Bulkanalysis by GD Spectrometry

Introduction

Bulk analysis is the determination of the elemental composition of a homogeneous solid sample. For Bulk analysis with the GD technique the outer layers of the material may be automatically removed by a the sputtering process prior to analysis. This avoids misleading results due to surface contamination such as oxidation or residuals of the sample preparation process. After this 'pre-burn' the material eroded in the continuous sputter process is analysed. The acquired intensities are integrated. This process may be repeated to improve statistics of the measurement and to check a possible in-homogeneous distribution of the elements in the solid sample.

GDS can handle bulk applications like sheet, powder, fastener, pressed, and mounted samples. Even small samples like wire (0.25 mm diameter) can be accurately tested. Thin sheets (down to 0.05 mm in thickness) may be successfully analysed using a back plate and cooling puck. Thinner sheets may be analysed after gluing and pressing them on a flat metal block. The glue avoids deformation of the sheet, pressing assures good thermal contact, thus avoiding excessive heating of the sample and the glue. Just about any sample form can be characterised by glow discharge technology, providing a good vacuum seal can be achieved.

The picture shows the typical external sample mount of a glow discharge spectrometer: here on a LECO GDS850A.

First published on the web: 3 August 2007

Author: Thomas Nelis.

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Application to stainless steel

An example of bulk analysis is shown to the right: the sample is a certified stainless steel no. 465/1. The certified concentrations (mass %) are shown along with the values measured by GD-OES.

The GD-OES analysis was the average of two burns, with three measures in each burn, an RF power of 70 W, a pre-integration time of 240 s and an integration time of 15 s.

The standard deviation estimated from the 2 times 3 measurements only partly represents the uncertainty of the measured values. The uncertainties of the calibration coeficients and possible drift correction factors must be included in the estimation of the total measurement uncertainty.

The composition of typical iron and steel products is given in the following table.

No.	Product Name	Content
	Unalloyed steels	Fe, plus many elements less than about 0.1 to 0.5%, but with Mn <= 1.8%
	Alloy steels	Fe, plus many elements with high concentrations, especially Co, Cr, Mn, Mo, Ni, W, but with C <= 2%
	Iron	Fe, plus some elements with high concentrations, especially Cr, Ni, but with 2% <= C <= 4.5%

First published on the web: 6 November 2000

Authors: Richard Payling & Thomas Nelis.

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Precision/Repeatability

The are many ways of measuring precision in GD-OES. A common method is to measure intensities repeatedly at the same place. This is known as 'internal precision'. An example is shown below for a stainless steel sample (465/1) measured four times. The experimental conditions were: RF power 70 W, pre-integration time 240 s, and integration time 15 s. The experiments were performed using a JY 5000 RF, Horiba Jobin Yvon, Longjumeau.

The first column shows the element and wavelength (nm), the second column the mean intensity ratioed to Fe 386, the third column the RSD %, and finally the four individual measurements. Fe 386 and Fi (ie, total light) are shown with * because they are the direct intensities, ie not ratioed.

Another common measure of precision is: 'external precision' where the same sample is measured repeatedly in different places. For homogeneous, well-polished samples the external precision approaches that of the 'internal precision'. If the external precision is significantly greater than the internal precision on the same sample, then the sample surface may need repolishing or the sample may not be homogeneous.

Short/Long Term Stability

Instruments may change slowly over weeks or months or years. These small, long term changes can be corrected, for example, by cleaning the window in the source, or by recalibration (also called drift correction).

But an instrument should be stable in the short term, e.g. over the time it takes to make a series of measurements, such as a calibration, i.e., over a period of at least some hours.

The GD source is a highly stable source and optical spectrometers are stable devices so the combination should give good short term stability.

Short term stability is a good test of the performance of an instrument. Instruments are routinely tested for their short term stability during production. An example is shown below where the intensities from a stainless steel sample have been measured repeatedly during a day to check any changes in intensity with time:

Clearly the instrument is performing well.

First published on the web: 8 October 2000.

Author: Richard Payling

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