Analytical Chemistry Chemical Cytometry Quantitates Superoxide

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Figure 4. Variation of MDA and FM with respect to PSA for 226 Ra standard solutions. and � events are correctly discriminated and a counting plateau, of almost constant efficiency (ca. 265%), is observed. In order to choose the optimum value within the efficiency plateau, both MDA and FM were plotted (Figure 4). In broad terms, MDA followed the efficiency curve, suggesting that, from a limit of detection perspective and within the plateau, best results would be achieved at higher PSA, as the background slowly decreases with the PSA parameter and stabilizes for the PSA ranging from 145 to 190. It is worth noticing that, as from PSA ) 135 the background rate is very low, the MDA is correspondingly low. Higher PSA values are discarded from the discussion as the efficiency there tends rapidly to zero. On the other hand, the FM shows a clear peak at PSA ) 145, and shows the optimum combination of efficiency and background, taking advantage of a high efficiency (and therefore sample throughput) and providing the lowest possible MDA. Counting Window. Due to the presence of many substances in the digestate, which may be partially soluble in the scintillator, the sample may show different levels of quenching, which also affect the position of the peaks. In order to compensate for the peak shift, we opted to define counting windows with a fixed width but a variable position in the spectrum, manually set to comprise the peaks of interest. The first strategy was to define a wide counting window of 150 channels. This window allowed to include, irrespective of the quenching level (quantified through the SQP(E) parameter), the three R peaks (Figure 1). The main advantage of this strategy is that counting efficiency is large, up to the maximum theoretical level of 300%, and above 200% for the real samples commonly analyzed. However, as the spectrum resolution does not allow us to distinguish other R impurities present in the sample and some � interferences, this method might lead to slight activity overestimation. In order to minimize the effect of background from impurities and interferences, the second strategy was to define a 50 channel wide counting window to only comprise the higher energy R peak ( 214 Po), which stands in an area of much smaller background and interference, 15 and is easily identified. However, in this case the maximum theoretical efficiency is only 100%. Calibration. Quenching is the only parameter affecting the counting configuration that is sample-dependent and cannot be easily controlled. Therefore, several 226 Ra standard solutions were quenched with CCl4 to values covering the expected SQP(E) 6850 <strong>Analytical</strong> <strong>Chemistry</strong>, Vol. 82, No. 16, August 15, 2010 Figure 5. Effect of quenching (SQP(E)) on R counting efficiency. Figure 6. Time stability test for sample quenching (SQP(E)). value in environmental samples (Figure 5). For both counting windows, efficiency is close to zero for SQP(E) < 575. Therefore, the calibration curves, obtained by regression analysis between the counting efficiency (y) and the SQP(E) parameter (x), were 150 channels: y ) (0.74 ( 0.06)x - (435 ( 51) and r ) 0.95 50 channels: y ) (0.21 ( 0.02)x - (119 ( 19) and r ) 0.91 Usually, LSC techniques involve the measurement of a batch of samples in cycles. As the typical activities expected in this type of work are low (of the order of 10 mBq), long-term counting, of the order of days for a single batch, is required. We successfully checked the stability of the scintillator through the monitoring of SQP(E) for a quenched tracer with an activity of 10 mBq during 4 days (Figure 6). Detection Limits. Each counting configuration (including counting window and quenching) shows different detection limits. In Table 1, we show the counting properties for a standard solution quenched to SQP(E) ) 850, which is an average value of sediment samples analyzed in our laboratory. The combination of the indirect detection of 226 Ra by 222 Rn emanation, the high resulting efficiency, and the ultralow background of Quantulus 1220 result in an extremely low MDA (0.29 Bq kg -1 ) for samples as low as 250 mg, a typical amount used when analyzing 210 Pb by R spectrometry. 10 Although both MDA are very low, and therefore any of the windows can be chosen, the larger efficiency (and therefore precision of measurements for the same counting time) favors the use of the largest window, which is the recommended value in this work. From a
Table 1. Counting Properties and Detection Limits for a 226 Ra Standard Solution a window (channels) efficiency (%) background (cpm) b spectrometric point of view, a smaller window and higher R energy minimize the possibility of interferences with other emissions and should be favored when expected sample activity or desired analytical throughput are not compromised. DISCUSSION Although this technique is well-suited to easily determine 226 Ra in small samples from most environmental matrixes, the system has been designed and optimized for small sediment samples, subsequent to 210 Pb determination through 210 Po in equilibrium. Proposed Method. We recommend operating in batches of about 12 samples, where both a reference material and a blank are processed and measured at the same time. The solution from which the Po source was deposited (about 15 mL) is recovered and, in order to destroy ascorbic acid and its degradation compounds, heated under reflux with 5 mL of concentrated HNO3 for half an hour or until a clear solution is obtained. This solution is then evaporated to incipient dryness, and small amounts of 0.5 M HCl are added to dissolve the residue. This operation is carried out two more times to ensure the elimination of HNO3, and the total volume is then adjusted to 10 mL in a 20 mL low-diffusion PE counting vial. Then 10 mL of OptiScint HiSafe cocktail is carefully added by avoiding disturbing the interface formed by the two immiscible liquids in the vial (Figure 7). The mixture is kept for 3 weeks in a dark temperaturecontrolled area, in order to wait for radioactive equilibrium and to minimize chemiluminescence, and is counted by liquid scintillation by using the following conditions: • Counting time: all samples of the batch are sequentially counted for 1 h each, during a minimum of 30 cycles. • Counting windows are approximately set to channels 750-900 and 850-900. However, best results are obtained by summing all spectra and visually adjusting the windows manually to their optimum value. • Counting mode is set to discriminate R and other pulses, with PSA ) 145. Counting conditions and calibration are equipment-sensitive, and therefore, the quenching calibration must be performed for each instrument. We also recommend carrying out some tests with 226 Ra tracer solutions to confirm that the chosen PSA value is correct for each instrument. The counting information needed to calculate the activity are sample count rate, background count rate, and quenching parameter (such as SQP(E) in Quantulus 1220). From the quenching value the efficiency can be obtained (Figure 5), and the sample activity finally calculated. Low-Activity Test. We tested the linearity and the low-activity response of the detector in the chosen configuration by measuring a set of 226 Ra unquenched tracer solutions (0.5 M HCl) with Ld (cpm) MDA (Bq kg -1 ) c 710-860 173 ± 12 0.0070 ± 0.0020 0.0076 0.29 810-860 54 ± 4 0.0024 ± 0.0012 0.26 0.59 a Activity ) 0.01 Bq. Quenched to SQP(E) ) 850. b Counting time of the background was 55 h. c MDA was calculated by assuming a typical sediment mass of 250 mg. Figure 7. Proposed procedure to measure 210 Po and 226 Ra in sediment samples. Table 2. Count Rates When Measuring 226 Ra Unquenched Standard Solutions by LSC sample activity (mBq) net count rate (cpm) 0.017 0.12
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Anal. Chem. 2010, 82, 6745-6750 Let
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purchased from Invitrogen-Molecular
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Figure 3. Electropherograms of TPP-
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Anal. Chem. 2010, 82, 6751-6755 Res
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(pH 8.0) with cysteine and cystamin
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Figure 4. Ion mobility mass spectra
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GOx glucose + O298 gluconic acid +
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coater at 2000 rpm for 20 s, and th
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Figure 5. Comparison of cyclic volt
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in channels with either no grooves
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indicators of atmospheric processin
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Figure 1. GC/MS total ion chromatog
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Table 2. Concentrations and Stable
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on a substrate are preferred. 20-24
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Figure 4. SERS analysis of NAADP co
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Anal. Chem. 2010, 82, 6775-6781 Hig
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tion, 2 µL of proprionaldehyde wer
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Figure 4. Analysis of 2a by HPLC-MS
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eaction of 1a with PBH can be condu
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Numerous references had demonstrate
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Figure 1. TEM images of the prepare
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Figure 4. Schematic representation
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esult in a big SPR signal change wi
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also reduces chemical noise, which
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Table 1. Extraction Yields, Liquid
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scanning of AMPP amides of the anal
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Anal. Chem. 2010, 82, 6797-6806 δ
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Page 75 and 76: Table 2. Concentrations and Ratios
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Page 99 and 100: Figure 2. Standards of (Pi)n, n ) 1
Page 101 and 102: Figure 5. Quantitative calibration
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Page 105: Figure 1. 226 Ra spectrum by liquid
Page 109 and 110: Table 3. Analysis of 226 Ra in Sedi
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Page 135 and 136: NaCl, phosphate buffer saline (PBS)
Page 137 and 138: Figure 2. Product ion mass spectra
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Page 141 and 142: Table 1. Intraday Precision and Acc
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Page 145 and 146: Figure 1. Infrared spectra of (A) u
Page 147 and 148: Figure 3. Cyclic voltammograms obta
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Page 153 and 154: Table 1. Chemical Structure, pKa Va
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Table 2. Linearity and Detection Li
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ascorbic acid (AA), uric acid (UA),
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the multielement capabilities, the
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RESULTS AND DISCUSSION Sulfur Detec
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Table 2. Molecular Properties and C
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Anal. Chem. 2010, 82, 6911-6918 Dir
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dilution and hybridization buffer.
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solution under appropriate incubati
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Figure 4. Standardization curve for
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Anal. Chem. 2010, 82, 6919-6925 Ele
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the ×10 objective, to have a large
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Figure 2. With a suitable removal o
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the NB signal in a much better foot
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Scheme 1. Reactions of Selenium Rea
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Figure 2. (a) ESI-MS spectrum showi
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Figure 4. (a) ESI-MS spectrum showi
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Anal. Chem. 2010, 82, 6933-6939 Dif
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trode 28 by a finite element using
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Figure 4. Comparison between simula
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Figure 6. Comparison between simula
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educed in the vicinity of double bo
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Figure 2. Normalized product ion ab
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Figure 4. EID (a) and IRMPD (b) of
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Anal. Chem. 2010, 82, 6947-6957 Ide
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Figure 1. Schematic flowchart showi
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difference, ppm compound Table 1. I
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isoforms, its successful use, in th
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Figure 5. Extracted ion current ESI
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m/z 1172.935 was observed for Ser14
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linked products via affinity tags.
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Scheme 2. Fragmentation Mechanism o
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Figure 1. (A) ESI-LTQ-CID-MS 2 prod
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Figure 3. (A) ESI-LTQ-CID-MS 2 prod
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Figure 5. (A) MALDI-TOF/TOF product
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Anal. Chem. 2010, 82, 6969-6975 Ana
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Figure 3. Equilibrium response as a
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Figure 6. The average measured resp
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for the fill time, and we find that
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nucleotide tails. 3-5 Thus, the amo
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allow the use of higher aptamer con
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quent ligation of the aptamers afte
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Anal. Chem. 2010, 82, 6983-6990 Imp
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Figure 2. Configuration editor wind
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Figure 4. Dependencies between volu
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Since the time for liquid expulsion
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Anal. Chem. 2010, 82, 6991-6999 Int
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Figure 1. Representation of the Car
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Table 2. Capillary Electrophoresis
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Figure 4. (A) Typical raw electroph
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field. DNA profiles were delivered
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Another approach to handle this lim
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(principal components, PCs) in whic
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Figure 5. Relevant score plots and
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CONCLUSIONS In this paper we have i
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electroactive-species loaded liposo
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Figure 2. Qdot-based FLFTS response
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Figure 6. Fluorescence imaging of Q
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Anal. Chem. 2010, 82, 7015-7020 Sel
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Table 1. Effect of 1 D-LC Condition
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Figure 3. Peak-production rate vers
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Anal. Chem. 2010, 82, 7021-7026 Cel
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luciferase (T7 control vector) was
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Figure 3. Inhibitory effects of luc
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Anal. Chem. 2010, 82, 7027-7034 Pat
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Electrochemistry. Electrochemical m
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Figure 2. SEM images of Au substrat
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Table 3. Film Thickness Measurement
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Anal. Chem. 2010, 82, 7035-7043 Qua
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Figure 1. (A) FL spectra of BSPOTPE
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Figure 4. (A) Variation in the FL i
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Figure 6. (A) FL spectra of BSPOTPE
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Scheme 1. Proposed Mechanism for Fl
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one or more drawbacks including poo
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Figure 2. Free fluorophores do not
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Anal. Chem. 2010, 82, 7049-7052 Dev
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Figure 2. Comparative analysis of d
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Analytical Chemistry Chemical Cytometry Quantitates Superoxide

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