What is autosampler?
In chromatographic experiments, the device that can quantitatively feed the sample into the column is called an injector. There are two types of injectors, they are manual injector and autosampler. The autosampler is an intelligent and automatic injection instrument. Users only need to set the injection parameters, and put in the sample to be tested, the instrument can complete the automatic injection process.
Working principle of autosampler
The injector requires good sealing, small dead volume, good repeatability, and small pressure and flow fluctuations caused by the chromatographic system when feeding. Nowadays, almost all manual injectors used in liquid chromatographs are six-way valve injectors with high-pressure resistance, good repeatability, and easy operation.
In general, an autosampler
consists of an inlet arm, an inlet needle, a sample tray, a cleaning system, a drive system, and a control system. Each part works closely with the other to drive the normal operation of the autosampler.
The sample feeder arm is loaded with the sample feeder needle, and the sample stations are evenly distributed in different radii on the circular sample tray with its rotation axis at the center of the circle. The inlet arm and the sample tray are controlled and signaled to produce a coordinated inlet action. Each rotation of the sample tray generates a signal, and the injection arm can only perform aspiration after receiving the signal, thus ensuring that the injection needle on the injection arm can accurately aspirate the target sample at different radii of the sample tray.
Type of autosampler
For inhalation autosamplers
, the sample is drawn into the sample ring by the suction generated by the syringe. This design of autosampler
was very popular in the early days because of its simplicity and reliability but later declined with the advent of two other designs.
The inhalation autosampler
syringe is attached to the waste liquid drain, and the sampling needle and corresponding piping are connected to the inlet port. The needle is inserted into the sample vial, followed by the syringe sucking back until the sample completely fills the sample ring and the circular gasket rotates to feed the sample.
The inhalation autosampler
has a simpler design, with only the sampling needle moving up and down along one axis. The sample vial is then placed under the needle by rotating the sample tray and the sample feed begins as required. The disadvantage of the inhalation autosampler is that there is a waste of sample because the sample must first fill the sampling needle and the connected pipeline before entering the sample ring, so there is a slight sample waste.
The design of the push-in autosampler is very similar to manual injection. The syringe moves to the sample vial to draw in a predetermined amount of sample, then moves to the inlet port and injects the sample into the sample ring. This design requires a much smaller sample volume than the inhalation design, and the syringe aspiration and discharge are done under the control of a stepper motor, allowing a high degree of precision and accuracy to be achieved. Under normal conditions, the deviation of the results is less than 0.5%. At the same time, the push-in autosampler also contains some features that the inhalation autosampler does not have, such as arbitrary sample sequence, multi-solvent cleaning function, intelligently programmed injection, etc. It meets the different injection needs of laboratory analysis.
Integral ring autosampler
The integral sample ring autosampler is more suitable for trace analysis with limited sample volume and basically no sample waste. The operation process is more complicated than the above two.
The sample is only present in the cleaned ring during the entire injection process of the integral ring autosampler, so the entire sample is injected without waste and the maximum volume of sample injected depends on the sample ring capacity.
Conveyor belt autosampler
The belt conveyor belt is equipped with an electro-hydraulic actuator-driven sampling scraper, which is fixed to a steel support frame with a control system box, and this is called conveyor belt autosampler.
Conveyor belt autosamplers are not affected by human factors, which can greatly improve the accuracy and representativeness of the sample. Through PLC control of the automatic sampling, the conveyor belt autosampler determines the sampling time, sampling frequency, and sampling volume according to the production quantity and species, which is fully controlled automatically. Therefore, it greatly improves the representativeness, timeliness, and accuracy of sample aggregation. At the same time, the instrument can continuously take multiple test samples online at regular intervals without interfering with the main production process, which improves the automation and efficiency of the process and reduces labor intensity and labor input.
Process of autosampler
The basic working process of the autosampler is sample extraction - injection - needle wash - re-extraction - re-injection, the actual operation sequence can be changed by software programming.
The specific detailed process is as follows.
a. The PC sends commands and parameters to the controller (PLC), which generates the corresponding signals to control the rotation of the sample tray motor to complete the positioning of the sample position number. The set commands and parameters are the number of samples installed, the position number, the sample volume, the sample feed speed, etc.
b. The displacement motor controls the syringe carriage to descend and the syringe needle reaches the position of the sample to be extracted.
c. The syringe is controlled by the extraction motor to lift the syringe puller to quantitatively extract the sample.
d. The pallet motor and the displacement motor control the syringe carriage and the sample chassis to reset in turn, completing the positioning of the sample inlet.
e. The displacement motor controls the syringe carriage to descend and insert the needle of the syringe into the chromatography inlet.
f. The injection control motor controls the rapid descent of the syringe puller to inject the liquid sample into the chromatograph.
g. The feed control motor, tray motor and displacement motor enter the syringe needle preparation state under the control of the controller (PLC) timing command.
h. Syringe needle washing process.
i. One injection process is finished.
Maintenance of autosampler
The popularity of HPLC analytical instruments and their improvement in automation has led to the replacement of the original manual mode with the automatic injection mode. Automatic injection mode has greatly improved the efficiency of analytical instruments, enhanced the accuracy of analysis, eliminated human error, and more and more units are using autosamplers.
However, once the failure of the autosampler occurs, it often makes it difficult to deal with. If you want to figure out the cause of the failure and effectively solve its failure, daily maintenance and care must be paid attention to.
A. Timely cleaning of the chromatographic system
For daily use, after analyzing the sample and cleaning the column, a PEEK tube needs to be attached to the original position where the column was attached. Then it needs to be cleaned with degassed pure water at a flow rate of 1.5 mL/min for 1.5 hours and then cleaned with methanol under the same conditions.
Replace the autosampler cleaning solution with methanol. Open the drain valve, aspirate over 50mL with a syringe, and close the drain valve again. Then perform a purge and rinse, repeat three times, and close the autosampler at the end.
Pure water cleaning chromatography system can remove the residual buffer salt in the pipeline, while methanol cleaning can prevent the system from breeding bacteria and microorganisms to block the pipeline, and also can wash away the insoluble substances.
B. Replace the septum cutout regularly
Replace the rubber pad in the Septum cutout, or rinse the port cap, regularly. There is a procedure for washing the needle in the automatic sample setup, so the rubber pad can easily get dirty after a period of time. Timely replacement to avoid contamination of the needle with dirt on the pad.
C. Replace the sample spacer regularly
Remove the inlet cap with a fixed wrench, take out the inlet liner cylinder sleeve inside, remove the old inlet pad, put in the new original sample, install the original sample in the inlet, tighten the nut by hand, and screw it again with a wrench about 15°.
D. Apply lubricant regularly
Use blotting paper to insert clean oil and debris on the lift shaft that controls the feed needle. Apply a small amount of lubricant to the lift shaft. After that, use absorbent paper to remove excess lubricant. Perform a sample test and listen to the crisp and regular sound when the lift shaft is working.
E. Cleaning with pure methanol for a long time without using the machine
Machines that have not been used for a long time can be cleaned with pure methanol instead of a cleaning solution. Syringe pumping about 100mL of cleaning solution, and then purge and rinse several times.
F. Timely replacement of metering pump sealing gasket
When replacing the metering pump seal gasket, unlike the pump, the user must first enter the instrument's diagnostic maintenance interface to retract the plunger rod inside the metering pump before disassembling the metering pump, otherwise, the plunger rod is in a compressed position when it is removed.
G. Through-valve maintenance
The speed of sample feeding should be fast. The syringe must be fed with a flat-tipped needle. In general, please do not disassemble the valve body. Open the valve only if the orifice is already blocked or if worn parts need to be replaced. Frequent cleaning of the inlet valve is required. When the gas chromatograph is not used for a long time or the ambient temperature is high, it leads to the volatilization of the solution in the residual drench solution inside the valve, and the salt concentration increases to form crystalline precipitation.
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