HOW TO USE THE
PICKFORD TEST SLIDE

The Pickford Phase Contrast Test Slide has been designed to be an alternative and eventual replacement for the HSE/NPL Mark II Phase Contrast Test Slide. It is patented in Australia, Great Britain and the United States of America.

The HSE/NPL Mark II Test Slide was designed 40 years ago using technology of the day, and has been invaluable in trying to standardise microscope and analyst performance around the world in relation to the estimation of airborne asbestos fibre concentrations.

The Pickford Test Slide has been designed using state of art, nanofabrication processes. It is of rugged stainless-steel construction; easier to use; based on pure phase objects designed to test an analyst and microscope’s limit of visibility; and is believed to comprise more stable optical components when compared with the HSE/NPL Mark II Slide.

Diagram 1 shows an exploded view of the Pickford Test Slide which consists of a coverslip, a special mounting adhesive, a two-piece stainless-steel slide, and four 0.5 mm thick quartz chips, each 8 mm x 4 mm that contain guidelines and 20 phase objects. Later Slides use only one chip that contains all the features of the four individual chips.

The phase objects are in the form of etched vertical grooves, 1 µm wide x 100 µm long, and spaced at 5 µm pitch – see Diagram 2.

The depths of the grooves are different for each Set ranging from 90 nm down to 5 nm which is at the limit of nanofabrication technology.

Around each set of phase objects are a series of radial and circular guidelines to assist the analyst locate the phase objects.

The most visible Set (Set A) is designed to simply show the nature of the phase objects to analysts.

Set B is equivalent to Band 5 of the HSE/NPL Mark II Slide, and the entirety of all phase objects should be faintly visible for every satisfactory analyst/microscope.

Set C is equivalent to Band 6 of the HSE/NPL Mark II Slide, and some phase objects may be partly visible or invisible, for satisfactory performance.

Set D is equivalent to Band 7 of the HSE/NPL Mark II Slide, and no phase objects should be visible for satisfactory performance.

Instructions for Use

There are two levels of instructions: ‘Brief Instructions’ designed for experienced analysts who are familiar with the use of the HSE/NPL Mark II Phase Contrast Test Slide. ‘Detailed Instructions’ are designed to assist new or less experienced analysts familiarize themselves with the application of the Pickford Test Slide.

Brief Instructions

Use the Pickford Phase Contrast Test Slide similarly to the HSE/NPL Mark II Phase Contrast Test Slide, making use of the radial and circular guidelines of the Pickford Test Slide to locate each of the Sets. Only the 40x phase objective is necessary instead of starting with the 10x objective, given the ease of locating the Sets.

The Pickford Test Slide has four Sets of phase object grooves compared with the seven blocks of phase object ridges of the HSE/NPL Mark II Test Slide, and must be used as follows:

Pickford Slide Set A is equivalent to HSE/NPL Mark II blocks 1 to 4
Pickford Slide Set B is equivalent to HSE/NPL Mark II block 5
Pickford Slide Set C is equivalent to HSE/NPL Mark II block 6
Pickford Slide Set D is equivalent to HSE/NPL Mark II block 7

Note. Disregard any small blemishes which may be present as a result of nanofabrication difficulties, as they do not affect the function of the Test Slide.

Detailed Instructions

Preliminary

Place the Pickford Test Slide on the microscope stage, and by eye use the stage controls to roughly centralise the objective on the centre of the left hand ‘chip’ immediately above the letter ‘A’ printed on the Slide.
First time analysts can use a 10x objective to start, and focus on any of the circular or radial guidelines. Then change to a 40x phase objective and check focus. (When more familiar, start off by using the 40x phase objective to locate and focus on the grid lines).
Optimise phase contrast conditions by using an auxiliary telescope or Bertrand lens to achieve Koehler illumination by centralising the field diaphragm and matching phase annuli rings in objective and condenser as usual.
Even though most analytical methods require the use of a green filter, practical experience and visibility testing show that it is not necessary, and its use is irrelevant when using the Test Slide unless required by the applicable Standard.

Set A

Adjust the stage and focus on the left hand ‘chip’ of the Slide window (Set A).
Re-position the stage aided by the radial and circular guidelines surrounding the phase objects, and completely centralise the image of the innermost circular guideline.
Focus on the Set A phase objects, which are in the form of vertical lines.
Familiarise yourself with the appearance of the 20 vertical lines, which repeat for the other Sets except they become more difficult to detect.

Set B

Retaining the 40x objective, move the stage to the left so that Set B is visible (i.e. immediately above the letter “B”), and centralise the image of the innermost circular guideline.
Without moving the stage, first focus on the innermost circular guideline.
Move your attention to the centre where the phase objects are located, focusing very finely up and down.
For satisfactory performance, it is necessary to be able to see the entirety of each of the 20 phase objects. For some objectives (eg plan-achromats, where most of the field of view is in focus), it may be possible to view each entire length without re-focusing. For other objectives, it may be necessary to continually re-focus along the length of the phase objects.
Apart from any minor manufacturing blemishes, if the entire length of each of the 20 phase objects cannot be seen, this is unsatisfactory.

Set C

Repeat Steps a to c of Set B and focus on the Set C phase objects immediately above the letter “C”.
For satisfactory performance, these phase objects can be either partly visible, or completely invisible.

Set D

Repeat Steps a to c of Set B and focus on the Set D phase objects immediately above the letter “D”.
For satisfactory performance, none of the 20 phase objects should be visible.

Corrective Action on Unsatisfactory Performance

If all of the Set B phase objects cannot be seen, the cause should be found and corrected prior to any further airborne asbestos fibre determinations as follows:

First, the microscope should be checked to ensure that it complies with the Membrane Filter Method (MFM) specifications and is properly adjusted, and

Second, if the analyst is not able to see the Set B phase objects, then appropriate corrective action should be taken before further analysis is undertaken.

If any of the Set D phase objects can be seen, the microscope should not be used as it does not comply with the MFM requirements – See Notes.

Note 1 – The Phase Contrast microscope is a qualitative instrument used in the biological field and has been adapted to airborne asbestos fibre measurement by ‘controlling’ its performance.

Note 2 – Objectives that have too high an absorption ring percentage or Numerical Aperture (NA) may be capable of increasing the visibility of finer asbestos fibres when compared with those visible by ‘standard’ microscopes used internationally for this technique.

Each Pickford Test Slide is certified by the Environmental Analysis Laboratory of the Southern Cross University as being equivalent in performance to that of the UK HSE/NPL Mark II Test Slide, as required by the UK HSE ‘Asbestos: Analysts Guide’, HSG248, 2021, Section A1.37.

HOW TO USE THE PICKFORD TEST SLIDE

Instructions for Use

Corrective Action on Unsatisfactory Performance

HAVE ANY QUESTIONS? GET IN TOUCH

HOW TO USE THE
PICKFORD TEST SLIDE

HAVE ANY QUESTIONS?
GET IN TOUCH