Absolute Detection Limits Research Articles

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 5μm, 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 820μL/min and the micromist nebulizer at 200μL/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 (4.77μL/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 0.29fg were achieved with satisfactory resolution.

Laser-based absorption spectroscopy as a technique for rapid in-line analysis of respired gas concentrations of O2 and CO2

The use of sidestream analyzers for respired gas analysis is almost universal. However, they are not ideal for measurements of respiratory gas exchange because the analyses are both temporally dissociated from measurements of respiratory flow and also not generally conducted under the same physical conditions. This study explores the possibility of constructing an all optical, fast response, in-line breath analyzer for oxygen and carbon dioxide. Using direct absorption spectroscopy with a diode laser operating at a wavelength near 2 μm, measurements of expired carbon dioxide concentrations were obtained with an absolute limit of detection of 0.04% at a time resolution of 10 ms. Simultaneously, cavity enhanced absorption spectroscopy at a wavelength near 760 nm was employed to obtain measurements of expired oxygen concentrations with an absolute limit of detection of 0.26% at a time resolution of 10 ms. We conclude that laser-based absorption spectroscopy is a promising technology for in-line analysis of respired carbon dioxide and oxygen concentrations.

Analytical Evaluation of a Reduced-Pressure Microwave-Induced Plasma Studied by Optical Emission Spectrometry Method

ABSTRACT The authors describe a system that utilizes a reduced-pressure (RP) air-cooled microwave-induced plasma (MIP) torch to interface an ultrasonic nebulizer (USN) with an optical emission spectrometer (OES). Argon was investigated as plasma gas. The analytical potential of such techniques was illustrated for the determination of elements. A univariate approach and simplex optimization procedure was used to achieve optimized conditions and derive analytical figures of merit. Analytical performance of the RP-MIP was characterized by determination of the limits of detection (LODs) and precision (RSDs) with the RP-MIP-OES observed at flow rate of 10 µL min−1 without removal of any matrix. The experimental concentration detection limits for simultaneous determination, calculated as the concentration giving a signal equal to three times the standard deviation of the blank (LOD, 3σblank criterion, peak height), were 15, 4.5, 6.2, 2.9, 31, 6.3, 3.1, 13, 5.4, and 33 n g mL−1 for Ba, Ca, Cd, Cu, Fe, Mg, Mn, Ni, Sr, and Zn, respectively. Absolute limits of detection were 167, 50, 68, 32, 350, 69, 34, 143, 59, and 363 pg for Ba, Ca, Cd, Cu, Fe, Mg, Mn, Ni, Sr, and Zn, respectively. The method offers relatively good precision (RSD ranged from 7 to 12%) for liquid analysis and microsampling capability. The accuracy of the method was verified by the use of digested certified reference materials (SRM 1648 (Urban Particulate Matter), IAEA 336 (Lichen), SRM 2710 (Montand Soil), INCT-SBF-4 (Soya Bean Flour)) and by aqueous standard calibration technique. The analyte concentrations in reference materials were in satisfactory agreement with the certified values. The method requires small amounts of reagents and reduces contamination and losses. In general, low-pressure argon discharges proved to be superior, in terms of detection limits (DLs), to atmospheric pressure MIPs for the excitation of the analyte atomic or ionic emission.

MPIC: A High-Throughput Analytical Method for Multiple DNA Targets

We describe the development of a novel combined approach for high-throughput analysis of multiple DNA targets based on multiplex Microdroplet PCR Implemented Capillary gel electrophoresis (MPIC), a two-step PCR amplification strategy. In the first step, the multiple target DNAs are preamplified using bipartite primers attached with universal tail sequences on their 5'-ends. Then, the preamplified templates are compartmentalized individually in the microdroplet of the PCR system, and multiple targets can be amplified in parallel, employing primers targeting their universal sequences. Subsequently, the resulting multiple products are analyzed by capillary gel electrophoresis (CGE). Using genetically modified organism (GMO) analysis as a model, 24 DNA targets can be simultaneously detected with a relative limit of detection of 0.1% (w/w) and absolute limit of detection of 39 target DNA copies. The described system provides a promising alternative for high-throughput analysis of multiple DNA targets.

Protein and peptide fractionation, enrichment and depletion: Tools for the complex proteome

The identification, quantitation and global characterisation of all proteins within a given proteome are extremely challenging. This is due to the absolute detection limits of technology as well as the dynamic range in expression of proteins; and the extreme diversity and heterogeneity of the proteome. To overcome such issues, the use of separation technologies has played a critical role in reducing sample complexity. To date, a plethora of chromatographic and electrophoretic fractionation tools have evolved over the years assisting in simplifying complex protein and peptide mixtures. Here, we review a range of these technologies highlighting the challenges of protein and peptide analysis in the context of proteome research and some of the advantages and disadvantages of present techniques.

Dielectric barrier discharge-plasma induced vaporization and its application to the determination of mercury by atomic fluorescence spectrometry

This paper describes a low-temperature dielectric barrier discharge (DBD)-plasma induced vaporization technique using mercury as a model analyte. The evaporation and atomization of dissolved mercury species in the sample solution can be achieved rapidly in one step, allowing mercury to be directly detected by atomic fluorescence spectrometry. The DBD plasma was generated concentrically in-between two quartz tube (outer tube: i.d. 5 mm and o.d. 6 mm, inner tube: i.d. 2 mm and o.d. 3 mm). A copper electrode was embedded inside the inner quartz tube and sample solution was applied onto the outer surface of the inner tube. The effects of operating parameters such as plasma power, plasma gas identity, plasma gas flow rate and interferences from concomitant elements have been investigated. The difference in the sensitivities of Hg(2+), methylmercury (MeHg) and ethylmercury (EtHg) was found to be negligible in the presence of formic acid (≥1% v/v). The analytical performance of the present technique was evaluated under optimized conditions. The limits of detection were calculated to be 0.02 ng mL(-1) for Hg(2+), MeHg and EtHg, and repeatability was 6.2%, 4.9% and 4.3% RSD (n = 11) for 1 ng mL(-1) of Hg(2+), MeHg and EtHg, respectively. This provides a simple mercury detection method for small-volume samples with an absolute limit of detection at femtogram level. The accuracy of the system was verified by the determination of mercury in reference materials including freeze-dried urine ZK020-2, simulated water matrix reference material GBW(E) 080392 and tuna fish GBW10029, and the concentration of mercury determined by the present method agreed well with the reference values.

Resonant laser-induced breakdown spectroscopy for analysis of lead traces in copper alloys

Resonant laser-induced breakdown spectroscopy (RLIBS) was investigated to improve the limit of detection (LoD) of lead traces in copper alloys. A 5-ns optical parametric oscillator laser pulse was used to ablate the sample (front edge of the pulse) and resonantly excite the lead atoms in the vapor plume (rear edge of the pulse). The laser was tuned to the Pb I 283.31 nm line and the Stokes direct-line fluorescence signal at 405.78 nm was recorded. The experiments were performed in air at atmospheric pressure. The influence of the main experimental parameters, namely the laser wavelength, laser fluence and acquisition delay, on the signal-to-noise ratio (SNR) for the Pb I 405.78 nm line was studied experimentally. We found that the best SNR for the Pb I 405.78 nm line was achieved for a laser fluence of about 1.4 J cm−2, corresponding to the onset of visible damage on the sample, and an acquisition delay of about 5 ns. When the laser was on resonance a Pb I 405.78 nm signal could however be observed for a fluence as low as 0.25 J cm−2. For fluences greater than about 5 J cm−2, similar results were obtained whether the laser wavelength was tuned on resonance or not. Under these optimal conditions, the relative LoD for lead was estimated to be about 8 parts per million, accumulating over 500 laser shots. This amounts to an improvement of about 11-fold when compared to non-resonant LIBS in typical conditions. The corresponding absolute LoD was estimated to be about 0.8 femtograms and was calculated from optical coherence tomography measurements of the crater profiles. The fact that the best performances of RLIBS were obtained near the onset of visible damage on the sample indicates that this approach is particularly suitable for minimally destructive elemental analysis.

Rapid Separation and Quantification of Iron in Uranium Nuclear Matrix by Capillary Zone Electrophoresis (CZE)

A method was developed for rapid separation and determination of iron by employing capillary zone elec-trophoresis (CZE) technique with direct UV detection. Iron could be separated from matrix uranium by di-rect injection of dissolved sample solution into capillary using a mixture of 10 mM HCl and 65 mM KCl (pH = 2) as background electrolyte (BGE) at an applied voltage of 15 kV. The developed method has a very high tolerance for the matrix element U (100 mg/mL) and as such may not need prior separation of iron from the matrix. Iron could be separated with better than 95% recovery. The method showed a linear calibration over a concentration range 1-50 ppm of Fe. The migration times for the iron peak were reproducible within 1% for both pure Fe(III) and in presence of matrix uranium (80 mg/mL). The precision (RSD, n = 22) of peak area obtained for 1ppm of iron was 3.5%. The limit of detection (LOD) (3 ) was 0.1 ppm and the ab-solute LOD was 9 × 10-14 g considering the sample injection volume of 1.5 nL. The developed method has been validated by separating and determining iron in two certified reference materials of U3O8. The method was applied for the determination of iron in different uranium based nuclear materials. The CZE method is versatile for routine analysis as it is simple, rapid and has simple sample preparation procedure.

Mercury speciation in whole blood by gas chromatography coupled to ICP-MS with a fast microwave-assisted sample preparation procedure

A simple and fast method is described for simultaneous determination of methylmercury (MeHg), ethylmercury (Et-Hg) and inorganic mercury (Ino-Hg) in blood samples by using capillary gas chromatography-inductively coupled plasma mass spectrometry (GC-ICP-MS) after derivatization and alkaline digestion. Closed-vessel microwave assisted digestion conditions with tetramethylammonium hydroxide (TMAH) have been optimized. Derivatization by using ethylation and propylation procedures have also been evaluated and compared. The absolute detection limits (using a 1 μL injection) obtained by GC-ICP-MS with ethylation were 40 fg for MeHg and Ino-Hg, respectively, and with propylation were 50, 20 and 50 fg for MeHg, Et-Hg and Ino-Hg, respectively. Method accuracy is traceable to Standard Reference Material (SRM) 966 Toxic Metals in Bovine Blood from the National Institute of Standards and Technology (NIST). Additional validation is provided based on the comparison of results obtained for mercury speciation in blood samples with the proposed procedure and with a previously reported LC-ICP-MS method. With the new proposed procedure no tedious clean-up steps are required and a considerable improvement of the time of analysis was achieved compared to other methods using GC separation.

Studies with a miniaturized microwave induced plasma for element specific detection in gas chromatographic separations of halogenated hydrocarbons

The possibilities of a helium microstrip microwave induced plasma, operated at less than 40 W and less than 250 mL min−1 of gas, as source for optical emission spectrometry (MSP-OES) were studied for the element specific detection of halogenated volatile organic compounds, such as chlorinated and brominated alkanes and arenes, after their separation by capillary gas chromatography (GC). After hyphenation of the GC with MSP-OES with the aid of a heated transfer line, different gas chromatographic parameters, like the carrier gas flow (1.4–3.3 mL min−1) and the oven temperature (100–200), as well as different plasma specific parameters, like the forward power (30–40 W) and the plasma gas flow (100–200 mL min−1), were optimized. In the next step, rotational and excitation temperatures of microstrip plasma were studied in dependence on analyte concentrations between 1 and 10% v/v in pentane and found to be in the order of 1250–1650, as measured with OH bands, and 6100–6900 K, as measured with Fe-lines from ferrocene vapor entered in the plasma gas, respectively. Further, it could be shown that the hyphenated technique GC-MSP-OES can be used for the determination of volatile halogenated organic compounds under the use of an internal standard with recovery rates at the order of 90–110% at the 2–5 mg g−1 level. The absolute limits of detection for volatile halogenated organic compounds, obtained with GC-MSP-OES and the Cl I 837.594 nm, Br I 827.244 nm and C I 247.856 nm, were found to be in the ng-range and the relative detection limits in pentane in the µL L−1 range.

Elemental analysis of airborne particulate matter using an electrical low-pressure impactor and laser ablation/inductively coupled plasma mass spectrometry

In this study, we investigated the elemental contents of airborne particulate matter, having dimensions in the nanometer ( 1.0 μm) size ranges, collected using an electrical low-pressure impactor (ELPI). We used laser ablation/inductively coupled plasma mass spectrometry (LA-ICP-MS) to analyze the airborne particles trapped on filter media and to determine their elemental compositions. To simulate the ELPI collection of samples, we prepared laboratory-made standard filter samples by placing drops of liquid standard solutions onto the filter. The absolute limits of detection ranged from 2 × 10−2 ng for Cd to 1.06 ng for Si. The mean relative standard deviation for five replicates using the standard filter ranged from 3.5 to 17.8%. To determine the accuracy of this protocol, we analyzed the airborne particles using a standard addition method. Comparison with the data obtained using a dichotomous sampler/ICP-MS method revealed that our proposed ELPI/LA-ICP-MS method allowed the elemental and physical characteristics of ultra-fine particles in the atmosphere to be monitored with suitable accuracy to satisfy the needs of environmental, safety, and health applications.

Sulfur determination in coal using molecular absorption in graphite filter vaporizer

The vaporization of sulfur containing samples in graphite vaporizers for atomic absorption spectrometry is accompanied by modification of sulfur by carbon and, respectively, appearance at high temperature of structured molecular absorption in 200–210nm wavelength range. It has been proposed to employ the spectrum for direct determination of sulfur in coal; soundness of the suggestion is evaluated by analysis of coal slurry using low resolution CCD spectrometer with continuum light source coupled to platform or filter furnace vaporizers. For coal in platform furnace losses of the analyte at low temperature and strong spectral background from the coal matrix hinder the determination. Both negative effects are significantly reduced in filter furnace, in which sample vapor efficiently interacts with carbon when transferred through the heated graphite filter. The method is verified by analysis of coals with sulfur content within 0.13–1.5% (m/m) range. The use of coal certified reference material for sulfur analyte addition to coal slurry permitted determination with random error 5–12%. Absolute and relative detection limits for sulfur in coal are 0.16μg and 0.02mass%, respectively.

SYBR Green Real-Time PCR Used to Detect Celery in Food

Celery was found to provoke human allergenic response in some countries. Labeling of celery ingredients was required by the European Union, and the threshold set at 10 mg/kg (0.001%). In our study, a celery mannitol transporter (Mat3) gene-based detection method was established by means of SYBR Green real-time PCR technique. No cross-reactivity was found between celery and the other food materials. Absolute detection limit (LODa), relative detection limit (LODr), and practical detection limit (LODp) of the method were determined through experiments on pure celery DNA, DNA mix, and spiked food samples. The method was able to detect 0.001% raw food sample and 0.01% heated food sample. The utility of the method was confirmed by the investigation of 13 commercial foods.

Determination of catechins and caffeine in proposed green tea standard reference materials by liquid chromatography-particle beam/electron ionization mass spectrometry (LC-PB/EIMS)

Presented here is the quantitative analysis of green tea NIST standard reference materials (SRMs) via liquid chromatography-particle beam/electron ionization mass spectrometry (LC-PB/EIMS). Three different NIST green tea standard reference materials (SRM 3254 Camellia sinesis Leaves, SRM 3255 C. sinesis Extract and SRM 3256 Green Tea-containing Oral Dosage Form) are characterized for the content of caffeine and a series of catechin species (gallic acid, catechin, epicatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate (EGCG)). The absolute limits of detection for caffeine and the catechin species were determined to be on the nanogram level. A reversed-phase chromatographic separation of the green tea reference materials was carried out on a commercial C 18 column using a gradient of water (containing 0.1% TFA) and 2:1 methanol:acetonitrile (containing 0.1%TFA) at 0.9 mL min −1 and an analysis time of 50 min. Quantification of caffeine and the catechin species was carried out using the standard addition and internal standard methods, with the latter providing appreciable improvements in precision and recovery.

Determination of Cisplatin 1,2-Intrastrand Guanine−Guanine DNA Adducts in Human Leukocytes by High-Performance Liquid Chromatography Coupled to Inductively Coupled Plasma Mass Spectrometry

Platinum-containing drugs are widely used to treat cancer in a variety of clinical settings. Their mode of action involves the formation of DNA adducts, which facilitate apoptosis in cancer cells. Cisplatin binds to the N7 position of the purine DNA bases forming intrastrand cross-links between either two adjacent guanines [cis-Pt(NH(3))(2)d(pGpG), 1,2-GG] or an adjacent adenine and guanine [cis-Pt(NH(3))(2)d(pApG), 1,2-AG)]. The cytotoxic efficacy for each of the different types of DNA adducts and the relationship between adduct levels in tumor cells and blood are not well understood. By using these Pt-containing adduct species as biomarkers, information on a patient's response to chemotherapy would be directly related to the mode of action of the drug. This type of analysis requires the most sensitive and specific methods available, to facilitate detection limits sufficient to measure the DNA adduct in the limited sample quantities available from patients. This was achieved in the current study by coupling a highly specific enzyme-based adduct isolation method with a sensitive detection system based on HPLC coupled to inductively coupled plasma mass spectrometry to measure the 1,2-GG cisplatin adducts formed in DNA. The method was developed and validated using calf thymus DNA and two different adenocarcinoma cell lines. The values for the limit of detection (LOD) and the limit of quantitation determined for the 1,2-GG cisplatin adduct were 0.21 and 0.67 fmol per microg DNA, respectively. This corresponds to an absolute LOD of 0.8 pg as Pt for the 1,2-GG adduct. Cisplatin-sensitive (H23) and -resistant (A549) tumor cells were exposed to the drug, and the 1,2-GG adduct levels were measured over a 24 h time period. The results showed a statistically significant (P < 0.05) higher concentration in the sensitive cells as compared to the resistant cells after repair for 7 h. Although the adduct concentration present fell at subsequent time points (12 and 24 h), the levels in each cell line were broadly similar. The protocol was then applied to the analysis of patient samples taken before and then 1 h after treatment. The 1,2-GG cisplatin adduct was present in the range from 113 to 1245 fg Pt per microg DNA in all of the patient samples taken after treatment. Although the adduct was not present at levels greater than the LOD in the initial pretreatment samples, trace amounts were discernible in some patient samples on their third treatment cycle.

Electrospray ionization mass spectrometry from discrete nanoliter‐sized sample volumes

We describe a method for nanoelectrospray ionization mass spectrometry (nESI-MS) of very small sample volumes. Nanoliter-sized sample droplets were taken up by suction into a nanoelectrospray needle from a silicon microchip prior to ESI. To avoid a rapid evaporation of the small sample volumes, all manipulation steps were performed under a cover of fluorocarbon liquid. Sample volumes down to 1.5 nL were successfully analyzed, and an absolute limit of detection of 105 attomole of insulin (chain B, oxidized) was obtained. The open access to the sample droplets on the silicon chip provides the possibility to add reagents to the sample droplets and perform chemical reactions under an extended period of time. This was demonstrated in an example where we performed a tryptic digestion of cytochrome C in a nanoliter-sized sample volume for 2.5 h, followed by monitoring the outcome of the reaction with nESI-MS. The technology was also utilized for tandem mass spectrometry (MS/MS) sequencing analysis of a 2 nL solution of angiotensin I.

Rapid and simple UPLC-MS/MS method for precise phytochelatin quantification in alga extracts

Quantitative phytochelatin (PC) analysis is, due to oxidation sensitivity of the PCs, matrix effects, and time consuming sample preparation, still a challenging analytical task. In this study, a rapid, simple, and sensitive method for precise determination of native PCs in crude extracts of the green alga Chlamydomonas reinhardtii was developed. Algae were exposed 48 h to 70 μM Cd. Coupling of ultra performance liquid chromatography and electrospray ionization tandem mass spectrometry with multi-reaction mode transitions for detection permitted the required short-time, high-resolution separation and detection specificity. Thus, under optimized chromatographic conditions, 10 thiol peptides were baseline-separated within 7 min. Relative detection limits in the nanomolar range in microliter sample volumes were achieved (corresponding to absolute detection limits at femtomole level). Next to glutathione (GSH), the most abundant cadmium-induced PCs in C. reinhardtii, namely CysGSH, PC(2), PC(3), CysPC(2), and CysPC(3), were quantified with high reproducibility at concentrations between 15 and 198 nmol g(-1) fresh weight. The biological variation of PC synthesis of nine independently grown alga cultures was determined to be on average 13.7%.

Characterization of the evanescent field profile and bound mass sensitivity of a label-free silicon photonic microring resonator biosensing platform

Silicon photonic microring resonators have emerged as a sensitive and highly multiplexed platform for real-time biomolecule detection. Herein, we profile the evanescent decay of device sensitivity towards molecular binding as a function of distance from the microring surface. By growing multilayers of electrostatically bound polymers extending from the sensor surface, we are able to empirically determine that the evanescent field intensity is characterized by a 1/e response decay distance of 63 nm. We then applied this knowledge to study the growth of biomolecular assemblies consisting of alternating layers of biotinylated antibody and streptavidin, which follow a more complex growth pattern. Additionally, by monitoring the shift in microring resonance wavelength upon the deposition of a radioactively labeled protein, the mass sensitivity of the ring resonator platform was determined to be 14.7 ± 6.7 [pg/mm 2]/Δpm. By extrapolating to the instrument noise baseline, the mass/area limit of detection is found to be 1.5 ± 0.7 pg/mm 2. Taking the small surface area of the microring sensor into consideration, this value corresponds to an absolute mass detection limit of 125 ag (i.e. 0.8 zmol of IgG), demonstrating the remarkable sensitivity of this promising label-free biomolecular sensing platform.

Automated liquid–solid extraction of pyrene from soil on centrifugal microfluidic devices

Organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) are present in the environment in increasing concentrations and so are of growing concern. Monitoring these species, particularly on-site, can be both difficult and expensive. This paper presents a novel miniaturised magnetically actuated liquid–solid extraction unit integrated in tandem with a filtration unit and a detection unit on a single centrifugal microfluidic device. A demonstration analyte, pyrene, was rapidly extracted and quantified by UV-absorbance from multiple soil samples. The system showed excellent performance for a system designed for field use. Characterization of two types of passive valves was performed along with an extraction time efficiency study. The system provides a factor of 150 reduction in sample weights and extraction solvent volumes and provides statistically similar recoveries to the conventional method with a pyrene detection limit of 1 ppm (0.03 μg absolute detection limit). The reduction in time and solvent and the potential for field use suggest that this device type may be valuable for environmental monitoring.

High precision determination of 228Ra and 228Ra/ 226Ra isotope ratio in natural waters by MC-ICPMS

A new methodology has been developed for precise and accurate determination of 228Ra and the radium isotope ratio ( 228Ra/ 226Ra) by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) in natural water. The method was tested on waters from Lake Kinneret (The Sea of Galilee) and saline springs that flow into it. The radium is pre-concentrated from 60 L of Lake Kinneret water (or 2 L of the saline springs water) by co-precipitation with MnO 2. The separation of radium from the matrix elements was conducted by BioRad ® AG50W-X8 and Sr Spec ® resins. The instrumental limit of detection for 226Ra and 228Ra was 5 fg mL −1 (0.05 ppq) or 0.1 and 30 dpm L −1, respectively, and the absolute limit of detection 0.005 fg. Internal precision of the radium isotope ratio ( 228Ra/ 226Ra) in Lake Kinneret for 228Ra concentration as low as 0.10 fg L −1 (0.06 dpm L −1) was 2.8% (2 σ) and external precision was 10% (2 σ, n = 9).