Frequent Questions

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?"

Yes and no. Chapter One (PDF, 33 pp, 274K) 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 (PDF, 6 pp, 36K) , or GPC using Method 3640A (PDF, 6 pp, 34K).)  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., Method 5030B (PDF, 21 pp, 174K) and Method 8260B (PDF, 86 pp, 444K)), 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.
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