AP Chemistry: Mass Spectrometry and Isotopes

AP Chemistry – Mass Spectrometry and Isotopes

Mass Spectrometry and Isotopes – AP Chemistry Study Guide

What This Is

Mass spectrometry (MS) is a lab technique that measures the mass-to-charge ratio (m/z) of ions to identify isotopes, determine atomic/molecular masses, and analyze chemical composition. On the AP exam, you’ll need to interpret mass spectra, calculate average atomic masses, and explain isotope abundance. Real-world example: Forensic scientists use MS to detect drugs or toxins in crime scenes, while archaeologists use it to date ancient artifacts (e.g., carbon-14 dating in Ötzi the Iceman).

Key Terms & Concepts

• Isotopes: Atoms of the same element with different numbers of neutrons (same protons, different mass numbers). Example: Carbon-12 (6 neutrons) vs. Carbon-14 (8 neutrons).
• Mass number (A): Total protons + neutrons in an atom. Written as a superscript (e.g., ¹²C).
• Atomic mass unit (amu): 1 amu = 1/12 the mass of a ¹²C atom (≈ 1.66 × 10⁻²⁴ g).
• Average atomic mass: Weighted average of all naturally occurring isotopes of an element. Formula: Average atomic mass = Σ (fractional abundance × isotopic mass)
• Mass spectrometer: Instrument that ionizes atoms/molecules, accelerates them in an electric field, deflects them in a magnetic field, and detects their m/z ratio.
• Mass spectrum: Graph of relative abundance (y-axis) vs. m/z ratio (x-axis). Peaks correspond to isotopes or fragments.
• Base peak: The tallest peak in a mass spectrum (most abundant ion).
• Molecular ion (M⁺): Peak representing the intact molecule (highest m/z in simple spectra).
• Fragmentation: Breaking of molecules into smaller ions during ionization (creates multiple peaks).
• Percent abundance: The natural occurrence of an isotope (e.g., ⁷⁹Br is ~50.7% of all bromine).
• Relative intensity: Height of a peak compared to the base peak (often set to 100%).

Step-by-Step: Interpreting a Mass Spectrum

1. Identify the molecular ion (M⁺):
2. Look for the highest m/z peak (not always the tallest). This is the mass of the intact molecule.

3. Example: For methane (CH₄), M⁺ = 16 amu.

4. List all peaks and their m/z values:

5. Note the m/z and relative intensity of each peak.

6. Example: Chlorine (Cl₂) has peaks at 70, 72, and 74 amu (from ³⁵Cl-³⁵Cl, ³⁵Cl-³⁷Cl, and ³⁷Cl-³⁷Cl).

7. Determine isotope patterns:

8. For elements with multiple isotopes (e.g., Cl, Br), peaks will form a characteristic pattern.

9. Example: Bromine (⁷⁹Br and ⁸¹Br) shows two equal-height peaks 2 amu apart.

10. Calculate average atomic mass (if given % abundance):

11. Multiply each isotopic mass by its fractional abundance and sum.

12. Example: Copper has 69.15% ⁶³Cu (62.93 amu) and 30.85% ⁶⁵Cu (64.93 amu).
    Average mass = (0.6915 × 62.93) + (0.3085 × 64.93) = 63.55 amu

13. Explain fragmentation (for molecules):

14. Smaller peaks = broken pieces of the molecule.
15. Example: Ethanol (CH₃CH₂OH) may fragment into CH₃⁺ (15 amu) or CH₂OH⁺ (31 amu).

Common Mistakes

• Mistake: Confusing mass number (protons + neutrons) with atomic mass (weighted average).
  Correction: Mass number is an integer (e.g., ¹²C = 12), while atomic mass is a decimal (e.g., carbon = 12.01 amu).

• Mistake: Assuming the tallest peak is always the molecular ion.
  Correction: The molecular ion is the highest m/z peak, but it may not be the tallest (e.g., in alcohols, the M⁺ peak is often weak).

• Mistake: Forgetting that diatomic elements (e.g., Cl₂, Br₂) show multiple peaks due to isotope combinations.
  Correction: For Cl₂, peaks at 70 (³⁵Cl-³⁵Cl), 72 (³⁵Cl-³⁷Cl), and 74 (³⁷Cl-³⁷Cl) appear in a 1:2:1 ratio.

• Mistake: Miscalculating average atomic mass by using percentages directly (e.g., 69.15% → 69.15 instead of 0.6915).
  Correction: Always convert percentages to decimals (divide by 100) before multiplying.

• Mistake: Ignoring fragmentation when interpreting spectra.
  Correction: Smaller peaks often represent broken pieces of the molecule (e.g., loss of H₂O or CH₃ groups).

AP Exam Insights

• FRQ Hotspot: You’ll likely get a mass spectrum and be asked to:
• Identify the molecular ion or isotopes.
• Calculate average atomic mass from % abundance.
• Explain fragmentation patterns (e.g., "Why is there a peak at 15 amu in methane’s spectrum?" → CH₃⁺ fragment).
• Multiple-Choice Traps:
• Isotope vs. ion: Isotopes differ in neutrons; ions differ in electrons.
• Peak height vs. m/z: The tallest peak isn’t always the molecular ion (e.g., in alcohols, the M⁺ peak is often small).
• Diatomic elements: Cl₂, Br₂, and I₂ show multiple peaks due to isotope combinations (e.g., Br₂ has 3 peaks in a 1:2:1 ratio).
• Tricky Distinction: Mass spectrometry measures mass/charge (m/z), not just mass. If an ion has a +2 charge, its m/z is half its actual mass.

Quick Check Questions

1. Multiple Choice: A mass spectrum of chlorine gas (Cl₂) shows peaks at 70, 72, and 74 amu. What is the ratio of their relative intensities?
   A) 1:1:1
   B) 1:2:1
   C) 3:1:3
   D) 9:6:1
   Answer: B) 1:2:1
   Explanation: Chlorine has two isotopes (³⁵Cl and ³⁷Cl) in a ~3:1 ratio, leading to Cl₂ peaks in a 1:2:1 ratio (³⁵Cl-³⁵Cl : ³⁵Cl-³⁷Cl : ³⁷Cl-³⁷Cl).

2. Short FRQ: The mass spectrum of an element shows two peaks at 63 amu and 65 amu with relative intensities of 69.15% and 30.85%, respectively. Calculate the average atomic mass of the element.
   Answer: 63.55 amu
   Explanation: (0.6915 × 63) + (0.3085 × 65) = 63.55 amu.

3. Multiple Choice: In the mass spectrum of ethanol (CH₃CH₂OH), a peak appears at 31 amu. Which fragment is most likely responsible?
   A) CH₃⁺
   B) CH₂OH⁺
   C) C₂H₅⁺
   D) OH⁺
   Answer: B) CH₂OH⁺
   Explanation: CH₂OH⁺ has a mass of 31 amu (12 + 2 + 16 + 1).

Last-Minute Cram Sheet

1. Isotopes = same protons, different neutrons (e.g., ¹²C vs. ¹⁴C).
2. Mass number (A) = protons + neutrons (integer).
3. Average atomic mass = Σ (fractional abundance × isotopic mass).
4. Mass spectrum: x-axis = m/z, y-axis = relative abundance.
5. Molecular ion (M⁺) = highest m/z peak (not always tallest).
6. Base peak = tallest peak (most abundant ion).
7. Diatomic elements (Cl₂, Br₂) show multiple peaks due to isotope combos.
8. Fragmentation = molecule breaks into smaller ions (e.g., CH₃⁺ from CH₄).
9. ⚠️ Charge matters! m/z = mass/charge (e.g., +2 ion → m/z = mass/2).
10. ⚠️ Convert % to decimals for average atomic mass calculations.