Hyphenation of capillary electrophoresis to inductively coupled plasma mass spectrometry as an element-specific detection method for metal speciation

A stepwise development for the use of capillary electrophoresis and inductively coupled plasma mass spectrometry (ICP-MS) for speciation investigations is presented. The high resolution power of CE is used for the separation of metal species, whereas ICP-MS is taken for element-specific detection with low detection limits. This contribution starts with an off-line combination of both instruments. Separation and identification of species in model solutions and real samples are shown by scanning UV detection at the CE unit with subsequent metal quantification in peak related fractions, applying electrothermal vaporization ICP-MS. Finally, first separations are demonstrated, using the on-line hyphenation with a laboratory-made nebulizer. Here, standard solutions are separated and monitored by UV and ICP-MS. Stability of electrical current during nebulization was checked and a possibly interfering suction flow was estimated. After optimization sufficient electropherograms were obtained. Advantages and problems are discussed for both modes.     

Development of chiral HPLC for selenoamino acids with ICP-MS detection:application to selenium nutritional supplements

The enantiomeric separation of three underivatized seleno-amino acids, D,L-selenocystine, and D,L-selenomethionine, and D,L-selenomethionine, with UV and ICP-MS detection is described. An HPLC column with a chiral crown ether stationary phase and a mobile phase of 0.10 M HCIO4 was used. Absolute detection limits obtained with UV detection ranged from 34.5 to 47.1 ng whereas those obtained with the plasma detector were ca. 40-400 times better. The separations with either detector were good, with the little detector effect on the resolution. Ten commercially available dietary selenium supplements were analyzed using the chiral column to identify and quantify the selenium species present with both detection modes. Selenium species were easily identified using ICP-MS detection, whereas UV detection was not viable because of interferences from the sample matrix and inadequate sensitivity. Selenium species that were unretained using the chiral column were identified using anion exchange chromatography. Total amounts in the samples were also measured using a conventional digestion and enzymatic digestion with ICP-MS detection.     

Coupling of capillary electrophoresis with ICP-MS for speciation investigations

An online hyphenation of capillary electrophoresis (CE) with inductively coupled plasma-mass spectrometry (ICP-MS) is developed, using a homemade nebulizer as the interface. The high resolution power of CE is used for the separation of metal species, whereas ICP-MS is taken for element specific detection with low detection limits. Metal species of standard solutions and real samples are separated and monitored by UV and ICP-MS. After optimization, electropherograms with high resolution were obtained, showing low detection limits around 1 μg/L (e.g. Pt-species) and very high resolution. Typical analysis times were below 22 min. Quality control aspects concerned species stability during the analytical procedure and stability of electrical current during nebulization. A possible interfering suction flow was estimated and found to be negligible. Received: 17 April 1996 / Revised: 12 June 1996 / Accepted: 20 June 1996     

Hadamard transform CE-UV detection for biological samples

A Hadamard transform-capillary electrophoresis-UV (HT-CE-UV) detection technique is described for the analysis of biological samples. Pseudorandom injections of sample and buffer according to a simplex matrix obtained from the corresponding Hadamard matrix is performed with conventional capillaries. Alternating injections are achieved with a novel capillary "T" connector created by drilling conventional capillary dimensions through a 1-cm diameter polycarbonate disc. This connector design coupled with a switching system allows for rapid, electrokinetic injections of solution into alternating sample and buffer capillary arms for UV detection. The standard mixtures of nitric oxide (NO) metabolites, nitrite and nitrate, dissolved in physiological saline solution are injected into the separation capillary according to an 83-element injection sequence to obtain a signal-to-noise ratio (S/N) enhancement of ca. 4.5 over a single injection. Nitrite, being the less concentrated metabolite in NO detection and thereby more difficult to detect, was calibrated with the HT-CE-UV method and a limit of detection (LOD) of 0.56 microM was obtained. Rat blood plasma was analyzed with this detection system and demonstrated to be comparable with NO metabolite concentrations of previously published results. This HT-CE-UV method is described where a unique reservoir tube design that contains 8-microL standard nitrite sample volumes is placed over the end of the capillary arm to explore low volume limits for biological samples.     

Derivatization, stabilization and detection of biogenic amines by cyclodextrin-modified capillary electrophoresis-laser-induced fluorescence detection

o-Phthalaldehyde (OPA) derivatives of eight biogenic amines were stabilized at 5 degrees C by forming inclusion complexes with methyl-beta-cyclodextrin (MBCD). The derivatives were separated and detected by cyclodextrin-modified capillary electrophoresis (CE) with UV or laser-induced fluorescence (LIF) detection. Using a borate buffer, pH 9.0 consisting of ethanol and a mixture of negatively charged sulfobutylether-beta-cyclodextrin and neutral MBCD, baseline separation of the eight OPA derivatives was achieved within 25 min with high separation efficiencies. The detection limits (S/N=3) obtained by UV and LIF detection were determined to be 10 microM and 0.250 microM, respectively. Glutamic acid was added after the initial derivatization step to neutralize residual OPA which otherwise caused a significant interference, particularly when analysis was performed around the detection limit of the OPA derivatives. Important biogenic amines in fish, wine and urine were then derivatized and determined by CE-LIF. In the case of sole and rainbow trout, the results obtained were validated by an enzymatic assay using putrescine oxidase.     

A simple and demountable capillary microflow nebulizer with a tapered tip for inductively coupled plasma mass spectrometry

A simple and demountable capillary microflow nebulizer (d-CMN) was developed for inductively coupled plasma mass spectrometry (ICP-MS). It consisted of a nebulizer body, a fused-silica capillary with a tapered tip and a polytetrafluoroethylene (PTFE) adapter. The gas orifice i.d., the solution capillary tip i.d. and its wall thickness were 200, 30, and 5 μm, respectively. The sensitivities, detection limits, precisions and the long-term stability with the d-CMN were evaluated. The experimental results indicated that its performances at low uptake rates were similar or better than those obtained with the conventional concentric nebulizer at 820 μL/min and the micromist nebulizer at 200 μL/min. The demountable construction of the d-CMN permitted that the blocked or broken solution capillary could be conveniently renewed. The low self-aspiration rate (4.77 μL/min) and the analytical characteristics comparable to commercial microflow nebulizers made the d-CMN a good choice for coupling capillary electrophoresis and microbore high-performance liquid chromatograph to ICP-MS. The proposed d-CMN was successfully applied in the iodine speciation by coupling microchip capillary electrophoresis (MCE) to ICP-MS. The absolute detection limits for iodide and iodate of 0.20 and 0.29 fg were achieved with satisfactory resolution.     

Determination of amitrole and urazole in water samples by capillary zone electrophoresis using simultaneous UV and amperometrical detection

In this paper, capillary zone electrophoresis with amperometric detection (CZE-AD) was first applied to the simultaneous separation and determination of amitrole and urazole in water samples. A simple end-column electrochemical detector was used in combination with a commercially available capillary electrophoresis instrument with UV detection. The effects of several important factors were investigated to find optimum conditions. A carbon disk electrode was used as working electrode. Separation and determination of these compounds in water samples were performed in 0.030 mol l(-1) acetate buffers at pH 4.5, 25 kV as separation voltage and the samples were introduced by hydrodynamic mode for 1.5 s. Most of the studies realized showed that the direct electrochemical detection is more sensitive and selective than UV detection. Under the optimum conditions, excellent linearity was observed between peak amperometric signal and analyte concentrations in the range of 0.19-1.35 mg l(-1) for amitrole and 0.20-1.62 mg l(-1) for urazole. The detection limits were 63 and 68 microg l(-1) for amitrole and urazole, respectively. The utility of this method was demonstrated by monitoring water samples, and the assay results were satisfactory. The detection limits using a previous preconcentration step for amitrole and urazole in spiked mineral water samples were 0.6 and 1.0 microg l(-1) for amitrole and urazole, respectively.     

Separation of perfluorocarboxylic acids using capillary electrophoresis with UV detection

A capillary electrophoretic method with UV detection for separation and quantitation of perfluorocarboxylic acids (PFCAs) from C6-PFCA to C12-PFCA has been developed. The optimization of measurement conditions included the choice of the most appropriate type and concentration of buffer in the background electrolyte (BGE), as well as the type and the content of an organic modifier. The optimal separation of investigated PFCAs was achieved with 50 mM phosphate buffer and 40% isopropanol in the BGE using direct UV detection. The optimum wavelength for direct UV detection was optimized at 190 nm. For indirect detection, several chromophores were studied. Five mM 3,5-Dinitrobenzoic acid (3,5-DNBA) in 20 mM phosphate buffer BGE and indirect UV detection at 280 nm gave the optimal detection and separation performance for the investigated PFCAs. The possibility of on-line preconcentration of solutes by stacking has been examined for indirect detection. The detection limits (LODs) determined for direct UV detection ranged from 2 microg/mL for C6-PFCA to 33 microg/mL for C12-PFCA. The LODs obtained for indirect UV detection were comparable to those obtained for direct UV detection.     

Trace and Ultratrace Element Determinations in Geological and Environmental Samples Using a New Generation High Ion Transmission Quadrupole ICP Mass Spectrometer

There is a continuous quest to enhance the limits of detection in ICP-MS for geoanalysis and geochemistry. Recently, a high transmission ICP-MS interface (S-mode) was designed to facilitate direct microanalysis of solids using a UV laser microprobe.     

The potential of inductively coupled plasma-mass spectrometric detection for capillary electrophoretic analysis of pesticides

In this work, the potential of inductively coupled plasma-mass spectrometry (ICP-MS) coupled to capillary electrophoresis (CE) to determine organophosphorus pesticides (OPPs) is demonstrated. Element specific detection of (31)P with ICP-MS is performed for the detection of OPPs. Three common OPPs, including glyphosate, glufosinate, and aminomethylphosphonic acid (AMPA), were analyzed by CE-ICP-MS to demonstrate its applicability for the analysis of OPPs. The advantages of using ICP-MS with respect to other common detectors, such as flame photometric detection (FPD), for CE analysis of OPPs are shown. Additionally, different CE separation conditions were studied to achieve complete baseline separation of the pesticide compounds in short migration times. Two CE buffer systems were evaluated for the separation of OPPs using ICP-MS detection. A buffer solution containing 40 mmol.L(-1) ammonium acetate at pH 9.0 and an applied voltage of +20 kV were finally selected leading to a separation time of 10.0 min. Both migration time and area relative standard deviations (%RSD) were evaluated and their respective values were in the intervals of 1.1-3.3% and 2.7-5.3%. Detection limits obtained with the CE-ICP-MS system were in the range of 0.11-0.19 mg.L(-1) (as compound) yielding an enhancement of 130- to 230-fold with respect to FPD. The proposed methodology was finally applied for the determination of the OPPs mentioned above in natural river water samples.     

Enantioselective determination of the organochlorine pesticide bromocyclen in spiked fish tissue using solid-phase microextraction coupled to gas chromatography with ECD and ICP-MS detection

A method for enantioselective determination of bromocyclen enantiomers in fish tissue has been developed. The enantiomers were resolved by capillary gas chromatography (GC) using a commercial chiral column (CP-Chirasil-Dex CB) and a temperature program from 50 degrees C (held for 1 min), raised to 140 degrees C at 40 degrees C min(-1) and then raised at 0.2 degrees C min(-1) to 155 degrees C. This enantioselective gas chromatographic separation was combined with a clean-up/enrichment procedure based on solid-phase microextraction (SPME). Under SPME optimized conditions, precision, linearity range and detection limits of the developed SPME-enantioselective GC procedure were evaluated and compared using two different detection systems: a classical electron-capture detection (ECD) and an element specific detection using inductively coupled plasma mass spectrometry (ICP-MS). The SPME-GC-ECD method exhibited an excellent sensitivity, with detection limits of 0.2 ng L(-1) for each enantiomer of bromocyclen. Although ICP-MS offered poorer detection limits (7 ng L(-1) as Br, equivalent to 36 ng L(-1) of each enantiomer) than conventional ECD detector, it proved to be clearly superior in terms of selectivity. The relative potential and performance of the two compared methods for real-life analysis has been illustrated by the determination of enantiomers of bromocyclen in spiked tissue extracts of trout.     

Determination of anions with capillary electrophoresis and indirect ultraviolet detection

A method is validated for the determination of anions with capillary electrophoresis (CE) in combination with indirect UV detection. The method described here is used for the analysis of eight of the most common anions (fluoride, chloride, bromide, sulphate, nitrate, nitrite, thiosulphate and phosphate). Next, the method is compared with a another buffer system for the determination of anions with CE and indirect UV detection. Typical limits of detection are obtained between 1 and 3 mg/l for the above-mentioned compounds. The repeatability and reproducibility of the system differs per compound and is, with the exception of fluoride and phosphate, between 4 and 6% and 5–10%, respectively. Linearity was observed between 1 and 10 mg/l. The method is applied for the determination of anions in drinking water, serum and urine.     

Two-dimensional on-line detection of brominated and iodinated volatile organic compounds by ECD and ICP-MS after GC separation

Inductively coupled plasma-mass spectrometry (ICP-MS) was coupled to a gas chromatographic (GC) system with electron capture detector (ECD), which enables relatively easy characterization and quantification of brominated and iodinated (halogenated) volatile organic compounds (HVOCs) in aquatic and air samples. The GC-ECD system is connected in series with an ICP-MS by a directly heated transfer line and an outlet port-hole for elimination of the ECD make-up gas during ignition of the plasma. The hyphenated GC-ECD/ICP-MS system provides high selectivity and sensitivity for monitoring individual HVOCs under fast chromatographic conditions. The ECD is most sensitive for the detection of chlorinated and brominated but the ICP-MS for iodinated compounds. The greatest advantage of the use of an ICP-MS is its element-specific detection, which allows clear identification of compounds in most cases. The absolute detection limits for ICP-MS are 0.5 pg for iodinated, 10 pg for brominated, and 50 pg for chlorinated HVOCs with the additional advantage that calibration is almost independent on different compounds of the same halogen. In contrast to that detection limits for ECD vary for the different halogenated compounds and lie in the range of 0.03-11 pg. The two-dimensional GC-ECD/ICP-MS instrumentation is compared with electron impact mass spectrometry (EI-MS) and microwave induced plasma atomic emission detection (MIP-AED). Even if EI-MS has additional power in identifying unknown peaks by its scan mode, the detection limits are much higher compared with GC-ECD/ICP-MS, whereas the selective ion monitoring mode (SIM) reaches similar detection limits. The MIP-AED detection limits are at the same level as EI-MS in the scan mode.     

Capillary supercritical fluid chromatography with ultraviolet multichannel detection of some pesticides and herbicides

The useful combination of capillary supercritical fluid chromatography with ultraviolet multichannel detection is demonstrated by the analysis of selected pesticides and herbicides. In this application the advantages of compound identification by ultraviolet spectrometry are appended to the separating capability of capillary supercritical fluid chromatography. A pseudo oncolumn detection approach is used. Compromises from the theoretically ideal conditions for both capillary SFC and the multichannel UV detector are made to achieve a practical interfacing of the chromatographic and spectrophotometric techniques. Detection limits are at the low nanogram levels.     

Enhancement of sensitivity in capillary supercritical fluid chromatography through optimization of injection and detection techniques

The reproducibility of peak areas as a function of the technique used for sample injection was investigated in capillary supercritical fluid chromatography (SFC). An injection technique has been developed to increase the volume of sample introduced into the capillary column. Using a modified time-split injection technique, long injection duration times were successfully applied to achieve lower detection limits. Analytes were effectively focused at the head of the analytical column using a unique pressure trap program. Because this on-column focusing was performed only by pressure and temperature programming, no instrumental modifications were necessary. Up to 1.0 μL of sample solution was injected onto 50 μm i.d. columns using this technique, with no observable peak splitting. Dual detection using ultraviolet (UV) absorption and flame ionization detection (FID) was performed in series, thereby avoiding the necessity of splitting the column effluent. For the compounds investigated (five nitroaromatics and one phthalate ester), the absolute sensitivity of the UV detector was significantly greater than that of the FID.     

Capillary electrophoresis of organic and inorganic cations with indirect UV detection

Summary The determination of alkali and alkaline earth metal ions by capillary electrophoresis using indirect UV detection is described. With the identical system it is also possible to determine short chain aliphatic amines and alkanol amines within 4 minutes. Indirect UV detection is achieved at 214 nm with a background electrolyte containing 5 mmol L^–1 imidazole. Linear calibration curves could be obtained for peak areas between 0.5 and 10 ppm. The detection limits are around 0.1 ppm (corresponding to 10 fmol in about 10 nL sample volume) for all cations and amines and 0.05 ppm for lithium. Practical applications demonstrate the applicability of this system in routine analysis.     

Determination of ebrotidine and its metabolites by capillary electrophoresis with UV and mass spectrometry detection

This study describes the application of capillary electrophoresis (CE) to the analysis of ebrotidine and its metabolites as an alternative analytical technique to liquid chromatography. Comparison between UV-diode array spectroscopy and mass spectrometry (MS) using an ion-trap system with electrospray ionization as detection systems has been performed. The quality parameters of the UV detection method were established, obtaining linear calibration curves over the range studied (8-200 mg ml(-1)), limits of detection between 3.4 and 4.3 microg ml(-1), and run-to-run and day-to-day precision lower than 14%. For these compounds the protonated species [M+H]+ and, in some cases, sodium adducts were observed in the MS spectra. Using MS coupled to CE, limits of detection were between 0.5 and 2.6 microg ml(-1).     

Determination of cannabinoids in hair using high-pH* non-aqueous electrolytes and electrochemical detection. Some aspects of sensitivity and selectivity

Non-aqueous capillary electrophoresis with electrochemical detection (NACE-ED) was applied to the determination of cannabinoids in hair. The effect of different electrolyte compositions on the selectivity of the separation of tetrahydrocannabinol (THC), cannabinol (CBN), cannabidiol (CBD) and tetrahydrocannabinol carboxylic acid (THCA) was studied. Complete electrophoretic resolution was obtained using a strongly basic background electrolyte consisting of 5 mM sodium hydroxide dissolved in acetonitrile-methanol (1:1). Electrochemical detection yielded well defined signals in the oxidation mode. In order to obtain low limits of detection experimental parameters, which determine the sensitivity and the noise level, were optimized. A crucial parameter for sensitive measurements using a wall-tube flow cell as end-column detector is the distance between the capillary outlet and the working electrode. The highest signal-to-noise ratio using a 50 microm I.D. capillary was obtained at a distance of 25 microm. When the capillary outlet was moved away from the working electrode, thus reducing the strength of the separation field present at the working electrode, a large low frequency noise developed. This rise was attributed to disturbances of the hydrodynamic pattern in the flow cell. Analytical aspects such as sensitivity, reproducibility and selectivity were addressed in this work. The precision of NACE-ED regarding migration time and peak height for a sample containing 1 microg/ml THC was 0.4% and 1.1% (RSD), respectively (n=5). The calibration curve was linear for concentrations ranging between 0.1 and 10 microg/ml (r=0.998). The limit of detection for THC was 37 ng/ml, which is almost two orders of magnitude lower when compared with on-column UV detection. The method was evaluated using hair samples containing cannabinoids as sample material.     

Capillary electrophoresis separation and permanganate chemiluminescence on-line detection of some alkaloids with beta-cyclodextrin as an additive.

Capillary electrophoresis has been used in combination with on-line permanganate chemiluminescence detection for the simultaneous determination of morphine, 6-monoacetylmorphine and heroin. It was found that beta-cyclodextrins could improve the separation efficiency and enhance the chemiluminescence signal. Improved sensitivity over capillary electrophoresis with UV detection was obtained. The procedure has detection limits of 23, 66 and 115 fmol for morphine, 6-monoacetylmorphine and heroin, respectively.     

Characterization of exopolysaccharides in marine colloids by capillary electrophoresis with indirect UV detection

A method was established using capillary electrophoresis with indirect UV detection for analysis of monosaccharides liberated from exopolysaccharides by acidic hydrolysis. Tangential flow filtration was used to isolate high molecular weight polysaccharides from seawater. The capillary electrophoresis method included the use of a background electrolyte consisting of 2,6-dimethoxyphenol and cetyltrimethylammonium bromide. Several neutral sugars commonly existing in marine polysaccharides were separated under optimized conditions. The relative standard deviations were between 1.3% and 2.3% for relative migration time and 1.3-2.5% for peak height. Detection limits (at S/N 3) were in the range of 27.2-47.8 μM. The proposed approach was applied to the analysis of hydrolyzed colloidal polysaccharides in seawater collected from the Baltic Sea. Nanomolar levels of liberated monosaccharides in seawater samples can be detected by preconcentration up to 30,000 times.