A high-efficiency cross-flow micronebulizer for inductively coupled plasma mass spectrometry

A pneumatically driven, high-efficiency cross-flow micronebulizer (HECFMN) is introduced for inductively coupled plasma (ICP) spectrometries. The HECFMN uses a smaller nozzle orifice for nebulizer gas (75 microm in diameter) and a replaceable and adjustable fused-silica capillary for sample uptake. The HECFMN is optimally operated over a wide range of sample uptake rate (5-120 microL/min) at a rf power of 1100 W and nebulizer gas flow rates of 0.8-1.0 L/min when a 50 microm i.d. by 150 microm o.d. capillary is used. The aerosol quality is qualitatively examined in a simple manner, and the transport efficiencies are determined by direct filter collection. Compared with conventional cross-flow nebulizers (CFNs), the HECFMN produces much smaller and more uniform droplets and thus provides much higher analyte transport efficiencies (generally 24-95%) at the sample uptake rates of 5-100 microL/min. Several analytical performance indexes are acquired using an Ar ICPMS system. The sensitivities and detection limits measured with the HECFMN at 50 microL/min sample uptake rate are comparable to or improved over those obtained with a conventional CFN consuming 1 mL/min sample, and the precisions with the HECFMN (typically 1.1-1.7% RSDs) are slightly better than those with the CFN (1.6-2.3% RSDs). The ratios of refractory oxide ion-to-singly charged ion (CeO+/Ce+) are typically in the range from 0.7 to 3.3% for the sample uptake rates of 5-100 microL/min. The free aspiration rate of the HECFMN is 8.9 microL/min for distilled deionized water at the nebulizer gas flow rate of 1.0 L/min without any effect of pressure. The features of the HECFMN suggest good potential for HECFMN use in interfacing ICPMS with capillary electrophoresis and microcolumn high-performance liquid chromatography.     

A high-efficiency cross-flow micronebulizer interface for capillary electrophoresis and inductively coupled plasma mass spectrometry.

A pneumatic nebulizer interface for capillary electrophoresis (CE) and inductively coupled plasma mass spectrometry (ICPMS) is reported. The interface is constructed using a high-efficiency cross-flow micronebulizer (HECFMN) and has the following features. (1) Makeup solutions can be fed to the interface by nebulizer self-aspiration and liquid gravity pressurization. (2) The liquid dead volume of the interface is approximately 65 nL, much smaller than those (200-2500 nL) reported for other interfaces. (3) The interface can be stably operated at a liquid flow rate down to 5 microL/min with a high analyte transport efficiency up to 95% to the plasma and (4) does not induce noticeable laminar flow in the CE capillary at typical nebulizer gas flow rates of 0.8-1.2 L/min. Because of these features, baseline resolution of 10 lanthanides with a CE-ICPMS system using the HECFMN interface is achieved, and detection limits and peak asymmetry are 0.05-1 microg/L and 0.93-1.23, respectively, improved significantly over those reported previously for a CE-ICPMS system using a high-efficiency nebulizer interface. Peak precision for the 10 lanthanides is in the range of 6.2-12.3% RSD (N = 5). Peak widths are from 9.1 s for 139La to 17.9 s for 175Lu. The effects of nebulizer gas flow rate, makeup solution flow rate, and spray chamber volume on CE-ICPMS signal intensity and separation are also evaluated for the HECFMN interface by the separation of Cr3+ and Cr2O7(2-).     

MicroSPE-nanoLC-ESI-MS/MS using 10-microm-i.d. silica-based monolithic columns for proteomics

Silica-based monolithic capillary columns (25 cm x 10 microm i.d.) with integrated nanoESI emitters have been developed to provide high-quality and robust microSPE-nanoLC-ESI-MS analyses. The integrated nanoESI emitter adds no dead volume to the LC separation, allowing stable electrospray operation at flow rates of approximately 10 nL/min. In an initial application with a linear ion trap MS, we identified 5510 unique peptides that covered 1443 distinct Shewanella oneidensis proteins from a 300-ng tryptic digest sample in a single 4-h LC-MS/MS analysis. The use of an integrated monolithic ESI emitter provided enhanced resistance to clogging and provided good run-to-run reproducibility.     

Identification of selenium-containing glutathione S-conjugates in a yeast extract by two-dimensional liquid chromatography with inductively coupled plasma MS and nanoelectrospray MS/MS detection

An approach for the identification of unknown selenium-containing biomolecules was developed, enabling the identification of selenodiglutathione (GS-Se-SG) and the mixed selenotrisulfide of glutathione and cysteinylglycine (GS-Se-SCG) in aqueous yeast extracts. The method consists of two-dimensional liquid chromatography, inductively coupled plasma mass spectrometry (ICPMS) and nanoelectrospray tandem mass spectrometry. Analytes were separated by size-exclusion chromatography followed by preconcentration and separation on a porous graphitic carbon HPLC column. The HPLC effluent was monitored for selenium by ICPMS, and two selenium-containing fractions were isolated and analyzed by nanoelectrospray MS. The nanoelectrospray technique has a low sample consumption of approximately 80 nL/min, enabling a preconcentration of the sample to a few microliters. Mass spectra of the two fractions showed the characteristic Se isotopic pattern centered at m/z 693.1 and 564.0 for the [M + H]+ 80Se ions. MS/MS spectra of adjacent parent ions confirmed the presence of Se. The two selenium species were identified as GS-Se-SG and GS-Se-SCG by collision induced dissociation (CID). The accurately measured masses of the most abundant 691 and 693 u parent ions are in good agreement (differences = 3 ppm) with the theoretical masses. To our knowledge, this is the first identification of GS-Se-SG and GS-Se-SCG in biological matrixes by MS/MS.     

High-efficiency nanoscale liquid chromatography coupled on-line with mass spectrometry using nanoelectrospray ionization for proteomics

We describe high-efficiency (peak capacities of approximately 10(3)) nanoscale (using column inner diameters down to 15 microm) liquid chromatography (nanoLC)/low flow rate electrospray (nanoESI) mass spectrometry (MS) for the sensitive analysis of complex global cellular protein enzymatic digests (i.e., proteomics). Using a liquid slurry packing method with carefully selected packing solvents, 87-cm-length capillaries having inner diameters of 14.9-74.5 microm were successfully packed with 3-microm C18-bonded porous (300-A pores) silica particles at a pressure of 18,000 psi. With a mobile-phase delivery pressure of 10,000 psi, these packed capillaries provided mobile-phase flow rates as low as approximately 20 nL/min at LC linear velocities of approximately 0.2 cm/s, which is near optimal for separation efficiency. To maintain chromatographic efficiency, unions with internal channel diameters as small as 10 microm were specially produced for connecting packed capillaries to replaceable nanoESI emitters having orifice diameters of 2-10 microm (depending on the packed capillary dimensions). Coupled on-line with a hybrid-quadrupole time-of-flight MS through the nanoESI interface, the nanoLC separations provided peak capacities of approximately 10(3) for proteome proteolytic polypeptide mixtures when a positive feedback switching valve was used for quantitatively introducing samples. Over a relatively large range of sample loadings (e.g., 5-100 ng, and 50-500 ng of cellular proteolytic peptides for 14.9- and 29.7-microm-i.d. packed capillaries, respectively), the nanoLC/nanoESI MS response for low-abundance components of the complex mixtures was found to increase linearly with sample loading. The nanoLC/nanoESI-MS sensitivity also increased linearly with decreasing flow rate (or approximately inversely proportional to the square of the capillary inner diameter) in the flow range of 20-400 nL/min. Thus, except at the lower loadings, decreasing the separation capillary inner diameter has an effect equivalent to increasing sample loading, which is important for sample-limited proteomic applications. No significant effects on recovery of eluting polypeptides were observed using porous C18 particles with surface pores of 300-A versus nonporous particles. Tandem MS analyses were also demonstrated using the high-efficiency nanoLC separations. Chromatographic elution time, MS response intensity, and mass measurement accuracy was examined between runs with a single column (with a single nanoESI emitter), between different columns (same and different inner diameters with different nanoESI emitters), and for different samples (various concentrations of cellular proteolytic peptides) and demonstrated robust and reproducible sensitive analyses for complex proteomic samples.     

A colloidal graphite-coated emitter for sheathless capillary electrophoresis/nanoelectrospray ionization mass spectrometry

A colloidal graphite-coated emitter is introduced for sheathless capillary electrophoresis/nanoelectrospray ionization time-of-flight mass spectrometry (CE/ESI-TOFMS). The conductive coating can be produced by brushing the capillary tip to construct a fine layer of 2-propanol-based colloidal graphite. The fabrication involves a single step and requires less than 2 min. Full cure properties develop in approximately 2 h at room temperature and then the tip is ready for use. The coated capillary tip is applied as a sheathless electrospray emitter. The emitter has proven to bear stable electrospray and excellent performance for 50 microm i.d. x 360 microm o.d. and 20 microm i.d. x 360 microm o.d. capillaries within the flow rate of 80-500 nL/min; continuous electrospray can last for over 200 h in positive mode. Baseline separation and structure elucidation of two clinically interesting basic drugs, risperidone and 9-hydroxyrisperidone, are achieved by coupling pressure-assisted CE to ESI-TOFMS using the described sheathless electrospray emitter with a bare fused-silica capillary at pH 6.7. It is found that the signal intensity of m/z in sheathless CE/ESI-TOFMS at pH 6.7 is approximately 50 times higher than that at pH 9.0 for the two analytes, although the electroosmotic flow (EOF) at pH 9.0 provides sufficient flow rate (approximately 150 nL/min) to maintain electrospray.     

Capillary column switching restricted-access media-liquid chromatography-electrospray ionization-tandem mass spectrometry system for simultaneous and direct analysis of drugs in biofluids

Capillary online restricted-access media-liquid chromatography-electrospray ionization-tandem mass spectrometry (RAM-LC-ESI-MS/MS) for direct analysis of drugs and metabolites spiked in biological fluids was developed. Using a column switching setup it was possible to perform effective sample preparation and analysis of raw biological fluids (plasma and urine) without matrix effects in the electrospray mass spectrometric detection step. The peak focusing efficiency of the extraction column was more effective in backflush compared to foreflush mode. The system was able to concentrate diminished samples of polar drugs and their metabolites reaching quantifiable results as low as 1 ng/mL utilizing a sample volume of only 333 nL of biofluids. New column hardware was developed to circumvent clogging problems experienced with plasma injections. The glass fiber filter frit, which is commonly used, was replaced with a short piece of 20 microm i.d. fused silica capillary. The extraction columns were able to handle up to 60 injections and showed a high loading capacity, making the saturation of the MS detector the limiting factor on the linear dynamic range. The simultaneous separation and detection of 10 drugs and metabolites was obtained in 8 min of analysis, including the online sample preparation and enrichment step.     

Preparation of 20-microm-i.d. silica-based monolithic columns and their performance for proteomics analyses

We describe the preparation and performance of high-efficiency 70 cm x 20 microm i.d. silica-based monolithic capillary LC columns. The monolithic columns at a mobile-phase pressure of 5000 psi provide flow rates of approximately 40 nL/min at a linear velocity of approximately 0.24 cm/s. The columns provide a separation peak capacity of approximately 420 in conjunction with both on-line coupling with microsolid-phase extraction and nanoelectrospray ionization-mass spectrometry. Performance was evaluated using a Shewanella oneidensis tryptic digest, and approximately 15-amol detection limits for peptides were obtained using a conventional ion trap and MS/MS for peptide identification. The sensitivity and separation efficiency enabled the identification of 2367 different peptides covering 855 distinct S. oneidensis proteins from a 2.5-microg tryptic digest sample in a single 10-h analysis. The number of identified peptides and proteins approximately doubled when the effective separation time was extended from 200 to 600 min. The number of identified peptides increased from 32 to 390 as the injection amount was increased from 0.5 to 100 ng. Both the run-to-run and column-to-column reproducibility for proteomic analyses were also evaluated.     

Anodic stripping voltammetry combined on-line with inductively coupled plasma-MS via a direct-injection high-efficiency nebulizer

A direct-injection high-efficiency nebulizer (DIHEN) is used to couple a thin-layer electrochemical flow cell on-line with an ICP-mass spectrometer to perform anodic stripping voltammetry (ASV) at a thin mercury film followed by subsequent ICPMS measurements for the stripped metal analytes. The resultant hyphenated technique (ASV-DIHEN-ICPMS) is capable of analyzing select heavy metals present at ultratrace levels (down to low-ppt to sub-ppt levels) that are lower than the detection limits obtained by conventional ICPMS. In addition to its good analytical performance, the technique offers other attractive features such as the ability to eliminate detrimental matrix effects that can compromise ICPMS analyses and the possibility of probing electrode reactions involving trace amounts metal species with ICPMS. For conducting ASV on-line with ICPMS, the DIHEN was found to be more advantageous than the microconcentric nebulizer in terms of minimizing memory effects and potential artifacts caused by the erosion of the Hg film into the flowing solution stream. Compared to a direct injection nebulizer (DIN), the DIHEN was easier to operate. Moreover, its simpler design and the lack of back pressure from the DIHEN capillary made it more compatible with coupling to the thin-layer electrochemical cell than a DIN system.     

Subnanoliter volume wall-jet cells combined with interdigitated microarray electrode and enzyme modified planar microelectrode

Miniaturized wall-jet type flow cells with an active volume of 0.042-15 nL were fabricated for use as highly sensitive electrochemical detectors for capillary electrophoresis/electrochemical detection and small on-line enzyme sensors. The cells consisted of three glass plates and a fused-silica capillary. Two of the plates had microfabricated flow channels and guide trenches for the capillary and working, reference, and counter electrodes. The other plate had a film electrode. When an interdigitated microarray electrode (total area, 66 microm x 64 microm; bandwidth and gap, 2 microm) was installed in the flow cell, the redox cycling enhanced the current at flow rates of less than 100 nL/min even though there were only eight pairs of microbands. A sharp dopamine peak enhanced by the redox cycling was observed when the cell was used for capillary electrophoresis. A square film electrode modified with glutamate oxidase and Os-poly(vinylpyridine) containing HRP was also installed in the flow cell and used to measure neurotransmitter release from cultured nerve cells. When the flow rate was relatively high, the response time of the modified electrode was comparable to that of a cylindrical carbon fiber electrode (33 microm o.d.) modified with the same enzyme and mediator. We observed a transient cathodic current response assigned to the glutamate release with the electrode in the flow cell in a suction mode measurement when we stimulated cultured nerve cells electrically with a dual microelectrode.     

Simplifying CE-MS operation. 2. Interfacing low-flow separation techniques to mass spectrometry using a porous tip

A robust, reproducible, and single-step interface design between low flow rate separation techniques, such as sheathless capillary electrophoresis (CE) and nanoliquid chromatography (nLC), and mass spectrometry (MS) using electrospray ionization (ESI), is introduced. In this design, the electrical connection to the capillary outlet was achieved through a porous tip at the capillary outlet. The porous section was created by removing 1-1.5 in. of the polyimide coating of the capillary and etching this section by 49% solution of HF until it is porous. The electrical connection to the capillary outlet is achieved simply by inserting the capillary outlet containing the porous tip into the existing ESI needle (metal sheath) and filling the needle with the background electrolyte. Redox reactions of water at the ESI needle and transport of these small ions through the porous tip into the capillary provides the electrical connection for the ESI and for the CE outlet electrode. The etching process reduces the wall thickness of the etched section, including the tip of the capillary, to 5-10 microm, which for a 20-30 microm i.d. capillary results in stable electrospray at approximately 1.5 kV. The design is suitable for interfacing a wide range of capillary sizes with a wide range of flow rates to MS via ESI, but it is especially useful for interfacing narrow (<30 microm i.d.) capillaries and low flow rates (<100 nL/min). The advantages of the porous tip design include the following: (1) its fabrication is reproducible, can be automated, and does not require any mechanical tools. (2) The etching process reduces the tip outer diameter and makes the capillary porous in one step. (3) The interface can be used for both nLC-MS and CE-MS. (4) If blocked or damaged, a small section of the tip can be etched off without any loss of performance. (5) The interface design leaves the capillary inner wall intact and, therefore, does not add any dead volume to the CE-MS or nLC-MS interface. (6) Bubble formation due to redox reactions of water at the high-voltage electrode is outside of the separation capillary and does not affect separation or MS performances. The performance of this interface is demonstrated by the analyses of amino acids, peptide, and protein mixtures.     

Separation of plutonium and neptunium species by capillary electrophoresis-inductively coupled plasma-mass spectrometry and application to natural groundwater samples

Capillary electrophoresis (CE) was coupled to ICPMS in order to combine the good performance of this separation technique with the high sensitivity of the ICPMS for the analysis of plutonium and neptunium oxidation states. The combination of a fused-silica capillary with a MicroMist AR 30-I-FM02 nebulizer and a Cinnabar small-volume cyclonic spray chamber yielded the best separation results. With this setup, it was possible to separate a model element mixture containing neptunium (NpO2(+)), uranium (UO2(2+)), lanthanum (La3+), and thorium (Th4+) in 1 M acetic acid. The same conditions were also suitable for the separation of various oxidation states of plutonium and neptunium in different aqueous samples. All separations were obtained within less than 15 min. A detection limit of 50 ppb identical with 2 x 10(-7) M (3-fold standard deviation of a blank) was achieved. To prove the negligible disturbance of the plutonium and neptunium redox equilibria during the CE separations, plutonium and neptunium speciation by CE-ICPMS in acidic solutions was compared with the results of UV/visible absorption spectroscopy and was found to be in good agreement. The CE-ICPMS system was also applied to study the reduction of Pu(VI) in a humic acid-containing groundwater at different pH values.     

Ultratrace and isotope analysis of long-lived radionuclides by inductively coupled plasma quadrupole mass spectrometry using a direct injection high efficiency nebulizer

The direct injection high efficiency nebulizer (DIHEN) was explored for the ultrasensitive determination of long-lived radionuclides ((226)Ra, (230)Th, (237)Np, (238)U, (239)Pu, and (241)Am) and for precise isotope analysis by inductively coupled plasma mass spectrometry (ICPMS). The DIHEN was used at low solution uptake rates (1-100 μL/min) without a spray chamber. Optimal sensitivity (e.g., (238)U, 230 MHz/ppm; (230)Th, 190 MHz/ppm; and (239)Pu, 184 MHz/ppm) was achieved at low nebulizer gas flow rates (0.16 L/min), high rf power (1450 W), and low solution uptake rates (100 μL/min). The optimum parameters varied slightly for the two DIHENs tested. The detection limits of long-lived radionuclides in aqueous solutions varied from 0.012 to 0.11 ng/L. The sensitivity of the DIHEN was improved by a factor of 3 to 5 compared with that of a microconcentric nebulizer (MicroMist used with a minicyclonic spray chamber at a solution uptake rate of 85 μL/min) and a factor of 1.5 to 4 compared with that of a conventional nebulizer (cross-flow used with a Scott type spray chamber at a solution uptake rate of 1 mL/min). The precision of the DIHEN ranged from 0.5 to 1.7% RSD (N = 3) for all measurements at the 10 ng/L concentration level (～3 pg sample size). The sensitivity decreased to 10 MHz/ppm at a solution uptake rate of 1 μL/min. The precision was about 5% RSD at a sample size of 30 fg for each long-lived radionuclide by the DIHEN-ICPMS method. The oxide to atom ratios were less than 0.05 (except ThO(+)/Th(+) ) and decreased under the optimum conditions in the following sequence: ThO(+)/Th(+) > UO(+)/U(+) > NpO(+)/Np(+) > PuO(+)/Pu(+) > AmO(+)/Am(+) > RaO(+)/Ra(+). Atomic and oxide ions were used as analyte ions for ultratrace and isotope analyses of long-lived radionuclides in environmental and radioactive waste samples. The analytical methods developed were applied to the determination of long-lived radionuclides and isotope ratio measurements in different radioactive waste and environmental samples using the DIHEN in combination with quadrupole ICPMS. For instance, the (240)Pu/(239)Pu isotope ratio was measured in a radioactive waste sample at a plutonium concentration of 12 ng/L. This demonstrates a main advantage of DIHEN-ICPMS compared with α-spectrometry, which cannot be used to selectively determine (239)Pu and (240)Pu because of similar α energies (5.244 and 5.255 MeV, respectively).     

On-line coupling of gel electrophoresis and inductively coupled plasma-sector field-mass spectrometry for the determination of dsDNA fragments

The on-line coupling of gel electrophoresis (GE) and inductively coupled plasma-mass spectrometry (ICPMS) is described for the first time. The new method combines the separation power of GE for large biomolecules and the high sensitivity and elemental selectivity of ICPMS. This coupling has been achieved by means of gels housed in glass tubes (length 2.5-20 cm; i.d. 0.5-5 mm). The gel is fixed by glass frits permeable for the analytes. After the electrophoretic separation at voltages up to 500 V, the analytes are transferred to the nebulizer of the ICPMS by an eluent stream, which is separated from the electrode chamber by a membrane. This filter blocks molecules with molecular masses larger than 500 Da. Using double-stranded DNA fragments, commercially available standard solutions were analyzed with on-line GE-ICPMS for the first time by monitoring (31)P(+) with a double-focusing mass spectrometer at a mass resolution of 4000.     

Microsphere entrapped emitters for sample preconcentration and electrospray ionization mass spectrometry

In this study a nano-electrospray emitter is constructed by precisely positioning entrapped octadecylsilane (ODS) particles within a photoinitiated polymer at the exit aperture of a capillary. Following poly-merization, the microsphere/polymer hybrid material is able to withstand pressures greater than 4000 psi for 1 cm length of material. Smaller microspheres (3 microm) patterned at the exit aperture of a capillary generated the most sensitive/stable electrospray from 100 to 1000 nL/min and moderately stable signal under 100 nL/min. Constant infusion of a standard PPG solution from a batch of eleven emitters resulted in a relatively small variance in total ion current (TIC) counts (8%). The entrapped microsphere emitter design yields an emitter that minimizes clogging and eliminates dead volume between the chromatographic bed and the electrospray emitter. The entrapped ODS microspheres can also be used for sample preparation prior to mass spectrometry (MS) analysis. We show the solid-phase extraction and preconcentration of 20-700 fmol of a peptide (leucine enkephalin) prior to MS analysis on an emitter with 1 cm of entrapped microspheres.     

Selective sampling of phosphopeptides for detection by MALDI mass spectrometry

The analysis of phosphopeptides by mass spectrometry (MS) is one of the most challenging tasks in proteomics. This is due to the lower isoelectric point (pI) of phosphopeptides, which leads to inefficient sample ionization in MS, particularly when competing with other peptides. The problem is compounded by the typical low abundance of phosphopeptides in biological samples. We describe here a simple nonsorptive method to isolate phosphopeptides based on their pI. A voltage is applied to selectively migrate the phosphopeptides into a capillary, which are negatively charged at acidic pH. The selectively sampled fraction is directly deposited onto MALDI sample target in nanoliter volumes (7-35 nL) for highly sensitive MS detection. No significant sample loss is evident in this procedure; hence, the MS was able to detect the isolated phosphopeptides at trace quantity. In this case, attomole-level detection limit is achieved for synthetic phosphopeptides (nM concentration and nL volume), from a mixture containing other peptides at up to 1 million times higher in concentration. Selective sampling was also applied to the tryptic digest of beta- and alpha-caseins to reveal the multiple phosphorylated peptides at the low-femtomole level using MALDI MS. Knowledge of pI based on the rejection/injection of peptides was found to be useful in peak assignment. To confirm the sequence of the selectively sampled peptides, fraction collection was performed for offline ESI MS/MS analysis.     

Nanoscale packed-capillary liquid chromatography coupled with mass spectrometry using a coaxial continuous-flow fast atom bombardment interface

Nanoscale packed-capillary liquid chromatography (LC) columns have been coupled with mass spectrometry (MS) using a coaxial continuous-flow fast atom bombardment interface. The combined system has been applied to the analysis of mixtures of peptides, including synthetic mixtures of bioactive peptides and tryptic digests of proteins. Nanoscale packed-capillary columns offer two principal advantages for LC/MS analysis--high chromatographic separation efficiencies and low mobile-phase flow rates. The high separation efficiencies facilitate the separation of complex mixtures, and the low mobile-phase flow rates reduce problems with coupling the LC effluent with the high-vacuum, high-voltage environment of sector MS ion sources. The columns used in this work were 50- or 75-micron i.d., 1-2 m long, packed with 10-micron C18 particles, using mobile-phase flow rates of 50-350 nL/min.     

Species-selective analysis by microcolumn multicapillary gas chromatography with inductively coupled plasma mass spectrometric detection.

A glass rod (5-20 cm long, 2 mm o.d.) containing more than 1200 parallel microchannels (< 40 microns i.d.) was converted into a high-resolution (> 100 theoretical plates cm-1) GC column by coating the inside of each channel in a way that compensated for the dispersion of the channel inner diameter. The columns were evaluated for the separation of mixtures of several organometallic (Hg, Sn, Pb) compounds prior to on-line sensitive metalselective detection by ICPMS. Chromatographic separation conditions were optimized to enable a rapid (within a maximum 30 s) multielemental speciation analysis. Absolute detection limits were 0.1 pg for Hg, 0.05 pg for Sn, and 0.03 pg for Pb using the carrier gas flows of approximately 200 mL min-1. The microcolumn multicapillary GC/ICPMS developed was applied to the analysis of a number of environmental samples. The results were validated with certified reference materials for tin (BCR477, PACS-2) and mercury (DORM-1, TORT-1).     

Application of flow field flow fractionation-ICPMS for the study of uranium binding in bacterial cell suspensions

Field flow fractionation (FFF) is a size-based separation technique applicable to biomolecules, colloids, and bacteria in solution. When interfaced with ICPMS on-line, elemental data can be collected concurrent with size distribution. We employed hyperlayer flow FFF (Fl FFF) methodology to separate cells of Shewanella oneidensis strain MR-1 from exopolymers present in washed cell suspensions. With a channel flow of 4 mL min-1 and a cross-flow of 0.4 mL min-1 cells eluted with a retention time of 4.7 min corresponding to an approximate equivalent spherical cell diameter of 0.8 microm. Cell suspensions were amended with increasing concentrations of U to establish an adsorption isotherm and with fixed U concentrations at varying pH to establish the pH dependence of sorption. A linear sorption isotherm was determined for U solution concentrations of 0.2-16 microM, maximum U sorption occurred at pH 5. A high molecular weight compound, presumably a cell exudate, was identified by Fl FFF-ICPMS. This cell exudate complexed U, and at elevated pH, the exudate appeared to have a greater affinity for U than cell surfaces. Thus, Fl FFF interfaced with ICPMS detection is a powerful analytical technique for metal sorption studies with bacteria; analysis can be carried out on small sample volumes (25 microL) and additional speciation information can be gained because of the versatile Fl FFF separation range and multielement detection capabilities of ICPMS.     

Improvements in LC/Electrospray Ion Trap Mass Spectrometry Performance Using an Off-Axis Nebulizer

Charged residues from the electrospray process have been hypothesized to limit the sensitivity and dynamic range of an ion trap mass spectrometry operation. Incorporation of an off-axis nebulizer (positioned 90-95° from the sampling orifice) was found to drastically reduce the detrimental effects caused by the charged particles or droplets compared to typical on-axis nebulization configurations (spraying 10-20° from sampling orifices). The off-axis nebulizer reduced total ion currents that enter the ion trap (through the reduction of charged residues) by a factor of 5-7 while resulting in an increase of analyte [M + H](+) signal by a factor of 6 compared to an on-axis sprayer at flow rates of 20 μL/min. At higher flow rates (e.g., 800 μL/min) these enhancements are more evident. At flows greater than 200 μL/min, off-axis nebulization reduced total ion current that enters the ion trap by a factor of 30 and resulted in a factor of more than 20 increase in [M + H](+) signal relative to on-axis nebulization. Incorporation of the off-axis nebulizer improved the detection limit and precision for determination of dihydroxyvitamin D(3) in plasma compared to on-axis nebulization. The LC/MS/MS detection limits obtained for the off-axis nebulizer on the ion trap was within a factor of 2 from the detection limit determined by the triple quadrupole. The relative standard deviation of the dihydroxyvitamin D(3) determination was less than 8% for both off-axis ion trap and triple-quadrupole determinations.