Cadmium detection limits

Fig. 23 Calibration experiment for the determination of cadmium. Detection limit and sensitivity are graphically evaluated...

Because of the increasing emphasis on monitoring of environmental cadmium the detemiination of extremely low concentrations of cadmium ion has been developed. Table 2 Hsts the most prevalent analytical techniques and the detection limits. In general, for soluble cadmium species, atomic absorption is the method of choice for detection of very low concentrations. Mobile prompt gamma in vivo activation analysis has been developed for the nondestmctive sampling of cadmium in biological samples (18). 

Table 2. Analytical Methods and Detection Limits for Cadmium lon ...

Johnson and Petty [129] reduced nitrate to nitrite with copperised cadmium, which was then determined as an azo dye. The method is automated by means of flow injection analysis technique. More than 75 determinations can be made per hour. The detection limit is 0.1 xmol/l, and precision is better than 1% at concentrations greater than 10 xmol/l. 

In the determination of cadmium in seawater, for both operational reasons and ease of interpretation of the results it is necessary to separate particulate material from the sample immediately after collection. The dissolved trace metal remaining will usually exist in a variety of states of complexation and possibly also of oxidation. These may respond differently in the method, except where direct analysis is possible with a technique using high-energy excitation, such that there is no discrimination between different states of the metal. The only technique of this type with sufficiently low detection limits is carbon furnace atomic absorption spectrometry, which is subject to interference effects from the large and varying content of dissolved salts. 

Guevremont et al. [117] studied the use of various matrix modifiers in the graphite furnace gas method of determination of cadmium in seawater. These included citric acid, lactic acid, aspartic acid, histidine, and EDTA. The addition of less than 1 mg of any of the compounds to 1 ml seawater significantly decreased matrix interference. Citric acid achieved the highest sensitivity and reduction of interference, with a detection limit of 0.01 pg cadmium per litre. 

Pruszkowska et al. [135] described a simple and direct method for the determination of cadmium in coastal water utilizing a platform graphite furnace and Zeeman background correction. The furnace conditions are summarised in Table 5.1. These workers obtained a detection limit of 0.013 pg/1 in 12 pi samples, or about 0.16 pg cadmium in the coastal seawater sample. The characteristic integrated amount was 0.35 pg cadmium per 0.0044 A s. A matrix modifier containing di-ammonium hydrogen phosphate and nitric acid was used. Concentrations of cadmium in coastal seawater were calculated directly from a calibration curve. Standards contained sodium chloride and the same matrix modifier as the samples. No interference from the matrix was observed. 

A Cis column loaded with sodium diethyldithiocarbamate has been used to extract copper and cadmium from seawater. Detection limits for analysis by graphite furnace atomic absorption spectrometry were 0.024 pg/1 and 0.004 xg/l, respectively [283]. 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

Chang et al. [952] used a miniature column packed with a chelating resin and an automatic online preconcentration system for electrothermal atomic absorption spectrometry to determine cadmium, cobalt, and nickel in seawater. Detection limits of 0.12,7 and 35 ng/1 were achieved for cadmium, cobalt, and nickel, respectively, with very small sample volume required (400-1800 xl). 

Cadmium, copper, and silver have been determined by an ammonium pyrrolidine dithiocarbamate chelation, followed by a methyl isobutyl ketone extraction of the metal chelate from the aqueous phase [677], and finally followed by graphite furnace atomic absorption spectrometry. The detection limits of this technique for 1% absorption were 0.03 pmol/1 (copper), 2 nmol/1 (cadmium), and 2 nmol/1 (silver). 

The application of the Spectroscan DC plasma emission spectrometer confirmed that for the determination of cadmium, chromium, copper, lead, nickel, and zinc in seawater the method was not sufficiently sensitive, as its detection limits just approach the levels found in seawater [731]. High concentrations of calcium and magnesium increased both the background and elemental line emission intensities. 

Berman et al. [735] have shown that if a seawater sample is subjected to 20-fold preconcentration by one of the above techniques, then reliable analysis can be performed by ICP-AES (i.e., concentration of the element in seawater is more than five times the detection limit of the method) for iron, manganese, zinc, copper, and nickel. Lead, cobalt, cadmium, chromium, and arsenic are below the detection limit and cannot be determined reliably by ICP-AES. These latter elements would need at least a hundredfold preconcentration before they could be reliably determined. 

Unlike halogenated solvents, it does not produce noxious substances in the inductively coupled plasma, has a very low aqueous solubility, and yields hundredfold concentration in one step. Detection limits ranged from 0.02 jtg/l (cadmium) to 0.6 pg/1 (lead). The results indicate that the proposed procedure should be useful for the precise determination of metals in oceanic water, although a higher sensitivity would be necessary for lead and cadmium. 

Liu et al. [955] developed an electrothermal vaporisation istope dilution -ICP-MS method for determining cadmium, mercury, and lead in seawater at 2, 5, and 1 ng/1 detection limits, respectively. 

Krznaric [799] studied the influence of surfactants (EDTA, NTA) on measurements of copper and cadmium in seawater by differential pulse ASV. Adsorption of surfactants onto the electrode surface were shown to change the kinetics of the overall electrode charge and mass transfer, resulting in altered detection limits. Possible implications for studies on metal speciation in polluted seawater with high surfactant contents are outlined. 

Andruzzi et al. [800,801] discussed the use of a long-lasting sessile-drop mercury electrode in the differential pulse ASV subtrace determination of zinc, cadmium, and lead in seawater. A repeatability of about 3% and a detection limit of 10 1() M were achieved for these three metals. 

Water samples from the Arctic Sea were analysed by the potentiometric stripping technique. Lead (II) and cadmium (II) were determined after preelectrolysis for 32 min at -1.1 V with respect to Ag/AgCl the detection limits were 0.06 and 0.04 nmol/1, respectively. Zinc (II) was determined after the ad-... 

Holzbecker and Ryan [825] determined these elements in seawater by neutron activation analysis after coprecipitation with lead phosphate. Lead phosphate gives no intense activities on irradiation, so it is a suitable matrix for trace metal determinations by neutron activation analysis. Precipitation of lead phosphate also brings down quantitatively the insoluble phosphates of silver (I), cadmium (II), chromium (III), copper (II), manganese (II), thorium (IV), uranium (VI), and zirconium (IV). Detection limits for each of these are given, and thorium and uranium determinations are described in detail. Gamma activity from 204Pb makes a useful internal standard to correct for geometry differences between samples, which for the lowest detection limits are counted close to the detector. 

Wu and Boyle [837] have developed a method using magnesium hydroxide coprecipitation and isotopic dilution mass spectrometry to determine lead, copper, and cadmium in 1 ml seawater samples, with detection limits of 1,40, and 5 pM, respectively. 

Although ICP-ES is a multielement technique, its inferior detection limits (relative to GFA-AS) necessitate the processing of relatively large volumes of seawater. 250 mL aliquots were found to be useful for the analysis of iron, manganese, zinc, copper, and nickel. Extension of the method to include cadmium, cobalt, chromium, and lead would require improvements in the preconcentration procedure. 

Apte and Gunn [23] used liquid-liquid extraction, involving 1 1 1 trichlor-ethane extraction of the ammonium pyrrolidine dithiocarbamates to concentrate copper, nickel, lead, and cadmium from estuary water. (Detection limits... 

Pongratz and Hunmann [19], using differential pulse anodic scanning voltammetry, found low levels of methyl cadmium compounds in the Atlantic Ocean. Levels in the South Atlantic were approximately 700 pg/1, and those in the North Atlantic were below the detection limit of the method, i.e., below 470 pg/1. It is believed that these compounds were formed as a result of biomethylation of inorganic cadmium. 

Hexavalent chromium is also a toxic compound (like lead, cadmium, mercury) and can be easily detected with UV spectrophotometry [20]. This system works for the quality control of electroplating treated wastewater with a detection limit of 5 pg It1. 

Continuous monitoring methods based on amperometric (Nikolic et al. 1992) or spectrophotometric (Kuban 1992 Ma and Liu 1992) techniques for the quantification of free cyanide are also available. Ion chromatography with amperometric determination provides good sensitivity (2 ppb) and selectivity for free cyanide and the weak complexes of cadmium and zinc (Rocklin and Johnson 1983). Postcolumn derivatization and fluorescence detection provides low detection limits as well (0.1 ppb) (Gamoh and Imamichi 1991). 

The technique is useful for the quantitation of many metals including lead, copper, mercury, cadmium and zinc with detection limits as low as lOpg. Its sensitivity makes it a very suitable method for trace metal analysis in biological samples. 

Molecular probe dyes for the determination of potassium, lithium, and sodium have been identified. Additionally, an NIR probe selective for potassium has been fabricated. The detection limits of this probe are in the ppm range. Lower detection limits may be achieved by varying the matrix which allows the entrapment of ions. Preliminary data for the detection of lead and cadmium demonstrate the potential capability of these probes for environmental applications. The development of OFMP for the detection of other ions of environmental interest such as Be2+, Hg2+, As3+, and Ni2+ is currently underway. 

The vendor claims that the following metals have been successfully treated to parts per biUion (ppb) and detection limit levels aluminum, arsenic, cadmium, chromium, cobalt, copper, iron, lead, manganese, mercury, molybdenum, nickel, selenium, silver, tin, uranium, vanadium, and zinc. The system is also able to remove ammonia, nitrates, phosphates, potassium, fluorides, and sodium. Studies have also been performed using Aqua-Fix to remove radionuchdes such as uranium from waste streams. 

According to the vendor, Cement-Lock technology has successfully removed polycyclic aromatic hydrocarbons (PAHs), PCBs, and tetrachlorodibenzo-1,4-dioxin (TCDD)/2,3,7,8-tetra-chlorodibenzofuran (TCDF) from soils and sediments in bench-scale tests. Metal concentrations were also reduced below detection limits in bench-scale tests. These metals included arsenic, cadmium, chromium, lead, nickel, mercury, and silver. 

According to the technology developer, geochemical fixation can treat dissolved hexavalent chromium and other metals in groundwater at concentrations ranging from the detection limit to several hundred parts per milhon. The developer asserts that geochemical attenuation can treat most of the common heavy metals, trace elements, and namral radionuclides that occur in groundwater, such as metal-cyanide complexes, arsenic, cadmium, chromium, copper, lead, selenium, uranium, and radium. 

The extremely low solubility of lead phosphate in water (about 6 x 10 15m) again suggests potentiometric analysis. Selig57,59 determined micro amounts of phosphate by precipitation with lead perchlorate in aqueous medium. The sample was buffered at pH 8.25-8.75 and a lead-selective electrode was used to establish the end-point. The detection limit is about 10 pg of phosphorus. Anions which form insoluble lead salts, such as molybdate, tungstate or chromate, interfere with the procedure. Similar direct potentiometric titrations of phosphate by precipitation as insoluble salts of lanthanum(III), copper(II) or cadmium(II) are suggested, the corresponding ion-selective electrodes being used to detect the end-point. 

---