4.3 Spectroscopy and Elemental Analysis

3.2 Spectroscopy and Elemental Analysis

Elemental analysis of workplace aerosols, metal fumes, mineral dusts, and inorganic chemical vapors requires high-precision optical, atomic, and diffraction techniques. Industrial hygienists must know the operational mechanisms, detection capabilities, interferences, and sample preparation protocols for each spectroscopic modality.

---

1. Fundamental Principles of Optical Spectroscopy

Quantitative absorption spectroscopy measures the attenuation of incident electromagnetic radiation as it passes through an absorbing medium containing target atoms or molecules.

The Beer-Lambert Law

$A = \varepsilon \cdot b \cdot c = -\log_{10}(T) = -\log_{10}\left(\frac{I}{I_0}\right)$

Where:

• A = Absorbance (dimensionless optical density)
• ε = Molar absorptivity / extinction coefficient (L·mol⁻¹·cm⁻¹)
• b = Absorption path length (cm)
• c = Analyte concentration (mol/L or µg/mL)
• T = Transmittance (I / I0, fraction of light transmitted)
• I0 = Intensity of incident light
• I = Intensity of transmitted light

  Incident Light (I₀)                                Transmitted Light (I)
  ───────────────────> [ Sample Cuvette / Cell ] ───> (I < I₀)
                         Path Length (b), Conc (c)

Practical Limits of Beer's Law

While theoretically linear, standard calibration curves deviate from linearity at elevated concentrations (typically A > 1.5 to 2.0, corresponding to < 1% - 3% light transmission) due to:

1. Electrostatic interactions between absorbing species at close distances altering charge distributions.
2. Stray light within the monochromator striking the detector without traversing the sample.
3. Chemical equilibria shifts (e.g., pH-dependent association/dissociation of chromophores).
4. Polychromatic light effects when spectral bandwidth exceeds the absorption band half-width. Optimal precision occurs within an absorbance range of 0.1 ≤ A ≤ 1.0.

---

2. Atomic Absorption Spectroscopy (AAS)

Atomic Absorption Spectroscopy measures the absorption of resonance optical radiation by ground-state free atoms in the gas phase.

Radiation Sources

• Hollow Cathode Lamp (HCL): The cathode is fabricated from the specific element being analyzed (e.g., lead, cadmium). Ionized inert filler gas (neon or argon) sputters metal atoms from the cathode, producing sharp, element-specific atomic emission spectral lines.
• Electrodeless Discharge Lamp (EDL): Uses radiofrequency energy to excite volatile elements (e.g., As, Se, Sb), providing higher radiant intensity and longer operating life.

Atomization Systems: Flame vs. Graphite Furnace

Interferences and Background Correction in AAS

1. Chemical Interferences: Formation of thermally refractory salts (e.g., calcium phosphate) in flames. Prevented by adding releasing agents (such as lanthanum chloride, LaCl3) which bind preferentially to phosphate, or using a hotter nitrous oxide-acetylene (N2O/C2H2) flame.
2. Matrix / Non-Specific Absorption: Caused by light scattering from smoke particles and molecular absorption from unvaporized matrix components.
   • Deuterium (D2) Arc Lamp Background Correction: Alternates broad-band UV continuum light (D2) with narrow-line HCL light. Subtracts non-specific broad-band background (effective between 190 nm and 350 nm).
   • Zeeman Effect Background Correction: Applies a strong magnetic field (0.8 - 1.0 Tesla) across the graphite tube or light path to split atomic energy levels into polarized components (σ and π components). Corrects for complex, high-background structured matrices across the full spectral range.

---

3. Inductively Coupled Plasma (ICP) Techniques

Inductively Coupled Plasma techniques use an argon plasma maintained at core temperatures of 6,000 to 10,000 K (surpassing the surface temperature of the sun). This high-energy thermal environment completely atomizes and ionizes sample aerosols, eliminating chemical refractory interferences.

[Sample Solution] ──> [Nebulizer & Spray Chamber] ──> [Argon Plasma Torch (6,000-10,000 K)]
                                                              │
                     ┌────────────────────────────────────────┴────────────────────────────────────────┐
                     ▼                                                                                 ▼
      [ICP-OES Optical Spectrometer]                                                    [ICP-MS Vacuum Interface & Mass Filter]
  (Measures Emitted Photon Wavelengths & Intensity)                                        (Measures Ion m/z Ratios & Counts)
  • Multi-element (30+ metals simultaneously)                                           • Ultra-trace multi-element (ppt level)
  • Linear Dynamic Range: 10^5 - 10^6                                                   • Isotopic analysis (e.g., 206Pb/208Pb ratios)

ICP-OES / ICP-AES (NIOSH Method 7300 Series)

• Principle: Excited atoms and ions emit characteristic optical photons upon returning to ground or lower electronic states. Emitted light is separated by an Echelle polychromator and detected simultaneously on charge-coupled device (CCD) or charge-injection device (CID) arrays.
• Industrial Hygiene Strengths: Simultaneous determination of 30+ metals in a single run (e.g., welding fumes containing Fe, Mn, Cr, Ni, Cu, Zn, Al, Ti). Wide linear dynamic range (10⁵ - 10⁶) eliminates frequent sample re-dilutions.
• Detection Limit: 1 - 10 µg/L (sub-ppm to low ppb).

ICP-MS (NIOSH Method 7304 / 7306)

• Principle: Plasma-generated positive ions (M⁺) are extracted through water-cooled nickel/platinum cones (sampler and skimmer) into a high-vacuum quadrupole or magnetic sector mass spectrometer.
• Industrial Hygiene Strengths: Exceptional sensitivity (parts-per-trillion, ppt, or ng/L), simultaneous multi-element capability, and isotope ratio determination.
• Primary Applications: Ultra-low regulatory monitoring, such as beryllium in aerospace/nuclear facilities (OSHA PEL = 0.2 µg/m³, Action Level = 0.1 µg/m³), arsenic, cadmium, and toxicological biomonitoring.
• Interferences: Polyatomic isobaric overlaps (e.g., ⁴⁰Ar³⁵Cl⁺ interfering with ⁷⁵As⁺). Corrected using Collision/Reaction Cells (CRC) pressurized with helium or hydrogen to selectively attenuate polyatomic ions via kinetic energy discrimination.

---

4. UV-Visible Spectrophotometry & Colorimetric Assays

UV-Vis spectrophotometry remains critical for specific inorganic compounds and functional groups that can be converted into highly absorbing colored complexes.

Hexavalent Chromium (Cr(VI)) Analysis (NIOSH Method 7600 / OSHA ID-215)

1. Hazard: Hexavalent chromium (CrO4²⁻, Cr2O7²⁻) is a potent human lung carcinogen with an OSHA PEL of 5 µg/m³ and an Action Level of 2.5 µg/m³.
2. Stabilization & Extraction: Air samples are collected on 37-mm, 5.0 µm polyvinyl chloride (PVC) filters. Extraction is performed using an alkaline buffer (2% NaOH / 3% Na2CO3 at pH 11.5) with heating (95°C). This high pH is essential to prevent the interconversion of Cr(VI) to Cr(III) or vice versa.
3. Derivatization Reaction: The extract is acidified with sulfuric acid and reacted with 1,5-diphenylcarbazide (DPC). Cr(VI) oxidizes DPC to diphenylcarbazone while being reduced to Cr(III), forming an intense magenta/violet coordination complex.
4. Measurement: Absorbance is measured spectrophotometrically at λ = 540 nm against a matrix-matched calibration curve.

Cr(VI) [Alkaline Extract @ pH 11.5] + 1,5-Diphenylcarbazide (DPC) + H2SO4 ──> [Cr(III)-Diphenylcarbazone Complex] (Magenta, λ = 540 nm)

---

5. Infrared Spectroscopy (FTIR) & X-Ray Diffraction (XRD) for Silica

Respirable crystalline silica (SiO2) inhalation causes silicosis and lung cancer. OSHA's Action Level is 25 µg/m³ and PEL is 50 µg/m³ as an 8-hour TWA. Analyzing respirable quartz, cristobalite, and tridymite requires structural/mineralogical techniques because elemental silicon methods (like ICP) cannot distinguish crystalline silica from amorphous silica or silicate clays.

                                    [Respirable Cyclone Sample on PVC Filter]
                                                        │
                         ┌──────────────────────────────┴──────────────────────────────┐
                         ▼                                                             ▼
            [FTIR Analysis - NIOSH 7602]                                  [XRD Analysis - NIOSH 7500 / OSHA ID-142]
  • Ashed & pressed into KBr pellet or redeposited              • Redeposited onto Silver Membrane Filter
  • Monitors Si-O bending doublet @ 798 & 780 cm^-1             • Measures Bragg Diffraction Peak (Quartz @ 26.66° 2θ)
  • Lower instrumentation cost, rapid turnaround                • Distinguishes Quartz, Cristobalite, and Tridymite
  • Subject to silicate mineral interference (kaolinite)        • Silver peak (38.12° 2θ) used for matrix absorption correction

Fourier Transform Infrared Spectroscopy (FTIR) (NIOSH Method 7602 / 7603)

• Principle: Absorption of mid-infrared radiation (4000 - 400 cm⁻¹) excites molecular vibrational modes (stretching and bending) associated with a changing dipole moment.
• Quartz Identification: Alpha-quartz displays a distinctive doublet absorbance peak at 798 cm⁻¹ and 780 cm⁻¹ arising from symmetrical Si-O-Si stretching vibrations.
• Interferences: Clay minerals (kaolinite, mica, feldspar) have overlapping absorption bands near 800 cm⁻¹. Thermal pretreatment (heating to 600°C to dehydroxylate kaolinite) or spectral subtraction is required to resolve quartz from interferences.

X-Ray Diffraction (XRD) (NIOSH Method 7500 / OSHA ID-142)

• Principle (Bragg's Law): $n\lambda = 2d \sin\theta$ Where λ is the X-ray wavelength (typically Cu K(α) = 1.5418 Å), d is the crystal lattice interplanar spacing, and θ is the diffraction angle.
• Silica Polymorph Diffraction Angles (2θ for Cu K(α)):
  • α-Quartz (primary analytical peak): 2θ = 26.66° (d = 3.34 Å); secondary confirmation peak at 2θ = 20.85° (d = 4.26 Å).
  • Cristobalite: 2θ = 21.98° (d = 4.04 Å).
  • Tridymite: 2θ = 21.62° (d = 4.11 Å).
• Sample Preparation & Matrix Correction:
  • The PVC filter is dissolved in tetrahydrofuran (THF) or low-temperature oxygen plasma ashed, and the mineral residue is redeposited onto a 25-mm, 0.45 µm silver membrane filter.
  • The silver substrate produces an intense reference diffraction peak at 2θ = 38.12° (d = 2.36 Å). Attenuation of this silver peak by the deposited sample dust layer provides a direct mathematical correction factor for matrix X-ray absorption, preventing underestimation in heavy dust loadings.

---

6. Analytical Methods Summary Table

---

7. Worked Example: Hexavalent Chromium UV-Vis Quantification

Worked Problem: An industrial hygienist collects a personal air sample on a welder grinding stainless steel using a 37-mm, 5.0 µm PVC filter loaded in a cassette at a flow rate of 2.00 L/min for an 8.0-hour shift (480 min). The laboratory extracts the filter into 15.0 mL of alkaline solution, develops the 1,5-diphenylcarbazide complex, and analyzes the sample at λ = 540 nm with a path length of 1.00 cm.

Analytical Data:

• Sample Absorbance (Asample): 0.215
• Field Blank Absorbance (Ablank): 0.005
• Calibration Curve Slope (m): 0.0820 absorbance units per (µg/mL)
• Calibration Intercept (y0): 0.000

Tasks:

1. Calculate the concentration of Cr(VI) in the 15.0 mL extract (µg/mL).
2. Determine the total mass of Cr(VI) collected on the filter (µg).
3. Calculate the sampled air volume in cubic meters (m³).
4. Determine the 8-hour TWA exposure concentration in µg/m³ and evaluate compliance against the OSHA PEL (5.0 µg/m³) and Action Level (2.5 µg/m³).

Step-by-Step Solution:

Step 1: Determine Extract Concentration (c) $A_{net} = A_{sample} - A_{blank} = 0.215 - 0.005 = 0.210$ Using the linear calibration equation Anet = m · c + y0: $c = \frac{A_{net}}{m} = \frac{0.210}{0.0820 \text{ mL}/\mu\text{g}} = 2.561 \ \mu\text{g/mL}$

Step 2: Calculate Total Mass on Filter (M) $M = c \times V_{extract} = 2.561 \ \mu\text{g/mL} \times 15.0 \text{ mL} = 38.415 \ \mu\text{g}$

Step 3: Calculate Air Sampling Volume (Vair) $V_{air} = 2.00 \text{ L/min} \times 480 \text{ min} = 960 \text{ L} = 0.960 \text{ m}^3$

Step 4: Calculate 8-hour TWA Concentration (CTWA) $C_{TWA} = \frac{M}{V_{air}} = \frac{38.415 \ \mu\text{g}}{0.960 \text{ m}^3} = 40.02 \ \mu\text{g/m}^3$

Regulatory Assessment: The worker's 8-hour TWA exposure of 40.02 µg/m³ exceeds the OSHA Permissible Exposure Limit (5.0 µg/m³) by a factor of 8. Immediate engineering controls (local exhaust ventilation) and respiratory protection are required.