Debunking FTIR Myths I: "FTIR Can't Identify Things in Mixtures"

I am not normally one to trash other chemical analysis techniques. But when people mouth untruths about FTIR it raises my dander and I feel compelled to respond. I have talked to several people lately who have said, “FTIR can’t be used to identify things in mixtures. It’s easier to shoot the sample into the GC-MS instead.” I am fully aware, and I always teach in my courses, that FTIR has difficulty with mixtures. However, as I also always teach, there are strategies for getting around the mixture analysis problem, some of which I have talked about in previous blog posts. These techniques include purifying the sample, mixture analysis software, subtraction, library searching, and the “process of elimination”. These techniques make analyzing mixtures doable, and frequently some of the components in a mixture can be identified from the infrared spectrum of a sample as a result. I have identified components in mixtures hundreds of times during my career using these tools. FTIR can be used to identify things in mixtures if the analysis if performed properly.

GC-MS stands for Gas Chromatography-Mass Spectrometry. In this technique the GC purifies mixtures into their components, and then the mass spectrometer identifies each component. Thus, it appears GC-MS does not suffer from a mixture problem like FTIR. Additionally, GC-MS is more sensitive than FTIR and does a better job of handling aqueous solutions. I have in fairness sung the praises of GC-MS.

Here is why the phrase “It’s easier to shoot the sample through the GC-MS” raises my ire. A GC-MS is always more expensive, more time consuming, and more difficult to use than an FTIR. This is why so many more labs have an FTIR than a GC-MS. Also, GC-MS does suffer from a mixture problem. The retention time of a peak in a gas chromatograph is not unique to that molecule. Molecules with different chemical structures can, by coincidence, elute at the same time. Thus, there is always the possibility that what we think is a pure component coming off a GC column may in fact contain two or more components. The other dirty little secret about a mass spectrum is that it does not provide a completely unique fingerprint of a molecule. Mass spectrometry does not distinguish between structural isomers. These are molecules that have the same chemical formula but different chemical structures. For example, a mass spectrometer normally can’t distinguish ortho-, meta-, and para- xylene from each other. These three molecules are easy to distinguish by FTIR. Infrared spectroscopy can distinguish between structural isomers, and provides a unique fingerprint for a given molecule. A mixture of structural isomers would be difficult to distinguish by MS. It could thus be argued that you can’t identify things in mixtures by GC-MS either.

There is really is no victor in the FTIR/GC-MS smackdown except hopefully the truth. Both techniques have their strengths and weaknesses, and you need to have an understanding of both of these to pick the right technique for the right sample. Since FTIR is faster, simpler, and cheaper than GC-MS I believe it should the first choice for analyzing unknown mixtures.

A Certification Program for FTIR Users

FTIRs have been used for decades to perform chemical analyses including in such critical applications as the analysis of drugs, criminal evidence, and in homeland defense. The user of an FTIR must prepare the sample appropriately, choose the correct scanning parameters, and interpret the data correctly to achieve a successful analysis. In short, a properly trained human being is key to achieving accurate FTIR results. However, there are no formal proficiency guidelines or certification programs that insure the competency of FTIR users and hence the quality of FTIR results…until now.

Starting in 2010 my company, Spectros Associates, will be offering a Certified FTIR User(CFU) program. To earn the CFU certificate FTIR users will be required to take a 5 day sequence of FTIR training courses consisting of my Fundamentals of FTIR, Hands-On FTIR Sample Preparation, and Infrared Spectral Interpretation I, II and III courses. Then, you must pass a written exam consisting of multiple choice questions and several unknown spectra that must be interpreted. Only upon completing the coursework and successfully passing the exam will the CFU certificate be awarded.

The advantage of this program to FTIR users is that it will be your proof to the world that you know how to do your job well. The knowledge you obtain in earning the CFU will make your FTIR analyses faster, better, and cheaper. This will make you more valuable to your employer and enhance your career. The certificate can be held up at review time as an accomplishment, perhaps to be rewarded with a higher salary. Your certification can be put on your resume as proof to potential employers that you possess an important skill set, and might make you more employable than those who are not certified. The CFU can also be used towards the continuing education requirement that some professional societies require of their members.

There are also advantages to employers of the CFU program. By certifying their workers they can rest assured they are getting the best FTIR analyses possible. If regulatory bodies ever question the quality of spectra, employers can point out that the data were measured by certified workers following best practices. Finally, employers hiring FTIR workers can rest assured that job applicants who have earned the CFU possess the skills needed to do the job well.

Please let me know what you think of thie Certified FTIR User idea by leaving a post below or e-mailing me at bcsmith@spectros1.com .

Infrared Spectroscopy in Outer Space!

Last month NASA announced the presence of a thin layer of water and hydroxyl ions on the surface of the moon. Not one, not two, but three different satellites recorded data to support this conclusion. And guess what technology the three satellites used to detect the water? Infrared Spectroscopy of course! As many of you know from looking at the spectra of liquid water or water vapor, this molecule absorbs strongly around 3500 cm-1, or since astronomers like to use wavelength, around 3 microns. The first evidence that there was water on the moon came from the analysis of the moon rocks brought back by the Apollo missions back in the 1970s (it was watching these missions as a kid that inspired me to become a scientist). Water was found in the rocks, but it was assumed that they had been contaminated with terrestrial moisture.

In 1999 NASA’s Cassini satellite flew past the moon on its way to Saturn and detected water as well. However, at the time the data were thought to be erroneous and were not published until this year. More recently, the Moon Mineralogy Mapper aboard the Indian Space Agencies’ Chandrayaan-1 spacecraft found evidence for water on the moon. In June of this year NASA’s Epoxi spacecraft, flying past the moon on its way to a comet encounter in 2010, also detected water on the moon. For all three of these spacecraft the spectrum of the moon was measured in infrared reflectance. A dispersive grating spectrometer was used rather than an FTIR, perhaps because FTIRs have moving parts and are hard to fix billions of miles from home. The spectra are measured in single beam mode since there is not an identical twin moon available devoid of water to provide a background spectrum. However, the spectra from equatorial regions, where it is known to be drier, can be subtracted from the spectra of polar regions to hopefully remove any contribution from the moon’s surface other than OH and water. Examples of the types of spectra measured are seen above.

Now, it’s not time to pack your swim trunks and head to the moon for a dip. The water and hydroxyl are estimated to be present in concentrations of about 1000 ppm. You would have to wring dry a ton of lunar soil to obtain a quart of lunar water. The water is thought to form when energetic protons in the solar wind collide with oxygen bearing minerals on the moon’s surface. More details at these NASA websites.
(cut and paste these URLs into your browser...blogger won't let me post links!).

http://science.nasa.gov/headlines/y2009/24sep_moonwater.htm

http://www.nasa.gov/topics/moonmars/features/moonm3-images.html

Abrasive Sampling: Spectra of Intractable Samples

There is a sampling technique used on powders and solids called Diffuse Reflectance Infrared Fourier Transform Spectroscopy, or DRIFTS for short. I think some graduate student spent several months thinking up that acronym. At any rate, the technique involves using a special sampling accessory that fits into your FTIR sample compartment to bounce the light off the sample. The light is then collected and sent to the IR detector (more on DRIFTS in a later blog post). Abrasive sampling is an interesting application of DRIFTS. Some FTIR accessory manufacturers make flat metal posts that fit into their DRIFTS accessories and SiC disks with an adhesive backing. The SiC disk is adhered to the metal post, and then the disk is rubbed against the sample to abrade off sample particles. The SiC disk with particles is then placed at the focal point of a DRIFTS accessory, the light is reflected off of the sample particles, and is collected and sent to the detector to obtain the sample spectrum. The background spectrum is run on a clean SiC disk.

The attached spectrum is of the white paint on a light fixture obtained by abrasive sampling using an Alpha spectrometer from Bruker Optics (http://www.brukeroptics.com/). This spectrum would have been difficult to obtain any other way. The light fixture was firmly attached to the ceiling and so could not be taken down. In theory, one could scrape a lot of the paint off and make a KBr pellet or cast a film of the paint. However, this would damage the light fixture, involve time consuming trial and error, and may still not work. With abrasive sampling, only a small hidden part of the light fixture was scratched, and the entire measurement process took about 2 minutes. Abrasive sampling is useful for spectra of large, intractable objects such as furniture, large pieces of plastic, or anything that is simply too big to be analyzed by normal FTIR sampling techniques. The beauty of abrasive sampling is that it is fast and easy. However, the SiC scatters the IR beam a lot, so abrasive sampling spectra can be noisy. This can sometimes be dealt with by increasing the number of scans, perhaps to as many as 256.

A note on the SiC disks. I have seen labs try to save a little money by going to the hardware store, buying SiC paper, and then using a cork borer to punch out SiC disks of the proper size. As long as these disks fit into the sample cup that came with your DRIFTS accessory this should at least in theory allow you to obtain abrasive sampling spectra.

Use your imagination…what type of applications might abrasive sampling have at your company?

ATR V: Depth Profiling Redux

This is the fifth in my intermittent installment series on Attenuated Total Reflectance (ATR), the sampling technique of choice for many FTIR samples. The second post in the series introduced an equation that determines the depth of penetration (DP) in the ATR experiment, a measure of how far the infrared beam penetrates into the sample. The most recent post in this series discussed how changing the refractive index of the ATR crystal can change the DP and allow spectra to be taken at different depths in samples non-destructively, which is called “Depth Profiling”. This blog post is subtitled “Depth Profiling Redux” because altering the angle of incidence of the infrared beam to the sample, called theta, can also alter DP. Examination of the DP equation shows that theta is in the denominator, so as theta goes up DP goes down. If we had some means of varying theta we could take spectra at different depths in a sample non-destructively i.e. perform depth profiling.

Fortunately, varying theta is not difficult. By adjusting the position of the mirror(s) involved in focusing the IR beam onto the ATR crystal, the angle of incidence of the beam at the sample can be adjusted. There exist ATR accessories where changing theta is simply a matter of moving one or more mirrors. The variable angle ATR accessory I use, the VeeMax from PIKE Technologies (details here: http://www.piketech.com/products/atr.html) allows theta to be adjusted by simply moving a knob up or down. This allows you to easily fine tune theta and hence easily fine tune the DP of your spectrum. This is, I feel, superior to adjusting the refractive index to change DP because in this case only certain fixed DPs are available to us depending upon the refractive indices of the ATR crystals mother nature provides us.

The attached figure shows the spectrum of a sample of polyethylene taken using 9 different angles of incidence varying between 42 and 70 degrees. Note how the peaks stack on top of each other; the absorbances are different sizes for the same sample because the DP for each spectrum is different. Adjusting theta to perform depth profiling will be useful for any sample where you would like to know how composition changes with depth. For example, this technique can be used on polymer laminates that consist of layers of different polymers. For example, a low DP scan can measure the spectrum of first layer. A high DP scan can measure the spectrum of the first and second layers. Subtracting this top layer spectrum from this spectrum will yield the spectrum of layer two non-destructively.

Fall 2009 Training Course Offerings

The purpose of this blog is to offer my readers advice on how to improve their chemical analyses. One of the best ways to improve your analyses is to take a training course. The purpose of this blog post is to alert you to upcoming training courses I will be teaching that you can take this year. These are mostly FTIR courses, along with some offerings of my Principles of Analytical Chemistry and Principles of Organic Chemistry courses.

There are a number of training course delivery options available to you here, including public and online training courses. Please see previous blog posts for the relative advantages and disadvantages of these course delivery options. For more info about Spectros Associates FTIR courses go here: http://www.spectros1.com/2009 .

Public FTIR Training Course Series Sponsored by Spectros Associates
Sept. 21-25 2009, Minneapolis MN
This is my 5-day series of FTIR courses. You can register for as many days as you wish. For more info or to register click here: http://www.spectros1.com/2009 .
Fundamentals of FTIR Sept. 21
Hands-On FTIR Sample Preparation Sept. 22
Infrared Spectral Interpretation I Sept. 23
Infrared Spectral Interpretation II Sept. 24
Infrared Spectral Interpretation III Sept. 25

Oct. 5-9, 2009, Cleveland OH
This is my 5-day series of FTIR courses. You can register for as many days as you wish. For more info or to register click here: http://www.spectros1.com/2009 .
Fundamentals of FTIR Oct. 5
Hands-On FTIR Sample Preparation Oct. 6
Infrared Spectral Interpretation I Oct. 7
Infrared Spectral Interpretation II Oct. 8
Infrared Spectral Interpretation III Oct. 9

FTIR Courses I Will be Teaching at Scientific Meetings
Federation of Analytical Chemistry and Spectroscopy Societies (FACSS) Meeting, Louisville KY. Oct. 19-20
I will be teaching IR Spectral interpretation I&II as a single two-day course. Details here: https://facss.org/contentmgr/showdetails.php/id/1450 .

Eastern Analytical Symposium (EAS) Meeting, Nov. 18-19, Somerset NJ.
I will be teaching IR Spectral Interpretation I on Nov. 18 and IR Spectral Interpretation II on Nov. 19. You can take each course by itself or take the pair. More info here: http://www.eas.org/education/ .

Online Courses I teach Sponsored by the American Chemical Society
For more info on these courses click here:
http://www.proed.acs.org/online_courses/online_courses.cfm .

Fourier Transform Infrared Spectroscopy: Three 2.5 hour online sessions, Sept. 14, 16, &18, 11:00 AM to 1:30 PM EDT. This is similar to the Fundamentals of FTIR course I teach.

Infrared Spectral Interpretation Basic: two 3.5 hour online sessions, Sept. 29 & 30, Noon to 3:30 PM EDT. This is similar to the Infrared Spectral Interpretation I course I teach.

Infrared Spectral Interpretation Intermediate: two 3.5 hour online sessions, Oct. 27 & 28, Noon to 3:30 PM EDT. This is similar to the Infrared Spectral Interpretation II course I teach.

Principles of Analytical Chemistry: three 2.5 hour online sessions. Sept. 28, Oct. 1 &2, Noon to 2:30 PM EDT.

Principles of Analytical Chemistry II: Three 2.5 hour online sessions, Nov.30, Dec. 2 &4. 11:00 AM to 1:30 PM EDT.

Public Principles of Chemistry Course Offerings
I teach these exclusively through the Center for Professional Innovation & Education: www.cfpie.com.

Principles of Organic Chemistry: Malvern PA, Nov. 10-11, 2009. More info here: http://www.cfpie.com/showitem.aspx?productid=108 .

Principles of Analytical Chemistry: Malvern PA, Nov. 12-13, 2009. More info here: http://www.cfpie.com/showitem.aspx?productid=107 .

I hope to see you at one or more of these courses soon!

R.I.P. Norm Colthup (1924-2009)

It might seem odd for me to be posting an obituary in a blog about infrared spectroscopy, but the passing of Norm Colthup is worth noting by anyone who has ever measured or interpreted an infrared spectrum. Norm spent most of his career working for American Cyanamid, now known as Cytec Industries. In the 1940s Norm was involved in designing and building one of the first infrared spectrometers used in industry. Subsequent to this he measured a number of spectra and saw a need for summarizing the peak positions of functional groups. He then invented the now well known Colthup Chart, which summarizes the wavenumber regions where different functional groups absorb in an easy-to-read format. It is safe to say that thousands of people around the world have used the Colthup chart since Norm developed it over 50 years ago.

Norm also co-authored two seminal books on infrared spectroscopy. I have learned a great deal from his books. Norm was the 1979 recipient of the Williams-Wright Award of the Coblentz Society, presented annually to an industrial spectroscopist who has made significant contributions to vibrational spectroscopy while working in industry (I am the current chair of Coblentz Society’s Williams-Wright committee. More on the 2010 winner in a later post). Norm also received the Maurice F. Hasler Award, presented by the Spectroscopy Society of Pittsburgh to a scientist having notable achievements in spectroscopy that have resulted in significant applications of broad utility. Norm also shared his knowledge and love of infrared spectroscopy by teaching short courses, which in part was an inspiration for my career as an FTIR short course instructor. Norm will be sorely missed by a large community of colleagues, family, and friends