Precision Of Peak Heights Research Articles

RP-HPLC Analysis of Phenolic Lignin Monomers with DAD and Coulometric Array Detection.

An high performance liquid chromatography (HPLC) method, applying diode array (DAD) and coulometric array detection simultaneously, was developed to determine monomeric phenolic compounds in lignin hydrolysates. To allow for additional mass spectrometric detection, a formic acid gradient was designed as an alternative to the non-volatile phosphate buffer gradient. Within a total run time of 28 min, 10 out of 14 analytes were baseline separated by the formic acid gradient, and 8 compounds were completely resolved by the phosphate gradient. The dependence of the coulometric detection on cell potential, potential step width, eluent pH and eluent composition was comprehensively tested. Detection limits ≤20 μg/L were achieved by DAD detection for nine analytes and by coulometric array detection for all analytes, demonstrating the superior sensitivity of electrochemical detection. DAD offered better linearity and reproducibility. Inter-day precision of peak heights spanned from 0.26 to 2.00% (formic acid gradient) and from 0.75 to 2.85% (phosphate buffer gradient) operating the DAD detector, but it exceeded 10% for several compounds applying the coulometric array detector. The simultaneous operation of both detectors offered an enhanced certainty of substance identification. The appropriateness of the method was confirmed by the analysis of various hydrolysates of lignin-containing materials.

A compact and low-cost miniaturized analysis system composed of microchip electrophoresis and chemiluminescence detection manipulated by a simple subatmospheric pressure fluid-driven device

A portable and low-cost miniaturized analysis system was proposed, in which microchip electrophoresis (MCE) and chemiluminescence (CL) were used as the separation and detection units, respectively. A porous monolithic plug was created in the separation channel of the microchip as a select valve, which prevented the pressure-driven CL reagents flowing back into the separation channel but allowed electrophoretic migration along the separation channel. The sensitivity was greatly enhanced by improving the mixing efficiency using a spiral detection channel with an increased length facing the photomultiplier tube (PMT). Peak width could be significantly reduced by increasing the flow rate of CL reagents. Putting grounding electrode before the detection channel significantly improved the reproducibility. A simple and compact subatmospheric pressure fluid-driven device was developed for manipulating the whole analytical process, including variable-volume sample loading, electrophoretic separation and CL reagents transportation. All miniaturized components for constructing a portable MCE-CL system are commercially available. This approach considerably simplified the operation and equipments for constructing an efficient MCE-CL system. Nine metal cations, such as Cr(III), Co(II), Cu(II), Ni(II), Au(III), Mn(II), Zn(II), Pt(II), Pb(II) were successfully separated within 200 s. Migration time precisions ranging from 0.39% for Cr(III) to 2.1% for Cu(II) were obtained for ten consecutive determinations with peak height precisions from 1.67% for Co(II) to 5.73% for Pb(II). Detection limits ranging from 7.5 x 10(-11) mol L(-1) for Co(II) to 8.3 x 10(-9) mol L(-1) for Pb(II) were achieved, which were about three orders lower than the present MCE-CL system. It has been applied for the determination of metals in tea. The results with the recoveries from 97.0% to 102.3% proved that the proposed MCE-CL system offers a number of benefits including miniaturization, high sensitivity and better resolution.

Microfluidic Picoliter-Scale Translational Spontaneous Sample Introduction for High-Speed Capillary Electrophoresis

A novel microfluidic picoliter-scale sample introduction approach was developed by combining the spontaneous injection approach with a capillary electrophoresis (CE) system based on a short capillary and slotted-vial array. A droplet splitting phenomenon at the capillary inlet end during the spontaneous sample introduction process was observed for the first time. On the basis of this phenomenon, a translational spontaneous injection approach was established to reduce sample injection volumes to the sub-100 pL range. A versatile high-speed capillary electrophoresis (HSCE) system was built on the basis of this sample injection approach with separation performance comparable to or even better than those reported in microfluidic chip-based CE systems. The HSCE system was composed of a short fused-silica capillary and an automated sample introduction system with slotted sample and buffer reservoirs and a computer-programmed translational stage. The translational spontaneous sample injection was performed by linearly moving the stage, allowing the capillary inlet first to enter the sample solution and then removing it. A droplet was left at the tip end and spontaneously drawn into the capillary by surface tension effect to achieve sample injection. The stage was continuously moved to allow the capillary inlet to be immersed into the buffer solution, and CE separation was performed by applying a high voltage between the buffer and waste reservoirs. With the use of the novel system, high-speed and efficient capillary zone electrophoresis (CZE) separation of a mixture of five fluorescein isothiocyanate (FITC) labeled amino acids was achieved within 5.4 s in a short capillary with a separation length of 15 mm, reaching separation efficiencies up to 0.40 microm plate height. Outstanding peak height precisions ranging from 1.2% to 3.7% RSD were achieved in 51 consecutive separations. By extension of the separation length to 50 mm, both high-speed and high-resolution CZE separation of eight FITC-labeled amino acids could be obtained in less than 21 s with theoretical plates ranging from 163,000 to 251,000 (corresponding to 0.31-0.20 microm plate heights). The present HSCE system also allowed fast chiral separations of FITC-labeled amino acids under micellar electrokinetic chromatography (MEKC) mode within 6.5 s.

Field amplified sample stacking coupled with chip-based capillary electrophoresis using negative pressure sample injection technique

A multi-T microchip for integrated field amplified sample stacking (FASS) with CE separation to increase the chip-based capillary electrophoresis (chip-based CE) sensitivity was developed. Volumetrically defined large sample plug was formed in one step within 5 s by the negative pressure in headspace of the two sealed sample waste reservoirs produced using a syringe pump equipped with a 3-way valve. Stacking and separation can proceed only by switching the 3-way valve to release the vacuum in headspace of the two sample waste reservoirs. This approach considerably simplified the operations and the equipments for FASS in chip-based CE systems. Migration time precisions of 3.3% and 1.3% RSD for rhodamine123 (Rh123) and fluorescien sodium salt (Flu) in the separation of a mixture of Flu and Rh123 were obtained for nine consecutive determinations with peak height precisions of 4.8% and 3.4% RSD, respectively. Compared with the chip-based CE on the cross microchip, the sensitivity for analysis of FlTC, FITC-labeled valine (Val) and Alanine (Ala) increased 55-, 41- and 43-fold, respectively.

Negative pressure pinched sample injection for microchip-based electrophoresis

A simple method for injecting well-defined non-biased sample plugs into the separation channel of a microfluidic chip-based capillary electrophoresis system was developed by a combination of flows generated by negative pressure, electrokinetic and hydrostatic forces. This was achieved by using only a single syringe pump and a single voltage supply at constant voltage. In the loading step, a partial vacuum in the headspace of a sealed sample waste reservoir was produced using a syringe pump equipped with a 3-way valve. Almost instantaneously, sample was drawn from the sample reservoir across the injection intersection to the sample waste reservoir by negative pressure. Simultaneously, buffer flow from the remaining two buffer reservoirs pinched the sample flow to form a well-defined sample plug at the channel intersection. In the subsequent separation stage, the vacuum in headspace of the sample waste reservoir was released to terminate all flows generated by negative pressure, and the sample plug at the channel intersection was electrokinetically injected into the separation channel under the potential applied along the separation channel. The liquid levels of the four reservoirs were optimized to prevent sample leakage during the separation stage. The approach considerably simplified the operations and equipment for pinched injection in chip-based CE, and improved the throughput. Migration time precisions of 3.3 and 1.5% RSD for rhodamine123 (Rh123) and fluorescein sodium (Flu) in the separation of a mixture of Flu and Rh123 were obtained for 56 consecutive determinations with peak height precisions of 6.2% and 4.4% RSD for Rh123 and Flu, respectively.

Peak Height Precision in Hadamard Transform Time-of-Flight Mass Spectra

Hadamard transform (HT) time-of-flight mass spectrometry (TOFMS) is a multiplexing technique that offers high duty cycle for the mass analysis of continuous ion sources. The multiplexing advantage is maximized when spectral noise is independent of signal intensity. For conditions in which shot noise predominates, the variance in each peak is a function of the population of all measured species. We develop expressions for the performance of a HT-TOF mass spectrometer based on Poissonian statistics for the arrival times of ions at the detector. These expressions and complementary probabilistic simulations are used to estimate the magnitude of the baseline noise as a function of mass spectral features and acquisition conditions. Experiment validates the predictions that noise depends on the total number of ions in the acquired spectrum, and the achieved signal-to-noise ratio for a given species depends on its relative population. We find that for HT-TOFMS experiments encoded with an n-order binary off-on sequence that contains N=2n−1 elements, the peak height precision, which is the peak intensity divided by its standard deviation, is greater than that of an equivalent conventional TOF experiment by a factor of √N/2 times the square root of the fractional abundance of the peak of interest. Thus, HT-TOFMS is superior to conventional TOF for all species whose fractional abundance Fi exceeds 2/N, which for a typical N value of 2047 corresponds to Fi > 0.001. HT-TOF mass spectra collected at 2500 per second demonstrates the method’s capability of monitoring transient processes not possible by conventional means.

A micro-chemiluminescence determination of cyanide in whole blood

A reactant volume self-controlled micro-device was presented and applied to the flow injection chemiluminescence (CL) for determination of cyanide in whole blood. A mini distiller was fabricated for cyanide extraction from the blood samples with the extraction efficiencies of cyanide ≥98%. A fluidic control platform with air driving was fabricated. The described system showed the features of easy fabrication, undiluted sample injection, safe analysis operation, and suitability for automatic cyanide analysis. The calibration curve showed linearity in the cyanide concentration range of 5.0×10 −7 to 5.0×10 −5 mol l −1 with the detection limits (3 σ) of 2.3×10 −7 mol l −1. CL peak-height precision was 1.9% R.S.D. ( n=11) at the 1.0×10 −6 mol l −1 cyanide level. The new devices were applied to the analysis of cyanide in rabbit whole blood samples and the results agreed well with those obtained from official method.

High-speed gas chromatography using synchronized dual-valve injection.

A novel injection technique for high-speed gas chromatography is demonstrated. Synchronized dual-valve injection is shown to provide peak widths as low as 1.5 ms (width at half-height) for an unretained analyte. This was achieved using a 0.5-m DB-5 column with an internal diameter of 100 microm and a film thickness of 0.4 microm operated at a temperature of 150 degrees C with a column absolute head pressure of 85 psi, resulting in a dead time of only t(o) = 26 ms ( approximately 1900 cm/s, 26 mL/min). Using the DB-5 column in a 1-m length under the same instrumental parameters, with a resulting linear flow velocity of 935 cm/s (12.7 mL/min, t(o) = 117 ms), a minimum peak width of 3.3 ms was obtained. During an isothermal separation, 10 analytes were separated in a time window of 400 ms. A rigorous comparison of experimental and theoretical band-broadening data based on the Golay equation showed that band broadening is limited almost entirely by the chromatographic band broadening terms expressed by the Golay equation and not by extra column band broadening due to the injection process. Synchronized dual-valve injection offers a rugged and inexpensive design, providing extremely reproducible injections with peak height precision of 2.4% (RSD) and low run-to-run variation in retention times, with an average standard deviation less than 0.1 ms. Herein, synchronized dual-valve injection is demonstrated as a proof of principle using high-speed diaphragm valves. It is foreseen that the injection technique could be readily implemented using a combination of thermal modulation and high-speed valve hardware, thus optimizing the mass transfer and not significantly sacrificing the limit of detection performance for high-speed GC. Further implications are that, if properly implemented, high-speed temperature programming coupled with this new technology should lead to very large peak capacities for approximately 1-s separations.

Combination of flow injection with capillary electrophoresis: 8. Miniaturized capillary electrophoresis system with flow injection sample introduction and electrogenerated chemiluminescence detection

The combined flow injection (FI)–capillary electrophoresis (CE) system was further exploited by coupling to an electrogenerated chemiluminescence (ECL) detection system. A low-cost miniaturized CE system was developed on a chip platform to provide easy interface both with FI sample introduction and with ECL detection. A falling-drop interface was employed to perform FI split-flow sample introduction while achieving electrical isolation from the CE high voltage. A plexiglas reservoir at the capillary outlet served as both the reaction and detection cell for the ECL reaction, with Ru(bpy) 3 2+ reagent continuously flowing through the cell. An optical fiber was positioned within the reservoir close to the capillary outlet for transferring the ECL emission to the PMT. The relative positions of the capillary outlet, working electrode and optical fiber as well as reagent renewal flow-rate were optimized to achieve both good sensitivity and separation efficiency under non-interrupted sampling conditions, involving large numbers of samples. An on-column joint often used in other works for isolating the ECL detection system from the CE separation voltage was not found necessary. The performance of the system was illustrated by the baseline separation of proline, valine and phenylalanine with a high throughput of 50 h −1 and plate height of 14 μm for proline under 147 V cm −1 field strength. Detection limits (3 σ) were 1.2, 50 and 25 μM and peak height precisions were 1.4, 5.4 and 4.3% R.S.D. ( n=9) for proline, valine and phenylalanine, respectively.

Capillary electrophoresis system with flow injection sample introduction and chemiluminescence detection on a chip platform.

A capillary electrophoresis (CE) system with chemiluminescence (CL) detection was combined with flow injection (FI) sample introduction on a chip platform. A falling-drop interface was applied to perform FI split-flow sample introduction while achieving electrical isolation from the CE high voltage. A tubular reservoir at the capillary outlet served as both the CL reaction and detection cell for the luminol-peroxide-metallic ion chemiluminescent reaction, with the luminol included in the separation buffer and CL reagent H2O2 continuously introduced into the outlet reservoir. An optical fiber was positioned within the outlet reservoir directly opposite, and 300 microns away from, the capillary outlet for collecting and transferring the generated CL to the PMT. The peak height signals and the separation efficiency were almost independent of the reagent flow-rate, making the system a robust one. The performance of the system was illustrated by the separation of Co(II) and Cu(II), achieving baseline separation in 60 s. Detection limits (3 sigma) were 1.25 x 10(-8) and 2.3 x 10(-6) mol dm-3 for Co(II) and Cu(II), respectively. Peak height precision was 1.9% RSD (n = 9) at the 10(-7) mol dm-3 Co level.

Combination of flow injection with capillary electrophoresis: Part 7. Microchip capillary electrophoresis system with flow injection sample introduction and amperometric detection

A microchip-based capillary electrophoresis (CE) system with amperometric detection, combined with flow injection (FI) sample introduction, was constructed from components readily available in the analytical laboratory. An H-channel configuration was used with separation capillary positioned between two tubular side-arms, and a falling-drop interface connected to one side-arm was developed to achieve electrical isolation between the FI and CE systems. End-column amperometric detection was accomplished with separation voltage decoupled from the detection system, employing a microdisk working electrode positioned immediately outside the capillary outlet in the other side-arm, which functioned as a large-volume reservoir. Sample dispersion in the FI system and the FI–CE interface was minimized by intercalating the sample zone between two air segments. Performance of the FI–CE amperometric system was demonstrated by separation of sugars. Separation of sucrose and glucose was performed with a 5 cm, 25 μm i.d. capillary with 1.7 kV separation voltage in about 60 s (40 μm plate height for glucose), achieving a sampling frequency of over 65 h −1. Responses for sucrose and glucose were linear in the range of 10–1000 μM with sensitivities of 0.011 and 0.025 nA μM −1. Detection limits ( S/ N=3) were 2 μM for sucrose and 1 μM for glucose. Peak height precisions were 2.1 and 2.4% R.S.D. ( n=9) for sucrose and glucose, respectively.

Indirect fluorescence detection of free cyanide and related compounds by capillary electrophoresis

A capillary electrophoretic method to detect CN − and some related compounds (CNO −, SCN −, and NO 3 −) using a prototype laser-based indirect fluorescence detector has been developed. This method avoids interferences from Cl −, SO 4 2−, CO 3 2−, HCO 3 −, and ClO − and allows the detection of CN − in less than 4 min and the detection of related compounds in less than 8 min with precise migration times (R.S.D. about 1%). The measured detection limits of CN − [5 × 10 −6 M with DR = 90 (dynamic reserved)] are close to the expected theoretical value [TR CN = 0.44 (transfer ratio)]. For the related compounds, the measured detection limits ranged from 10 −5 to 2 × 10 −6 M, depending on noise conditions (DR ⩾ 300). The results showed that the chosen analytical conditions were appropriate and the detection limits could be improved using techniques to further decrease the noise (probably due to mechanical vibrations). Peak-height precision was about 10% R.S.D. and log-log regressions had slopes lower than one. These results seem to be due to instabilities in the indirect signal. The analysis of a quasi-real sample has been achieved, allowing identification of the related compounds present in a CN −/SCN − sample oxidized with ClO − and determination of the CNO − formed in this alkaline oxidation.

Appropriate calibration functions for capillary electrophoresis I. Precision and sensitivity using peak areas and heights

Calibration functions for CE can be calculated by using peak heights or areas. The relationship between peak heights and concentrations is non-linear, but can well be approximated by a parabolic function. The precision of peak heights is often better than of areas. However, the sensitivity of the calibration function decreases at higher concentrations. Thus calibration functions calculated by using areas lead to a better precision of the analytical result, which is estimated by the slope-normalized standard deviation.

Flow-injection and liquid chromatographic determination of aluminum based on its fluorimetric reaction with 8-hydroxyquinoline-5-sulphonic acid in a micellar medium

Aluminum reacts with 8-hydroxyquinoline-5-sulphonic acids (HQS) in cationic micelles of cetyltrimethylammonium bromide (CTAB) to form a strongly fluorescent compound. In the micellar media the formation of the Al—HQS complex is very fast, allowing a useful continuousdetermination of aluminum in flow systems. At pH 6.3, for 2 × 10 -4 M HQS and 2 × 10 -3 M CTAB the detection limit for a luminum by flow injection is 0.1 ng ml -1. The calibration is linear up to 100 ng ml -1 and the peak-height precision is 2% at the 10 ng ml -1 level. Interfences are greatly decreased compared with the batch method. The reaction has also been applied to the post-column detection by ion chromatography of aluminium in fresh and high-salinity wates and for speciation of aluminium in blodd serum after separation of serum proteins by ion-exchange liquid chromatography.

Determination of fluoride in drinking water and sea water by aluminium monofluoride molecular absorption spectrometry using an electrothermal graphite furnace.

A method is presented for the determination of trace levels of fluoride in drinking water and sea water, based on the formation of aluminium monofluoride in an electrothermal graphite furnace, followed by molecular absorption at 227.45 nm. Two linear intervals extending over wide F– concentration ranges (0–0.2 and 0.2–0.4 µg ml–1 for the peak-height relationship and 0–1 and 1–2 µg ml–1 for the peak-area relationship) were found.The proposed method was applied to natural and artificial drinking and sea waters from a variety of sources and with different saline contents. The detection limit was 8 ng ml–1 and the determination limit 20 ng ml–1. The peak-height precision at 0.2 and 0.4 µg ml–1 of F– was 5% and 7%, respectively.F– contents can be measured with a simple dilution. The use of a deuterium lamp allows simultaneous background correction. Interferences from cations, acidic media and anions were investigated.

Reduction of matrix interferences in furnace atomic absorption with the L'vov Platform

Use of a modified L'vov Platform and ammonium phosphate as a matrix modifier greatly reduced matrix interferences in a commercial Massmann-type atomic absorption furnace. Platforms were readily fabricated from furnace tubes and, once positioned in the furnace, caused no inconvenience in operation. Two volatile elements (Pb, Cd), two of intermediate volatility (Co, Cr) and two which form stable oxides (Al, Sn) were tested in natural water and selected synthetic matrices. In every case for which there was a significant matrix effect during atomization from the tube wall, the platform and platform plus modifier gave improved performance. With lead, for example, an average ratio of 0.48 ± 0.11 was found when the slope of the standard additions plot for six different natural water samples was compared to the slope of the standard working curve in dilute acid. The average slope ratio between the natural water matrices and the dilute acid matrix was 0.94 ± 0.03 with the L'vov Platform and 0.96 ± 0.03 with the platform and matrix modifier. In none of the cases studied did the use of the platform or platform plus modifier cause an interference problem where none existed while atomizing from the tube wall. An additional benefit of the platform was a factor of about two improvement in peak height precision.