The platform: ELFA, sections, pipetting, the optical scanner, electronics, printer and LIS

Name every major assembly of the mini VIDAS, explain what it contributes to a result and know which board controls it, so that any later fault can be placed on the right block.

What the instrument actually does

The mini VIDAS is a small, closed immunoassay analyser built around enzyme-linked fluorescent assay, ELFA. You met fluorescence readers and sectioned analysers in Level 2; here we stay with this one machine. The operator does very little: pipettes the sample into a well of a ready-made reagent strip, puts the strip into a tray, puts the matching coated tip into the block above it, closes the door and starts the section. Everything after that, from piercing the foil to printing the result, is the instrument's work.

Two consumables carry the chemistry. The reagent strip is a row of ten wells under a foil seal: one well for the sample, eight with reagents, wash and substrate, and an optical cuvette at the end. A bar code on its label tells the instrument the assay, the lot and the expiry date. The SPR, the solid phase receptacle, looks like a pipette tip; its inner wall is coated at the factory with antibody or antigen. It is at the same time the reaction surface and the pipette tip, so the instrument draws liquids up into it and pushes them back, well after well. A coloured dot and a three-letter code tie each SPR to its strip; a mismatched pair cannot give a valid result, whatever the hardware does.

The sequence at concept level is short. Sample is cycled through the SPR so the analyte binds to the wall, wash removes what did not bind, an enzyme-labelled conjugate binds to the captured analyte, another wash follows, and substrate is cycled through. The enzyme turns the substrate into a fluorescent product, which is finally pushed into the cuvette and read by the optics. More bound analyte means more enzyme, more product and more light. All liquid waste stays in the strip, which is why the instrument has no waste bottle, no tubing and no wash station. Remember that when you compare it with the XP-300: here the fluidics are dry air, pistons and seals.

Two sections, each a small analyser

Look at the front and you see two sections, A and B. Each has its own SPR block behind a door, its own reagent strip tray that slides in and out, its own six-channel pipettor in a tower above the tray, its own heaters and its own controller board. Each section takes six single strips or three dual strips, so the instrument runs up to twelve tests at once. All six positions in a section follow the same protocol at the same time, which is why the software refuses to mix incompatible assays in one section. A green lamp above each door tells the operator whether the section is idle, running or finished and waiting to be unloaded.

The design matters to you because the sections are independent. If one section fails mechanically, the operator can take it out of use and keep running on the other. When you fault-find, the first question is always whether the problem is in one section or in both: one section points at that section's tray, tower, pump, heaters, sensors or controller; both sections point at what they share, the scanner and optics, the computer, the power supply, the reagent lot or the room.

Temperature is controlled in two places per section. The SPR block is a thermally isolated block heated by a surface heating element with a thermistor in it, and the tray carrying the strips has its own identical heater and thermistor. Both run at 37 °C, with tolerances and section-to-section matching limits that the service manual states and that you will check in lesson 2. The heaters are powered from low-voltage AC rather than mains, and a separate controller keeps the power supply enclosure at a steady temperature to help hold the air inside the cabinet stable. The room matters too: the manual asks for an ambient temperature range and clear space around the cabinet, and an instrument placed under an air-conditioning vent or in afternoon sun will show it in temperature errors and drifting results.

Pipetting: pistons, diaphragms and seals

The pipettor of each section is one assembly with six channels that move together but share no liquid. Each channel is an air-displacement piston sealed by a rolling diaphragm, often called a bellofram, which keeps the effective piston area constant over the stroke. A linear stepper motor with a lead screw drives a lever arm and a pressure plate that pushes all six pistons at once. The SPR sits on the bottom of each channel in a resilient grommet seal. The whole pump assembly rides up and down in the tower on two drive screws, so the SPRs can pierce the foil and dip into whichever well the tray has moved underneath.

Three things follow for service. First, there is nothing to calibrate in the field: volume accuracy comes from machined parts, and the manufacturer's way to prove it is the quality control test strip, QCT, which you will meet in lesson 2. Second, the weakest point is the grommet seal. A worn, contaminated or damaged grommet lets air in, the SPR does not pick up or hold the right volume, and results and wash efficiency suffer without any mechanical error. The manual lists grommet replacement in the yearly maintenance and whenever they are contaminated or damaged. Third, the diaphragms are fragile. If the pump arm is allowed to spring up while the motor is off, they can be damaged; lesson 5 deals with that during motor replacement.

Optical sensors watch the pipetting movement during a run, and if the motion deviates from what the protocol commanded, the section stops and reports an error. Every moving assembly in a section, pump, tower and tray, is driven by a stepper motor whose position is counted from a home sensor. Most of those home sensors are reflective: they look at a shiny surface on the arm, the pump plate or the tray. Dirt, chipped paint or a faded reflective cover on that surface is enough to make a healthy section report a jam.

The optical scanner

One optical head serves both sections. It sits on a carriage that runs along a rail at the base of the instrument, driven by a stepper motor and a lead screw, and it carries three things: the fluorometer, the bar code reader that reads the strip labels, and a tray edge sensor that finds the exact position of each tray. A flag on the carriage and an optical sensor give the scanner its home position. The cables to the head run through a flexible chain, the E-chain, which must move without kinks.

The fluorometer is a single-channel, ratio design. A xenon flash lamp fires a series of flashes through a narrow-band filter in the near ultraviolet and a lens onto the cuvette. The fluorescent product emits blue light, which a second lens, at right angles to the excitation, collects through a narrow-band blue filter onto a sensitive PIN photodiode. A beam splitter sends part of the excitation light to a reference photodiode, and the electronics divide signal by reference, so that ageing of the lamp is compensated. Peak detectors hold each pulse, an analogue-to-digital converter on the scanner board digitises them, the highest and lowest flashes are discarded and the rest averaged into a relative fluorescence value.

The flash lamp needs a high-voltage supply on the carriage, and the lamp emits UV. The manual warns of both hazards on the scanner assembly: shock from the high voltage and eye damage from the UV. Keep the covers in place when the instrument is powered and do not look into the head while the lamp flashes.

Between sections A and B sits a solid fluorescent reference in a standard cuvette, the auto-calibration standard. After a field calibration with the manufacturer's liquid calibration strips, its reading is stored on the scanner board, and from then on the instrument reads the solid standard periodically and trims its own gain to hold that value. This is what keeps the optics stable between engineer visits, and it is why errors about the solid standard, its bracket or its stored value stop the instrument from running assays. Lesson 2 covers the logic and the limits.

Electronics: who controls what

There are three levels of controller, and knowing them is half of fault-finding. At the top is the VIDAS Internal Computer, the VIC board, mounted on the right end panel. It runs the menus, stores protocols, master lot data, standards, configuration and results, drives the display, keypad, speaker and printers, and talks to the LIS. It has flash memory, a memory card slot at the rear for software and data, and a real-time clock with battery backup.

In the middle is the scanner board, mounted inside the top cover, which the manual also calls the system controller. It receives protocols from the VIC and hands them to the sections, runs the scanner motor, the bar code reader and the tray edge sensor, digitises the fluorescence readings, runs the auto-calibration and the start-up self-tests, and reports temperatures. It has a serial port to the VIC and a second serial port for a diagnostic terminal, which is the engineer's window into the instrument.

At the bottom are two identical incubator boards, one per section, also inside the top cover. Each runs the pump, tower and tray motors of its section, reads its sensors, runs the two temperature controllers and shows sensor and temperature states on LEDs. Address jumpers tell each board which section it is, so a replacement board must be jumpered like the old one. On each tower sits a bay interface board, which simply connects the incubator board to the heaters, thermistors, motors, sensors and front lamp of that section.

The power supply sits at the rear of the base: a toroidal transformer, a switching supply for the logic rails and a power supply board that distributes and generates the other voltages, including the low-voltage AC for the heaters and the supply for the printer. It carries test points for every rail and a fan, and you will measure it at every maintenance visit.

Operator interface, printer and LIS

The operator uses a liquid crystal display with selection keys beside it, a numeric keypad, arrow keys and three function keys: one for help and error display, one to go back a screen, one to undo the last entry. When a serious error occurs the instrument beeps and can flash the display; the help key shows the error code and message. A built-in test menu lets the operator, and you, check the display, keys, speaker and printer.

Results print on an internal thermal printer on top of the display assembly; the 2000 service manual names two makes, an older and a later one, and later production may differ again, so check which one you have before ordering parts. An external parallel printer can be selected instead. An optional hand-held bar code wand connects to a rear port.

For the laboratory system the mini VIDAS offers a one-way serial interface from a stated software version onward; the 2000 manual says so, and bioMérieux's current product page still lists a uni-directional interface for mini VIDAS, but check the software level and any middleware on site: it sends results to the LIS or host and receives nothing back. The settings, communication parameters, timeouts and data format, follow the LIS vendor's specification, and a test message can be sent from the configuration menu. One-way means no worklist download, so sample identification depends on what the operator enters or scans. Treat the link as a patient data channel: results leaving the instrument carry identifiers, and changes to it are agreed with the laboratory and its IT staff.

On-site checklist

• Instrument model, serial number and software versions noted
• Fault placed in one section or both sections
• Consumables checked: strip and SPR paired, in date, correct positions
• Location checked: ambient temperature, clearances, no vents or direct sun
• Printer type and LIS connection identified

Check yourself