Distribution analyses of pharmacologically relevant high molecular compounds using secondary ion mass spectrometry (SIMS)

Journal of

MOLECULAR STRUCTURE Journal of Molecular Structure 348 (1995) 397400

Distribution Compounds

Analyses of Pharmacologically Relevant High Molecular Using Secondary Ion Mass Spectrometry (SIMS)

M. Trapp” and H. Stahlb “RTG Mikroanalyse GmbH Berlin, Ostendstr. 2-14, 12459 Berlin, BRD bSchering Aktieng esellschaft, Pharmazeutische Entwicklung, PSF 6503 11, 13342 Berlin, BRD

Abstract A palmitic acid (PA) / galactose mixture was investigated concerning the distribution of PA in galactose by means of SIMS. The results show, that also at a low average concentration of PA (O,l%) its distribution can be measured qualitatively. The size of the PA particles could be estimated by 3-dimensional depth profiling.

1. Problem For special applications in medical diagnostics it is veryimportant to have small PA particles in a galactose matrix. A rather complicate process is to be used to get such a substance. To optimize this technological process the final distribution and size of the PA particles must be known.

2. Method For many years the Secondary Ion Mass Spectrometry (SIMS) is a very useful method to investigate solid matter especially inorganic materials. In recent years this method is also used to analyse organic substances, because the detection limits for impurities are very good [l]. Additionally structural information can be received. SIMS is divided in more or less two classes, the static and the dynamic SIMS. Both methods use a primary ion beam, mostly oxygen or cesium, to bombard the surface of the sample. The impinging primary particles erode the surface and generate simultaneously so called secondary ions, which are detected by a mass spectrometer. In static SIMS very low primary ion densities are used,. to minimize the destruction of the sample. But to maintain a high detection efficiency, it is necessary to erode a rather large area which leads to a loss of local lateral information. The primary ion density at dynamic SIMS is much higher. The surface will be changed rapidly by the bombardment with results in a fractionation of large organic molecules. In this case the lateral resolution is up to one micrometer with a good depth 0022-2860/95/$09.50 Q 1995 Elsevier Science B.V SSDI 0022-2860(95)08672-2

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398

resolution in the order of 5 nm. For more information about SIMS see[ 2,3 1.Using dynamic SIMS it is necessary to look for stable molecules in the mass spectra which characterize the substances under investigation.

3. Experimental All the mearurements described here represent a dynamical SIMS mode. They are performed with an IMS-4F mass spectrometer [CAMECA France]. This instrument offers imaging capabilities allowing 3-dimensional element mapping. As primary ions Cs'was used. A test sample was prepared, with consists ofpressed galactose with 100 10

.2 E d

=

1

Of1 0,Ol 0,001 24

29

34

39

44

49

54

59

Mass

Figure 1. Mass spectrum secondary ions

of PA, neg.

Figure 2. images of: C- (left); COOH- (right) of the test sample. upper part: galactose lower part: PA

a PA

drop on it. A mass spectrum of PA is shown in Fig.1. It is obvious, that there is a significant intensity at mass 45, with represents the COOHmolecule. Fig. 2 shows an image of the interface between PA an galactose for C- and COOH-. The size of all images is 150 pm x 150 pm. A line scan perpendicular to the interface from PA to galactose I is given in Fig. 3. The intensity ratio of the COOH 60 40 20 0 intensity of PA and galactose is more then a factor Distance [pm] of 10. It can be seen that this mass can be used for _ localizing the PA. It must be emphasized here, that Figure 3. Line scan from the COOHapriory the system contains only these two organic image in Fig. 2 from PA to galactose compounds. This is true for the test sample as well perpendicular to the interface as the PA/ galactose system. Otherwise there were no possibility to solve the problem in this way. To get information about the size of the PA particles the sputter rate has to be determined. It was lnm/sec. In Fig.4 a subset of image planes from the PA / galactose system is given. It presents beside a reference image for C the COOH distributions ofthree different depth regions. The bright points indicate, where the PA is located. A depth profile through all image planes from the marked point in Fig. 4 is given in Fig. 5 . The 50% intensity of the maximum can be used for the estimation of the size of the particle.

Figure 4. first: C- -reference image, second to forth: COOH- image at indicated depth region

I

120,

w

.Jf

~_ ._- ~~~

--

4. Results

COOH Depth profile

2

4

Depth b-d

6

The measurements discussed above give an impression of the features of PA in this special system. The size of the PA particles and their distribution confirm assumptions, deduced from technological experience. The distribution is very inhomogenious. The size is in the order of 1 pm.

5. Conclusions Figure 5. Depth profile of a PA particle, indicated in Fig. 4

The application of dynamic SIMS was the only way to get reasonable results at this problem. The preparationofthe samples is very easy and the time necessary for these measurements is rather low. If there are characteristic atoms in admixtures of organic systems and their distribution have to be analysed, the measurements become still easier than above, because then the fractionation of the high molecular compounds under the bombardment of the primary ions can be neglected.

References 1. 2.

3.

Biology of the Cell, Vol.74 (1992) 1, Editions Scientifiques Elsevier Paris A Benninghoven; F.G. Rtidenauer; H.W. Werner: ‘Secondary Ion Mass Spectrometry: Basic Concepts, Instrumental Aspects, Applications and Trends’ Wiley and Sons, New York 1987 A. Benninghoven, Analytical Chemistry 65( 1993) 630A - 640A