Determination and study on dissipation and residue determination of cyhalofop-butyl and its metabolite using HPLC-MS/MS in a rice ecosystem

Cyhalofop-butyl is an aryloxyphenoxypropionate postemergence herbicide with good control of barnyard grass in rice paddies. In this study, method for the determination of cyhalofop-butyl and its metabolite was developed with high-performance liquid chromatography tandem mass spectrometry. Dissipation and residue levels of cyhalofop-butyl and its metabolite in rice ecosystems were also investigated. Recoveries and relative standard deviations of cyhalofop-butyl and cyhalofop acid in six matrices at three spiking levels ranged from 76.1 to 107.5 % and 1.1 to 8.2 %, respectively. The limit of quantitation (LOQ) of cyhalofop-butyl and cyhalofop acid was 0.01 mg/kg in paddy water, paddy soil, rice plant, rice straw, rice hulls, and husked rice. For field experiments, the results showed that cyhalofop-butyl degraded to cyhalofop acid quickly, and the half-lives of cyhalofop acid in paddy water, paddy soil, and rice plant were 1.01–1.53, 0.88–0.97, and 2.09–2.42 days, respectively. Ultimate residues of cyhalofop-butyl and its metabolite in the rice samples were not detectable or below 0.01 mg/kg at harvest.     

Dissipation behavior and risk assessment of butralin in soybean and soil under field conditions

Dissipation behavior, final residue, and risk assessment of butralin in soybean, green soybean, plant, and soil were investigated. Butralin residues were extracted with acetonitrile and then soybean samples were detected with gas chromatography-mass spectrometer (GC-MS) and soil samples were determined with GC with nitrogen phosphorous detector (GC-NPD). The limit of quantification (LOQ) of the method was 0.01 mg/kg for soybean, green soybean, plant, and soil. Average recoveries ranged from 90.4 ~ 98.2% for green soybean, 86.2 ~ 86.6% for soybean, 86.0 ~ 98.8% for plant, and 85.0 ~ 106.8% for soil. The relative standard deviations (RSDs) were 2.0 ~ 7.2% for green soybean, 2.0 ~ 3.0% for soybean, 3.1 ~ 8.1% for plant, and 1.8 ~ 6.6% for soil. Half-lives of butralin in soil samples varied in the range of 11–22 days. At harvest time, final residues of butralin in soybean and green soybean were lower than LOQ. Risk assessment demonstrated that, at recommended dosage and frequency, butralin would not induce significant harm on humans. The study could be used as a quantitative basis for application of butralin on soybean.     

Comparative assessment of pesticide residues in grain, soil, and water from IPM and non-IPM trials of basmati rice

The integrated pest management (IPM) modules of pesticide schedule on Basmati rice were validated at field experiments conducted in Northern India for consecutive 3 years (2005–2008). The pesticide residues were found below the detectable limit (<0.01–0.001 mg/kg) in soil and irrigation water samples of Kaithal region. In Dehra Dun region of Uttrakhand, the residues of carbendazim in rice grains and soil were detected below <0.01 mg/kg level. In second year experiments (2006–2007), only four non-IPM soil samples indicated the presence of chlorpyrifos and endosulfan in the range of ND <0.001 to 0.07 mg/kg, out of 45 samples analyzed. Carbendazim applied as seed treatment at Dehradun and Kaithal field trials was found below detectable limit in both IPM and non-IPM rice grains (<0.01 mg/kg) and irrigation water (0.01 μl/ml). Chlorpyrifos was detected in five water samples from Kaithal and one from Pant Nagar in the range of 0.003–0.006 μl/L, α- and β-isomer of endosulfan in the range of 0.005–0.03, and 0.005–0.02 μl/ml, respectively, in one sample from Pant Nagar and two from Kaithal, out of a total of 22 samples. In the region of Uttrakhand and Uttar Pradesh during 2007–2008, four non-IPM samples of soil indicated trace levels of endosulfan, out of 16 samples analyzed. The residues were detected below detection limit for carbendazim (<0.01 mg/kg) in soil samples of Dehradun IPM fields and for endosulfan and carbendazim (0.001–0.01 μl/L) in water samples each from IPM and non-IPM fields of Uttar Pradesh. The results of 3-year trials of IPM module indicated basmati rice as safe and economical with pesticide residue-free rice grains.     

Pesticide residue analysis of soil,water, and grain of IPM basmati rice

The main aim of the present investigations was to compare the pesticide load in integrated pest management (IPM) with non-IPM crops of rice fields. The harvest samples of Basmati rice grain, soil, and irrigation water, from IPM and non-IPM field trials, at villages in northern India, were analyzed using multi-pesticide residue method. The field experiments were conducted for three consecutive years (2008–2011) for the successful validation of the modules, synthesized for Basmati rice, at these locations. Residues of tricyclazole, propiconazole, hexconazole, lambda cyhalothrin, pretilachlor chlorpyrifos, DDVP, carbendazim, and imidacloprid were analyzed from two locations, Dudhli village of Dehradun, Uttrakhand and Saboli and Aterna village of Sonepat, Haryana. The pesticide residues were observed below detectable limit (BDL) (<0.001–0.05 μg/g) in all 24 samples of rice grains and soil under IPM and non-IPM trials. Residues were below detection level (<0.001–0.05 μg/L) in irrigation water samples (2008–09). Residues of tricyclazole and carbendazim, analyzed from same locations, revealed pesticide residues as BDL (<0.001–0.05 μg/g) in all 40 samples of Basmati rice grains and soil. It was also observed as BDL (<0.001–0.05 μg/L) for 12 water samples (2009–2010). The residues of tricyclazole, propioconazole, chlorpyrifos, hexaconazole, pretilachlor, and λ-cyhalothrin were also found as BDL (<0.001–0.05 μg/g) in 40 samples of Basmati rice grains and soil and 12 water samples (<0.001–0.05 μg/L) (2010–2011).     

Dissipation kinetics and assessment of processing factor for chlorpyrifos and lambda-cyhalothrin in cardamom

The dissipation kinetics and method for estimation of residues of chlorpyrifos and lambda-cyhalothrin in cardamom were studied and developed. The limit of detection and limit of quantitation arrived for the compounds were 0.01 and 0.025 μg?g^?1, respectively. Gas chromatographic response of chlorpyrifos and lambda-cyhalothrin residues was linear in the range of 0.01–0.50 μg?g^?1 and the mean recovery obtained was 97.3 % for chlorpyrifos and 98.9 % for lambda-cyhalothrin with satisfactory relative standard deviation values. The mean initial residues of chlorpyrifos applied at a concentration of 0.05 % in cardamom was 2.5 μg?g^?1 and the residue was 8.1 μg?g^?1 after processing, with a processing factor of 3.24, while lambda-cyhalothrin when applied at 0.0025 % resulted in initial residues of 1.63 μg?g^?1 that magnified to 4.86 μg?g^?1 on curing, with a processing factor of 2.98. The half-life of chlorpyrifos was in the range of 5.1–5.24 days while that of lambda-cyhalothrin was in the range of 4.40–4.55 days. The processing factor arrived at in the above experiment lead to the conclusion that the residues of chlorpyrifos got magnified to 3.24–3.68 times and that of lambda-cyhalothrin got magnified to 2.98–3.46 times of initial residues, consequent to loss of weight due to dehydration during curing.     

Persistence behaviour of imidacloprid and its metabolites in soil under sugarcane

The persistence and metabolism of imidacloprid in soil under sugarcane were studied following application of imidacloprid at 20 and 80 g active ingredient (a.i.) ha^?1. Soil samples were collected at different time intervals (7, 15, 30, 45, 60 and 90 days after application), and the residues of imidacloprid and its metabolites (6-chloronicotinic acid, nitrosimine, imidacloprid-NTG, olefin, urea and 5-hydroxy) were quantified by high-performance liquid chromatography. In soil, the total imidacloprid residues were mainly constituted by the parent compound followed by 6-chloronicotinic acid, nitrosimine and imidacloprid-NTG metabolites. Maximum residues of imidacloprid and its metabolites were 4.29 and 7.81 mg kg^?1 in soil samples collected 7 days after the application of imidacloprid at 20 and 80 g a.i. ha^?1, respectively. At both doses, these residues declined to below the detectable limit in soil after 90 days of application. Olefin, urea and 5-hydroxy metabolites were not detected in soil. Dissipation of total imidacloprid residues did not follow the first-order kinetics with a coefficient of determination value of 0.883 and 0.838 for the recommended dose and four times the recommended dose, respectively. The half-life (T _1/2) value of total imidacloprid was observed to be 10.64 and 10.10 days for the recommended dose and four times the recommended dose, respectively.     

Persistence and risk assessment of spiromesifen on tomato in India: a multilocational study

Supervised field trials were conducted at four different agro-climatic locations of India to evaluate the dissipation pattern and risk assessment of spiromesifen on tomato. Spiromesifen 240 SC was sprayed on tomato at 150 and 300 g a.i.?ha^?1. Samples of tomato fruits were drawn at 0, 1, 3, 5, 7, 10 and 15 days after treatment and soil at 15 days after treatment. Quantification of residues was done on gas chromatograph–mass spectrophotometer in selective ion monitoring mode in the mass range of 271–274 (m/z). The limit of quantification of the method was found to be 0.05 mg kg^?1, while the limit of determination was 0.015 mg kg^?1. Residues were found below the LOQ of 0.05 mg kg^?1 in 10 days at both the doses of application at all the locations. Spiromesifen dissipated with a half-life of 0.93–1.38 days at the recommended rate of application and 1.04–1.34 days at the double the rate of application. Residues of spiromesifen in soil were detectable level (<0.05 mg kg^?1) after 15 days of treatment. A preharvest interval (PHI) of 1 day has been recommended on tomato on the basis of data generated under All India Network Project on Pesticide Residues. Spiromesifen 240 SC has been registered for its use on tomato by Central Insecticide Board and Registration Committee, Ministry of Agriculture, Government of India. The maximum residue limit (MRL) of spiromesifen on tomato has been fixed by Food Safety Standard Authority of India, Ministry of Health and Family Welfare, Government of India as 0.3 μg/g after its risk assessment.     

Persistence and bioaccumulation of oxyfluorfen residues in onion

A field study was conducted to determine persistence and bioaccumulation of oxyflorfen residues in onion crop at two growth stages. Oxyfluorfen (23.5% EC) was sprayed at 250 and 500 g ai/ha on the crop (variety, N53). Mature onion and soil samples were collected at harvest. Green onion were collected at 55 days from each treated and control plot and analyzed for oxyfluorfen residues by a validated high-performance liquid chromatography method with an accepted recovery of 78–92% at the minimum detectable concentration of 0.003 μg g^???1. Analysis showed 0.015 and 0.005 μg g^???1 residues of oxyfluorfen at 250 g a.i. ha^???1 rate in green and mature onion samples, respectively; however, at 500 g a.i.ha^???1 rates, 0.025 and 0.011 μg g^???1 of oxyfluorfen residues were detected in green and mature onion samples, respectively. Soil samples collected at harvest showed 0.003 and 0.003 μg g^???1 of oxyfluorfen residues at the doses 250 and 500 g a.i. ha^???1, respectively. From the study, a pre-harvest interval of 118 days for onion crop after the herbicide application is suggested.     

Persistence of chlorantraniliprole granule formulation in sugarcane field soil

Chlorantraniliprole, an anthranilic diamide insecticide with novel mode of action, is found effective against several lepidopteran as well as coleopteran, dipteran, and hemipteran pests. The present studies were carried out to study the persistence pattern of chlorantraniliprole on sugarcane field soil following application of granule formulation. The residues of chlorantraniliprole were estimated using high performance liquid chromatograph (HPLC) and confirmed by liquid chromatograph-mass spectrometry (LC-MS/MS). Following application of chlorantraniliprole (Ferterra 0.4G) at 100 and 200 g a.i. ha^?1, the average initial deposits of chlorantraniliprole were observed to be 0.88 and 1.59 mg kg^?1, respectively. These residues dissipated below the limit of quantification (LOQ) of 0.01 mg kg^?1 after 56 days of the application of insecticides at both the dosages. The half-life values (t _1/2) of chlorantraniliprole were worked out to be 8.36 and 8.25 days, at recommended and double the recommended dosages, respectively.     

Persistence of fipronil and its metabolites in soil under field conditions

Fipronil belongs to phenylpyrazole class of chemical compounds. Degradation of fipronil in sandy loam soil was investigated under field conditions by applying fipronil (Regent 5 % SC) at 50 (T ₁) and 100 g a.i. ha^?1 (T ₂) in field. Samples were drawn periodically in triplicate on 0 (1 h after treatment), 1, 3, 7, 10, 15, 30, 60, and 90 days after treatment and analyzed on GC-ECD system equipped with capillary column. The residues of fipronil in both the doses dissipated in the range of 93.33–100 % in 90 days. Limit of detection (LOD) and limit of determination (LODe/LOQ) were 0.0003 and 0.001 mg kg^?1, respectively. Dissipation followed a biphasic first-order kinetics with half-life values of 10.81 and 9.97 days for fipronil alone and 8.14 and 13.05 days for fipronil along with metabolites in soil at (T ₁) and (T ₂) treatments, respectively.     

Residual behavior of quizalofop ethyl on onion (Allium cepa L.)

Quizalofop ethyl, a phenoxy propionate herbicide, is used for postemergence control of annual and perennial grass weeds in broad-leaved crops in India. The experiments were designed to study the dissipation kinetics of quizalofop ethyl on onion for two seasons. A simple, rapid, and sensitive method for estimation of quizalofop ethyl residues in onion and soil was developed and validated. The recoveries of quizalofop ethyl residues from onion and soil at different spiking level range from 84.81 to 92.68 %. The limit of quantification of this method was found to be 0.01 μg g^?1. The risk assessment through consumption of the onion in comparison to its acceptable daily intake which is an important parameter for the safety of the consumer was also evaluated. Standardized methodology supported by recovery studies was adopted to estimate residues of quizalofop ethyl on onion and soil. The average initial deposits of quizalofop ethyl on onion were observed to be 0.25 and 0.33 mg kg^?1, following single application of the herbicide at 50 g active ingredient (a.i.) ha^?1 during 2009 and 2010, respectively. The half-life values (T _1/2) of quizalofop ethyl on onion crop were worked out to be 0.85 and 0.79 days, respectively, during 2009 and 2010. At harvest time, the residues of quizalofop ethyl on onion and soil were found to be below the determination limit of 0.01 mg kg^?1 following single application of the herbicide at 50 and 100 g a.i. ha^?1 for both the periods.     

Dissipation kinetics and residues of triazolopyrimidine herbicides flumetsulam and florasulam in corn ecosystem

The dynamic and residues of florasulam and flumetsulam in corn field ecosystem were investigated using quick, easy, cheap, effective, rugged, and safe (QuEChERS) procedure with high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). The limits of quantification (LOQs) of the proposed method ranged from 0.005 to 0.01 mg/kg. Mean recoveries and relative standard deviations (RSDs) of the two compounds in all samples at three spiking levels ranged 94–110 % and 2.0–9.2 %, respectively. Florasulam and flumetsulam degradation followed first-order kinetics with half-lives 1.7–2.9 and 3.3–8.7 days in soil and 1.3–1.8 and 0.9–1.7 days in plant, respectively. The residues in all the samples were found to be less than the LOQs at preharvest intervals of 53 and 78 days. The results suggest that the combined use of florasulam and flumetsulam on corn is considered to be safe under the recommended conditions and can be utilized for establishing the maximum residue limit (MRL) of florasulam in corn in China.     

Safety evaluation of flubendiamide and its metabolites on cabbage and persistence in soil in different agroclimatic zones of India

Supervised field trials following good agricultural practices were conducted at the research farms of four agricultural universities located at four different agroclimatic zones of India to evaluate the persistence and dissipation of flubendiamide and its metabolite, des-iodo flubendiamide, on cabbage. Two spray applications of flubendiamide 480 SC of standard and double dose at the rate of 24 and 48 g a.i. ha^?1 were given to the crop at a 15-day interval, and the residues of flubendiamide 2 h after spray were found in the range of 0.107–0.33 and 0.20–0.49 mg kg^?1 at respective doses. Residue of des-iodo flubendiamide was not detected in any cabbage sample during study period. No residues were found in the soil samples collected from all treated fields after 15 days of application. On the basis of data generated under All India Network Project on Pesticide Residues, a preharvest interval (PHI) of 10 days has been recommended, and the flubendiamide 480 SC has been registered for its use on cabbage by Central Insecticide Board and Registration Committee, Ministry of Agriculture, Government of India. The maximum residue limit (MRL) of flubendiamide on cabbage has been fixed by the Ministry of Health and Family Welfare, Government of India, under Food Safety Standard Authority of India as 0.05 μg/g after its risk assessment.     

Persistence of dicofol residues in cotton lint seed, and soil

A supervised field trial was conducted at the CCS Haryana Agricultural University, Hisar to assess the residues of dicofol on cotton, during Kharif season, 2008. Dicofol (Kelthane 18.5EC) was applied at 500 g a.i./ha (T(1)) and 1,000 g a.i./ha T(2)) after 105 days of sowing of cotton crop (Varity Cotton/H-1226). Soil samples were collected on 0 (1 h after treatment), 3, 7, 10, 15, 30, and 60 days after spray and cotton samples were collected at harvest. Samples were processed and residues were quantified by GC-ECD system equipped with capillary column. Limit of detection and limit of quantification (LOQ) were 0.001 and 0.010 mg kg(?-1), respectively, for soil and LOQ for cotton lint and seed was 0.020 mg kg(?-1). Initial residues of 0.588 and 1.182 mg kg(?-1) in soil reached below detectable level (BDL) of 0.010 mg kg(?-1) in T(1) and to the level of BDL (0.010 mg kg(?-1)) in T(2) at harvest (60 days after treatment). In 60 days, residues dissipated almost completely (100 and >99%) in both the treatments. Half-life period was calculated as 8.57 days at single dose and 8.69 days at double dose in soil. Residues of dicofol were detected in cotton lint to the levels of 0.292 and 0.653 mg kg(?-1) and in seed 0.051 and 0.090 mg kg(?-1) in T(1) and T(2) doses, respectively at harvest. Residues in cotton seed were below MRL value of 0.01 mg kg(?-1) in both the doses.     

Metabolic degradation of imidacloprid in paddy field soil

The metabolic degradation and persistence of imidacloprid in paddy field soil were investigated following two applications of imidacloprid at 20 and 80 g a.i. ha^?1 at an interval of 10 days. The soil samples were collected at various time intervals. The limit of quantification for the analysis of imidacloprid and its metabolites was obtained at the concentration of 0.01 mg kg^?1. The initial deposits of total imidacloprid were found to be 0.44 and 1.61 mg kg^?1 following second applications. These residues could not be detected after 60 and 90 days following second applications of imidacloprid at lower and higher dosages, respectively. In soil, urea metabolite was found to be the maximum, followed by olefine, nitrosimine, 6-chloronicotinic acid, 5-hydroxy and nitroguanidine. The half-life values (t _1/2) of imidacloprid were worked out to be 12.04 and 11.14 days, respectively, when applied at lower and higher doses, respectively.     

Dissipation and residue of triforine in strawberry and soil

Two independent field trials were performed in Guangdong and Hubei, China in 2011 to investigate the dissipation and residue levels of triforine in strawberry and soil. A fast and simple method using gas chromatography with electron capture detector was developed and validated to determine triforine levels in strawberry and soil. The average recovery of triforine in strawberry ranged from 87.46 to 104.32 % with a relative standard deviation (RSD) of 0.72 to 4.54 %; that in soil ranged from 83.82 to 103.01 % with an RSD of 3.89 to 4.36 %. The limit of quantification of the proposed method was 0.01 mg/kg for both strawberry and soil. The results suggest that the triforine dissipation curves followed the first-order kinetic. The half-lives of triforine in strawberry from Guangdong and Hubei were 3.58 and 4.42 days, respectively; those in soil were 3.53 and 4.10 days, respectively. The terminal residues of triforine in strawberry ranged from 0.032 to 0.264 mg/kg at preharvest intervals of 0.5, 1, and 3 days. These values are lower than the maximum residue limit of 1 mg/kg in strawberry set by the Codex Alimentarius Commission.     

Residues and dissipation of trifloxystrobin and its metabolite in tomatoes and soil

A simple residue analytical method using the quick, easy, cheap, effective, rugged, and safe (QuEChERS) procedure for the determination of trifloxystrobin and its metabolite trifloxystrobin acid (CGA321113) in tomato and soil was developed using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). The limits of detection were 0.0005 mg/kg for trifloxystrobin and 0.001 mg/kg for trifloxystrobin acid, respectively. The average recoveries in tomato and soil ranged from 73–99 % for trifloxystrobin and 75–109 % for trifloxystrobin acid, with relative standard deviations below 15 %. The method was then used to study the dissipation and residues in tomato and soil. The dissipation half-lives of trifloxystrobin in tomato were 2.9 days (Beijing) and 5.4 days (Shandong), while in soil were 1.9 days (Beijing) and 3.0 days (Shandong), respectively. The final results showed that the major residue compound was trifloxystrobin in tomato whereas it was its metabolite, trifloxystrobin acid, in soil. The final residues of total trifloxystrobin (including trifloxystrobin acid) were below the EU maximum residue limit of 0.5 mg kg^?1 in tomato 3 days after the treatment.     

Dissipation and residue of metalaxyl and cymoxanil in pepper and soil

A simple and accurate method of determining metalaxyl and cymoxanil in pepper and soil was developed by ultra-performance liquid chromatography–photodiode array detection. The limits of detection were 0.015 mg/kg for metalaxyl and 0.003 mg/kg for cymoxanil. The limits of quantification were 0.05 mg/kg for metalaxyl in pepper and soil as well as 0.01 mg/kg for cymoxanil in pepper and soil. Recoveries of pepper and soil were investigated at three spiking levels and ranged within 77.52 to 102.05 % for metalaxyl and 87.15 to 103.21 % for cymoxanil, with relative standard deviations below 9.30 %. For field experiments, the half-lives of metalaxyl were 3.2 to 3.9 days in pepper and 4.4 to 9.5 days in soil at the three experimental locations in China. At harvest, pepper samples were found to contain metalaxyl and cymoxanil well below the maximum residue limit MRLs of the European Union (EU) following the recommended dosage and the interval of 21 days after last application.     

Dissipation of S-metolachlor in plant and soil and effect on enzymatic activities

The present study aimed at evaluating the dissipation of S-metolachlor (S-MET) at three doses in maize growing on diverse physico-chemical properties of soil. The effect of herbicide on dehydrogenase (DHA) and acid phosphatase (ACP) activity was estimated. A modified QuEChERS method using LC-MS/MS has been developed. The limit of quantification (0.001 mg kg^?1) and detection (0.0005 mg kg^?1) were very low for soil and maize samples. The mean recoveries and RSDs for the six spiked levels (0.001–0.5 mg kg^?1) were 91.3 and 5.8%. The biggest differences in concentration of S-MET in maize were observed between the 28th and 63rd days. The dissipation of S-MET in the alkaline soil was the slowest between the 2nd and 7th days, and in the acidic soil between the 5th and 11th days. DT₅₀ of S-MET calculated according to the first-order kinetics model was 11.1–14.7 days (soil) and 9.6–13.9 days (maize). The enzymatic activity of soil was higher in the acidic environment. One observed the significant positive correlation of ACP with pH of soil and contents of potassium and magnesium and negative with contents of phosphorus and organic carbon. The results indicated that at harvest time, the residues of S-MET in maize were well below the safety limit for maize. The findings of this study will foster the research on main parameters influencing the dissipation in maize ecosystems.     

A simple extraction procedure for determination of total mercury in crude oil

The determination of mercury in crude oil and petroleum products is particularly difficult due to the volatile nature of both mercury and the matrix, which may lead to significant loss of the analyte. A simple extraction method for total mercury has been developed to determine total mercury in crude oil using cold vapor atomic fluorescence spectrometry. The homogenized crude oil sample was diluted to 5, 10, and 20 % (w/w) in toluene. The diluted crude oil samples were spiked with 10 and 40 μg/kg (w/w). The samples were extracted using an oxidant/acid solution, BrCl/HCl. The mercury was extracted into the aqueous phase; the ionic mercury was then reduced to volatile elemental mercury (Hg⁰) by stannous chloride (SnCl₂). The mercury vapor was detected by Merlin cold vapor atomic fluorescence spectrometry at a 253.7-nm wavelength. The average recoveries for mercury in spiked diluted crude oil (10 and 40 μg/kg, w/w) were between 96 and 103 %, respectively, in 5 and 10 % spiked diluted crude oil. Whereas, low recoveries (<50 %) were recorded in 20 % diluted spiked crude oil. The method detection limit was calculated as t _{(0.01)(n ? 1)}?×?SD where t is the student's value for 99 % confidence level and standard deviation estimate with n???1 degrees of freedom. The method detection limit was found to be 0.38 μg/kg based on 5 g of diluted crude oil sample. The method is sensitive enough to determine low levels of mercury in crude oil.