Liquid Microjunction Surface Sampling Probe Fluid Dynamics: Computational and Experimental Analysis of Coaxial Intercapillary Positioning Effects on Sample Manipulation

A coaxial geometry liquid microjunction surface sampling probe (LMJ-SSP) enables direct extraction of analytes from surfaces for subsequent analysis by techniques like mass spectrometry. Solution dynamics at the probe-to-sample surface interface in the LMJ-SSP has been suspected to influence sampling efficiency and dispersion but has not been rigorously investigated. The effect on flow dynamics and analyte transport to the mass spectrometer caused by coaxial retraction of the inner and outer capillaries from each other and the surface during sampling with a LMJ-SSP was investigated using computational fluid dynamics and experimentation. A transparent LMJ-SSP was constructed to provide the means for visual observation of the dynamics of the surface sampling process. Visual observation, computational fluid dynamics (CFD) analysis, and experimental results revealed that inner capillary axial retraction from the flush position relative to the outer capillary transitioned the probe from a continuous sampling and injection mode through an intermediate regime to sample plug formation mode caused by eddy currents at the sampling end of the probe. The potential for analytical implementation of these newly discovered probe operational modes is discussed.     

High-efficiency headspace sampling of volatile organic compounds in explosives using capillary microextraction of volatiles (CMV) coupled to gas chromatography–mass spectrometry (GC-MS)

A novel geometry configuration based on sorbent-coated glass microfibers packed within a glass capillary is used to sample volatile organic compounds, dynamically, in the headspace of an open system or in a partially open system to achieve quantitative extraction of the available volatiles of explosives with negligible breakthrough. Air is sampled through the newly developed sorbent-packed 2 cm long, 2 mm diameter capillary microextraction of volatiles (CMV) and subsequently introduced into a commercially available thermal desorption probe fitted directly into a GC injection port. A sorbent coating surface area of ～5?×?10^?2 m² or 5,000 times greater than that of a single solid-phase microextraction (SPME) fiber allows for fast (30 s), flow-through sampling of relatively large volumes using sampling flow rates of ～1.5 L/min. A direct comparison of the new CMV extraction to a static (equilibrium) SPME extraction of the same headspace sample yields a 30 times improvement in sensitivity for the CMV when sampling nitroglycerine (NG), 2,4-dinitrotoluene (2,4-DNT), and diphenylamine (DPA) in a mixture containing a total mass of 500 ng of each analyte, when spiked into a liter-volume container. Calibration curves were established for all compounds studied, and the recovery was determined to be ～1 % or better after only 1 min of sampling time. Quantitative analysis is also possible using this extraction technique when the sampling temperature, flow rate, and time are kept constant between calibration curves and the sample.     

Optimization of dynamic pH junction for the sensitive determination of amino acids in urine by capillary electrophoresis

A capillary electrophoresis method with UV-absorbance detection was studied and optimized for the determination of underivatized amino acids in urine. To improve concentration sensitivity the utility of in-capillary analyte stacking via dynamic pH junction was investigated with phenylalanine (Phe) and tyrosine (Tyr) as model amino acids. Before sample injection, a plug of ammonium hydroxide solution was injected to enable analyte concentration. Samples were 1:1 (v/v) mixed with background electrolyte (1 M formic acid) prior to injection. The effect of the injected sample volume, and the injected ammonium hydroxide volume and concentration on analyte stacking and separation performance was investigated. The optimal volume of ammonium hydroxide depended on the injected sample volume. Using a dynamic pH junction good resolution (1.4) was obtained for a sample injection volume of 10% of the capillary (196 nl) with Phe and Tyr dissolved in water. Limits of detection (LODs) were 0.036 and 0.049 μM for Phe and Tyr, respectively. For urine samples, the optimized procedure comprised a 1.7-nl injection of 12.5% ammonium hydroxide, followed by a 196-nl injection of urine spiked with Phe and Tyr. Satisfactory resolution was obtained and amino acid peak widths at half height were only 1.6 s indicating efficient stacking. Calibration plots for Phe and Tyr in urine showed good linearity (R(2) > 0.96) in the concentration range 10-175 μM, and LODs for Phe and Tyr were 0.054 and 0.019 μM, respectively. RSDs for peak area and migration time for Phe and Tyr were below 7.5% and 0.75%, respectively.     

Self-aspirating atmospheric pressure chemical ionization source for direct sampling of analytes on surfaces and in liquid solutions

A self-aspirating heated nebulizer probe is described and demonstrated for use in the direct analysis of analytes on surfaces and in liquid samples by atmospheric pressure chemical ionization (APCI) mass spectrometry. Functionality and performance of the probe as a self-aspirating APCI source is demonstrated using reserpine and progesterone as test compounds. The utility of the probe to sample analytes directly from surfaces was demonstrated first by scanning development lanes of a reversed-phase thin-layer chromatography plate in which a three-component dye mixture, viz., Fat Red 7B, Solvent Green 3, and Solvent Blue 35, was spotted and the components were separated. Development lanes were scanned by the sampling probe operated under computer control (x, y plane) while full-scan mass spectra were recorded using a quadrupole ion trap mass spectrometer. In addition, the ability to sample the surface of pharmaceutical tablets (viz., Extra Strength Tylenol and Evista tablets) and to detect the active ingredients (acetaminophen and raloxifene, respectively) selectively was demonstrated using tandem mass spectrometry (MS/MS). Finally, the capability to sample analyte solutions from the wells of a 384-well microtiter plate and to perform quantitative analyses using MS/MS detection was illustrated with cotinine standards spiked with cotinine-d3 as an internal standard.     

Electrochemically initiated tagging of thiols using an electrospray ionization based liquid microjunction surface sampling probe two‐electrode cell

This paper reports on the conversion of a liquid microjunction surface sampling probe (LMJ‐SSP) into a two‐electrode electrochemical cell using a conductive sample surface and the probe as the two electrodes with an appropriate battery powered circuit. With this LMJ‐SSP, two‐electrode cell arrangement, tagging of analyte thiol functionalities (in this case peptide cysteine residues) with hydroquinone tags was initiated electrochemically using a hydroquinone‐doped solution when the analyte either was initially in solution or was sampled from a surface. Efficient tagging (～90%), at flow rates of 5–10 µL/min, could be achieved for up to at least two cysteines on a peptide. The high tagging efficiency observed was explained with a simple kinetic model. In general, the incorporation of a two‐electrode electrochemical cell, or other multiple electrode arrangement, into the LMJ‐SSP is expected to add to the versatility of this approach for surface sampling and ionization coupled with mass spectrometric detection. Published in 2009 by John Wiley & Sons, Ltd.     

Simple approach for elimination of blank peak effects in flow-injection analysis of samples containing trace analyte and an excess of another solute

A blank peak effect in FIA systems often found for samples containing an excess of solute other than the analyte has been studied. Emphasis has been given to trace level determinations and a simple approach for elimination of this effect is presented. The present approach is based on the use of a large sample injection volume, which results in a portion of the sample plug being undiluted with carrier and hence prevents the formation of refractive index gradients in the undiluted portion. The quantitative performance of this approach was demonstrated with the determination of iron in the presence of 0.20M sodium chloride and cobalt in the presence of 0.10M glucose. No significant difference in the accuracy, precision, and limit of detection was observed between samples for iron both in the presence and absence of sodium chloride and for cobalt in the presence and absence of glucose. Despite using a large sample volume (0.89 ml for a 400-cm sample loop), the sample throughput was about 25/hr.     

Numerical analysis of an electrokinetic double-focusing injection technique for microchip CE

The injection techniques in electrophoresis microchips play an important role in the sample-handling process, whose characteristics determine the separation performance achieved, and the shape of a sample plug delivered into the separation channel has a great impact on the high-quality separation performance as well. This paper describes a numerical investigation of different electrokinetic injection techniques to deliver a sample plug within electrophoresis microchips. A novel double-focusing injection system is designed and fabricated, which involves four accessory arm channels in which symmetrical focusing potentials are loaded to form a unique parallel electric field distribution in the intersection of injection channel and separation channel. The parallel electric field effectuates virtual walls to confine the spreading of a sample plug at the intersection and prevents sample leakage into separation channel during the dispensing step. The key features of this technique over other injection techniques are the abilities to generate regular and nondistorted shape of sample plugs and deliver the variable-volume sample plugs by electrokinetic focusing. The detection peak in the proposed injection system is uniform regardless of the position of the detection probe in the separation channel, and the peak resolution is greatly enhanced. Finally, the double-focusing injection technique shows the flexibility in detection position and ensures improved signal sensitivity with good peak resolution due to the delivered high-quality sample plug.     

A multilayer poly(dimethylsiloxane) electrospray ionization emitter for sample injection and online mass spectrometric detection

An ESI emitter made of poly(dimethylsiloxane) interfaces on-chip sample preparation with MS detection. The unique multilayer design allows both the analyte and the spray solutions to reside on the device simultaneously in discrete microfluidic environments that are spatially separated by a polycarbonate track-etched, nanocapillary array membrane (NCAM). In direct spray mode, voltage is applied to the microchannel containing a spray solution delivered via a syringe pump. For injection, the spray potential is lowered and a voltage is applied that forward biases the membrane and permits the analyte to enter the spray channel. Once the injection is complete, the bias potential is switched off, and the spray voltage is increased to generate the ESI of the injected analyte plug. Consecutive injections of a 10 microM bovine insulin solution are reproducible and produce sample plugs with limited band broadening and high quality mass spectra. Peptide signals are observed following transport through the NCAM, even when the peptide is dissolved in solutions containing up to 20% seawater. The multilayer emitter shows great potential for performing multidimensional chemical manipulations on-chip, followed by direct ESI with negligible dead volume for online MS analysis.     

The effect of beta-cyclodextrin on liquid chromatography/electrospray-mass spectrometry analysis of hydrophobic drug molecules

Cyclodextrins (CDs) are widely used in the pharmaceutical industry for their capability of improving bioavailability, solubility, or stability of drugs via the formation of soluble inclusion complexes. CDs have also been widely used in various chemical analysis methods. In this work, liquid chromatography/electrospray mass spectrometry (LC/ESI-MS) analysis for four different drugs (imipramine, desipramine, propranolol, and naproxen) that form inclusion complexes with CDs was performed in the presence and absence of beta-CD. These drugs are subject to nonspecific adsorption when brought into contact with plastics, such as HPLC tubing, sample collection and preparation apparatus, etc. Inclusion of the CD in the samples reduces this nonspecific adsorption due to competitive complex formation between the CD and the analyte. ESI-MS ion intensities increased when beta-CD was included in the sample with concentrations up to 1% (w:v), with a diverter valve installed post LC column. The degree of increased ion signal correlated with the beta-cyclodextrin:analyte binding constant. beta-CD appeared to elute within the void volume time and was observed in a full spectrum scan among the different analyte samples with up to 0.01% beta-CD injected directly to the LC/MS system with the diverter valve switched inline with the mass spectrometer. The use of the diverter valve allowed for direct injection of samples containing up to 1% beta-CD to the LC/MS without any deterioration of analyte ion signal.     

Impact of reservoir potentials on the analyte behavior in microchip electrophoresis: computer simulation and experimental validation for DNA fragments

Fundamental understanding of the impact of reservoir potentials on the analyte behavior on the microfluidic chips is an important issue in microchip electrophoresis (MCE) for suitable injection and separation of analytes, since the applied potentials may significantly affect the shape of sample plug, sample leakage from the injection channel to the separation channel, injected sample amount, and separation efficiency. This study addressed this issue for the case of a conventional cross-geometry microchip with four reservoirs using computer simulations, the results of which were verified by the analysis of DNA fragments. For the microchip with a definite structure and migration distance, the injected sample amount was shown to be the vital parameter for improving the limit of detection and resolution. During injection, the shape of the sample plug could be adjusted by varying the reservoir potentials. It was demonstrated that a "magnified injection" (applying high voltage on the three reservoirs to the sample reservoir) is useful to enhance the detection sensitivity depending on the analyte composition, although such injection was previously avoided because of introducing too large amounts of the analyte in comparison with two established modes, floating and pinched injection. Optimal magnified injection was proved to improve the sensitivity for about 4 times over that of pinched injection for the analysis of DNA step ladders using microchip gel electrophoresis (MCGE). Sample leakage of DNA fragments could be suppressed by applying a high positive voltage on injection channel during separation, but the voltage degraded the injected amount and resolution.     

Uncertainty in analyte mass for samples containing small numbers of particles.

A sampling equation was derived that relates the standard deviation in analyte mass to the number of particles in the sample, the fractions of the different types of particles in the mixture and the masses and analyte concentrations of the individual particles. The equation, which is applicable to samples containing any number of particles, was verified by sampling and analysis of two cereal grain mixtures for manganese, potassium, chlorine and magnesium, and by Monte Carlo computer simulation. Comparison of the sampling precision of analyte mass with the analytical measurement precision was also studied, and it was shown that use of the equation allows the calculation of the minimum number of particles required to hold the sampling relative standard deviation to that of the analytical measurement.     

High-performance thin-layer chromatography plate blotting for liquid microjunction surface sampling probe mass spectrometric analysis of analytes separated on a wettable phase plate

A blotting method that transfers analytes separated on wettable high‐performance thin‐layer chromatography (HPTLC) plates to a hydrophobic reversed‐phase C8 HPLTC plate suitable for analysis with a liquid microjunction surface sampling probe electrospray ionization mass spectrometry system was described and demonstrated. The simple blotting procedure transfers the analyte from the wettable plate to the topmost surface of a rigidly backed, easy‐to‐mount hydrophobic substrate that already has been proven viable for analysis by this sampling probe/mass spectrometry system. The utility of the approach was demonstrated by the analysis of a four‐component peptide mixture originally separated on a ProteoChrom® HPTLC cellulose sheet and then blotted onto the reversed‐phase HPTLC plate. Published in 2011 by John Wiley & Sons, Ltd.     

气相色谱仪的两种液体样品自动进样器的兼容设计与研制

设计和构建了多轴多模式、闭环伺服控制系统的研发平台,实现了用于气相色谱仪的不同结构的两种液体自动进样器的并行设计与研制。基于兼容与互换的思路,开发出一体化嵌入式控制驱动模块,采用相同的直流电机、编码器,电气与机械接口统一标准,实现了上述组件在两种机型间的直接代换。研制的标准型110个样品位和平台型40个样品位两种液体自动进样器,与气相色谱仪联机,采用1μL手动进样针,对于0.5μL十六烷-异辛烷标准液体样品,6个不同瓶位连续6次进样测试,重复进样的信号峰面积RSD分别为1.1%和1.5%,取0.1,0.3,0.5,0.7和0.9μL进样,峰面积与进样量的线性相关系数为0.9947。     

Direct injection of seawater for the analysis of nitroaromatic explosives and their degradation products by micellar electrokinetic chromatography

Practical considerations for the injection and separation of nitroaromatic explosives in seawater sample matrices are discussed. The use of high surfactant concentrations and long electrokinetic injections allows for improved detection limits. Sensitivity was enhanced by two mechanisms, improved stacking at the detector-side of the sample plug and desorption of analyte from the capillary wall by surfactant-containing BGE from the inlet side of the sample plug. Calculated limits of detection (S/N = 3) for analytes prepared in pure seawater were 70–800 ppb with injection times varying from 5 to 100 s.     

Time-resolved measurements of individual ion cloud signals to investigate space-charge effects in plasma mass spectrometry

A new approach to directly monitor space charge induced effects due to high concentrations of efficiently ionized elements in inductively coupled plasma mass spectrometry (ICP-MS) is described. The broadening of ion clouds produced from individual, monodisperse drops of sample is measured by using time-resolved ICP-MS. The extent of broadening due to high concentrations of Pb in the sample is related inversely to the analyte mass. For the lightest analyte investigated, Li⁺, the relative width of the time-resolved analyte peak increases and then shows a dip in the center as the Pb concentration is increased to 500 and then 1500 µg/mL. The initial results of experiments that investigated chemical matrix effects as a function of concomitant species concentration, analyte mass, and sampling location in ICP-MS are consistent with space-charge effects.     

The active and passive sampling of benzene, toluene, ethyl benzene and xylenes compounds using the inside needle capillary adsorption trap device

A new and simple method of solventless extraction of volatile organic compounds (VOCs) from air is presented. The sampling device has an adsorbing carbon coating on the interior surface of a hollow needle, and is called the inside needle capillary adsorption trap (INCAT). This paper describes a study of the reproducibility in the preparation and sampling of the INCAT device. In addition, this paper examines the effects of sample volume in active sampling and exposure time in passive sampling on the analyte adsorption. Analysis was achieved by sampling the air from an environmental chamber doped with benzene, toluene, ethyl benzene and xylenes (BTEX) compounds. Initial rates of adsorption were found to vary among the different compounds, but ranged from 0.0099 to 0.016 nmol h(-1) for passive sampling and from 2.2 to 10 nmol h(-1) for active sampling. Analysis was done by thermal desorption of the adsorbed compounds directly into a gas chromatograph injection port. Quantification of the analysis was done by comparison to actively sampled activated carbon solid phase extraction (SPE) measurements.     

pH-mediated acid stacking with reverse pressure for the analysis of cationic pharmaceuticals in capillary electrophoresis

When using capillary electrophoresis (CE) for the analysis of biological samples, it is often necessary to employ techniques to overcome peak-broadening that results from having a high-conductivity sample matrix. To improve the concentration detection limits and separation efficiency of cationic pharmaceuticals in CE, pH-mediated acid stacking was performed to electrofocus the sample, improving separation sensitivity for the analyzed cations by 60-fold. However, this method introduces a large titrated acid plug into the capillary. To overcome the limitations this low-conductivity plug poses to stacking, the plug was removed prior to the separation step by applying reverse pressure to force it out of the anode of the capillary. Employing this technique allows for roughly twice the volume of sample to be injected. A maximum sample injection time of 240 s was attainable with baseline peak resolution compared to a maximum sample injection time of 120 s without reverse pressure, leading to a twofold decrease in the limits of detection of the analytes used. Separation efficiency overall is also improved when utilizing the reverse pressure step. For example, a 60 s sample injection time results in 94,000 theoretical plates as compared to 60,500 theoretical plates without reverse pressure. This reverse-pressure method was used for detection and quantitation of several cationic pharmaceuticals that were prepared in Ringer's solution to simulate microdialysis sampling conditions.     

Measurement and Visualization of Mass Transport for the Flowing Atmospheric Pressure Afterglow (FAPA) Ambient Mass-Spectrometry Source

Ambient desorption/ionization mass spectrometry (ADI-MS) has developed into an important analytical field over the last 9 years. The ability to analyze samples under ambient conditions while retaining the sensitivity and specificity of mass spectrometry has led to numerous applications and a corresponding jump in the popularity of this field. Despite the great potential of ADI-MS, problems remain in the areas of ion identification and quantification. Difficulties with ion identification can be solved through modified instrumentation, including accurate-mass or MS/MS capabilities for analyte identification. More difficult problems include quantification because of the ambient nature of the sampling process. To characterize and improve sample volatilization, ionization, and introduction into the mass spectrometer interface, a method of visualizing mass transport into the mass spectrometer is needed. Schlieren imaging is a well-established technique that renders small changes in refractive index visible. Here, schlieren imaging was used to visualize helium flow from a plasma-based ADI-MS source into a mass spectrometer while ion signals were recorded. Optimal sample positions for melting-point capillary and transmission-mode (stainless steel mesh) introduction were found to be near (within 1 mm of) the mass spectrometer inlet. Additionally, the orientation of the sampled surface plays a significant role. More efficient mass transport resulted for analyte deposits directly facing the MS inlet. Different surfaces (glass slide and rough surface) were also examined; for both it was found that the optimal position is immediately beneath the MS inlet.

Study of the factors affecting the performance of microextraction by packed sorbent (MEPS) using liquid scintillation counter and liquid chromatography-tandem mass spectrometry

Microextraction by packed sorbent (MEPS) is a new technique for sample preparation that can be connected on-line with LC or GC. In MEPS, approximately 1-2 mg of the solid packing material is inserted into a syringe (100-250 μL) as a plug. Sample preparation takes place on the packed bed. The bed can be packed or coated to provide selective and suitable sampling conditions. The new method is very promising for extraction of drugs and metabolites from biological samples.In this paper, some factors affecting the performance of MEPS such as recovery, carry-over, leakage, washing volume and elution volume were studied using C18 and hydroxylated polystyrene-divinylbenzene copolymer (ENV+) as sorbents. Radioactively labelled bupivacaine in plasma samples was used as test analyte. For the extraction of this drug, using methanol/water 95:5 (v/v) (0.25% ammonium hydroxide) was used as elution solvent. The analyte response increased with increasing the elution volume and it was linear upp up to 100 μL utilizing liquid scintillation counter. Further, for concentrating the sample, we found that MEPS may be used such that the sample can be drawn through the needle, up and down, several times. The analyte leakage increases as the volume washing increases, though higher washing volumes may also result in cleaner extracts. To eliminate analyte carry-over, the sorbents were washed first with 3 × 250 μL elution solution and then with 3 × 250 μL washing solution. In addition, the reproducibility measurements show relatively good relative standard deviation (RSD) % values concerning analyte recovery and analyte leakage. The present study provides an understanding of basic aspects when optimizing methods for MEPS. In this study, MEPS was used off-line with liquid scintillation counter and on-line with LC-MS/MS.