Hitachi L7450 HPLC Diode-Array-Detector Teardown

This is a complete teardown of a Hitachi L7450 HPLC Diode-Array-Detector. HPLC is short for High Performance Liquid Chromatography. The DAD uses UV light to detect a color spectrum through a liquid flow cell into a linear detector. This liquid analysis method requires a wide range of apparatuses, starting with mixing, pumps, injection, temperature oven columns and at last a detector. I have previously done a teardown of a temperature column for HPLC, the Thermo Scientific Dionex TCC-3000SD Standard Thermostatted Column. For the related gas chromatography technology, I have done a teardown of a Agilent Markes UNITY Thermal Desorber.

Hitachi L7450 HPLC working principle

The DAD is the last apparatus in the chain of units used for HPLC. A typical setup can be seen here along with the condition of the unit when I got hold of it. It had taken a lot of damage to the front and sides.

The working principle of the HPLC detector is illustrated below. The detector is using ultraviolet light with a diffraction mirror to shine light through a flow cell. The light exiting from behind the flow cell is captured by a diode array detector with a filter for the diffracted light. According to the chromatograph (the resulting graph/image), the liquids building blocks can be determined. The array of photodiodes enables simultaneous acquisition across a range of wavelengths.

Fluid mechanical parts and detection

The UV light is generated with a deuterium lamp, also just called a D2 lamp. The lamp is mounted inside a light tight chamber, to avoid light contamination coming from other places than through the focus lens in the side of the light source chamber.

A deuterium lamp is contructed with a tungsten filament. The filament is not the light source as in a regular lamp. The filament is heated before a arc can ignite. The ignition voltages for firing the arc is around 300 to 500 volt. The voltage can then be dropped, once the arc is established. The arc excites the molecular deuterium gas to a higher state, as the deuterium transmissions back to its initial state, UV light is emitted. This process continues over and over and results in the constant UV output.

Inside the chamber, there is firstly the flow cell with a 4nm slit. Secondly, the output from the flow cell goes to a diffraction grating which goes to a half-mirror. From here light is either going to the detector or the detector can see the reference area. This reference area is the patch of dark foam/cloth on the side of the chamber.

The photodiode array (PDA) is a linear row of discrete photodiodes in a single IC. Combining this with control circuitry, filters and software, we get a Diode Array Detector (DAD). The detector is able to, simultaneously, measure the lamps/detectors entire wavelength response of the sampling, which is typical 1 second.

A flow cell is a very simple mechanical piece. It is most often made from 10 mm stainless steel block with a 1 mm hole drilled through. This results in a 8 µL volume with quartz windows in each end. The light shines through the flow cell and all the known diffractions from air, quartz and flow cell side walls are known, so what is left is the diffraction from the mobile phase liquid being analyzed.

Power supplies, controllers and software

The power supply is a Nemic-Lambda, made in Japan! The model number UNK-5/A and voltages suggests its a rather special power supply. It has 6 channel output and is specifically made for arc lamps. 5 VDC 2 A on channel 1. 12 VDC 0.2 A on channel 2. -12 VDC 0.2 A on channel 3. 24 VDC 1.0 A on channel 4. 10 VDC 1.2 A on channel 5. Channel 6 is the special arc lamp channel that has a high voltage pulse of 310 V for 2 us. After arc ignition, voltage decreases to 70 VDC 0.29 A.

The circuit boards for the Hitachi L7450 is designed around a motherboard, CPU board and detector board structure. The internal bus communication seems to be controlled by three Xilinx XC2018 PC68C FPGA. These are 5 VDC supplied and have around 600 – 1500 logic elements. The CPU is Hitachi HD641016CP8 16-bit RISC microprocessor with 1 kb RAM, DMA controler, serial communications, interrupt controller, peripheral controller, and memory access control devices on a single chip. It is based on CMOS technology.

Mads Barnkob

Electrician, programmer, experimenter and amateur scientist with experience in industry automation, programming and all kinds of high voltage generating electronics. Administrator of and the high voltage community forum

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