Precision of Infrared Spectrometers in Routine Use EUGENE CHILDERS AND G . W. STRUTHERS Polychemicals Department, E . I . d u Pont de Nemours & Co., Znc., Charleston, W . Va.
The routine use of infrared spectrometers for quantitative analytical measurements is a new technique in many industrial laboratories. A series of tests is described which evaluate (1) the reproducibility of these instruments on a dayto-day basis; (2) the linearity of the absorbance scales of both the single- and double-beam type instruments; and (3) the precision of a quantitative infrared method when applied by one analyst, by several analysts in one laboratory, and by several analysts in two laboratories. These tests indicate that for duplicate determinations the expected error would be less than 0.1% absolute.
T
HE use of infrared spectrometers in plant laboratories where they may be utilized for routine process control is a somewhat recent development. At the D u Pont Co.’s Belle Works, there are now in operation three infrared spectrometers: one PerkinElmer Model 21 double-beam spectrometer and two PerkinElmer 12-C single-beam spectrometers. The operating characteristics of these instruments are of interest primarily from the standpoint of reproducibility and linearity on a day-to-day basis. I t is realized that better reproducibility and linearity can be attained, when needed, by careful and more frequent adjustment of the instruments. This is more permissible when they are used in research studies than when used in a plant area on a round-the-clock basis by nontechnical personnel. A series of precision studies to evaluate the following characteristics of the single-beam and double-beam instruments has recently been completed. The short-term reproducibility of the two instruments when applied to a laboratory analysis currently in routine use has been evaluated. This study was designed to evaluate the instrument error inherent in a method in ivhich a standard comparison technique is used-i.e., a standard material is scanned along with each series of samples in a period of only a few hours. The long-term reproducibility of the tmo instruments when applied to the same routiiie laboratory analysis has also been evaluated. This study was designed to estimate the inherent instrument error in a niethod used over a long period of time. This would be analogous to a scheme of analysis in which a curve or absorbance index, along 11-ith a cell calibration, is used for the analytical calculation. Table I.
Reproducibility Tests on .\lode1 12-C SingleBeam Spectrometer
Absorbance Average 0.200 0,200 0.200 0,200 0,200 0.199 9-30-52 0.202 0.202 0.201 0.201 0,202 Date 9-29-52
10-1-52
10-2-52
10-3-52
0.203 0 196 0.196 0 195 0 197 0 196 0.197 0.199 0.199 0,202 0.200 0.200 0 I98 0 200 0 199 0,200
Date 10-7-52
Absorbance Average 0,199 0.199 0,200 0.197 0.199
10-8-52
0.196
10-13-52
0.199
10-20-52
0,200 0.195 0.197 0 193 0 197 0.197 0 200 0 200 0 200 0 200 0 201 0.200 0.199
0.196
0 200
10-22-52
0.199 0 200 0 199
REPRODUCIBILITY T E S T S
Single-Beam Spectrometer. For the short-term tests, a solution containing approximately 1.5y0 cyclohexanone in cyclohexane was placed in a 1-mm. sodium chloride cell. The instrument gain was adjusted t o give a base line of approximately 90% transmittance a t 1 2 . 7 ~ . The zero was set with a lithium fluoride shutter, and the spectrum was recorded from 12.7 microns to 14.0 microns a t a speed of approximately 1 minute per micron. The wave length was then manually reset to 12.7 microns and the process repeated. Five scans were obtained by this method without changing any of the instrument settings. This is analogous to a comparison technique in quantitative infrared analysis. From the scans, the absorbance of the 13.35-micron cyclohexanone band was measured by the standard base line technique. This process was repeated nine times over a period of approximately one month, giving a total of 50 determinations. Table I shows the values obtained. The values of interest in this case are the deviations of each of the five determinations from the average for that day. From the sum of the squares of each deviation from its daily average, the precision can be calculated on the basis of a short lapsed period of time. This gives a standard deviation of 0.0009 absorbance unit or 0.45% relative a t the absorbance level of 0.2. This is a 95% confidence limit of 0.9% relative. The absolute absorbance error a t the 0.4 level would not be much greater; hence the relative error would be about half the value obtained here. For the long-term reproducibility of the Model 12-C, the averages of the daily determinations were used as shown in Table I. This average of 0.199 absorbance unit with a standard deviation of 0.0019 gives a 95% confidence limit of 1.9% relative a t the 0.2 absorbance level. At the 0.4 absorbance level, this value would be approximately half or about 1% ’ relative. Double-Beam Spectrometer. I n reproducibility studies on the ;\lode121 double-beam instrument, the sample of cyclohexanone in cyclohexane was placed in a 1-mm. cell in the sample beam, with cyclohexane in a 0.9-mm. cell in the reference. The base line was again set a t about 907’ transmittance. The gain, automatic suppression, and zero were set, and the sample was scanned from 12.7 to 14 microns five times in succession without altering any instrument conditions. This process was repeated nine times over a period of about one month, yielding again a total of 50 determinations. The values obtained are shown in Table 11.
0.200
0.200 0 201 0.199
The use of absorbance indices or extinction coefficients for the final calculation is becoming more popular than the conventional reference curve. This method of calculating is rapid and is valid since the rock salt cells, when used with most organic liquids in an essentially anhydrous system, exhibit practically no changes in thickness after several weeks of normal use. In most cases where absorbance indices have been employed, errors arising from changes in cell thickness have remained within the error of the analytical methods when monthly cell calibrations were practiced.
0 200
0 199 0 201 0 200
-
Again using the deviation of each determination from its daily average and summing the squares of these deviations, the short1,311
ANALYTICAL CHEMISTRY
1312 Table 11. Reproducibility Tests on Model 21 DoubleBeam Spectrometer Date Absorbance 9-29-52 0.240 0.239 0.238 0.239 0.238 10-1-52 0.233 0.232 0.232 0.233 0.233 10-2-52 0 233 0 234 0 234 0 232 0 234
.4verage 0 239
Date 10-8-52
Absorbance 0 223 0 226 0 231 0 226 0 224
Average 0.226
0 233
10-13-52
0.237 0.232 0.234 0.233 0.231
0.233
0 233
10-14-52
0 236 0 234 0 235 0.232 0 233
0 234
10-3-52
0.232 0.230 0.230 0.228 0,229
0.230
10-20-62
0.232 0.232 0.232 0.233 0.236
0 233
10-7-52
0.225 0.224 0.224 0.224 0.223
0 224
10-22-52
0.233 0.230 0.233 0,238 0.238
0.234
_____
Single-Beam Spectrometer. In measurement on Model 12-C, a solution of approximately 1.5% cyclohexanone in cyclohexane was placed in a 1-mm. cell in the instrument and the solution scanned across the 13.35-micron band. The gain was set so the base line fell near the 0.02 absorbance point on the scale. The gain was then reset to place the base line at 0.06 absorbance unit on scale, and the scan was repeated. This process was repeated until nine scans were obtained, the last curve having a base line a t 0.7 absorbance unit with the peak of the absorption curve a t 0.9. The absorbancies of each of the curves was obtained and these values compared. Double-Beam Spectrometer. In the case of the Model 21, the double-beam instrument, the same process of scanning was used, the only variation being that the base line was set Kith the optical balance comb in the sample beam. Nine scans n-ere completed in this manner with a base line ranging from 0.01 to 0.62 absorbance unit. The values obtained from this series of studies are shown in Table IV.
Table I\-. Model 12-C Effective Base line absorbance absorbance 0.02 0.201 0.06 0.201 0.12 0.199
n
term precision was calculatetl. This shows a standard deviation of 0.0017 absorbance unit or O.i% relative. At the 0.4 ahqorbance level this value should be about 0.4%. The long-term reproducibility for the Model 21 calculated from the daily averages is 0.007 absorbance unit or a standard deviation of 1.7% relative at the 0.2.3 absorbance level. .it the 0.4 level, this value will be ahout 1%. The results of the over-all test are shown in Table 111. Note that in the case of short-term reproducibility the Model 21 showed about 1.5 times as much inherent error as the Model 12C . In the case of the long-term test, Model 21 showed almost twice as much error as Model 1 2 4 . I n each case however, the errors are quite small. These values are within the limits of most of the other errors found in our quantitative infrared techniques.
Table 111. Reproducibility Calculations Standard Deviation, I Relatiye, %
Instruiiient
A s Level
A s unit
P & E 12-C
P&E21
0.200 0.232
0.0009 0.0017
0 4.; 0 70
P & E 12-C
Long Term 0.200 0.232
0.0019 0.0043
0.95 l,85
Short Term
P&E21
SCALE LINEARITY
In addition to the aspect of reproducibility of these instruments when applied to routine quantitative analysk, some consideration was given to the linearity of the transmittance or absorbance scale. When simple comparison techniques are used in an analysis and it is assumed that the standard and sample exhibit approximately the same absorption a t the analytical frequency, large deviations in scale linearity can be tolerated. This is also true for absorption us. concentration reference curves since they automatically compensate for deviations in scale linearity. When absorbance indices or extinction coefficients are used over wide concentration ranges, horvever, it is imperative that the adsorbance scale be linear. A comparison was made between the values for scale linearity of the Perkin-Elmer Models 12-C and 21.
Average = 0 199 Range = 0.005
198
Scale Linearity Model 21 Effective Base line absorbance absorbance 0.01 0,243 0.04 0.245 0.09 0.250
n mn
Average = 0 , 2 5 0 Range = 0.014
I t will be observed in thi- ca'e that the range of variation in scde linearity is mole than tn ice that obtained with the singlebeam instrument, I t is believed that the major portion of the difference observed in the scale linearity between the two instruments is due to an electronic aspect of the Model 21. Since this investigation, the Perkin-Elmer Corp. has introduced an electronic modification of the Model 21 designed to overcome this poorer performance. Although long-range observations have not been conducted by this laboratory, preliminary studies indicate that the electronic change has significantlv improvel the pel formance of the instrument. I t should be noted in the case of both instruments, however, that the range of base line level or peak absorption level is much greater than that which would be encountered in the average analytical measurement. For most work, the use of absorbance indices would not be hampered by this deviation in scale linearity. On the basis of these observations, it has been recommended that laboratories be supplied with both single-beam and double-beam instruments whenever possible. In analytical studies where solvents and atmospheric absorption permit, the Model 12-C spectrometer is preferred for rapid routine quantitative analysis, because of its slightly greater reproducibility and ruggedness. In a large number of cases, however, atmospheric interference renders the single-beam instrument almost useless, and more frequently, double-beam methods may be required bemuse of solvent limitations and interfering substance9. PRECISION
-4significant adjunct to this study has been the evaluation of a routine infrared analysis with respect to precision. This concerns the reproducibility of an analysis by one individual; by a single laboratory where several individuals use the same procedure and instrument; and by different laboratories, in which case the instruments used also become variables. A study was designed to encompass all these possible variables in addition to the time factor. The analysis coiisiqted of the determination of two components,
V O L U M E 25, NO. 9, S E P T E M B E R 1 9 5 3 Table V.
1313
Precision Differences between Two Laboratories Average,
; C
10 9
9
Cvclohexsno,l A
10 11 10 4
B
C D
70
3.42 3.39 3,42
o
ni 0 02 0 03
0.02 0.07 0.09
3 3 3 3
0 03 0 04
0 03 0 01
0 0 0 0
0.04 0 03 0 02
0 11 0 07 0.06
39 32 30 28
11
10 07 04
Laborstorv 2
Cyclohexanone E F
10 10 10
G
Laboratory
2 0 Limits,
nc
Laboratory 1
Cyclohexanone
Table \-I.
.\v. Dev.,
%
KO.Detns.
Analyst
3 35 3 38 3 3’)
Averaged Precision Differences between Two Laboratories h-o. 13etns.
1 2
28 30
1 2
35 30
Average, 7% AY. Dev.. % Cyclohexanone 3.41 3 3i
2 0 Limits, 7%
0 02 0 04
0.07 0.09
0 04 0 04
0.11 0.10
Cyclohexanol
.
_
_
Table VII. ~
3.33 3.35
~
Consrltnent h-o. Deins. C~cloIrexanonr 68 Cyclohexanol 65
Over-all Reproducibility Average. 3.39 3 31
Av. Dei-., % 0 03 0 0.5
2 0 Limits, % 0.09 0.11
cyclohexanone and cyclohexanol in cyclohexane. A synthetic blend was prepared consisting of approximately 3.4% each of cj-clohexanone and cyclohexanol dissolved in pure cyclohexane. The synthetic sample was divided into 65 portions, half of which were analyzed by three chemists in the technical laboratory and the other half by three analysts in a plant laboratory who had been trained t80use the infrared spectrometer. Each of the six individuals made at least nine determinations for cyclohexanone and cyclohexanol in the synthetic hydrocarbon sample. To cover the time variahle, the folloxing testing sequence was established: Kot more than two samples n-ere analyzed by each laboratory during any %hour period. The individual analyst conducted not more than one analysis during any 24-hour period. The study required about one month for completion and all infrarrd measurements were made with the single-beam instrument, using sodium chloride optics. The analytical procedures used v-ere as follows:
Cyclohexanone. The cyclohesanone was extract,ed from the hydrocarbon sample with aqueous sodium bisulfite as the watersoluble addition product. From this aqueous extract, the cyclohexanone was regenerated by neutralization with strong sodium hydroxide. The liberated ketone xas ext,racted from this aqueous salt solution with pure cyclohexane, in which medium it x-as measured a t 13.35 microns. Cyclohexanol. This material was estracted from the sample with conrentrated hydrochloric acid. This did not constitute a
b
chemical reaction but was merely use of selective solubility. The extract was neutralized with sodium hydroxide, and the cyclohexanol was extracted from this aqueous salt solution with carbon disulfide. The cyclohexanol was determined in this carbon disulfide estract by infrared absorption a t 10.35 microns. Values for both components were calculated using absorbance indices, each laboratory having its own value for these constants. The results of this series of determinations are shown in Tablea V and F?. Table V depicts the precision by individuals in the two laboratories. Inspection of the precision of each analyst indicates significant differences in the ability of individuals to reproduce their own results. For example, analyst G shows a 2-sigma variation of 0.04y0 for cyclohexanol, while analyst F in the same laboratory shows 0.14%. Subsequent tests have shown that most of this variation is due to personal error in the extraction steps. Analyst D analyzed four samples of cyclohexanol in an effort to help resolve a discrepancy in this extraction scheme which seemed inherent between analysts. His values are included in the final over-all calculation for reproducibility. Whenever practical, chemical treatment of samples is used prior to infrared measurement to remove inteifering compounds. This technique is considered more desirable than optical compensation for interferences using the double-beam principle, because of the difficulty of training routine analysts to use properly the compensation techniques and because possible fluctuations in the interference level from sample to sample add further complications. Table T? shows some final calculations for the precision by laboratories, and Table VI1 shows the over-all reproducibility of the methods when used by either laboratory. The precision between laboratories is not significantly different. This is particularly desirable from the standpoint of coordination and standardization of analytical methods in company laboratories. The precision figures indicated are based on single determinations I n most laboratories, including this one, analytical determinations are made in duplicate This would reduce these error values to about 0.05 and 0.08% absolute. I n the case of cyclohexanone. this represents a 95% confidence limit of 1.9% relative, which value combines all sources of error. JT‘hen one considers that only one sample out of 20 when analyzed in duplicate is suhject to this error, the precision iq quite satisfactory R E C h I V E D for review M a y 11, 19S3. Accepted June 26, 1953 Presented March 6, 1953, a t the Pittsburgh ConfereRce on Analytical Chemistry a n d Applied Syectroscopy.
Potentiometric Polarography-Correction I n the article on “Potentiometric Polarography” [ANAL.
CHEJI., 25, 1160 (1953)] on page 1160, in the first column of the abstract, the word “electrom:tRnt,tic.” Fhould read “electromotive.’ ’ The last sentence of the first p a r a g a p h of the text was inserted in response to a reviewer’s suggestion and was applicable a t the time of writing. However, automatic recording instrumentation has been developed for the current-scanning method and the application t o the dropping mercury elect’rode has proved to be very interesting and advantageous in several respect,s. This R. N. i i n m s work will he reported in the new future.
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