Ndung’u et al. 2004 — Lead in vinegar by ICP-MS and GFAAS: sample preparation evaluation
This study measured lead concentrations in 59 commercial vinegars purchased from retail stores in California using both ICP-MS and graphite furnace atomic absorption spectrometry (GFAAS), with the primary objective of identifying the most reliable sample preparation protocol. Direct analysis without digestion produced erroneously high and poorly reproducible results for both instruments due to organic matrix interferences; nitric acid digestion with UV photolysis was identified as the optimal preparation method, producing excellent agreement between ICP-MS and GFAAS (R = 0.997). The study confirmed that balsamic vinegars — particularly traditionally aged varieties — carry substantially higher lead concentrations than other vinegar types, with the highest single sample (307 µg/L, traditionally barrel-aged) far exceeding the mean, implicating production and storage processes as the primary contamination pathway.
Key numbers
Balsamic vinegar (n=52): range 14.9–307 µg/L, mean 68 ± 56 µg/L, reported as µg/L (equivalent to µg/kg for aqueous matrices, i.e. ppb).
Sub-grouped balsamic results (Table 4):
- Balsamic group 1 (n=36, younger): range 15–68 µg/L, mean 44 µg/L
- Balsamic group 2 (n=12, mid-age): range 73–110 µg/L, mean 85 µg/L
- Balsamic group 3 (n=4, older/traditional): range 174–179 µg/L, mean 173 µg/L
Single-sample extremes:
- Highest balsamic: 307 ± 19.5 µg/L (traditionally barrel-aged, reportedly 25-year process)
- Second highest balsamic: ~257 µg/L (aged 18 years)
Wine vinegar (n=4): range 36–62 µg/L, mean 50 µg/L Garlic vinegar (n=1): 276 µg/L Apple cider vinegar (n=1): 15 µg/L Rice vinegar (n=1): 6.6 µg/L
All vinegars combined (n=59): range 7–307 µg/L, mean 64 ± 54 µg/L
Method detection limit (GFAAS after acid/heat digestion): 0.12 µg/L. Spike recovery: 96 ± 5% (n=6). NIST 1640 SRM recovery: 97.4 ± 1.3%. RSD for duplicates: <8%.
Methods (brief)
ICP-MS: ThermoFinnigan Element magnetic sector high-resolution ICP-MS, low resolution (r=300), 209Bi internal standard. GFAAS: Perkin-Elmer SIMAA 6000 with Zeeman background correction, Mg(NO3)2/NH4H2PO4 chemical modifier, optimized atomization at 600°C ash/1300°C atomize (lower than manufacturer defaults, necessary due to organolead volatility). Sample preparation compared: (1) simple aqueous dilution (poor precision, high bias); (2) nitric acid heating block digestion (effective, clean); (3) nitric acid + H2O2 UV photolysis (fastest, most complete, but H2O2 reagent blank Pb ~15 µg/L limits use for low-concentration samples). Recommended protocol: nitric acid digestion with UV photolysis, without H2O2 for samples <50 µg/L Pb. Samples from California retail (glass, plastic, ceramic bottles).
Key finding on organolead: GFAAS showed a sharp absorbance drop between 700–800°C in vinegar digests not seen in SRM, indicating the presence of relatively volatile organolead compounds; this required lowering the ashing temperature from the manufacturer’s recommended 800°C to 600°C.
Implications
Certification: Balsamic vinegar should be treated as a high-lead-risk matrix distinct from other vinegar types. Traditional barrel-aged balsamic presents lead concentrations (up to 307 µg/L) that may be relevant to product ingredient risk. Production and storage (wood barrels, metal fittings) are implicated as the primary lead source, not agricultural inputs. This is methodologically important: ICP-MS and GFAAS methods require acid digestion with temperature optimization for vinegar matrices.
Courses: Illustrates that direct liquid injection is unreliable for vinegar metal analysis; acid digestion with UV photolysis is the validated approach. The organolead volatility issue is a noteworthy pitfall in method validation for this matrix.
App: Lead contamination in vinegar products varies by type (balsamic > wine > garlic > apple cider/rice); the range across all types is wide (7–307 µg/L). The ingredient contamination profile for balsamic-vinegar should reflect a much higher lead burden than generic vinegar.