Interfacing capillary electrophoresis with inductively coupled plasma mass spectrometry by direct injection nebulization for selenium speciation

Abstract

A demountable direct injection high efficiency nebulizer operating at low sample uptake rates was developed and used for coupling of capillary electrophoresis (CE) with inductively coupled plasma mass spectrometry (ICP-MS). When the nebulizer was used for continuous sample introduction, detection limits of 20 and 1 ng L⁻¹ were obtained for ⁸²Se and ¹⁰³Rh, respectively, at sample uptake rates of 10–30 μL min⁻¹, based on three times the standard deviation of blank solution (3σ_b, n = 10). The nebulizer was used as part of the interface for coupling of CE with ICP-MS and applied for speciation of aqueous selenium standards. The interface was operated in the self-aspirating mode with a sheath liquid uptake of 10 μL min⁻¹. The CE-ICP-MS system resulted in baseline separation of selenate, selenite, selenocystine and selenomethionine within a total analysis time of 5.4 min. Detection limits were in the sub µg Se L⁻¹ range, corresponding to absolute detection limits in the range 25–125 fg selenium. Repeatability (n = 6) expressed as relative standard deviations with respect to migration times, peak heights and peak areas were better than 1.6, 6.7 and 6.0%, respectively.

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Introduction

In recent years, there has been increasing interest in selenium speciation. Selenium is an essential trace element as a constituent of selenoproteins. The total selenium concentrations in biological tissue are typically <100 µg L⁻¹ and several chemical forms have been detected in biological samples. In human plasma, three selenium-containing plasma proteins, selenoprotein P, extracellular glutathione peroxidase and serum albumin, have been separated by tandem chromatography with on-line ICP-MS detection.¹ In human urine, at least five selenium compounds have been separated by ion exchange chromatography and two of the minor constituents were identified as trimethylselenonium and selenite.²

Capillary electrophoresis with on-line ICP-MS detection would be a versatile alternative for speciation studies. Unfortunately, the utility of CE-ICP-MS for selenium speciation in biological material is limited by low sensitivity, unless preconcentration techniques are used. Michalke and Schramel³ interfaced capillary isoelectric focusing with ICP-MS for speciation of selenium in human plasma. They observed a multitude of peaks at concentrations close to the detection limit. When the interface was used for CE-ICP-MS speciation of selenium standards, detection limits of 10–50 µg L⁻¹ were obtained.

Interfacing of CE with ICP-MS is not straightforward, and was first reported by Olesik et al.⁴ five years ago. Since then, about 30 reports concerning coupling of CE to ICP-MS have appeared in the literature, and recently the technique has been reviewed by Sutton and Caruso.⁵ It is generally accepted that an interface should provide a stable electrical connection to the outlet end of the capillary, prevent introduction laminar flow in the capillary, and offer a high sample transfer efficiency. Most reported interfaces have used liquid junctions to establish the electrical circuit. Alternatively, metallic conductors have been attached directly to the capillary outlet.^4,6 Common pneumatic nebulizers used to interface CE with ICP often possess a suction effect, known as the natural self-aspiration. Unless counterbalanced, this suction effect will cause a laminar flow in the capillary and compromise the high efficiency of CE. Several methods have been applied for counterbalancing of laminar flow in the capillary, including pumping of sheath liquid at matched flow rates,^7–16 operating the nebulizer in self-aspirating mode with a leveled sheath liquid reservoir,^16–20 placing a sol–gel frit in the sample inlet end of the capillary²¹ or reducing the pressure at the capillary inlet buffer vial.^16,22

The limited amount of sample injected into the capillary typically, less than 100 nL, necessitates the use of highly efficient sample introduction systems for interfacing CE with ICP-MS. Interfaces with high sample transfer efficiencies have been obtained by using microconcentric nebulizers (MCNs)in combination with spray chambers, e.g., the MicroMist,¹² the MCN^16,17,19,20 and the high-efficiency nebulizer (HEN).^8,11 In addition, hydride generation,²³ ultrasonic nebulization^24,25 andmodified/laboratory-made microconcentric nebulizers^{6,9–11,18,21,26} have been applied.

Sample transfer efficiencies reaching 100% can be obtained by using direct injection type nebulizers such as the direct injection nebulizer (DIN) and the direct injection high efficiency nebulizer (DIHEN). The DIN has been applied successfully for coupling of CE with ICP-MS by Liu et al.⁷ However, the versatility of the DIN is restricted by its high cost and requirements for a gas displacement pump for sample delivery and a coaxial carrier gas in addition to the nebulizing gas. The DIHEN is a simple low cost concentric nebulizer and it has recently been applied for coupling of CE with ICP-MS by Majidi et al.¹³ Improvements of detection limits by a factor two compared with the cross-flow nebulizer were reported. This renders the DIHEN a promising sample introduction system for coupling of CE with ICP-MS.

Optimum conditions for the DIHEN are obtained at a sample uptake rate of 85 μL min⁻¹ when used for continuous sample introduction,²⁷ whereas the size of the electroosmotic flow in the capillary is typically less than 1 µL min⁻¹. The aim of this study was to develop a CE-ICP-MS interface based on a direct injection high efficiency nebulizer operating at low sample uptake rates in order to obtain adequate sensitivity for selenium speciation in biological material.

Experimental

ICP-MS system

PE-SCIEX Elan 6000 instrument (Perkin-Elmer SCIEX, Thornhill, ON, Canada) configured for a demountable torch was used. A PEEK adaptor (article No. 50-0002, Spetec, Erding, Germany) was used to accommodate the interface within the torch. Instrumental settings are given in Table 1. Peak areas were computed by a Turbochrom Workstation (Perkin-Elmer).

Table 1 Operating conditions for the ICP-MS instrument

Method	ICP-MS	CE-ICP-MS
a External argon supply.
Sampling and skimmer cones	Platinum
Argon flow rate—
Plasma gas	15 L min^−1
Auxiliary gas	1.2 L min^−1
Nebulization^a	0.2 L min^−1
Rf power supply	750–1250 W	1050 W
Lens voltage	6.75 V
Data acquisition—
Dwell time	300 ms	300 ms
Sweeps per reading	10	1
Readings per replicate	1	450
Replicates	10	1
Isotope monitored	^82Se and ^103Rh	^82Se

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CE system

A Waters (Milford, MA, USA) Quanta 4000 CE-instrument was used for capillary electrophoresis; the instrument was operated with open door and defeated door interlock. Capillaries of 75 µm id, 375 µm od and length 75 cm (Poly Micro Technologies, Phoenix, AZ, USA) were conditioned by a 20 min flush with 1 M sodium hydroxide, a 20 min flush with 0.1 M sodium hydroxide, a 20 min flush with water-purified with a Milli-Q system (Millipore, Bedford, MA, USA) and finally a 20 min flush with the run buffer. Samples were injected hydrostatically at 981 mbar s (9.81 mbar at 100 s) corresponding to a sample volume of 100 nL. Separation was performed at a run voltage of −30 kV. The run buffer at the cathode reservoir was replaced for every fifth run, and prior to analysis the capillary was flushed with run buffer for 1 min at 2 bar.

CE-ICP-MS interface

The interface is illustrated in Fig. 1. PEEK components for the body were machined in-house. Shields were drawn in-house from borosilicate glass tubes (6 mm od, 3 mm id) to give orificediameters of 100±10 µm. Sample capillaries were drawn in-house from borosilicate glass tubes (1.4 mm od, 0.95 mm id) to give short taperings with ids of 40±10 µm and a typical wall thickness of 5–10 µm. These dimensions resulted in a gas annulus area of approximately 0.005 mm². Tefzel^® ferrules with PEEK lock rings, PEEK nuts for 1/16 in tubing and the barbed tubing adapter were obtained from Upchurch Scientific (Oak Harbor, WA, USA). The platinum tube (syringe needle) was obtained from Aldrich. Teflon tubing sleeves and rubber O-rings, size 5.28 × 1.78, 1.42 × 1.53 and 2.17 × 0.81 mm, were standard types.

Fig. 1 Schematic view of the interface.

The sample capillary was secured within the body by a ferrule with a compression ring and O-rings were placed on both sides of the ferrule. With this arrangement, the position of the sample capillary tip could be adjusted to 0.5 mm with respect to the nozzle surface. Adjustment of the sample capillary position was performed at a constant nebulizer gas back-pressure of 4–5 bar, allowing the nebulizer to self-aspirate water. Adjustment was performed to give a fine aerosol that was completely centered within the external torch tube. This was typically obtained when the sample capillary tip exceeded the nozzle surface by 0.1 mm.

The nebulizer was accommodated within the torch with the nozzle positioned 2 mm below the intermediate torch tube, in accordance with directions given²⁷ for the DIHEN. An external argon supply was used for nebulization since the gas back-pressure was beyond the working range of the ICP-MS gas system. An optimum nebulizer gas flow rate of 0.2 L min⁻¹ was obtained at a gas back-pressure of 5 bar and maintained throughout. Under these conditions, the natural self-aspiration rate of the nebulizer was determined to be 10±1 µL min⁻¹.

For simplicity, the rear part of the body was replaced by a standard nut during characterization of the nebulizer, and a 0.58 mm id × 0.965 mm od polyethylene tube (Becton Dickinson, Sparks, MD, USA) was inserted into the sample capillary. An infusion syringe pump (Model KDS101, KD Scientific, New Hope, PA, USA) was used to obtain a pulsation free flow in the range 2.5–30 µL min⁻¹.

For CE-ICP-MS analysis, a fused silica capillary was inserted into the tapering of the sample capillary, leaving enough space to allow for unhindered sheath liquid flow. Prior to speciation analysis, instrumental parameters for the ICP-MS system were optimized for ⁸²Se determinations by forcing a 100 µg Se L⁻¹ solution in 10% methanol through the CE capillary at a constant back-pressure of 2 bar. The electrical circuit was established by aspirating the sheath liquid through a grounded platinum tube.

Chemicals and reagents

Purified water from a Milli-Q deionization unit was used throughout. Nitric acid, sodium hydroxide, aqueous ammonia, and methanol, all of analytical-reagent grade, were purchased from Merck (Darmstadt, Germany). A 1000 mg Rh L⁻¹ stock standard solution as [RhCl₆]³⁻ in 10% hydrochloric acid was obtained from Perkin-Elmer. Selenomethionine (Aldrich, Milwaukee, WI, USA), selenocystine (Aldrich), sodium selenate (Merck), and sodium selenite (Merck) stock standard solutions each containing 100 mg Se L⁻¹ were prepared by dissolution in 5 mM aqueous ammonia. Each solution was standardized against a selenium dioxide Titrisol standard (Merck) using external calibration. Organic selenium standards were stored in 1.00 mL aliquots at −18 °C and inorganic selenium standards were kept at 5 °C. Working standard solutions were prepared immediately before analysis by appropriate dilution of the stock standard solutions. For continuous sample introduction, a 10 µg Rh L⁻¹ standard in 0.67% HNO₃ and a 100 µg Se L⁻¹ standard in 0.67% HNO₃ with methanol added to a final concentration of 10% were used. The run buffer for CE-ICP-MS speciation analysis consisted of 25 mM nitric acid and 0.5 mM cetyltrimethylammonium hydroxide (Fluka, Buchs, Switzerland) adjusted to pH 9.25 using aqueous ammonia. A solution of 10 mM nitric acid in 10% methanol was used as sheath liquid.

Results and discussion

Optimization of ICP-MS instrumental parameters

To examine the performance of the nebulizer at low sample uptake rates, the influence of sample uptake on the rf power, sensitivities and detection limits were examined by monitoring the ⁸²Se and ¹⁰³Rh isotopes. ⁸²Se was the target isotope and ¹⁰³Rh was included as a reference.

Selenium measurements

The ⁸²Se isotope (8.73%) was used in order to avoid the isobaric ⁴⁰Ar–⁴⁰Ar interference associated with the most abundant ⁸⁰Se isotope (49.61%). Further, ⁸²Se was found to provide a better signal-to-noise ratio than the more abundant ⁷⁸Se isotope (23.78%). Having a high ionization energy of 940.7 kJ mol⁻¹, selenium will only be 30% ionized in the plasma under normal conditions. The signal intensity for selenium can be enhanced by addition of organic modifiers to the sample matrix and operating the plasma at higher rf powers, a phenomenon known as the carbon enhancement effect. One explanation of the effect involves charge transfer from selenium to ionized carbon, bringing selenium into the ionized first excited state.²⁸ The effect of the methanol concentration on the sensitivity was examined by monitoring the intensities of 100 µg Se L⁻¹ selenium standard solutions. Measurements were performed at a sample uptake rate of 10 µL min⁻¹ using individually optimized rf power settings. Compared with an aqueous standard solution, signal enhancement by factors of 1.7, 1.9, 2.4, 2.8 and 2.9 was obtained at methanol concentrations of 3, 5, 10, 20 and 30%, respectively. Thus, an enhancement factor close to the theoretically predicted value of 3 was obtained at 20–30% methanol. However, when these methanol concentrations were used at higher sample uptake rates, it was not possible to reach an rf power optimum within the working range of the rf generator. A methanol concentration of 10% was found to be a good compromise at sample uptake rates in the range 2.5–30 µL min⁻¹, and this concentration was maintained throughout. In a recent study by Gammelgaard and Jøns,²⁹ optimum concentrations of 3% methanol or 5% acetic acid were reported to yield a sixfold signal enhancement for selenium with the standard cross-flow nebulizer/Scott-type double-pass spray chamber combination. Therefore, the optimum methanol concentration found in this work was beyond the optimum concentration reported for the conventional cross-flow nebulizer and may be explained by a reduced organic solvent load of the plasma.

Influence of sample uptake rates on the rf power

The effect of the sample uptake rates on the optimum rf power was examined using the automated optimization procedure at a step value of 50 W. Results obtained for ¹⁰³Rh and ⁸²Se are illustrated in Fig. 2 and show that the optimum rf power was roughly proportional to the sample uptake rate. The trace for ⁸²Se was shifted towards a higher rf power as a consequence of the added organic modifier and the higher ionization energy of ⁸²Se. The shift was more pronounced at higher methanol concentrations and, as a consequence, an optimum in terms of the rf power was observed. Generally, the optimum rf powers found in this work were much lower than reported²⁷ for the DIHEN, where an optimum rf-power of 1500 W was observed for ¹⁰³Rh at a sample uptake rate of 11 µL min⁻¹.

Fig. 2 Influence of the the sample uptake rate on the rf power optimum for ¹⁰³Rh (■) and ⁸²Se (○). Optimizations were performed on a 100 µg Se L⁻¹ standard in 0.67% HNO₃ with methanol added to a final concentration of 10% and a 10 µg Rh L⁻¹ standard in 0.67% HNO₃, respectively.

Influence of sample uptake rate on sensitivities and precision

The influence of sample uptake rates on the relative sensitivities for ¹⁰³Rh and ⁸²Se are illustrated in Fig. 3. The relative sensitivities for both isotopes were roughly proportional to the sample uptake rate in the range 2.5–20 µL min⁻¹, and tended to fade at higher sample flow rates (20–30 µL min⁻¹). The precision expressed as relative standard deviation was calculated from ten replicates, each integrated for 3 s, and was found to be better than 2.5% for both isotopes at sample uptake rates of 2.5–30 µL min⁻¹.

Fig. 3 Influence of sample uptake rate on the relative sensitivities for ¹⁰³Rh (■) and ⁸²Se (○). Measurements were performed at individual optimized RF-powers in accordance with Fig. 2.

Influence of sample uptake rates on detection limits

The influence of sample uptake rates on the relative detection limits obtained for ⁸²Se and ¹⁰³Rh is illustrated in Fig. 4. Detection limits calculated on the basis of three times the standard deviation of a blank solution (3σ_b, n = 10) were nearly constant at sample flow rates of 10–30 µL min⁻¹ but increaseddrastically at sample uptake rates below the natural self-aspiration rate of 10 µL min⁻¹.

Fig. 4 Influence of sample uptake rate on the detection limits for ¹⁰³Rh (■) and ⁸²Se (○), calculated on the basis of three times the standard deviation of blank intensities, n = 10.

Comparison with the DIHEN

For comparison, sensitivity, precision and detection limits obtained for ¹⁰³Rh and ⁸²Se at a sample uptake rate of 10 µL min⁻¹ were compared with data reported²⁷ for the DIHEN at sample uptake rates of 11 and 85 µL min⁻¹.

The relative sensitivity obtained for ¹⁰³Rh at a sample uptake rate of 10 µL min⁻¹ was improved by one order of magnitude when compared with the reported²⁷ value obtained with the DIHEN at a sample uptake rate of 11 µL min⁻¹. The precision and detection limits obtained for both isotopes at a sample uptake rate of 10 µL min⁻¹ were comparable to those reported²⁷ for the DIHEN at a sample uptake rate of 85 µL min⁻¹. The significant increase in sensitivity and decrease in detection limits observed for ⁸²Se relies partly on the carbon enhancement effect. The enhanced performance of this nebulizer at low sample uptake rates is thought to originate from reduction of the nozzle dimensions. Compared with the DIHEN, the sample capillary id was reduced from ∼80 μm to ∼40 µm, the sample capillary wall thickness was reduced from ∼30 to ∼10 µm and the gas annulus area was reduced from ∼0.01 to ∼0.005 mm². However, further investigations are needed in order to understand how these modifications affect critical aerosol parameters such as droplet size and droplet velocities.

In conclusion, we have obtained an efficient sample introduction system for coupling CE with ICP-MS. In addition, the demountable construction offers advantages of easy replacement of clogged sample capillaries and worn out parts.

CE-ICP-MS system

The performance of the interface was examined by CE-ICP-MS speciation analysis of selenate, selenite, selenocystine and selenomethionine. Separation of anionic species was performed at pH 9.25 with reversed polarity. Cetyltrimethylammonium hydroxide was added to the run buffer in order to direct the electroosmotic flow toward the detector. Self-aspiration of sheath liquid from a leveled reservoir was chosen for its simplicity.

A typical electropherogram obtained with the CE-ICP-MS system is presented in Fig. 5, and shows that baseline separation of all the selenium compounds was obtained within at a total analysis time of 5.4 min (including 100 s hydrostatic sampling). Peak widths at the baseline of less than 3 s were obtained for the inorganic selenium compounds. This corresponds to an efficiency of 60 000 theoretical plates for the CE-ICP-MS system.

Fig. 5 Electropherogram obtained from a standard solution containing 10 µg Se L⁻¹ of each compound. Migration order: selenate, selenite, selenocystine and selenomethionine.

Control for laminar flow

To reduce laminar flow in the capillary during self-aspiration, the sheath liquid transfer line was constructed to have a sectional area of at least 0.25 mm² (from the sheath liquid reservoir to the outlet of the capillary). The size of the laminar flow in the capillary during nebulization was determined by injecting a plug of a highly colored Bromophenol Blue solution.

After 30 min, the plug had moved 10 cm, corresponding to a laminar flow of 15 nL min⁻¹. Judged on the basis of the electropherogram presented in Fig. 5, a laminar flow of this rate caused no significant dispersion and no effort was made to suppress it further. However, the additional amount of analyte injected during sampling was included in calculations of absolute detection limits.

Repeatability

The repeatability of the CE-ICP-MS system with respect to migration times, peak heights and peak areas expressed as relative standard deviations (RSDs) (n = 6) shown in Table 2 is comparable to previously reported^12,20,25,30 values obtained by CE-ICP-MS. The RSD with respect to peak areas decreased systematically with increasing migration times, which might be a consequence of the data acquisition method used. At a dwell time of 300 ms, the first migrating peak is represented by only eight data points, whereas nearly twice as many data points represent the last migrating peak.

Table 2 Repeatabillity and detection limits obtained in CE-ICP-MS selenium speciation

Species	Repeatability [RSD (%)]^a			Detection limit^b
	Migration time	Peak height	Peak area	Relative/µg Se L^−1	Absolute/fg Se
a Precision expressed as relative standard deviations were calculated from six replicate determinations of a standard solution containing 10 µg Se L^−1 of each compound. b Relative detection limits were calculated as concentrations that will give signals equivalent to three times the peak-to-peak noise of the baseline, based on the electropherogram obtained from a standard solution containing 0.5 µg Se L^−1 of each compound. Absolute detection limits were calculated on the basis of a total sample volume of 125 nL (100 + 25).
Selenate	1.6	6.7	6.0	0.2	25
Selenite	1.4	3.5	4.1	0.2	25
Selenocystine	1.5	4.0	3.9	1.0	125
Selenomethionine	1.6	4.6	3.1	0.5	65

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Limits of detection (LODs)

The signal-to-noise ratios suggest that LODs close to 0.5 µg Se L⁻¹ could be obtained. To give more accurate figures for LOD, speciation analysis of a standard solution containing 0.5 µg Se L⁻¹ of each species was performed. The resulting electropherogram is presented in Fig. 6. LODs calculated as the concentrations that will give signals equivalent to three times the peak-to-peak noise of the baseline are presented in Table 2. An LOD of 0.2 µg Se L⁻¹, corresponding to an absolute LOD of 25 fg Se at 125 nL sampling, was obtained for selenate and selenite. An LOD of this low level for selenate and selenite by CE-ICP-MS speciation has only been achieved by costly sample introduction systems such as the DIN⁷ and ultrasonic nebulization²⁴ (no absolute LOD were given). An absolute LOD of 25 fg Se for selenate and selenite using CE-ICP-MS has to our knowledge never been reported before. The lowest absolute LODs previously reported³⁰ for selenate and selenite are 395 fg and 174 fg Se, respectively. Thus, an improvement of absolute LOD by one order of magnitude was achieved. The LOD obtained for selenocystine and selenomethionine was improved by between one and two orders of magnitude whencompared with previously reported³¹ values of 35 and50 µg Se L⁻¹ for selenocystine and selenomethionine, respectively (no absolute LOD was given).

Fig. 6 Electropherogram obtained from a standard solution containing 0.5 µg Se L⁻¹ of each compound. Migration order as in Fig. 5.

Conclusion

This paper has presented a highly efficient sample introduction system developed specifically for coupling of CE with ICP-MS. The interface allows improved detection limits for selenium in CE-ICP-MS speciation without affecting the stability. The LODs obtained for selenium standards were nearly three orders of magnitude bellow the selenium concentration in biological material. Application of the interface to selenium speciation in plasma and urine samples is under investigation and will be published separately.

ack

The authors are grateful to Torben Lindholm Hansen for his precise machining of the PEEK parts used for construction of the interface.

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