The accuracy and precision of chromatographic results depend to a large extent on the repeatability of precise injection volumes into the chromatographic systems. Such volumes generally fall in µl range. Other contributing factors responsible for the consistency of results are operational parameters such as mobile phase composition and flow rate, control of the temperature of the column, purity of mobile phase solvents or gases, freedom from leaks, detector wavelength and efficiency of the column. The present article covers in detail the role of automated injections in achieving the desired objectives.
The commonly adopted means of sample introduction are:
- Manual syringe injection
- Injection using valves with fixed volume sample loops
- Automated sample introduction with autosamplers
Manual Syringe Injection
Syringe injection is the most common sample introduction practice. It is the most economical option which permits manual volume selection and introduction into the system. However, the selected volume is dependent on personal judgment and there is the possibility of errors from person to person or even the same person making repeat injections. Syringe injection can be relied upon only if proper care is taken in use and handling of syringes. Such requirements are time-consuming and even experienced chromatographers cannot guarantee repeatability of injected volumes. Syringes are delicate devices so require careful handling during use and also elaborate cleaning before subsequent use.
Injections using valves with fixed volume sample loops
The next popular device is a fixed volume loop injector. The advantage offered is constant injection volume which is decided by the volume of the sample loop. However, if the loop is not flushed adequately by the mobile phase the residues left over from earlier analysis can result in undesired impurity peaks in the chromatogram.
Automated Sample introduction using autosamplers
The best available option is automated injection with the help of autosampler. It offers several advantages over manual injections.
The injection volume is set by the software and therefore is free of personal errors. The volume can be reset, if required, to meet the analysis requirements.
The number of repeat injections per vial can be optimized with the help of software to get the desired reproducibility of results. All this is possible without human intervention. Such high precision injections are not possible if repeat injections are carried out manually.
The autosampler tray has numbered slots for positioning of sample vials. The autosampler is pre-programmed to select the vials in the desired sequence for making injections. The sequence can be modified during runs to handle priority samples. This is a big benefit as it eliminates selection of vials manually.
It is essential to wash the autosampler syringe and needle with blanks in between repeat injections. The number of wash cycles and their sequence can be programmed as per analysis requirements.
Injection of standards
It is essential to inject standard solutions in between sample injections to help verify the system performance. The number of standards to be injected and their sequence can be programmed in advance.
A large number of samples
In a single loading, 100 or even more samples or standards can be loaded in numbered positions of the autosampler tray. Injections from numbered positions follow the planned sequence and errors of selection from such a large number of vials manually are avoided.
The biggest advantage offered by autosamplers is time-saving. After the samples are prepared and loaded and the analysis sequence is set the analyst is free to attend to other tasks. Auto samples operate even overnight when no one is around. This feature offers significantly to increasing the laboratory sample throughputs.