The development of an automated continuous measurement system for the monitoring of HCHO and CH3CHO in the atmosphere by using an annular diffusion scrubber coupled to HPLC

An automated continuous measurement system for the monitoring of formaldehyde (HCHO) and acetaldehyde (CH₃CHO) in the urban atmosphere was developed by using an annular diffusion scrubber in conjunction with a high-performance liquid chromatograph (HPLC). With this technique, atmospheric HCHO and CH₃CHO were effectively collected by the annular diffusion scrubber which consists of a porous polytetrafluoroethylene (PTFE) tube disposed concentrically within a Pyrex-glass tube and a scrubbing solution. 2,4-Dinitrophenylhydrazine (DNPH) was selected as the scrubbing solution for collecting HCHO and CH₃CHO, which are derivatized to 2,4-dinitrophenylhydrazone-formaldehyde (DNPH-HCHO) and 2,4-dinitrophenylhydrazone-acetaldehyde (DNPH-CH₃CHO), respectively. An aliquot of the sample solution was automatically injected into an HPLC equipped with a semi-micro ODS column and a UV-VIS detector for separating and determining DNPH-HCHO and DNPH-CH₃CHO. All the operations are sequenced by a programmable controller, and automated continuous measurements are performed with a typical temporal resolution of 1 h. The collection efficiencies of HCHO and CH₃CHO were about 97% and 93%, respectively, at an air flow rate of 0.2 L/min. The lower detection limits (3σ of the blank hydrazones) of HCHO and CH₃CHO were 0.05 ppbv and 0.10 ppbv, respectively, in the case of 12-L air sample volume. Analytical response of a standard solution of DNPH-HCHO and DNPH-CH₃CHO by the HPLC during a 10-day continuous measurement was unchanged and the relative standard deviation (RSD) was < 1.0%. Interferences from O₃ and NO₂ were insignificant in this annular diffusion scrubber method. Both for HCHO and CH₃CHO measurements, concentrations from this developed system well agreed with those measured by a DNPH Silica cartridge method.     

The development of an automated continuous measurement system for the monitoring of HCHO and CH3CHO in the atmosphere by using an annular diffusion scrubber coupled to HPLC

An automated continuous measurement system for the monitoring of formaldehyde (HCHO) and acetaldehyde (CH₃CHO) in the urban atmosphere was developed by using an annular diffusion scrubber in conjunction with a high-performance liquid chromatograph (HPLC). With this technique, atmospheric HCHO and CH₃CHO were effectively collected by the annular diffusion scrubber which consists of a porous polytetrafluoroethylene (PTFE) tube disposed concentrically within a Pyrex-glass tube and a scrubbing solution. 2,4-Dinitrophenylhydrazine (DNPH) was selected as the scrubbing solution for collecting HCHO and CH₃CHO, which are derivatized to 2,4-dinitrophenylhydrazone-formaldehyde (DNPH-HCHO) and 2,4-dinitrophenylhydrazone-acetaldehyde (DNPH-CH₃CHO), respectively. An aliquot of the sample solution was automatically injected into an HPLC equipped with a semi-micro ODS column and a UV-VIS detector for separating and determining DNPH-HCHO and DNPH-CH₃CHO. All the operations are sequenced by a programmable controller, and automated continuous measurements are performed with a typical temporal resolution of 1 h. The collection efficiencies of HCHO and CH₃CHO were about 97% and 93%, respectively, at an air flow rate of 0.2 L/min. The lower detection limits (3σ of the blank hydrazones) of HCHO and CH₃CHO were 0.05 ppbv and 0.10 ppbv, respectively, in the case of 12-L air sample volume. Analytical response of a standard solution of DNPH-HCHO and DNPH-CH₃CHO by the HPLC during a 10-day continuous measurement was unchanged and the relative standard deviation (RSD) was < 1.0%. Interferences from O₃ and NO₂ were insignificant in this annular diffusion scrubber method. Both for HCHO and CH₃CHO measurements, concentrations from this developed system well agreed with those measured by a DNPH Silica cartridge method. Received: 15 July 1998 / Revised: 5 October 1998 / Accepted: 7 October 1998     

Automated measurement system for H2O2 in the atmosphere by diffusion scrubber sampling and HPLC analysis of Ti(IV)-PAR-H2O2 complex

An automated selective measurement system for monitoring hydrogen peroxide (H2O2) in the atmosphere was developed (using a diffusion scrubber coupled to a high performance liquid chromatography) due to the importance of H2O2 in understanding tropospheric chemistry and its harmful effects on vegetation. H2O2 in the atmosphere was effectively collected by a diffusion scrubber, which consisted of a hydrophobic porous polytetrafluorethylene (PTFE) tube positioned concentrically within a Pyrex glass tube. Titanium(IV)-4-(2-pyridylazo)resorcinol (Ti(IV)-PAR) solution acidified at pH 2.2 was used as the scrubbing solution for the diffusion scrubber. After the collection of the air sample by the diffusion scrubber, the pH value of the Ti(IV)-PAR scrubbing solution was adjusted to pH 11.9 to form a stable complex of Ti(IV)-PAR-H2O2. An aliquot of the sample solution was injected into a high performance liquid chromatograph equipped with a semi-micro-reversed-phase column and a spectrophotometric detector set at 508 nm for separating and determining the Ti(IV)-PAR-H2O2 complex. The automated measurement could be performed at 60 min intervals. The collection efficiency of H2O2 was higher than 98% at an air flow rate of 1.0 l min-1. The detection limit (3 sigma of the blank value) of H2O2 was 9 parts per trillion by volume (pptv) for an air sampling volume of 55.1. The interference from coexisting O3 or SO2 in the atmosphere was negligible during the collection of H2O2 by the diffusion scrubber. The developed automated measurement system was suitable for monitoring H2O2 in the atmosphere.     

Development of an automated monitoring system for various gas-phase organic carbonyls in ambient air

An automated monitoring system for various C₁ to C₅ gas-phase organic carbonyls in ambient air is described. The system consists of a parallel plate diffusion scrubber (PPDS), which is coupled with a high-performance liquid chromatography–ultraviolet (HPLC–UV) system using an automated injection valve. Compared with an annular diffusion scrubber (DS) employed so far for gas-phase carbonyl monitoring, PPDS shows an improved collection efficiency for formaldehyde, acetaldehyde, propionaldehyde, and acetone with >97% at an airflow rate of 0.5?L/min. High gas–liquid concentration ratios of PPDS and an optimised HPLC–UV system allow limits of detection (LOD) in a range of 80–500?pptv. A low liquid hold-up volume of the PPDS results in a short response time of about 10?min. Additionally, the optimised analysis time for 13 carbonyl compounds containing calibration standard enables brief measurement intervals of 25?min. The developed PPDS–HPLC system shows its reliability from urban site monitoring in Seoul, South Korea.     

Simultaneous determination of HCHO,CH3CHO and O(x) in ambient air by hydrazine reagent and hplc

Formaldehyde, acetaldehyde, ozone and nitrogen dioxide in ambient air are simultaneously collected on silica gel cartridges coated with 1-methyl-1-(2,4-dinitrophenyl)hydrazine (MDNPH), where the two aldehydes are derivatized to their respective hydrazones, while the two oxidants are converted into N-methyl-2,4-dinitroaniline (MDNA). The three products are then separated and quantified by HPLC with UV detection at 360 nm. The stoichiometric factors of the MDNPH reactions with O3 and NO2 in laboratory tests correspond to 2.0 +/- 0.1 moles of MDNA per mole of O(x) (O3 + NO2). The limits of detection (LOD) are 0.7 ppbv HCHO, 0.8 ppbv CH3CHO and 1.6 ppbv O(x) for 30 L (1 h) air sampled. The sampling performance is insensitive to relative humidities encountered in real atmospheres. When compared with Sep-Pak DNPH silica cartridges as well as with ozone photometric and nitrogen dioxide chemiluminescent analyzers, the proposed chromatographic method demonstrates a very good accuracy (12% for HCHO, 14% for CH3CHO and 7% for O(x), on the average) under field sampling conditions at concentrations lower than 3 and 1 ppbv, for HCHO and CH3CHO, respectively and ranging from 28 to 62 ppbv for O(x).     

高效液相色谱法测定卷烟粘合剂中甲醛、乙醛的含量

提出了预衍生化高效液相色谱法测定卷烟粘合剂中甲醛、乙醛含量的方法。卷烟粘合剂中醛经纯净水提取,离心机离心后分取部分上层清液,与2,4-二硝基苯肼衍生反应,生成的甲醛和乙醛的2,4-二硝基苯腙,用C18柱进行分离,用乙腈与水的混合物作流动相。在358nm紫外波长下检测,用醛衍生物配制标准定量。甲醛和乙醛在0.10～10.0mg·mL-1范围内呈线性。甲醛加标回收率在98.3%～118.9%之间,乙醛加标回收率在79.2%～89.2%之间,相对标准偏差(n=5)均小于5%。     

The determination of aldehydes in the exhaust gases of LPG fuelled engines

Summary The exhaust gas of a LPG fuelled engine is drawn through two bubblers in series in an ice bath, and filled with saturated 2,4-dinitrophenylhydrazine in 2M HCl. After heating the derivatives are extracted with toluene-cyclohexane and 1l samples injected on-column on a OV1 capillary column. Using an FID the lower limit of detection is 15–18 pg for formaldehyde (about 8–10 ppbv for a 16l exhaust sample). Taking the blank into account, the limit is about 40 ppbv.The exhaust gases of a LPG-fuelled engine contain formaldehyde, acetaldehyde, propionaldehyde, acrolein and acetone. Carbonyl compounds of more than 3 C-atoms were not found in detectable amounts. The engine was rund under stoichiometric, lean and rich air/fuel conditions. Under rich conditions the concentrations of the aldehydes were: formaldehyde 2.8 ppm, acetaldehyde 1.3 ppm, propionaldehyde 0.06 ppm, acrolein 0.03 ppm, acetone 0.17 ppm; under stoichiometric conditions: 4.5, 1.6, 0.10, 0.03 and 0.18 ppm respectively; under lean conditions 17.0, 2.9, 0.13, 0.07 and 0.27 ppm respectively. These figures demonstrate the necessity of measuring aldehydes in exhaust gases of LPG-fuelled engines.     

Improving continuous chemiluminescence determination method of formaldehyde in the atmosphere: reducing interference of acetaldehyde and others by using iodoform reaction

ABSTRACT

The continuous and selective determination method of formaldehyde (HCHO) in ambient air using chemiluminescence method has been developed. The counter current flow tube was used to collect gaseous formaldehyde. The major interferences of HCHO determination from acetaldehyde, ethanol, and ferrous ion were removed by applying iodoform reaction. Effect of acetaldehyde on chemiluminescence signal of formaldehyde at the same concentration was reduced from 19 to 0.3% by applying iodoform reaction. Subsequently, HCHO was online detected by measuring chemiluminescence produced from the reaction of HCHO, gallic acid, H₂O_2, and KOH. The limit of detection (S/N = 3) was 4.5 ppbv in air. The calibration graph was linear up to 6.25 ppmv. HCHO concentration measured by the present method showed good agreement with that obtained by the 2–4 DNPH-HPLC method.     

HPLC–MS determination of the oxidation products of the reaction between α- and β-pinene and OH radicals

Biogenic non-methane hydrocarbons such as isoprene, alpha-pinene, and beta-pinene, are emitted by forests in very large quantities. To evaluate the role of alpha- and beta-pinene and their contribution to the global production of trace gases and especially aerosol precursors, a study of the oxidation mechanism of alpha- and beta-pinene with hydroxyl radicals must be conducted.The degradation products of both monoterpenes with hydroxyl radicals were identified and quantified in a fast-flow reactor. The products were collected on a liquid-nitrogen trap coated with a 2,4-DNPH solution to which two internal standards (benzaldehyde-2,4-DNPH and tolualdehyde-2,4-DNPH) had been added. The collection method was based on the in situ conversion of aldehyde and/or ketone compounds to their 2,4-dinitrophenylhydrazone derivatives. The derivatives were analyzed by HPLC-MS using APCI(-). TIC chromatograms and mass spectral data for the various oxidation products are presented.For alpha-pinene, pinonaldehyde is the most important degradation product, with smaller amounts of acetone, formaldehyde, campholenealdehyde, and acetaldehyde. For beta-pinene, nopinone and formaldehyde are the most abundant products, of almost equal importance, whereas acetone and acetaldehyde are minor compounds.     

Fluorimetric flow-injection analysis of total amounts of aldehydes in auto exhaust gas and thermal degradation emission gas with cyclohexane-1,3-dione

A simple flow-injection (FI) spectrofluorimetric method for the assay of total volatile aldehydes in auto exhaust gas and emission gas from thermal degradation was developed. Aldehydes, such as formaldehyde, acetaldehyde, propionaldehyde and n-butyraldehyde, reacted with cyclohexane-1,3-dione (CHD) to form more strongly fluorescent compounds. A two-channel flow system was assembled. Distilled water and 0.02% CHD were delivered at 0.75 ml min(-1). The optimum conditions were pH 5 (2.2 M CH(3)COONH(4)-CH(3)COOH buffer solution), reaction temperature 70 degrees C, reaction coil length 0.5 mm i.d. x 7 m, cooling coil length 2 m, sample size 60 microl, excitation and emission wavelengths, 376 nm and 452 nm. Aldehydes in sample gas (10 1) were collected by passing the gas at a flow rate of 0.5 1 min(-1) through two impingers connected in series. 10 ml of methanol was used as an absorbent and diluted sample solution was injected into the carrier stream. The calibration graph was linear in the range 100-1000 ppb. The detection limit was 30 ppb and a sampling frequency of 30 h(-1) was attained. Relative standard deviation for 10 standard formaldehyde solutions (500 ppb) was 1.5%. This rapid and simple FI method was applied to the determination of the total amount of aldehydes, calculated as formaldehyde, in auto exhaust gas and emission gas from the thermal degradation of polymers. The method is useful for monitoring aldehyde emissions and investigating the removal effect of aldehydes from various sources.     

Determination of volatile carbonyl compounds in clean air

Summary An improved analytical procedure has been developed for the detection of formaldehyde, acetaldehyde, acetone and other volatile carbonyls in clean air. For sampling, 2,4-dinitrophenyl-hydrazine (DNPH) coated silica gel cartridges were used. DNPH reacts with carbonyls and forms carbonyl hydrazones which are extracted with acetonitrile and subsequently separated by reversed phase HPLC. Sampling flow rates up to 3.5 l/min were tested. The quantification limit of the complete sampling and analytical procedure is 60 ng carbonyl which corresponds to a mixing ratio of 1 ppbv HCHO in a 45 l air sample taken during a sampling time of 13 min. Carbonyl mixing ratios down to 0.1 ppbv can be determined. The collection efficiency and the elution recovery range between 96 and 100%; the precision is ±5% for HCHO and ±4% for CH₃CHO at mixing ratios of 1 ppbv. This technique can also be applied for the determination of aldehydes and ketones in the aqueous phase, e.g. cloud and fog water. In this case, carbonyls were converted to hydrazones simply by mixing the aqueous sample with an acidified DNPH solution. After 40 min reaction time, the hydrazones were analysed by HPLC. The detection limit was 0.2 mol HCHO/l. Possible interference caused by ozone and NO₂ was eliminated by using KI filters connected in series with the DNPH-coated cartridges. The analytical procedure was tested at a mountain measuring station and proved to be a suitable method for monitoring carbonyl compounds in clean air.     

Determination of aldehydes in fish by high-performance liquid chromatography

A new analytical method to determine trace volatile aldehydes isolated from the headspace of fish meat at room temperature by high-performance liquid chromatography (HPLC) in the form of 2,4-dinitrophenylhydrazone (DNPHo) derivatives has been developed. Aliquots (50 g) of the fish purée were introduced into a 500-mL glass recipient and were purged with N2 for 40 min through two SEP-PAK C18 cartridges (connected in series) coated with an acid solution of 2,4-dinitrophenylhydrazine. The cartridges were then eluted with acetonitrile (2 mL) and the 2,4-DNPHo formed was quantitated by HPLC-UV analysis using a Zorbax C18 column. The isolated compounds from the dynamic headspace sampling of four kinds of fish species were saturated aldehydes, formaldehyde, acetaldehyde, propanal, butanal, pentanal, and hexanal. Under optimized conditions the detection limits of the HPLC method were in the range of 0.75 nmol/g (formaldehyde) to 2.19 nmol/g (hexanal). The calibration curves were linear in the concentration range from 1.3 nmol/mL to 12.5 nmol/mL. Propanal and acetaldehyde were the major carbonyl compounds identified (ranging from 3.9 nmol/g and 10 nmol/g). This study has revealed the widespread occurrence of formaldehyde, acetaldehyde, propanal, butanal, pentanal, and hexanal in fish meat.     

Determination of formaldehyde and acetaldehyde in mainstream cigarette smoke by high-performance liquid chromatography

A method is described for the determination of formaldehyde and acetaldehyde in mainstream cigarette smoke. This involved the collection and reaction of the aldehydes with 2,4-dinitrophenylhydrazine in aqueous acetonitrile. The high-performance liquid chromatographic separation and measurement of the various components directly in this reaction solution eliminated the need for a clean-up stage. Cigarette yields of greater than 5 micrograms of formaldehyde and 50 micrograms of acetaldehyde could be determined to estimated relative standard deviations of 0.07 and 0.05, respectively.     

催化氧化处理工业废气中高浓度甲醛

制备了用于温和条件下催化氧化去除工业废气中高浓度甲醛(HCHO)的1%Pt-4%CeO₂/AC催化剂. 将高浓度甲醛的催化氧化过程与双甘膦氧化制备草甘膦的反应过程集成在一起,使草甘膦合成过程中产生和排放出来的甲醛(100-300 mg/m³)在通过催化剂床层时被完全除去. 系统研究了温度、空速和甲醛含量对甲醛去除率的影响. 在气体空速(GHSV)低于20000 h^-1时废气中几乎所有的甲醛都被氧化,处理后的废气中的甲醛含量低于0.1 mg/m³,甲醛的转化率为99.1%-100%. 当GHSV为30000- h^-1,催化剂床层温度为12℃时,生产废气通过催化剂床层后的甲醛含量小于1.5 mg/m³,甲醛的转化率为97.56%-99.99%. 1%Pt-4%CeO₂/AC催化剂的中试试验结果表明,处理后最终尾气中甲醛含量小于10 mg/m³,有效地防止了甲醛对人们健康的危害,具有良好的产业化前景.     

Evaluation of Monolithic Columns for Determination of Formaldehyde and Acetaldehyde in Sugar Cane Spirits by High-Performance Liquid Chromatography

Abstract

High-Performance Liquid Chromatography (HPLC) conditions are described for separation of 2,4-dinitrophenylhydrazone (2,4-DNPH) derivatives of carbonyl compounds in a 10 cm long C₁₈ reversed phase monolithic column. Using a linear gradient from 40 to 77% acetonitrile (acetonitrile-water system), the separation was achieved in about 10 min—a time significantly shorter than that obtained with a packed particles column. The method was applied for determination of formaldehyde and acetaldehyde in Brazilian sugar cane spirits. The linear dynamic range was between 30 and 600 µg L^?1, and the detection limits were 8 and 4 µg L^?1 for formaldehyde and acetaldehyde, respectively.     

Identification of the oxidation products of the reaction between alpha-pinene and hydroxyl radicals by gas and high-performance liquid chromatography with mass spectrometric detection

In this paper an identification method is described for determining the degradation products of the reaction of alpha-pinene with hydroxyl radicals. The study is carried out in a fast-flow reactor equipped with a specially designed microwave cavity (type Surfatron) allowing to operate at pressures up to 100 Torr (1 Torr=133.322 Pa). The semivolatile products are collected on a liquid nitrogen trap (LN2 trap) and the batch samples are subsequently analysed by GC-MS and HPLC-MS. Some samples were also collected directly on a LpDNPH-cartridge, followed by HPLC-MS analysis. When experiments were carried out at 50 Torr both GC-MS and HPLC-MS measurements showed that campholenealdehyde and pinonaldehyde were identified as condensable oxidation products for the alpha-pinene/OH reaction, with pinonaldehyde being the main product. Furthermore, the LN2 trap collection method based on the in situ conversion of aldehydes and ketones to their 2,4-dinitrophenylhydrazone derivatives allowed the determination of formaldehyde, acetaldehyde and acetone. Although formaldehyde and acetone are present in small amounts in blank samples it could be established that formaldehyde and acetone are also formed in the alpha-pinene/OH reaction.     

Micro gas analysis system for measurement of atmospheric hydrogen sulfide and sulfur dioxide

A honeycomb structure microchannel scrubber was developed to achieve efficient and stable gas collection. A thin porous membrane was pasted on a microchannel by the adhesive force of a fresh polydimethylsiloxane surface. The microchannel scrubber achieved much more efficient gas collection than conventional impingers and diffusion scrubbers. Two sets of the microchannel scrubbers and detectors were integrated in a 10 cm x 9 cm plastic board to create a micro gas analysis system (microGAS) for simultaneous measurements of H2S and SO2. The whole system including a battery was incorporated in a carrying case 34 cm W x 16 cm D x 17 cm H for use in the field. Liquid flows at 30 microl min(-1) were obtained by bimetal micropumps. The estimated detection limits were 0.1 ppbv for H2S and 1 ppbv for SO2. The system was demonstrated for real atmospheric gas analysis, and the results agreed well with data concurrently obtained by ion chromatography coupled with a cylindrical diffusion scrubber. The system we developed allowed automated continuous analyses in the field and achieved a much higher time resolution compared to those by ion chromatographic analysis.     

Bio-sniffers for ethanol and acetaldehyde using carbon and Ag/AgCl coated electrodes

Monitoring of ethanol and acetaldehyde in expired gas after drinking is an effective method for assessment of alcohol metabolic function. We have developed bioelectronic gas sensors (bio-sniffers) for convenient measurement of ethanol and acetaldehyde. The bio-sniffers were fabricated using a simple process and inexpensive method. The process consisted of coating carbon and Ag/AgCl on a filter paper using screen-printing, and immobilizing alcohol oxidase and aldehyde dehydrogenase on the carbon electrode. These bio-sniffers showed good response to ethanol and acetaldehyde vapor and were used to sense these vapors in the concentration range 1.0 to 100 ppm and 0.2 to 4.0 ppm, respectively. The calibration ranges cover the alcohol and acetaldehyde concentration in breath air after drinking. The bio-sniffer for acetaldehyde is applicable also to evaluate other aldehydes (i.e. formaldehyde as volatile organic compounds).

     

Product detection of the CH radical reaction with acetaldehyde

The reaction of the methylidyne radical (CH) with acetaldehyde (CH(3)CHO) is studied at room temperature and at a pressure of 4 Torr (533.3 Pa) using a multiplexed photoionization mass spectrometer coupled to the tunable vacuum ultraviolet synchrotron radiation of the Advanced Light Source at Lawrence Berkeley National Laboratory. The CH radicals are generated by 248 nm multiphoton photolysis of CHBr(3) and react with acetaldehyde in an excess of helium and nitrogen gas flow. Five reaction exit channels are observed corresponding to elimination of methylene (CH(2)), elimination of a formyl radical (HCO), elimination of carbon monoxide (CO), elimination of a methyl radical (CH(3)), and elimination of a hydrogen atom. Analysis of the photoionization yields versus photon energy for the reaction of CH and CD radicals with acetaldehyde and CH radical with partially deuterated acetaldehyde (CD(3)CHO) provides fine details about the reaction mechanism. The CH(2) elimination channel is found to preferentially form the acetyl radical by removal of the aldehydic hydrogen. The insertion of the CH radical into a C-H bond of the methyl group of acetaldehyde is likely to lead to a C(3)H(5)O reaction intermediate that can isomerize by β-hydrogen transfer of the aldehydic hydrogen atom and dissociate to form acrolein + H or ketene + CH(3), which are observed directly. Cycloaddition of the radical onto the carbonyl group is likely to lead to the formation of the observed products, methylketene, methyleneoxirane, and acrolein.     

HPLC-UV method development and validation for the determination of low level formaldehyde in a drug substance

A reversed-phase high-performance liquid chromatographic method (HPLC) with diode-array detection is developed and validated for the quantitative determination of formaldehyde in a drug substance. Formaldehyde (HCHO) is reacted with 2,4-dinitrophenylhydrazine (DNPH) to form a Schiff base (HCHO-DNPH derivatization product), which has an absorbing maximum (lambda max) at 360 nm. The HPLC method employs a C8, 3-microm particle size analytical column (150 mm x 4.6 mm), 15-microL injection volume, column temperature controlled at 30 degrees C, detection at 360 nm, and a water-acetonitrile (55:45, v/v) mobile phase at a flow rate of 1 mL/min. These conditions resolve the HCHO-DNPH product from unreacted DNPH, the drug substance and related impurities, as well as diluent peaks within 20 min. The retention time of the HCHO-DNPH product is approximately 6.4 min. The method is linear, accurate in the specified range (0.33-333 ppm), and robust based on analyte (HCHO-DNPH derivatization product) stability in standard and sample. Detection limit is 0.03 ng (0.1 ppm).