Collecting time

There are a number of observations to be drawn from the above fomuila the relative uncertainty can be reduced to an arbitrarily small value by increasing T, but because the relative uncertainty is proportional to /s/f, a reduction in relative uncertainty by a factor of two requires a factor of four increase in collection time. The relative uncertainty can also be reduced by reducing At. Flere, it is understood that At is the smallest time window that just includes all of the signal. At can be decreased by using the fastest possible detectors, preamplifiers and discriminators and minimizing time dispersion in the section of the experiment ahead of the detectors. 

Improved sensitivities can be attained by the use of longer collection times, more efficient mass transport or pulsed wavefomis to eliminate charging currents from the small faradic currents. Major problems with these methods are the toxicity of mercury, which makes the analysis less attractive from an eiivironmental point of view, and surface fouling, which coimnonly occurs during the analysis of a complex solution matrix. Several methods have been reported for the improvement of the pre-concentration step [17,18]. The latter is, in fact. 

Area Detectors. A two-dimensional or area detector attached to a powder diffractometer can gready decrease data collection time. Many diffraction appHcations require so much time with a conventional detector that they are only feasible if an area detector is attached to the iastmment. The Siemens General Area Detector Diffraction System (GADDS) uses a multiwire area detector (Fig. 17). This detector measures an x- and ajy-position for each x-ray photon detected. The appHcations foUow. 

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

Sample Handling System. Venous or capillary blood, urine, and cerebrospinal fluid are specimens routinely used in medical diagnostic testing. Of these biological fluids, the use of venous blood is by far the most prevalent. Collection devices such as syringes and partial vacuum test tubes, eg, Vacutainer, are used to draw ten milliliters or less of venous blood. At collection time, the test tubes are carefully labeled for later identification. 

Other reactor sources with instruments like D4C but with much lower flux, and hence longer data collection times, are the Laboratoire Leon Brillouin (LLB, Saclay, France), on the instrument 7C2, and the NFL (Studsvik, Sweden), on the instrument SLAD... 

The emphasis that the FQPA placed on the assessment of pesticide residues in drinking water, for example, led to the collection and analysis of data on the effects of drinking water treatment processes on pesticide residues. These data were presented to the FIFRA Science Advisory Board to highlight the variability in the effects of treatment on different kinds of pesticides and the products formed and the variability of treatment processes employed at different locations and at different collection time intervals at an individual location. These complexities led to the current proposal... 

Prior to use, each GPC column should be calibrated in order to establish the correct eluate collection time. In addition, the column should be equilibrated for at least 2h prior to use. 

Solvent system Flow rate Injection volume Collection time... 

FIGURE 5. PCP urinary excretion rate versus the midpoint of the urine collection time in three dogs... 

It must be assumed that urine collections were accurately timed and that complete urine specimens were obtained at each collection time. It is also assumed that the assay procedure is accurate and reproducible. 

Before collecting data, at least two lean/rich cycles of 15-min lean and 5-min rich were completed for the given reaction condition. These cycle times were chosen so as the effluent from all reactors reached steady state. After the initial lean/rich cycles were completed, IR spectra were collected continuously during the switch from fuel rich to fuel lean and then back again to fuel rich. The collection time in the fuel lean and fuel rich phases was maintained at 15 and 5 min, respectively. The catalyst was tested for SNS at all the different reaction conditions and the qualitative discussion of the results can be found in [75], Quantitative analysis of the data required the application of statistical methods to separate the effects of the six factors and their interactions from the inherent noise in the data. Table 11.5 presents the coefficient for all the normalized parameters which were statistically significant. It includes the estimated coefficients for the linear model, similar to Eqn (2), of how SNS is affected by the reaction conditions. 

Thermal drift. It is essential that the STM can acquire images rapidly (i.e. 10 seconds) since thermal drifts due to electrolyte cooling, expanding electrodes, etc. will degrade lateral resolution over longer collection times. 

Figure 3.22 Reflection X-ray dilTractogram from platinised Pt in 1 M H2S04 in the double layer region. Data collection time 3 x 104 seconds. From Fleischmann and Mao (1987).

MONKEY, Macaca fascicularis 1 or 10 mg/kg BW of trans-chlordane given once weekly for 5 weeks by subcutaneous injection. Adipose tissue, blood, and skin lipids analyzed for up to 20 weeks after the last injection trans-Chlordane and oxychlordane were detected in all tissues. In blood and adipose tissue, trans-chlordane decreased rapidly and oxychlordane increased gradually until a plateau was reached. Good correlations were determined for all chemicals between blood and adipose tissue, regardless of collection time and dose level, and between skin lipids and adipose tissue. At the high dose, trans-chlordane reached a maximum of 35 mg/kg FW in adipose tissue, but was not detectable after 20 weeks. The oxychlordane concentration in adipose tissue of the high-dose group was 25 mg/kg FW after the last injection, and 18 mg/kg FW after 20 weeks (Sasaki et al. 1992)... 

A second problem in whole molecule mass spectrometry is that fluctuations in ion current may introduce substantial errors. Recall that ions of different m/z are not measured simultaneously in whole molecule mass spectrometry. If the ion current is not stable (and it commonly fluctuates in El sources), then after first peak (say m/z = 112 in our example) is measured, and instrumental parameters are changed in order to focus the next peak (m/z = 114) on the collector, the ion current of this second peak may no longer correspond to that existing at the time the first peak was measured. One can try to switch the detector from peak to peak more rapidly but that shortens the collection time for each peak, fewer ions will be counted, and errors in counting statistics will increase. Normally this problem is dealt with by statistical... 

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