Data generated from quality-control (QC) samples are a requisite for evaluating the quality of the sampling (environmental samples) and processing techniques, as well as data from the actual samples themselves. Without QC data, environmental-sample data can not be adequately interpreted because the errors associated with the sample data are unknown. Some of the various types of QC samples collected by the USGS Illinois Water Science Center are described in the following section. Procedures have been established for the storage of water-quality-control data within the USGS. These procedures allow for storage of all derived QC data and are identified so that they can be related to corresponding environmental samples.  These data are not presented in this report but are available from the Science Center.

BLANK SAMPLES--Blank samples are collected and analyzed to ensue that environmental samples have not been contaminated during the data-collection process. The blank solution used to develop specific types of blank samples is a solution that is free of the analytes of interest. Any measured-value signal in a blank sample for an analyte (a specific component measured in a chemical analysis) that was absent in the blank solution is believed to be due to contamination.  Many types of blank samples are possible, each designed to segregate a different part of the data-collection process. The types of blank samples collected by the USGS Illinois Water Science Center are:

Source solution blank - a blank solution that is transferred to a sample bottle in an area of the office laboratory in a clean and protected atmosphere with respect to target analytes.

Ambient blank - a blank solution that is put in the same type of bottle used for an environmental sample, kept with the set of sample bottles before sample collection, and opened at the site and exposed to the ambient conditions.

Field blank - a blank solution that is subjected to all aspects of sample collection, field-processing preservation, transportation, and laboratory handling as an environmental sample.

Trip blank - a blank solution that is put in the same type of bottle used for an environmental sample and kept with the set of sample bottles before and after sample collection.

Equipment blank - a blank solution that is processed through some or all equipment used for collecting and processing an environmental sample (similar to a field blank but normally done in the more controlled conditions of the office than the field).

Sampler blank - a blank solution that is poured or pumped through the same field sampler used for collecting an environmental sample.

Filter blank - a blank solution that is filtered in the same manner and through the same filter apparatus used for an environmental sample.

Splitter blank - a blank solution that is mixed and separated using a field splitter in the same manner and through the same apparatus used for an environmental sample.

Preservation blank - a blank solution that is treated with the sampler preservatives used for an environmental sample.

REFERENCE SAMPLES--Reference material is a solution or material prepared by a laboratory. The reference material composition is certified for one or more properties so that it can be used to assess a measurement method. Samples of reference material are submitted for analysis to ensure that an analytical method is accurate for the known properties of the reference material. Generally, the selected reference material properties are similar to the environmental-sample properties.

REPLICATE SAMPLES--Replicate samples are a set of environmental samples collected in a manner such that the samples are thought to be essentially identical in composition. Replicate is the general case for which a duplicate is the special case consisting of two samples. Replicate samples are collected and analyzed to establish the amount of variability in the data contributed by some part of the collection and analytical process. Many types of replicate samples are possible, each of which may yield slightly different results in a dynamic hydrologic setting, such as a flowing stream. The types of replicate samples collected in the USGS Illinois Water Science Center are:

Concurrent sample - a type of replicate sample in which the samples are collected simultaneously with two or more samplers or by using one sampler and alternating collection of samples into two or more compositing containers. 

Sequential sample - a type of replicate sample in which the samples are collected one after the other, typically over a short time.

Split sample - a type of replicate sample in which a sample is split into subsamples; each subsample is contemporaneous in time and space.

SPIKE SAMPLES--Spike samples are samples to which known quantities of a solution with one or more well-established analyte concentrations have been added. These samples are analyzed to determine the extent of matrix interference or degradation on the analyte concentration during sample processing and analysis.