Student’s Guide
Maple Syrup
Mass spectrometry analysis
Olivier Tardif-Paradis
Mathieu Riopel
Cégep Garneau
Maple Syrup
Background
In Québec, the return of the warm weather at the end of winter also signals sugaring off time. The maple syrup produced in our good old sugar shacks is one of our most treasured traditions. And in fact, about 75% of all the maple syrup consumed in the entire world is produced right here in Québec.
But with the high demand and drive for expansion and quick profits, some maple culturists sometimes dilute their maple syrup with other, less expensive sugar syrups, like corn syrup. This adulteration process is illegal, since it misleads the consumer and hurts the maple culture industry. And yet this alteration of our national syrup is hard to detect, since it generally changes neither the flavour nor the colour of the syrup. Our maple syrup is therefore subject to frequent controls to certify its quality.
One of the most common techniques for checking the authenticity of syrup is to analyse the isotopic ratios using mass spectrometry. In brief, this technique measures the relative concentrations of different carbon isotopes in various substances. Since corn syrup has slightly less 13C than maple syrup, its presence is detected when a maple syrup sample has a 13C ratio that is lower than normal. This isotopic signature specific to maple syrup can only be measured by highly sensitive devices, though, such as mass spectrometers.
Your role is to analyse a maple syrup sample using mass spectrometry. To do this, you will have to determine the different settings to use on the spectrometer and carry out an appropriate analysis of the data collected. A detailed description of the spectrometer will be given to you, along with a basic spectrum of the sample to analyse.
Isotopic ratios – standards and definitions
The isotopic signature of a substance is established using the concentration ratios of certain stable isotopes found in the substance. For example, the standardratio between the isotopes 13C and 12C is
which means that the quantity of 13C atoms generally found in a substance is far lower than the quantity of 12C atoms. Different substances have an isotopic ratio for carbon that is slightly different. For example, a given plant might have an isotopic ratio of
or slightly higher than the standard. The isotopic signature of this substance is then calculated from its isotopic ratio and the standard ratio, using the following formula:
The isotopic signature is therefore basically a deviation expressed in per mil (‰). For the example cited above, the result is δ13 C = –30.
Pure maple syrup has an isotopic signature of δ13 C = –23.81 whereas sugar syrup has a ratio of .[1]
Three-step cycle
List all the relevant information you gathered when you read the problem. Based on this information, state what you need to know to solve the problem. As you discover new information, you should summarize and update the relevant information you have gathered and ask new questions.
List the following:
What we know / To determine / SummaryLe spectromètre de masse
Here is a scale diagram of the mass spectrometer that will be used for the analysis.
Technical detail
Ionization and acceleration
The sample is fed into the spectrometer as a gas. An electron cannon ionizes the atoms, which acquire a positive charge (q = +e). These ions are then accelerated and directed toward the velocity selector.
For this problem, your analysis will concentrate on the velocity selector and the magnetic deflector. We will assume therefore that the ionization and acceleration phase go normally and generate accelerated, positively charged ions that can be analysed by the rest of the spectrometer.
Furthermore, we will assume that the spectrometer will be used to analyse atoms produced in the ionization phase. This is a simplification, because in reality, it is the sugar molecules in the maple syrup sample that are analysed, and they contain several atoms.
Velocity selector
The velocity selector is in a uniform fixed magnetic field of 300 G directed outward. A potential difference, , can be applied between the parallel plates of the velocity selector. This potential difference is varied during the analysis. Only the ions that maintain a rectilinear trajectory (straight) in the velocity selector can reach the magnetic deflector.
Magnetic deflector
The magnetic deflector is in a uniform fixed magnetic field of 300G directed outward. The ions are deviated by 90º from their original direction. Only the electrons on the central trajectory in the deflector can reach the detector due to the collimators at the end of the deflector.
Detector
The ions reach the detector, an electron multiplier, and generate a current proportional to their number. It is the intensity of this current based on the potential difference, , that is used to develop the mass spectrum
Low-resolution spectrum
An initial analysis of the sample was carried out to check that the spectrometer is functioning properly. It shows the signal measured in terms of . The resolution is weak and is unable to differentiate isotopes of the same element.
Use this graph to help you determine the settings required to analyse the carbon isotopic ratios.
Questions
Calculate the range of voltages, ∆Vs, required for an accurate analysis of the carbon-12 and -13 isotopes.
1)Do these values correspond to one of the peaks in the low-resolution graph?
2)Which elements correspond to the other peaks?
Once you have accurately determined the appropriate range of voltages, ∆Vs, give these values to your teacher, who will give you the high-resolution carbon analysis.
PBL/Student’s Guide1
PBL/Student’s Guide1
High-resolution spectrum
Based on the high-resolution graph, determine the isotopic ratio δ13 C of carbon-13. According to these measurements, does the quality of the maple syrup analysed meet the regulations?
Note: The quantity of ions detected for a specific mass is proportional to the area under the curve of the corresponding peak.
APP/Guide de l’élève1
[1]A. Hope Jaren et al. An isotopic method for quantifying sweeteners derived from corn and sugar cane. The American Journal of Clinical Nutrition 84(2006): 1380.