Blanks contamination control

In addition, all Phase I TIE tests should include system blanks or controls to detect toxic artifacts added during the effluent characterization manipulations. Common sources of toxicity artifacts include i) excessive ionic strength resulting from addition of acid/base during pH adjustments, ii) contaminated reagents,... 

The major quality parameters to be addressed during sample preparation are listed in Table 1.4. These are accuracy, precision, extraction efficiency (or recovery), and contamination control. These quality issues also need to be addressed during the analysis that follows sample preparation. Accuracy is determined by the analysis of evaluation samples. Samples of known concentrations are analyzed to demonstrate that quantitative results are close to the true value. The precision is measured by running replicates. When many samples are to be analyzed, the precision needs to be checked periodically to ensure the stability of the process. Contamination is a serious issue, especially in trace measurements such as environmental analysis. The running of various blanks ensures that contamination has not occurred at any step, or that if it has, where it occurred. As mentioned before, the detection limits, sensitivity, and other important parameters depend on the recovery. The efficiency of sample preparation steps such as extraction and cleanup must be checked to ensure that the analytes are being recovered from the sample. 

Other types of blanks may be employed as the situation demands. It should be noted that blanks are effective only in identifying contamination. They do not account for various errors that might exist. Blanks are seldom used to correct for contamination. More often, a blank above a predetermined value is used to reject analytical data, making reanalysis and even resampling necessary. The laboratory SOPs should identify the blanks necessary for contamination control. 

Trichloro-compounds—Fujiwara test. To 1 ml of urine add 1 ml of sodium hydroxide solution and 1 ml of pyridine, and heat in a boiling water-bath for 2 minutes. The development of a red colour in the pyridine layer indicates ingestion of a trichloro-compound. A blank urine sample and an authentic solution of trichloroacetic acid should be tested at tile same time, both blank and control solutions being treated in similar fashion to the sample, because contamination of the atmosphere with laboratory reagents may give positive results. 

Negative control DNA samples should be included for every amplicon being analyzed and positive control DNA samples should be included when available (see Note 5). A negative control is a sample with no sequence change in the amplicon and a positive control is a sample with a known sequence change in the amplicon being analyzed. A blank (H2O) control should also be included to check for PCR contamination. 

Preparing and running laboratory blanks, laboratory control samples, and field and trip blanks. These blanks serve to assess whether or not samples may have become contaminated during sampling and sample transport. 

Colorimetric methods for the determination of DIP, nitrate, nitrite, ammonium, and silicate are well established, with both manual and automatic procedures well understood and able to provide adequate sensitivity for most purposes. These methods can readily be used at sea and in the future probably in situ. International intercalibration exercises have shown that many laboratories now have the analytical expertise to measure nutrient concentrations at ambient levels, but this is not true of all laboratories and the analysis still requires careful analytical procedures that recognize the importance of contamination control, blank correction, and the complications arising from the saltwater matrix. Although there have been a number of such intercalibration studies, there is no widely available standard reference material for nutrient analysis at present. 

The types and amount of quality control used during the field component of a study can vary depending on the data requirements of the stu. At a minimum, field blanks should be used to identify any contamination either through direct contact or airborne exposure of the sample. Other quality control samples to be considered include equipment blanks, if the same sampling equipment is repetitively used, trip blanks (contaminant-free water samples which accompany the field collected samples from the field to the laboratory but are not exposed to the air), and positive control... 

The analytical uncertainty should be reduced to one-third or less of sampling uncertainty (16). Poor results obtained because of reagent contamination, operator errors ia procedure or data handling, biased methods, and so on, can be controlled by proper use of blanks, standards, and reference samples. 

In another example, urine samples were extracted with MIP phases imprinted with clenbuterol in order to determine the concentration of this (3-agonist, which is known to be misused in animal breeding and thus is occasionally found as a food contaminant. Recovery rates of up to 75% were observed for spiked samples when extracting the clenbuterol. However, in subsequent control experiments clenbuterol was detected also in non-spiked blank urine samples, and further experiments lead to the conclusion that the clenbuterol used as template permanently bled from the Mi-polymer. Consequently, the authors decided to use in the future a structural analogue as template instead of clenbuterol in order to avoid this problem [44]. 

Quality Assurance/Quality Control. QA/QC measures included field blanks, solvent blanks, method blanks, matrix spikes, and surrogates. Percent recovery was determined using three surrogate compounds (nitrobenzene-d5, 2-fluorobiphenyl, d-terphenyl-diQ and matrix spikes (naphthalene, pyrene, benzo[ghi]perylene) the recoveries ranged from 80 to 102%. Separate calibration models were built for each of the 16 PAHs using internal standards (naphthalene-dg, phenanthrene-dio, perylene-di2). Validation was performed using a contaminated river sediment (SRM 1944) obtained from NIST (Gaithersburg, MD) accuracy was <20% for each of the 16 analytes. 

First Control Run. A large number (7 to 15) of sets of standards and blanks are run and the results tabulated, as in the trial runs. These data are then plotted (responses vs. concentration for all data points, on one graph) and the means, standard deviations, RSDs, the slope, y-intercept, and correlation coefficient are determined. The smaller the value of the y-intercept, the better (the less chance for a contamination or interference problem). The closer the slope is to 1, the better (the more sensitive). At higher concentrations, the standard deviation should get larger, and the RSDs should get smaller (while approaching some limit). If the RSDs are between 30% and 100%, a close approach to the detection limit is indicated. 

Next let us consider those difficulties associated with the determination of the amount of material deposited on the surface. We have already noted that the method of depositing insoluble monolayers by spreading permits the accurate determination of n. Since the spreading technique requires solvent volatility, care must be exercised to prevent the stock solutions from changing concentration due to evaporation prior to their application to the surface. Also, precise microvolumetric methods must be used to dispense the solution on the aqueous surface since the quantity used is small. The solvent (as well as the solute) must be free from contaminants. There is also the possibility that the solvent will extract spreadable contaminants from the waxed surfaces of the float, barriers, and tray. Some workers advocate addition and evaporation of one drop at a time to minimize this. Oily contaminants may also reach the water surface from the fingers and from the atmosphere. These last sources are particularly hard to control Tests for reproducibility and blank compressions (i.e., moving the barrier toward the float on a clean surface) are the best evidence of their absence. 

Analysis by GC-MS of the contaminating peaks in eluates from a blank control column revealed ethyl hexadecanoate, methylbutyl phthalate, stearic acid, methyl 4-methyldodecanoate, dioctyl phthalate (two peaks), and an alkyl benzene. An eighth peak corresponded to an isomer of geranyl citronellal [(H3C)2C=CHCH2CH2C(CH3)= CHCH2CH2C(CH3)=CHCH2CH2C(CH3)CH2CHO]. The A-162 eluate showed bis(2-ethylhexyl) phthalate. 

The purpose of trip blanks is to assess the collected sample representativeness by determining whether contaminants have been introduced into the samples while they were handled in the field and in transit, i.e. in coolers with ice transported from the site to the analytical laboratory. A possible mechanism of such contamination is the ability of some volatile compounds, such as methylene chloride or chlorofluor-ocarbons (Freons), to penetrate the PTFE-lined septum and dissolve in water. Potential sources of this type of contamination are either ambient volatile contaminants or the VOCs that could be emanating from the samples themselves, causing sample cross-contamination. To eliminate ambient contamination, samples must not be exposed to atmospheres containing organic vapors. Cross-contamination is best controlled by such QA measures as sample segregation and proper packaging. 

A preparation batch is a group of up to 20 field samples, prepared together for the same analysis using the same lots of reagents and associated with common QC samples. In addition to field samples, a preparation batch must, at a minimum, include a method (extraction or digestion) blank, an LCS, and an LCSD. Other laboratory QC checks may be part of the preparation batch, such as an MS/MSD pair or a laboratory duplicate. If laboratory QC checks in a preparation batch meet the laboratory acceptance criteria, the batch is considered be in a state of control and every sample in it is acceptable, provided that individual QC checks are also acceptable. If the method blank and the samples in a preparation batch show contamination that makes sample results inconclusive or if the LCS and LCSD recoveries are not acceptable, the whole batch may be prepared again. 

Each sample batch contains replicates of two sets of standard calibrators which are placed at the beginning and end of the batch to bracket study samples and QC samples. The total numbers of QC samples from all levels should be >5 % of the number of unknown samples in the run and at a minimum of N= 2 at each level or N= 3 for Dilution QC. Minimum three system suitability test samples (SST) should be included in the beginning of the batch (1) SST-LLOQ sample to evaluate the signal to noise ratio of the instrument on the day of the analysis. (2) SST-QCO sample (a blank sample is fortified with IS) to evaluate any potential contamination of IS solution. (3) SST-Control blank (a double blank sample) to evaluate any interference or contamination of the blank matrix lot. 

When using a pneumatic nebulizer, an unheated spray chamber, and a quadrupole mass spectrometer, ICP-MS detection limits are 1 part per trillion or less for 40 to 60 elements (Table 3.4) in clean solutions. Detection limits in the parts per quadrillion range can be obtained for many elements with higher-efficiency sample introduction systems and/or a magnetic sector mass spectrometer used in low-resolution mode. Blank levels, spectral overlaps, and control of sample contamination during preparation, storage, and analysis often prohibit attainment of the ultimate detection limits. 

Method blank—an analytical control consisting of all reagents, internal standards and surrogate standards, which is carried through the entire analytical procedure. The method blank is used to define the level of laboratory background and reagent contamination. 

The organization of the analytical process is another point. A systematic contamination is unveiled when analyzing the procedural blanks. Because of the systematic nature of the procedure, one can reasonably assume that, given an equal treatment of all the samples, the same error occurs for every sample. In this way, the error can be brought under control and corrected. 

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