Sample loop, calibration

Sample loop calibrators are commonly used for low concentration analytes that are difficult to prepare or are unstable in gas bottles. They are also extremely flexible, in that new compounds can be calibrated upon acquisition of a sample of the pure compound in liquid form. Sample loops are basically small heated known-volume vessels into which a small amount of liquid standard is injecSecL The sample vaporizes, and the entire volume is then pumped past the inlet, into which a small amount of the vapor is drawn for calibration. Drawbacks of sample loops include the lack of automation for unattended calibration, and inaccuracies inherent in making manual injections of nanoliter-scale liquid volumes. For ambient air monitoring of toxic compounds, one should bear in mind the safety imphcations of handling syringes that contain potentially hazardous analytes. 

Samples from the 40-mL VOA vials were split for chromate and PCE analyses and typically analyzed within 48 hours of collection. Chromate concentration was determined via an HPLC method using a Gilson Model 116 UV detector set at 365 nm and a 2- by 150-mm Waters Nova-Pak C18 60A HPLC column packed with 4-pm particles. The mobile phase consisted of 5-mM tert-butylammonium hydrogen sulfate buffered to pH 4.4 with NaOH with 10% acetonitrile (v/v) as a modifier. The eluent flow rate was 0.8 mL min-i Samples were filtered through a0.45-pm filter as they were injected by an Alcott 708 autosampler with a 0.1-mL sample loop. The typical run time was 4 min with a calibration range of 0.05 to 20 mg L 1 (0.001 - 0.38mmol L 1) Cr as chromate. 

A dry packed column with porous material was used for the characterization according to size of the PVAc latex samples. The packing employed was CPG (Controlled Pore Glass), 2000 A, 200-400 mesh size. Deionized water with 0.8 gr/lit Aerosol O.T. (dioctyl sodium sulphosuccinate), 0.8 gr/lit sodium nitrate and 0.4 gr/lit sodium azide served as the carrier fluid under a constant flowrate. The sample loop volume was 10 pC A Beckman UV detector operating at 254 nm was connected at the column outlet to monitor particle size. A particle size-mean retention volume calibration curve was constructed from commercially available polystyrene standards. For reasons of comparison, the samples previously characterized by turbidity spectra were also characterized by SEC. A number of injections were repeated to check for the reproducibility of the method. 

Some valve systems incorporate a calibrated sample loop that is filled with test solution for transfer to the column in the mobile phase. In other systems, test solution is transferred to a cavity by syringe and then switched into the mobile phase. 

The calibration method most often used in ion chromatography is direct comparison of the peak area in an unknown sample with that of a solution with a known content of the same substance. This method requires the injection of constant volumes under constant chromatographic conditions. Errors in the sample delivery, however, are almost excluded upon application of a sample loop valve. A prerequisite is the existence of reference compounds for all sample components to be analyzed. In practice, several different standard solutions in the investigated concentration range are prepared and chromatographed [8], When the resulting peak area is plotted versus the concentration of the standards, one obtains a substance-specific calibration function. 

The integrated UV detector signal produced by each of the aromatic hydrocarbons was determined to be proportional to its concentration. Individual response factors were found for each compound by first replacing the extractor column with a calibrated sample loop and then injecting acetonitrile solutions with known concentrations of the individual hydrocarbons. Details concerning the loop calibration technique and preparation of the acetonitrile solutions are given in Appendices A-3 and A-4. 

The accuracy of the liquid chromatographic analysis of the generated solutions is limited by the uncertainties involved with the calibration of the sample loop, with the preparation of standard acetonitrile solutions of the PAHs, and with the volumetric measurement of the amount of saturated solution sampled for a given analysis. The random errors associated with each of these processes have been estimated to be less than 1.2%, 0.1%, and 1.0%, respectively. A detailed explanation of how each of these estimates was made is presented elsewhere (39). Quadratic addition of these random errors yields a minimum uncertainty of 1.6% for the quantitative analysis of the generated saturated solutions and hence a potential accuracy of greater than 98% for the method. 

A-3. Calibration of the Sample Loop. The volume of the sample loop was determined by an indirect gravimetric procedure. The loop was initially filled with mercury. The mercury was swept from the loop into a tarred weighing dish with approximately 1 mL of pentane. Since mercury and pentane are not miscible, the bulk of the pentane was decanted and the remainder was allowed to evaporate. This process was repeated four times, yielding the following results ... 

C to hydrogenate the preadsorbed CO. After 30 minutes of reaction time, the entire contents of the reactor are flushed directly into a Varian 3400 GC and the concentration of CH4 is analyzed with a flame ionization detector. No higher hydrocarbons or oxygenated hydrocarbons were detected. The amount of methane was quantified by comparing the integrated area of the methane in the reactor gas to Ae integrated areas of methane samples admitted fi om a calibrated volume sample loop. 

Plots of phase angle difference in the interferometer arms vs. time are related to heat-production vs. time, and this in turn is related to the concentration of the species responsible for heat production. Typical instrument output for the urea/urease system is shown in Figure 3. Calibration curves can be constructed as shown in Figure The system is quite stable, and reasonably sensitive. Minimum detectable levels of urea are 5 mM, compared to the 0.1-5 mM limits for traditional detectors. Over extended time periods (7 days) the relative standard deviation at 5 mM concentrations is better than 5 /.. The optimum FIA conditions were around 1.0 ml/min flow rate, with a sample loop of 0.1-0.25 ml. 

The peroXide/cata1ase system was selected for study because of the difficulties found with the fiber optic sensor described above. The FIA carrier contained Triton-X as a surface active agent to reduce bubble formation. Typical FIA flow characteristics were ml/min, and a sample loop of 80 uL. Typical sensor output is shown in Figure 7. The calibration curve developed for the perox-ide/catalase system is linear from O.OO M to 1.OM (Figure 8). The error bars shown in the calibration curve were calculated by multiplying the standard deviation by the 95 /. confidence limit for a set of data with four degrees of freedom. The correlation coefficient of the line is... 

Quality control laboratories may be equipped with instruments which indicate MW or MWD directly. The most common technique used for this purpose is gel permeation chromatography (GPC). Infrequent analysis is the rule. Such off-line measurements are most often used to update inferential models, or to effect open loop control of the polymerization by manual process adjustments based on GPC results. On-line GPC is available its application is not yet common in industrial practice, but applications of on-line measurement of MWD by GPC have been reported [26]. The difficulties encountered with on-line GPC are the maintenance, sampling and calibration problems associated with any process chromatography application. In addition, a compromise must be made between resolution of the MWD and time of analysis. As a rule of thumb, it is possible to determine an accurate average molecular weight in under 10 minutes. Determination of the MWD can take considerably longer. 

A calibration curve must be recorded for each component of interest. The peak areas/heights of the sample chromatogram are then compared with those in the calibration curve. All chromatographic conditions must remain absolutely constant, for both sample and reference substances. If the volume of injected sample and injected reference solution is constant (which is usually the case if a sample loop is applied), the calculation can be carried out as follows ... 

Fig. 8-2. Schematic illustration of the coulometric Ct determination setup. The different parts are (1) seawater dispensing system, (2) acid adder, (3) extraction tower, (4) sample gas purifier, (5) coul-ometer and (6) gas loop calibration system.

To calibrate the coulometric system, including the response factor of the coulometer, a gas loop calibration system can be used. In principle it consists of a chromatography valve with sample loops of different sizes that can be filled with a calibration gas of high purity (> 99.99 % pure CO2). Prior to injection of the caUbration gas into the system, the temperature and pressure have to be measured accurately, as they have a significant impact on the amount of CO2 in the gas loop. 

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