Second detector confirmation

Second detector confirmation is another compound confirmation technique. Two detectors with selectivity to different functional groups are connected in series to one column or in parallel to two columns. For example, two detectors, a UV/VIS detector and a fluorometer, connected in series to the HPLC column are used in the EPA Method 8310 for analysis of PAH compounds. If the second detector is connected to a second column of a dissimilar polarity, then the confirmation becomes even more reliable. An example of such a configuration is organophosphorus pesticides analysis the samples may be initially analyzed with an NPD, and then confirmed on a different column with an ECD or a FPD. 

The non-selective FID does not provide the second detector confirmation for BTEX, whereas the PID has only a marginal selectivity to aromatic compounds. In a typical analysis of gasoline in water by EPA Method 8021, benzene and MTBE results tend to be biased high due to coelution with other constituents false positive results are also common. 

Second column and second detector confirmation are not always necessary. If the source of contamination is known and there is certainty that the contaminants are, in fact, present at the site, confirmation of compound identification may not be necessary. For example, the confirmation of pesticides in the samples collected at a pesticide manufacturing facility is not needed as the source of contamination is known. Confirmation of pesticide concentrations in this case is still necessary. 

Petroleum fuel analysis does not require second column or second detector confirmation. Fuels are identified based on their fingerprints or characteristic patterns of multiple peaks similar to ones shown in Figure 2.5. Each peak represents an individual chemical constituent, and each fuel has a unique combination of these constituents forming a characteristic pattern or a fingerprint. The fingerprints obtained... 

Test methods that analyze individual compounds (e.g., benzene-toluene-ethylbenzene-xylene mixtures and PAHs) are generally applied to detect the presence of an additive or to provide concentration data needed to estimate environmental and health risks that are associated with individual compounds. Common constituent measurement techniques include gas chromatography with second-column confirmation, gas chromatography with multiple selective detectors, and gas chromatography with mass spectrometry detection (GC/MS) (EPA 8240). 

Individual compound identification in all GC methods with the exception of GC/MS relies on the compound retention time and the response from a selective or non-selective detector. There is always a degree of uncertainty in a compound s identity and quantity, particularly when non-selective detectors are used or when the sample matrix contains interfering chemicals. To reduce this uncertainty, confirmation with a second column or a second detector is necessary. Analyses conducted with universal detectors (mass spectrometer or diode array) do not require confirmation, as they provide highly reliable compound identification. 

Qualitative compound confirmation may be performed either with a second column or with a second detector. Second column confirmation technique consists of analyzing the sample on two columns with dissimilar polarities. Each column is calibrated with the same standards, and the same calibration acceptance criteria are applied. For the presence of a compound to be confirmed, its retention time values obtained from each column must fall into respective retention time windows. If a peak falls within the retention time window on one column, but not on the second column, the compound is not considered confirmed and should not be reported. 

Second column confirmation is an imperfect technique, prone to false positive detection, particularly if non-selective or low selectivity detectors are used. Even the most selective detectors may not be fully capable of correctly identifying target analytes in complex environmental matrices as illustrated in Example 4.12. 

The complexities of the second detector and second column confirmation are illustrated in Example 4.13. 

Example 4.13 Second detector/second column confirmation... 

Request that the laboratory confirm all chromatography results with a second column or a second detector, when required by the method. (Laboratories do not automatically do this.)... 

This detector responds to infrared emissions in at least two wavelengths. Typically a CO2 reference at 4.45 microns is established and a second reference channel that is away from the CO2 and H2O wavelengths is made. It requires that the two signals received are confirmed as are synchronous and that the ratio between both signals is correct. 

The use of two separate electrical or mechanical zones of detectors, both of which must be actuated before the confirmation of a fire or gas detection. For example, the detectors in one zone could all be placed on the north side of a protected area, and positioned to view the protected area looking south, while the detectors in the second zone would be located on the south side and positioned to view the northern area. Requiring both zones to be actuated reduces the probability of a false alarm activated by a false alarm source such as welding operations, from either the north or the south outside the protected area. However this method is not effective if the zone facing away from the source, sees the radiation. Another method of cross zoning is to have one set of detectors cover the area to be protected and another set located to face away from the protected area to intercept external sources of nuisance UV. If welding or lighting should occur outside the protected area, activation of the alarm for the protected area would be inhibited by second... 

Instrumentation for revealing the presence of bulk quantities of concealed drugs will differ from those developed to find evidence of minute quantities on surfaces. Bulk detection is concerned with amounts ranging from grams to kilograms [4], Bulk detection is done by manual inspection, X-ray, CT scans, and acoustic inspection. X-ray or CT scanners used as bulk detectors have sensitivity of 2-10 g, and suspect items are subsequently confirmed by chemical analysis. Hand-held acoustic inspection instruments such as the Acoustic Inspection Device (AID) and the Ultrasonic Pulse Echo (UPE) developed by Pacific Northwest National Laboratories/Battelle, can be used for analysis of cargo liquids in sealed containers of various sizes within seconds [5]. The acoustical velocity and attenuation of multiple echoes returned to the instrument is evaluated by software which compares the data to the shipping manifest. 

Radicals are generated at a tubular electrode and are then transported by laminar flow into the ESR cavity which, as a downstream detector, is analogous to a second electrode. The theoretical response for the cases where the radicals are stable or decompose by first- or second-order kinetics has been derived and experimentally confirmed [126, 301, 302]. The flow-rate dependence is different for each of the three situations which provides a diagnostic for the type of kinetics. Further information may be obtained from galvanostatic transients which allow the elucidation of electrode and radical surface processes [303]. Very recently, an in situ channel tube electrode has been described for electrochemical ESR which also allows shorter-lived species to be observed and smaller surface coverages to be analysed [304—306]. 

Confirmation with a second column of a dissimilar polarity and a second, selective detector is necessary for the correct identification and quantitation of BTEX and oxygenated additives. And yet, even when a dissimilar column and a second PID are used in confirmation analysis, false positive detection of MTBE often takes place. 

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