Comparison of different calibration approaches in the application of thermal desorption technique: A test on gaseous reduced sulfur compounds

In this study, the reliability of thermal desorbing technique was investigated using the gaseous standards of five reduced sulfur compounds (RSCs: hydrogen sulfide, methane thiol, dimethyl sulfide, carbon disulfide, and dimethyl disulfide). A series of calibration experiments for RSCs were performed using gas chromatography (GC) with pulsed flame photometric detector (PFPD) that is interfaced with a thermal desorber (TD) unit. These calibration data were evaluated by means of two contrasting concepts: fixed standard concentration method (FSC: variable volumetric injection of standard gases prepared at a given concentration) and fixed standard volume method (FSV: injection of multiple standards with varying concentrations at a given volume). When the results of both methods were compared, RSCs generally showed enhanced sensitivity with increasing concentration (FSC) and sample loading volume (FSV). This study highlights that TD-based calibration properties are practically undistinguishable between different sample transfer approaches (e.g., FSV and FSC). As a result, the calibration properties of RSCs derived by thermal desorption technique are greatly distinguished from those of direct injection into GC.     

Effects of injection volume change on gas chromatographic sensitivity determined with two contrasting calibration approaches for volatile organic compounds

In this study, the effects of injection volume change on gas chromatographic detection properties have been evaluated using gas-phase standards containing three aromatic volatile organic compounds (VOC): benzene, toluene, and xylene (commonly called BTX). To examine such effects on GC sensitivity, a series of calibration data sets were obtained using standards of three concentration values (3, 6, and 10 ppm) at each of five selected injection volumes (20 to 1000 microL). The results were initially examined in terms of the fixed standard volume (FSV) approach to allow the direct comparison of calibration patterns between different injection volumes. Identical data sets were then re-organized so that the calibration data could also be compared across variable injection volumes for a given standard concentration (at all three concentrations), i.e. by the fixed standard concentration (FSC) approach. The results of our comparative analysis between the FSV and the FSC approaches indicate that the calibration patterns of VOC are highly sensitive to changes in injection volume or injection-related conditions. It is thus suggested that the former approach is more reasonable for reducing uncertainties associated with the GC-based quantification of atmospheric pollutants.     

The properties of calibration errors in the analysis of reduced sulfur compounds by the combination of a loop injection system and gas chromatography with pulsed flame photometric detection

In order to evaluate the extent of analytical biases involved in the GC calibration, we conducted a series of experiments to examine the calibration methods of trace gas components. For the purpose of this comparative study, gaseous standards of reduced sulfur compounds (RSC) including hydrogen sulfide (H₂S), methanethiol (CH₃SH), dimethyl sulfide (DMS), carbon disulfide (CS₂), and dimethyl disulfide (DMDS) were calibrated by the combination of a GC/PFPD technique and a loop-injection method. In the course of this study, two different types of calibration methods were tested and compared: incremental-injection of a given standard with the fixed standard concentration (FSC) versus injection of multiple standards (with different concentrations) at the fixed standard volume (FSV). In the case of the FSV calibration, a notable increase in the GC sensitivity is apparent with decreasing loop size (or injection volume). For instance, the calibration slope for RSC obtained using a 10 μl loop system was approximately three times higher than that for a 250 μl one. However, the results obtained by the FSC method exhibit much poorer sensitivity than its counterpart with slight differences in their sensitivities across different standard concentrations (due to such factors as the matrix effect from varying injection volumes). Thus, the overall results of this study confirm that the detailed characterization of the selected calibration method (e.g., the use of FSV approach relative to FSC) is of primary significance to perform an accurate quantification of trace gases.     

A combined application of thermal desorber and gas chromatography to the analysis of gaseous carbonyls with the aid of two internal standards

In this study, a series of GC calibration experiments were conducted to examine the feasibility of the thermal desorption approach for the quantification of five carbonyl compounds (acetaldehyde, propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde) in conjunction with two internal standard compounds. The gaseous working standards of carbonyls were calibrated with the aid of thermal desorption as a function of standard concentration and of loading volume. The detection properties were then compared against two types of external calibration data sets derived by fixed standard volume and fixed standard concentration approach. According to this comparison, the fixed standard volume-based calibration of carbonyls should be more sensitive and reliable than its fixed standard concentration counterpart. Moreover, the use of internal standard can improve the analytical reliability of aromatics and some carbonyls to a considerable extent. Our preliminary test on real samples, however, indicates that the performance of internal calibration, when tested using samples of varying dilution ranges, can be moderately different from that derivable from standard gases. It thus suggests that the reliability of calibration approaches should be examined carefully with the considerations on the interactive relationships between the compound-specific properties and the operation conditions of the instrumental setups.     

Effect of standard phase differences between gas and liquid and the resulting experimental bias in the analysis of gaseous volatile organic compounds

Liquid- or gas-phase standards can be used for the analysis of VOCs in air. Once the accuracy is secured in the standard preparation stage, the use of gas-phase standard should be more reliable with the least matrix effect. However, it is not difficult to find that the liquid-phase standard is used more preferably in many laboratories for several reasons (e.g., low expense, easy handling, etc.). As such, one needs to accurately evaluate any possible bias stemming from the use of different standard phases. To this end, standards for 8 VOCs consisting of 4 aromatic compounds (benzene (B), toluene (T), styrene (S) and p-xylene (p-X)) and 4 others (methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), butyl acetate (BuAc), and isobutyl alcohol (i-BuAl)) were prepared in both liquid and gas phases. Each standard was analyzed by the initial collection on the adsorption tube and by the combined application of thermal-desorption–gas chromatography–mass spectrometry (TD/GC/MS). The results indicated that experimental bias between the two phases, if expressed in terms of percent difference (PD), was very low in many target VOCs (B (1.09%), T (2.41%), p-X (3.64%), MEK (6.76%), and MIBK (0.17%)), while it was not in some targets (e.g., >10%: e.g., S, i-BuAl, and BuAc). In an ancillary experiment, biases were evaluated further by (1) calibrating gaseous samples against liquid phase standard and via (2) comparison between two different types of gas phase standards. In conclusion, treatment of different standards (e.g., between the same or different phases) will inevitably induce biases in most VOCs, although certain volatiles (e.g., benzene, MIBK, etc.) are virtually unaffected by such variables in a practical sense.     

The Sensitivity of Gas Chromatography-Headspace Solid Phase Microextraction in Relationship with Relative Volatility of Volatile Organic Compounds

In order to explore the analytical performance of Headspace-Solid-phase Microextraction (HS-SPME), the sensitivity of gas chromatography (GC)-Mass Spectrometry (MS) determinations was examined in terms of calibration slopes, that is, response factor values of selected volatile organic compounds (VOC). The HS-SPME was applied to extract two kinds of gaseous VOC analytes, namely group I (methyl ethyl ketone, isobutyl alcohol, methyl isobutyl ketone, and butyl acetate, all having high water solubility) and group II (benzene, toluene, styrene, and xylene, all having moderate water solubility) from water solutions. The results, derived by both external and internal calibration, were then evaluated by considering headspace sample volume and solute volatility. In the case of solutes consisting of group I, sensitivity seems to increase with increasing HS size, although there are no such discernible patterns for group II solutes. The observed relative patterns in extraction efficiency may be accounted for by the differences in intermolecular forces present between the compounds of groups I and II and the possible effects of diffusion kinetics of the VOCs to the SPME fiber or competitive adsorption between different VOCs. As such, sensitivity of HS-SPME is tightly affected by the air-water partitioning properties of the target compounds and the response of SPME to such properties.     

Syn/anti isomerization of 2,4-dinitrophenylhydrazones in the determination of airborne unsymmetrical aldehydes and ketones using 2,4-dinitrophenylhydrazine derivation

Aldehydes and ketones readily react with 2,4-dinitrophenylhydrazine (2,4-DNPH) to form the corresponding hydrazones. This reaction has been frequently used for the quantification of airborne carbonyl compounds. Since unsymmetrical aldehydes and ketones are known to form isomeric 2,4-dinitrophenylhydrazones (syn/ anti-isomers), the influence of isomerization on the practicability and accuracy of the 2,4-DNPH-method using 2,4-dinitrophenylhydrazine-coated solid sorbent samplers has been studied with three ketones (methyl ethyl ketone (MEK), methyl isopropyl ketone (MIPK), and methyl isobutyl ketone (MIBK)). With all three ketones the reaction with 2,4-DNPH resulted in mixtures of the isomeric hydrazones which were separated by HPLC and GC and identified by mass spectroscopy and ¹H nuclear magnetic resonance spectroscopy. The isomers show similar chromatographic behaviour in HPLC as well as in GC, thus leading to problems in quantification and interpretation of chromatographic results.     

Simultaneous determination of odorous volatile organic compounds with gas chromatography and a thermal desorber: A case study on methyl ethyl ketone, methyl isobutyl ketone, butyl acetate, toluene, and xylene

In this study, a series of experiments were conducted to examine the feasibility of the gas chromatographic approach for the quantification of several odorous volatile organic compounds (VOCs) in environmental samples which included methyl ethyl ketone, isobutyl alcohol, methyl isobutyl ketone, and butyl acetate plus benzene, toluene, and xylene (namely, BTX). The gaseous working standards (WS) of seven compounds were initially calibrated at varying concentration ranges by direct injection (DI) into GC injector. The detection properties of these compounds were then tested with a thermal desorber (TD). The relative sensitivities of three aromatic VOCs differed greatly between DI and TD methods. In contrast, four polar VOCs tend to consistently exhibit relative enhancement in response factors with increasing molecular mass (an exception of butyl acetate), regardless of method. The TD-based analysis was reliable enough to detect all target VOCs below their odor threshold values with their detection limit (DL) values. This TD method, when tested against a number of environmental samples collected from several industrial facilities, confirmed the presence of these odorous VOCs at a wide concentration range.     

Comparative analysis of odorous volatile organic compounds between direct injection and solid-phase microextraction: Development and validation of a gas chromatography–mass spectrometry-based methodology

In this study, the feasibility of GC–MS was evaluated for the quantification of odorous volatile organic compounds (VOCs) in environmental samples. These included methyl ethyl ketone, isobutyl alcohol, methyl isobutyl ketone, and butyl acetate plus benzene, toluene, and xylene (BTX). For this purpose, the gaseous standard for these VOCs were analyzed by GC–MS with the aid of both direct injection (DI) into the GC injector and solid-phase microextraction (SPME). The liquid phase standard prepared independently was tested additionally by the DI method as a reference to gaseous calibration. The detection limit (DL) values, when tested for basic quality assurance in this study, showed large differences between DI (0.002–0.007 ng) and SPME method (1.03–1.81 ng) in terms of absolute mass. The DL values, when expressed in terms of concentration (v/v), showed considerable improvement in SPME (below 0.40 nmol mol⁻¹) relative to the DI method (∼6–15 nmol mol⁻¹). The reliability of the GC–MS method was further validated through an analysis of real environmental samples collected from an industrial area.     

Slope comparison method (SCM) for the determination of trace amounts of silicate in ultrapurified water

A sensitive analytical method for the determination of trace amounts of silicate in ultrapurified water was developed. The method is based on the formation of an ion associate of molybdosilicate with malachite green (MG) and the collection of the ion associate on a tiny membrane filter (diameter: 5 mm, and effective filtering diameter: 1 mm). The ion associate formed on the membrane filter is dissolved together with the membrane filter in 1 ml of methyl cellosolve (MC) and the absorbance of MC solution is measured at 627 nm by a flow injection-spectrophotometric detection technique. In this method, silicate in the original sample (ultrapurified water) is concentrated as the ion associate into a small volume of MC to get high sensitivity. As sample concentration takes place, the small amounts of silicate contained in the reagents used also become concentrated as the ion associate into MC. The original sample volumes are varied and evaporated to an identical volume. Therefore, the reagent added is fixed to the same volume. The absorbance increase linearly with increase in the original sample volume will be due only to silicate in the original samples (ultrapurified water). The resulting slopes obtained by varying the sample volumes are compared with the slope of the calibration graph, and thus named the slope comparison method (SCM). The SCM facilitates a more sensitive and accurate evaluation of silicate concentration in the samples than either common calibration method (CCM) or standard addition method (SAM) because it compensates for the influence of trace amounts of silicate contained in chemicals, reagent solution and solvent used. The calibration graph was constructed from 0 to 0.25 ng ml⁻¹ of Si and the detection limit was 10 pg ml⁻¹ (ppt) when 30 ml of samples was used. The standard deviation and relative standard deviation from six measurements of the reagent blanks were 0.0012 and 3.5%, respectively.     

Flame atomic absorption spectrometry of silver and gold in copper intermediates

The effect of hydrochloric acid and salts from the sample on the solubility of methyl isobutyl ketone (MIBK) in aqueous phase and the effect of hydrochloric acid concentration on the MIBK extraction of iron(III) and gold(III) were studied. As a result, an improved method by flame atomic absorption sepctrometry for determination of silver and gold in copper intermediates was developed, employing a more efficient removal of iron and special standards depending upon the concentration of salts introduced by different samples.     

Comparison of storage stability of odorous VOCs in polyester aluminum and polyvinyl fluoride Tedlar bags

Whole air sampling using containers such as flexible bags or rigid canisters is commonly used to collect samples of volatile organic compounds (VOC) in air. The objective of this study was to compare the stability of polyester aluminum (PEA) and polyvinyl fluoride (PVF, brand name Tedlar^®) bags for gaseous VOC sampling. Eight VOC standards (benzene, toluene, p-xylene, styrene, methyl ethyl ketone, methyl isobutyl ketone, butyl acetate, and isobutyl alcohol) were placed into each bag at storage times of 0, 2, and 3 days prior to analyses by gas chromatography/mass spectrometry (GC/MS). From each bag representing each storage day, samples of 3 different mass loadings were withdrawn and analyzed to derive response factors (RF) of each chemical between the slope of the GC response (y-axis) vs. loaded mass (x-axis). The relative recoveries (RR) of VOC, if derived by dividing RF value of a given storage day by that of 0 day, varied by time, bag type, and VOC type. If the RR values after three days are compared, those of methyl isobutyl ketone were the highest with 96 (PVF) and 99% (PEA); however, the results of isobutyl alcohol were highly contrasting between the two bags with 31 and 94%, respectively. Differences in RR values between the two bag types increased with storage time, such that RR of PEA bags (88 ± 10%) were superior to those of PVF bags (73 ± 22%) after three days, demonstrating that VOC in PEA bags were more stable than in PVF bags.     

Experimental approach to assess sorptive loss properties of volatile organic compounds in the sampling bag system

In this study, the sorptive loss patterns for volatile organic compounds were evaluated by gaseous standards containing 13 compounds (benzene, toluene, styrene, p‐xylene, methyl ethyl ketone, methyl isobutyl ketone, isobutyl alcohol, butyl acetate, acetaldehyde, propionaldehyde, butyraldehyde, isovaleraldehyde, and valeraldehyde). The gaseous standards, prepared initially at two contrasting concentration levels (40 and 4000 ppb) in a polyester aluminum bag, were measured after two consecutive transfers into empty bags. It indicates that the percent loss patterns, if assessed for all 13 target compounds, are affected most sensitively by the initial concentration levels of samples to yield 2.62 ± 2.22% (at 40 ppb) and 9.57 ± 6.74% (at 4000 ppb). Moreover, the sorptive loss patterns at high concentration samples (4000 ppb) tend to increase in relation with increasing molecular weight of target compounds, although such pattern disappears in low concentration samples (40 ppb). The observed loss patterns, if evaluated in relation to some key parameters like concentration or compound type, suggest the possibility that the sorptive loss of target compounds in storage media can occur in a predictable manner.     

Determination of Polymer Molecular Weight by GPC in the Low Molecular Weight Region

Abstract

In order to determine the molecular weight of a polymer by GPC in the low MW region, a secondary calibration method can be established when polymer standards of the analysed polymer are not available (1). The efficiency of this method has been checked for the system poly(methyl methacrylate) (PMMA-methyl ethyl ketone (MEK) using polystyrene (PS) standards. When the solvent is poor for one of the polymers, limitations arise due to the secondary separation effects. Furthermore, a GPC method is proposed for the determination of the Dondos-Benoīt constants (A₁ A₂) which must be known for the above-mentioned calibration. This method uses either two or at least one polydisperse polymer sample.     

气相色谱／质谱法鉴定甲基异丁基酮产品中的杂质组分

采用ＧＣ／ＭＳ联用技术分离出甲基异丁基酮产品中６个杂质峰。将标准图谱库，质量色谱图等手段结合起来鉴定出７种杂质，并用标样，双柱对部分ＧＣ／ＭＳ的结果进行了证实，为产品质量控制提供依据。     

Equilibrium in-fiber standardization method for determination of sample volume by solid phase microextraction

This paper describes an in-fiber standardization method by Solid Phase Microextraction (SPME) for the determination of a sample volume. After loading a specific amount of standard, the volumetric standard, on a PDMS-coated fiber (n(0)), the fiber was exposed in the headspace of sample vials containing different volumes of water. The amount of standard that remained on the fiber after equilibrium (n(f)), which was determined with gas chromatography/mass spectrometry (GC/MS), depends on the volume of the sample. Naphthalene, 1-methylnaphthalene, 2-methylnaphthalene, 1-ethylnaphthalene, and 2-ethylnaphthalene were chosen as volumetric standards based on theoretical calculations. The effect of loading time, exposure time, and exposure temperature were investigated. The effect of the matrix was also studied, analyzing both water and wine samples. Precision and accuracy of the method were obtained for each standard in both water and wine. The partition coefficients of the compounds between the fiber and the sample (K(fs)) and between the headspace and the sample (K(hs)) were obtained by plotting n(0)/n(f)versus sample volume.     

甲基异丁基甲酮萃取-火焰原子吸收光谱法测定氧化锑中的磷

研究了MIBK萃取-火焰原子吸收光谱法测定氧化锑中的磷。在盐酸介质中,磷酸根与钼酸铵形成磷钼物质的量之比为1∶12磷钼杂多酸,用酒石酸钾钠掩蔽锑,以甲基异丁基甲酮为萃取试剂,用火焰原子吸收光谱法测定有机相中磷钼杂多酸中的钼,据此建立了一种间接测定氧化锑中微量磷的新方法。在优化的实验条件下,当磷的含量在0.055~1.00 mg/L范围内时,其吸光度与浓度呈良好的线性关系;方法的检出限为0.011 mg/L;对4个样品进行测定的回收率在97.8%~106.2%之间;相对标准偏差为2.6%~4.2%。用于氧化锑中微量磷的测定,结果满意。     

Study of Analytical Methods for Iron Determination in Complex Organic Liquids by Atomic Absorption Spectrometry

Abstract

In the determination of iron in complex organic liquids by atomic absorption spectrometry (A. A. S.), methods of sample preparation, such as dilution with an organic solvent and sample pretreatment to destroy organic material, are investigated. Moreover, methods of analysis using calibration curve and standard additions are presented. The possible cause of error associated with iron determination in organic samples by flame (F-A. A. S.) and graphite furnace (GF-A. A. S.) atomic absorption spectrometry are discussed.

From all of these studies, the use of graphite furnace atomic absorption spectrometry after sample dilution with methyl isobutyl ketone, and the use of the method of standard additions are advised for iron determination.     

The fundamental properties of the direct injection method in the analysis of gaseous reduced sulfur by gas chromatography with a pulsed flame photometric detector

In this study, the fundamental aspects of gas chromatography with a pulsed flame photometric detector were investigated through the calibration of gaseous reduced sulfur compounds based on the direct injection method. Gaseous standards of five reduced sulfur compounds (hydrogen sulfide, methane thiol, dimethyl sulfide, carbon disulfide, and dimethyl disulfide) were calibrated as a function of injection volume and concentration level. The results were evaluated by means of two contrasting calibration approaches: fixed standard concentration method (variable volumetric injection of standard gases prepared at a given concentration) and fixed standard volume method (injection of multiple standards with varying concentrations at a given volume). The optimum detection limit values of reduced sulfur compounds, when estimated at 100 μL of injection volume, ranged from 2.37 pg (carbon disulfide) to 4.89 pg (dimethyl sulfide). Although these detection limit values improved gradually with decreasing injection volume, the minimum detectable concentration (e.g., in nmol mol⁻¹ scale) remained constant due to a balance by the sample volume reduction. The linearity property of pulsed flame photometric detector also appeared to vary dynamically with changes in its sensitivity. According to this study, the performance of pulsed flame photometric detector, when tested by direct injection method, is highly reliable to precisely describe the behavior of reduced sulfur compounds above ∼20 nmol mol⁻¹.