JOURNAL OF RAMAN SPECTROSCOPY, VOL. 26, 325-326 (1995)

SHORT COMMUNICATION

Accurate Relative Calibration of a Multi-Channel Raman Spectrometer

Patrice Huguet and Robert Gaufres* Laboratoire de Spectroscopie Moleculaire, Universite de Montpellier 11, Place Eugene Bataillon, 34095 Montpellier Cedex 5, France

A method of restoring a given spectral position of a multi-channel Raman spectrometer is described. With a spectrometer, the wavenumber counter of which is graduated in cm-’ and the dispersion on the detector is about 0.7-0.9 cm-’ per photodiode, the standard deviation about a mean position is shown to be 0.15 cm-’ when this method is used.

INTRODUCTION

Spectral subtraction is nowadays a common procedure in Raman spectroscopy, in order, for instance, to isolate the spectrum of a component from that of a mixture. This sort of operation is often incorrectly referred to as ‘Raman difference spectroscopy’. Savoie et al.’ have per- tinently suggested that ‘Raman spectral subtraction’ should be clearly distinguished from ‘Raman difference spectroscopy’, in which both terms of the difference are simultaneously recorded. A third possibility is found in ‘indirect Raman difference spectroscopy’, a term we have recently proposed where the spectra to be com- pared are not simultaneously recorded, but each of them is simultaneously recorded with a common refer- ence.2

Raman spectral subtraction cannot be as accurate a procedure as true difference Raman spectroscopy, espe- cially if several days or months have elapsed between the recordings. In any case, in multi-channel spectrom- etry, as soon as the spectral position of the spectrometer has been changed, it is difficult to find the former cali- bration again within less than 1 cm-‘. This accuracy may be insufficient for good spectral subtraction.

In other experimental situations, the possibility of restoring as accurately as possible for a new experiment the former calibration is of prime interest. In recent papers, we have illustrated the relevance of the absolute area of a difference spectrum in various problems. In addition to its simple mathematical proper tie^,^ which may be used for the determination of spectral shifts,2 this quantity allows us to allot a number to the alter- ation of a profile. By these indirect means, we have been able to follow a conformational change as a function of temperature4 or to determine the temperature of a gas.’ An accurate determination of the absolute area of a dif- ference spectrum implies that the calibration of the spectrometer can be reproduced as accurately as pos- sible.

* Author to whom correspondence should be addressed.

CCC 0377-0486/95/040325-02 0 1995 by John Wiley & Sons, Ltd

We have also shown that, by measuring the inte- grated intensity ratio of two hydrogen rotation lines, in recordings obtained for two spectral positions of the spectrometer shifted by 1 cm-’, the numerical values are significantly different.’ As the rotational lines are fairly sharp, these discrepancies may be explained by the fact that the main part of the intensity is concen- trated on a few photodiodes, so that the profile is ill- defined and its area may be changed under a calibration shift. Local irregularities of the sensitivity of the photo- diodes should also be considered, for similar reasons. These drawbacks may be removed in this case by the use of the ‘scanning multi-channel technique’,6 but actually we need only intensity ratios for different tem- peratures of the sample, which do not depend on the definition of the profile of the lines provided that it remains the same from one recording to another.

For all these applications of Raman spectrometry, we have had to set up a method for restoring the spectral calibration of the spectrometer within less than 1 cm-’. We must emphasize that this method is in no way rele- vant to metrology, but allows one to reproduce any spectral position of the spectrometer with an accuracy better by about one order of magnitude than that obtained with the procedure generally designed by the manufacturer. In other words, it is simple procedure that any user of a multi-channel spectrometer can bring into play when good reproducibility of the relative cali- bration is required.

EXPERIMENTAL AND RESULTS ~ ~~~~

An OMARS 89 spectrometer from Dilor was used. In all the experiments it was used in the ‘normal spectral mode’, the two gratings of the fore monochromator operating in the subtractive mode. The dispersion on the diode array was about 0.7-0.9 cm-’ per pixel, owing to the spectral range. The wavenumber counters are graduated in cm-’. The motion of the gratings is driven by means of manually operated motors, with various rates available. For the calibration adjustment, the software provided by the manufacturer sets out a

Received 26 March 1994 Accepted 7 January 1995

326 P. HUGUET AND R. GAUFRES

Table 1. intermediate results in the calculation of the standard deviation from the mean position Number of the set

Parameter 1 2 3 4 5 6 7 8 9 10 11

Average shift from the 0.051 4 --0.0232 0.01 45 -0.21 45 -0.3777 0.1 602 -0.0083 -0.2985 -0.1 245 -0.1 301 -0.1 587 reference spectrum/cm-'

shifts in the setlcm-'

mean position/cm-'

Standard deviation of the 0.1 125* 0.021 4 0.0230 0.01 68 0.01 83 0.021 2 0.0236 0.01 23 0.01 16 0.01 11 0.0099

Average shift from the 0.1732 0.0986 0.2263 -0.0927 -0.2539 -0.0384 0.1 136 -0.1 767 -0.0027 -0.0083 -0.0369

" A n aberrant shift in this set accounts for the atypical standard deviation.

vertical line at the centre of the spectral field. With a 5 12-diode array, this line corresponds to the diode number 255. A reference spectral line is displayed on the monitor and the spectral position of the gratings is then adjusted in order that the profile of the line is symmetri- cally shared by the vertical line. The counters may be then adjusted to give back the wavenumber correspond- ing to the reference spectral line. However, generally, the centre of the spectral field, set by the wavelength of the reference spectral line, is inadequate for the intended Raman recording, the gratings must be moved, a new spectral position given by the wavenumber counters must be set out and the accuracy of the calibration is partially lost.

To overcome this drawback, we proceed as follows. A convenient spectral field for the intended recording is first chosen. In most cases, an emission line of a neon spectral lamp appears somewhere in this spectral field. We have developed a subroutine which allows a vertical line corresponding to any photodiode of the array to be drawn on the monitor, characterized by its number, entered directly from the keyboard. This line is chosen to be as close as possible to the maximum of the spec- tral line. The position of the gratings is then moved, at the slowest rate, to obtain a symmetrical division of the spectral line by the vertical line. The calibration of the spectrometer is then defined by the number of the chosen photodiode, and may be accurately reproduced at any time, ignoring the wavenumber indicated by the

counters. Obviously, this is a matter of relative cali- bration and not of absolute calibration, which is gener- ally known with less accuracy.

More recently, we acquired a new version of the soft- ware written by the manufacturer, which allows one to display an index at a desired pixel of the monitor, the number of the corresponding diode also being given. This facility may be used in the same way as the posi- tioning of a vertical line.

To evaluate the reproducibility of the relative cali- bration by such a method, we carried out 11 recordings of a Raman band (the v2 band of benzene), adjusting the spectral position of the spectrometer by means of a neon line (Aair = 540.056 nm). We then substantially changed the spectral position and came back to the initial position, adjusted by means of the neon line. Eleven sets of 11 recordings were performed, a new adjustment of the spectral position being made between each set. The analysis of the results was made as follows. A spectrum of the first set was chosen as refer- ence, and the shifts between this reference and each of the 120 other recordings were determined according to the method of absolute area.2 An average shift for each set of 11 recordings with respect to the reference spec- trum was obtained. Finally, the mean value of the shifts of the sets was taken as the origin, and a standard devi- ation was calculated. The main results of this calcu- lation are given in Table 1. The standard deviation from the mean spectral position was found to be 0.146 cm-'.

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