E23 · Frozen specimen workflow

Cryo-SEM: Preserve Before You Observe

Can you distinguish freezing damage, frost, charging and beam damage? Yes, if you know the specimen's history, because each preparation step leaves its own kind of mark. Ice crystals mean the specimen was too thick to freeze fast enough. Frost is water vapor that condensed on the cold surface, for example during transfer. Charging points to a coating too thin to carry charge away, and beam damage grows with the dose received while imaging.

Take a specimen through freezing, fracture, sublimation, coating, transfer and imaging, in that order. Then trace each flaw in the final image back to the step that caused it.

Assigned freezing preservation $\text{—}$

Preparation structural retention $\text{—}$

Transfer frost depth $\text{—}$

Assigned beam retention $\text{—}$

More measurements and target

Remaining masking surface ice $\text{—}$

Sublimated equivalent ice depth $\text{—}$

Committed coating thickness $\text{—}$

Accumulated acquired dose $\text{—}$

Negative specimen potential $\text{—}$

Accumulated recorded exposure $\text{—}$

Selected-cell expected collected electrons $\text{—}$

Experiment target $\text{—}$

Simulated examples; each plot identifies its data source.

Freezing: 0 plunge, 1 high pressure

1

Sublimation temperature

T_e

-105\,\mathrm{{}^\circ C}

Sublimation duration

t_e

40\,\mathrm{s}

Conductive coating thickness

t_c

3\,\mathrm{nm}

Transfer stage temperature

T_s

-150\,\mathrm{{}^\circ C}

Advanced settings

Initial hydrated specimen thickness

t_s

100\,\mathrm{\mu m}

Residual water pressure multiplier

p_w

1\,\mathrm{\times10^{-5}Pa}

Cold transfer duration

t_t

60\,\mathrm{s}

Dose added by each acquisition

D

0.8\,\mathrm{\frac{electrons}{nm^2}}

Acquisition beam current

I

50\,\mathrm{pA}

Selected specimen cell

k

1176

Collected-signal display gain only

G

1

Learning goal and model limits

Freeze · prepare · transfer · acquire

Preserve what you want to see

Choose Start a new preparation, then carry out the six steps below. Target: freezing and preparation each retain at least 90 percent of the structure. Surface ice and frost are each at most 5 nm, and the coating is 1 to 8 nm thick. Acquire a nonzero dose while keeping beam retention at least 90 percent and the potential magnitude at most 5 V. Presets load finished examples.

Model assumptions

This is a teaching workflow, not a preparation protocol, and its images are not measured micrographs. Freezing preservation, coating conductivity and beam damage follow assigned stand-in formulas. Surface ice exchange uses a flat-interface law with unit accommodation, fixed reservoirs and Murphy–Koop saturation. Heat transfer, nucleation, fracture mechanics, the microstructure of deposited layers and calibrated kinetics are left out. Each step keeps the settings it was done with, and later controls cannot repair earlier damage. Sizes and hardware are schematic.

Start with the essentials

Focus question: Can you distinguish freezing damage, frost, charging and beam damage?
One-sentence intuition: The history of preparation is part of the image.

Core mathematical model

Assigned freezing preservation

P_f=\exp\!\left[-\left(\frac{t_s}{\ell_f}\right)^4\right]

The preservation depth is assigned: 10 micrometres for plunge freezing and 200 for high-pressure freezing. Thicker specimens keep less structure. The factor is a stand-in, not a probability or a promise of vitrification.

Surface ice exchange

\begin{aligned}\dot h&=\frac{1}{\rho_i}\sqrt{\frac{m_w}{2\pi k_B}}\\&\quad\times\left[\frac{p_w}{\sqrt{T_g}}-\frac{p_{\rm sat,i}(T_s)}{\sqrt{T_s}}\right]\end{aligned}

A positive thickness change means frost builds up; a negative one means ice sublimes. The gas and the specimen each keep their own temperature, and the reservoirs stay fixed.

Dose adds up with every image

\begin{aligned}D_{\rm total}&=\sum_j D_j\\B&=\exp(-D_{\rm total}/D_c)\end{aligned}

Each new image adds to the total dose, and structure fades exponentially with that total. The characteristic damage dose is assigned as ten electrons per square nanometre. Display gain does not change the dose.

Common difficulties

What the model leaves out

Typical misconceptionIf my settings pass the task, they will work on a real instrument.

Better mental modelThis is a teaching workflow, not a preparation protocol, and its images are not measured micrographs. Freezing preservation, coating conductivity and beam damage follow assigned stand-in formulas. Surface ice exchange uses a flat-interface law with unit accommodation, fixed reservoirs and Murphy–Koop saturation. Heat transfer, nucleation, fracture mechanics, the microstructure of deposited layers and calibrated kinetics are left out. Each step keeps the settings it was done with, and later controls cannot repair earlier damage. Sizes and hardware are schematic.

Run the experiment

01
Freeze and fracture
Choose Start a new preparation, select the freezing method and specimen thickness, then freeze and fracture the specimen.
What to observe: Changing the method after freezing cannot repair ice-crystal damage.
02
Reveal and coat
Choose sublimation temperature and time, commit etching, then coat the exposed face.
What to observe: Remove too little ice and it hides detail; remove too much and structure is lost.
03
Protect the transfer
Set cold-stage temperature, residual water pressure and transfer time, then commit transfer.
What to observe: Frost picked up during transfer is a separate artifact from the ice crystals formed during freezing.
04
Acquire sparingly
Acquire, check the target, then acquire again or change display gain.
What to observe: Each new image adds dose that cannot be taken back; display gain adds none.

Core mathematical model

Assigned freezing preservation

Surface ice exchange

Dose adds up with every image

Common difficulties

What the model leaves out

Run the experiment

Freeze and fracture

Reveal and coat

Protect the transfer

Acquire sparingly