Instrument bias range

Hence, by applying a bias emf corresponding to 2.3026RT/F pH, or by offsetting the mechanical zero of the potentiometer by pH units, the zero shift is compensated for any temperature within the range (about 20° C) over which pH is fairly constant. Modern instruments often include the possibility of varying the pH, bias (to allow the use of different electrode systems) by incorporating the isopotential bias into the meter circuits. 

It is not surprising that the data produced as total petroleum hydrocarbons (EPA 418.1) suffer from several shortcomings as an index of potential ground-water contamination or health risk. In fact, it does not actually measure the total petroleum hydrocarbons in the sample but rather, measures a specific range of hydrocarbon compounds. This is caused by limitations of the extraction process (solvents used and the concentration steps) and the reference standards used for instrumental analysis. The method specifically states that it does not accurately measure the lighter fractions of gasoline [benzene-toluene-ethylbenzene-xylenes fraction (BTEX)], which should include the benzene-toluene-ethylbenzene-xylenes fraction. Further, the method was originally a method for water samples that has been modified for solids, and it is subject to bias. 

Cost If the choice of method comes down to either Method A or Method B after having examined all facets of the work (detection limit, bias and accuracy, useful range, and capacity), the final decision may very well depend on cost. Cost includes all components of a budget (i.e., personnel, training, instrumentation, equipment, maintenance, supplies, overhead, and costs of using an outside laboratory for some or all of the work). All other factors being equal, if Method A is less expensive than Method B, then Method A should be chosen. 

LC/MS analysis was conducted on a Fisons VG Quattro II mass spectrometer with electrospray introduction capabilities (Fisons Instruments, Beverly, MA). A +3900 volt bias was placed on the discharge needle, the electron multiplier was held at -650 volts, and the source temperature was held 150°C. A mass range of either 600 to 1200 or 1200 to 2100 Daltons was monitored in all analyses. The following voltages were maintained on the ion optics HV Lens, -500 V cone, 50 V skimmer, 1.5 V RF lens, 200 V. Chromatographic conditions were identical to those described above. 

In principle, the Kramers-Kronig relations can be used to determine whether the impedance spectrum of a given system has been influenced by bias errors caused, for example, by instrumental artifacts or time-dependent phenomena. Although this information is critical to the analysis of impedance data, the Kramers-Kronig relations have not found widespread use in the analysis and interpretation of electrochemical impedance spectroscopy data due to difficulties with their application. The integral relations require data for frequencies ranging from zero to infinity, but the experimental frequency range is necessarily constrained by instrumental limitations or by noise attributable to the instability of the electrode. 

The analytical measurement range (measuring interval, reportable range) is the analyte concentration range over which the measurements are within the declared tolerances for imprecision and bias of the method. In practice, the upper limit is often set by the linearity limit of the instrument response and the lower limit corresponds to the lower limit of quantitation (LoQ—see below). Usually, it is presumed that the specifications of the method apply throughout the analytical measurement range. However, there may... 

---