Preparation and Maintenance of Instrument

There are several different designs of GDOES instruments available on the market and they range in size from small bench-top units to large laboratory systems, depending mainly on the focal length of the spectrometer. The largest GDOES instrument ever built, one of the first but no longer made, had a focal length of 2.5 m. Today, spectrometers vary between 150 mm and 1 m in focal length. Despite these variations in size, they all have many features in common (shown in block form in Figure 1.1):

• glow discharge source

• optical spectrometer(s)

• vacuum pump(s)

• electronics

• computer and printer

• services: electricity, ultra-high purity argon

Some have additional services: water cooling, high purity nitrogen (for spectrometer), compressed air (for pneumatic valves) and a computer network/Internet connection.

Before beginning work on an instrument, the first thing to do is to check that each of these components is up and running. Walk around the instrument. Check that the area is tidy and the cables are connected. Check the services are working: electricity, gas(es) and cooling water. If necessary, check that the vacuum pump(s) is working. Check that the GD source is clean and ready and the computer is on. Run a common bulk sample for several minutes to 'condition' the source, i.e. warm and clean the source through normal operation. If necessary, check the alignment of the spectrometer(s). The instrument should now be ready.

Glow discharge spectrometers, like most analytical instruments, need regular maintenance and preparation for measurements. So, besides this initial inspection, consider regular maintenance checks. The purpose of maintenance is at least two-fold:

• to avoid serious trouble with the instrument and

• to avoid bad/wrong measurements

Checking the instrument before starting the analysis not only avoids trouble during the measurement, but also helps the operator to concentrate on the task of getting excellent results, as it liberates the mind from constant worries such as:

• Is my computer system stable?

• Is there enough argon?

• Do we have a leak in the source?

It is beyond the scope of this book to give detailed instructions for the maintenance of all instruments currently available on the market. For details on routine maintenance specific to your instrument, refer to the user's manual supplied with your instrument. In the following, we will provide general advice on maintenance procedures that will help save time in the long run and improve the overall quality of the analyses.

1 Before Starting Measurements

We will now consider preparation in more detail. Before starting to operate your instrument, make sure the computer and the software are in stable condition. This implies:

• if necessary, restart the system

• check the available space on the hard disk regularly

• archive the data you definitely do not want to lose

Checking the different supplies, such as gas, water and electricity, before starting analysis should become a habit. The argon supply pressure must be in the range specified by the instrument manufacturer. A different argon supply pressure may seriously affect the performance of the argon pressure regulation in the discharge chamber. When using gas bottles, be aware that the last 10% of the bottle may no longer meet the quality required. When replacing argon bottles, avoid getting air into the argon gas line.

Make sure the cooling system(s) is operating properly:

• check the water supply for open (non-return) systems or

• check the level of the cooling liquid for closed (recirculating) systems

• check for particles in the closed cooling liquid; if necessary replace the liquid

Make sure no power failure has occurred since you last operated the instrument, as this may seriously affect the condition of the instrument. For example, a long time without power may have affected the thermal regulation of the spectrometer, so the instrument may need time to stabilise. A common guide for larger, heated spectrometers is 1 hour stabilising time, up to a maximum of 8 hours, for each hour without electrical power.

When the instrument has not been used for some time, it is generally recommended to run a few dummy samples, before optimal stability can be achieved. This effect may be more or less important, depending on the design of the instrument.

2 Routine Checks

Perform regular routine analyses on samples of known chemical composition. If your instrument is used for depth profiling, these samples should include homogeneous reference materials as well as coated materials (see, e.g. Figure 1.2).

The purpose of these tests is to assess, verify and finally establish in written or recorded form the current performance of the instrument. They also help to detect possible malfunctions of the instrument, prior to analysis. Another result of routine checks on samples of known composition is the possibility of measuring the long-term precision and accuracy of the instrument for a given application, and the frequency required for drift corrections.

For bulk analysis, we suggest you analyse two different homogeneous samples, one a low-alloyed material and the other a high-alloyed material. The chosen materials should have a large number of specified elements. In particular, the elements contained in air, namely N, O, C and H, should be checked to make sure no air leak has occurred, as only a small amount of air in the discharge chamber can drastically change the nature of the glow discharge plasma, for all elements.

Analysing a coated sample allows you to monitor the sputtering rate and depth resolution. The sample used for routine checks should therefore be homogeneous in coating thickness and should have clearly defined interfaces between the different layers.

3 Monitoring Key Parameters Over Time

To monitor the performance of the instrument and to detect malfunctions before they become significant in the analytical results, the following key parameters should be monitored over time:

• intensities

• source impedance

An example is shown in Figure 1.3. These charts allow quick troubleshooting and can be used to plan preventative maintenance, thereby reducing unscheduled downtime.

Monitoring the intensities of the elements, N, O, C and H, overtime, if they...