Does further clean-up reduce the matrix enhancement effect in gas chromatographic analysis of pesticide residues in food?

Sample extracts of apples, peas, green beans, oranges, raspberries, clementines, carrots, and wheat obtained using the Food and Drug Administration (acetone extraction) and Canadian Pest Management Regulatory Agency (acetonitrile extraction) multiresidue methods for pesticides were subjected to clean-up using different solid-phase extraction (SPE) cartridges in an attempt to reduce or eliminate the matrix enhancement effect. The matrix enhancement effect is related to the blocking of active sites on the injector liner by matrix components, thereby increasing signal in the presence of matrix versus standards in solvent in which the pesticides themselves interact with the active sites. Graphitized carbon black (GCB) was often used in combination with various anion-exchange SPE cartridges. The extracts were then spiked with organophosphorus insecticides. These process standards were then compared to standards in acetone of the same concentration using gas chromatography with flame photometric detection or ion trap mass spectrometric detection. Sample matrix enhancement varied from little to no effect for some pesticides (e.g. chlorpyrifos, malathion) to >200% in the case of certain susceptible pesticides. The GCB removed color components but showed little effect in reducing matrix enhancement by itself. The anion-exchange cartridges in combination with GCB or not, substantially reduced the matrix enhancement effect but did not eliminate it.

Chromatographic methods of analysis of antibiotics in milk.

The widespread use of antibiotics in dairy cattle management may result in the presence of antibiotic residues in milk. While rapid screening tests are commonly used to detect the presence of antibiotics in milk, more accurate chromatographic methods are required by government regulatory agencies to identify and confirm the identity and quantity of antibiotic present. This paper review recent developments in the chromatographic determination of antibiotic residues in milk.

Determination of organochlorine and organophosphorus pesticide residues in eggs using a solid phase extraction cleanup.

A multiresidue solid phase extraction (SPE) method for the isolation and subsequent gas chromatographic determination of nonpolar organochlorine and polar organophosphorus pesticide residues in eggs is described. The method uses an acetonitrile extraction followed by an SPE cleanup using graphitized carbon black and aminopropyl SPE columns. Organophosphorus pesticides are determined by gas chromatography with flame photometric detection. After further cleanup of the extract using Florisil SPE columns, organochlorine pesticides are determined by gas chromatography with electron capture detection. Studies were performed using eggs containing both fortified and incurred pesticide residues. The average recoveries were 86-108% for 8 fortified organochlorine pesticide residues and 61-149% for 28 fortified organophosphorus pesticide residues.

Stability of tetracycline antibiotics in raw milk under laboratory storage conditions.

Raw milk samples collected from bulk milk tankers may be screened for the presence of tetracycline antibiotics using rapid screening tests. If tetracycline residues are detected, the milk may be shipped to a laboratory for high-pressure liquid chromatography (HPLC) analysis. Because the milk may be shipped on ice blocks, it is important to know whether tetracycline residues are stable at that temperature and for how long. Control raw milk samples fortified with 50 ppb each chlortetracycline, demeclocycline, methacycline hydrochloride, minocycline, oxytetracycline, and tetracycline were incubated at 4 degrees C or 25 degrees C, then analyzed using a metal chelate affinity chromatography extraction and HPLC. No loss of tetracycline was observed after 48 h of storage at 4 degrees C or 24 h at 25 degrees C. Losses ranging from 4 to 13% and 0 to 18% were noted after 72 h at 4 degrees C and 48 h at 25 degrees C, respectively.

Screening of nonfatty fish for organochlorine pesticide residues by solid-phase extraction cleanup: interlaboratory study.

Six U.S. Food and Drug Administration laboratories participated in an Interlaboratory trial of a solid-phase extraction cleanup method for determination of pesticides in nonfatty seafood products. The participants analyzed control and fortified (about 0.050 ppm lindane, heptachlor epoxide, p,p'-DDE, and endrin) croaker and flounder samples and a sea trout sample containing incurred p,p'-DDE residues. Mean recoveries of the fortified residues from the fish ranged from 89.1 to 107.8%. The within-laboratory coefficients of variation (CVs) ranged from 4.2 to 8.5%, and the among-laboratory CVs ranged from 10.9 to 26.5%. The 6 laboratories reported a mean value of 0.040 ppm p,p'-DDE in a fish sample which contained incurred residues. The same value (0.040 ppm) was obtained by using official methodology. The within-laboratory CVs ranged from 3.5 to 18.3%, and the among-laboratory CV was 17.3%.

Multiresidue determination of abamectin, doramectin, ivermectin, and moxidectin in milk using liquid chromatography and fluorescence detection.

Abamectin, doramectin, ivermectin, and moxidectin are macrocyclic lactones derived from soil dwelling actinomycetes, and are very effective against nematode, insect, and arthropod infestations. These compounds, known as endectocides, have been approved for use in beef cattle in the United States; however, they are currently not approved for use in dairy cattle. Abamectin, doramectin, ivermectin, and moxidectin residues were isolated from milk by a series of liquid-liquid extraction steps, derivatized with trifluoroacetic anhydride, and determined by liquid chromatography with fluorescence detection. Recovery studies were performed in 2 laboratories. Recoveries of > 80% (1-30 ng/mL) were achieved for all 4 compounds.

Florisil solid-phase extraction cartridges for cleanup of organochlorine pesticide residues in foods.

Florisil solid-phase extraction cartridges were evaluated for cleanup of organochlorine pesticide residues in food extracts. Elution patterns and recoveries were determined for 24 organochlorine pesticides. A range of elution solvents was evaluated. A 2% ethyl ether-petroleum ether eluant optimized overall recoveries while minimizing interferences from coextractants.

Determination of organochlorine pesticide and polychlorinated biphenyl residues in fatty fish by tandem solid-phase extraction cleanup.

A rapid, multiresidue solid-phase extraction (SPE) technique for determination of organochlorine pesticide and polychlorinated biphenyl (PCB) residues in nonfatty fish was modified for use with fatty fish. In the modified procedures, samples are extracted with acetonitrile, and the extract is cleaned up with both C18 and Florisil SPE columns. Residues are determined by gas chromatography with electron capture detection. The original method was modified for use with fatty fish by reducing the amount of tissue extracted and by using an improved Florisil SPE cleanup. Recovery data are presented for 24 fortified organochlorine pesticide residues (0.12 ppm) and 3 fortified PCB residues (0.80 ppm) from flounder, bluefish, and shad samples, which contained 0.8, 5.4, and 22.6% fat, respectively. For the 3 types of fish, recoveries of 23 of 24 fortified organochlorine pesticide residues ranged from 55 to 129%, and recoveries of 3 fortified PCB residues ranged from 55 to 104%. There were no significant differences in recovery based on fish species and/or fat content for the majority of residues studied. This SPE method and the official AOAC method yielded comparable results for fish containing incurred organochlorine residues.

Screening procedure for organochlorine and organophosphorus pesticide residues in eggs using a solid-phase extraction cleanup and gas chromatographic detection.

A solid-phase extraction (SPE) screening procedure for the isolation and gas chromatographic (GC) determination of organochlorine and organophosphorus pesticide residues in eggs is described. Eggs are extracted with acetonitrile. The extract is subjected to a cleanup on tandem C18 and Florisil SPE columns. Organochlorine and organophosphorus pesticide residues are determined by GC with electron capture and flame photometric detection, respectively. Because the injected extracts are free from matrix interferences, the amount of residue present is easy to calculate. The average recoveries of 9 spiked organochlorine pesticide residues (0.01-1.0 ppm) ranged from 80.9 to 91.1%. The average recoveries of 7 spiked organophosphorus pesticide residues (0.02-0.50 ppm) ranged from 80.3 to 89.5%. The SPE method results in a 90% reduction in organic solvent consumption and an 85% reduction in hazardous waste production compared with the AOAC methodology.

Determination of organochlorine and organophosphorus pesticide residues in low moisture, nonfatty products using a solid phase extraction cleanup and gas chromatography.

A multiresidue solid-phase extraction (SPE) method for the isolation and subsequent gas chromatographic determination of organochlorine and organophosphorus pesticide residues in low-moisture, nonfatty products is described. Residues are extracted from samples with an acetonitrile/water mixture. Cleanup of the extract is performed using graphitized carbon black and anion exchange SPE columns, and analysis is performed by gas chromatography with Hall electrolytic conductivity and flame photometric detection. Recovery data was obtained by fortifying corn, oats and wheat with pesticides. The average recoveries were 79-123% for eight organochlorine and 51-122% for 28 organophosphorus pesticide residues. The limit of quantitation for chlorpyriphos was 0.05 ppm using the Hall electrolytic conductivity detector and < 0.005 ppm using the flame photometric detector.

The effect of storage at 4 degrees C on the stability of ampicillin residues in raw milk.

Raw milk samples collected from tanker trucks are routinely screened for beta-lactam antibiotic drug residues using rapid screening tests. If drug residues are detected, the milk may be shipped on ice blocks to a laboratory for further analysis. A study was conducted to determine the stability of ampicillin in raw milk stored at +4 degrees C in order to predict if shipping the milk would result in the degradation of ampicillin residues. Milk samples were spiked with 20 ppb ampicillin, stored at +4 degrees C and -70 degrees C for 1-6 days, and then analysed by HPLC with fluorescence detection. No loss of ampicillin residues was found in milk stored at +4 degrees C for 1-6 days.

Screening procedure for organochlorine and organophosphorus pesticide residues in milk using matrix solid phase dispersion (MSPD) extraction and gas chromatographic determination.

A rapid technique for the extraction and gas chromatographic determination of five organochlorine and five organophosphorus pesticide residues in milk is described. Milk (5.0 ml) is blended with 2.0 g of C18 [octadecylsilyl-derivatized silica] and 1.5 ml acetonitrile in a syringe barrel. After the aqueous phase is removed from the column by vacuum aspiration, the pesticide residues are eluted from the C18/milk matrix with acetonitrile which is then eluted through a Florisil solid phase extraction (SPE) column. The acetonitrile is evaporated under nitrogen and the residue is dissolved in petroleum ether. This extract is directly analysed for organophosphorus pesticides by gas chromatography with flame photometric detection. After further clean-up of the extract on a mini-Florisil column, the organochlorine pesticide residues are determined by gas chromatography with electron capture detection. Grade A homogenized and raw milk samples were fortified with five organochlorine and five organophosphorus pesticide residues. The average recoveries of fortified organochlorine pesticide residues (2.0-20 ppb) ranged from 76.0% to 97.8%. The average recoveries of fortified organophosphorus pesticide residues (10-50 ppb) ranged from 75.0% to 104.5%. The MSPD and the AOAC International multiresidue method for pesticides in milk produced comparable results for milk samples containing incurred organochlorine pesticide residues. The use of the MSPD method results in a 90% reduction in organic solvent consumption and a 95% reduction in the hazardous waste generated when compared with the AOAC method.

Particle beam liquid chromatography/mass spectrometry with negative ion chemical ionization for the confirmation of ivermectin residue in bovine milk and liver.

Particle beam liquid chromatography/mass spectrometry (LC/MS) using negative ion chemical ionization was applied to the analysis of ivermectin residue in bovine milk and liver. Samples were prepared by liquid/liquid extraction followed by alumina B solid-phase extraction clean-up. On-line LC/MS of extracts was carried out on a C-18 bonded silica column. Signals were observed from on-column injections of 4 ng dihydro-avermectin B1a (H2B1a) in extracts equivalent to 2 ml milk or 0.3 g liver. The specificity required for a regulatory confirmation procedure was achieved by monitoring the H2B1a molecular ion and four fragment ions. Ion chromatogram peak areas were at least three times greater than control samples integrated over the same time window. Coeluting matrix compounds enhanced the response and altered the abundance pattern of H2B1a. To compensate for this matrix effect, control milk extracts were spiked with H2B1a standard and used for the abundance matching for the abundance matching requirement of regulatory confirmation.

Rapid screening for organochlorine and organophosphorus pesticides in milk using C18 and graphitized carbon black solid phase extraction cleanup.

A rapid, multiresidue, solid phase extraction (SPE) technique for the isolation and gas chromatographic determination of organochlorine and moderately polar organophosphorus pesticide residues in milk is described. Milk is sonicated with an acetonitrile-acetone-methanol mixture and centrifuged. The supernatant is subjected to a cleanup using both C18 and graphitized carbon black SPE columns. The pesticide residues are determined by gas chromatography with electron capture and flame photometric detection. The method required minimal volumes of solvent and resulted in the production of minimal volumes of hazardous waste.

A highly sensitive gas chromatographic determination of levamisole in milk.

An efficient extraction and sensitive gas chromatographic method is described for the determination of the anthelminthic drug levamisole in milk. Levamisole was extracted from alkaline milk with ethyl acetate. Clean-up of the extract was by a series of liquid-liquid extraction steps. Levamisole residues in the extract were determined by gas chromatography with a nitrogen-phosphorus detector. This method was satisfactory for determining levamisole residues in milk as low as 0.5 ng/g. Mean recoveries of 0.5-10.0 ng/g fortified milk samples ranged from 84.5 to 95.2%. Five replicate analyses performed on a milk containing incurred levamisole residues yielded a mean of 3.34 ng/g levamisole with a CV of 3.0%.

Gas chromatographic determination of incurred dimetridazole residues in swine tissues.

A gas chromatographic method for determination of 2-hydroxymethyl-1-methyl-5-nitroimidazole (DMZOH), the hydroxy metabolite of dimetridazole, in swime muscle has been developed. The method uses cleanup steps similar to those of an earlier polarographic method. The present method is capable of quantitating levels as low as 2 ppb and detecting less than 1 ppb. Recoveries from 30 control tissues spiked at 1, 2, or 4 ppb averaged 80.4%. Performance of the method in incurred tissue was documented and limited data on the depletion of the metabolite in muscle were generated. The muscle of swine given 150 ppm dimetridazole in feed for 14 days contained less than 1 ppb DMZOH at 12 h withdrawal time.