Best Practices with the MZ5 Mid-IR Spectrometer
All scientists know that in practice, unknown factors can affect observations negatively. For that reason, employing best practices with every experiment is integral, even when you think “in theory” such diligence should not be necessary. In a recent feasibility test for determining ethanol concentrations in gasoline using the Ocean Optics MZ5 spectrometer, unsound experimental practices led to unexpected results and unusable data. This article will help demonstrate that a spectrometer is only as good as the experimental techniques used.
In the initial feasibility test for ethanol concentration detection in gasoline, a gradient of ethanol and the hydrocarbon iso-octane were created from pure ethanol to pure iso-octane “gasoline.” Concentrations were then evaluated with the MZ5 spectrometer. The Ocean MZ5 is a miniature ATR spectrometer with measurement capabilities from 1818-909 cm−1 (5.5-11 μm). This fully self-contained instrument -- including sample interface, light source and detector -- provides a repeatable, compact, fast and scalable alternative to traditional FTIR spectroscopy.
The following mixtures were made according to the volume of ethanol:
Each concentration was sealed in a vial and agitated prior to application to the MZ5 ATR crystal with a pipette. Standard device settings were used on the MZ5: 8 Hz pulse rate, 100% light intensity, 40 warm-up scans, then 5 measurements of 400 average scans for each concentration. Each measurement of 400 scans takes about 50 seconds. Since ethanol and iso-octane are volatile substances (likely to evaporate), ample amounts of sample (3 ml) were placed on the crystal to prevent full evaporation during the 5 minutes of measurements.
After normalizing the spectrum of each concentration, the repeatability was not as expected. To demonstrate this, Figure 1 is a plot of the normalized MIR spectrum of ethanol and iso-octane; Figure 2 shows the first measurement subtracted from the measurement of each concentration. The inverse shape of the iso-octane spectrum appears as negative absorption values, and worsens as the concentration of iso-octane increases. Meaning simply that there is physically less iso-octane in the last measurement than in the first, with the issue worsening as the concentration of iso-octane increases.
Figure 1. Plots of the normalized mid-IR spectrum for ethanol and iso-octane.
Figure 2. Results of the first measurement subtracted from the concentration measurement of each mixture.
Figure 3. Placing the crystal protector in place after each application of sample minimizes evaporation effects.
Having compared for each concentration the boiling point, vapor pressure and heat of vaporization (factors that help establish volatility1) and finding them similar, this result was not expected. The volatility of molecular mixtures of these same components is being studied to optimize car engines and help minimize greenhouse gases2, so it is more complex than we can cover here. What is clear is the data from the first experiment could not be used to determine a calibration curve with any certainty.
To test for evaporation effects, the experiment was repeated with the crystal protector moved into place immediately after application of each concentration (Figure 3). If you compare the two plots (with and without crystal protector) on the same scale it is clear the evaporation effects have been eliminated (Figures 4-5).
Figure 4. Without the crystal cover in place, evaporation effects lead to inaccurate absorbance measurements.
Figure 5. With the crystal cover in place, evaporation effects are virtually eliminated.
Any measurement tool is only as good as the techniques used; spectrometers are no exception. Ocean Optics highly recommends you use a barrier to avoid evaporation effects while taking measurements with the MZ5 spectrometer, even if you do not think your sample is volatile. Take care to not overfill the sample or get liquids on the crystal cover. There is less of a concern for highly polar liquids such as aqueous mixtures and liquids with large molecules like oils, but it is better to be safe than sorry. But don’t worry, the MZ5 miniature ATR spectrometer has you covered.
- “Volatility (Chemistry).” Wikipedia, Wikimedia Foundation, 15 Jan. 2019, en.wikipedia.org/wiki/Volatility_(chemistry).
- Corsetti, Stella, et al. “Probing the Evaporation Dynamics of Ethanol/Gasoline Biofuel Blends Using Single Droplet Manipulation Techniques.” The Journal of Physical Chemistry A, vol. 119, no. 51, 2015, pp. 12797–12804., doi:10.1021/acs.jpca.5b10098.
- “2,2,4-Trimethylpentane.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine,15 Jan. 2019, pubchem.ncbi.nlm.nih.gov/compound/2_2_4-Trimethylpentane. https://pubchem.ncbi.nlm.nih.gov/compound/2_2_4-Trimethylpentane#section=Top
- “Chegg.com.” Lifetime Physical Fitness and Wellness: A Personalized Program 12th Edition | Rent 9781111990015 | Chegg.com, CENGAGE Learning, www.chegg.com/homework-help/table-b2-appendix-b-provides-parameters-computing-vapor-pres-chapter-1-problem-3p-solution-9781259696527-exc.
For more information on using spectroscopy for determine concentrations of constituents please see: