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Batch Sizes And QC Questions
- The Frequencies of QC Analyses
- How to Define a Batch
- The Purpose of the Matrix Spike Sample Compared to the Laboratory Control Sample
- Insufficient Sample Volume
The Frequencies of QC Analyses
Question: Why do so many of the QC operations (MS, MSD, LCS, blanks, etc.) have to be run "once for every 20 samples?"
Answer: The 1 per 20 (5%) frequency is a default value that has been used in many EPA programs for many years. EPA also recognizes that other frequencies may be appropriate under other circumstances. For example, in the case of a long-term monitoring project involving a small number of analyses of a sample matrix that does not change, it should not be necessary to prove that the method applies to the matrix each time that samples are collected and analyzed. Therefore, the MS/MSD analyses could be run less frequently than 1 every 20 samples. To that end, EPA recommends that, if another frequency for the QC analyses is chosen, it be clearly documented in a sampling and analysis plan that is reviewed and approved by the relevant regulatory authority.
How to Define a Batch
Question: The methods talk about different kinds of "batches" and tie the frequencies of QC samples to various batches. Is there a single definition of a "batch?"
Answer: Yes and no. Chapter One defines a "batch" as
A group of samples which behave similarly with respect to the sampling or testing procedures being employed and which are processed as a unit..."
Because of the many sample processing steps that can be involved in a given analysis, there are a number of types of batches, each of which may drive the frequency of QC operations. For example:
Extraction or digestion batch - A group of up to 20 samples extracted together (organics) or digested together (metals) using the same techniques. In order to demonstrate that the extraction or digestion equipment used for these samples does not result in contamination of the samples, you need to prepare a method blank with each batch of samples extracted or digested together. In order to demonstrate that you could perform the sample preparation procedures in a clean matrix, you also need to prepare one laboratory control sample (LCS) with each such batch. The method blank, LCS, and any spiked samples (MS) or matrix spike duplicates (MSD) do not count in the total of 20 samples in the extraction or digestion batch.
Matrix spike batch - A group of up to 20 field samples of the same matrix (e.g., water, soil, sediment, waste) being analyzed for the same constituents may be associated with a single matrix spike (MS) sample of a matrix spike/matrix spike duplicate (MS/MSD) pair. Under ideal circumstances, the samples should all be from the same site or very similar sites, where the analytes of interest are the same. The MS or MS/MSD results from the spiked sample(s) are used as a measure of the potential bias and precision of the results from the unspiked field samples and therefore, as a measure of the applicability of the methods employed to the sample matrix. Unless otherwise specified in a method, a QA plan, or a sampling and analysis plan, the samples in a matrix spike batch all do not need to be analyzed at the same time, or on the same instrumentation. In other words, the MS/MSD results may apply to samples extracted, digested, and/or analyzed on different shifts, provided that the same techniques were employed.
Cleanup batch - A group of up to 20 samples or sample extracts (including QC aliquots) that undergo a given cleanup procedure (i.e., sulfur cleanup using Method 3660B, or GPC using Method 3640A). If all the samples in a single extraction batch (see above) undergo the cleanup procedure, then the method blank and LCS prepared above will also go through the cleanup procedure. However, if only some of the samples are subjected to cleanup, then you need an additional blank that is carried through the cleanup procedure, in order to be able to distinguish possible contamination introduced during the cleanup from other possible contamination sources. If the LCS and MS/MSD aliquots are subjected to the same cleanup procedure, then they count in the total of 20 samples or extracts for this batch.
Analysis batch - A group of up to 20 samples, sample extracts, or sample digestates (including QC aliquots), that are analyzed together on the same instrument. For the analysis of volatiles, there may be no sample preparation equipment other than that attached directly to the determinative instrument (e.g., Methods 5030B and 8260B), so the analysis batch drives the frequency of the method blank and LCS for volatiles, as well as the frequency of calibration verification standards for methods using external standard calibration. The limit of 20 in the analysis batch includes all the analyses, including the method blank, LCS, MS, and MSD, so that an analysis batch for volatiles will include fewer than 20 field samples. However, as noted above, the MS/MSD may be analyzed on another shift or other equivalent instrument.
For extractable organics and metals, the analysis batch will drive the frequency of the calibration verification standard analyses, except where a given method specifies a higher frequency for the verifications (e.g., after every 10 samples). The limit of 20 in the analysis batch includes all the analyses, including the method blank, LCS, MS, and MSD, so that an analysis batch will include fewer than 20 field samples. Some organic methods do not count any instrument blanks in this total, since these aliquots of clean solvent are simply designed to prevent cross-contamination between samples.
As noted above, a QA plan or a sampling and analysis plan for a given project may contain requirements that restrict batch sizes or definitions for that project.
The Purpose of the Matrix Spike
Question: What is the purpose of analyzing the matrix spike (MS) sample versus analyzing the laboratory control sample (LCS) and why should we run both?
Answer: The MS/MSD results are an important measure of the performance of the method relative to the specific sample matrix of interest. EPA believes that such a demonstration is an important aspect of an overall quality assurance program, and is particularly important for the RCRA program, where a wide range of different matrices are subject to regulation.
The primary purpose of these MS/MSD analyses is to establish the applicability of the overall analytical approach (e.g., preparative, cleanup, and determinative methods) to the specific sample matrix from the site of interest.
The primary purpose of the laboratory control sample (LCS) is to demonstrate that the laboratory can perform the overall analytical approach in a matrix free of interferences (e.g, in reagent water, clean sand, or another suitable reference matrix).
Therefore, the LCS results should be used in conjunction with MS/MSD results to separate issues of laboratory performance and "matrix effects."
Unfortunately, some may believe that the MS/MSD results can and should routinely be used to evaluate performance of an individual laboratory. This was not EPA's intent in specifying that MS/MSD analyses be performed at a 5% frequency.
EPA believes that consistent trends in MS/MSD results can be of some use in evaluating laboratory performance, as are trends in surrogate recoveries, LCS recoveries, and other QC data. However, the appropriate use of a single set of MS/MSD results is to evaluate method performance in the matrix of interest, not to evaluate laboratory performance.
Insufficient Sample Volume
Question: The client did not indicate which sample should be used for the matrix spike, or did not provide additional sample volume for spiking. What should we do?
Answer: The first answer is "call the client as soon as the samples are received and the problem is noted." In some cases, the problem is an oversight on the part of the samplers. It may also be due to difficulties in obtaining sufficient volume, such as from a poorly producing groundwater well. It may also be that the additional volume was shipped separately. Therefore, call the client and determine what they intended to do.
If the client does not specify a particular sample to be spiked, but all samples have enough volume, then choose a sample that is similar to many others in the group. Do NOT choose the cleanest looking sample, or a trip blank, or field blank, since such samples will not tell you much about the other field samples.
In other instances, the client simply may assume that the laboratory will prepare the MS/MSD from some other sample prepared at the same time. Unfortunately, this situation can lead to MS/MSD results for a matrix that is only marginally related to the samples in question. If this occurs, the utility of the MS/MSD results is severely limited and may simply increase the cost of the analyses without providing any real benefit to the client or the ultimate data user.
Due to the importance of the relationship between the matrices of the MS/MSD and the field samples, EPA stresses that an MS/MSD pair (or a spiked sample and a duplicate sample) should be prepared from additional volumes of the material collected from the site in question. Each MS/MSD will require that additional sample volume from the site be provided to the laboratory by the field sampling personnel. EPA further recommends that data users should be routinely provided with the MS/MSD results from only those QC samples associated with the field samples from the same site.