Mass Spectroscopy Primer, Part I: Introduction

Mass spectroscopy is used in cannabis analysis to determine pesticides, mold toxins, and potency. Mass spectrometers are the most expensive and complex to operate instruments found in cannabis labs. In this first installment of a series on mass spectroscopy (MS), I will discuss the basics of how a mass spectrometer works and what the data it generates mean.

Introduction to Mass Spectroscopy

Mass spectrometers take molecules, break them into ionized bits, sort them by their mass, count the number of ions of a given mass, and then present us the results in a plot of ion abundance versus mass (1).

My original field of expertise for over 30 years has been spectroscopy. I define spectroscopy as the study of the interaction of electromagnetic (EM) radiation, or light, with matter (2). EM radiation is composed of an electric wave and a magnetic wave undulating in mutually perpendicular directions. An important property that distinguishes different types of EM radiation from each other is their wavelength. Simply defined, wavelength is the distance between adjacent crests or adjacent troughs of a wave. There are different types of spectroscopies depending upon the wavelength or light that is used to examine samples as shown in Figure 1.

So, given the name of the field we are about to study, you may be wondering what wavelengths are used in this application. The answer is none of the above! MS does not use light at all, and why it was ever given the moniker spectroscopy is beyond me.

A block diagram of how a mass spectrometer works is seen in Figure 2.

In MS, the sample must be in the gas phase before it is introduced into the instrument. The molecules are ionized using one of several ionization methods in what is called an ion source. Recall that an ion is an atom or molecule with a charge on it. It is imperative that the molecular fragments be ionized or else the spectrometer will not be able to analyze them. The mass analyzer sorts the ions generated by mass/charge ratio or m/z. The mass analyzer can be set to only allow ions of a specific m/z to pass through, which it does typically with magnetic or electric fields. The number of ions of a specific m/z that pass through the instrument are counted by a detector. Measuring a mass spectrum then is a matter of sweeping the instrument through different m/z values, counting the ions collected, and then making a plot of the two quantities. Certain scientists, myself included, chide mas spectroscopists by saying that determining molecular structures by smashing molecules into pieces is like determining how an expensive watch works by hitting it with a hammer and looking at the pieces. There is some truth to this, but the amount of information we can obtain from a mass spectrum is remarkable. One can determine molecular structure, the functional groups present, molecular weight, and molecular formula. It is perhaps the most information-rich technique that we will discuss in this column series.

MS is very important in cannabis analysis. MS instruments are often interfaced with gas chromatographs giving us GC-MS, or liquid chromatographs (LC) giving us LC-MS. The beauty of GC-MS and LC-MS instruments, often called hyphenated techniques because of the hyphen in their names, is that they can take complex mixtures, purify them into their components, identify the molecules present, and then quantify them. GC-MS and LC-MS systems are widely used in cannabis labs to analyze for pesticides and toxins such as those present in mold. These instruments can also be used for potency analysis, but there are simpler and cheaper ways of doing this, such as by chromatography or infrared spectroscopy as I have discussed previously (3,4).

How Do We Make Ions?

In MS, we must first generate a beam of molecules so they can enter the instrument. This can be done by injecting the sample with a syringe, but much more commonly we take as the source the effluent from a gas chromatograph or liquid chromatograph. We then need to turn the beam of molecules into a beam of ions so they can traverse the instrument, be separated by mass, and be detected. If the pressure inside the instrument is relatively high – let’s say 1 atmosphere – the molecular or ion beam will collide with the gas phase molecules present inside the instrument, the beam will be deflected in random directions, and the molecules will never be ionized, or the ions will never be detected. This is why the interior of mass spectrometers are evacuated to a pressure of 1x10^-6 atmosphere or lower, so the molecular and ion beams have a free path from the sample introduction port and ion source to the detector. The pumps and seals needed to maintain this vacuum level add cost and complexity to the design and use of mass spectrometers.

As the field of MS has developed, there have been many innovations in how to generate a beam of ions. However, one of the first ionization techniques developed and perhaps an easy one to understand is called electron impact ionization. An illustration of an electron impact ionization source is seen in Figure 3.

At the top of the figure is a filament, simply a hot wire, which gives off electrons when heated to a high enough temperature. Note there is a north pole of a magnet above the filament and a south pole at the bottom of the ion source. This sets up a magnetic field causing the electrons to move from top to bottom as seen in the figure. In Figure 3, the molecular beam is moving from behind you and through the plane of the page as indicated by the grey circle. At the center of the source, the molecules and electrons collide generating ionized molecular fragments. You can think of the impact process like a chemical reaction illustrated below, where M = Molecule and e = Electron:

M + e^-=> M+ & 2 e^-

The repeller to the left of the source forces the ions to the right, while the extraction electrodes to the right pull the ions into the body of the instrument.

Mass Analysis

Now that we have generated our beam of ions, how do we sort them by mass? For now we will discuss the tried-and-true method of using a magnetic field to sort ions of different mass, otherwise known as magnetic sector mass spectroscopy. There are other ways to sort ions by their m/z values I will discuss in future columns. To understand how a magnetic sector instrument works we need to understand how a charged particle such as an ion behaves in the presence of a magnetic field. This is illustrated in Figure 4.

The magnetic field, with strength B, is pointing into the plane of the page as represented by the + sign, which is an arrow tail. The ion is moving towards the top of the page with velocity (v), and we will assume this particle has charge (q). An ion moving through a magnetic field will experience a force (F) as illustrated by the vector pointing to the right in Figure 4. The magnitude of this force is given by Equation 1, where F = force, q = charge, v = velocity, and B = magnetic field strength:

Thus, according to Equation 1, the force on an ion moving through a magnetic field depends upon the velocity of the ion, the charge on the ion, and the magnetic field strength.

An illustration of how a magnetic sector MS separates ions is seen in Figure 5.

In Equation 1, the velocity (v), depends upon the mass of the ion amongst other things. All other conditions being equal, a massive ion will move more slowly through a mass spectrometer than a light ion. Note the tube through which the ion beam passes is curved, and we will call this the ion tube. Note that for an ion to make it to the detector it must follow a path, or describe an arc, to be technical, where it follows the curvature of the ion tube. We would then say that the radius of curvature of the ion’s path through the ion tube must equal that of the tube itself for the ion to be detected. For a given magnetic field strength (B), ions with a large mass will move slowly and be deflected weakly and collide with the outside wall of the ion tube. Ions with a small mass that are moving quickly will be deflected strongly and collide with the inside wall of the ion tube. Only ions of a specific mass will have the correct radius of curvature to make it to the detector and be counted. Measuring a mass spectrum then with a magnetic sector instrument is a matter of scanning through different values of the magnetic field strength (B) and counting the number of ions of a given m/z that make it to the detector. The resultant mass spectrum is then a plot of ion abundance on the y-axis versus m/z on the x-axis.

A Mass Spectrum

An example of a simple mass spectrum, that of carbon dioxide, is seen in Figure 6.

Note that the x-axis is in “m/z” units, not simply mass and this is an important distinction. Technically, a mass spectrometer does not separate ions based on their mass but by their mass to charge ratio or m/z. It can happen, for example, that an ion with a mass of 100 and a charge of 1 and hence with an m/z of 100 will be detected at the same time as an ion with a mass of 200, a charge of 2, and hence also have m/z = 100. Note that the y-axis here is “% Relative Intensity.” If we plotted the raw signal this scale would be labeled “ion abundance” which is a direct count of the number of ions detected at a specific m/z. It is convenient though to divide the intensity of each individual peak by that of the intensity of the largest peak, in this case m/z = 44, multiply by 100, then plot the y-axis in % Relative Intensity units as seen.

CO₂ has a molecular weight of 44. Note in Figure 6, that the peak with the highest m/z, and also the most intense peak, has m/z = 44. This is called the molecular ion peak (M+ peak), or parent ion peak, which in this case is from a CO₂ molecule with a single positive charge on it. Molecular ion peaks are often seen in mass spectra and are very useful because they tell us the molecular weight of an analyte. Note that there are also peaks with m/z values of 12 and 18 in the spectrum. These are simply singly charged carbon and oxygen atoms, respectively. The peak at m/z = 28 is more interesting. Its peak position tells us this molecular fragment has a mass of 28. However, if we subtract its mass from that of the molecular ion, we get 44 – 28 = 16, and 16 is the atomic mass of oxygen. What this tells us is that this ion was formed when the parent ion, CO₂+ lost an oxygen atom. So, what is interesting about interpreting mass spectra is that the peak positions carry information, as does how far away a peak is from the parent ion peak. The upshot here is that mass spectra give us functional group information as we will see in future articles.

Conclusions

A mass spectrometer works by smashing molecules into ionized bits and then analyzes the resulting ions by mass/charge ratio. These instruments must be evacuated to low pressure to allow beams of molecules and ions to pass through them. A basic mass spectrometer consists of a sample injection port, an ion source, a mass analyzer, and detector. A common ionization method is electron impact ionization, and a common mass analyzer is a magnetic sector. A simple mass spectrum was introduced and we defined what the molecular ion is and how to interpret mass spectra using m/z values of ions, and how far they fall from the molecular ion peak.

References

1. Skoog, D.; and West, D., Fundamentals of Analytical Chemistry, Academic Press, 1975.
2. Smith, B.C., Fundamentals of Fourier Transform Infrared Spectroscopy 2nd Edition, CRC Press, 2011.
3. Giese, M.W.; Lewis, M.A.; Giese, L.; and Smith, K.M., Journal of AOAC International, 2015, 98(6)1503.
4. Smith, B.C.; and Fucetola, C., Distinguishing Hemp from Marijuana by Mid-Infrared Spectroscopy, Cannabis Science and Technology, 2020, 3(6), 24-38.

About the Columnist

Brian C. Smith, PhD, is Founder, CEO, and Chief Technical Officer of Big Sur Scientific. He is the inventor of the BSS series of patented mid-infrared based cannabis analyzers. Dr. Smith has done pioneering research and published numerous peer-reviewed papers on the application of mid-infrared spectroscopy to cannabis analysis, and sits on the editorial board of Cannabis Science and Technology. He has worked as a laboratory director for a cannabis extractor, as an analytical chemist for Waters Associates and PerkinElmer, and as an analytical instrument salesperson. He has more than 30 years of experience in chemical analysis and has written three books on the subject. Dr. Smith earned his PhD on physical chemistry from Dartmouth College. Direct correspondence to: brian@bigsurscientific.com.

How to Cite this Article

Smith, B., Mass Spectroscopy Primer, Part I: Introduction, Cannabis Science and Technology, 2025, 8(1), 6-9.