Surrogate standards

Before sample preparation, surrogate compounds must be added to the matrix. These are used to evaluate the efficiency of recovery of sample for any analytical method. Surrogate standards are often brominated, fluorinated, or isotopically labeled compounds that are not expected to be present in environmental media. If the surrogates are detected by GC/MS within the specified range, it is... 

Due to the predicted and previously detected low concentrations of pesticides in environmental samples (usually around the nanogram per liter level), a preconcentration step of the water samples is necessary prior to measurement. In this way, a preconcentration factor of several orders of magnitude (200-1,000-fold) is mandatory to reach the low detection limits necessary for the identification of pesticides, especially in complex wastewater samples. Also, the use of surrogate standards (e.g., triphenyl phosphate) added before the extraction step is a common practice in order to account for possible errors during the extraction process and for quantitative purposes. The commonly used extraction methods for polar compounds from water matrices involve isolation using liquid-liquid extraction (LLE) and solid-phase extraction (SPE), which are commented on below. Other methods such as semipermeable membrane devices (SPMD) are also mentioned. 

M [8]. Ferguson et al. [10] have shown that matrix effects may result in severe ion suppression in APEO analysis (this phenomenon occurs particularly in negative mode electrospray ionisation). They also demonstrated that the effect may be accounted for appropriately when using internal and surrogate standards, which closely mimic the ionisation behaviour of the analytes. In Ferguson s study, specially... 

Most of the development work on organic pollutants has resulted from the use of GC-MS and synthesis of authentic standards or surrogate standards. However, with advances in other techniques it is clear that this field will benefit by making greater use of alternative identification and characterization methods. The following is a summary of some advances and instrument combinations ... 

There are obvious inadequacies in the established values. First, it was assumed that the oxidant standard is a surrogate standard for photochemical oxidants, with ozone as the indicator. However, it is clear that very low concentrations of specific irritants, such as peroxyacetyl-nitrate (PAN), are sufficient to cause eye irritation. Second, some of the statistical techniques used to determine the lowest concentration at which effects are observed were inconsistent and undocumented. A hockey stick fimction was sometimes used to find an effect threshold. 

Incorrect Surrogate standards (SS) and internal standards (IS) were used. 

Correct Surrogate standards and internal standards were used. 

Sample Extraction and Cleanup. The sample extraction method has been described previously (27, 28). Prior to extraction, 100 pL of a dio-fluoranthene surrogate standard solution (0.82 ng/pL) was added to each sample. The XAD-2 resin was extracted for 24 h in a Soxhlet apparatus with -350 mL hexane/acetone (50/50 v/v). The extract volume was then reduced to -100 pL using rotary evaporation followed by evaporation... 

Spike blanks, samples and standards, ready for extraction with surrogate standard. Spike each calibration standard matrix with appropriate amount of standard for the calibration curve standards and each QC sample. Vortex the standard curve samples and QC samples for approximately 5 sec. 

Calibration standards can be of two types external standards and internal standards. With external standards, multiple concentrations of the standards are injected, areas are measured, and a calibration curve is platted. Unknown samples are then injected, chromatograms run, and areas are calculated and compared with the calibration curves to determine amounts of each compound present. With internal standards, known amounts of an internal standard are added to each known concentration of standard compound and areas or peak height response factors relative to those of the internal standard are calculated. When unknowns are run, a known amount of internal standard is added to the unknown sample, response factors are calculated relative to the internal standards, and amounts of each unknown present are calculated from the standards calibration factors. Internal standards are usually used to correct for variations in injection size due to different operators and injection techniques. Internal standards can also be used to correct for extraction variation in GC/MS target compound quantitation, this standard is referred to as a surrogate standard. Generally, an internal standard is used for one purpose or the other, not both at the same time. 

Dichlorophcnylacctic acid is recommended as a surrogate standard in the U.S. EPA Method 8151. 

Aqueous samples are extracted with methylene chloride by liquid-liquid extraction. The extract is concentrated and then exchanged to hexane. Soils, sediments, and solid wastes are extracted by sonication or Soxhlett extraction. Samples should be spiked with one or more surrogate standard solution to determine the accuracy of analysis. Some of the internal standards mentioned above may also be used as surrogates. If only the PCBs are to be analyzed, hexane instead of methylene chloride may be used throughout. Oil samples may be... 

Another tool that enables us to evaluate analytical accuracy of organic analyses is surrogate standards. These are compounds that do not naturally occur in the environment and that are similar in chemical nature and behavior to target analytes. In organic compound analysis, known amounts of surrogate standards are added to each sample prior to extraction. The comparison of surrogate standard recoveries to laboratory control limits permits the laboratory to monitor the efficacy of extraction and to measure the accuracy of analysis for each individual sample. 

The laboratory will introduce the required internal and surrogate standards through the septum with a small gauge syringe and then place the vials on the autosampler for analysis. Matrix spikes will be prepared in a similar manner. 

Prior to the preparation of field and laboratory QC samples for organic compound analysis, sample preparation personnel amend them with surrogate standards to monitor the efficacy of extraction for each sample matrix. Surrogate standards are organic compounds that are similar in chemical behavior to the target analytes and that are not expected to be present in samples. 

As a standard reporting practice, most laboratories also include results for laboratory QC checks (method blanks, surrogate standards, LCS/LCSD, MS/MSD) and their respective acceptance criteria. Per client s request, the laboratory will provide comprehensive data packages for data validation. 

The use of LIMS makes internal review process rapid, reliable, and concurrent with data reporting. While Tier 1 reviewers typically work with hardcopy data, Tier 2 reviewers often access appropriate LIMS modules and review sample and QC data before final results are printed. They will also review the accompanying laboratory documentation that is available in hardcopy form only. Computerized confirmation of numeric acceptance criteria, such as the recoveries of laboratory QC check samples or surrogate standards, significantly reduces time and effort during review. Tier 3 Review is usually conducted on a hardcopy final data after analytical results and support documentation have been compiled into a single package. 

Similar to target analytes, surrogate standards must be calibrated with multipoint calibration curves. 

Analysis of surrogate standards and laboratory control samples to measure analytical accuracy... 

Surrogate standard recovery measures analytical accuracy for each individual sample. Approved methods for organic compound analysis usually recommend the surrogate standard selection. Similar to target analytes, multipoint calibration curves are prepared for surrogate standards. 

The importance of surrogate standard recovery in evaluating data quality cannot be overemphasized. Laboratories evaluate the efficacy of extraction of each individual sample based on the surrogate standard recovery. If batch QC checks are acceptable, but the individual sample surrogate standard recovery is not, the validity of sample results is questionable. Results of organic compound analysis performed without surrogate standards cannot be considered definitive. 

Similar to LCS recoveries, surrogate standard recoveries should be monitored by the laboratory and plotted as control charts. The EPA recommends the use of in-house laboratory control limits for surrogate standards recoveries for all organic compound analyses (EPA, 1996a). The exception is the CLP SOW, which specifies these limits for soil and water analysis. Unless affected by matrix interferences, surrogate standard recoveries normally have relatively narrow control limits, 65-135 percent for most organic compound analysis. (Many laboratories, however, default to arbitrary limits of 50-150 percent for GC analyses, instead of using statistical control limits.)... 

For GC/MS analyses, some laboratories use surrogate standard recovery limits from outdated versions of EPA Methods 8260 and 8270. These recovery limits, shown in Example 4.18, are fairly close to the statistically derived control limits at most laboratories and can be safely used in the evaluation of data quality. The surrogate... 

Example 4.18 Surrogate standard recovery limits for GC/MS methods... 

These surrogate standard recoveries are from the CLP SOW (EPA, 1999d). They are also acceptable for EPA Method 8260 and 8270. 

Surrogate Standard Percent recovery, water Percent recovery, soil... 

Surrogate standard Recovery as determined by laboratory control charts. 

Are surrogate standard recoveries in each sample acceptable ... 

Did matrix interferences or sample dilutions affect the surrogate standard recoveries and reporting limits ... 

GC/MS methods are the only published methods that include the surrogate standard recovery limit guidance. Similar to LCS, acceptance criteria for surrogate standard recoveries of all other organic analysis methods are the laboratory control limits. The limits for internal standard recovery in GC/MS analysis are specified by the method and cannot be changed by the laboratory. Acceptance criteria for matrix interference detection techniques in trace element analyses, discussed in Chapter 4.4.4.5, are also specified in the analytical methods. 

The chemist reviews results of each analysis and determines whether data qualification is needed. Typical deficiencies that turn definitive data points into estimated ones include insufficient surrogate standard recoveries the absence of second column confirmation and the quantitation performed outside the calibration curve. The chemist may even reject the data based on low surrogate standard recoveries. Example 5.6 shows how surrogate standard recoveries may affect the validity of analytical results. 

The laboratory control limits of the BFB surrogate standard in EPA Method 8021 are 56 to 143 percent. The chemist may use the following rationale for qualifying individual sample data with the surrogate standard recoveries outside these limits ... 

The surrogate standard recoveries for some samples may be outside the control limits due to sample dilutions or matrix interferences. Such samples are usually clearly identified in the Case Narrative or in the laboratory reports, and their data are not qualified. For example, if a surrogate standard recovery is below 10 percent due to dilution, the result will not be rejected and will not be qualified as an estimated value. The chemist, however, may request from the laboratory the raw data for this sample in order to verify whether the dilution was justified and the interferences were truly present. 

A surrogate standard recovery that is above the upper control limit is not necessarily an indication of a high bias in the sample result. It may indicate matrix interference, a preparation error, or a measurement error. 

Surrogate standard recovery report (concentration spiked, percent recovered, and recovery control limits) / / /... 

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