Signal enhancement in adsorptive stripping voltammetry of Pt by forced convection during the measurement step
Introduction
Adsorptive stripping voltammetry of metals and metal complexes is known as one of the most sensitive electroanalytical methods. Especially, if the adsorptive accumulation is coupled with catalytic reactions, extremely sensitive measurements are possible for some trace metals, and also for organic compounds [1].
The ‘Pt–formazone-method’ belongs to this class of voltammetric methods. It has been first described in 1986 by Zhao and Freiser [2], who investigated the in situ formation of the Pt(II)–formazone-complex in sulfuric acid electrolyte containing hydrazine and formaldehyde, adsorption of this complex at the dropping mercury electrode (DME), and catalytic hydrogen evolution. The mechanism of the catalytic reaction has been investigated in some detail already by Zhao and Freiser. They found that the catalytic reaction occurs only in acidic solution, and they suggested a catalytic hydrogen formation from the protonated Pt–formazone-complex at the electrode surface, while at the same time Pt(II) is reduced to Pt(0). This proposed mechanism has some similarities to that of the well-known Brdicka reaction [3], dealing with the voltammetric determination of sulfur-containing proteins via complexation with cobalt. For this reaction, a complete mechanism has been presented recently [4]. In 1987, Wang et al. demonstrated that the sensitivity of the Pt–formazone-method can be enhanced significantly by using a hanging mercury drop electrode (HMDE) instead of the DME, i.e. by controlled interfacial accumulation of the Pt complex for longer times at a stationary electrode [5]. Later, the method was slightly modified and adapted by several groups [6], [7], [8], and has been used extensively for determination of ultratrace platinum concentrations in environmental, and biological matrices. One of the most challenging tasks for the formazone-method is the determination of platinum–nucleotide-ratios in DNA after chemotherapy with platinum-based cytostatica in clinical studies. This requires the determination of picograms of platinum in micrograms of isolated DNA [9], which is very close to the absolute detection limit of the method (2 pg Pt [10]).
It is well known that the adsorptive enrichment step depends strongly upon the mass transport, e.g. stirring rate, but stirring in the measurement step is normally not done with mercury drop electrodes. Such electrodes may be used as hydrodynamic detector electrodes in chromatography [11], [12], and even a rotating mercury drop electrode has been proposed [13], but generally mercury film electrodes (based on the rotating disk electrode) have been preferred for experiments with forced convection. The performance of such mercury film electrodes has been compared and discussed already with respect to different stripping methods [14], and the highest sensitivity was obtained with a slowly rotating electrode; but no adsorptive stripping methods were included. Some very efficient methods for exploiting convection phenomena are the use of ultrasound [15], and the use of heated wire electrodes [16], but these mechanisms are only applicable to robust electrodes. Although other types of electrodes are available, the best sensitivity for platinum is still obtained with the HMDE.
During experiments, which were undertaken to further improve the detection limit of the Pt–formazone method for the above-mentioned clinical applications, we discovered that forced convection in the measurement step leads to considerable enhancement of the catalytic hydrogen wave. The impact of this enhancement effect on detection limit and sensitivity of the method is investigated quantitatively in the present paper. Furthermore, experimental results are presented, which support the assumption that the enhancement effect is particularly useful for adsorptive stripping methods with catalytic hydrogen evolution.
Section snippets
Chemicals
The Pt standard solution (‘Specpure’, 1 mg/mL in 20% HCl) was obtained from Alfa Aesar. Working standards were prepared daily by diluting this solution with 0.36 mol/L sulfuric acid to the desired range. Nickel, cobalt, and lead standards were prepared by appropriate dilution of Merck Titrisol® standards.
All other chemicals and solvents used for voltammetry were of Merck p.a. purity, and only ultrapure water (Seralpure Delta UV) was used.
Voltammetric equipment
Two voltammetric systems were used, which differ in
Signal enhancement
The effect of stirring upon the voltammetric signal in the platinum–formazone-system (catalytic hydrogen wave) is shown in Fig. 1. The voltammograms of Fig. 1A were recorded using the standard procedure as described in the experimental section, i.e. accumulation with stirring for 60s at −0.6 V, and then scanning the potential in quiescent solution down to −1 V. The peak current for the 200 pg Pt(II) standard (corresponding to a concentration of 13.3 ng/L or 68.4 pmol/L) is 302 nA, and the background
Conclusions
The observed enhancement effect of forced convection on the voltammetric signal in the Pt–formazone method offers the possibility of up to 30 times better sensitivity and up to 10 times lower detection limit for Pt. Since in the present experiments only 60 s enrichment time were used, even lower detection limits are possible at the cost of longer analysis time. However, at present most voltammetric instruments are not ideally suited for exploiting this enhancement effect. So, it is to be
References (18)
J. Electroanal. Chem.
(2001)- et al.
J. Electroanal. Chem.
(1987) - et al.
Anal. Chim. Acta
(1997) - et al.
Anal. Chim. Acta
(1985) - et al.
Electrochem. Commun.
(1999) Analytical Electrochemistry
(1994)- et al.
Anal. Chem.
(1986) Coll. Czech. Chem. Commun.
(1933)- et al.
Fresenius J. Anal. Chem.
(1989)
Cited by (11)
Preparation and characterization of magnetic nanoparticles for the on-line determination of gold, palladium, and platinum in mine samples based on flow injection micro-column preconcentration coupled with graphite furnace atomic absorption spectrometry
2014, TalantaCitation Excerpt :Direct determinations of trace or ultra-trace levels of metals in geological, metallurgical, and environmental samples are difficult because of their extremely low concentrations together with very complicated sample matrices [1,2]. Thus, a separation and preconcentration procedure is required even with the high sensitivity and selectivity of modern analytical techniques, such as graphite furnace atomic absorption spectrometry (GFAAS) [3,4], inductively coupled plasma optical emission spectrometry (ICP-OES) [5–7], inductively coupled plasma mass spectrometry (ICP-MS) [8–10], adsorptive stripping voltammetry (ASV) [11] or neutron activation analysis (INAA) [12]. Although GFAAS shows similar shortcomings, it is available in most laboratories and has a chance to be in common use over traditional atomic spectrometries.
Nanomolar simultaneous determination of levodopa and serotonin at a novel carbon ionic liquid electrode modified with Co(OH)<inf>2</inf> nanoparticles and multi-walled carbon nanotubes
2013, Electrochimica ActaCitation Excerpt :Neurotransmitters play a significant role in the research of Parkinson's disease [1]. l-Dopa is currently the therapeutic drug in the treatment of Parkinson's disease and required by the brain to produce dopamine [2] which compensates the deficiency of dopamine in the organism and decreases the symptoms of Parkinson's disease. Various analytical methods have been developed for l-dopa determination, such as spectrophotometry [3,4], liquid chromatography [5,6], and capillary zone electrophoresis [7].
Increased sensitivity of anodic stripping voltammetry at the hanging mercury drop electrode by ultracathodic deposition
2011, Analytica Chimica ActaCitation Excerpt :The deposition potential to be applied varies with several factors including type of analyte, solvent and pH, among others; however, in ASV, the common rule is the application of a potential around 300 mV more negative than the peak potential of the least easily reduced analyte to be analyzed. Generally, deposition is enhanced by stirring, which increases mass transport by convection [10]. Eq. (2) (stripping step) describes the anodic stripping where the metals are reoxidized to their cationic form, thus enabling quantification since current intensity is proportional to bulk concentration.
Evaluation of environmental cytotoxic drug contamination in a clinical setting
2017, Journal of the American Animal Hospital Association