Jun
Yin
,
Bin
Hu
*,
Man
He
,
Mingming
Zheng
and
Yu-Qi
Feng
*
Department of Chemistry, Wuhan University, Wuhan, 430072, P. R. China. E-mail: binhu@whu.edu.cn; yqfeng@public.wh.hb.cn; Fax: +86-27-68754067
First published on 24th October 2008
A novel method of polymer monolith microextraction (PMME) using a poly(acrylamide-vinylpyridine-N,N′-methylene bisacrylamide) (AA-VP-Bis) monolithic column combined with electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) for the determination of trace Cd, Tl, and Pb with Pd as chemical modifier has been proposed. Several factors that influence the microextraction efficiency including pH value, sample flow rate, extraction time, sample volume, eluent volume, and coexisting ions, were investigated and the optimal microextraction conditions were established. The chemical modification of Pd in ETV-ICP-MS has been studied and the results indicated that the pyrolysis temperature for the target analytes could be increased up to 600 °C and the signal intensities were increased by 4.1, 2.1, and 13.8 times for Cd, Tl, and Pb, respectively. Under the optimized conditions, with a consumption of 0.4 mL sample, the limits of detection (LODs) were 1.1, 0.5, and 0.2 ng L−1 for Cd, Tl, and Pb, and the relative standard deviations (RSDs) were 10.3, 9.7, and 9.2% (C = 0.05 µg L−1, n = 5) for Cd, Tl, and Pb, respectively. The proposed method was successfully applied to the determination of trace Cd, Tl, and Pb in human serum and urine samples, and the recovery for the spiked samples were in the range of 90–110%. In order to validate the proposed method, certified reference material of GBW09103 human urine was analyzed, and the determined values were in good agreement with the certified values. The poly(AA-VP-Bis) monolith can be used more than 40 times without decrease in the extraction efficiency.
It should be addressed that the current study on SPE is mainly focused on two aspects: new functional adsorbents and miniaturization of extraction equipment. The properties of adsorbents decide the analytical performance and application range of the method. Therefore, the exploration of new functional adsorbents with good mass transferability, pH stability, good bioadaptability, and high adsorption capacity has become one of the hotspots in the study of SPE. Some new functional adsorbents, such as ion imprinted materials,4 nanometer-sized materials,5 monolithic materials,6 have already demonstrated their outstanding adsorption performance in SPE. In the past few years, more and more efforts have been directed towards the miniaturization of analytical instrument, in order to greatly reduce reagent consumption, increase the sample throughput, and achieve in-situ, on-line, in-vivo, and real-time analysis. Similarly, the miniaturization of extraction equipment, the conveniency of operation as well as the ability of easily combining with other analytical instrument have become the foreland of SPE researches. Currently, some miroextraction methods, such as solid phase microextraction (SPME),7capillary microextraction (CME),8 stirring bar sorptive extraction (SBSE),9 and microextraction in packed syringe (MEPS),10 have attracted the attention of analysts.
Monolithic material11,12 is a new type of separation medium with high speed, high efficiency and high throughput, and has been widely applied as the stationary phase in various chromatographic methods such as gas chromatography (GC), high performance liquid chromatography (HPLC) and capillary electrochromatography (CEC). The applications of monolithic materials in non-chromatographic methods,13,14 especially in the field of sample pretreatment,14 have also attracted the attention of analysts in recent years. The history of monolithic material in SPE is not very long. The first paper describing this entirely new approach was published in 1998. To achieve the desired increase in surface area, Xie et al.15 used commercial 80% divinylbenzene (remaining 20% being ethylstyrene) as the only monomer and dodecanol with toluene as porogens to obtain monolith with a surface area of 400 m2/g resulting from the presence of mesopores and micropores by thermally polymerizing them in a PEEK tube. Despite this very high surface area, such monoliths had excellent hydrodynamic properties because of the 6 µm large through pores. Since then, monolithic material was rapidly accepted as the excellent adsorption materials for solid phase extraction of different analytes from various samples. Schaller et al.16 successfully separated three antidepressants by capillary electrophoresis (CE) with in-line SPE using a monolithic adsorbent, and the enrichment factors were over 500. In 2006, Feng's group proposed a new type of miniaturized syringe extraction method named polymer monolith microextraction (PMME),17 which offers several attractive features. These include the advantages of the polymer monolith, such as easy preparation with good control of porosity, convective mass transfer procedure, bioadaptability, and pH stability; and the advantages of the extraction device, such as convenience, flexibility, and easy operation. PMME combined with CE, HPLC and LC-MS, respectively, was successfully applied in the analysis of opiates in human urine as well as other fields.18–21 However, no correlative research work has been found in inorganic elemental analysis up to date.
Atomic spectrometry techniques, such as flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ETAAS), inductively coupled optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS), are the most used analytical techniques for inorganic elemental analysis. Among them, ICP-MS has been accepted as a powerful technique for trace element analysis due to its outstanding advantages, such as low detection limits, wide linear dynamic range, multi-element detection capability, high sample throughput,22 and has a widespread application in an extensive field such as life science, food science, clinical chemistry, and geological chemistry. However, conventional pneumatic nebulization (PN)-ICP-MS has disadvantages of low intake efficiency (less than 2%), large sample consumption and difficult to analyze high salt-containing and viscous samples.
Electrothermal vaporization (ETV),23,24 as a sample introduction technique for ICP-MS, has the following outstanding advantages: (1) very high transport efficiency (∼80%); (2) vaporization and excitation/ionization processes are separated; (3) low sample consumption in microgram or microliter level; (4) the sensitivity and selectivity of the method could be further improved by using chemical modifier. Therefore, it has consistently received considerable attention in atomic/mass spectrometry.23,24 In spite of these notable advantages, in some environmental and biological samples with complex matrix, the level of the heavy metal ions is fairly low and the major constituents can cause matrix effects and spectral interference. In order to circumvent the above problems, the utilization of chemical modifier or the use of efficient separation/preconcentration has been proven to be very effective for ETV-ICP-MS.
As mentioned above, PMME is a novel miniaturized sample pretreatment technique and ETV-ICP-MS is a micro-sampling and high sensitivity method. If PMME is combined with ETV-ICP-MS, it will be extremely suitable for the determination of ultra-trace elements in samples with a complex matrix, especially a sample with a limited available volume, for example, biological fluids.
Cadmium is a non-essential toxic element and acute cadmium poisoning may produce degenerative changes in renal tubular cells. Cadmium has also been demonstrated to inhibit many enzymes and competes with calcium metabolism and alter phosphorylation patterns.25Thallium occurs in the environment naturally in small amounts. Excess of thallium would cause gastrointestinal irritation and disorders of the nervous system after exposure for relatively short times. In the long term, this has the potential to cause effects such as changes in blood chemistry, damage to liver, kidney, intestinal, and testicular tissue, and hair loss.26 Also, lead fulfils no essential function in the human body, and its toxic effect is primarily the inactivation of enzymes and proteinsvia the binding to sulfydryl groups.27 They are all considered to be high toxicity for human health and can be considered as biomonitors whose concentration in human body would well indicate human exposure to the background of harmful chemicals in the environment. Serum and urine analysis are often used for monitoring environmental and occupational exposure to trace elements because serum and urine are the most readily available biological fluid and most trace elements are mainly excreted via urine.
The aim of this work is to develop a new method of PMME-ETV-ICP-MS for the determination of trace Cd, Tl, and Pb in human serum and urine with Pd as chemical modifier. The factors that influence the extraction efficiency and the vaporization behaviors of analytes were systematically investigated, and the optimal operation conditions were established. The proposed method was applied to the determination of trace Cd, Tl and Pb in human serum and urine with satisfactory results.
ICP-MS plasma | |
---|---|
Rf power | 1250 W |
Outer gas flow rate | 15 L min−1 |
Intermediate gas flow rate | 0.9 L min−1 |
Nebulizer gas flow rate | 0.7 L min−1 |
Sampling depth | 7.0 mm |
Sampler/skimmer diameter orifice | Nickel 1.0 mm/0.4 mm |
Time-resolved data acquisition | |
Scanning mode | Peak-hopping |
Dwell time | 20 ms |
Integration mode | Peak area |
Points per spectral peak | 1 |
Elements and istopes | 111Cd, 205Tl, 208Pb |
Electrothermal vaporizer | |
Sample volume | 10 µL |
Carrier gas flow rate | 0.4 L min−1 |
Drying step | 100 °C ramp 10 s \ hold 10 s |
Pyrolysis step | 600 °C ramp 10 s \ hold 10 s |
Vaporization step | 2200 °C hold 4 s |
Cleaning step | 2500 °C hold 3 s |
Acrylamide (AA) was purchased from Tianjin Chemical Plant (Tianjin, China). 4-Vinylpyridine (4-VP) was obtained from Acros (Sweden). N,N′-Methylene bisacrylamide (Bis), 2,2′-azobis(2-methylpropionitrile) (AIBN), dodecanol and DMSO were obtained from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China) and were of analytical reagent grade.
The stock solutions of Cd, Tl, and Pb (1 g L−1) were prepared from their nitrate of analytical reagent grade (Shanghai Chemical, Shanghai, China) and diluting to a certain volume with high purity deionized water. Highly pure grade HNO3 was distilled before used. Certified reference material of human urine GBW09103 was obtained from Beijing Medical University (Beijing, China). Pd(NO3)3 (Shanghai Chemical, Shanghai, China) and other chemical reagents used in this work were of analytical grade, and high purity deionized water was used throughout.
4 mL urine sample was transferred into PTFE insert, and 4 mL HNO3 was added. The insert was placed into the bottom of the digestion vessel and the digestion vessel was then closed and positioned inside the microwave oven. After that, the heating program was performed as 5 min at 15 atm and 200 °C in 600W. After cooling down, the vessel was opened and clear solution was observed. The digest was then heated to near dryness by a hot plate, and the residue was dissolved with high purity deionized water, and then diluted to 4 mL. Before used, the solution was adjusted to the required pH value with NH3·H2O–HNO3.
4 mL human serum sample was also prepared using above microwave digestion procedure. The vessel was opened after cooling down, and then H2O2 (30%, 0.5 mL) was added dropwise into the PTFE insert. The clear solution was heated to near dryness by a plate heater, and the residue was dissolved with high purity deionized water, and then diluted to 4 mL. Before used, the solution was adjusted to the required pH value with NH3·H2O–HNO3.
The resulting polymer monolith was taken for FT-IR, elemental analysis, SEM and N2 adsorption characterization.29 The spectra of FT-IR indicated the presence of acrylamide and pyridyl groups. The composition of the polymer monolith includes 56.4% (C), 13.2% (N) and 6.0% (H). In SEM, the interconnected skeletons and interconnected textural pores of the monolith are easily observed, and the monolith was attached tightly to the inner-wall of the capillary. The average skeleton pore size of the monolith determined by N2 adsorption method was 3.3 nm.
A WZB-50 syringe infusion pump was employed for the delivery of solution, and the whole PMME procedure included four steps: (1) Precondition. The syringe was filled with 0.15 mL of 0.1 mol L−1 phosphate buffer solution (pH = 7.0), and then ejected through the monolithic capillary at 0.2 mL min−1 by the infusion pump; (2) Preconcentration. 0.4 mL sample solution was ejected through the monolithic capillary at 0.2 mL min−1; (3) Washing. 0.05 mL high purity deionized water was ejected through the monolithic capillary at 0.1 mL min−1; (4) Elution. 0.05 mL HNO3 (1.0 mol L−1) was ejected through the monolithic capillary at 0.2 mL min−1. A whole PMME procedure could be finished in 4 min. The above analytical process could result in an 8-fold preconcentration factor.
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Fig. 1 Effect of pH value on the adsorption of several metal ions on polymer monolith. Concentration of the target analyte: 0.5 µg L−1; sample volume: 0.4 mL; sample flow rate: 0.2 mL min−1. |
According to the theory of hard and soft acids and bases, the interaction force between soft acid and soft base, or hard acid and hard base, are stronger than the interaction force between soft acid and hard base, or hard acid and soft base. Since the amidocyanogen and pyridine in poly(AA-VP-Bis) monolith are nearly soft bases, they have only a slight interaction force with the hard acid metal ions such as Mn, which is coincident with the experimental results.
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Fig. 2 Effects of eluent concentration and volume on the recoveries of Cd. Cd: 0.5 µg L−1; sample volume: 0.4 mL; sample flow rate: 0.2 mL min−1; elution flow rate: 0.2 mL min−1. |
The elution flow rate was also studied, and no obvious difference was observed with the elution flow rate varying in the range of 0.05–0.25 mL min−1. In this work, 0.05 mL of 1.0 mol L−1 HNO3 with the elution flow rate of 0.20 mL min−1 was employed to simultaneously elute the analytes retained on the polymer monolith.
The optimized PMME operation parameters are summarized in Table 2.
The regeneration of polymer monolith is one of the key factors in evaluating the performance of PMME. For the regeneration of the polymer monolith, 1.0 mol L−1 HNO3 solution and high purity deionized water were passed through the column. By this treatment, the poly(AA-VP-Bis) monolith could be re-used for more than 40 times with good stability (recovery higher than 90% for the target elements) under the optimized conditions.
Coexisting ions | Tolerance limit (mg L−1) | Concentration range in human serum33 (mg L−1) | Concentration range in urine33 (mg L−1) |
---|---|---|---|
Note:a mL L−1.b µg L−1. | |||
K+ | > 10,000 | 140–215 | ≈ 1.9 |
Na+ | > 10,000 | 2500–3560a | ≈ 2200 |
Ca2+ | 500 | 91–106 | ≈ 120 |
Mg2+ | 500 | 17–22 | ≈ 90 |
Mn2+ | > 5,000 | 0.1–2.9b | 0.1–20 b |
Ni2+ | > 5,000 | 0.05–1.3 b | 0.06–8 b |
Cu2+ | 2 | 0.6–1.4 | 42–50 b |
Zn2+ | 2 | 0.6–1.2 | 0.27–0.85 |
Al3+ | 2 | 0.5–8 b | 2.3–110 b |
Co2+ | 4 | 0.08–0.45 b | 0.2–135 b |
Fe3+ | 4 | ≈ 0.17 | |
Cl − | > 10,000 | ||
Br − | > 10,000 | 3.5–5.5 b | ≈ 5 |
SO42−, H2PO4−, CH3COO− | > 10,000 |
For comparison, the detection limits of the proposed method and the other similar methods reported in the literatures for Cd, Tl and Pb are listed in Table 4. As can be seen, the detection limits obtained by the proposed method are better than that obtained by isotope dilution (ID)-ETV-ICP-MS,34,35 slurry sampling ETV-ICP-MS,36capillary microextraction (CME)-ICP-MS,37 flow injection knotted reactor (FI-KR)-ICP-MS,38 flow injection solid phase extraction (FI-SPE)-ICP-MS,5 large-bore direct injection high efficiency nebulizer (LB-DIHEN)-ICP-MS,39 cloud extraction (CPE)-ETAAS40 and SPE-ICP-OES,41 but poorer than that obtained by single drop microextraction (SDME)-ETV-ICP-MS.42
Analytical method | Sample | Limits of detection (ng L−1) | |||
---|---|---|---|---|---|
Cd | Tl | Pb | |||
a ID: isotope dilution; SS: slurry sampling; CME: capillary microextraction; KR: knotted reactor; SPE: solid phase extraction; LB-DIHEN: large-bore direct injection high efficiency nebulizer; SDME: single drop microextraction; CPE: cloud point extraction. | |||||
This method | PMME-ETV-ICP-MS | Urine and serum | 1.1 | 0.5 | 0.2 |
Ref. 34 | aETV-ID-ICP-MS | Urine | 20 | — | 5 |
Ref. 35 | aID ETV-ICP-MS | Fuel alcohol | 80 | 1.0 | 50 |
Ref. 36 | a SS-ETV-ICP-MS | Sediment | 10 | 60 | 400 |
Ref. 37 | aCME-ICP-MS | Urine | 4.5 | — | 3.7 |
Ref. 38 | aKR-FI-ICP-MS | Natural water | 1.1 | — | 4.5 |
Ref. 5 | aSPE-FI-ICP-MS | Natural water | 79 | — | 27 |
Ref. 39 | aLB-DIHEN-ICP-MS | Urine | 10; 3 | — | 90; 6 |
Ref. 40 | aCPE-ETAAS | Urine | 8 | — | — |
Ref. 41 | aSPE-ICP-OES | Urine, blood and serum | 300 | — | 1100 |
Ref. 42 | aSDME-ETV-ICP-MS | Urine and water | 0.27; 0.16 | — | — |
The proposed method was also applied to the analysis of real human serum and urine samples. The analytical results, together with the recoveries for the spiked samples, are given in Table 6. As can be seen, the concentrations of Cd, Tl and Pb in non-spiked serum sample are 0.09, 0.08 and 0.11 µg L−1, respectively, and the concentrations for Cd, Tl and Pb in non-spiked urine are 38, 0.37 and 29 µg L−1. The Cd, Tl and Pb concentrations in non-spiked human serum and urine samples obtained by the proposed method are within the range reported by Caroli et al.33 for normal human serum and urine. The recoveries for the target analytes in spiked serum and urine ranged from 90% to 110%.
Sample | Element | Added (µg L−1) | Founda (µg L−1) | Recovery (%) | Average level (µg L−1)33 |
---|---|---|---|---|---|
a Values expressed as mean ± s (n = 3). | |||||
Human serum | Cd | 0 | 0.09 ± 0.01 | — | 0.04–0.4 |
0.1 | 0.20 ± 0.01 | 110 | |||
0.2 | 0.28 ± 0.02 | 95 | |||
0 | 0.08 ± 0.01 | — | 0.05–0.3 | ||
Tl | 0.1 | 0.17 ± 0.02 | 90 | ||
0.2 | 0.27 ± 0.02 | 95 | |||
0 | 0.11 ± 0.01 | — | — | ||
Pb | 0.1 | 0.22 ± 0.02 | 110 | ||
0.2 | 0.29 ± 0.02 | 90 | |||
Human urine | Cd | 0 | 38 ± 2 | — | 0.4–70 |
50 | 89 ± 4 | 102 | |||
100 | 135 ± 6 | 97 | |||
0 | 0.37 ± 0.03 | — | 0.02–8.9 | ||
Tl | 0.5 | 0.91 ± 0.05 | 108 | ||
1.0 | 1.41 ± 0.09 | 104 | |||
0 | 29 ± 2 | — | 12–30 | ||
Pb | 50 | 76 ± 3 | 94 | ||
100 | 126 ± 5 | 97 |
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