Methodology and Quality Assurance/Quality Control of Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 119

extraction of the analyte as a result of an improper procedure. Two types of
''spiking'' QC samples are mainly used for the QA/QC purpose to assess sample
preparation procedures.

Spiked sample/matrix spike: A small quantity of a known concentration of
analyte stock solution is added (spiked or fortified) to the sample (or aliquot
of the sample) before the sample preparation. This sample then undergoes
digestion or extraction and subject to analysis. The concentration of stock
solution should be high enough so that only a small quantity of the stock
solution is needed and the volume change of the sample becomes negligible.
A general guideline should be followed regarding how much spiking
chemical should be added. In general, the spike addition should produce a
minimum level of 10 times and a maximum level of 100 times of the
instrument detection limits (IDL). The concentration of each analyte in the
spiking solution should be approximately 3-5 times the level expected in the
sample (Csuros, 1994). A practical approach is to spike the concentration to
reach near the midrange calibration point. Results of spiked samples are
evaluated based on the percentage recovery for the analytical accuracy. Refer
to Eq. 2.5 in Chapter 2 and the example below for the calculation of
percentage recovery. The volume of spike stock solution that should be
added to the sample can be calculated according to:

C 1 V 1 ¼ C 2 V 2 ð5 : 1Þ

where C 1 and C 2 are the concentration of the spike stock solution and the
desired spike concentration, respectively. V 2 ¼volume of the sample to be
spiked, and V 1 is the volume of stock solution to be calculated.
Surrogate spikes: Surrogates are organic compounds (nontargeted com-
pound) that are similar to analytes of interest in chemical composition,
extraction, and chromatography, but that are not normally found in the
environment. Surrogate spikes are used to trace organic determination
methods such as GC, GC/MS, and HPLC. These compounds are spiked
into all blanks, standards, samples, and spiked samples prior to purging or
extraction according to the appropriate methods. The analyst should make
sure that surrogate standard should not interfere with target analytes.
Surrogate samples are used to assess retention times, percentage recovery
(accuracy), and method performance.

5.4.2 Quality Control Procedures During Analysis

Types of Laboratory QA/QC Samples

In principle, there are mainly three types of QC samples used for the QA/QC
purpose during laboratory analysis. They are: (a) blanks used to assess any potential
contamination; (b) spikes used to obtain the percentage recovery and therefore
the accuracy; and (c) replicates (duplicates) used to determine the analytical