Mass Spectrometry
This part of the mass spectrometer is responsible for turning molecules into charged particles.
Ionization source
The x-axis of an IR spectrum is usually labeled in these units.
wavenumbers (cm⁻¹)
The “signal” in signal-to-noise ratio represents this part of the measurement.
analyte response or desired peak
If a molecule with a molar mass of 44 g/mol produces a parent ion with a charge of +1, what is its m/z value?
44
A compound is analyzed using both FT-IR and mass spectrometry.
The FT-IR spectrum shows a strong absorption at 1715 cm⁻¹ and a broad signal near 2500–3300 cm⁻¹.
In the mass spectrum, the molecular ion peak appears at m/z = 60 with a base peak at m/z = 43.
The signal intensity for the molecular ion is 10 units and the noise level is 0.25 units.
Based on these data:
Identify the most likely compound.
Calculate the signal-to-noise ratio for the molecular ion peak.
Explain how doubling the number of scans would affect the S/N ratio.
Likely compound: Acetic acid (CH₃COOH)
IR 1715 cm⁻¹ → C=O stretch (carboxylic acid)
Broad 2500–3300 cm⁻¹ → O–H stretch (acidic OH)
m/z 60 → molecular ion (M⁺ = 60)
m/z 43 → CH₃CO⁺ fragment
S/N ratio = 10 / 0.25 = 40:1
Doubling the number of scans improves S/N by √2 → new S/N ≈ 56.6:1
The x-axis of a mass spectrum represents this value, which determines how ions are separated.
mass-to-charge ratio (m/z)
This region of the spectrum (4000–1500 cm⁻¹) is used to identify functional groups.
functional group region
Increasing this number of scans can improve the signal-to-noise ratio.
The number of accumulations or measurements
A peak is observed at m/z 88 and is doubly charged. What is the molecular weight of the compound?
176 (m/z × charge = 88 × 2)
In an electron impact (EI) spectrum, the peak with the highest m/z value corresponds to this ion.
molecular ion (M⁺)
A broad absorption around 3200–3600 cm⁻¹ typically indicates this functional group.
An –OH group?
The signal-to-noise ratio increases by the square root of this factor.
The number of scans (√N rule)
The C=O stretch in an IR spectrum appears at 1720 cm⁻¹. What is the approximate wavelength (in µm)? (Use λ = 10⁴ / ν̅)
5.81 µm
This type of fragmentation results when a molecule breaks into two charged fragments.
homolytic cleavage
The “fingerprint region” lies below this wavenumber and is unique for every compound.
1500 cm⁻¹
Baseline fluctuations or random electronic disturbances are examples of this.
Noise
An FT-IR signal has a noise level of 0.5 and a peak intensity of 10. Calculate the signal-to-noise ratio.
20:1
This type of analyzer separates ions by oscillating them through a quadrupole field.
quadrupole mass analyzer
The “FT” in FT-IR refers to this mathematical process that converts time-domain data to frequency-domain data.
Fourier Transform
To double the signal-to-noise ratio, you must increase the number of scans by this factor.
four (4× the scans)
In a mass spectrometry run, the current signal-to-noise ratio is 5:1. How many times more scans are needed to increase S/N to 20:1?
16× more scans
(since (S₂/S₁)² = N₂/N₁ → (20/5)² = 16)