Assessing Analytical Methods for the Rapid Detection of Lead Adulteration in the Global Spice Market

Abstract

Lead adulteration of spices, primarily via Pb chromate compounds, has been documented globally as a growing public health concern. Currently, Pb detection in spices relies primarily on expensive and time-consuming laboratory analyses. Advancing rapid Pb detection methods, inclusive of their accuracy and precision, would improve field assessments by food safety inspectors, stakeholders, and the public in the hope of reducing Pb exposure risks at its source. Here, we present two field procedures for Pb detection: portable X-ray fluorescence analysis (pXRF) and a simple colorimetric test. We assess their efficacy to detect Pb and its chemical form in seven spice types, including powders, spice-salt mixtures, and dried roots, compared to the proven laboratory technique, inductively coupled plasma mass spectrometry (ICP-MS). Lead concentrations measured using pXRF and ICP-MS were within 5% of each other for spice powders and 24% for dried roots. By pXRF, spice samples were analyzed within collection plastic bags without preparation, resulting in a detection limit of 2 mg Pb/kg for spice powders, which is comparable to national food standards. The colorimetric test utilized here targets hexavalent chromium, making the method selective to Pb chromate adulteration assuming that this is its dominant source in spices. Color development, and thus detection, was observed when Pb concentrations exceeded approximately 5-70 mg/kg in dried turmeric roots and 1000 mg/kg in spice powders; however, it was ineffective for the spice-salt mixture. We show that pXRF analysis and a colorimetric assay provide information that may improve field decisions about Pb adulteration in a range of spice types, helping to minimize Pb exposure.

Keywords: ICP-MS; Pb; adulteration; colorimetry; lead chromate; pXRF; rapid detection; spices.

Figure 1

Average pXRF versus ICP-MS measurements of total Pb concentrations (mg/kg) and Deming regression line (dashed) for (A) all powder samples of various spice types, (B) powder spices with Pb concentrations less than 300 mg/kg, and (C) turmeric root samples. Note that the axis bounds differ by panel. Sample measurements below the limit of detection (2 mg/kg) on pXRF were plotted as 0. Error bars represent the standard error of the mean of three measurements. Regression details are reported in Table S3.

Figure 2

Average molar Pb/Cr ratio versus Pb concentrations (mg/kg) measured by (A) ICP-MS and (B) pXRF. Symbols and colors are based on the spice type, and error bars represent the standard error of three sample measurements. Chromium concentrations were below the instrumental limit of detection for all red chili, coriander, and blue fenugreek powder samples, and therefore, molar Pb/Cr ratios could not be calculated by pXRF. Turmeric powder (n = 6), svaneti salts (n = 4), khmeli suneli (n = 3), and dried turmeric roots (n = 3) with undetectable Cr by pXRF are shown with molar Pb/Cr ratios of 0 (could not be calculated) (Table S1).

Figure 3

(A) Color rankings of the DPC method for turmeric roots (n = 11) by total Pb concentration (mg/kg; ICP-MS). Each bar represents a spice sample. (B,C) Photographs of turmeric root samples (bars with asterisks) with DPC (left) and blank (right) reagents after 45 min of color development showing (B) slight (medium) and (C) strong (high) color change. Light yellow, light violet, and dark violet bar colors represent no (low), slight (medium), and strong (high) color change after 45 min of color development using the DPC method, respectively.

Figure 4

DPC color rankings by total Pb concentration (mg/kg; ICP-MS) for (A) all powder spice types (except red chili powder and svaneti salt), (B) khmeli suneli, (C) turmeric, (D) blue fenugreek, (E) coriander, (F) svaneti salt mixture, and (G) red chili powder samples. Each bar represents a spice sample. Light yellow, light violet, and dark violet bar colors represent no (low), slight (medium), and strong (high) color change after 45 min of color development using the DPC method, respectively. Striped light violet bars correspond to turmeric powder with mixed color results (see Figure S9). Note that the y-axis bounds differ by panel.

Figure 5

UV–vis absorbance values at 540 nm using the DPC method versus ICP-MS measurements of total Pb concentration (mg/kg) of pigment–turmeric mixtures. The powder samples were analyzed by DPC (A) before (slope p-value = 4.4 × 10^–7, 95% CI: 0.00021–0.00024; intercept p-value = 0.65) and (B) after ashing in a muffle furnace (slope p-value = 0.0009, 95% CI: 0.009–0.017; intercept p-value = 0.53). Photographs of DPC-spiked (C) unashed and (D) ashed samples.