Auto-analysers: Parts, Functioning and Disadvantages

In this article we will discuss about Auto-analysers used for Biochemical Analysis of Hospital Specimens:- 1. Parts of Auto-analysers 2. Functioning of Auto-analysers 3. Disadvantages.

Parts of Auto-analysers:

These instruments are involved in the automation to the routine biochemical analysis of hospital specimens. About 60 to 300 samples of blood or serum per hour for as many as 18 constituents can be analysed by the auto-analysers.

The principal parts of an auto-analyser are the followings:

i. Simpler.

ii. Proportionating pump.

iii. Dialyzer.

iv. Constant temperature.

v. Flow through colorimeter.

vi. Recorder.

i. Simpler:

(a) This module holds the batch of samples awaiting analysis in separate cups on a circular tray which is rotated at intervals.

(b) A probe connected by plastic tubing to the proportionating pump enters each sample serially.

(c) The volume of sample aspirated is deter­mined by the pumping rate and the ad­justable dwell time of the probe in the sam­ple.

(d) As the probe movement into the sample cup was slow, the volume aspirated var­ied somewhat with the depth to which this was filled.

(e) The transit time between reservoir and sample is short.

ii. Proportionating Pump:

(a) This module determines the relative flow rates of sample and all reagents and re­places the use of different sizes of pipettes in manual methods.

(b) The pumping technique involves the peristaltic action produced by a series of rollers passing along an array of parallel plastic “Pump tubes”.

(c) Each roller compresses all tubes so that the rate flow in each tube is proportional to the square of the pump tube diameter.

(d) Colour-coded tubes with a range of nomi­nal diameters and pumping rates are avail­able in three materials.

(e) The normal tygon tubing is suitable for most reagents but Acid-flex tubing is pre­ferred for reagents containing corrosive acid and Salvaflex tubing for certain or­ganic reagents.

iii. Dialyser:

(a) This module achieves the separation of small and large molecules by allowing the former to pass through a semipermeable membrane from the donor (sample) stream of liquid and air bubbles to a recipient , stream of liquid again segmented by air bubbles.

(b) The dialysis rate depends on the tempera­ture but complete passage of small mol­ecules into the recipient stream is rarely achieved and may be only a few percent of the total.

(c) The analytical process then requires that a constant fraction should dialyse and this is not always the case when simple aque­ous and protein-containing solutions are compared.

(d) It is important to ensure that the two streams flow in the same direction.

(e) Care must be taken to ensure that the out­put from the recipient stream is the one which enters the remainder of the analyti­cal system. If the sample stream is greatly diluted, the difference may not be seen easily.

iv. Constant Temperature:

(a) It is to maintain the reaction mixture at a constant temperature for a defined time to bring about the required chemical change under controlled conditions.

(b) The incubator bath consists of a glass de­lay coil mounted in a thermostatically controlled oil bath. This is sealed and stirred constantly.

(c) Most baths are set at 37°C or 95°C but some have adjustable thermo-regulators which allow operation up to 120°C or even higher.

v. Flew through colorimeter:

(a) The colorimeter is to measure the inten­sity of colour produced in the reaction and to provide a graphical display of change in colour with time.

(b) The use of double beam spectrophotom­eter is costly and is rarely justified by ana­lytical requirements. The single beam colorimeters have insufficient stability to operate reliably over the long period re­quired.

(c) The Auto-analyser MKI colorimeter com­bines double beam operation with inter­ference filters to select the wavelength.

vi. Recorder:

The servo-potentiometer recorder is used to record the ratio of the responses from the two detectors and these responses are proportional to the inten­sity of light reaching the detectors.

Functioning of Auto-analysers:

i. The flow of the analytical stream is di­rected through plastic tubing from one module to another.

ii. The samples are then loaded into the cup of the sampler and the channels of the proportionating pump aspirate them as well as dilute them.

iii. The diluted serum samples are led through one of the dialyser unit.

iv. The pump introduces suitable reagents through the other side of the dialyser.

v The two streams run side by side being separated only by dialysing membrane.

vi. A portion of dialyzable constituents of se­rum passes across the membrane to the rea­gent stream.

vii. Further treatment like incubation at suit­able temperature is given in the constant temperature bath.

viii. The intensity of the colour developed is measured in a colorimeter and recorded in the recorder.

ix. Suitable standards are treated in the same way.

Lately, multiple analysers have been in­troduced with multiple channels to esti­mate as many as 18 selected constituents including the non-dialyzable proteins, en­zymes like alkaline phosphatase, triacylglycerol’s and electrolytes. About 60 to 300 samples can be investigated per hour simultaneously for 18 parameters.

Auto-analysers have gained much popu­larity for routine biochemical analysis in clinical laboratories for their speed, flex­ibility of the methodology, increased quan­tum of clinical biochemical analysis, easy operation, and accuracy of results.

Disadvantages of Auto-analysers:

i. Prohibited cost.

ii. “Carry-over” errors.

iii. The usual hazards of any complicated equipment.