Anal. Chem. 1985, 57,857-861 (8) Lorber, A.; Goldbart, 2.; Eldan, M. Anal. Cbem. 1984, 56, 43-48. (9) Lorber, A. Anal. Chem. 1984, 56, 1404-1409. (IO) Lawson, C. L.; Hanson, R. J. “Solving Least Squares Problems”: Prentlce-Hail: Englewood, NJ, 1974. (1 1) Malinowskl, E. R.; Howery, D.G. “Factor Analysis in Chemistry”; Wiley: New York, 1980. (12) Lorber, A. Anal. Chem. 1984, 56, 1004-1010.
857
(13) Boumans, P. W. J. M.; McKenna, R. J.; Bosveld, M. Spectrochim. Acta, Par! 8 1981, 368, 1031-1058.
RECEIVED for review August 20, 1984. Accepted November 26, 1984.
Determination of Gallium in Sediment, Coal, Coal Fly Ash, and Botanical Samples by Graphite Furnace Atomic Absorption Spectrometry Using Nickel Matrix Modification Shan Xiao-quan, Y u a n Zhi-neng, a n d Ni Zhe-ming* Institute of Environmental Chemistry, Academia Sinica, P.O. Box 934, Beijing, People’s Republic of China
A method has been developed for the determination of gallium in environmental samples at the level of 0.069-58 pg/g by graphite furnace atomic absorption spectrometry using nickel matrix modification. The sensitivity for determining gallium was Improved by a factor of 6 in the presence of nickel as compared to that of pure gallium standard. The related mechanism of enhancement effect is discussed. The suppression effect of perchloric acid on the determination of gallium was probably due to the formation of GaCi orlglnatlng from the condensed phase reaction at low HCIO, concentration and the vapor phase reaction would also be recognized to occur at higher HCIO, concentration.
Gallium is not found as a major constituent of minerals and is widely distributed in nature. The concentration of gallium in silicate rocks is normally in the range of 10 to 100 pg/g (I). The methods present in use for the determination of gallium in rocks, ores, metals, and other inorganic materials have included flame atomic absorption spectrometry after solvent extraction ( 2 )or anion exchange separation (3) and graphite furnace atomic absorption spectrometry using conventional graphite tube atomization ( 4 ) ,Zr-coating technique (5),metal atomizer (6), or atomization from platform (7). In general, graphite furnace atomic absorption spectrometry is sensitive enough to be applicable to a variety of samples. However, severe interferences are frequently encounterred. Pelosi and Attolini (4) stated that gallium absorption signal was masked by 1000- and 5000-fold amounts of zinc and indium and selective volatilization with thermal programming was unsuccessful, and a solvent extraction procedure was employed to determine gallium impurities in semiconducting materials. Nakamura et al. (8)reported that nitric acid, hydrochloric acid and phosphoric acid as well as a variety of common salts seriously influenced the results. When EDTA was used, the interference of hydrochloric acid and nitric acid was suppressed completely and the interference of phosphoric acid was suppressed partly. Koirtyohann et al. (9) investigated the interference effect of perchloric acid on the determination of the group 3 elements and 0.5 M HCIOl caused over 95% reduction in peak absorbance for gallium. This suppression effectpersisted even though the furnace tube was heated far above the boiling point of perchloric acid during the ashing step. The authors assumed that the acid or one of its decomposition products reacted with graphite to form ther-
mostable products. Decomposition or release of this residual product resulted in a gas-phase reaction which inhibited atomization of gallium. This suppression effect was removed by addition of a certain carbonate or ammonium sulfate. Nevertheless, no molecular form of analyte escape was established. In order to reduce such interferences or to match the sample matrices, solvent extraction (6) or standard addition techniques ( I , 4 , 8) were frequently used in sample analyses. The purpose of the present study is to develop a method for determining gallium in sediments, coal, coal fly ash, and botanical samples by graphite furnace atomic absorption spectrometry using nickel matrix modification. In the presence of nickel the tolerable charring temperature for gallium was raised to 1200 “C, and the sensitivity was improved by a factor of 6 compared to that of aqueous gallium standard. In addition, the interferrences from sample matrices were greatly reduced. And the machanism of enhancement effect of nickel on the determination of gallium and the interference from perchloric acid was discussed. EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Model 4000 atomic absorption spectrometer, equipped with a Model HGA-400 graphite furnace and Model 056 chart recorder, was employed for the measurement of gallium absorbances at a resonance line of 287.4 nm under the conditions of “gas stop” and “maximum power”. The spectral bandwidth was set at 0.7 nm. A hollow cathode lamp of gallium was operated at 15 mA. Deuterium arc background correction facility was used throughout. A 2OvL Eppendorf microliter pipet fitted with disposable polypropylene tips was employed to introduce sample solution into the graphite tube atomizer. Reagents. Gallium stock solution, 1000 Fg/mL, was prepared by dissolving 0.100 g of gallium (99.999%, Shanghai Chemical Co., China) in 10 mL of 7 M nitric acid. The solution was boiled to expel nitrogen oxide and diluted to 100 mL with deionized water. Working standards were prepared by appropriate dilution with 0.1 M nitric acid. Nickel solution, 5 mg/mL, and ammonium sulfate solution, 25 mg/mL, were prepared by dissolving suitable amounts of nickel nitrate and ammonium sulfate (analytical reagent grade) in demineralized water. All other chemicals used in this study were of analytical reagent grade. Procedures. (1)Decomposition of Peach Leaves and Tomato Leaues. The method used to decompose 300 mg of sample by pressure decomposition was basically the same as that given in ref 10 except that 2 mL of 67% nitric acid, 1 mL of 72% perchloric acid, and 1 mL of 35% hydrofluoric acid were used and the final
0003-2700/85/0357-0857$01.50/0 D 1985 American Chemical Society
858
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
Table I. Experimental Conditions for Measurement of Molecular Absorption GaO analyte
GaCl
NiClz
+
+
tolerable
+
1pg of Ga 0.5 pg of Ga 20 fig of Ni 50 pg of Ni 50 ,ug of Ni 20 pL of 0.1 M 10 p L of ~ ~ 1 0 0.2 M HC104 244.5" 248.1" 346.P'
+
wavelength, nm drying temp, "C ramp/ holding time, s charring temp, OC ramp/ holding time, s vaporization temp, O C ramp/ holding. cleaning temp, O C ramp/ holding time, s
Table 11. Comparison of Various Matrix Modifiers for Gallium (20 pL of 0.025 pg/mL of Ga and Various Matrix Modifiers)
~
matrix modifier
concn, charring re1 mg/mL temp, O C absorbance
as added
none Zn2+
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.1 1.0 1.0
V6+
110
110
110
Cr6+ Ba2+
1/30
1/30
1/30
Srz+
300
1100
250
1/30
4/30
1/30
varying
varying
varying
0/10
0/10
0110
2650
2650
2650
1/6
1/6
1/6
co*+
Mo6+ AP+ Mg2+ PdZ+ Ca2+ Ni2+
900 900 900 900 1000 1000 1000 1000 1000 1000 1100 1100 1200
1.0 4.1 5.6 6.8 4.9 4.9 5.7 5.3 5.8 6.5 4.7 4.9 6.1
*Lys-C peptide cleaved with trypsin. 0.40 [
nFrom ref 11. bFrom ref 12.
~
RESULTS AND DISCUSSION Selection of Matrix Modifiers. In order to search suitable matrix modifiers, a variety of metal ions were tested. The absorbance obtained by injecting 20 p L of 0.025 ,ug/mL of gallium aqueous standard was set as unity, and the relative absorbances of gallium in various matrix modifiers were
i i
C
0
'?o
0.20 -
n VI Q
volume of sample solution was defined as 5.0 mL for the determination of gallium by graphite furnace atomic absorption spectrometry. (2)Decomposition of Drainage Sediments, Coal, and Coal Fly Ash. The methodlwas similar to procedure 1but the operation of standing overnight was omitted before the pressure attack procedure. In addition, the final content obtained by the pressure decomposition method was transferred to a 50-mL volumetric flask and diluted to the mark with 0.1 M nitric acid. (3)Determination of Gallium. According to gallium content in drainage sediment, coal, and coal fly ash, the digested sample solution prepared by procedure 2 was appropriately diluted, 20 ,uLof the sample solution was introduced into the pyrolytically coated graphite furnace along with the same volume of a mixture of 1 mg/mL of nickel and 5 mg/mL of ammonium sulfate, the sample was dried at 110 OC for 30 s, charred at 1100 0C for 30 s, atomized at 2400 "C for 5 s using "maximum power" mode, and gallium absorbances were measured under the condition of "argon gas flow interrupted". Finally the tube was cleaned at 2650 "C for 5 s. (4)Measurement of Molecular Absorption of GaC1, GaO, and NiClZ. When the Perkin-Elmer Model 4000 atomic absorption spectrometer was used for the measurement of molecular absorbance, the spectral bandwidth was set at 0.07 nm and a deuterium arc lamp or tungsten halide lamp was used as a continuous light source. The wavelengths for measuring GaCl, GaO, and NiClz were 248.1, 244.5, and 346.8 nm (11, 12), respectively. The experimental procedures and the operation conditions are summarized in Table I. First, the analyte solutions were introduced into the graphite furnace followed by drying and charring and then molecular absorption was measured at the vaporization stage using "maximum power" and "gas stop" modes in order to make the absorption. Finally a cleaning stage was employed. (5) Measurement of Appearance Temperature of Gallium i n the Absence or Presence of Matrix Modifiers. The procedure used for this purpose was the same as that described by Campbell and Ottaway (13).
0.30 .
u
0.1 0 0:i
0 LOO 800 1200 1600 2000
2600
Temperature ,"C Flgure 1. Effectof ashing and atomization temperature on the atomic absorption for gallium in the absence or presence of nickel: (0)0.8 ng of Ga: ( 0 )0.8 ng of Ga 20 pg of Ni.
+
calculated, and all of these results are summarized in Table 11. As cah be seen from the data, the sensitivity for determining gallium is slightly higher in the presence of chromium and magnesium than in the presence of nickel. However, more severe interferences are encountered using chromium and magnesium since only lower charring temperatures were tolerated. Considering the sufficiently high sensitivity for determining gallium and the highest allowable charring temperature of 1200 "C, only nickel was used in the remainder of this study. The effect of charring and atomization temperature on gallium absorbances was examined, and the results are schematically shown in Figure 1. The left-side branches refer to the effect of charring temperatures on gallium absorbances obtained under the optimum atomization temperature in the absence or presence of nickel. The right-side branches refer to the effect of atomization temperatures on gallium absorbances with or without addition of nickel under the optimum charring temperature in each case. In the presence of nickel the tolerable charring temperature for gallium could be raised up to 1200 "C and the sensitivity improved by a factor of 6. The appearance temperatures of gallium in the absence and presence of nickel were 1100 and 1530 "C, respectively. The mechanism of the enhancement effect of nickel matrix modification on the determination of gallium is ascribed to the formation of more thermostable solid solution and/or alloys (14), thus resulting in reduction of analyte loss in preatomization stage. In order to verify this assumption the molecular absorption of GaO was measured following procedure 4 and the results are shown in Figure 2. When no nickel was added, the molecular absorption of GaO increased with increasing vaporization temperature from 800 to 1300 "C,
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
: z
0.25 0.20
II
c
0.20
n
& 0.1 5
g ; 0.1 0
nL
VI
0
n
.-"
0.10
5
0.05
n 0.10
E
0.0001
Temperature , O C
Effect of vaporization temperature on the molecular absorption of GaO in the absence or presence of nickel: (0)1 pg of Ga; (0)1 pg of Ga + 50 pg of Ni. 0.10
0.01
0.001
0.1
1.0
o
=
HClOk Concentration , M
Effect of HCIO, on atomic absorption of Ga and molecular absorption of GaCI: atomic absorption, (0)0.5 ng of Ga + 20 pg of Ni 20 p L of HCIO,, (0)0.5 ng of Ga + 20 pg of Ni + 100 pg of (NH4),S04 20 p L of HC104; molecular absorption, (A)0.5 pug of Ga 50 pg of Ni 20 p L of HCIO,. Flgure 4.
+
+
1
:
-0 -0u 2
0 Flgure 2.
; 0
O
0.1 5
,-
v
;0.20
0
6
1 0.30
0.25
859
+
+
L
0.5
1.0
15
2.0
n
0.1 0
Concentration of Ni , mg/ml
$b
-
Flgure 3. Dependence of the absorbance for 0.8 ng of gallium on the concentration of the nickel solution added (20 pL).
9 0
reached a maximum a t 1300 "C, and then decreased with further increase in temperature; there was a plateau over the temperature range of 1600-2400 "C. However, a very small absorption of GaO and very little change in the absorption of GaO was observed when vaporization temperature was varied over the above range if nickel was used as a matrix modifier. The stablilizing effect of nickel depended upon the amounts added, and this is shown in Figure 3. When the charring temperature of 1100 "C was used and 20 pL of various concentrations of nickel solution was added to the graphite atomizer, the absorbance for 0.8 ng of gallium increased with increasing nickel concentration from 0.02 to 0.5 mg/mL, then a nearly constant absorbance was obtained over the range 0.5-1.5 mg/mL, and a slight reduction was observed with further increase in nickel Concentration. Therefore 20 pL of 1mg/mL of nickel was used in the remainder of this study. Study of Interferences. In order to examine the applicability of the recommended method for the real sample analyses, a series of experiments were undertaken to test the interference effects of a variety of foreign ions. Twenty milliliters of 0.020 pg/mL of gallium solution containing 1 mg/mL of nickel and 5 mg/mL of ammonium sulfate and the same volume of various foreign ions was introduced to the pyrolytically coated graphite furnace and procedure 3 was followed. The interference effect of each foreign ion was estimated by reference to the absorbance obtained by the same amount of gallium standard. It was found that there were no interferences from 0.05 mg/mL of Li+ and P, 0.10 mg/mL of Fe3+,Sb5+,and Te6+,0.25 mg/mL of K+, Na+, Mg2+,Cr6+, BO3*, and Si032-,1.0 mg/mL of Sr2+,Mn2+,Co2+,Ca2+,Ba2+, Zn2+, Cu2+,Pb2+,Cd2+,Bi3+,As3+,T13+,Se4+,and V5+. Since perchloric acid was frequently used in sample preparation and its suppression effect on the determination of gallium by graphite furnace atomic absorption spectrometry was not fully elucidated, a series of experiments were conducted to study the mechanism of perchloric acid interference and to explain the reason of effectiveness of excess ammonium sulfate in overcoming this interference. As can be seen from
1200
2000
1600
2100
Temperature , 'C Flgure 5. Relationship between vaporization temperature and the molecular absorption of GaCl in the absence or presence of ammonium sulfate: ( 0 )0.5 pg of Ga 50 pg of NI 10 p L of 0.25 M HCIO,; (0)0.5 pg of Ga 50 pg of Ni 100 pg of (NH,),SO, + 10 p L of
0.25 M HCIO,.
+
+
+
+
Figure 4 gallium atomic absorbances decreased with increasing perchloric acid concentration even when nickel was employed as a matrix modifier. However, no interference at 0.3 M HC104 and only a reduction of 7% in peak absorbance were observed when excess ammonium sulfate was added. To elucidate the mechanism of the suppression effect of perchloric acid on gallium, the molecular absorption of GaCl was measured a t 248.1 nm as a function of perchloric acid concentration or the vaporization temperature using a deuterium arc lamp. The results are shown in Figures 4 and 5, respectively. As the concentration of perchloric acid increased, the absorption of GaCl increased while atomic absorption of gallium decreased. This fact indicated that the suppression effect of perchloric acid was primarily due to the formation of GaCl by the following reactions:
-
HC104
+ 3C1- + 6 0 2
-+ +-
Ga(C104)3
Ga*
Ga*
C1*
C1*
OH
+ 3/202
GaCl
(1) (2) (3)
Similar reactions have been used to elucidate the mechanism of perchloric acid interference with the determination of indium by flame atomic absorption spectrometry (15). Koirtyohann et al. (9) reported that the interference effect of perchloric acid persisted a t very high temperatures. This finding was confirmed in this study. Molecular absorbance of GaCl increased with increasing vaporization temperature from 1200 to 1800 "C and reached a maximum at 1800 "C which was higher than the appearance temperature of 1530 "C for gallium in the presence of nickel. Contrastly, the molecular absorbance of GaCl was remarkably reduced when
860
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
N
0.50
$ 100
I
a,
0.20
800
1200
1600
2000
5
75
n o
50
.-c 25 [L
.
0 0.001
2LOO
01
001
HCIOL
10
concentration , M
Flgure 7. Comparison of HCIO, interference from two-troughs platform
Temperature , O C
Figure 6. Effect of vaporization temperature on the molecular ab20 pL of 0.1 M HCIO,. sorption of NICI,: (0)20 p g of Ni
+
(0)with that from the conventional platform (0): 0.5 ng of Ga -t 20 p g of Ni 4- 10 pL of HC104.
Table 111. Recovery Study
excess ammonium sulfate was added although the trends remain the same. The reason for the effectiveness of ammonium sulfate in removing the interference of perchloric acid was probably due to the following reaction which was recognized to be a useful means of destroying perchlorate (16) 3C104-
+ 8NH4+
-
3C1-
+ 4Nz + 12H20 + 8Hf
(4)
Therefore, no suppression effect was observed over the concentration of perchloric acid ranging from 0.0001 to 0.3 M, but some reduction in gallium absorbance existed when the concentration of perchloric acid exceeded this limit. In addition to the analyte loss as GaC1, the less important factor resulting in suppression effect in the presence of both perchloric acid and nickel was the loss of nickel as NiClz at charring stage (Figure 6). Because the stabilizing effect of nickel depends on the amount of nickel present in the graphite tube (Figure 3), the nickel remaining after charring at 1200 "C was insufficient to stabilize gallium. If more nickel was added, the suppression effect would become less severe (not given in Figure 4). Koirtyohann et al. (9) concluded that the interference of perchloric acid was due to the gas-phase reaction when a relatively high concentration of perchloric acid was used. It is difficult to establish whether GaCl being lost during atomization would have originated from condensed phase or vapor phase interaction. In order to make this investigation, a homemade platform with two troughs was used, the breadth and depth of each trough were the same as those of the standard platform while the length was only half that of the standard platform. Gallium aqueous standard solution containing nickel was added in one of the troughs and perchloric acid of various concentrations was added in the other to avoid the physical contact of the two solutions, or a mixture of the two solutions was added in the single trough of a standard platform. The solutions were dried under infrared lamp, the platform was inserted into the graphite tube, and the normal furnace cycle was run. The absorbances obtained by using the above two types of platforms for gallium in the presence or absence of perchloric acid were compared and the results are shown in Figure 7 . There is no interference over the concentration of perchloric acid ranging from 0.001 to 0.3 M if the two-trough platform was used, although the suppression effect was obvious when higher concentrations of perchloric acid were added. However, the suppression effect was most marked on the standard platform. It seems that the interference of perchloric acid on gallium determination was mainly due to the condensed phase reaction, though the gas phase reaction may also occur at higher concentrations of perchloric acid. Recovery Study. Varied known amounts of pure gallium standards were added to drainage sediments, coal fly ash, and
sample 81-101,
amt amt Ga total Ga weighed, Ga in added, found, recovery, mg sample wg wg wg % 100
1.95
river sediment (China) 82-201,
3.0 3.9
105 98
3.0
4.8
95
100
2.81
1.0 2.0 3.0
3.75 4.69 5.69
94 94 96
200
0.078
0.050 0.100
0.128 0.175
100 97
0.150
0.235
105
coal fly ash (China) 82-301,
1.0 2.0
peach leaves (China)
Table IV. Determination of Gallium in Samples
sample NBS SRM 1633a coal fly ash 82-201, coal fly ash (China) NBS SRM 1632a coal GSD-8, drainage sediment (China) 81-101, river sediment (China) NBSSRM 1573 tomato leaves 82-301, peach leaves (China)
gallium content, pg/g informaamt tion weighed, mg this work value lit. value 100 (0.2 mg/mL)
58.7 56.3
100 (0.4 mg/mL) 100 (1.0 mg/mL) 100 (1.0 mg/mL)
28.1 28.3 8.0 8.3 11.0 11.0
100 (0.5 mg/mL) -.
19.5 18.5
300 (60 mg/mL)
0.083
200 (20 mg/mL)
0.375 0.390
58
8.49 10.7
22.0 (7) 0.069 (17)
peach leaves, and then the entire procedure was carried out. The recovery was estimated by reference to the calibration curve constructed from gallium standards and an average recovery of 94 to 105% was achieved (Table 111). Determination of Gallium in Real Samples. Since no serious interferences were encountered and quantitative recoveries were obtained, the recommended method has been applied to the determinations of gallium in a variety of samples omitting the standard addition method or any separation procedures which were frequently used in the literature. The results of sample analyses are summarized in Table IV, and
Anal. Chem. 1985, 57,861-864
a good agreement was obtained with the information values reported by NBS or other workers (7, 17). The relative standard deviation was 3.7% for 11 replicate determinations of a drainage sediment in which gallium content was found to be 10.8 pg/g. Registry No. Ni, 7440-02-0; Ga, 7440-55-3.
LITERATURE CITED (1) Langmyhr, F. J.; Rasrnussen, S. Anal. Chim. Acta 1974, 7 2 , 79-84. (2) Lypka, G. N.; Chow, A. Anal. Chlm. Acta 1972, 6 0 , 85-70. (3) Korkisch, J.; Steffan, I.; Nonaka, J. Anal. Chim. Acta 1979, 109, 181- 185. (4) Pelosi, C.; Attolini, G. Anal. Chim. Acta 1976, 84, 179-183. (5) Kuga, K. Bunsekl Kagaku 1981, 3 0 , 529-534. 16) Ohta. K.: Suzuki. M. Anal. Chim. Acta 1976. 85.83-88.
861
Koirtyohann, S. R.; Glass, E. D.; Lichte, F. E. Appl. Spectrosc. 1981, 35, 22-26. Shan, Xiao-quan; NI, Zhe-ming; Zhang, Li At. Spectrosc. 1984, 5 , 1-4. Dittrich, K.; Schneider, S. Specfrochim. Acta, Part 8 1979, 3 4 8 , 257-268. Yasuda, S.; Kakiyama, H. Anal. Chim. Acta 1976, 8 4 , 291-298. Campbell, W. C.; Ottaway, J. M. Talanta 1974, 21, 837-844. Wade, K.; Banister, A. J. I n “Comprehensive Inorganic Chemistry”; Bailar, J. C. Jr., EmelBus, H. J., Nyholm, R., Trotman-Dickenson, A. F., Eds.; Pergamon Press: New York, 1973; Vol. 1, p 1074. Haraguchi, H.; Fuwa, K. Bull. Chem. SOC. Jpn. 1975, 4 8 , 305613059. Burns, D. T.; Townshend, A.; Carter, A. H. “Inorganic Reaction Chemistry”; Eiiis Horwood Limited: England, 1981; Vol. 2, Part A, Chapter 14, p 138. Gladney, E. Anal. Chim. Acta 1980, 118, 385-396.
Determination of Formaldehyde with the Thermal Lens Effect Jan A. Alfheim and Cooper H. Langford* Department of Chemistry, Concordia University, 1455 de Maisonneuve, W e s t , Montreal, Quebec H3G 1M8, Canada
The laser thermal lens anaiysls of formaldehyde is reported. Two systems were tested, the first using a single laser In comblnatlon wlth a single diode detector and the second uslng two lasers In conjunctlon wlth a photodiode array detector. The formaldehyde solutions were prepared for colorimetry uslng the NIOSH method based on chromotroplc acid. Improved sensitivity over standard absorptlon techniques Is observed with enhancement factors up to 20.0 based on detectlon of absorptlvity as low as 9 X cm-‘ whlch corresponds to a concentratlon of 1.5 X lo-* M. With collection efficiency of 95% for sampllng solutlons, this supports facile detection of formaldehyde in the parts-per-billlon reglon in air.
Current interest in the quantitative analysis of formaldehyde stems from its potential as a human health hazard (1). Formaldehyde polymers are used in the fabrication of wood products and home insulation and are known to emit low concentrations of formaldehyde into the surrounding environment ( 2 , 3). A recent trend in pollution analysis is toward decrease of the sample volume by means of an increase of analytical sensitivity. Thermal lens analysis is attractive in this context because it “improves” classic and well-tested methods. Several methods to analyse formaldehyde have been developed (4-6) using both chromatographic and optical techniques. A close examination of the pararosaniline method (7) has shown it to be p H dependent (8) and seriously effected by SO2 ( 9 ) ,thus limiting its potential. In this study the NIOSH method for formaldehyde analysis has been modified to improve its sensitivity from 0.1-2 ppm to the parts-per-billion level where it can compete with GC and HPLC methods (4-6). The modifications to technique include diluting the chromotropic acid solutions 10-fold and then analyzing the samples with a laser thermal lens (LTL) spectrometer rather than a standard absorption spectrophotometer. A thermal lens works on the principle that the passage of a laser beam through a material with a finite optical absorption
generates thermal energy which heats the sample. The temperature gradient causes a refractive index gradient. For a Gaussian laser intensity distribution, a well-defined tranverse gradient in the refractive index will be established. In most materials dnldt is negative and thus this gradient has the same optical effect on the laser beam as a diverging lens. It should be noted that a thermal lens is a function of true absorption in the same fashion as other calorimetric techniques and not absorption plus scattering as in transmittance techniques. The intensity of the thermal lens effect is proportional to the absorbed light energy. LTL is a very sensitive method ( 1 0 , l l )which has already been used to measure pollutants in the micrograms per liter and lesser regions in both liquid and gaseous samples (12,13). In keeping with the goal of minimizing apparatus complexity, both simple single and more complex dual laser systems were used in our experiments to evaluate their detection capabilities for quantitative formaldehyde determination. The admittedly primitive single laser experiments were performed to demonstrate that using the most basic of components to design a system one can still obtain relatively high sensitivity. The dual laser experiments were designed to demonstrate the true potential of LTL as a means of trace pollutant analysis, specifically formaldehyde. We encourage laboratories with one laser available to consider using the thermal lens effect.
EXPERIMENTAL SECTION Instrument. Figure 1A shows a block diagram for the simplified experimental setup. The optical train and lasers were fixed to a workbench which was isolated from building vibrations with a three-ply “sandwich” consisting of a sheet of cork, a piece of sheet metal, and a layer of sponge rubber. The Coherent CR-6 argon ion laser (A) was used to drive the Coherent CR-590 dye laser (B) which was run at 150 mW at X = 600 nm. The power was measured in front of the cell holder to account for power loss due to reflections of preceding optical components. The beam was elevated to the height of the optical train by means of a dual mirror system (C). The lens (D)which brought the light to a focus has a focal length of 23 cm. A manual shutter (E) was used to block the pump beam. The cell holder (F)was placed one confocal
0003-2700/85/0357-0861$01.50/00 1985 American Chemlcai Society
