Is that band weak  or very weak?

2. Is that band weak or very weak?

The Editor

Late last year we received an e-mail from Richard Duerst asking what relationship existed between absorbance and band intensities on the old and well established very weak/medium/very strong scales. I didn’t know, but I have enquired. I have spoken to several elderly spectroscopists and compared their thoughts with my own experience.

In years gone by when the intensity scale was devised, everyone plotted infrared and other absorption spectra on a % transmission scale. If absorbances were required suitably scaled chart paper was used and the absorbance duly read off the chart. Relatively few instruments could display absorbance on a linear scale until computer outputting became common.

On the % transmission scale everyone had their own way of describing bands – the whole subject was completely subjective. However, some rough and ready agreement can be reached by comparing experience. In the figure I draw a set of peaks and label them with verbal strength descriptions. I have checked these with Bill Maddams (an author of several feature articles in IJVS and a very experienced, but now retired spectroscopist). On the RH margin I have written in the absorbance scale using the well known formula.

I  = eEcl      

       I0

Whence for a given spectrum where c & l are fixed throughout

   – Log 10      I       = 2.303Ecl        (1)
                          
  I0

The relationship between the absorbance and the % transmission is logarithmic and of course the negative sign inverts the spectrum. The conversion is that

   Absorbance = Log 10  I0                          (2)
       I

Bands which absorb ‘completely’ are always described as very strong (sometimes very, very strong), but the meaning is as impossible to quantify as it is to compare the musical descriptions ff and fff! At the other extreme, the weakest bands people used to see were limited by the noise in the spectra – noise far worse that we see today. Two percentage points was about the reliable limit so these bands were described as very weak. Below this level imagination and or a high alcohol level in the blood stream became useful.

I remember that when I was a Postgrad, we used a Hilger H800 Prism instrument. Below 700cm-1the noise was pretty awful, but my supervisor Don Powell was studying metal amine complexes. Now the nPt-N in platinum II amino complexes can be very weak indeed and Don announced one day he had “found the band”. He showed me a Honewell chart and pointed to this world stopping feature – I couldn’t see a thing! Tipping the paper up and looking alongthe trace was the trick I was told very firmly. To be honest, I still couldn’t see “the band”, but as we all know PhD Supervisors like customers are always right. Later on, Donald ‘got his act together’ and “the band” he saw was confirmed. Phew!

So we get the descriptions in Figure 1 and hence can read off the absorbance values as an equivalent scale. 

Figure 1. Graphical Representation of band strengths 
% Transmission & Absorbance

Designation

%Transmission

Absorbance

VVS
VS
S
M
MW
W
VW
VVW

>5
~25
~50
~60
~75
~85
~95
>98

>1.3
~0.6
~0.3
~0.2
~0.1
~0.07
~0.02
~0.01

Table 1. Equivalence of verbal band strengths, 
%transmission and absorbance values. 
Last two lines see script below.

As spectrometers (or to be pedantic interferometers) have such excellent stray light and S:N characteristics these days, the scale above which refers to the historic literature could well be extended to include Extremely Strong and Extremely Weak.

REF: P.J.Hendra, Internet J. Vib. Spec.[www.irdg.org/ijvs] 5, 1, 2 (2001)