Autosampler large-volume

Measurement of pH and conductivity for a range of aqueous samples is common in most water laboratories. Surprisingly, many laboratories still use manual methods for these measurements. TTie automation of these techniques is not easy. Two approaches to solve this automation problem, descrihed in this section, both used the same large volume autosampler. 

Stan, H.J. Linkerhagner, M. Large-volume injection 10. in residue analysis with capillary gas chromatography using a conventional autosampler and injection by programmed-temperature vaporization with solvent venting. J. Chromatogr. A. 1996, 727, 275. 

Polystyrene (PS) 2-mL Delves-type autosampler cups for small-volume autosamplers or 15-mL PP or PS tubes for large-volume autosamplers cleaned by presoaking in 1% nitric acid overnight, rinsed with ultrapure water and dried in a clean area. 

Volatile sample pans and crimpers Vapor pressure pans and crimpers Large-volume stainless steel pans and crimpers Reusable high-pressure capsules and sealers Autosampler system pans and universal crimpers... 

FIGURE 14.12 Fully automated nano HPLC-MS with autosampler injection of large sample volume using two parallel online sample trapping, concentrating, desalting, and filtering columns. Pumps 1 and 2 transfer sample from loop of the autosampler into trap column 1 while pumps 3 and 4 elute sample in trap column 2 into the analytical column then to MS. 

When the system which was to be linked to the autosampler system was purchased, it was found that the conductivity cell was excessively large and primarily designed to be fitted to a A" pipehne for analytical purposes this was clearly too large. However, the pH cell provided an Ingold standard electrode system with dimensions which allowed it to be placed within the conductivity sensor, as shown in Fig. 7.21. It is therefore possible to configure a flow cell in which the conductivity sensor houses the pH electrode. The latter also serves to take up most of the cell volume so that the wash-out requirements are not too excessive. The cell is filled by taking a sample from the autosampler with pump 1 hquid fills the cell and overflows via the weir arrangement and the top of the cell. 

Most of the autosamplers have a piston metering S3rrmge t)rpe pump to suck the preestablished sample volume into a line and then transfer it to the relatively large loop ( 100 ml) in a standard six-port valve. The simplest autosamplers utilize the special vials with pressuarization caps. A special plunger with a needle, push the cap down to the vial and displace the sample through the needle into the valve loop. Most of the autosamplers are microprocessor controlled and can serve as a master controller for the whole instrument... 

Samples can be injected automatically with mechanical devices that are often placed on top of gas chromatographs. These autosamplers mimic the human injection process just described using syringes. After flushing with solvent, they draw up the required sample several times from a sealed vial and then inject a fixed volume into the standard GC inlet. Autosamplers consist of a tray which holds a large number of samples, standards, and wash solvents, all of which are rotated into position under the syringe as needed. They can run unattended and thus allow many samples to be run overnight. Autosamplers provide better precision than manual injection— typically 0.2% relative standard deviation (RSD). 

If the mass spectrometer is equipped with an LC system, then fast fingerprint MS analyses can be performed by injection of a series of different samples by the autosampler unit. This system will allow automated operation of a large series of samples. In addition, it is possible to utilize the flexibility that lies in the use of different mobile phases and the fast variation between them. The LC-MS system is simply operated as an ordinary LC-MS system, but without an analytical column. Typical injection volumes would be between 0.1 and 3 pL, and mobile phase flows between 0.1 and 0.2 mlV min. With this flow, a chromatographic peak will appear between 0.3 and 0.8 min after injection. Total time per analysis will be 1-2 min per injection. With a scan range of m/z 65-1500, a sufficient number of scan spectra (>15 spectra) will be captured for further data processing. Normally, it is preferred to perform multiple injections from the same sample, and the number of injections could be from 3 up to 10, depending on the study. With multiple injections, the precision of the analytical system can be controlled directly from the score plots and, of course, be evaluated relatively to the differences between the samples. 

The most important source of error in analyses by the method just described is usually the uncertainly in the volume of sample occasionally, the rate of injection is also a factor. Samples are usually small ( 1 pL), and the uncertainties associated with injection of a reproducible volume of this size with a microsyringe may amount to several percent relative. The situation is even worse in GC, where the sample must be injected into a heated sample port here, evaporation from the needle tip may lead to large variations in the volume injected. Errors in sample volume can be reduced to perhaps 1% to 2% relative by means of autosamplers or a rotary sample valve such as that described in Chapter 27. 

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