Analyst, July 1995, Vol. 120 2041

Utilization of Thermal Decomposition of Immobilized Compounds for the Generation of Gaseous Standard Mixtures Used in the Calibration of Gas Analysers

~~ ~~~ ~~ ~~ ~

Piotr Konieczkaa, Jacek NamieSnika, Andrzej Przyjaznyb, Elibieta Lubocha and Jan F. Biernata a Chemistry Faculty, Technical University of Gdarisk, Gdarisk, Poland

G M I Engineering and Management Institute, Flint, MI , U S A

A new method of generation of gaseous standard mixtures has been developed in which the measured component is generated by the thermal decomposition of a substance chemically bonded to the surface of silica gel (modified silica gel). The method enables preparation of a standard mixture (generation of a measured component) immediately before the calibration step. Consequently, it can be applied primarily for the generation of standard mixtures containing volatile, malodorous, unstable and toxic compounds. The results of studies on the application of this method to the generation of multicomponent standard mixtures (mixtures of CO and C 0 2 in nitrogen) are presented. Keywords: Gaseous standard mixture generation; carbon dioxide; carbon monoxide; detector, calibration; modified silica gel

Introduction

Calibration is the basis of almost every analytical method. There is often a need for the determination of a wide variety of chemical species (analytes) at ever decreasing concentration levels (traces and ultratraces) in increasingly complex mat- rices. 1 Widespread application of instrumental analytical methods has attracted attention to the calibration of detectors, analysers and monitors used in measurements.

In the case of analysis of gaseous mixtures, standard solutions called gaseous standard mixtures are used at the calibration stage. In addition, these mixtures find use in all kinds of model studies of adsorption, desorption, catalytic processes, etc. The lowering of concentration levels of analytes, even down to ppq (parts per quadrillion), affected the development of methods of preparation of standard mixtures. Owing to such low concentrations of the analytes in samples, the accuracy of generation of correspondingly low analyte concentrations in gaseous standard mixtures used at the calibration step has become crucial.

An additional problem in preparing gaseous mixtures is their instability, which is particularly important in the case of mixtures containing reactive, unstable or toxic analytes. The search for new methods of generation of standard mixtures continues in the direction of such a preparation of a mixture that would ensure the smallest error in determining its composition. The International Standards Organization has developed and published a number of methods of preparation of calibration gas mixtures.2 Primary standards for calibration gas mixtures can also be obtained from various national standard organizations, such as NIST.

The methods for generation of standard mixtures that, to a large extent, eliminate the errors in determination include those in which a mixture is prepared immediately prior to the

calibration step, and the concentration of the analyte can be accurately determined based upon the function relating to the output signal to the measured amounts (process parameters, etc.).3

Calibration Under ideal conditions, calibration should result in an accurate and precise determination of the amount of the analyte in a sample. The accuracy of a calibration step greatly affects the final result of a quantitative analysis. In an extreme case, inaccurate calibration may lead to false results.

A calibration process may be divided into several steps:4 (1) preparation of standard solutions (mixtures), the

knowledge of analyte concentration in a standard with an accuracy greater than the class of a calibrated instrument is required;

(2) performing measurements for standard solutions; (3) determination of the dependence of output signal on the

analyte concentration for standards.

Methods of Calibration

The most important criteria for selecting a calibration technique include:

(1) type of measuring device and its characteristics; (2) number of samples to be analysed; (3) possibility of preparation of a wide variety of standard

(4) required accuracy of determining the analyte concentra-

( 5 ) degree of matrix complexity of the analysed sample. Obviously, the kind of calibration greatly affects the

solutions;

tion in a sample;

accuracy of determination.5

Linear calibration

In the case of linear calibration, the calibration step involves the determination of the output signal as a function of the analyte concentration for a series of standards and plotting the results using the method of least squares.

Calibration by the method of limiting solutions is a variant of linear calibration. Measurements are carried out for two standards for which the analytical signal of a measuring device is higher and lower than the signal for the analyte in the analysed sample. The narrower the range of concentrations of limiting solutions, the more accurate the result of analysis. The method can also be used (using an appropriately narrow concentration range of limiting solutions) for the calibration of devices having the output signal linear (or close to linear) only over a certain narrow concentration range.

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2042 Analyst, July 1995, Vol. 120

Nonlinear calibration

If the relationship between the analytical signal and the analyte concentration is nonlinear, linearization of the results of measurements obtained for standards is used most fre- quently. Computer programs are employed to fit the experimental data to a function relating the analytical signal to the analyte concentration. The algorithm of such programs also contains the selection of best function available, and the quality of the fit is measured by the correlation coefficient determined from the shape of the function after linearization.

If we can deduce the form of mathematical function for a given analytical method, then the calibration is performed by fitting the experimental data to this function and finding, on this basis, the empirical parameters of the resulting equation.

Calibration by the standard additions method

In the case of a very complex sample matrix, the standard additions method is most commonly used for calibration. The method involves adding one or more increments of a standard solution to a specific volume of the sample. Measurements are made on the original sample and after each addition and a plot of the dependence of the analytical signal on the concentration of the standard added prepared. If small increments of the standard are used, the approximation of the results by a straight line does not contribute a significant error. The resulting straight line can then be extrapolated to zero analytical signal, yielding the concentration of the analyte.6

The standard additions method is also used in a number of modifications, such as the method of sample addition to standard, the method of sample dilution, or the method of sample concentration.

Gaseous Standard Mixtures

The rapid development of monitoring and determination of environmental pollutants has resulted in an increased demand for suitable gaseous standard mixtures. Their preparation poses many technical, methodical and instrumental problems. These problems have been considered in many papers. 1.7-9 Many research centres continue extensive studies on the development of new methods and devices for the generation of gaseous standard mixtures.

The terminology related to standard gaseous mixture can also be found in a document prepared by the International Standards Organization.

Utilization of Immobilized Compounds as a Source of the Analytes in Gaseous Standard Mixtures

Following long-term investigations and utilization of gaseous standard mixtures obtained by such methods as diffusion,lO permeation,” electrolysis,12 chemical reactions,l3,14 and auto- dilution,*5 in 1990 we began to examine the possibility of using thermal decomposition of immobilized compounds as a source of the measured components (analytes) in gaseous standard mixtures. The principle of this method of generation of gaseous components is as follows:

A sample of silica gel, the surface of which has been suitably modified by chemically bonding to it a specific compound, is placed in a glass tube which has been previously silanized. The silanization process tends to eliminate such detrimental effects as adsorption, desorption, and chemical reactions on the surface of glass by the deactivation of active sites.16

The glass tube is then inserted into a tube furnace and heated electrically to an appropriate temperature. By heating such a chemically modified silica gel, a new compound is generated, which can be used as the analyte in the standard

mixture being prepared. Next, a stream of purified diluent gas is passed at a specific and constant flow rate through the silica gel bed. The obtained mixture after drying (when necessary) is directed to a detector, and the generated signal is amplified, processed and recorded.

In our investigations, batches of silica gel chemically modified in different ways by bonding to its surface various chemical compounds were used.l7 The chemical compound providing the coverage (modification) of the surface of silica gel was selected in such a way as to generate (by way of thermal decomposition) the desired volatile product which could be used as the analyte in the mixture obtained by slushing the silica gel bed with a stream of diluent gas. In addition, the gel bed should be protected from the possibility of adsorption of the product generated by thermal decomposi- tion. Another important consideration was to make sure that no other compound beside the desired component was produced by thermal decomposition. This was essential owing to the fact that the modified silica gel was to be used for the generation of gaseous standard mixtures containing a specific gaseous product as the analyte.

The apparatus used for the generation of analytes by the described method is shown in Fig. 1.9 In the case of using modified silica gel for the generation of standard mixtures, the following parameters affect the composition of the mixture being prepared: mass of the gel; temperature of thermal decomposition; flow rate of the diluent gas; and the degree of surface coverage of modified silica gel. Obviously, the variation in each of these parameters can only take place within certain specified intervals. Hence, the lowest allowed temperature of generating a gaseous component will be the one at which the reaction of thermal decomposition of the immobilized compound is just starting, whereas the highest allowed temperature will be the one above which the process of thermal degradation of the silica gel bed will take place. It is preferred that the thermal decomposition of the compound chemically bonded to the surface of silica gel will be initiated at temperatures higher than ambient temperature. This will facilitate the storage of the prepared batch of silica gel. Furthermore, during checking the zero point of the instrument being calibrated, the zero gas can be passed through the tube

Fig. 1 Schematic diagram of the appa ra tu~ :~ 1, cylinder with compressed nitrogen; 2, gas purifying system (deoxidizer, scrubber with ascarite and magnesium perchlorate); 3, tube with gel sample; 4, tube furnace; 5, temperature controller; 6, drier; 7, NDIR detector; and 8, recorder.

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Analyst, July 1995, Vol. 120 2043

with silica gel without the risk of eluting even trace amounts of the analytes. This simplifies the design of apparatus for the generation of gaseous standard mixtures and calibration of

instruments. Specific values of the flow rate of diluent gas and the mass of silica gel will depend upon instrumental parameters (detection limit of the calibrated detector), the capacity of the tube with a sample of silica gel, the possibility of variation of the flow rate of diluent gas and the time required for a single measurement and the corresponding throughput).

The method described above has, in our opinion, several significant advantages when compared with other, known methods of generation of gaseous standard mixtures. The most important advantages include:

(1) the possibility of preparing gaseous standard mixtures containing volatile, malodorous , unstable or toxic compounds as the analytes;

(2) the preparation of a standard mixture immediately before the calibration step;

(3) high precision of determination of concentration of the analyte in the mixture; (4) the possibility of preparing standard mixtures with a

wide range of concentration of the analyte; (5) the possibility of adjusting the concentration of the

analyte in a mixture by varying several parameters of the process of generation of the mixture (temperature of thermal decomposition, mass of the gel sample, flow rate of the diluent

So far, the method has been successfully applied to the generation of such components as thiols,lg isothiocyanates,l9 and carbon dioxide.9 This paper presents the utilization of the above method for the generation of multicomponent standard mixtures (containing more than one analyte).

gas) *

Generation of Multicomponent Standard Mixtures by Thermal Decomposition of Chemically Modified Silica Gel Previously9~1gJ9 we presented the results of studies on the method of preparing single-component standard mixtures. However, the availability of suitable multicomponent stan- dard mixtures would also be of great advantage, particularly when using chromatographic methods for the separation and final determination of the analytes in gaseous mixtures.

The results of investigations of the method of generation of multicomponent gaseous standard mixtures by thermal decomposition of compounds bonded to the surface of silica gel are presented in Table 1. A very important consideration when generating multicomponent mixtures based on one batch of the silica gel is that each of the components be released in the same (or overlapping) temperature range of thermal decomposition.

Silica gel the surface of which was chemically modified in such a way as to yield CO and C 0 2 as volatile components upon decomposition of the bonded compounds during heating was used in this study. The liberation of CO and CO2 as a result of thermal decomposition of the substance immobilized on the silica gel surface allows the release, from a given mass of silica, of known and constant total amounts of CO and C02 (the ratio of total amounts of C02 and CO is constant for a given batch of the gel).

Apparatus and Reagents

The following apparatus (Fig. 1) was used: non-dispersive IR (NDIR) detector (VEB Junkalor, Greiz-Dolau, Germany) with CO and C 0 2 cuvettes; oxygen remover (Omnisfera, Gdansk, Poland) packed with Antyoxo I1 bed (COBRABiD, Warsaw, Poland). Antyoxo II is an oxygen-removing sub- stance of catalytic-sorption type; copper compounds coated on a ceramic support (kaolin, sodium silicate) are the active component. Other components are silver, manganese, iron and palladium compounds, which play the role of activators and modifiers. Operating conditions of oxygen remover: temperature of about 200 "C, residual oxygen concentration <1 ppm (v/v); temperature controller (Mera-Tronik, Wro- clew, Poland); purpose-designed tube furnace; flow meter (R & D Separations, Cordova, USA); amperostat OH 404/A (Radelkis, Budapest, Hungary); x-t recorder, TL-4200 (Lab- oratorni Pristroje, Prague, Czechoslovakia).

The following chemicals were used: MN Kiesegel 60 silica gel (Macherey Nagel, Diiren, Germany), 35-70 mesh, specific surface area about 200 m2 g-1; hexamethyldisilazane (Serva, Heidelberg, Germany); trimethylchlorosilane (Serva); nit- rogen (technical) , (Polgaz, Gdansk, Poland); magnesium perchlorate (for microanalysis), 14-22 mesh (Merck, Darm- stadt, Germany); Ascarite (for microanalysis), 14-22 mesh

Table 1 Summary of studies on the method of preparation of gaseous standard mixtures by thermal decomposition of immobilized compounds

No. Type of immobilized compound S I I

I1 1 -SiCH2NHC-SCH3

2 -SiCH2NHC-SC3H7

S

?

Amount released Generated volatile Calibrated per unit mass of compound detector gel/mg g-1 Ref.

CH3SH FID 13.2 18

C3H7SH FID, FPD 17.4* 18 15.S

3 -SiC3H6SC-NH-CH2-CH=CH2 CH2KH-CH2NCS FID 5.2 19 II S II

4 -SiC3H6SCNH-C4H9 -Si-C3H60CH2CHCH20H

I

C4H9NCS FID 6.5 19

5 00CCH2COOH co2 -Si-C3H60CH2CHCH20H

I

NDIR 41 9

6 bOCCOOH co + c02 NDIR 65, C02 - 44, co

* Results obtained for two different batches of silica gel.

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2044 Analyst, July 1995, Vol. 120

(Merck); oxalic acid (analytical-reagent grade, POCh, Lublin, Poland); sulfuric acid (analytical-reagent grade, POCh); sodium hydroxide (analytical-reagent grade, POCh) .

Modification of Silica Gel Surface

Silica gel used for the generation of CO and C02 was prepared as follows: a solution of 2 cm3 of y-glycidoxypropyltriethoxy- silane in 30 cm3 of dried toluene was added to 20 g of silica gel dried at 120 "C. The obtained mixture was refluxed for 6 h. The product was filtered, washed successively with toluene,

Fig. 2 Scheme of synthesis of a compound chemically bonded to the silica gel surface and its thermal decomposition.

8.0

7.0

6.0

c 5 3.0 8

1. 2.

acetone, and diethyl ether, and then suspended in a solution of 1.8 g of oxalic acid in 25 cm3 of diethyl ether. After 24 h the obtained product was filtered, washed with water and methanol, and dried. Fig. 2 shows chemical reactions used in the modification of silica gel.

Procedure

Measurements were carried out using the apparatus shown in Fig. 1 with an NDIR detector being employed as a measuring device. Based upon the previously established conditions of thermal decomposition for each of the two components (temperatures of thermal decomposition of 200-290 and 170-280 "C for CO;! and CO, respectively), a temperature of 240 "C was selected for thermal decomposition.

A flow rate of 30 cm3 min-1 was used for the diluent gas (deoxygenated nitrogen) in all the experiments.

Five measurements were carried out for each of the two analytes, the mass of the silica gel being varied each time.

Prior to analyses, the detector was calibrated using: (1) gaseous mixtures of C 0 2 in nitrogen prepared by electrolysis of the saturated aqueous solution of oxalic acid and bubbling a stream of nitrogen through the solution; or (2) gaseous mixtures of CO in nitrogen prepared by a static (volumetric) method using a 8.4% by volume gaseous mixture of CO in nitrogen and pure air.20

The results obtained are shown in Figs. 3 and 4 and in Table 2.

Additionally, the total amount of carbon monoxide and carbon dioxide which was released from unit mass of the gel was determined. To this end, the obtained C 0 2 (including the C 0 2 produced by oxidation of CO) was absorbed on the Ascarite bed placed in an absorber. This process was camed out under the following conditions: temperature, 250 "C; flow rate, 35 cm3 min-1; mass of silica gel, about 0.4 g (weighed to 0.00001 g); and time 150 min.

The amount of absorbed CO and C 0 2 was determined by a gravimetric method. The amount of C02 was determined directly from the increase in mass of the absorber after passing a stream of the generated mixture. The mass of CO, on the

3.

1

1 1

i -1 a 4.

St

I 0 150 300 - 0 150 300

Time of mixture generatiods

Fig. 3 Dependence of the carbon monoxide content in nitrogen on duration of thermal decomposition. 1, T = 240 "C; V = 30.4 cm3 min-l; rn = 0.23700 g; c = 4.6exp(-0.0079t); r = -0.998; 2, T = 240 "C; V = 30.6 cm3 min-1; rn = 0.33076 g; c = 5.7exp(-0.0080t); r = -0.999; 3, T = 240 "C; V = 30.3 cm3 min-1; m = 0.39695 g; c = 6.0exp(-0.0081t); r = -1.OOO; 4, T = 240 "C; V = 30.7 cm3 min-1; rn = 0.43321 g; c = 6.2exp(-0.0075t); r = -0.999; 5, T = 240 "C; V = 30.4 cm3 min-1; rn = 0.47132 g; c = 6.6exp (-0.0081t); r = -1.OOO.

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Analyst, July 1995, Vol. 120 2045

other hand, was determined indirectly, by first passing the stream of the gaseous mixture past a bed of the Korbl catalyst. The amount of liberated CO was found from the difference in mass of adsorbed C02 in the measurement with and without passing the mixture through the Korbl catalyst. When using this method to determine the amount of CO and C02, the procedure was identical to that used for the determination of carbon content by classical elemental analysis.21

The results, being an average of five measurements, were 65 k 1.6 and 44 k 1.5 mg of CO g-1 of bed for COz and CO, respectively.

The ratio of total masses C02/C0 calculated from the experimental data is 1.48 or, accounting for the experimental errors, 1.48 k 0.09. The same ratio calculated from molecular masses (stoichiometry) is 1.57 which is within the confidence interval of the result obtained from the experimental data. Note, that the decomposition reaction of the substance immobilized on the silica gel surface takes place in two steps: (1) a decarbonylation reaction and (2) a decarboxylation reaction. This is clearly demonstrated by the temperatures of liberation of the two compounds: CO and C02. Conse- quently, the ratio of amounts of liberated C 0 2 and CO at a certain time of generation of the mixture may vary for different temperatures of thermal decomposition of the modified silica gel.

"$ 8.0 m

E 7.0

\ .z n 6.0

6 0

C 0 .- c)

3.0 c C

0 fj 2.0 0

1.

0.0 150 300

Discussion and Conciusions

The use of thermal decomposition of compounds chemically bonded to the silica gel surface as a source of gaseous analytes allows the generation of standard mixtures in which the concentration of measured component(s), although not con- stant in the course of mixture generation, can be accurately described by a suitable relationship (see Table 2 and Figs. 3 and 4). Consequently, it is possible to determine the analyte concentration at any moment of the generation of a gaseous standard mixture. Batches of silica gel with a suitably modified surface can be conveniently stored at ambient temperature, and the mixture is generated only when the calibration of a measuring device (detector, analyser, monitor) is to be carried out. This novel method of preparation of gaseous standard mixtures has a particular applicability when the measured components in a gaseous mixture are toxic, unstable, reactive, or malodorous.

The major advantages of the proposed method are the possibility of generation of standard mixtures with a wide range of concentrations of the analyte and of varying this concentration by the adjustment of parameters of the process of preparation of the mixtures (temperature of thermal decomposition, mass of silica gel used, flow rate of the diluent gas). Other advantages include the possibility of generation of

1

3

0 150 300 Time of mixture generationh

Fig. 4 Dependence of the carbon dioxide content in nitrogen on duration of thermal decomposition. 1, T = 240 "C; V = 30.4 cm3 min-1; m = 0.23700 g; c = 5.6exp(-O.Wt); r = -0.996; 2, T = 240 "C; V = 30.6 cm3 min-1; m = 0.33076 g; c = 7.2exp(-0.0042t); r = -0.997; 3, T = 240 "C; V = 30.3 cm3 min-l; m = 0.39695 g; c = 7.4exp(-0.0050t); r = -0.999; 4, T = 240 "C; V = 30.7 cm3 min-'; m = 0.43321 g; c = 7.5exp(-0.0046t); r = -0.998; 5, T = 240 "C; V = 30.4 cm3 min-1; rn = 0.47132 g; c = 8.8exp (-0.005Ot); r = -1.OOO.

Table 2 Parameters of generation of CO and C02 based on thermal decomposition of modified silica gel and parameters of the curves describing the dependence of concentration of the analyte being released upon time

Carbon monoxide mixtures Carbon dioxide mixtures

No. TPC Vtcm3 min-1 m/g A* B* r A* B' r 1 240 30.4 0.23700 4.6 -0.0079 -0.998 5.6 -0.0048 -0.996 2 240 30.6 0.33076 5.7 -0.0080 -0.999 7.2 -0.0042 -0.997 3 240 30.3 0.39695 6.0 -0.0081 -1.OOO 7.4 -0.0050 -0.999 4 240 30.7 0.43321 6.2 -0.0075 -0.999 7.5 -0.0046 -0.998 5 240 30.4 0.47132 6.6 -0.0081 -1.OOO 8.8 -0.0050 -1.OOO

* A, B, empirical constants in the equations obtained [c = Aexp(Bt)].

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2046 Analyst, July 1995, Vol. 120

standard mixtures immediately before the calibration step using a simple apparatus as well as the stability of stored batches of modified silica gel (especially when the measured component is only released at temperatures considerably higher than ambient temperature) which substantially improves the accuracy of preparation of standard mixtures. The determination of the amount of substance which can be released per unit mass of modified silica gel allows the use of the gel as a source of the analyte. Furthermore, the determination of this amount enables the use of the gel as a quantitative standard for the calibration based on a single point (e.g. , in a thermal desorption method).

This work was supported by a grant from the Committee on Scientific Research (937489). Financial support of the Tech- nical University of Gdansk is also acknowledged.

References

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NamieSnik, J., J. Chromatogr., 1984, 300, 79. NamieSnik, J., Konieczka, P., Chrzanowski, W., and Biernat, J . F., Chem. Anal. (Warsaw), 1994,39, 245. Konieczka, P., Makarewicz, J . , Luboch, E., NamieSnik, J . , and Biernat, J . F., Chem. Anal. (Warsaw), 1994, 39, 179. NamieSnik, J., Torres, L., Koziowski, E., and Mathieu, J., J. Chromatogr., 1981,208, 553. Janicki, W., Wolska, L., Gorecki, T., and NamieSnik, J., Chem. Anal. (Warsaw), 1993,38,423. NamieSnik, J., and Kozlowski, E., Chem. Anal. (Warsaw), 1980, 25, 793. NamieSnik, J., Bownik, M., and Kozlowski, E., Analusis, 1982, 10, 145. Przyjazny, A., J. Chromatogr., 1984, 292, 189. NamieSnik, J., and Konieczka, P., Chem. Anal. (Warsaw), 1991, 36, 357. Lee, M. L., and Wright, B. W., J. Chrornatogr., 1980,184,235. Biernat, J. F., Konieczka, P., Tarbet, B. J . , Bradshaw, J. S., and Izatt, R. M., Sep. PuriJ Methods, 1994,23, 77. Konieczka, P., NamieSnik, J., and Biernat, J . F., J. Chromat- ogr., 1991, 540, 449. Konieczka, P., Luboch, E., NamieSnik, J., and Biernat, J. F., Anal. Chim. Acta, 1992, 265, 127. Bownik, M., Pomiary Autom.-Kontrola, 1971, 10, 443. Bobranski, B., Analiza IloSciowa Zwiqzkdw Organicznych, PWN, Warszawa 1979.

Paper 51003996 Received January 21, 1995

Accepted February 24, 1995

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