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• Ocular micrometer disc: This is placed inside the objective (10×). This has calibration
without any unit. It is matched with the stage micrometer to provide the measure of
each division.
• Stage micrometer: The stage micrometer is a calibrated micrometer slide that is used to
calibrate the ocular.
• Lens paper
• Immersion oil
Procedure
1. Put the high-power objective (100×) in place. If your microscope is not par focal (i.e.,
when the objective remains more or loss in focus when changed from a low-power objective to a higher power objective), raise the nose piece before changing to the higher
power objective and re focus. Before changing the objective make sure that the object
examined is in the middle of the eld, so that it is not lost after changing the objective.
2. Take out the eye piece. Unscrew the eye lens of the ocular.
3. Place the ocular micrometer with the engraved scale face-down in the ocular. Use lens
paper to handle the micrometer disc. Do not touch the lens.
4. Replace the lens carefully.
5. Place the ocular with the micrometer in the ocular tube of the microscope.
6. Put the calibrated stage micrometer on the stage of the microscope and focus on the
scale. You should be able to clearly distinguish the 0.1- and 0.01-mm subdivisions. Adjust the stage micrometer so that the 0-mm line coincides with the 0-mm line of the
ocular micrometer.
7. Look for another set of lines where the scale of the stage micrometer coincides with that
of the ocular micrometer. This set of lines should be as far away from the 0-mm line as
possible (Figure 4.38b). The distance between the two coinciding sets of lines varies,
depending on the magnication of the objective of the microscope. Count the number
of 0.1-mm subdivisions of the stage micrometer scale between the 0-line and the second
set of coinciding lines.
8. Count the number of subdivisions of the ocular micrometer scale between the 0-line and
the second set of coinciding lines.
9. Calculate the proportion of a millimeter that is measured by a single ocular unit using
the following formula:
Medical Laboratory Technology: Volume 1
Example
With a high-power objective (40×), the calculation is as follows
Note Corresponding objectives should not be exchanged for a calibrated objective, but must
be separately calibrated. The ocular containing the micrometer disc should be stored until
required. Each microscope that is to be used for measuring the size of organisms must be
individually calibrated.
Routine care and maintenance of a microscope
A microscope is an expensive and delicate piece of equipment which, if properly maintained,
can render long years of service.
Introduction to Laboratory Equipment and Basic Laboratory Operations
109
Cleaning of Lenses
Lenses are cleaned in a special way in order to avoid dust-scratches. If the lens is taken out for
cleaning (eye piece or objective) choose a place which is free from air draft and is not dusty.
The eye piece is pulled out while the objective is unscrewed from the nose piece. During
cleaning, rst blow o the dust particles from the surface with the help of a rubber bulb
or paintbrush. Rubbing with a lens paper or toilet paper or with a soft old non-uy cloth
follows this. Use chamois leather, if it is available. While cleaning lenses breathing on it by
mouth is a good practice but do not clean it by spiing or blowing on it by mouth and never
touch a lens or mirror with the ngers.
The oil-immersion objective may require more frequent cleaning. Make sure that there is
no immersion oil left on the oil-immersion objective after use. Use clean soft tissue paper for
removing the oil by repeated gentle rubbing on the surface. Move the cloth across and not
circularly. The optical surface may be nally cleaned with a special solution, consisting of the
following:
• 90% petroleum ether (boiling point 60–80°C)
• 20% 2-propanol
Caution Do not use 95% ethanol, xylene or toluene for cleaning the lenses, since these substances dissolve the cement. The dripping solvent may seep in and dissolve the cement holding the lens in the socket. If at all, soak a piece of soft tissue in the solution and rub the paper
lightly and gently on the surface of the lens. They can, however, be used for cleaning mirrors.
A piece of chamois leather or a non-uy rag is a preferred choice over toilet paper or tissue
paper.
• By viewing from the side, a complete cleaning of the lens surface can be checked
(Figure 4.39).
Precautions
The following precautions must be observed at all times while handling the microscope:
• Carry the microscope by holding its limb with one hand with the other hand under the
foot rest (Figure 4.33). Never swing the microscope while carrying it.
• Never dip the objectives in organic solvents, as this might cause the lenses to become
detached.
• Never use ordinary paper to clean the lenses. It will scratch.
• Never touch the lenses with your ngers. It will leave nger prints.
• Never clean the support or the stage with xylene or acetone.
• Never clean the lenses of the eye pieces and objectives with cloth or paper. This might
remove the anti-reecting coating. Use soft camel hairbrush, a ne paint brush or a
blower instead.
• Before storing the microscope after the day’s work, clean the lenses.
• Never leave the microscope without the eye pieces unless the openings are plugged.
• Never press the objective on to the slide, since both the slide and the objective might
break. Take care when focussing the microscope.
• Keep the mechanical stage clean.
• Do not dismantle the optical components, as this might cause misalignment. The optical
surfaces should be cleaned with lens cleaning tissue or soft tissue paper.
• Never put the microscope away with immersion oil on the objective. Remove any oil on
the lenses. This must be done every day after work. Mild soap solution is suitable for
most cleaning.
• Use organic solvents only in accordance with the manufacturer’s recommendations.
• When changing the bulb, avoid touching the glass with your ngers, as ngerprints
reduce the intensity of illumination.
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Medical Laboratory Technology: Volume 1
Figure 4.39 Maintenance of microscope: (a) Eye piece and the objective must be kept clean and
protected from dirt, (b) Never leave the oil on the oil-immersion objective when the
microscope is stored, (c) While viewing through the microscope if you see a dirty eld, turn
the eye piece and see if the dirt is moving, (d) If it moves, take out the eye piece and clean.
Always keep the microscope covered, (e) Never take out the objective without covering
the tube, (f-i) If the objective looks smudged or dirty against reected light, clean it with
lens paper soaked with xylol and nally with dry lens paper; do not use alcohol or spirit for
cleaning lenses, (j) Soft camel hair brush is recommended for cleaning the objective
• To maximize the lifespan of bulbs, adjust the voltage with a dimmer switch to give the
lowest required light intensity.
• If the mains voltage uctuates excessively, use a voltage stabilizer.
• In hot dry climates, the main problem is dust. Avoid all dust accumulation by keeping
the microscope in an airtight plastic cover when not in use.
• At the end of the day’s work, clean the microscope thoroughly by blowing air over it
with a rubber bulb. Also wipe o dust from the lens surface with a soft camel hairbrush
or a ne paintbrush or a blower. If dust particles remain on the surface of the objective,
clean it with special lens tissue paper. The microscope must be cleaned daily to get rid
of the dust, its worst enemy.
Introduction to Laboratory Equipment and Basic Laboratory Operations
• Never allow direct sunlight to fall on the microscope; put a plastic cover on it when not
in use during the day. A common way of storing it is under a bell jar resting on a glass
plate with a sealed edge (use grease), and with a desiccant inside the bell jar. Alternatively keep it inside a heated cupboard (heat the cupboard with a 40 W lamp). Never
store the microscope in its wooden box.
• Areas with high humidity might grow fungi on the microscope, lens surfaces, grooves
of the screws and under the paint. This might render the instrument useless. To prevent
this from happening, always keep the microscope in airtight plastic cover when not in
use. Inside the cover keep a dish lled with blue silica to dry the air under the cover.
(The silica turns red when it has lost its capacity to absorb moisture from the air. It can
be simply regenerated by heating in a hot air oven or over a re.)
• While working with the microscope, do not pull out the object-slide from the stage with-
out swinging out the oil-immersion objective; the slide might scratch the objective. Do
not use the oil-immersion objective on a wet mount. If this is essential, use a cover slip.
Also remember not to push the oil-immersion objective through the slide; it might crack
both the slide as well as the objective. It is easy to prepare another slide but dicult to
replace an expensive oil-immersion objective.
• Heavy contamination can be removed with mild soap solutions. Grease and oil can be
removed with the special cleaning solution of distilled water and 95% ethanol (50:50).
Caution This is not suitable for cleaning the optical surface. The microscope must be
cleaned daily.
• The mechanical parts (coarse adjustments screw, ne adjustment screw, condenser
focussing and mechanical stage) should be periodically cleaned and lubricated with
machine oil to make them run freely.
111
Centrifuges
Centrifuges are devices by which a suspension of solid material in liquid phase is spun at
high speed in order to separate the liquid phase from the solid phase. The principle of centrifugation is that when a body is rotated in a circular movement at a high speed, it creates a
force that drives the body away from the centre of the circular movement (Figure 4.40). This is
called centrifugal force. At the time of centrifugation, the centrifugal force of spinning pushes
the solid particles of higher density ‘outwards’, which pack in the narrow boom of the centrifuge tube and form a pellet. The packing of the solid particles facilitates the separation of
the supernatant which is then decanted out.
Types of centrifuges
The common laboratory centrifuge is used for the separation of serum, precipitates and sediments of various body uids. The haematology laboratory, however, uses a dierent kind of
centrifuge for determining haematocrit, while the blood bank uses the high-speed angle-head
centrifuge. All centrifuges must be properly used in order to ensure longer years of service.
Most modern centrifuges are electrically operated. Hand-driven (manual) centrifuges are
seen in areas where either electricity is not available or the supply of electricity is not dependable. Hand centrifuges (Figure 4.40) can hold only two to four centrifuge tubes and run at a
slow speed. The most common uses of hand-driven centrifuges are to obtain urinary sediment for microscopic examination and for the concentration of parasites in faecal material.
They are not as safe as the electrically operated ones.
The two most common types of centrifuges are table-top model and oor model. The oor
models are large, often with the arrangements for refrigeration and are seen in the blood
bank. The table-top laboratory centrifuges can be broadly classied as: (a) free-oating type or
horizontal type [Figure 4.40 (b–d)], and (b) angle-head type (Figure 4.41b). In the free-oating
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Medical Laboratory Technology: Volume 1
Figure 4.40 (a) Principle of centrifugation, (b and c) Free-oating centrifuges – electrically operated,
and (d) Hand operated
type, the centrifuge tubes holding the material to be centrifuged stay in a vertical position
when the centrifuge is at rest (Figures 4.42); but assume the horizontal position when the
centrifuge revolves. As a result, the sediment surface stays in a straight line at right angles
to the centrifuge tube wall. The free-oating type centrifuge does not hold more than four to
eight centrifuge tubes (15 mL) and the maximum speed that it can aain is about 1800 rpm
(or 2000 G). In the case of the angle-head centrifuge, the tubes are held in a rigid position at a
xed angle of 45°. Either having the trunnions xed at an angle or using a cone-shaped solid
head does this. The angle-head centrifuge holds more centrifuge tubes, can run at a higher
speed, the sediment is laid at an angle and there is also less chance that the sediment will
be disturbed when the centrifuge stops (Figures 4.42 and 4.43). The angle-head centrifuge,
which can aain high speed in a short time, is preferred in the blood bank in order to observe
haemagglutination.
Introduction to Laboratory Equipment and Basic Laboratory Operations
Figure 4.41 (a) (a) Balance the opposite-side-sockets before starting the centrifuge, (b) The tubes
need not be identical. An unbalanced centrifuge (by weight) will vibrate and may
cause breakage of the centrifuge tubes and damage to the centrifuge head.
113
Figure 4.41 (b) Swing out packing as compared to angle head packing
Components of a clinical centrifuge
The clinical centrifuge (Figure 4.42) has three basic components—the central shaft which rotates at high speed and is driven by a hidden motor; the head which is xed to the shaft and
carries the centrifuge tubes in a bucket or cup or hole and the chamber (bowl) in which the
shaft with the centrifuge spins. Electrical centrifuges have a rheostat to regulate the speed and
some of them are provided with a timer or a tachometer which measures the rate of spinning.
The centrifuge has a lid, which must be closed during the running of the centrifuge. It is a
good practice to keep the lid closed all the time, even when the centrifuge is not working.
The centrifuge tubes are either made of glass or of plastic; they should be able to withstand
the stress during centrifugation. Make sure that the soft rubber cushions are present at the
base of the sockets. If they are missing, replace them promptly. Their absence might cause the
centrifuge tubes to break.
114
Figure 4.42 Swing motion of (a) free-oating and (b) angular-head centrifuges, (c and d) The tubes
show their respective packing of sediment
Medical Laboratory Technology: Volume 1
Procedure of centrifugation
1. Place the centrifuge on a skid-free padding or rubber cushion so that it does not slide
away. Make sure that the centrifuge is away from the edge of the table. Also check the
soft rubber cushions placed at the boom of the sockets before running the machine.
While working with infectious material, keep the centrifuge inside the hood and keep
the sockets closed with a lid.
2. Label the centrifuge tubes before lling in the uid to be centrifuged. Use a wax pencil
and make sure that the number does not rub o.
3. The centrifuge must be balanced before running (Figure 4.41a). In other words, the centrifuge tubes put in opposite sockets must be of equal weight. Failure to balance the load
in a centrifuge can lead to severe vibration of the centrifuge with possible loss of samples and hence shortens the life of the machine. Ordinarily, balancing is done by taking
equal volumes of uids in a pair of centrifuge tubes, and they are put in opposite sockets. If the number of centrifuge tubes with specimens is odd, take an empty tube and ll
it with water to match. If the uids are of dierent densities, matching by volume is not
recommended; weigh the tubes on a physical balance before puing them in opposite
sockets.
4. Close the lid and start the motor. Never run the centrifuge without closing the lid.
5. Gradually increase the speed until the desired speed is reached. Put on the timer if it is
provided with the centrifuge or use an alarm clock.
6. Stop the centrifuge after the desired period. Let the centrifuge stop gradually. If brakes
are provided, use them only sparingly, or else the sediment will be disturbed. Never try
to stop the centrifuge by holding the shaft.
7. Remove the tubes slowly and carefully without disturbing the pellet.
Maintenance of centrifuge
Maintenance of the centrifuge should be carried out according to the manufacturer’s
directions. If lubrication has to be done on a regular basis, include this in the maintenance
schedule. The centrifuge and the work area around it should be kept clean at all times.
Check that the soft cushions at the base of the sockets are in place before running the
centrifuge. Never take the cushions out unless for cleaning. Glass tubes break due to the
pressure on the metal. Cracked or damaged centrifuge tubes should not be used. They may
not be able to withstand the stresses of centrifugal force and valuable samples can be lost.
Whenever a centrifuge tube breaks inside the centrifuge cup, it is most important that both
the cup and the rubber cushion in the cup be cleaned well to prevent further breakage by
glass particles left behind. After cleaning the bucket (or socket), make sure that the cushions
Introduction to Laboratory Equipment and Basic Laboratory Operations
115
are back. Clean the bowl regularly with phenol-soaked absorbent paper. In case of breakage
of centrifuge tubes containing microbiological specimens, the bowl of the centrifuge must
be treated with a disinfectant (5% phenol or Lysol). While cleaning, wear a pair of gloves
and disinfect the gloves after the clean-up. Soak the cup in phenol water for 1-2 h before it
is washed.
Standardization of centrifuge
Centrifugal force is the principal factor that determines the separation of the solid from the
liquid phase in a suspension. The centrifugal force depends on speed as well as on the size
of the centrifuge head. The usual expression of revolutions per minute (rpm) gives only the
centrifugation speed and does not actually express the centrifugal force. The centrifugation
speed and the centrifugal force are, however, directly proportional.
Formula to convert RPM of centrifuge to RCF or g-force (or G-force)
The force exerted on a particle in a centrifuge is a simple function of the rotation speed of the
centrifuge and the radius of rotation (R).
The actual equation is:
RCF or G-force = 1.12 × R × (rpm/1000)
Here, RCF is the relative centrifugal force or g (G);
R = radius of rotation measured from center of the
pivot to the boom of the centrifuge tube (It is 240 mm
in Figure 4.43a).
Where, R = radius in millimeters (mm), rpm =
revolutions per minute (shown on the tachometer
provided with the centrifuge)
The use of a nomogram (Figure 4.43b) is more
convenient for determining the RCF if the radius of
the centrifuge head and the rate of spinning (rpm) are
known.
Example r = 9 cm; RCF recommended for the test = 1500 G; the rpm to be set is equivalent
to about 3700, which is found out by laying a straight edge that touches 9 cm and 1500 G and
passes over 3700 rpm.
The time of centrifugation is an important consideration in applying centrifugal force and
is the period of time required to move the heavier particles to the boom of the tube before
the lighter particles, at the requisite speed. Hence, the tubes are spun for a specied period
to obtain the desired eect. If the centrifuge is unable to accomplish the set goal, the time of
centrifugation is changed. For example, if the haematocrit value of normal blood (male) is
found to be 52% for the centrifuge in use, increase the time until the standard value of 47% is
reached. Keep this time as constant for comparing with the test specimens.
2
Figure 4.43(a)
Balances
A balance is an important instrument in the laboratory which measures the weight of a substance. Balances are of dierent sensitivities depending on their use. The physical balance
is less sensitive than the analytical balance. For routine laboratory purposes, the sensitivity
of a balance can be considered to be the smallest mass that can be weighed accurately. For
example, a physical balance may require more weight to move the pointer than the analytical
balance.
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Medical Laboratory Technology: Volume 1
Figure 4.43(b) (a and b) Common laboratory centrifuge with angular head, (c) Distribution of particles
in suspension, (d) Eect of centrifugation on the sedimentation of the particles, (e)
Use of nomogram for G value; connect the value of the radius (cm) and the revolution
per minute; alternatively use the formula
Most of the balances in current use in developing countries are the old-fashioned dou-
ble-pan balances (Figure 4.44). The substance is put on a pan which is counter-balanced by
known weights on the other side of the pivot. Rider is used to add smaller weights. In recent
years, single-pan balances have been introduced which are replacing the older double-pan
balances (Figure 4.44). These have internal counterweights, which are added or removed by
turning a knob on the outside of the balance case. More improved balances have a digital read
out and electronic operation of the weighing process.
Introduction to Laboratory Equipment and Basic Laboratory Operations
117
Figure 4.44 Dierent types of analytical balances: (a and b) Double-pan balances, (c) Single-pan
automatic electric balance
Physical balance
Physical balances are used for relatively crude weight measurements with accuracy up to
10–100 mg. They are faster and easier to weigh on and are cheaper than analytical balances.
The triple beam balance is more common in clinical laboratories (Figure 4.45). It has the
advantage of not requiring a large set of weights like the older type of double-pan balance.
Components of a triple beam balance
The base of the balance holds the pan (Figure 4.45), the beams and the pillar. The pillar is
located on the side of the balance which has a ‘0’ mark for tarring and balancing. The container with the weight of the substance on the pan balances the weights on the beams. The
pointer swings on the pillar of the balance and when rested to ‘0’, indicates that the weight