Source: Lessons 1-2, CC2LB Lecture (Concepcion / Dacanay, BMLS3-4)
Tags: spectrophotometer, spectrophotometry, Beer's law, absorbance, transmittance, cuvette, monochromator, photodetector, centrifuge, ultracentrifuge, pipette, air displacement, positive displacement, clinical chemistry lab instrumentation
Difficulty: Introductory to Intermediate Prerequisites: Basic understanding of electromagnetic radiation and the visible light spectrum. Familiarity with clinical laboratory workflow is helpful but not required.
This material covers the core instruments you will use in a clinical chemistry laboratory: the spectrophotometer, the centrifuge, and the pipette. These are the workhorses of any chem lab rotation, and understanding how they function (not just how to press the buttons) is what separates competent practice from guesswork. The spectrophotometer section is the heaviest, because nearly every quantitative assay in clinical chemistry depends on light absorption measurements. If you can explain Beer's law and trace the light path from source to readout, you are in good shape for both the practical and the written exam.
Spectrophotometers measure how much light a solution absorbs to work out the concentration of a substance in it; absorbance is directly proportional to concentration (Beer's law). The instrument's light path runs from a source through a monochromator, a cuvette holding the sample, and into a photodetector. Centrifuges separate blood components by spinning, and pipettes deliver precise sample volumes; knowing their types and limitations matters for accurate, reproducible results.
Spectrophotometer
An instrument that measures the intensity of light absorbed by a sample solution. Think of it as a device that shines a specific colour of light through your sample and asks, "How much of this light did the sample swallow?"
Spectrophotometry
The measurement of light transmitted by a solution to determine the concentration of the absorbing substance. In simple terms, it is the principle behind the spectrophotometer: more substance means more light absorbed.
Transmittance (T)
The fraction of incident light that passes through the sample and reaches the detector. Think of it as "how much light got through." Higher concentration of absorbing substance means lower transmittance.
Absorbance (A)
The parameter that increases linearly with concentration; directly proportional to the amount of light-absorbing substance. In simple terms, absorbance is the measurement you actually use for quantitative analysis because it scales in a straight line with concentration.
Monochromator
A component that isolates a specific wavelength of light from a polychromatic source. Think of it as a filter that picks out one precise colour of light for the measurement.
Cuvette
Also called an absorption cell, analytical cell, or sample cell. The transparent container that holds the solution being measured. In simple terms, it is the little tube or box you place in the instrument.
Photodetector
A device that detects transmitted light and converts it into an electrical (photoelectric) signal. Think of it as the instrument's "eye."
Stray light
Any wavelength of light reaching the detector that is outside the band selected by the monochromator. In simple terms, unwanted light that sneaks through and throws off your reading. It is the most common cause of loss of linearity at high analyte concentrations.
Beer's law (Beer-Lambert law)
The principle that absorbance is directly proportional to both the concentration of the absorbing substance and the path length of light through the solution. In simple terms, it is the mathematical reason spectrophotometry works for quantitative analysis.
Bandpass
The total range of wavelengths transmitted through the exit slit. The narrower the bandpass, the greater the spectral purity (resolution) of the instrument.
Centrifuge
An instrument that separates blood or other fluid components by spinning samples at high speed. Think of it as a very fast spinner that sorts heavier components to the bottom.
Air displacement pipette
A pipette that uses a piston and air cushion to draw liquid into a disposable tip. The piston does not contact the liquid.
Positive displacement pipette
A pipette where the piston moves directly inside the tip (like a syringe), contacting or nearly contacting the liquid. Useful for viscous or volatile reagents.
Reagent
A solution of known composition used in a chemical analysis.
Unknown (sample)
The sample solution being analysed in the laboratory.
Reconstitution
The process of adding a specified volume of distilled water to a lyophilised (freeze-dried) material to return it to liquid form.
A spectrophotometer measures how much light a sample absorbs. The core relationship to remember:
A sample with a high concentration of light-absorbing substance absorbs more light, so the light passing through has lower intensity (lower transmittance).
Absorbance increases linearly with concentration, which is why it is the parameter used for quantitative analysis.
Transmittance and absorbance are inversely related.
UV-visible spectrophotometer: operates over the ultraviolet range (185-400 nm) and the visible range (400-700 nm).
Infrared (IR) spectrophotometer: operates over the infrared range (700-1500 nm).
Single-beam spectrophotometer
Simplest type. Makes one measurement at a time at one specified wavelength.
Operates between 325-1000 nm.
Light travels in a single direction. The test solution and the blank are read in the same cuvette, one after the other.
Light path: light source, lens, grating (monochromator), wavelength selector, sample cuvette, photodetector, digital meter.
Double-beam spectrophotometer
Splits the monochromatic light into two beams: one passes through the sample, the other through a reference solution.
The reference beam corrects for variation in light source intensity, giving more accurate and precise measurements.
Operates between 185-1000 nm.
Two photocells split the light from the monochromator into the two beams.
Two sub-types of double-beam instrument:
Double-beam in space: uses two separate photodetectors, one for the sample beam and one for the reference beam.
Double-beam in time: uses a single photodetector. A chopper alternately directs light first through the sample cuvette, then through the reference cuvette.
Stable source of radiant energy
Filter (or monochromator)
Sample holder (cuvette)
Radiation detector (photodetector)
Signal processor
Readout device
Light/radiant source
Provides polychromatic light (multiple wavelengths). Must generate sufficient energy to measure the analyte of interest.
Continuum sources emit radiation across a broad range of wavelengths:
Tungsten lamp: visible and near-infrared region.
Deuterium lamp: UV radiation.
Xenon discharge lamp: UV and visible region (continuous).
Line sources emit limited, discrete wavelengths:
Mercury and sodium vapour lamps (UV and visible).
Hollow cathode lamps.
Entrance slit
Minimises unwanted light entering the system. Reduces stray light, which is the most common cause of loss of linearity at high analyte concentration.
Monochromator
Isolates specific wavelengths from the polychromatic source.
Prisms
Diffraction gratings: most commonly used; better resolution than prisms.
Filters: simplest and least expensive, but less precise.
Exit slit
Controls the width of the light beam (bandpass). The narrower the bandpass, the greater the spectral purity (resolution). For accurate absorbance measurements, the exit slit should produce a bandpass of less than one-fifth the natural bandpass of the spectrophotometer.
Cuvette (absorption cell / sample cell)
Holds the solution to be measured.
Round cuvettes: common but subject to lens-effect error.
Square cuvettes: preferred because they produce less error from lens-effect orientation.
Glass cuvettes: for measurements in the visible range.
Quartz cuvettes: for measurements in the UV range (glass absorbs UV light, so it cannot be used).
Photodetectors
Convert transmitted light into an electrical signal.
Barrier cell: simplest, least expensive, temperature-sensitive.
Photocell: contains cathode and anode in a glass case.
Photomultiplier: most sensitive photodetector. Avoid exposing it to intense or room light, as this causes irreversible damage to the photoelectric surface.
Photodiode: less sensitive than a photomultiplier but has excellent linearity.
Meter / readout device
Displays the output. Types include galvanometer, ammeter, and LED display.
Contaminating substances and stray light are the main sources of interference.
Stray light is controlled by the entrance slit.
In qualitative analysis, stray light causes an incorrect absorption spectrum.
In quantitative analysis, stray light produces a falsely higher absorbance result.
Three parameters are checked when calibrating a spectrophotometer:
Wavelength (photometric) accuracy: the instrument measures at the wavelength it is set to. Assessed using special glass optical filters:
Didymium glass: broad absorption peak around 600 nm.
Holmium oxide: multiple absorption peaks, with a peak at 360 nm.
Absorbance checking: uses glass filter solutions with known absorbance values.
Linearity: the ability of the photometric system to yield a linear relationship between radiant power at the detector and concentration (Beer's law). Determined using optical filters or solutions with known absorbance values at a given wavelength. Stray light effects are evaluated with special cutoff filters.
Turn on the spectrophotometer 10 to 15 minutes before use (warm-up period).
Calibrate the device each time before use.
Select the maximum wavelength that the solution can absorb.
The sample must not contain any substance that can dissociate, react, or change during measurement.
Sample preparation should be within the acceptable concentration range, since absorbance depends on concentration.
Centrifuges separate blood components by spinning. This matters because using the wrong specimen type affects results: whole blood used for glucose determination gives a result 10-15% lower than serum, so uniformity of specimen type is essential.
Types of centrifuge
Horizontal (swinging-bucket) centrifuge: the tube sits in a horizontal position during rotation. Maximum speed is about 3000 rpm; above that, excessive heat builds up from air friction.
Fixed-angle (angle-head) centrifuge: allows particles to sediment more rapidly. The rotor holds tubes at a fixed 52-degree angle.
Ultracentrifuge: a high-speed, refrigerated centrifuge used to separate layers of different specific gravities, such as lipoproteins. It is the reference method for lipoprotein quantitation. The refrigerated chamber counters the heat produced at high speed.
Automatic pipettes dispense pre-rated sample volumes.
Parts of a micropipette: plunger, volume-adjusting knob, tip ejector button, ejector arm, digital volume display window, plastic shaft, tip cone, pipette tip.
Air displacement pipette
Uses a piston and air cushion to draw the sample into a disposable tip.
The piston does not come in contact with the liquid.
The polypropylene tip is single-use and attached to the pipette barrel.
Positive displacement pipette
The piston moves directly inside the pipet tip or barrel, much like a hypodermic syringe.
Does not require a different tip for each use.
Uses a capillary tip (siliconised glass, glass, or plastic). A Teflon tip fits tightly inside the capillary, making it useful when the reagent reacts with plastics.
Reusable, and carryover is negligible with proper maintenance.
Reagent: solutions used in chemical analysis.
Unknown: the sample solution being analysed in the laboratory.
Pipetting: the process of using a pipette to transfer liquid.
Reconstitution: adding a specified volume of distilled water to lyophilised QC material to return it to liquid form. The material will be more turbid after reconstitution; diluent should be added carefully and mixed.
Lyophilisation: the opposite of reconstitution, i.e. dehydrating a sample to powder form (freeze-drying).
Control: a material that resembles the unknown specimen and contains various substances of known concentration, assayed by the usual clinical lab methods. Assayed daily to verify that the machine gives accurate results.
Standard: a substance of known composition whose value is established by analytical procedures.
Calibrator: a secondary standard; a substance with a known value used to calibrate an instrument or laboratory procedure.
No formulas are tested in Lesson 1 (Instrumentation). The key quantitative relationships are:
Absorbance is directly proportional to concentration (Beer's law).
Transmittance is inversely related to absorbance.
The relevant formulas (standard deviation, coefficient of variation, Levey-Jennings ranges) appear in the companion document on Calibration and Quality Control.
Light path diagrams to remember:
Single beam: Light source, lens, grating (monochromator), wavelength selector, sample cuvette, photodetector, digital meter.
Double beam (in space): Light source, lens, grating, divider, two wavelength selectors, sample cuvette + reference sample, two photodetectors, digital meter.
Spectrophotometry is the backbone of routine clinical chemistry: glucose, cholesterol, liver enzymes, kidney function markers, and dozens of other analytes are measured by shining light through a reacted sample and reading the absorbance. When you run a metabolic panel on a patient, the analyser is essentially a fast, automated spectrophotometer.
Centrifuges are used every time a lab needs serum or plasma from a whole-blood sample, which is most of the time. The ultracentrifuge has a specialised role in lipoprotein fractionation, the reference method for quantifying HDL, LDL, and VLDL.
Students often think transmittance is the same as absorbance. They are inversely related: high absorbance means low transmittance.
Students confuse the monochromator with the filter. A filter is a simple, inexpensive type of monochromator, but diffraction gratings (the most commonly used monochromator) offer much better resolution.
Students assume any cuvette works at any wavelength. Glass cuvettes absorb UV light, so quartz cuvettes must be used for UV-range measurements.
Students forget that the photomultiplier, despite being the most sensitive photodetector, is fragile: exposure to room light can cause irreversible damage.
⚠️ Know the light path for both single-beam and double-beam spectrophotometers. Exams commonly ask you to trace the sequence from light source to readout.
⚠️ Beer's law (absorbance is proportional to concentration) is the conceptual foundation. Expect questions that test whether you understand this relationship.
⚠️ Stray light is the most common cause of loss of linearity at high analyte concentration. This is a frequently tested fact.
⚠️ Know which lamp covers which wavelength range: tungsten for visible/IR, deuterium for UV, xenon for UV and visible.
⚠️ Square cuvettes are preferred over round cuvettes (less lens-effect error). Glass is for visible range; quartz is for UV range.
⚠️ The photomultiplier is the most sensitive photodetector but must never be exposed to room light.
⚠️ The ultracentrifuge is the reference method for lipoprotein quantitation.
⚠️ Air displacement pipettes: piston does not contact the liquid. Positive displacement pipettes: piston does (like a syringe).
True or False: Absorbance is inversely proportional to concentration. (False. Absorbance is directly proportional to concentration.)
True or False: A single-beam spectrophotometer operates between 185-1000 nm. (False. It operates between 325-1000 nm. The double-beam instrument covers 185-1000 nm.)
Fill in the blank: The most common cause of loss of linearity at high analyte concentration is ______. (Stray light.)
True or False: Glass cuvettes are suitable for UV-range measurements. (False. Quartz cuvettes are required for UV measurements.)
Fill in the blank: The reference method for lipoprotein quantitation uses a/an ______. (Ultracentrifuge.)
Q: What is the principle behind spectrophotometry?
A: Spectrophotometry measures the light transmitted by a solution to determine the concentration of the light-absorbing substance. Absorbance is directly proportional to concentration (Beer's law).
Q: A sample with a high concentration of analyte is measured and the absorbance reading is unexpectedly low (non-linear). What is the most likely cause?
A: Stray light. It is the most common cause of loss of linearity at high analyte concentration.
Q: Why is a double-beam spectrophotometer considered more accurate than a single-beam instrument?
A: The second beam passes through a reference solution and corrects for variations in light source intensity, ensuring more uniform light distribution and more precise measurements.
Q: Which type of cuvette should be used when measuring absorbance in the UV range, and why?
A: Quartz cuvettes, because glass absorbs UV light and would interfere with the measurement.
Q: Differentiate between an air displacement pipette and a positive displacement pipette.
A: In an air displacement pipette, a piston creates suction via an air cushion and the piston does not contact the liquid; it uses a disposable tip. In a positive displacement pipette, the piston moves directly in the tip (like a syringe), may contact the liquid, and the capillary tip is reusable.
Q: A student notices the spectrophotometer is giving erratic readings immediately after switching it on. What preventive measure was likely skipped?
A: The warm-up period. The spectrophotometer should be turned on 10 to 15 minutes before use to allow the light source to stabilise.
Q: Name the three types of centrifuge and state one distinguishing feature of each.
A: Horizontal (swinging-bucket): tube rotates to horizontal position, max ~3000 rpm. Fixed-angle (angle-head): tube at 52-degree angle, sediments particles more rapidly. Ultracentrifuge: high-speed, refrigerated, used for lipoprotein separation.
Beer's law and the spectrophotometer connect directly to Lesson 2 (Calibration and Quality Control), where you will use absorbance data to calculate means, standard deviations, and Levey-Jennings charts. Understanding what the instrument actually measures is necessary before you can evaluate whether its output is in control.
Centrifuge technique links to specimen processing in haematology and coagulation studies: the same principles of sedimentation and specific gravity apply.
Pipetting accuracy underpins every volumetric step in clinical chemistry, from reagent preparation to sample dilution. Errors here cascade through every downstream measurement.
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