample concentration in units of grams per milliliter.
- To calculate a percent optical purity (or enantiomeric excess):
The value for the pure enantiomer can be obtained from literature or it can be measured using a neat sample, considering the purity of that sample as described by the manufacturer.
Care and Maintenance
Go Direct Polarimeter contains a small lithium-ion battery. The system is designed to consume very little power and not put heavy demands on the battery. Although the battery is warranted for one year, the expected battery life should be several years. Replacement batteries are available from Vernier (order code: GDX-BAT-650).
Storage and Maintenance
To store Go Direct Polarimeter for extended periods of time, put the device in sleep mode by holding the button down for at least three seconds. The red LED will stop flashing to show that the unit is in sleep mode. Over several months, the battery will discharge but will not be damaged. After such storage, charge the device for a few hours, and the unit will be ready for use.
Exposing the sensor to temperatures below –15°C or over 45°C will damage the sensor. Additionally, temperatures over 35°C (95°F) will reduce the battery's lifespan. If possible, store the device in an area that is not exposed to temperature extremes.
Important: Go Direct Polarimeter is neither waterproof, nor is it water resistant and should never be immersed in water.
If any liquid gets into the device, immediately power the unit down (press and hold the power button for more than three seconds). Disconnect the sensor and charging cable, and remove the battery. Allow the device to dry thoroughly before attempting to use the device again. Do not attempt to dry using an external heat source.
Note: Damage due to liquids is not covered under warranty.