Water, vessels, balances and centrifuges

15 min

Choose a vessel and cleaning process that will not alter the analyte
Tell balance readability from repeatability measured with mass standards
Calculate relative centrifugal force from radius and rotational speed
Choose resistivity, organic-carbon, or plate-count checks for the water an assay needs

Try first

A serum specimen for a zinc level needs an aliquot sent to a reference laboratory for trace-metal testing. Which container do you use?

A clean glass tube from the washed-glassware shelf

The next section explains it.

A plastic tube from the general supply

The next section explains it.

A tube certified for trace-metal work, as the reference laboratory specifies

Right. The next section explains why.

I don't know yet

The next section explains it.

Get the idea

Choose a vessel that leaves the analyte alone

A container can adsorb an analyte, release additives, or let gas and vapor through. The effect depends on the material, the analyte, the contact time and the temperature.¹ Glass adsorbs metal ions, so ordinary glass is a poor container for trace-metal work. Labels such as certified-clean or low-binding are separate product properties, each checked against its own specification.

Rinse washed ware repeatedly with water of the required grade. Compare the pH of the final rinse with the pH of the incoming rinse water. An alkaline drift means detergent is left behind. Residue can shift pH, suppress enzymes and add optical background.¹

Readability and repeatability

Readability is the smallest step the balance displays. Repeatability is how closely repeated weighings of one load agree. Linearity is the response across the range. A balance that reads to 0.1 mg can scatter far more than that when drafts, vibration, static or poor leveling disturb it.¹ Checks with traceable mass standards at masses across the working range show how well it weighs.^2,3

Set the force the procedure names

RCF = 1.118 × 10⁻⁵ × r × rpm²

Here r is the radius in centimeters from the axis of rotation to the point in the tube the procedure names. The rotor manual lists it for each tube position. Radius enters the formula once. Speed enters as its square. The same rpm in a rotor of another radius gives another force, so program the RCF, or convert it to rpm with each rotor's own radius.¹

Water that suits the assay

CLSI GP40 sorts laboratory water by its use, and the laboratory validates that the water it selects suits each purpose. Each monitored parameter detects one kind of contamination.⁴

Check	Detects
Resistivity	Ionic contamination
Total organic carbon	Organic contamination
Viable plate count	Microbial burden

Storage and distribution can recontaminate purified water, so monitoring samples are taken at the point of use.⁴

References

Bishop ML, Fody EP, Van Siclen C, Mistler JM, Moy M. Clinical Chemistry: Principles, Techniques, and Correlations. 9th ed. Jones & Bartlett Learning; 2023.
ASTM International. Standard Specification for Laboratory Weights and Precision Mass Standards. ASTM E617-23. ASTM International; 2023.
Bruce SS, Possolo A, Watters RL Jr. Metrological Traceability: Frequently Asked Questions and NIST Policy. NIST Technical Note 2156. National Institute of Standards and Technology; 2021. Accessed September 26, 2026.
Clinical and Laboratory Standards Institute. Preparation and Testing of Reagent Water in the Medical Laboratory. 5th ed. CLSI guideline GP40. Clinical and Laboratory Standards Institute; 2024. Accessed September 26, 2026.

Watch one

The serum procedure specifies 1,400 × g. The swinging-bucket rotor's manual lists a maximum radius of 16.0 cm, and the procedure names the maximum radius. The centrifuge's run card says 2,800 rpm. Does 2,800 rpm give the required force in this rotor?

Take the radius: r = 16.0 cm.
The procedure says which radius applies, and the manual gives its value for this rotor.
Square the speed: 2,800² = 7,840,000.
Speed enters the formula as its square, so it is worked out first.
Calculate: RCF = 1.118 × 10⁻⁵ × 16.0 × 7,840,000 = 1,402 × g (rounded).
The constant and the radius multiply the squared speed.
Compare: 1,402 × g meets the specified 1,400 × g.
The procedure names a force, so the calculated force is compared with it.

Yes. In this rotor 2,800 rpm gives about 1,402 × g, which meets the procedure's 1,400 × g.

Your turn

Problem 1 of 3

Two rotors spin at the same rpm. One has an effective radius of 5 cm and the other 10 cm. How does the RCF at 10 cm compare with the RCF at 5 cm?

It is the same

Incorrect. RCF depends on radius as well as speed, so the same rpm gives a different force in a rotor of a different radius.

It is twice as high

Correct. In RCF = 1.118 × 10⁻⁵ × r × rpm², radius is a direct multiplier, so going from 5 cm to 10 cm at the same rpm doubles RCF.

It is four times as high

Incorrect. Only the speed term is squared. Radius enters the formula once, so doubling it doubles RCF, and doubling rpm would quadruple it.

Hint

Look at which term in the formula is squared.
Hold rpm fixed and see what happens to the product when only r changes.

Review Centrifugation

Problem 2 of 3

An analytical balance reads to 0.1 mg. The procedure requires a repeatability standard deviation of 0.2 mg or less. Ten weighings of a 20 g mass standard give a standard deviation of 0.6 mg. The balance stands beside an open door to the corridor. What do you conclude?

The balance reads to 0.1 mg, so it passes

Readability is only the size of the last displayed step. The repeated weighings scatter with a standard deviation of 0.6 mg, three times the procedure's limit.

Took balance readability as its weighing accuracy

Readability is only the size of the last displayed step. Drafts, vibration, static, poor leveling, or nonlinearity can bias or scatter results far beyond that step. Repeatability and working-range checks with traceable mass standards show how well the balance weighs.

Repeat the check with a 1 g standard, since 20 g is too heavy to judge this balance

Checks use standards at masses across the working range, and 20 g lies well within an analytical balance's capacity. A lighter standard would not remove the scatter.

The balance fails repeatability, and the cause, such as the draft, is found and corrected before it is used

A standard deviation of 0.6 mg exceeds the 0.2 mg limit. Drafts from the open door are a likely cause, and a repeat check after the cause is corrected shows whether the balance is fit for use.

Hint

Readability and repeatability answer different questions.
Compare the measured standard deviation with the procedure's limit.
Think about what an open door does to the air around the pan.

Review Balances, desiccators, and mass standards

Problem 3 of 3

A fixed-angle rotor has an effective radius of 15.0 cm at the point the procedure names. It spins at 2,500 rpm. What is the relative centrifugal force, to the nearest whole number?

Show the answer

1,048 × g

RCF = 1.118 × 10−5 × 15.0 × 2,5002 = 1.118 × 10−5 × 15.0 × 6,250,000 = 1,048 × g.

Review Centrifugation

Use it

The chemistry section is preparing reagents for a new, sensitive enzymatic assay. Its instructions require water low in organic carbon and microbes.
The water system's point-of-use resistivity monitor reads 18.0 MΩ·cm, within its limit.
The reagent glassware comes from the washer. The final rinse reads pH 8.4, and the incoming rinse water reads pH 6.8.
A new centrifuge arrives with a swinging-bucket rotor whose radius at the point the procedure names is 18.0 cm. The serum procedure specifies 1,300 × g for 10 min. The old rotor, 13.0 cm, ran it at 3,000 rpm.

Decision 1 of 3

Which evidence shows that the water suits the new assay?

The resistivity of 18.0 MΩ·cm at the point of use

Resistivity measures ionic contamination only. Water with high resistivity can still carry organic compounds and microorganisms, which the assay's instructions limit.

Took high resistivity to mean all contaminants were absent

Resistivity measures ionic contamination only. Water with high resistivity can still carry organic compounds or microorganisms, which total organic carbon testing and plate counts detect, and these can interfere with sensitive assays. Monitoring at the point of use shows what the assay receives.

Organic carbon and plate counts on water sampled at the purifier's outlet

Storage and distribution can recontaminate water after the purifier. A sample from the point of use shows the water the assay receives.

Organic carbon and plate counts on water sampled at the point of use, compared with the assay's requirements

Total organic carbon detects organic contamination and plate counts detect microbes, the two things the instructions limit. Sampling at the point of use tests the water the assay receives.

Review Reagents and laboratory water

Decision 2 of 3

What do you do with the washed glassware?

Rinse it again with water of the required grade until the final rinse pH matches the incoming water

A final rinse at pH 8.4 against incoming water at pH 6.8 is an alkaline drift, the sign of detergent left on the glass. Residue can suppress the enzyme and add optical background, so the glass is rinsed until the drift is gone.

Use it, since it looks clean and dry

Detergent residue is invisible. The rinse pH shows it is there, and it can shift pH, suppress enzymes and add optical background in the assay.

Dry it in the oven to drive off the residue

Oven drying leaves the detergent on the glass. Rinsing with water of the required grade removes it.

Review Containers, cleaning, and compatibility

Decision 3 of 3

What do you program for the serum spin on the new centrifuge?

3,000 rpm, as on the old centrifuge

At 18.0 cm, 3,000 rpm gives 1.118 × 10−5 × 18.0 × 3,0002 = 1,811 × g, about 40% more force than the procedure specifies.

Copied rpm to a rotor with a different radius

RCF depends on radius as well as rpm, so the same rpm in a rotor of different radius gives a different force. Copying a speed between rotors under- or over-spins the specimen and changes the separation. Convert the procedure's RCF to rpm with each rotor's own radius.

4,150 rpm, the old speed scaled up by the ratio of the radii

A larger radius already gives more force at the same speed, so the speed has to go down. Scaling it up drives the force far past 1,300 × g.

1,300 × g, which is about 2,540 rpm in this rotor

Programming the procedure's force works in any rotor. Converted with this rotor's 18.0 cm radius, 1.118 × 10−5 × 18.0 × 2,5402 = 1,298 × g.

Review Centrifugation

Each decision rested on a measurement matched to its question:

Point-of-use organic carbon and plate counts judge water for an assay that limits organics and microbes. Resistivity covers ions only.
A final rinse more alkaline than the incoming water shows detergent left on the glass.
The new rotor's own 18.0 cm radius sets the speed. The old rotor's 3,000 rpm gave 1,308 × g only because its radius was 13.0 cm.

Keep

Sources checked September 26, 2026

Add your earlier progress to your account?

Study progress is waiting to be saved

Water, vessels, balances and centrifuges

Try first

Get the idea

Choose a vessel that leaves the analyte alone

Readability and repeatability

Set the force the procedure names

Water that suits the assay

Watch one

Your turn

Use it

Results

Keep

The rest of this step