Spectrophotometric Determination of Lead

The authors developed a reliable spectrophotometric method for determining and measuring trace amounts of lead in various samples.
Apr 02, 2008
Volume 32, Issue 4

Lead has long been recognized as a hazardous metal to fish and other wildlife (1). For humans, lead is a neurotoxic metallic element that can be absorbed by the body primarily through the lungs and stomach. Lead poisoning occurs when repeated exposure to a lead-containing environment happens so that lead is slowly accumulated in bones and tissues.

Exposure to high concentrations of lead can cause serious health problems, including nervous system dysfunction of fetuses and infants, hemotoxic effects, reproductive dysfunction, gastrointestinal tract alterations, nephropathies, and Alzheimer's disease. Lead poisoning is related to anemia, because the activity of heme synthetic enzymes can be inhibited by lead ions (2). Lead is also a risk factor for hypertension in women (3). If lead poisoning is left untreated, it may damage kidneys, the nervous system, and the brain. Because of the possibility of permanent impairment, lead poisoning is particularly dangerous during infant development and for children under 7 years of age, who can absorb 50% of lead ingested, compared with adults who only absorb 10% of it (4). In 1985, the Centers for Disease Control defined childhood lead poisoning as blood lead levels >25 μg/L or 25 ppb. This definition has been revised down to 10 ppb based on new scientific evidence and observations that show children with lead levels as low as 10 ppb have decreased intelligence and slower neurological development (5).

As a metal that has been widely used in industry for many years, lead is omnipresent in the environment—either from natural resources or from pollutants. In the United States, the average lead background level in soil is 7– 40 ppm (5–7). The maximum allowable lead level in food is 2 ppm (6). In agricultural products, food, raw dietary supplements, and pharmaceutical materials, lead usually exists at levels <1 ppm. To determine lead at ppm or sub-ppm levels, some analytical instruments with high-sensitivity detection capabilities are available, including inductively coupled plasma mass spectroscopy (ICP-MS) and graphite furnace atomic absorption spectroscopy (GFAAS) (8–13). Ion chromatography also has been used for lead determination; however, its sensitivity is not as good as that of ICP-MS and GFAAS (14–15).

Although their sensitivities make ICP-MS and GFAAS the methods of choice for lead analysis, they are very expensive. For example, a typical ICP-MS instrument ranges in price from $250,000 to $750,000, and daily operational cost can be more than $1000. In addition, because only a small amount of sample solution can be introduced into the instrument, both ICP-MS and GFAAS require a very clean working environment, which adds maintenance cost to operate the instruments, especially an ICP-MS. Therefore, many analytical laboratories cannot afford an ICP-MS. Moreover, for laboratories in which lead analysis is not a routine task, it is not economically worthwhile to own a GFAAS instrument, even though it is less expensive (more than $20,000). Those laboratories prefer sending their samples to other laboratories for lead determination, even though sending samples to an outside laboratory costs money and time.

To analyze low-level lead quickly, simply, and less expensively, a few analytical methods for lead determination have been developed in the past 10 years. Di Nezio et al. have reported a preconcentration technique involving flow injection spectrophotometry to detect lead in natural water samples (16). Other examples include a dibromo-p-methylsulfonazo spectrophotometric method to detect lead in biological samples, a resonance Rayleigh scattering method to detect lead in tap-water samples, and a fluorescent method to detect lead in soft drinks (17–19). However, these methods have their shortcomings: either they could only be used for water or liquid samples, or their detection limits were too high, or they were not tolerant of other common metal ions.

The method reported in this article is based on the complex formation of lead with dithizone or diphenylthiocarbazone. Although dithizone has been used for lead analysis for more than half a century, limited research has been conducted on the sample digestion and few studies have been conducted on reagent recycling (20–23).

Application of this complex reagent is very limited. A colorimetric method based on a lead-dithizone complex is the procedure recommended for lead analysis according to the US Pharmacopeia (24). Unfortunately, this method can be used only for simple samples and has serious shortcomings. For instance, the method is invalid for samples containing lead at levels <5 ppm. The shortcomings of the USP method include:

  • It is not suitable for salts of alkali earth elements because a heavy precipitation is formed during the sample digestion process
  • For many samples the digestion process is extremely time-consuming—it can take several hours or even days
  • Detection limit is too high for almost all common samples
  • The method generates too much waste of dithizone chloroform solution, which is both environmentally and economically unacceptable.

We discuss a practical UV–vis spectrophotometric method for measuring trace amounts of lead (e.g., <1 ppm). The method is applicable to all organic and inorganic samples and eliminates all the shortcomings mentioned for the USP method. It is simple, economical (<$2/sample), reliable, and versatile for any type of sample. The procedure has been validated by standard addition method and a ICP-MS method. Its sensitivity is at sub-ppm levels, similar as those of ICP-MS and GFAAS. Using this method, lead in pharmaceutical, agricultural, food, and raw dietary supplement samples have been determined.

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