Resources

Measurement Basics

Product Data Sheets

Anodic Stripping Voltammetry (ASV)

Accuracy and Approved Methods

Measurement Basics

Cleaning Containers and Equipment

Any trace metals measurement is only as good as the background contamination present in the sample containers and starting chemicals and on the electrodes used in the measurement. In critical measurements, all sample containers and electrodes should be cleaned in a nitric/hydrochloric acid solution. The solution we recommend is made by adding 0.5 mL concentrated nitric acid and 1.5 mL concentrated hydrochloric acid to 100 mL of deionized or distilled water. Particular care should be taken with fritted reference electrodes so that they do not act as a source of contamination.

Sample preparation

Sample preparation varies greatly with the particular samples involved and the desired type of measurement. If the total metal content is desired the sample must first be digested to extract all metals from the sample "matrix". Typical procedures include wet and dry acid digestion, UV digestion, and microwave digestion. If only the electrochemically active metals are to be measured, no digestion is needed.

Particulars of trace metals measurements

This is best performed using Anodic Stripping Voltammetry (ASV) in a strong electrolyte so that measurement currents are easily supported and electrical noise is reduced. Thus sample preparation specific to voltammetric analysis centers on selection of the proper supporting electrolyte. With Nano-Band electrodes, typical measurements do not require either the addition of mercury to the sample and oxygen removal from the sample.
Proven electrolytes include 0.05 M potassium biphthalate buffer (pH = 4.0), 0.1 M sodium acetate-acetic acid buffer (pH = 4.6), and solutions of nitric acid to pH = 2. The phthalate and acetate buffers are available commercially in liquid form and the phthalate buffer is available in capsule form.

Default Settings File (default.tma)

Using the phthalate buffer and thin-film mercury/iridium Nano-Band electrodes, the default settings file shipped with the instrument (default.tma) measures Zn, Cd, Pb, or Cu simultaneously, at levels of 50 to 300 ppb. If you need to set these values manually, they are as follows:

Waveform Square Wave
Background subtraction Enabled
Preclean time 2 seconds
Preclean voltage +200 mV
Background plate time 1 second
Foreground plate time 20 seconds
Plate voltage -1300 mV
Range ±1.25 mA
Frequency 1000 Hz
Start voltage 1350 mV
Stop voltage +200 mV
Averaging 16
Step voltage 5 mV
Square wave amp. 50 mV

The resulting measurement will produce stripping peaks for each of the above metals at their characteristic voltages. These are -900, -600, -400, and -50 mV (± 50 mV) vs. Ag/AgCl for Zn, Cd, Pb, and Cu, respectively. When run uncalibrated and with no interferences, the sensitivity of ASV stripping peak heights is approximately 1.5 nA/ppb ± 50% for Cd, Pb, and Cu, and 1 nA/ppb ± 50% for Zn. Other metals have similar sensitivities.

For solutions containing more than 200 ppb, shorter plate times can be used, but the same plate time should be used when building a calibration curve or performing the method of standard additions. Note that plate times less than 5 seconds are discouraged due to their sensitivity to convection currents in the sample. For solutions containing in excess of 1 - 2 ppm, the sample should be diluted to keep total stripping peak height less than 2 mA.

For solutions with less than 50 ppb metal concentration, the plate time should be increased roughly in inverse proportion to the metal concentration. A 1 ppb detection limit (peak height 10 × above the background noise) can easily be achieved with a 120 second plate time.

Calibration

Calibration can be achieved through the method of standard additions or through the construction of calibration curves using known additions to a blank sample matrix. Regardless of the method you prefer, read-back accuracy of 10% is easily achieved.

Simultaneous Measurement of Zinc, Cadmium, Lead, and Copper

The image below is a screen shot of the Nano-Band Explorer software. All four metals were measured simultaneously, and the measurement took 17 seconds. The sample being tested was DI water with 1-4ppb of each metal added. The sample was prepared via acidification to pH 4.5 with acetate buffer, with no stirring and no deoxygenation. We used an Iridium electrode for our thin-film mercury electrode.

Arsenic Measurement with Bare Iridium Electrode

The sample being tested was DI water with 150ppb of arsenic added. The sample was prepared via acidification to pH 4.5 with 1M sulfuric acid, with no stirring and no deoxygenation. We used a bare Iridium electrode.