Certification methods for standard specimens and for evaluating the parameters of different groups of

optical analytical instruments according to their photometric, spectral, and analytical characteristics are

considered. Results are provided for studying certification test methods and calibration provisions for

atomic-absorption spectrometers, flame photometers, fluorimeters, refractometers, and also provisions for

checking photometric instruments and dispersed medium analyzers.

Key words: verification scheme, evaluation procedure, standardized metrological characteristics,

verification provisions, calibration, standard specimens, optical analytical instruments.

Procedural bases for certification tests on optical analytical instruments (OAI) are governed by factors that are com-

mon for analytical instruments and specific for OAI, in particular by the optical method, measured values, design scheme,

purpose, etc. In scale, it is possible to separate OAI, calibrated in units of substance concentration and in optical (transmis-

sion factor, refractive index, etc.) or relative units.

Optical analytical instrument-concentrators, based on spectrophotometric, refractometric, fluorimetric and other meth-

ods, are commercial automatic instruments used in monitoring systems for production processes, and part of laboratory instru-

ments. Standardized metrological characteristics (SMC) are chosen of concentration meters as for concentration measurement

provisions (range of measurements, errors, and others, expressed in units or functions of substance concentration). Metrological

provision for these instruments is based on certified specimens of substances, mixtures or standard specimens (SS). The main

problem in controlling errors for OAI-concentration meters is the choice of a standard and the error in its determination [1, 2].

The basis of calibration for instrument-concentrators is a mixture of known composition, certified mainly by ana-

lytical chemistry. Metrologically this is determined by a possible instrument–medium–instrument verification scheme,

according to which the media, instruments, and devices adopted in analytical chemistry are moved to a higher order. The ver-

ification scheme is based on the following prerequisites:

• methods and devices of analytical chemistry used for certification of calibrated mixtures, SS, should be “standards”

since the composition of mixtures is determined not by single measurements, but the scale of the instrument for a

large number of determinations;

• results of analyses performed by analytical chemistry should have results that are more accurate by a factor of not

less than two to three than those obtained by analytical instruments that correspond to the requirements of accura-

cy coordination; this condition is not always fulfilled since analytical instruments often make it possible to obtain

greater sensitivity and accuracy;

• extensive distribution of verification mixtures, and SS, gives rise to the requirement for certifying mixtures in dif-

ferent laboratories under conditions of “interlaboratory” error, and therefore quality control for analysis in a given

laboratory and in others together with procedural approaches in establishing certification errors is important.

Measurement Techniques, Vol. 48, No. 3, 2005

CERTIFICATION TEST METHODS AND PROVISIONS

FOR OPTICAL ANALYTICAL INSTRUMENTS

PHYSICOCHEMICAL MEASUREMENTS

M. A. Karabegov UDC 535.8.001.4(018)

Translated from Izmeritel’naya Tekhnika, No. 3, pp. 63–68, March, 2005. Original article submitted November 17,

2004.

0543-1972/05/4803-0299©2005 Springer Science+Business Media, Inc. 299

Referral of a method (equipment) to a standard requires consideration of a number of factors:

• with good reproducibility of the results of analyses,cases are possible of differences between them and the actual

content of a substance; good reproducibilty of the results of analyses cannot generally be a criterion for accuracy;

• agreement of the results of analyses performed by two different methods cannot be a guarantee of accuracy; cases are

known of almost the same result obtained by different methods but somewhat greater or less than the actual results;

• the average value of the results of analyses obtained by two different methods is not always correct; often one

method gives better results due to different systematic errors;

• in determining the concentration of a substance, there is no method of analysis that gives the most correct results

(“standard” method),and this is normal for different methods depending on concentration,associated substances,etc.

Analysis of these factors indicates that this version of a certif ication scheme has limitations for determining and

maintaining unification, and retaining the accuracy of analytical measurements.

Reference to analysis as a source of information about substance composition is valid in determining an unknown

substance composition. An alternative approach may be formulated in setting up the task of obtaining a mixture of know com-

position. Composing a mixture is a technical operation that may be carried out and controlled without using methods of ana-

lytical chemistry. On the other hand, any pure substance is an individual system with specific properties deviation from which

indicates the presence of extraneous impurities. These deviations may be expressed in the form of theoretical relationships

that make it possible to determine the amount of impurities. Thus it is possible to obtain calibrated (verif ied) mixtures of

known composition on an instrument basis without using analytical chemistry.

This approach may be presented in the form of another version of a verification scheme whose realization requires that

the composition of the verification mixture is characterized as more accurate than in analytical instruments. The practice of prepar-

ing pure substances and mixtures of prescribed composition,development of methods for determining the level of purity based on

the rules of thermodynamics,physical chemistry and others, makes it possible to prepare mixtures of prescribed composition with

the greatest accuracy. This approach has been proven and applied practically in creating SS, verification mixtures,and resolving

metrological problems. Work has been carried out in determining the errors in analyses by comparing their results with the pre-

scribed composition of a mixture prepared from pure substances. These mixtures and SS are used in checking methods of ana-

lytical chemistry and calibrating OAI providing unification and accuracy of analytical measurements. Thus, this verification

scheme envisages instrument calibration and verification system for OAI, use of certified equivalents and standard instruments as

a means of verification, it limits the participation of chemists and analysts in the system for calibration and verification of instru-

ments. At the same time, in view of the development of methodology and equipment for analytical chemistry and also the variety

of analytical tasks in branches of industry, ecology, energy generation, and agriculture, in valid cases it is desirable to use the

methodology for each of the verification schemes or an integrated approach including components of two verification schemes.

Verif ication provisions for OAI-concentration meters may be SS, standard substances,or mixtures of known com-

position that are keepers and transmitters of the sizes of instrument scale units,standard equivalents of measured units,stan-

dard or other instruments of high accuracy.

A particular source of instrument errors connected with the characteristics of calibration specimens is the certainty

of the composition–property relationship used in an instrument as a basis of measurement. Study and realization of the rela-

tionship may be carried out through stages of choosing the method or the composition–property relationship,determination

of this relationship in quantitative expressions and quantitative expression of the composition–output parameter relationship

that serves as a basis for instrument calibration. In this stage, errors may possibly appear that may be called measurement and

scale reproduction errors, respectively depending on the error of the means being calibrated and the certainty of the compo-

sition–property relationship. The best case is when there is a strict rule as the basis for the composition–property relationship.

With lack of such a rule, or the impossibility of expressing it in mathematical form, an empirically bound composition–prop-

erty relationship is set as the basis of the scale. Measurement errors may have a different value in the overall instrument error

for cases when the measurement error for the output parameter:

• is an order of magnitude lower than that measured for this parameter due to errors in determining the composition

of the calibration specimens; in this case, instrument error will be determined by these errors and the main prob-

lem will determining and controlling them;

300

• is commensurate with the error of determining the composition of the calibration specimens; in this case, the error

should be a component in the overall instrument error;

• is much higher than the error for determining the composition of the calibration specimens and the latter may be

ignored in determining the overall instrument error.

Measurement and scale reproduction errors are built into the overall error of an instrument and they do not depend

on instrument quality.

Optical analytical instruments with a scale in optical or relative units are the majority of laboratory instruments and

a considerable part of industrial instruments. They relate to spectrophotometers for the visible, UV- and IR-regions of the

spectrum,atomic-absorption (AA) spectrometers,photometers, including flame, photometric-counting (PC) particle analyz-

ers, refractometers, fluorimeters, etc. These instruments are used in order to determine the optical parameters or concentra-

tion of substances on the basis of a calibrated characteristic “instrument reading (optical parameter)–concentration.” An

important factor of the functioning of instruments of this type in measuring the concentration of substances is construction

and use of a calibration characteristic by the instrument user. Therefore, for effective use of an instrument the evaluation pro-

cedure for the metrological state of an OAI f or the user, realized by the manufacturer, should contain complete information

about the most important properties of the instrument.

Currently systematic methodology for certif ication tests and the degree of its treatment is not the same for different

groups of AOI [3, 4]. Sufficiently complete procedures have been developed for spectrophotometers and photometers,whose

evaluation of the metrological state is accomplished for a collection of spectral and photometric SMC. Spectral SMC may be

chosen from the series; spectral range, limits for the permissible error and the random error for setting up wavelength,sepa-

rate spectral ranges,scattered emission,etc. Existence of a state verif ication scheme for measurement provisions for wave-

length,verif ication provisions (spectral lamps with a linear spectrum, etc.) and unified procedures make it possible to pro-

vide unification of spectral characteristic measurements for OAI. Photometric SMC may be chosen from the series:range of

measurements for transmission coefficient (optical density),limits for the permissible error and random error of measuring

transmission coefficient (optical density),noise level, reading stability, etc. Photometric SMC are determined by means of

neutral reducers. Calibration of neutral reducers is carried out in accordance with the state verif ication scheme for measure-

ment provisions of spectral and integral coefficients of directed transmission in the wavelength range 0.2–20.0 µm, diffusion

and mirror reflections in the wavelength range 0.3–2.5 µm [5, 6].

The existence of special components (atomization, fluorescence, scattering, etc.) in some groups of OAI in schemes

for forming information signals also gives rise to a requirement of evaluating the metrological state of instruments with ana-

lytical characteristics connected with the method used. The main analytical SMC may be selected from the series:sensitivi-

ty or characteristic concentration (amount),detection limits,limit of the permissible random error in measuring a test sub-

stance under optimum conditions,the degree of compensation for nonselective absorption (for AA-instruments),instability

of sensitivity or characteristic concentration, etc.

A common information parameter for spectrophotometers (photometers), AA-spectrometers, fluorimeters, tur-

bidimeters, nepheleometers, is the intensity of emission in some wavelength range. This is a prerequisite of the procedural

approach, i.e. the metrological characteristic of some OAI properties is expressed by means of parameters of chemical,fluo-

rescence, scattering, and other specimens in accordance with a photometric scale. In this respect the sensitivity characteris-

tic index for OAI instruments is “characteristic concentration” that is determined as the “concentration of an element in a

sample causing a change in the absorption parameter compared with absorption from a blank test by 0.0044 of an optical den-

sity unit (1% absorption).” This informative characteristic connects analytical and photometric parameters and taking account

of the metrological provision of the photometric characteristic it facilitates a unit of measurement. It is desirable to use pro-

cedurally a similar for other OAI, for example, fluorimeters and nephelometers,with provision of the possibility of using neu-

tral reducers calibrated with respect to transmission coefficient. In particular, this makes it possible under production or oper-

ational conditions to carry out some tests for instruments with respect to the photometric scale without special (chemical)

samples. The method has been proved in development,production,and in carrying out state tests for a number of instruments,

and it has demonstrated its efficiency.

301

For spectral and photometric SMC, there are units (for wavelength they are fractions,microns,nanometers, etc.,

for transmission coefficient they are percentage, etc.), and procedures and verif ication provisions have been developed

(linear spectrum sources,neutral reducers, etc.). Analytical SMC are specific and the methodology for determining and

evaluating them have not been developed sufficiently. They should make it possible to evaluate the comprehensive quali-

ties of OAI reflecting the specific nature of the optophysical method used:for AA-spectrometers in a complex with atom-

izers, for fluorimeters taking account of the spectral-energy parameters of the fluorescence excitation and recording sys-

tem, for turbidimeters and nephelometers taking account of parameters of the illumination system scattering (reflection)

of emission in dispersed media and recording scattered (reflected) emission in dispersed media and recording scattered

(reflected) emission,etc.

A plan for a verif ication scheme, basic SMC and a scheme for evaluating the metrological state of AA-spectrome-

ters have been developed [7,8].

Photometric SMC. Determination conditions and means of verif ication: lamp with a hollow cathode (LHC) for

Ca1 422.7 nm,three neutral reducers (start, end, and middle range of measurements).

The range of optical density measurements is determined by the region of its values for which the limits of permis-

sible errors are standardized.

The limit of permissible error for an instrument

∆A = Ao – Ai max, i = 1, 2, 3,

where Ao is optical density of the calibration reducer; Ai is the result of its measurement.

The limit of permissible random error for an instrument

where A is mean arithmetic value of the results of optical density measurements of the calibration reducer; n = 20 is the num-

ber of measurements.

Spectral SMC. Determination conditions and means of verif ication: UV- and the visible region, for example LHC

for Zn 1 213.8 nm,Ca 1 422.7 nm,Li 1 670.8 nm.

The spectral range for measurements is determined by the region of the wavelengths within whose limits the limits

for the permissible instrument error are standardized.

The limit of the permissible error for wavelengths of the device

∆λ = λ 0 – λi max, i = 1, 2, 3,

where λ0 is nominal wavelength of the source of a linear spectrum; λi is the index for the scale of wavelengths correspond-

ing to the maximum emission intensity.

The separated spectral interval is determined by the difference between values of wavelength corresponding to two

points of the smooth part of the spectral function whose ordinates equal half the maximum.

Analytical SMC. Characteristic concentrations Cchar and quantity qchar are determined as values of concentration

(quantity) of an element in a sample providing absorption equal to 0.0044 of an optical density unit,

Cchar = 0.0044C/Asa– Abla, qchar = 0.0044q/Asa– Abla,

where C and q are concentration and quantity of an element in a verif ication sample; Asais optical density of the verif ication

sample; Abla is optical density of a blank sample. Determination conditions and means of verif ication: proven samples with

S

A A

n

ii

n

=

−

−=∑ ( )

,

2

1

1

302

a known content of Cd, Al, and blanks,values of C and q are selected from the condition A ≤ 0.2, the atomizers are a flame

graphite furnace, capsule-lamp,LHC for Cd 1 228.8 nm,Al 1 309.3 nm,fuel mixtures AA for Cd, ANO for Al (AA is acety-

lene/air, ANO is acetylene/nitrous oxide).

The detection limit is determined as the least concentration Cmin or quantity qmin of an element in a pure sample

whose optical density with a confidence level of not less than 0.9 exceeds the optical density of the blank sample:

Cmin = 3S0Cchar /0.0044; qmin = 3S0qchar /0.0044;

where S0 is mean square deviation (MSD) of the results of measuring optical density for a blank specimen; Ai bla is its mea-

sured result; Abla is mean arithmetic value of the results of measuring optical density for the blank sample; n = 20 is the num-

ber of measurements. Determination conditions and means of verif ication: proven blank specimens,LHC for Cd 1 228.8 nm,

Al 1 309.3 nm.

The limit of the permissible relative MSD for a random component of instrument error in measuring optical densi-

ty of an atomic vapor

where Aisais the result of measuring the optical density of an atomic vapor; Asais mean arithmetic value of the its measured

results; n = 20 is the number of measurements. Determination conditions and means of verif ication: proven samples with a

known content of Cd, integration time about 5 sec, values of C and q are selected from the condition 0.4 < A < 0.5,LHC for

Cd 1 228.8 nm.

The instability of characteristic concentration

where Ctchar is characteristic concentration determined in terms of time interval t after determining Cchar. Determination con-

ditions and means of verif ication: samples with a known content of Cd and blanks,LHC for Cd 1 228.8 nm.

The level of nonselective absorption compensation

where A1NaCl is the mean arithmetic value of the optical density of the atomic vapor of verif ication sample containing NaCl

without compensation; A2NaCl is the same value with compensation. Determination conditions and means of verif ication:

proven samples with a known content of Cd and NaCl,concentration of NaCl is selected from the condition ACNaCl= 0.2–0.3,

LHC for Cd 1 228.8 nm.

Producers of AA-equipment evaluate instruments taking account of production conditions,the requirements of the

customer for an agreed range of elements and characteristics, in a unit with a mercury-hydride generator, with a continu-

ous-flow-injection device, in an emission regime, etc. For example, for AA-spectrometers of the KVANT (Korték, Russia)

family checking of spectral and photometric characteristics is carried out in the stages of production and approval cycles,

KA

A= 2

1100NaCl

NaCl,

∆ =−

CC Ct

t

charchar char

charC100 ,

SA

A A

n

ii

n

0

2

11

1=

−

−=∑

sa

sa sa( )

,

S

A A

n

ii

n

0

2

1

1=

−

−=∑ ( )

,

bla bla

303

and the limits of detection and characteristic concentration for different (more than 50) elements are controlled. Within

metrological tests in an AA regime, analytical characteristics are evaluated: for flame models KVANT-2 and KVANT-2AT

it is the relative MSD and systematic error for solutions of Cu,Pb, Al, etc., and the supplementary error for correction of

background absorption for the wavelength Cd 228.8 nm:for the electrothermal Zeeman model KVANT-ZÉTA it is the

detection limits,relative MSD and systematic error for solutions of Cd, Cr, etc. Use of an AA-spectrometer in solving ana-

lytical problems should be accomplished on the basis of special measurement procedures using specimens of known com-

position and (or) SS.

The main purpose of flame photometers is determination of the concentration of an element in solution,and the main

information parameter is the intensity of emission of an element in the flame. In view of the fluctuating nature of a flame, the

main analytical SMC are determined by means of statistical methods,i.e., limits of detection for chemical elements,limits of

permissible values for random error, etc. Proven chemical samples are used as a means of verif ication. The procedure for cer-

tif ication tests is given in [9].

For fluorimeters [10, 11], spectral, photometric, and analytical SMC have been developed. The sensitivity charac-

teristic has been determined in the form of the ratio of concentration for a test substance per unit of absorption (transmission).

A means of verif ication for fluorimeters includes chemical samples,spectral lamps,and neutral reducers. As chemical test

samples apart from the traditional quinine-sulfate, fluoroscein,and rhodamine-B, studies and recommendations have been

made for 2-naphthol in a urotrapin buffer with pH 4.75 ± 0.05 (for the UV – and blue region), 3-amino-phthalimide in

0.05mole/liter solution of sulfuric acid (for the green region), and a complex of aluminum with eriochrome blue-black R in

N-propanol (for the red region).

For liquid refractometers, a plan for a verif ication scheme has been created [12]. For commercial refractometers,

SMC and means of verif ication (calibration liquids,standard instruments have been developed providing evaluation of the

error of determining the refractive index n = 105–106. The procedure for certif ication tests is given in [13]. Currently there is

a state verif ication scheme for measurement provisions of the refractive index of solid and liquid transparent substances [5].

For a refractometer with the measurement range 10–5 and a sensitivity of 10–7, a procedure has been developed for certif ica-

tion tests in a continuous-flow regime.

For calibration of photometric instruments, reducers have been created [14–16] from neutral glass (UNS collection),

glass with application of chromium in the form of streaks (streak reducers ShNO) and quartz with application of palladium

KNO-P, titanium KNO-T, and nickel KNO-N. The collection UNS has been proven with an error of 0.3% in the range

380–750 nm. Reducers ShNO have been prepared by deposition of chromium on glass substrates followed by application of

photolithography. They have been studied in a UIM-23 microscope and an SF-26 spectrophotometer, and the proving error

was 0.5%. Reducers KNO have been prepared by vacuum application for a continuous layer of palladium,titanium,nickel

on a quartz substrate. The quartz is transparent in the region 170–2800 nm,and palladium,titanium,and nickel were select-

ed for the parameter of neutrality of the spectral characteristic, good adhesion with the substrate, stability with time towards

the action of external factors,and stability. Reducers were studied for spectrophotometers SF-18,SF-26,5270 Bekman,OSF.

Stability was checked over three years and changes did not exceed 1%. Zonal nonuniformity was checked at 4 and 25 points

of the surface and it did not exceed 0.1%. The collection was certif ied with an error of 0.3%. In studies for atomic-absorp-

tion spectrometers S-115 NPO Analitpribor, 5000 Perkin-Elmer, and 180-80 Hitachi, the difference in values of transmission

coefficient for KNO-P and KNO-N did not exceed ±1.5%,and for KNO-T they did not exceed ±3%.

Liquid and solid specimens for calibrating dispersed media analyzers. In studying the stability of pyrex glass sus-

pensions[17, 18] over time and with repeated preparations,the transmission coefficients for suspensions of different glasses

differed little and there was good reproducibility with time. The dependence of transmission coefficient on volume concen-

tration agrees with the Bouguer–Lambert–Beer equation in the optical density range 0–1. The intensity of scattered emission

due to optical density and volume concentration, and the change in characteristic scattering curve for volume concentrations

of 25,50,75 and 100% and angles from 45 to 90° are characterized by a linear dependence. Calibration specimens of pyrex

glass suspensions have been used for testing and standardizing nephelometer and turbidimeter parameters.

Cloudy glasses(CG) [17–20]. Studies of the scattering and absorbing properties of CG were carried out in compar-

ison with model cloudy media,i.e., a suspension of kaolin in formazin. For all specimens,the characteristic curve was drawn

304

out along the direction of the beam and the maximum intensity was up to 20°. The transmission coefficient for a koalin sus-

pension in formazin with concentration 0–500 mg/liter varied from 96 to 10% (SF-10,λ = 650 nm,layer 5 mm). The depen-

dence of transmission coefficient on glass thickness and comparison of the characteristic curves for scattering and transmis-

sion coefficient of CG and a suspension of kaolin in formazin demonstrated the possibility of using CG in tests and stan-

dardizing turbidimeter parameters with measurements with measurements of dispersed media cloudiness within the limits

0–500 mg/liter. Studies of CG specimens for nephelometry were carried out in a device making it possible to record scatter-

ing forward to 30°, at an angle of 90°, and back. Scattering properties were compared for CG and a suspension of kaolin in

formazin. Values were obtained for the thickness of CG Nos. 1644,1646,and 1648 with which at the angle selected an inten-

sity of scattered emission is provided corresponding to the different cloudiness of dispersed media within the limits

0–500mg/liter. The dimensions and shape of glass specimens were selected for a specific instrument construction and oper-

ating principle. Proving of CG specimens should be carried out in equipment that makes it possible to measure the spectral

coefficient of brightness in different directions and the spectral natural attenuation index.

Method and means of calibrating photometric-counter analyzers for dispersed media[21]. The measured values of

analyzers are the dimensions and numerical concentration of suspended particles. In the optical scheme of the analyzer there

is a calibration cell with a shaped optical recording zone. A particle with a certif ied size if placed in the cell. The particle is

brought into reciprocal motion and with a prescribed frequency it crosses the recording zone. Emission pulses of the same

amplitude are formed that are then converted in electrical pulses. A concentration scale is formed with respect to the number

of pulses,and the dimensional scale is formed with respect to pulse amplitude [21]. Characteristics of the particle analyzers

are established, checked, and corrected by means of the calibration cell. Analyzer conditions are controlled according to the

conditions in [22]:range of particle dimensions for the groups 3–5,5–10,10–25,25–50,50–100 µm or more, concentration

0–2500 particles/ml,classes of frequency for the standard, 3–17,and calculation error 3%.

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22. GOST 17216–2001,Industrial Frequency. Classes of Liquid Frequency.

306