Spikes and spiking

If the sample matrix is complex, it may be necessary to determine if there are any interference effects from the matrix, on the analyte response. This is usually done by spiking the sample with a known amount of analyte. Two equal portions of sample are taken and an appropriate quantity of analyte is added to one to effectively double the absorbance. A similar quantity of analyte is added to water to make a spike-alone solution. Readings are taken for sample, sample-plus-spike and spike-alone solutions and the amount of interference calculated as a percentage enhancement or... 

Table 4 Bias Corrected Isotope Ratio Measurements in the Unspiked Sample, Isotopically Enriched Spike, and Spiked Sample for Three Oil Leachate Samples as Determined by Glow Discharge Mass Spectroscopy Solution Residue Method...

RECOVERIES OF ORGANOTIN COMPOUNDS FROM 500 MG OF NON-SPIKED AND SPIKED MUSSELS. RESULTS ARE THE AVERAGE OF FIVE DIFFERENT EXPERIMENTS [8]... 

Table 55 Recoveries of organotin compounds from 500 mg of non-spiked and spiked mussels"...

Present in citronella and valerian oils, tur penline, ginger, rosemary and spike oils. It is produced artificially by the elimination of hydrogen chloride from bornyl chloride (artifi cial camphor) or from isobornyl chloride, by the dehydrogenation of borneol and isobor-neol and by the action of elhanoic anhydride on bornylamine. Chiral. 

Balk M W and Fleming G R 1985 Dependence of the coherence spike on the material dephasIng time In pump-probe experiments J. Chem. Phys. 83 4300-7... 

Figure Cl.5.2. Fluorescence excitation spectra (cps = counts per second) of pentacene in /i-teriDhenyl at 1.5 K. (A) Broad scan of the inhomogeneously broadened electronic origin. The spikes are repeatable features each due to a different single molecule. The laser detuning is relative to the line centre at 592.321 nm. (B) Expansion of a 2 GHz region of this scan showing several single molecules. (C) Low-power scan of a single molecule at 592.407 nm showing the lifetime-limited width of 7.8 MHz and a Lorentzian fit. Reprinted with pennission from Moemer [198]. Copyright 1994 American Association for the Advancement of Science.

Figure C3.3.12. The energy-transfer-probability-distribution function P(E, E ) (see figure C3.3.2 and figure C3.3.11) for two molecules, pyrazine and hexafluorobenzene, excited at 248 nm, arising from collisions with carbon dioxide molecules. Both collisions that leave the carbon dioxide bath molecule in its ground vibrationless state, OO O, and those that excite the 00 1 vibrational state (2349 cm ), have been included in computing this probability. The spikes in the distribution arise from excitation of the carbon dioxide bath 00 1 vibrational mode.

Going back to our case and recalling that x(

conjugate functions, namely, iTn((p) where nr((p) = V T 2 + Tjj + T 3- In Figure 13a and b we present tn(conical intersections and they occur at points where the circles cross their axis line. 

Here, I(co) is the Fourier transform of the above C(t) and AEq f is the adiabatic electronic energy difference (i.e., the energy difference between the v = 0 level in the final electronic state and the v = 0 level in the initial electronic state) for the electronic transition of interest. The above C(t) clearly contains Franck-Condon factors as well as time dependence exp(icOfvjvt + iAEi ft/h) that produces 5-function spikes at each electronic-vibrational transition frequency and rotational time dependence contained in the time correlation function quantity <5ir Eg ii,f(Re) Eg ii,f(Re,t)... 

Daskalakis and co-workers recently evaluated several procedures for digesting the tissues of oysters and mussels prior to analyzing the samples for silver. One of the methods used to evaluate the procedure is a spike recovery in which a known amount of silver is added... 

A third spectrophotometric method for the quantitative determination of the concentration of in blood yields an Sjamp of 0.193 for a 1.00-mL sample of blood that has been diluted to 5.00 mb. A second 1.00-mL sample is spiked with 1.00 )J,L of a 1560-ppb Pb + standard and diluted to 5.00 mb, yielding an Sspike of 0.419. Determine the concentration of Pb + in the original sample of blood. 

A fifth spectrophotometric method for the quantitative determination of the concentration of Pb + in blood uses a multiple-point standard addition based on equation 5.6. The original blood sample has a volume of 1.00 mb, and the standard used for spiking the sample has a concentration of 1560 ppb Pb +. All samples were diluted to 5.00 mb before measuring the signal. A calibration curve of Sjpike versus Vj is described by... 

Since a standard additions calibration curve is constructed in the sample, it cannot be extended to the analysis of another sample. Each sample, therefore, requires its own standard additions calibration curve. This is a serious drawback to the routine application of the method of standard additions, particularly in laboratories that must handle many samples or that require a quick turnaround time. For example, suppose you need to analyze ten samples using a three-point calibration curve. For a normal calibration curve using external standards, only 13 solutions need to be analyzed (3 standards and 10 samples). Using the method of standard additions, however, requires the analysis of 30 solutions, since each of the 10 samples must be analyzed three times (once before spiking and two times after adding successive spikes). 

A sixth spectrophotometric method for the quantitative determination of Pb + levels in blood uses CQ+ as an internal standard. A standard containing 1.75 ppb Pb + and 2.25 ppb CQ+ yields a ratio of Sa/Sis of 2.37. A sample of blood is spiked with the same concentration of Cu +, giving a signal ratio of 1.80. Determine the concentration of Pb + in the sample of blood. 

To determine the concentration of analyte in a sample, a standard additions was performed. A 5.00-mL portion of the sample was analyzed and then successive 0.10-mL spikes of a 600.0-ppb standard of the analyte... 

Ion-selective electrodes can be incorporated in flow cells to monitor the concentration of an analyte in standards and samples that are pumped through the flow cell. As the analyte passes through the cell, a potential spike is recorded instead of a steady-state potential. The concentration of K+ in serum has been determined in this fashion, using standards prepared in a matrix of 0.014 M NaCl. ... 

With conventional nonspectroscopic detectors, other methods must be used to identify the solutes. One approach is to spike the sample by adding an aliquot of a suspected analyte and looking for an increase in peak height. Retention times also can be compared with values measured for standards, provided that the operating conditions are identical. Because of the difficulty of exactly matching such conditions, tables of retention times are of limited utility. 

Caffeine in coffee, tea, and soda is determined by a solid-phase microextraction using an uncoated silica fiber, followed by a GC analysis using a capillary SPB-5 column with an MS detector. Standard solutions are spiked with G3 caffeine as an internal standard. 

The most useful methods for quality assessment are those that are coordinated by the laboratory and that provide the analyst with immediate feedback about the system s state of statistical control. Internal methods of quality assessment included in this section are the analysis of duplicate samples, the analysis of blanks, the analysis of standard samples, and spike recoveries. 

Spike Recoveries One of the most important quality assessment tools is the recovery of a known addition, or spike, of analyte to a method blank, field blank, or sample. To determine a spike recovery, the blank or sample is split into two portions, and a known amount of a standard solution of the analyte is added to one portion. The concentration of the analyte is determined for both the spiked, F, and unspiked portions, I, and the percent recovery, %R, is calculated as... 

A spike recovery for the analysis of chloride in well water was performed by adding 5.00 mb of a 25,000-ppm solution of Ck to a 500-mL volumetric flask and diluting to volume with the sample. Analysis of the sample and the spiked sample resulted in chloride concentrations of 183 ppm and 409 ppm, respectively. Determine the percent recovery of the spike. 

Spike recoveries on method blanks and field blanks are used to evaluate the general performance of an analytical procedure. The concentration of analyte added to the blank should be between 5 and 50 times the method s detection limit. Systematic errors occurring during sampling and transport will result in an unacceptable recovery for the field blank, but not for the method blank. Systematic errors occurring in the laboratory, however, will affect the recoveries for both the field and method blanks. 

Spike recoveries for samples are used to detect systematic errors due to the sample matrix or the stability of the sample after its collection. Ideally, samples should be spiked in the field at a concentration between 1 and 10 times the expected concentration of the analyte or 5 to 50 times the method s detection limit, whichever is larger. If the recovery for a field spike is unacceptable, then a sample is spiked in the laboratory and analyzed immediately. If the recovery for the laboratory spike is acceptable, then the poor recovery for the field spike may be due to the sample s deterioration during storage. When the recovery for the laboratory spike also is unacceptable, the most probable cause is a matrix-dependent relationship between the analytical signal and the concentration of the analyte. In this case the samples should be analyzed by the method of standard additions. Typical limits for acceptable spike recoveries for the analysis of waters and wastewaters are shown in Table 15.1. ... 

With a prescriptive approach to quality assessment, duplicate samples, blanks, standards, and spike recoveries are measured following a specific protocol. The result for each analysis is then compared with a single predetermined limit. If this limit is exceeded, an appropriate corrective action is taken. Prescriptive approaches to quality assurance are common for programs and laboratories subject to federal regulation. For example, the Food and Drug Administration (FDA) specifies quality assurance practices that must be followed by laboratories analyzing products regulated by the FDA. 

A good example of a prescriptive approach to quality assessment is the protocol outlined in Figure 15.2, published by the Environmental Protection Agency (EPA) for laboratories involved in monitoring studies of water and wastewater. Independent samples A and B are collected simultaneously at the sample site. Sample A is split into two equal-volume samples, and labeled Ai and A2. Sample B is also split into two equal-volume samples, one of which, Bsf, is spiked with a known amount of analyte. A field blank. Dp, also is spiked with the same amount of analyte. All five samples (Ai, A2, B, Bsf, and Dp) are preserved if necessary and transported to the laboratory for analysis. 

The first sample to be analyzed is the field blank. If its spike recovery is unacceptable, indicating that a systematic error is present, then a laboratory method blank. Dp, is prepared and analyzed. If the spike recovery for the method blank is also unsatisfactory, then the systematic error originated in the laboratory. An acceptable spike recovery for the method blank, however, indicates that the systematic error occurred in the field or during transport to the laboratory. Systematic errors in the laboratory can be corrected, and the analysis continued. Any systematic errors occurring in the field, however, cast uncertainty on the quality of the samples, making it necessary to collect new samples. 

If the spike recovery for Bsf is acceptable, or if the result for sample B is below the method s detection limit or outside the range of 0.1 to 10 times the amount of analyte spiked in Bsf, then the duplicate samples Ai and A2 are analyzed. The results for Ai and A2 are discarded if the difference between their values is excessive. If the difference between the results for Ai and A2 is within the accepted limits, then the results for samples Ai and B are compared. Since samples collected from the same sampling site at the same time should be identical in composition, the results are discarded if the difference between their values is unsatisfactory, and accepted if the difference is satisfactory. 

In a performance-based approach to quality assurance, a laboratory is free to use its experience to determine the best way to gather and monitor quality assessment data. The quality assessment methods remain the same (duplicate samples, blanks, standards, and spike recoveries) since they provide the necessary information about precision and bias. What the laboratory can control, however, is the frequency with which quality assessment samples are analyzed, and the conditions indicating when an analytical system is no longer in a state of statistical control. Furthermore, a performance-based approach to quality assessment allows a laboratory to determine if an analytical system is in danger of drifting out of statistical control. Corrective measures are then taken before further problems develop. 

Using Control Charts for Quality Assurance Control charts play an important role in a performance-based program of quality assurance because they provide an easily interpreted picture of the statistical state of an analytical system. Quality assessment samples such as blanks, standards, and spike recoveries can be monitored with property control charts. A precision control chart can be used to monitor duplicate samples. 

Another important quality assessment tool, which provides an ongoing evaluation of an analysis, is a control chart. A control chart plots a property, such as a spike recovery, as a function of time. Results exceeding warning and control limits, or unusual patterns of data indicate that an analysis is no longer under statistical control. 

The use of several QA/QC methods is described in this article, including control charts for monitoring the concentration of solutions of thiosulfate that have been prepared and stored with and without proper preservation the use of method blanks and standard samples to determine the presence of determinate error and to establish single-operator characteristics and the use of spiked samples and recoveries to identify the presence of determinate errors associated with collecting and analyzing samples. 

Analysis of the sample before and after the spike gave signals of0.456 for B and f.03 for sample Bsl- Considering these data, what is the most likely source of the systematic error ... 

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