Over its 30-year history, PDP has generated close to 42 million pesticide/commodity data points from the analysis of almost 311,000 samples over 126 different commodities. That’s a long way from the start of the program in May 1991 when three States (FL, NY, and WA) collected three commodities (grapes, lettuce, and potatoes) for a total of 35 samples per month.

In 1991, samples were analyzed for 11 pesticides and only detected values were reported. In 2021, nine States collected 14 commodities each month for a total of 826 samples per month. Samples are analyzed for up to 515 residues, and both detects and non-detects are reported.

Reporting non-detects was an early major program development. For the years 1991–1993, the testing laboratories only reported data for the confirmed detection of pesticide residue. Beginning in 1994, testing laboratories began reporting all validated pesticide residues results including non-detects which is key for application of monitoring data in exposure assessment. The number of reported pesticide/commodity data points increased significantly. In 1993, the analysis of 7328 samples yielded 10,329 discrete pesticide/commodity data points (1.41 data points per sample). In 1994 with non-detects reported, 7,589 samples yielded 504,296 discrete pesticide/commodity data points (66.6 data points per sample). In 2020, the analysis of 9600 samples yielded 2,602,551 discrete pesticide/commodity data points (271.1 data points per sample; Fig. 1).

Average (mean) number of PDP results reported per sample by year.

Trans-shipping was a major program development that affected both sampling and testing. Prior to trans-shipping, the testing laboratory would analyze the samples for all commodities collected in their State. Because different commodities could not be combined into a single analytical set, the testing laboratories had to run smaller sets including required quality assurance samples with each set. This prevented economy-of-scale and reduced productivity. Additionally, switching between the different commodities had the potential to impact proficiency as matrix effects vary dramatically (e.g., testing grapes is very different than spinach). In October 1994, a pilot project between three States (CO, MI, and WA) began to send samples from all the States to a single testing laboratory, thereby allowing for the creation of larger sample sets. By 1997, all States were trans-shipping samples. This increased proficiency and productivity while also substantially reducing mandatory method validation and quality assurance costs.

Throughout the entire history of PDP, there has been a continued drive to lower detection limits and increase the number of pesticide residues included in the screens. For example, in 1996 the mean limit of detection for thiabendazole was 53.5 parts-per-billion (ppb) while in 2020 the mean limit of detection was 3.9 ppb. Simultaneously, there has been an increase in the number of compounds included in the screens (Fig. 1). Because PDP data are geared for dietary risk assessments, the current laboratory methods are optimized for detection limits in the low ppb.

The multi-residue analytical methods employed at PDP laboratories have evolved over the years. While both gas and liquid chromatography instruments have been used throughout the entirety of the program, the detectors coupled to those have changed considerably. Initially, determinations by gas chromatography included the use of electron capture detector (ECD), flame photometric detector (FPD), electrolytic conductivity detector (ELCD), and flame ionization detector (FID) along with mass spectrometry (MS) and liquid chromatography determinations primarily used post column derivatization and ultraviolet detectors. Gradually, the laboratories made the switch to single-stage MS or tandem mass spectrometry (MS/MS) and by 2012 all testing laboratories had made the move to all MS confirmation.

The extraction methods used by the PDP laboratories have evolved along with the analytical methods. Variations of the Luke [14] extraction, a solid phase extraction based on the Agriculture and Agri-Food Canada method [15], and a multi-residue extraction method developed by the California Department of Food and Agriculture (CDFA) [16] were initially used by the PDP laboratories. Currently, all use variations of the QuEChERS [17] method. Procedures for inspecting, storing, and processing samples are specified in the PDP LABOP SOP [18]. Upon receipt, samples are inspected for acceptability and damage and discarded if deemed inedible. Samples can be stored for specified periods of time and temperature prior to preparation. Samples are processed to mimic consumer practices: the sample is rinsed, ends are trimmed, and inedible portions (e.g., stems seeds, peels, rinds) are removed. The prepared food samples are then chopped, blended, or mixed until homogenized. Homogenates are either extracted immediately or held at −40 °C until extraction.

Just as the extraction and analytical methods have evolved over the years, so have the program SOPs [7]. During 2009–10, PDP merged the requirements of 48 SOPs into four overarching documents: administration, laboratory operations, data reporting, and QA/QC. This consolidation removed reduncancies and inconsistencies. In 2016, the requirements from nine sampling SOPs were consolidated into a single document.

PDP has conducted many special projects during the past 30 years. From 1997 through 2000, PDP conducted surveys of single servings of apples, peaches, pears, and potatoes. Surveys of single servings are designed for commodities in which one unit may comprise a serving for a single meal/snack. In surveys of single servings, the samples are processed differently than routine PDP composite samples. For routine PDP samples, the entire 5 lb. sample is homogenized into a composite sample, whereas, for surveys of single servings, a single unit (e.g., an apple) is taken from the 5 lb. sample and homogenized. Details regarding sample collection, preparation, analysis and results can be found in the respective PDP Annual Summaries [19].

PDP conducted a special triazoles survey in 2003–2004 [20]. EPA requested data for triazole pesticides and their common metabolites as part of a review of new tolerance applications. PDP partnered with the United States Triazole Task Force (USTTF) to test apples, bananas, eggs, grapes, milk, peaches, peanut butter, strawberries, and wheat. Sample analysis was split between PDP laboratories and USTTF contract laboratories.

In 2005, PDP responded to a data request from EPA to collect 306 soybean samples slated for export and test for 14 fungicides used to treat soybean rust and two insecticides used to control the Chinese aphid [21]. The USDA Grain Inspection, Packers, and Stockyard Administration (GIPSA) laboratory developed a specialized method to optimize the recovery of target compounds using a solid phase extraction and liquid chromatography/MS/MS instrumentation.

In 2009, PDP conducted a special survey where 387 organic lettuce samples [20, 22] were collected at routine PDP sampling sites. The samples were tested for 57 parent pesticides, metabolites, and isomers with an emphasis on compounds used in organic farming.

In 1998 and 1999, PDP teamed with the Corn Refiners Association to test corn syrup. In 1998, 298 one-quart high fructose corn sugar samples were collected by plant personnel at one of the 17 participating plants that represented over 95% of corn syrup production. The samples were sent to a PDP laboratory and the testing profile included 83 pesticides and 26 metabolites, degrades, or isomers [23]. In 1999, a total of 156 dextrose equivalent corn syrup samples were collected by plant personnel at one of the eight participating refineries. The samples were sent to a PDP laboratory and the testing profile included 82 pesticides and 21 metabolites, degrades, or isomers [24].

Details for all PDP special projects can be found by visiting the “Special Projects” tab of the PDP website [20] and details on the results of the special projects and compounds detected can be found in the corresponding PDP Annual Summary reports [19].