Spectrophotometer calibration is a very important part of any lab’s routine.  You need to be sure the data that is being recorded is 100% accurate.  This is where wavelength calibration using a holmium oxide filter comes in.  First, I want to go over the history of holmium glass and explain why a holmium oxide filter is so important.

Beware of Self-Calibrating Machines

Something that people should be aware of is that some machines come with a system calibration built in.  Now this may work to some degree for photometric accuracy, but it does not work for wavelength accuracy.  A machine may have wavelength accuracy feature built it, but it only checks two wavelengths at best.  A holmium oxide filter is absolutely needed to get the full range of values needed to fully validate a spectrophotometer. Moreover, independent validation by NIST-traceable calibration standards is required by most ISO and GMP laboratory guidelines and internal calibration alone with lead to a noncompliance.

History of Holmium Glass

Holmium glass has been reliably used for spectrophotometer calibration for over 40 years.  You want a robust material that will give you consistent results year after year.  This material has proven to have no spectral variations making it the perfect choice for this type of calibration. 

NIST has given approval to use holmium glass material as a calibration standard back in the 1970s.  Throughout this huge time span, NIST has published results stating that, they have never seen any spectral shifts of the certified values for this material. 

On top of that, in the NIST document published in 2007 by David W. Allen (see link here), Mr. Allen writes “Based on the extensive experience that NIST has with this material and its long-term stability, NIST will no longer recommend the recertification of these standards.  Wow that is amazing!  For NIST to come out and make a statement like this is something I’ve never seen before.

So now we can see that holmium glass is amazing for calibration standards, so let’s go ahead and talk about why someone needs a holmium oxide filter.

What does a Holmium Oxide Filter Do?

A holmium oxide filter allows a spectrophotometer user the ability to scan a wavelength range from 200-700 nm.  This range covers the ultra-violet (UV) and the entire visible (VIS) range.  When the machine is set to wavelength scan mode, benchmark peaks that are unique to holmium glass will appear on the spectrophotometers’ display. 

These peaks need to be compared to what is on the certificate of calibration from the manufacturer of the calibration standards.  Keep in mind that not all machines show every peak on the certificate.  If some peaks do not show up in your scan, you need to contact the manufacturer of your spectrophotometer.

Step-by-Step Guide to Using a Holmium Oxide Filter

What we are going to go over can be viewed in the video below.  (Please give it a thumbs up!)

• Step 1:  Turn on your spectrophotometer and let it warm up for 45 minutes.
• Step 2:  Set your machine for Wavelength Scan mode.
• Step 3:  Set the scan range for 200-700 nm.
• Step 4:  Perform a baseline scan with nothing in the cuvette holder.
• Step 5:  Insert the WAV-1 UV/VIS holmium glass filter or wavelength filter of your choice.
• Step 6:  Run the scan.
• Step 7:  Compare the results from the spectrophotometer to the data on the certificate of calibration.

If you have any further questions, please leave us a comment below or send an e-mail to info@fireflysci.com.

Here’s to your success!

The FireflySci Team