Mass spectrometry and hyphenated methods

A customer of Kingfisher Bioenergy reports a faint off-odor in a shipment of fuel ethanol. The gas chromatograph shows a small unknown peak between isobutanol and isoamyl alcohol, at a retention time that matches nothing in the lab's standards. Retention time can suggest an identity, but it cannot prove one. To identify the compound, the lab sends the sample to its gas chromatograph-mass spectrometer, which records a mass spectrum of every peak as it leaves the column. Within an hour, a library search names the unknown, and the spectrum's isotope pattern and fragments confirm it. Mass spectrometry is the most powerful identification tool in the analytical lab, and coupled to chromatography it is also one of the most sensitive ways to measure trace compounds.

What a mass spectrometer does

A mass spectrometer does three things. It turns molecules into gas-phase ions, separates the ions according to their mass-to-charge ratio, m/z, and counts them. The output is a mass spectrum, a plot of ion abundance against m/z. Most ions carry a single charge, so m/z is usually the ion's mass in daltons (atomic mass units). Because separation requires ions to travel without colliding with gas molecules, the analyzer operates under high vacuum, about \(10^{-5}\) Pa or lower.

Ionization

Electron ionization (EI) is the classic method for gas chromatography. Molecules leaving the column pass through a beam of electrons accelerated to 70 eV, which knock an electron out of each molecule to form a radical cation, the molecular ion \(M^{+\bullet}\). The 70 eV electrons deliver far more energy than is needed, so many molecular ions break apart into fragment ions in patterns that are characteristic and highly reproducible. Because every instrument uses 70 eV, EI spectra can be compared with large reference libraries containing hundreds of thousands of compounds. The drawback is that for some compounds the molecular ion fragments so completely that it barely appears.

Chemical ionization uses a reagent gas such as methane or ammonia, which is ionized first and then transfers a proton to the analyte gently, giving a strong \([M+H]^+\) ion and little fragmentation, which helps confirm molecular mass. For liquid chromatography, electrospray ionization (ESI) is dominant. The column effluent is sprayed from a fine needle held at several kilovolts, forming charged droplets that shrink as solvent evaporates until ions are released into the gas phase. ESI is gentle enough to ionize sugars, mycotoxins, proteins and other fragile molecules, usually as \([M+H]^+\) or \([M-H]^-\), and it can put many charges on large molecules such as proteins.

Mass analyzers

Resolving power describes how well an analyzer separates ions of nearly equal mass. A quadrupole has unit resolution, distinguishing m/z 46 from 47 but not 46.04 from 46.07. High-resolution instruments such as TOF and orbital traps can measure m/z to a few parts per million, which is enough to tell which elemental formula an ion has. Carbon monoxide and nitrogen both have a nominal mass of 28, but their exact masses are 27.9949 and 28.0061, easily distinguished at high resolution.

Reading a mass spectrum

The most intense peak in a spectrum is the base peak, set to 100 percent, and other peaks are scaled to it. The molecular ion, when present, gives the molar mass. Fragments reveal structure, because bonds break in predictable ways. Alcohols lose a hydrogen or alkyl group next to the oxygen to form stable oxygen-containing cations: ethanol (molar mass 46) shows a small molecular ion at m/z 46, a peak at 45 from loss of a hydrogen atom, and a base peak at 31 from \(\text{CH}_2\text{OH}^+\), formed when the methyl group breaks off. Ethyl acetate (88) shows a weak molecular ion at 88, fragments at 70 and 61, and a base peak at 43 from the acetyl ion \(\text{CH}_3\textKingfisher Bioenergy^+\). Losses of 15 (methyl), 18 (water) and 28 (ethene or carbon monoxide) are common and worth recognizing.

The nitrogen rule is a quick check: a molecule with an odd number of nitrogen atoms has an odd molar mass, and one with zero or an even number of nitrogen atoms has an even molar mass. A molecular ion at m/z 59 therefore suggests one nitrogen.

Isotope patterns

Most elements have more than one stable isotope, and their natural abundances leave fingerprints in mass spectra. About 1.07 percent of carbon is carbon-13, so a molecule with n carbon atoms has an M+1 peak of roughly \(1.1n\) percent of the molecular ion. That lets the analyst estimate the number of carbons: an M+1 peak of 8.7 percent suggests about eight carbons. Chlorine and bromine are even more distinctive. Chlorine is about 76 percent chlorine-35 and 24 percent chlorine-37, so one chlorine gives an M+2 peak about one-third the height of M; bromine's two isotopes are nearly equal, so one bromine gives M and M+2 peaks of almost equal height. Two or three halogens give patterns predicted by the binomial expansion.

Measuring with mass spectrometry

Identification is only half the job. Once the lab knows the off-odor compound is propyl acetate, the customer wants to know how much is present, and the plant wants to know whether other shipments contain it. GC-MS quantitation works like any other chromatographic calibration, with standards and usually an internal standard, but it measures the area of one characteristic ion rather than the whole peak. A quantifier ion, chosen for its intensity and freedom from interference, gives the number. One or two qualifier ions confirm identity: their ratio to the quantifier must match the ratio in a pure standard within a set tolerance, typically 20 percent, or the result is flagged.

Mass spectrometers need regular care. The ion source gradually becomes coated with sample residues, which lowers sensitivity, and the instrument is tuned against a reference compound to check mass calibration and ion ratios before each sequence. The vacuum pumps, the GC column's bleed and the carrier gas purity all affect the background. Because a mass spectrometer is expensive and needs skilled operators, many plants keep only GC and HPLC instruments in-house and send identification work to a contract laboratory. Kingfisher Bioenergy bought its GC-MS after a year of recurring odor complaints made the contract lab's turnaround too slow.

Exact mass

High-resolution instruments report m/z to four or more decimal places. Comparing the measured mass with the theoretical exact mass of a proposed formula, calculated from the monoisotopic masses of the most abundant isotopes, gives the mass error in parts per million:

An error within about 5 ppm, together with a sensible isotope pattern, usually narrows a small molecule down to one or a few elemental formulas. The program below calculates carbon-13 M+1 ratios for three compounds found in fuel ethanol, the chlorine isotope patterns in the figure, and the mass error for protonated ethanol measured on a high-resolution instrument.

# Mass spectrometry: isotope patterns, exact mass error and fragment checks
from math import comb
# 1. M+1 peak from carbon-13 (natural abundance about 1.07 percent)
p13 = 0.0107
for name, nC in [('ethanol C2H6O', 2), ('isoamyl alcohol C5H12O', 5), ('ethyl hexanoate C8H16O2', 8)]:
    ratio = nC * p13 / (1 - p13) * 100
    print(f"{name:26s} M+1 / M = {ratio:.1f} %")

# 2. Chlorine pattern: Cl-35 75.8 %, Cl-37 24.2 %
def cl_pattern(n):
    p, q = 0.758, 0.242
    peaks = [comb(n, k) * p ** (n - k) * q ** k for k in range(n + 1)]
    top = max(peaks)
    return [round(x / top * 100) for x in peaks]
for n in (1, 2, 3):
    print(f"{n} Cl: M, M+2, ... = {cl_pattern(n)}")

# 3. Exact mass and ppm error for a high-resolution measurement
mono = {'C': 12.000000, 'H': 1.007825, 'O': 15.994915, 'N': 14.003074}
def mass(f): return sum(mono[e] * k for e, k in f.items())
ion = mass({'C': 2, 'H': 6, 'O': 1}) + 1.007276          # protonated ethanol [M+H]+
measured = 47.04913
print(f"theoretical [M+H]+ = {ion:.5f}, error = {(measured - ion) / ion * 1e6:.1f} ppm")

ethanol C2H6O              M+1 / M = 2.2 %
isoamyl alcohol C5H12O     M+1 / M = 5.4 %
ethyl hexanoate C8H16O2    M+1 / M = 8.7 %
1 Cl: M, M+2, ... = [100, 32]
2 Cl: M, M+2, ... = [100, 64, 10]
3 Cl: M, M+2, ... = [100, 96, 31, 3]
theoretical [M+H]+ = 47.04914, error = -0.2 ppm

Hyphenated methods

Coupling a separation to a spectroscopic detector creates a hyphenated method, and the combination is far more powerful than either alone. In gas chromatography-mass spectrometry (GC-MS), the column effluent flows directly into the ion source, and the instrument records a full spectrum several times per second. The total ion chromatogram looks like an ordinary chromatogram, but every point carries a spectrum, so each peak can be identified by library search. When the target compounds are known, the instrument can record only a few characteristic ions (selected ion monitoring), which raises sensitivity a hundredfold by spending all its time on the ions that matter.

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) on a triple quadrupole instrument is the standard for trace quantitation in complex samples. The first quadrupole selects the analyte's parent ion, the collision cell breaks it into fragments, and the third quadrupole passes only a chosen fragment. This multiple reaction monitoring is so selective that compounds can be measured at micrograms per kilogram in feed, food and environmental samples with little cleanup. It is how reference labs measure a dozen mycotoxins in distillers grains in one run, using isotopically labeled internal standards, versions of each toxin made with carbon-13, which behave identically in the column and ion source but appear at a different mass.

Other hyphenated methods include GC with infrared detection, LC with diode-array UV detection, which records a UV spectrum for every peak to check purity, and the ICP-MS coupled to liquid chromatography used to measure the different chemical forms of arsenic or selenium. In each case, one technique separates and the other identifies.