Pesticide Residue Samples

Amy Hernandez  |  9/30/2009 4:49:01 AM

The Pesticide Residue section is located on the second floor of the Agricultural Chemistry building. Six full-time employees analyze an average of 1,800 samples each year. Samples are received from various sources -- including researchers in other departments of the university system, state government and the general public. Employees attend seminars and workshops throughout the year at government agencies and professional organization meetings to learn of new methods and instrumentation used in pesticide analysis. The lab participates in a "check sample" program conducted by the State of Wisconsin, in which unknown pesticides are detected and quantitated, to test the laboratory's performance. Over 200 pesticide standards at residue levels are maintained for routine use in sample analysis. "State-of-the-art" instrumentation allows analysts to detect the presence of pesticide residues often at the part per billion range. To prevent contamination from other sections' samples, separate facilities for glassware, sample preparation and storage are dedicated for pesticide residue use.

Types of Samples Analyzed

Damage/Enforcement Samples

Complaints by the public of damage to plants, humans, animals or property that may involve the use of pesticides are investigated by Louisiana Department of Agriculture and Forestry (LDAF) inspectors. Samples taken for analysis come in a variety of matrices including foliage, water, soil, clothing, animal tissue, animal feeds and gauze swabs of areas in a home.

Environmental Monitoring Samples

Louisiana Department of Agriculture and Forestry (LDAF) monitors the environment for pesticide residues during the growing and harvesting seasons to establish levels of agricultural pesticides. These results aid in the investigation of fish kills and other incidents.

The Water Monitoring Project, which has been conducted each year since 1989, monitors surface and ground waters of Louisiana. Forty-seven sites are tested March through October in alternate months for approximately 35 agricultural pesticides. 

Soil samples submitted to LDAF's Organic Farming Certification are analyzed for the presence of pesticides before a certification is issued.

Structural Pesticide Samples

Louisiana Department of Agriculture and Forestry investigates complaints of structural damage to homes by termites. Samples of soil and foundations are analyzed to determine the presences of termiticides, including Fipronil, Imidacloprid, Bifenthrin, Permethrin, Cypermethrin. Inspectors also take soil samples from new construction sites. These samples are analyzed to determine if termiticides have been applied in accordance with the label.

Research Samples

Louisiana Agricultural Experiment Station staff and Louisiana State University graduate students in various departments conduct research on pesticides. Samples range in description from test-plot soils, dolphin tissue, runoff water to turfgrass. The graduate students are instructed in methods of preparation and analysis of the samples and perform many of the analyses themselves under supervision of staff.

Miscellaneous Samples

The general public also submits samples for analysis of pesticide residues present. Samples range in nature from well water to car surfaces to clothing. Fees are based on analyses requested.

Types of Pesticides Analyzed

Arranged by Method of Analysis and Matrix

Sample Extraction

The Extraction Lab of the Pesticide Residue Section uses various methods to extract pesticides from the sample matrix. Sources for extraction methods include the Association of Official Analytical Chemists (AOAC), the Environmental Protection Agency (EPA) and the manufacturers of the pesticide.

The sample must first be prepared for extraction depending on the matrix. For example, soils are dried and then ground. Animals submitted for analysis must first be dissected to obtain the organs or tissues where pesticides accumulate such as the brain, liver and adipose tissues.

The basic theory of extraction is to separate the pesticide from the sample matrix through a series of chemical reactions and solvent separations. Extraction methods are dependent on both the sample matrix and the pesticide targeted.

Depending on the sample matrix, targeted pesticides and amount of sample submitted, the extraction lab may perform several extractions and clean-up steps on a single sample.

Quality control

Quality control is ensured by the preparation of a "blank" sample and a "spiked" sample for every set of samples extracted for that particular matrix and the targeted pesticide. A blank sample is prepared by using "clean" soil, laboratory cotton swabs or distilled laboratory water and extracting it along with the samples. This will allow the analyst to see the effect of the extraction procedure on that matrix. It is not always possible to acquire a blank sample for each set. In these cases, a reagent blank is run to test all reagents used in the extraction. A "spike" or "fortified" sample is necessary to prove the efficiency of the extraction method for the targeted pesticide. A known amount of pesticide is added to the matrix and then is tested to determine what percentage of the pesticide applied was recovered by extraction. A low percentage of pesticide recovered indicates that an alternative method of extraction must be used (if available) to ensure that the targeted pesticide is extracted from the actual sample. Likewise, a high percentage of pesticide recovered from the spiked sample indicates that it is reasonably certain that a high percentage of the targeted pesticide, if present in the sample, will be extracted as well.

Once extraction of the sample is complete, analysis of the extractant begins in the Analysis Lab

Sample Analysis

Samples are analyzed for pesticide residues using chromatography. In general terms, analysis of a sample by chromatography begins with the separation of a sample into its component analytes by a chromatographic column based on the analytes' chemical composition (weight, structure, affinity for column composition, etc.), rate of the column's carrier flow and temperature of the column. The detection of a pesticide in a sample by chromatography must be confirmed. Confirmation involves detecting the pesticide on a different chromatographic column and/or a different type detector. This laboratory uses three confirmations as proof that the pesticide is present. It is preferred that one of the confirmations be performed using mass spectrometer detection.

This laboratory employs four main types of chromatography: Gas Chromatography (GC), Gas Chromatography with Mass Spectrometer Detection (GC-MS), High Pressure Liquid Chromatography (HPLC) and High Pressure Liquid Chromatography with MS/MS detectors. GC and GC-MS use inert gas as the carrier for the chromatographic column while HPLC uses liquid under pressure. Each type of chromatography has its own specialty detectors. The chemical nature of the pesticide targeted and its relative sensitivity determine which type of chromatography is used. However, when possible, all pesticides found are confirmed by GC-MS.

See the Instrumentation page for a detailed list of instruments in the Pesticide Residue Analysis Lab.

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