How to specify a spectrometer

Tuesday, October 30, 2018

Know the application of the analytical instrument

Specification of design qualification
(DQ) is highly important and often underestimated. Through the DQ, a customer can ensure that the instrument has the necessary functions and performance criteria to be used for his or her application. A proper design qualification will also help to separate the important from the less important features.

To determine design requirements, it is helpful if customers can specify the application they want to use the spectrometer for. Ask yourself: What is the primary function or the primary problem I want to solve? Do I need to test fuels for material specification? To optimize my blending process? Or to comply with governmental or transport regulations? Specifications should follow this purpose, and reflect the user requirements. 

Design qualification should also include very basic design questions. For example,  should a fuel truck driver be able to measure fuel quality with the instrument? Or do you purchase the analyzer for a trained spectroscopist? The requirements for these user groups will be fairly different.

The core purpose

A first step for specification is to find the purpose. Spectrometers can measure many liquids, but a qualified spectrometer is already set up to fit a specific purpose. For example, a Grabner fuel analyzer is already set up to measure gasoline, diesel and jet fuel. It is not set up to measure waste water or solvents.

Still, the instrument is able to measure 100+ fuel parameters right from the spot by using proven and robust fuel data from sources all around the world for analysis. Not all of these parameters will be relevant for your application, typically a limited set of parameters needs to be analyzed. So it is important to find out if the relevant parameters are covered in the standard setup or if a method for a new parameter can be added to the analyzer.

It is also important to define which type of fuels you want to use within the analyzer. Your instrument might be able to measure finished gasolines, but can you also test non-regular fuels like Naptha or base gasoline without oxygenates?

There may be some modifications required within the basic analyzer setup. Your supplier should be able to help you with setting up a dedicated Naphta database to improve your results. 

Specifying Operation and Performance of a FTIR-analyzer

Operational Qualification (OQ) is far more difficult to verify and to specify than the design qualification. Performance criteria determines which technical aspects the instrument must fulfill to be able to achieve the required operational performance. For example, operation requires that an instrument uses less than 25 mL sample for a typical run. For the reasons of speed and accuracy, it may be required to use a FTIR analyzer with an optical resolution of 4 cm-1. For a good signal and reliable absorbance detection, the instrument should have a Ge-coated KBr as material for the beamsplitter. The cell should be made of ZnSe and the light source should have at least 1000°C. To quantify FAME in Diesel, the instrument should use a 100 µm cell as this is required by Diesel specifications.

During OQ, performance requirements must be testable, when using a calibrated instrument and traceable standards. A requirement may be to check to see if the instrument is able to achieve repeatability according to ASTM D5845 oxygenate specifications when 4% EtoH concentration is measured in gasoline. Or if the limit of detection complies with the requirements of EN 238 benzene requirements during daily analytical use.

Specify sensible criteria

As said, a good specification is designed along the purpose of analyzer use. This also includes technical specifications. Often, specifications look to good to be true or are irrelevant to the job they are intended for. Below are examples of specifications that are not related to the purpose.

Example 1: A customer is specifying a Limit of Detection (LOD) of 1g/L for Aniline measurements for an FTIR spectrometer using a 100 µm cell. In contrast, the relevant goverrnmental standard acknowledges, that spectrometers will not be able to reliably identify or quantify Aniline content below 3g/L. 

Example 2: When optical resolution of a mid-infrared analyzer is discussed, usually the perception prevails: The better the resolution, the better the result. But does a resolution of 2 cm-1 have advantages above a resolution of 4 cm-1? Could be, as component peaks that are similar to each other can be differentiated more easily (FAME and FAEE). On the other hand, another fact holds true: The better the resolution, the higher the spectrum noise will be. Thus, the optimal resolution allows for a robust identification of fuel components in mid-IR spectrum and reduces baseline noise to a reasonably low level. 

Specify your requirements 
The number of available spectrometers is increasing and the instruments are becoming more and more complex. Thus it is important to extend specifications on supplier requirements. Ideally, the sales person of the supplier works as trusted adviser for the customer and helps with finding meaningful applications and specifications. He enters into the discussion with the customer to find out:

• What application the analyzer is used for
• Which specifications are required to achieve required performance
• If an existing analyzer setup has to be modified to comply with these specifications
• Which limits cannot be achieved



At Petrolab Company, We offer demos at a customer site and runs tests, perform IQ and OQ with the customer and provide overall training and after sales support, answer application questions and aid with basic trouble shooting through after sales support processes.